EP 0767740 B1
Europaisches Patentamt
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J
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European Patent Office
Office europeen des brevets
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EP
0 767
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E U R O P E A N PATENT S P E C I F I C A T I O N
(12)
ation and mention
mention
(45) Date of publication
of the grant of the patent:
22.12.1999 Bulletin 1999/51
(51) int. CI.6: B41J 13/00,
B41J 1 3 / 0 8
(86) International application number:
PCT/US96/04818
(21) Application number: 96912622.6
(87) International publication number:
WO 96/32290 (17.10.1996 Gazette 1996/46)
(22) Date of filing: 10.04.1996
(54) MODULAR DIGITAL PRINTING
MODULARE DIGITALE DRUCKTECHNIK
IMPRESSION NUMERIQUE MODULAIRE
(56) References cited:
EP-A- 0 317 762
US-A-5150167
(84) Designated Contracting States:
DEGB
(30) Priority: 12.04.1995 AU PN2331 95
12.04.1995 AU PN233295
(43) Date of publication of application:
16.04.1997 Bulletin 1997/16
(73) Proprietor:
EASTMAN KODAK COMPANY
Rochester, New York 14650-2201 (US)
(72) Inventor: SILVERBROOK, Kia
Leichhardt, NSW 2040 (AU)
(74) Representative:
Reichert, Werner R, Dr. rer.nat., Dipl.-Phys. et al
Kodak Aktiengesellschaft,
Patent Department
70323 Stuttgart (DE)
US-A- 4 694 307
• PATENT ABSTRACTS OF JAPAN vol. 014, no.
486 (M-1038), 23 October 1990 & JP,A,02 196679
(NEC CORP), 3 August 1990,
• PATENT ABSTRACTS OF JAPAN vol. 009, no.
114 (M-380), 18 May 1985 & JP,A,60 000980
(TATEISHI DENKI KK), 7 January 1985,
• PATENT ABSTRACTS OF JAPAN vol. 008, no.
242 (P-311), 7 November 1984 & JP,A,59 116840
(CASIO KEISANKI KK), 5 July 1984,
• PATENT ABSTRACTS OF JAPAN vol. 94, no. 01 2
& JP,A,06 340137 (HITACHI LTD), 13 December
1994,
• SEYBOLD volume 24 Number 13 13/03/95 The
Omnius Press
• SEYBOLD volume 22 Number 20 12/07/93
Indigo's E-Print
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Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give
notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in
a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art.
99(1) European Patent Convention).
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EP 0 767 740 B1
Description
Field of the Invention
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[0001 ] The present invention relates to computer controlled and in particular to digital printing with a plurality of cooperative modular printer devices.
Background of the invention
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[0002] At present, most high volume full color printing is performed by web fed and sheet fed offset color presses.
These machines print color pages using four printing plates, one for each of the four color components used in process
printing; cyan, magenta, yellow, and black (CMYK). While these machines are highly efficient in printing large volumes
of color pages, it is difficult, time consuming, and expensive to change the image being printed. When a new image is
to be printed, color separations of the image must be created. Then proof sheets are created, to verify the quality and
color of the printed image. These are usually created by a photographic process wing the color separations created for
the printing press. When the proof sheets are approved, four printing plates must be etched with the color separation
images. Offset presses are also large and expensive and required extensive technical knowledge to operate effectively.
Many technical parameters, such as dot gain, registration, and screen angles must be carefully controlled to obtain
acceptable results. If the print run is greater than 10,000 copies, the set-up costs of the press can be effectively amortized over the volume printed. However, the cost and time required to set up a color press mean that only rarely is fewer
than 500 copies of a page printed, If fewer than one hundred copies of a page are to be printed, then color copiers are
generally used.
[0003] There is increasing recognition in the industry of the need for digital color printing presses, which are capable
of printing high quality color pages directly from computer data, without requiring photographic and platemaking processes. These are considered to be most cost effective for print runs of between 100 copies and 10,000 copies.
[0004] A digital color printing press accepts a digital version of the page from a computer system, and directly prints
the color images. Many technologies have been developed to directly print color pages from digital information, but
none yet are cost effective for medium or high volume color printing.
[0005] One such technology presently on the market is digital laser electrophotographic color printing. However, the
throughput and image quality of this system is inadequate for medium volume printing. As the system uses a single
scanned laser beam to generate the image, the throughput is inherently limited by the modulation rate, intensity, and
scanning rate of the laser. Other electrophotographic based approaches have been developed and marketed with success in some lower throughput regions of the 100 to 10,000 copies range.
[0006] While such machines as in U.S. Patent No. 5,150,167 are viable for short run printing, they are not suitable as
replacements for offset presses for medium or large run printing. The throughput is substantially lower, and cost per
page substantially higher, than offset printing for print runs in excess of a few thousand copies. Although these
machines can be used in parallel to increase the overall printing throughput, the cost of these systems is quite high. The
capital cost combined with the high cost per page makes parallel systems not cost competitive with traditional offset
printing for medium or large print runs.
[0007] Thus, there is a widely recognized need for a high speed digitally controlled printing system able to produce
high quality images using standard paper and low cost inks, that is able to compete effectively against mechanical technologies for medium and high volume printing.
Summary of the invention
[0008] My concurrently filed application, entitled "A Liquid Ink Printing Apparatus and System" describes new methods and apparatus that afford significant improvements toward overcoming the prior art problems discussed above.
Those inventions offer important advantages, e.g., in regard to drop size and placement accuracy, as to printing speeds
attainable, as to power usage, as to durability and operative thermal stresses encountered and as to other printer performance characteristics, as well as in regard to manufacturability and the characteristics of useful inks. One important
purpose of the present invention is to further enhance the structures and methods described in that patent application
and thereby contribute to the advancement of printing technology.
[0009] One object of the present invention is to provide a digital color printing press characterized by a plurality of
printing modules being adapted to be cascaded to achieve a higher total printing rate.
[001 0] Thus, in one aspect, the present invention constitutes a digital printing system comprising a plurality of digital
printer modules, each including means for supporting and feeding a print medium from a supply station through a print
path and from a print path outlet, means for pronging upon said medium during its movement through said print path,
and sheet conveyor means for transporting sheets from said print path outlet along a module transport segment to a
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module egress, said modules being interconnected in a serial array wherein the module egress of upstream modules
are coupled to the print sheet outlet region of the adjacent downstream modules so that a stack of print sheets builds
up upon the coupled conveyor means as the stack passes along the transport segments, from the first module to the
last module.
[0011] One preferred feature of the invention is that the paper supply is a roll on a removable frame that includes
wheels mounted on the underside.
[001 2] Another preferred feature of the invention is that the paper transport between the printing modules is also modular.
[001 3] Another preferred form of the invention is a digital color printing press comprising:
(a) means for connecting to a raster image processing computer to receive data for producing a plurality of digitally
halftoned binary page images;
(b) a plurality of digital page memories for storing such binary page image data;
(c) a plurality of liquid ink printing heads;
(d) a paper transport system which moves a marking medium past said printing heads as the page image is being
printed; and
(e) an ink reservoir and ink pressure regulation system which maintains ink flow to the said heads.
[0014] Another preferred feature of the invention is that the printing heads are fixed at the same height.
[001 5] Another preferred feature of the invention is that there is a single ink reservoir for each color which supplies all
of the said printing heads.
[001 6] Another preferred embodiment provides at least two printing heads per module adapted to print simultaneously
on opposite sides of the print medium, respectively.
Brief Description of the Drawings
[0017]
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Figure 1(a) shows a simplified block schematic diagram of one exemplary printing apparatus according to the
present invention.
Figure 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention.
Figures 2(a) to 2(f) show fluid dynamic simulations of drop selection.
Figure 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of
the invention.
Figure 3(b) shows successive meniscus positions during drop selection and separation.
Figure 3(c) shows the temperatures at various points during a drop selection cycle.
Figure 3(d) shows measured surface tension versus temperature curves for various ink additives.
Figure 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of
figure 3(c)
Figure 4 shows a block schematic diagram of print head drive circuitry for practice of the invention.
Figure 5 shows projected manufacturing yields for an A4 page width color print head embodying features of the
invention, with and without fault tolerance.
Figure 6 shows a schematic system diagram of one preferred digital printing configuration using digital color printing modules.
Figure 7 is a simplified schematic of one preferred digital color printing press module
Figure 8 shows a simplified schematic diagram of a single printing head driver system of a digital color printing
press using printing technology of the Figure 1 system.
Figure 9 shows the major modules and the paper path of a single printing module.
Figure 10 shows three modules of a high volume printing line.
Figure 11(a) shows a modular printing line printing a ten sheet document.
Figure 11(b) shows the occurrence of a faulty printing module in the printing line of figure 11(a).
Figure 11(c) shows the operation of the printing line in a fault tolerant manner.
Figure 12(a) shows a modular printing line with a bidirectional data connection between adjacent printing modules.
Figure 12(b) shows data transferred 'downstream' from a faulty printing module immediately after detection of the
fault
Figure 12(c) shows data transferred 'upstream' to restore normal operation after a fault has been corrected.
Figure 13 is a simplified schematic of a digital color printing press module which includes high speed data links to
adjacent printing modules.
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Figure 14 is an external view showing the approximate size of a line of eight digital color printing modules.
Detailed Description of Preferred Embodiments
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[0018] In one general aspect, the invention constitutes a drop-on-demand printing mechanism wherein the means of
selecting drops to be printed produces a difference in position between selected drops and drops which are not
selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body
of ink, and wherein an alternative means is provided to cause separation of the selected drops from the body of ink.
[001 9] The separation of drop selection means from drop separation means significantly reduces the energy required
to select which ink drops are to be printed. Only the drop selection means must be driven by individual signals to each
nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles.
[0020] The drop selection means may be chosen from, but is not limited to, the following list:
1)
2)
3)
4)
Electrothermal reduction of surface tension of pressurized ink
Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection
Piezoelectric, with insufficient volume change to cause drop ejection
Electrostatic attraction with one electrode per nozzle
The drop separation means may be chosen from, but is not limited to, the following list:
[0021]
1)
2)
3)
4)
Proximity (recording medium in close proximity to print head)
Proximity with oscillating ink pressure
Electrostatic attraction
Magnetic attraction
[0022] The table "DOD printing technology targets" shows some desirable characteristics of drop on demand printing
technology. The table also lists some methods by which some embodiments described herein, or in other of my related
applications, provide improvements over the prior art.
DOD printing technology targets
[0023]
Target
Method of achieving improvement over prior art
High speed operation
Practical, low cost, pagewidth printing heads with more than 10,000
nozzles. Monolithic A4 pagewidth print heads can be manufactured
using standard 300 mm (12") silicon wafers
High image quality
High resolution (800 dpi is sufficient for most applications), six color
process to reduce image noise
Full color operation
Halftoned process color at 800 dpi using stochastic screening
Ink flexibility
Low operating ink temperature and no requirement for bubble formation
Low power requirements
Low power operation results from drop selection means not being
required to fully eject drop
Low cost
Monolithic print head without aperture plate, high manufacturing
yield, small number of electrical connections, use of modified existing CMOS manufacturing facilities
High manufacturing yield
Integrated fault tolerance in printing head
High reliability
Integrated fault tolerance in printing head. Elimination of cavitation
and kogation. Reduction of thermal shock.
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Target
Method of achieving improvement over prior art
Small number of electrical connections
Shift registers, control logic, and drive circuitry can be integrated on
a monolithic print head using standard CMOS processes
Use of existing VLSI manufacturing facilities
CMOS compatibility. This can be achieved because the heater drive
power is less is than 1% of Thermal Ink Jet heater drive power
Electronic collation
A new page compression system which can achieve 100:1 compression with insignificant image degradation, resulting in a compressed
data rate low enough to allow real-time printing of any combination
of thousands of pages stored on a low cost magnetic disk drive.
[0024] In thermal ink jet (TIJ) and piezoelectric inkjet systems, a drop velocity of approximately 10 meters per second
is preferred to ensure that the selected ink drops overcome ink surface tension, separate from the body of the ink, and
strike the recording medium. These systems have a very low efficiency of conversion of electrical energy into drop
kinetic energy. The efficiency of TIJ systems is approximately 0.02%). This means that the drive circuits for TIJ print
heads must switch high currents. The drive circuits for piezoelectric ink jet heads must either switch high voltages, or
drive highly capacitive loads. The total power consumption of pagewidth TIJ printheads is also very high. An 800 dpi A4
full color pagewidth TIJ print head printing a four color black image in one second would consume approximately 6 kW
of electrical power, most of which is converted to waste heat. The difficulties of removal of this amount of heat precludes
the production of low cost, high speed, high resolution compact pagewidth TIJ systems.
[0025] One important feature of embodiments of the invention is a means of significantly reducing the energy required
to select which ink drops are to be printed. This is achieved by separating the means for selecting ink drops from the
means for ensuring that selected drops separate from the body of ink and form dots on the recording medium. Only the
drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or
condition applied simultaneously to all nozzles.
[0026] The table "Drop selection means" shows some of the possible means for selecting drops in accordance with
the invention. The drop selection means is only required to create sufficient change in the position of selected drops
that the drop separation means can discriminate between selected and unselected drops.
Drop selection means
[0027]
Method
Advantage
Limitation
1. Electrothermal reduction of surface tension of pressurized ink
Low temperature increase and low
Requires ink pressure regulating
mechanism. Ink surface tension
drop selection energy. Can be used
with many ink types. Simple fabrica- must reduce substantially as tempertion. CMOS drive circuits can be fab- ature increases
ricated on same substrate
2. Electrothermal reduction of ink
viscosity, combined with oscillating ink pressure
Medium drop selection energy, suita- Requires ink pressure oscillation
ble for hot melt and oil based inks.
mechanism. Ink must have a large
decrease in viscosity as temperature
Simple fabrication. CMOS drive circuits can be fabricated on same sub- increases
strate
3. Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection
Well known technology, simple fabrication, bipolar drive circuits can be
fabricated on same substrate
High drop selection energy, requires
water based ink, problems with
kogation, cavitation, thermal stress
4. Piezoelectric, with insufficient
volume change to cause drop ejection
Many types of ink base can be used
High manufacturing cost, incompatible with integrated circuit processes,
high drive voltage, mechanical complexity, bulky
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Method
Advantage
Limitation
5. Electrostatic attraction with one
electrode per nozzle
Simple electrode fabrication
Nozzle pitch must be relatively large.
Crosstalk between adjacent electric
fields. Requires high voltage drive
circuits
[0028] Other drop selection means may also be used.
[0029] The preferred drop selection means for water based inks is method 1: "Electrothermal reduction of surface tension of pressurized ink". This drop selection means provides many advantages over other systems, including; low
power operation (approximately 1% of TIJ), compatibility with CMOS VLSI chip fabrication, low voltage operation
(approx. 10 V), high nozzle density, low temperature operation, and wide range of suitable ink formulations. The ink
must exhibit a reduction in surface tension with increasing temperature.
[0030] The preferred drop selection means for hot melt or oil based inks is method 2: "Electrothermal reduction of ink
viscosity, combined with oscillating ink pressure". This drop selection means is particularly suited for use with inks
which exhibit a large reduction of viscosity with increasing temperature, but only a small reduction in surface tension.
This occurs particularly with non-polar ink carriers with relatively high molecular weight This is especially applicable to
hot melt and oil based inks.
[0031 ] The table "Drop separation means" shows some of the possible methods for separating selected drops from
the body of ink, and ensuring that the selected drops form dots on the printing medium. The drop separation means
discriminates between selected drops and unselected drops to ensure that unselected drops do not form dots on the
printing medium.
Drop separation means
[0032]
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Means
Advantage
Limitation
1. Electrostatic attraction
Can print on rough surfaces, simple
implementation
Requires high voltage power supply
2. AC electric field
Higher field strength is possible than
electrostatic, operating margins can
be increased, ink pressure reduced,
and dust accumulation is reduced
Requires high voltage AC power supply synchronized to drop ejection
phase. Multiple drop phase operation
is difficult
3. Proximity (printhead in close
proximity to, but not touching,
recording medium)
Very small spot sizes can be
achieved. Very low power dissipation. High drop position accuracy
Requires print medium to be very
close to print head surface, not suitable for rough print media, usually
requires transfer roller or belt
4. Transfer Proximity (print head is
in close proximity to a transfer
roller or belt
Very small spot sizes can be
achieved, very low power dissipation, high accuracy, can print on
rough paper
Not compact due to size of transfer
roller or transfer belt.
5. Proximity with oscillating ink
pressure
Useful for hot melt inks using viscosity reduction drop selection method,
reduces possibility of nozzle clogging, can use pigments instead of
dyes
Requires print medium to be very
close to print head surface, not suitable for rough print media. Requires
ink pressure oscillation apparatus
6. Magnetic attraction
Can print on rough surfaces. Low
power if permanent magnets are
used
Requires uniform high magnetic field
strength, requires magnetic ink
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[0033] Other drop separation means may also be used.
[0034] The preferred drop separation means depends upon the intended use. For most applications, method 1: "Electrostatic attraction", or method 2: "AC electric field" are most appropriate. For applications where smooth coated paper
or film is used, and very high speed is not essential, method 3: "Proximity" may be appropriate. For high speed, high
quality systems, method 4: "Transfer proximity" can be used. Method 6: "Magnetic attraction" is appropriate for portable
printing systems where the print medium is too rough for proximity printing, and the high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances.
[0035] A simplified schematic diagram of one preferred printing system according to the invention appears in Figure
1(a).
[0036] An image source 52 may be raster image data from a scanner or computer, or outline image data in the form
of a page description language (PDL), or other forms of digital image representation. This image data is converted to a
pixel-mapped page image by the image processing system 53. This may be a raster image processor (RIP) in the case
of PDL image data, or may be pixel image manipulation in the case of raster image data. Continuous tone data produced by the image processing unit 53 is halftoned. Halftoning is performed by the Digital Halftoning unit 54. Halftoned
bitmap image data is stored in the image memory 72. Depending upon the printer and system configuration, the image
memory 72 may be a full page memory, or a band memory. Heater control circuits 71 read data from the image memory
72 and apply time-varying electrical pulses to the nozzle heaters (103 in figure 1(b)) that are part of the print head 50.
These pulses are applied at an appropriate time, and to the appropriate nozzle, so that selected drops will form spots
on the recording medium 51 in the appropriate position designated by the data in the image memory 72.
[0037] The recording medium 51 is moved relative to the head 50 by a paper transport system 65, which is electronically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315. The paper
transport system shown in figure 1(a) is schematic only, and many different mechanical configurations are possible. In
the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50.
However, in the case of scanning print systems, it is usually most convenient to move the head 50 along one axis (the
sub-scanning direction) and the recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion. The microcontroller 315 may also control the ink pressure regulator 63 and the heater control circuits 71 .
[0038] For printing using surface tension reduction, ink is contained in an ink reservoir 64 under pressure. In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a
drop. A constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink
pressure regulator 63. Alternatively, for larger printing systems, the ink pressure can be very accurately generated and
controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink
level can be regulated by a simple float valve (not shown).
[0039] For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and the ink pressure is caused to oscillate. The means of producing this oscillation may be a piezoelectric actuator mounted in the ink
channels (not shown).
[0040] When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51 , while unselected drops remain part of the body of ink.
[0041 ] The ink is distributed to the back surface of the head 50 by an ink channel device 75. The ink preferably flows
through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the nozzles
and actuators are situated. In the case of thermal selection, the nozzle actuators are electrothermal heaters.
[0042] In some types of printers according to the invention, an external field 74 is required to ensure that the selected
drop separates from the body of the ink and moves towards the recording medium 51 . A convenient external field 74 is
a constant electric field, as the ink is easily made to be electrically conductive. In this case, the paper guide or platen
67 can be made of electrically conductive material and used as one electrode generating the electric field. The other
electrode can be the head 50 itself. Another embodiment uses proximity of the print medium as a means of discriminating between selected drops and unselected drops.
[0043] For small drop sizes gravitational force on the ink drop is very small; approximately 10"4 of the surface tension
forces, so gravity can be ignored in most cases. This allows the print head 50 and recording medium 51 to be oriented
in any direction in relation to the local gravitational field. This is an important requirement for portable printers.
[0044] Figure 1(b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the
invention, fabricated using a modified CMOS process. The nozzle is etched in a substrate 101, which may be silicon,
glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and
data distribution circuitry may be formed in the surface semiconducting layer. Single crystal silicon (SCS) substrates
have several advantages, including:
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2)
3)
4)
High
Print
SCS
SCS
performance drive transistors and other circuitry can be fabricated in SCS;
heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment;
has high mechanical strength and rigidity; and
has a high thermal conductivity.
[0045] In this example, the nozzle is of cylindrical form, with the heater 103 forming an annulus. The nozzle tip 104 is
formed from silicon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry. The nozzle tip is
passivated with silicon nitride. The protruding nozzle tip controls the contact point of the pressurized ink 100 on the print
head surface. The print head surface is also hydrophobized to prevent accidental spread of ink across the front of the
print head.
[0046]
Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in
shape, dimensions, and materials used. Monolithic nozzles etched from the substrate upon which the heater and drive
electronics are formed have the advantage of not requiring an orifice plate. The elimination of the orifice plate has significant cost savings in manufacture and assembly. Recent methods for eliminating orifice plates include the use of 'vortex' actuators such as those described in Domoto et al US Pat No. 4,580,158, 1986, assigned to Xerox, and Miller et al
US Pat No. 5,371 ,527, 1994 assigned to Hewlett-Packard. These, however are complex to actuate, and difficult to fabricate. The preferred method for elimination of orifice plates for print heads of the invention is in corporation of the orifice
into the actuator substrate.
[0047] This type of nozzle may be used for print heads using various techniques for drop separation.
Operation with Electrostatic Drop Separation
[0048] As a first example, operation using thermal reduction of surface tension and electrostatic drop separation is
shown in figure 2.
[0049] Figure 2 shows the results of energy transport and fluid dynamic simulations performed using FIDAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois, USA. This simulation
is of a thermal drop selection nozzle embodiment with a diameter of 8 urn, at an ambient temperature of 30°C. The total
energy applied to the heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air
pressure, and the ink viscosity at 30°C is 1.84 cPs. The ink is water based, and includes a sol of 0.1% palmitic acid to
achieve an enhanced decrease in surface tension with increasing temperature. A cross section of the nozzle tip from
the central axis of the nozzle to a radial distance of 40 urn is shown. Heat flow in the various materials of the nozzle,
including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated
using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 us.
[0050] Figure 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no
ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of
the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus.
[0051 ] Figure 2(b) shows thermal contours at 5°C intervals 5 us after the start of the heater energizing pulse. When
the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes
the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus. This drives a convective flow
which rapidly transports this heat over pan of the free surface of the ink at the nozzle tip. It is necessary for the heat to
be distributed over the ink surface, and not just where the ink is in contact with the heater. This is because viscous drag
against the solid heater prevents the ink directly in contact with the heater from moving.
[0052] Figure 2(c) shows thermal contours at 5°C intervals 10 us after the start of the heater energizing pulse. The
increase in temperature causes a decrease in surface tension, disturbing the equilibrium of forces. As the entire meniscus has been heated, the ink begins to flow.
[0053] Figure 2(d) shows thermal contours at 5°C intervals 20 us after the start of the heater energizing pulse. The
ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head. The electrostatic field becomes concentrated by the protruding conductive ink drop.
[0054] Figure 2(e) shows thermal contours at 5°C intervals 30 us after the start of the heater energizing pulse, which
is also 6 us after the end of the heater pulse, as the heater pulse duration is 24 us. The nozzle tip has rapidly cooled
due to conduction through the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water
cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium.
Were the heater pulse significantly shorter (less than 16 us in this case) the ink would not accelerate towards the print
medium, but would instead return to the nozzle.
[0055] Figure 2(f) shows thermal contours at 5°C intervals 26 us after the end of the heater pulse. The temperature
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at the nozzle tip is now less than 5°C above ambient temperature. This causes an increase in surface tension around
the nozzle tip. When the rate at which the ink is drawn from the nozzle exceeds the viscously limited rate of ink flow
through the nozzle, the ink in the region of the nozzle tip 'necks', and the selected drop separates from the body of ink.
The selected drop then travels to the recording medium under the influence of the external electrostatic field. The
meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the
next ink drop. One ink drop is selected, separated and forms a spot on the recording medium for each heat pulse. As
the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved.
[0056] Figure 3(a) shows successive meniscus positions during the drop selection cycle at 5 us intervals, starting at
the beginning of the heater energizing pulse.
[0057] Figure 3(b) is a graph of meniscus position versus time, showing the movement of the point at the centre of
the meniscus. The heater pulse starts 10 us into the simulation.
[0058] Figure 3(c) shows the resultant curve of temperature with respect to time at various points in the nozzle. The
vertical axis of the graph is temperature, in units of 100°C. The horizontal axis of the graph is lime, in units of 10 us. The
temperature curve shown in figure 3(b) was calculated by FIDAR using 0.1 us time steps. The local ambient temperature is 30 degrees C. Temperature histories at three points are shown:
A - Nozzle tip: This shows the temperature history at the circle of contact between the passivation layer, the ink,
and air.
B - Meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and the centre of the
meniscus.
C - Chip surface: This is at a point on the print head surface 20 urn from the centre of the nozzle. The temperature
only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures.
[0059] Figure 3(e) shows the power applied to the heater. Optimum operation requires a sharp rise in temperature at
the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration
of the pulse, and a rapid fall in temperature at the end of the pulse. To achieve this, the average energy applied to the
heater is varied over the duration of the pulse. In this case, the variation is achieved by pulse frequency modulation of
0. 1 us sub-pulses, each with an energy of 4 nJ. The peak power applied to the heater is 40 mW, and the average power
over the duration of the heater pulse is 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be
varied without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control
over the power applied to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable
for minimizing the effect of radio frequency interference (RFI).
35
Inks with a negative temperature coefficient of surface tension
40
The requirement for the surface tension of the ink to decrease with increasing temperature is not a major
[0060]
restriction, as most pure liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and
Shields is satisfactory for many liquids:
45
so
55
[0061] Where jT is the surface tension at temperature T, k is a constant, Tc is the critical temperature of the liquid,
M is the molar mass of the liquid, x is the degree of association of the liquid, and p is the density of the liquid. This equation indicates that the surface tension of most liquids fails to zero as the temperature reaches the critical temperature
of the liquid. For most liquids, the critical temperature is substantially above the boiling point at atmospheric pressure,
so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperature, the admixture of surfactants is recommended.
[0062] The choice of surfactant is important. For example, water based ink for thermal ink jet printers often contains
isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying. Isopropyl alcohol has a boiling
point of 82.4°C, lower than that of water. As the temperature rises, the alcohol evaporates faster than the water,
decreasing the alcohol concentration and causing an increase in surface tension. A surfactant such as 1-Hexanol (b.p.
158°C) can be used to reverse this effect, and achieve a surface tension which decreases slightly with temperature.
However, a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude. A
9
EP 0 767 740 B1
surface tension decrease of 20 mN/m over a 30°C temperature range is preferred to achieve large operating margins,
while as little as 10mN/m can be used to achieve operation of the print head according to the present invention.
5
10
15
Inks With Large -Ayj
[0063] Several methods may be used to achieve a large negative change in surface tension with increasing temperature. Two such methods are:
1) The ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at
a threshold temperature. Particle sizes less than 1,000 A are desirable. Suitable surfactant melting points for a
water based ink are between 50°C and 90°C, and preferably between 60°C and 80°C.
2) The ink may contain an oil/water microemulsion with a phase inversion temperature (PIT) which is above the
maximum ambient temperature, but below the boiling point of the ink. For stability, the PIT of the microemulsion is
preferably 20°C or more above the maximum non-operating temperature encountered by the ink. A PIT of approximately 80°C is suitable.
Inks with Surfactant Sols
20
35
40
45
[0064] Inks can be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as:
Name
Formula
m.p.
Synonym
Tetradecanoic acid
CH3(CH2)12COOH
58°C
Myristic acid
Hexadecanoic acid
CH3(CH2)14COOH
63°C
Palmitic acid
Octadecanoic acid
CH3(CH2)15COOH
71°C
Stearic acid
Eicosanoic acid
CH3(CH2)16COOH
77°C
Arachidic acid
Docosanoic acid
CH3(CH2)20COOH
80°C
Behenicacid
[0065] As the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxylic acid with a melting point slightly above the desired drop selection temperature. A good
example is Arachidic acid.
[0066] These carboxylic acids are available in high purity and at low cost. The amount of surfactant required is very
small, so the cost of adding them to the ink is insignificant. A mixture of carboxylic acids with slightly varying chain
lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less
than the pure acid.
[0067] It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic acids. Surfactants with
branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of the surfactant. It is desirable that the polar
end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent
flocculation. In the case of carboxylic acids, this can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide.
Preparation of Inks with Surfactant Sols
so
55
[0068] The surfactant sol can be prepared separately at high concentration, and added to the ink in the required concentration.
[0069] An example process for creating the surfactant sol is as follows:
1) Add the carboxylic acid to purified water in an oxygen free atmosphere.
2) Heat the mixture to above the melting point of the carboxylic acid. The water can be brought to a boil.
3) Ultrasonicate the mixture, until the typical size of the carboxylic acid droplets is between 100A and 1,000A.
4) Allow the mixture to cool.
5) Decant the larger particles from the top of the mixture.
10
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6) Add an alkali such as NaOH to ionize the carboxylic acid molecules on the surface of the particles. A pH of
approximately 8 is suitable. This step is not absolutely necessary, but helps stabilize the sol.
7) Centrifuge the sol. As the density of the carboxylic acid is lower than water, smaller particles will accumulate at
the outside of the centrifuge, and larger particles in the centre.
8) Filter the sol using a microporous filter to eliminate any particles above 5000 A.
9) Add the surfactant sol to the ink preparation. The sol is required only in very dilute concentration.
[0070] The ink preparation will also contain either dye(s) or pigment(s), bactericidal agents, agents to enhance the
electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required.
[0071] Anti-foaming agents will generally not be required, as there is no bubble formation during the drop ejection
process.
Cationic surfactant sols
15
[0072] Inks made with anionic surfactant sols are generally unsuitable for use with cationic dyes or pigments. This is
because the cationic dye or pigment may precipitate or flocculate with the anionic surfactant. To allow the use of cationic
dyes and pigments, a cationic surfactant sol is required. The family of alkylamines is suitable for this purpose.
[0073] Various suitable alkylamines are shown in the following table:
20
Name
30
35
40
45
so
55
Formula
Synonym
Hexadecylamine
CH3(CH2)i4CH2NH2
Palmityl amine
Octadecylamine
CH3(CH)16CH2NH2
Stearyl amine
Eicosylamine
CH3(CH2)18CH2NH2
Arachidyl amine
Docosylamine
CH3(CH)20CH2NH2
Behenyl amine
[0074] The method of preparation of cationic surfactant sols is essentially similar to that of anionic surfactant sols,
except that an acid instead of an alkali is used to adjust the pH balance and increase the charge on the surfactant particles. A pH of 6 using HCI is suitable.
Microemulsion Based Inks
[0075] An alternative means of achieving a large reduction in surface tension as some temperature threshold is to
base the ink on a microemulsion. A microemulsion is chosen with a phase inversion temperature (PIT) around the
desired ejection threshold temperature. Below the PIT, the microemulsion is oil in water (O/W), and above the PIT the
microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significantly above the PIT, the surfactant prefers a high curvature surface around water. At temperatures close to the PIT, the microemulsion forms a continuous 'sponge' of topologically
connected water and oil.
[0076] There are two mechanisms whereby this reduces the surface tension. Around the PIT, the surfactant prefers
surfaces with very low curvature. As a result, surfactant molecules migrate to the ink/air interface, which has a curvature
which is much less than the curvature of the oil emulsion. This lowers the surface tension of the water. Above the phase
inversion temperature, the microemulsion changes from O/W to W/O, and therefore the ink/air interface changes from
water/air to oil/air. The oil/air interface has a lower surface tension.
[0077] There is a wide range of possibilities for the preparation of microemulsion based inks.
[0078] For fast drop ejection, it is preferable to chose a low viscosity oil.
[0079] In many instances, water is a suitable polar solvent. However, in some cases different polar solvents may be
required. In these cases, polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable.
[0080] The surfactant can be chosen to result in a phase inversion temperature in the desired range. For example,
surfactants of the group poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula:
CnH2n+i C4H6(CH2CH20)mOH) can be used. The hydrophilicity of the surfactant can be increased by increasing m, and
the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable.
11
EP 0 767 740 B1
[0081 ] Low cost commercial preparations are the result of a polymerization of various molar ratios of ethylene oxide
and alkyl phenols, and the exact number of oxyethylene groups varies around the chosen mean. These commercial
preparations are adequate, and highly pure surfactants with a specific number of oxyethylene groups are not required.
[0082] The formula for this surfactant is C8H17C4H6(CH2CH20)nOH (avenge n=10).
[0083] Synonyms include Octoxynol-10, PEG-10 octyl phenyl ether and POE (10) octyl phenyl ether
[0084] The HLB is 13.6, the melting point is 7°C, and the cloud point is 65°C.
Commercial preparations of this surfactant are available under various brand names. Suppliers and brand
[0085]
names are listed in the following table:
Trade name
Supplier
AkyporoxOP100
Chem-YGmbH
Alkasurf OP-10
Rhone-Poulenc Surfactants and Specialties
Dehydrophen POP 10
Pulcra SA
Hyonic OP-10
Henkel Corp.
Iconol OP-10
BASF Corp.
Igepal O
Rhone-Poulenc France
Macol OP-10
PPG Industries
Malorphen 81 0
Huls AG
Nikkol OP-10
Nikko Chem. Co. Ltd.
Renex 750
ICI Americas Inc.
Rexol 45/10
Hart Chemical Ltd.
Synperonic OP10
ICI PLC
TericXIO
ICI Australia
[0086] These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute
less than 10 cents per liter to prepared microemulsion ink with a 5% surfactant concentration.
[0087] Other suitable ethoxylated alkyl phenols include those listed in the following table:
Trivial name
[0088]
Formula
HLB
Cloud point
Nonoxynol-9
C9H19C4H6(CH2CH20).9OH
13
54°C
Nonoxynol-10
C9H19C4H6(CH2CH2O).10OH
13.2
62°C
Nonoxynol-11
CgH^C^CHaCHaOJ.nOH
13.8
72°C
Nonoxynol-12
C9H19C4H6(CH2CH20).12OH
14.5
81 °C
Octoxynol-9
C8H17C4H6(CH2CH20).9OH
12.1
61 °C
Octoxynol-10
C8H17C4H6(CH2CH2O).10OH
13.6
65°C
Octoxynol-12
C8H17C4H6(CH2CH20)-12OH
14.6
88°C
Dodoxynol-10
C12H25C4H6(CH2CH2O).10OH
12.6
42°C
Dodoxynol-11
C^H^Hg^HgCHgOJ.nOH
13.5
56°C
Dodoxynol-14
C12H25C4H6(CH2CH20).14OH
14.5
87°C
Microemulsion based inks have advantages other than surface tension control:
EP 0 767 740 B1
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1) Micro-emulsions are thermodynamically stable, and will not separate. Therefore, the storage time can be very
long. This is especially significant for office and portable printers, which may be used sporadically.
2) The microemulsion will form spontaneously with a particular drop size, and does not require extensive stirring,
centrifuging, or filtering to ensure a particular range of emulsified oil drop sizes.
3) The amount of oil contained in the ink can be quite high, so dyes which are soluble in oil or soluble in water, or
both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to
obtain specific colors.
4) Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets.
5) The use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium.
6) The viscosity of microemulsions is very low.
7) The requirement for humectants can be reduced or eliminated.
Dyes and pigments in microemulsion based inks
15
20
25
[0089] Oil in water mixtures can have high oil contents - as high as 40% - and still form O/W microemulsions. This
allows a high dye or pigment loading.
[0090] Mixtures of dyes and pigments can be used. An example of a microemulsion based ink mixture with both dye
and pigment is as follows:
1)70% water
2) 5% water soluble dye
3) 5% surfactant
4) 10% oil
5) 10% oil miscible pigment
[0091] The following table shows the nine basic combinations of colorants in the oil and water phases of the microemulsion that may be used.
Combination
so
55
Colorant in water phase
Colorant in oil phase
1
none
oil miscible pigment
2
none
oil soluble dye
3
water soluble dye
none
4
water soluble dye
oil miscible pigment
5
water soluble dye
oil soluble dye
6
pigment dispersed in water
none
7
pigment dispersed in water
oil miscible pigment
8
pigment dispersed in water
oil soluble dye
9
none
none
[0092] The ninth combination, with no colorants, is useful for printing transparent coatings, UV ink, and selective gloss
highlights.
[0093] As many dyes are amphiphilic, large quantities of dyes can also be solubilized in the oil-water boundary layer
as this layer has a very large surface area.
[0094] It is also possible to have multiple dyes or pigments in each phase, and to have a mixture of dyes and pigments
in each phase.
[0095] When using multiple dyes or pigments the absorption spectrum of the resultant ink will be the weighted average of the absorption spectra of the different colorants used. This presents two problems:
1) The absorption spectrum will tend to become broader, as the absorption peaks of both colorants are averaged.
This has a tendency to 'muddy' the colors. To obtain brilliant color, careful choice of dyes and pigments based on
their absorption spectra, not just their human-perceptible color, needs to be made.
13
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10
15
20
2) The color of the ink may be different on different substrates. If a dye and a pigment are used in combination, the
color of the dye will tend to have a smaller contribution to the printed ink color on more absorptive papers, as the
dye will be absorbed into the paper, while the pigment will tend to 'sit on top' of the paper. This may be used as an
advantage in some circumstances.
Surfactants with a Krafft point in the drop selection temperature range
[0096] For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and the
solution contains essentially no micelles. Above the Krafft temperature micelle formation becomes possible and there
is a rapid increase in solubility of the surfactant. If the critical micelle concentration (CMC) exceeds the solubility of a
surfactant at a particular temperature, then the minimum surface tension will be achieved at the point of maximum solubility, rather than at the CMC. Surfactants are usually much less effective below the Krafft point
[0097] This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At
ambient temperatures, only a portion of the surfactant is in solution. When the nozzle heater is turned on, the temperature rises, and more of the surfactant goes into solution, decreasing the surface tension.
[0098] A surfactant should be chosen with a Krafft point which is near the top of the range of temperatures to which
the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature.
[0099] The concentration of surfactant should be approximately equal to the CMC at the Krafft point. In this manner,
the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount
at ambient temperatures.
[0100] The following table shows some commercially available surfactants with Krafft points in the desired range.
Formula
Krafft point
C16H33S03"Na+
57°C
C18H37S03"Na+
70°C
C16H33S04"Na+
45°C
Na+-04S(CH2)16S04-Na+
44.9°C
K+-04S(CH)16S04K+
55°C
C16H33CH(CH3)C4H6S03-Na+
60.8°C
Surfactants with a cloud point in the drop selection temperature range
40
45
50
55
[0101] Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension falls with increasing temperature. At low temperatures, the POE chain is hydrophilic, and maintains the surfactant
in solution. As the temperature increases, the structured water around the POE section of the molecule is disrupted,
and the POE section becomes hydrophobic. The surfactant is increasingly rejected by the water at higher temperatures,
resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension. The temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point of that surfactant. POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C
[0102] Polyoxypropylene (POP) can be combined with POE in POE/POP block copolymers to lower the cloud point
of POE chains without introducing a strong hydrophobicity at low temperatures.
[0103] Two main configurations of symmetrical POE/POP block copolymers are available. These are:
1) Surfactants with POE segments at the ends of the molecules, and a POP segment in the centre, such as the
poloxamer class of surfactants (generically CAS 9003-1 1-6)
2) Surfactants with POP segments at the ends of the molecules, and a POE segment in the centre, such as the
meroxapol class of surfactants (generically also CAS 9003-1 1-6)
[0104] Some commercially available varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud point above 40°C and below 100°C are shown in the following table:
14
EP 0 767 740 B1
5
w
Trivial name
BASF Trade name
35
40
45
so
55
Cloud point
Pluronic10R5
HO(CHCH3CH20).7(CH2CH20).22-(CHCH3CH20).
7OH
50.9
69°C
Meroxapol 108
Pluronic10R8
HO(CHCH3CH20).7(CH2CH20).91 -(CHCH3CH20).
7OH
54.1
99°C
Meroxapol 178
Pluronic17R8
HO(CHCH3CH20).12(CH2CH20).136-(CHCH3CH20).
12OH
47.3
81°C
Meroxapol 258
Pluronic25R8
HO(CHCH3CH20).18(CH2CH20).163-(CHCH3CH20).
18OH
46.1
80°C
Poloxamer 105
Pluronic L35
HO(CH2CH20).ir
(CHCH3CH20).16-(CH2CH20).
11OH
48.8
77°C
Poloxamer 124
Pluronic L44
HO(CH2CH20).ir
(CHCH3CH20).21-(CH2CH20).
11OH
45.3
65°C
25
30
Surface Tension
(mN/m)
Meroxapol 105
15
20
Formula
[0105] Other varieties of poloxamer and meroxapol can readily be synthesized using well known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between
40°C and 100°C, and preferably between 60°C and 80°C.
[0106]
Meroxapol [HO(CHCH3CH20)x(CH2CH20)y(CHCH3CH20)zOH] varieties where the average x and z are
approximately 4, and the average y is approximately 15 may be suitable.
[0107] If salts are used to increase the electrical conductivity of the ink, then the effect of this salt on the cloud point
of the surfactant should be considered.
[0108] The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I"), as this makes
more water molecules available to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as CI", OH"), as fewer water molecules are available to
form hydrogen bonds. Bromide ions have relatively little effect. The ink composition can be 'tuned' for a desired temperature range by altering the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice
of salts (e.g CI" to Br" to I") that are added to increase electrical conductivity. NaCI is likely to be the best choice of salts
to increase ink conductivity, due to low cost and non-toxicity. NaCI slightly lowers the cloud point of nonionic surfactants.
Hot Melt Inks
[0109] The ink need not be in a liquid state at room temperature. Solid 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so that the surface
tension of the molten ink decreases with temperature. A decrease of approximately 2 mN/m will be typical of many such
preparations using waxes and other substances. However, a reduction in surface tension of approximately 20 mN/m is
desirable in order to achieve good operating margins when relying on a reduction in surface tension rather than a reduction in viscosity.
[0110] The temperature difference between quiescent temperature and drop selection temperature may be greater
for a hot melt ink than for a water based ink, as water based inks are constrained by the boiling point of the water.
[0111] The ink must be liquid at the quiescent temperature. The quiescent temperature should be higher than the
highest ambient temperature likely to be encountered by the printed page. The quiescent temperature should also be
as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the
quiescent and the drop ejection temperatures. A quiescent temperature between 60°C and 90°C is generally suitable,
though other temperatures may be used. A drop ejection temperature of between 160°C and 200°C is generally suita-
15
EP 0 767 740 B1
ble.
[01 12] There are several methods of achieving an enhanced reduction in surface tension with increasing temperature.
5
1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to the hot melt ink while in the liquid
phase.
2) A polar/non-polar microemulsion with a PIT which is preferably at least 20°C above the melting points of both the
polar and non-polar compounds.
10
[01 13] To achieve a large reduction in surface tension with temperature, it is desirable that the hot melt ink carrier have
a relatively large surface tension (above 30 mN/m) when at the quiescent temperature. This generally excludes alkanes
such as waxes. Suitable materials will generally have a strong intermolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a melting point of 88°C.
15
Surface tension reduction of various solutions
[01 14] Figure 3(d) shows the measured effect of temperature on the surface tension of various aqueous preparations
containing the following additives:
20
25
30
35
40
45
1)0.1%
2) 0.1%
3) 0.1%
4) 0.1%
5) 0.1%
sol of Stearic Acid
sol of Palmitic acid
solution of Pluronic 10R5 (trade mark of BASF)
solution of Pluronic L35 (trade mark of BASF)
solution of Pluronic L44 (trade mark of BASF)
Operation Using Reduction of Viscosity
[01 15] As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink, is as follows. Prior to operation of the printer, solid ink is melted in the reservoir
64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperature at
which the ink 100 is liquid, but exhibits a relatively high viscosity (for example, approximately 100 cP). The Ink 100 is
retained in the nozzle by the surface tension of the ink. The ink 100 is formulated so that the viscosity of the ink reduces
with increasing temperature. The ink pressure oscillates at a frequency which is an integral multiple of the drop ejection
frequency from the nozzle. The ink pressure oscillation causes oscillations of the ink meniscus at the nozzle tips, but
this oscillation is small due to the high ink viscosity. At the normal operating temperature, these oscillations are of insufficient amplitude to result in drop separation. When the heater 103 is energized, the ink forming the selected drop is
heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in
the ink meniscus moving further during the high pressure part of the ink pressure cycle. The recording medium 51 is
arranged sufficiently close to the print head 50 so that the selected drops contact the recording medium 51 , but sufficiently far away that the unselected drops do not contact the recording medium 51 . Upon contact with the recording
medium 51 , part of the selected drop freezes, and attaches to the recording medium. As the ink pressure falls, ink
begins to move back into the nozzle. The body of ink separates from the ink which is frozen onto the recording medium.
The meniscus of the ink 100 at the nozzle tip then returns to low amplitude oscillation. The viscosity of the ink increases
to its quiescent level as remaining heat is dissipated to the bulk ink and print head. One ink drop is selected, separated
and forms a spot on the recording medium 51 for each heat pulse. As the heat pulses are electrically controlled, drop
on demand ink jet operation can be achieved.
Image Processing for Print Heads
so
55
[01 16] An objective of printing systems according to the invention is to attain a print quality which is equal to that which
people are accustomed to in quality color publications printed using offset printing. This can be achieved using a print
resolution of approximately 1,600 dpi. However, 1,600 dpi printing is difficult and expensive to achieve. Similar results
can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yellow and
black. This color model is herein called CC'MM'YK. Where high quality monochrome image printing is also required,
two bits per pixel can also be used for black. This color model is herein called CC'MM'YKK'.
16
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Applications Using Print Heads According to this Invention
5
10
[01 17] Printing apparatus and methods of this invention are suitable for a wide range of applications, including (but
not limited to) the following: color and monochrome office printing, short run digital printing, high speed digital printing,
process color printing, spot color printing, offset press supplemental printing, low cost printers using scanning print
heads, high speed printers using pagewidth print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimile and copying machines, label
printing, large format plotters, photographic duplication, printers for digital photographic processing, portable printers
incorporated into digital 'instant' cameras, video printing, printing of PhotoCD images, portable printers for 'Personal
Digital Assistants', wallpaper printing, indoor sign printing, billboard printing, and fabric printing.
Compensation of Print Heads for Environmental Conditions
15
20
25
30
35
40
[0118] It is desirable that drop on demand printing systems have consistent and predictable ink drop size and position.
Unwanted variation in ink drop size and position causes variations in the optical density of the resultant print, reducing
the perceived print quality. These variations should be kept to a small proportion of the nominal ink drop volume and
pixel spacing respectively. Many environmental variables can be compensated to reduce their effect to insignificant levels. Active compensation of some factors can be achieved by varying the power applied to the nozzle heaters.
[01 19] An optimum temperature profile for one print head embodiment involves an instantaneous raising of the active
region of the nozzle tip to the ejection temperature, maintenance of this region at the ejection temperature for the duration of the pulse, and instantaneous cooling of the region to the ambient temperature.
[0120] This optimum is not achievable due to the stored heat capacities and thermal conductivities of the various
materials used in the fabrication of the nozzles in accordance with the invention. However, improved performance can
be achieved by shaping the power pulse using curves which can be derived by iterative refinement of finite element simulation of the print head. The power applied to the heater can be varied in time by various techniques, including, but not
limited to:
1) Varying the voltage applied to the heater
2) Modulating the width of a series of short pulses (PWM)
3) Modulating the frequency of a series of short pulses (PFM)
[0121] To obtain accurate results, a transient fluid dynamic simulation with free surface modeling is required, as convection in the ink, and ink flow, significantly affect on the temperature achieved with a specific power curve.
[01 22] By the incorporation of appropriate digital circuitry on the print head substrate, it is practical to individually control the power applied to each nozzle. One way to achieve this is by 'broadcasting' a variety of different digital pulse
trains across the print head chip, and selecting the appropriate pulse train for each nozzle using multiplexing circuits.
[0123] An example of the environmental factors which may be compensated for is listed in the table "Compensation
for environmental factors". This table identifies which environmental factors are best compensated globally (for the
entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle.
Compensation for environmental factors
[0124]
Factor compensated
Scope
Sensing or user control
method
Compensation mechanism
Ambient Temperature
Global
Temperature sensor mounted
on print head
Power supply voltage or global
PFM patterns
Power supply voltage fluctuation
with number of active nozzles
Global
Predictive active nozzle count
based on print data
Power supply voltage or global
PFM patterns
Local heat build-up with successive nozzle actuation
Per nozzle
Predictive active nozzle count
based on print data
Selection of appropriate PFM
pattern for each printed drop
17
EP 0 767 740 B1
(continued)
Factor compensated
Scope
Sensing or user control
method
Compensation mechanism
Drop size control for multiple bits
per pixel
Per nozzle
Image data
Selection of appropriate PFM
pattern for each printed drop
Nozzle geometry variations
between wafers
Per chip
Factory measurement, dataf ile
supplied with print head
Global PFM patterns per print
head chip
Heater resistivity variations
between wafers
Per chip
Factory measurement, dataf ile
supplied with print head
Global PFM patterns per print
head chip
User image intensity adjustment
Global
User selection
Power supply voltage, electrostatic acceleration voltage, or ink
pressure
Ink surface tension reduction
method and threshold temperature
Global
Ink cartridge sensor or user
selection
Global PFM patterns
Ink viscosity
Global
Ink cartridge sensor or user
selection
Global PFM patterns and/or
clock rate
Ink dye or pigment concentration
Global
Ink cartridge sensor or user
selection
Global PFM patterns
Ink response time
Global
Ink cartridge sensor or user
selection
Global PFM patterns
[0125] Most applications will not require compensation for all of these variables. Some variables have a minor effect,
and compensation is only necessary where very high image quality is required.
Print head drive circuits
[0126] Figure 4 is a block schematic diagram showing electronic operation of an example head driver circuit in accordance with this invention. This control circuit uses analog modulation of the power supply voltage applied to the print
head to achieve heater power modulation, and does not have individual control of the power applied to each nozzle.
Figure 4 shows a block diagram for a system using an 800 dpi pagewidth print head which prints process color using
the CC'MM'YK color model. The print head 50 has a total of 79,488 nozzles, with 39,744 main nozzles and 39,744
redundant nozzles. The main and redundant nozzles are divided into six colors, and each color is divided into 8 drive
phases. Each drive phase has a shift register which converts the serial data from a head control ASIC 400 into parallel
data for enabling heater drive circuits. There is a total of 96 shift registers, each providing data for 828 nozzles. Each
shift register is composed of 828 shift register stages 217, the outputs of which are logically anded with phase enable
signal by a nand gate 215. The output of the nand gate 215 drives an inverting buffer 216, which in turn controls the
drive transistor 201. The drive transistor 201 actuates the electrothermal heater 200, which may be a heater 103 as
shown in figure 1(b). To maintain the shifted data valid during the enable pulse, the clock to the shift register is stopped
the enable pulse is active by a clock stopper 218, which is shown as a single gate for clarity, but is preferably any of a
range of well known glitch free clock control circuits. Stopping the clock of the shift register removes the requirement for
a parallel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400.
Data is routed to either the main nozzles or the redundant nozzles by the data router 219 depending on the state of the
appropriate signal of the fault status bus.
[0127] The print head shown in figure 4 is simplified, and does not show various means of improving manufacturing
yield, such as block fault tolerance. Drive circuits for different configurations of print head can readily be derived from
the apparatus disclosed herein.
[0128] Digital information representing patterns of dots to be printed on the recording medium is stored in the Page
or Band memory 1513, which may be the same as the Image memory 72 in figure 1(a). Data in 32 bit words representing dots of one color is read from the Page or Band memory 1513 using addresses selected by the address mux 417
and control signals generated by the Memory Interface 418. These addresses are generated by Address generators
41 1, which forms part of the 'Per color circuits' 410, for which there is one for each of the six color components. The
addresses are generated based on the positions of the nozzles in relation to the print medium. As the relative position
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EP 0 767 740 B1
5
10
15
20
25
30
35
40
45
so
of the nozzles may be different for different print heads, the Address generators 41 1 are preferably made programmable. The Address generators 41 1 normally generate the address corresponding to the position of the main nozzles.
However, when faulty nozzles are present, locations of blocks of nozzles containing faults can be marked in the Fault
Map RAM 412. The Fault Map RAM 412 is read as the page is printed. If the memory indicates a fault in the block of
nozzles, the address is altered so that the Address generators 41 1 generate the address corresponding to the position
of the redundant nozzles. Data read from the Page or Band memory 1513 is latched by the latch 413 and converted to
four sequential bytes by the multiplexer 414. Timing of these bytes is adjusted to match that of data representing other
colors by the FIFO 415. This data is then buffered by the buffer 430 to form the 48 bit main data bus to the print head
50. The data is buffered as the print head may be located a relatively long distance from the head control ASIC. Data
from the Fault Map RAM 41 2 also forms the input to the FIFO 41 6. The timing of this data is matched to the data output
of the FIFO 415, and buffered by the buffer 431 to form the fault status bus.
[0129] The programmable power supply 320 provides power for the head 50. The voltage of the power supply 320 is
controlled by the DAC 313, which is part of a RAM and DAC combination (RAMDAC) 316. The RAMDAC 316 contains
a dual port RAM 31 7. The contents of the dual port RAM 31 7 are programmed by the Microcontroller 315. Temperature
is compensated by changing the contents of the dual port RAM 317. These values are calculated by the microcontroller
315 based on temperature sensed by a thermal sensor 300. The thermal sensor 300 signal connects to the Analog to
Digital Converter (ADC) 31 1. The ADC 31 1 is preferably incorporated in the Microcontroller 315.
[0130] The Head Control ASIC 400 contains control circuits for thermal lag compensation and print density. Thermal
lag compensation requires that the power supply voltage to the head 50 is a rapidly time-varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320
to produce this voltage. An analog time varying programming voltage is produced by the DAC 313 based upon data
read from the dual port RAM 317. The data is read according to an address produced by the counter 403. The counter
403 produces one complete cycle of addresses during the period of one enable pulse. This synchronization is ensured,
as the counter 403 is clocked by the system clock 408, and the top count of the counter 403 is used to clock the enable
counter 404. The count from the enable counter 404 is then decoded by the decoder 405 and buffered by the buffer 432
to produce the enable pulses for the head 50. The counter 403 may include a prescaler if the number of states in the
count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately
compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between
the counter 403 and the dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To allow
non-linear timing of these states the counter 403 may also include a ROM or other device which causes the counter 403
to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used.
[0131] For print density compensation, the printing density is detected by counting the number of pixels to which a
drop is to be printed ('on' pixels) in each enable period. The 'on' pixels are counted by the On pixel counters 402. There
is one On pixel counter 402 for each of the eight enable phases. The number of enable phases in a print head in accordance with the invention depend upon the specific design. Four, eight, and sixteen are convenient numbers, though there
is no requirement that the number of enable phases is a power of two. The On Pixel Counters 402 can be composed of
combinatorial logic pixel counters 420 which determine how many bits in a nibble of data are on. This number is then
accumulated by the adder 421 and accumulator 422. A latch 423 holds the accumulated value valid for the duration of
the enable pulse. The multiplexer 401 selects the output of the latch 423 which corresponds to the current enable
phase, as determined by the enable counter 404. The output of the multiplexer 401 forms part of the address of the dual
port RAM 317. An exact count of the number of 'on' pixels is not necessary, and the most significant four bits of this
count are adequate.
[0132] Combining the four bits of thermal lag compensation address and the four bits of print density compensation
address means that the dual port RAM 317 has an 8 bit address. This means that the dual port RAM 317 contains 256
numbers, which are in a two dimensional array. These two dimensions are time (for thermal lag compensation) and print
density. A third dimension - temperature - can be included. As the ambient temperature of the head varies only slowly,
the microcontroller 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print
density at the current temperature. Periodically (for example, a few times a second), the microcontroller senses the current head temperature and calculates this matrix.
[0133] The clock to the print head 50 is generated from the system clock 408 by the Head clock generator 407, and
buffered by the buffer 406. To facilitate testing of the Head control ASIC, JTAG test circuits 499 may be included.
Comparison with thermal ink jet technology
55
[0134] The table "Comparison between Thermal ink jet and Present Invention" compares the aspects of printing in
accordance with the present invention with thermal ink jet printing technology.
[0135] A direct comparison is made between the present invention and thermal ink jet technology because both are
drop on demand systems which operate using thermal actuators and liquid ink. Although they may appear similar, the
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two technologies operate on different principles.
[0136] Thermal ink jet printers use the following fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in liquid ink. Rapid and consistent bubble formation can be achieved by superheating the ink, so that sufficient heat is transferred to the ink before bubble nucleation is
complete. For water based ink, ink temperatures of approximately 280°C to 400°C are required. The bubble formation
causes a pressure wave which forces a drop of ink from the aperture with high velocity. The bubble then collapses,
drawing ink from the ink reservoir to re-fill the nozzle. Thermal ink jet printing has been highly successful commercially
due to the high nozzle packing density and the use of well established integrated circuit manufacturing techniques.
However, thermal ink jet printing technology faces significant technical problems including multi-part precision fabrication, device yield, image resolution, 'pepper' noise, printing speed, drive transistor power, waste power dissipation, satellite drop formation, thermal stress, differential thermal expansion, kogation, cavitation, rectified diffusion, and
difficulties in ink formulation.
[0137] Printing in accordance with the present invention has many of the advantages of thermal ink jet printing, and
completely or substantially eliminates many of the inherent problems of thermal ink jet technology.
Comparison between Thermal ink jet and Present Invention
[0138]
Thermal Ink-Jet
Present Invention
Drop selection mechanism
Drop ejected by pressure wave
caused by thermally induced bubble
Choice of surface tension or viscosity
reduction mechanisms
Drop separation mechanism
Same as drop selection mechanism
Choice of proximity, electrostatic,
magnetic, and other methods
Basic ink carrier
Water
Water, microemulsion, alcohol, glycol,
or hot melt
Head construction
Precision assembly of nozzle plate,
ink channel, and substrate
Monolithic
Per copy printing cost
Very high due to limited print head life
and expensive inks
Can be low due to permanent print
heads and wide range of possible inks
Satellite drop formation
Significant problem which degrades
image quality
No satellite drop formation
Operating ink temperature
280°C to 400°C (high temperature
limits dye use and ink formulation)
Approx. 70°C (depends upon ink formulation)
Peak heater temperature
400°C to 1,000°C (high temperature
reduces device life)
Approx. 130°C
Cavitation (heater erosion by bubble collapse)
Serious problem limiting head life
None (no bubbles are formed)
Kogation (coating of heater by ink
ash)
Serious problem limiting head life
and ink formulation
None (water based ink temperature
does not exceed 100°C)
Rectified diffusion (formation of
ink bubbles due to pressure
cycles)
Serious problem limiting ink formulation
Does not occur as the ink pressure
does not go negative
Resonance
Serious problem limiting nozzle
design and repetition rate
Very small effect as pressure waves
are small
Practical resolution
Approx. 800 dpi max.
Approx. 1,600 dpi max.
Self-cooling operation
No (high energy required)
Yes: printed ink carries away drop
selection energy
Drop ejection velocity
High (approx. 10 m/sec)
Low (approx. 1 m/sec)
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(continued)
Thermal Ink-Jet
Present Invention
Crosstalk
Serious problem requiring careful
acoustic design, which limits nozzle
refill rate.
Low velocities and pressures associated with drop ejection make crosstalk
very small.
Operating thermal stress
Serious problem limiting print-head
life.
Low: maximum temperature increase
approx. 90°C at centre of heater.
Manufacturing thermal stress
Serious problem limiting print-head
size,
Same as standard CMOS manufacturing process.
Drop selection energy
Approx. 20 nJ
Approx. 270 nJ
Heater pulse period
Approx. 2-3 us
Approx. 15-30 us
Average heater pulse power
Approx. 8 Watts per heater.
Approx. 12 mW per heater. This is
more than 500 times less than Thermal Ink-Jet.
Heater pulse voltage
Typically approx. 40V.
Approx. 5 to 10V.
Heater peak pulse current
Typically approx. 200 mA per heater.
This requires bipolar or very large
MOS drive transistors.
Approx. 4 mA per heater. This allows
the use of small MOS drive transistors.
Fault tolerance
Not implemented. Not practical for
edge shooter type.
Simple implementation results in better yield and reliability
Constraints on ink composition
Many constraints including kogation,
nucleation, etc.
Temperature coefficient of surface
tension or viscosity must be negative.
Ink pressure
Atmospheric pressure or less
Approx. 1.1 atm
Integrated drive circuitry
Bipolar circuitry usually required due
to high drive current
CMOS, nMOS, or bipolar
Differential thermal expansion
Significant problem for large print
heads
Monolithic construction reduces problem
Pagewidth print heads
Major problems with yield, cost, precision construction, head life, and
power dissipation
High yield, low cost and long life due
to fault tolerance. Self cooling due to
low power dissipation.
Yield and Fault Tolerance
[01 39] In most cases, monolithic integrated circuits cannot be repaired if they are not completely functional when manufactured. The percentage of operational devices which are produced from a wafer run is known as the yield. Yield has
a direct influence on manufacturing cost. A device with a yield of 5% is effectively ten times more expensive to manufacture than an identical device with a yield of 50%.
[0140] There are three major yield measurements:
1) Fab yield
2) Wafer sort yield
3) Final test yield
[0141 ] For large die, it is typically the wafer sort yield which is the most serious limitation on total yield. Full pagewidth
color heads in accordance with this invention are very large in comparison with typical VLSI circuits. Good wafer sort
yield is critical to the cost-effective manufacture of such heads.
[0142] Figure 5 is a graph of wafer sort yield versus defect density for a monolithic full width color A4 head embodiment of the invention. The head is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is calculated according
to Murphy's method, which is a widely used yield prediction method. With a defect density of one defect per square cm,
Murphy's method predicts a yield less than 1%. This means that more than 99% of heads fabricated would have to be
discarded. This low yield is highly undesirable, as the print head manufacturing cost becomes unacceptably high.
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EP 0 767 740 B1
[0143] Murphy's method approximates the effect of an uneven distribution of defects. Figure 5 also includes a graph
of non fault tolerant yield 197 which explicitly models the clustering of defects by introducing a defect clustering factor.
The defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of the manufacturing
process. The defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case
yield projections closely match Murphy's method.
[0144] A solution to the problem of low yield is to incorporate fault tolerance by including redundant functional units
on the chip which are used to replace faulty functional units.
[0145] In memory chips and most Wafer Scale Integration (WSI) devices, the physical location of redundant sub-units
on the chip is not important. However, in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to the page being printed. To be able to print a dot in the same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators
can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in the same position as the faulty actuator, the data timing to the redundant actuator can be altered
to compensate for the displacement in the scan direction.
[0146] To allow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100%
redundancy. The requirement for 100% redundancy would normally more than double the chip area, dramatically
reducing the primary yield before substituting redundant units, and thus eliminating most of the advantages of fault tolerance.
[0147] However, with print head embodiments according to this invention, the minimum physical dimensions of the
head chip are determined by the width of the page being printed, the fragility of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip. The minimum practical size for
a full width, full color head for printing A4 size paper is approximately 215 mm x 5 mm. This size allows the inclusion of
100% redundancy without significantly increasing chin area, when using 1.5 urn CMOS fabrication technology. Therefore, a high level of fault tolerance can be included without significantly decreasing primary yield.
[0148] When fault tolerance is included in a device, standard yield equations cannot be used. Instead, the mechanisms and degree of fault tolerance must be specifically analyzed and included in the yield equation. Figure 5 shows
the fault tolerant sort yield 199 for a full width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in figure 5 indicates that thoroughly implemented fault tolerance
can increase wafer sort yield from under 1% to more than 90% under identical manufacturing conditions. This can
reduce the manufacturing cost by a factor of 100.
[0149] Fault tolerance is highly recommended to improve yield and reliability of print heads containing thousands of
printing nozzles, and thereby make pagewidth printing heads practical. However, fault tolerance is not to be taken as an
essential part of the present invention.
Modular high speed digital color printing presses
[0150] Modular high speed digital color printing press can be constructed using drop on demand printing technology
such as, e.g., coincident forces, liquid ink printing in accordance with my concurrently filed applications.
[0151] Such printers can accept information supplied by an external raster image processor (RIP) in the form of a
halftoned raster at 600 dots per inch. This is stored in a bi-level page memory. Many printing modules can be supplied
with information from a single RIP, and can print simultaneously. The contents of the page memory can then be printed
using the printing head.
[0152] This system has a number of advantages over alternative technologies. These include:
1) Modularity: printing speed can be increased by adding low cost modules.
2) Small size: each printing module can be compact
3) Consistency: the image quality generated is consistent, as each dot is digitally controlled.
4) Reliability: the system is fault tolerant, increasing reliability.
5) Perfect registration: the four process colors are printed using a monolithic silicon printing head. The nozzles of
this head can be fabricated with a relative position tolerance of less than one micron. This eliminates the need to
align four color passes, as is usually required.
6) High quality with lower resolution: the amount of ink deposited is directly proportional to the number of dots
printed. The position of each dot is also controlled. Therefore it is not necessary to use clustered dot ordered dithering to digitally halftone the continuous tone images. Instead, computer optimized dispersed dot ordered dithering
can be used. Combined with seven color printing, photographic image quality equivalent to that achieved by conventional presses using up to 1,800 dots per inch can be achieved using only 600 dots per inch. This reduces the
time and cost of the raster image processing (RIPping) required, as well as reducing the cost and increasing the
22
EP 0 767 740 B1
speed of the printing process.
7) Implicit collation: if a number of printer modules are set up to simultaneously print successive sheets of a multipage color document such as a magazine, then the result can be automatically collated without requiring special
equipment
8) Flexibility: The image to be printed can be changed instantly.
[0153] Table 1, "Example product specifications," shows the specifications of one possible configuration of a high performance color printing module using coincident forces, liquid ink printing technology.
Example product specifications
[0154]
[0155]
Configuration
Floor standing, web fed
Web width
420 mm
Printer type
4 x LIFT A4 page width printing heads
Number of nozzles
158,976 active nozzles, 158,976 spare nozzles
Printing speed
128 A4 ppm duplex (37 A3 sheets per minute)
Printer resolution
800 dpi
Dimensions (W X D X H)
600 X 600 X 2,000 mm
Reliability
Fault tolerant at print head and module level
Page description language
Adobe Postscript* level 2
Connectivity
100 BaseT Ethernet
Some other features of the printing system are:
1) The heads from both sides of the paper are at the same horizontal level, allowing the ink pressure for both heads
to be identical when fed from a common ink reservoir where the ink pressure is determined by ink column height.
2) The paper movement conveyor belt is modular, allowing entirely modular construction of a multi-unit printing line.
3) The roll of blank paper is mounted on a frame which can be simply wheeled into the printing module whenever
the paper needs to be replenished.
4) The roll of blank paper can be at ground level, underneath the printing heat, drying region, paper cutter, and document conveyor belt. This arrangement has the significant advantage that the paper roll can be simply wheeled into
place when the paper requires changing, without requiring a fork-lift truck or special machinery.
[0156] The table "LIFT head type Web-6-800" (see Appendix A) is a summary of some characteristics of an example
full color two chip LIFT printing head suitable for high speed web-fed A3 printing. A single printing module of the digital
color printing press uses two of these print heads to print the four pages of a magazine sheet simultaneously.
Modular printing system description
[0157] Figure 6 shows a simplified system configuration for a high speed color publishing and printing system. Text is
created, images are scanned, graphics are created, and pages are laid out using computer based color publishing
workstations 576. These can be based on personal computers such as the Apple Macintosh and IBM PC, or on workstations such as those manufactured by Sun and Hewlett-Packard. Alternatively, they can be purpose built publishing
workstations. Information is communicated between these workstations using a digital communications local area network 577 such as Ethernet or FDDI. Information can also be brought into the system using wide area networks such as
ISDN, or by physical media such as floppy disks, hard disks, optical disks, CDROMs, magnetic tape, and so forth. This
information may be in the form or raster images, such as TIFF files and Scitex files, page description language files
such as Adobe Postscript, or native files from computer application programs such as Aldus Pagemaker, Quark
Express, or Adobe Photoshop. Color images can be acquired using an image input device 579 such as a drum scanner,
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EP 0 767 740 B1
a flatbed scanner, or a slide scanner and incorporated in the page layouts. Proofing devices, such as low volume color
printers and copiers can be incorporated into the network. Also appropriate for color publishing is PhotoCD jukeboxes
or other image libraries.
[0158] When the page layout is completed, it is sent to the raster image processor (RIP) 552. The raster image processor converts the page layout information (which is typically in the form of a page description language) into a raster
image. This module also performs halftoning, to convert the continuous tone image data from the scanned photographs, graphics and other sources into bi-level image data.
[0159] There are several Page Description Languages (PDLs) in common use. These include Adobe's PostScript language and Hewlett Packard's PCL5. The raster image processor can either support a single PDL, or an automatic PDL
selector can detect the PDL being used from the data stream, and send the PDL data to an appropriate PDL interpreter.
Other non-PDL image formats are also commonly used in the professional Pre-press and printing markets. These
include the formats used by digital pre-press machines, such as Scitex format, Linotype-Hell format, and Crosf ield format.
[0160] The PDL interpreter can interpret a scan-line rendering PDL. Such interpreters can create the page image in
scan-line order, without reference to a frame memory. The continuous tone data can be produced in raster order, so
may be error diffused before being stored in a bi-level image memory. For highest quality, the digital halftoning algorithm
can be vector error diffusion. This operates by selecting the closest printable color in three dimensional color space to
the desired color. The difference between the desired color and this printable color is determined. This difference is then
diffused to neighboring pixels. The vector error diffusion function accepts a raster ordered continuous tone (typically 24
bit per pixel) input image and generates a bi-level output with one bit per color per pixel (four bits for CMYK, 6 bits for
CC'MM'YK, 7 bits for CC'MM'YKK'). This is then stored in a bi-level page memory. In the case of a 800 dpi, A3 color,
with four colors the Bi-level page memory requires approximately 58 MBytes per page (when not compressed). With six
colors the Bi-level page memory requires approximately 88 MBytes per page. The bi-level page memory can be implemented in DRAM. An alternative to providing a full Bi-level page memory is to use a compression scheme, and provide
a compressed page memory, a real-time expansion system, and a bi-level band memory. This can reduce the memory
requirements significantly. The Bi-level page memory or compressed page memory may be a section of the main memory of the raster image processor. The functions of the raster image processor are primarily to interpret the PDL. The
raster image processor may also perform the digital halftoning. Alternatively, this may be performed by digital hardware
in the form of an ASIC. However, this function is relatively simple when compared to the PDL interpretation, and can
readily be performed by the processor.
[0161] PDL interpreters which require random access to a page memory cannot use error diffusion as a means of
halftoning, as error diffusion requires access to the continuous tone information in scan-line order. A practical solution
is to use ordered dithering instead of error diffusion. PDL interpreters in current use typically use a clustered dot
ordered dither to reduce the effects of non-linear dot addition that occurs with laser printers and offset printing. However, dot addition using the printing process is substantially linear, so dispersed dot ordered dithering can be used.
Computer optimized dispersed dot ordered dither provides a substantially better image quality than clustered dot
ordered dither, and more efficient to calculate than error diffusion.
[0162] Once a binary image of the page has been created, it can be sent to the appropriate digital color printing module 574 for printing. A single page can be changed at a time, or both sides of the sheet can be changed. It is also possible to change only a portion of a page. This has application for personalizing color printed documents for mass
mailing. The data is transferred by a digital data link 578. If the data must be changed quickly, this should be a high
speed data link. 116 MBytes of information must be transferred to change a complete sheet when printed with seven
colors. The high speed data link may be FDDI, which can theoretically transfer the data in less than 12 seconds. In practice, however, longer data transmission times are likely. SCSI is also a possible data transfer system. However, the long
physical distances and high electrical noise environments of a large printing establishment means that much care must
be taken to ensure data integrity if SCSI is used.
[0163] Figure 7 shows a simplified block diagram of a single digital color printing module 574. A computer interface
551 accepts data from the raster image processor 552 via the high speed data link 578. This data is stored in the bilevel page memory of the appropriate print head, page memory and driver module 550. There are two modules 550,
one for each side of the sheet. Pressure regulators 63 maintain pressure in ink reservoirs 64. Pressure regulators and
ink reservoirs are required for each of the printing colors. Each of the ink colors is supplied to each of the full color printing heads in the modules 550. A paper transport system 65 moves the paper 51 passed the fixed heads.
[0164] Figure 8 is a schematic process diagram of a printer head, memory, and driver module 550 according to one
preferred embodiment of the invention. The computer interface 551 writes the binary image of the page to the bi-level
page memory 505. When a page is to be printed, the bi-level page memory 505 is read in real-time. This data is then
processed by the data phasing and fault tolerance system 506. This unit provides the appropriate delays to synchronize
the print data with the offset positions of the nozzle of the printing head. It also provides alternate data paths for fault
tolerance, to compensate for blocked nozzles, faulty nozzles or faulty circuits in the head.
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[0165] The printing head 50 prints the image 60 composed of a multitude of ink drops onto a recording medium 51 .
This medium will typically be paper, but can also be coated plastic film, cloth, or most other substantially flat surfaces
which will accept ink drops.
[0166] The bi-level image processed by the data phasing and fault tolerance circuit 506 provides the pixel data in the
correct sequence to the data shift registers 56. Data sequencing is required to compensated for the nozzle arrangement
and the movement of the paper. When the data has been loaded into the shift registers, it is presented in parallel to the
heater driver circuits 57. At the correct time, these driver circuits will electronically connect the corresponding heaters
58 with the voltage pulse generated by the pulse shaper circuit 61 and the voltage regulator 62. The heaters 58 heat
the tip of the nozzles 59, reducing the attraction of the ink to the nozzle surface material. Ink drops 60 escape from the
nozzles in a pattern which corresponds to the digital impulses which have been applied to the heater driver circuits. The
ink drops 60 fall under the influence of gravity or another field type towards the paper 51 . The various subsystems are
coordinated under the control of one or more control microcomputers 51 1.
[0167] Figure 9 shows a simplified mechanical schematic diagram of a possible implementation of the invention. This
diagram is schematic only, and is not intended to represent an actual recommended physical arrangement. The design
of paper transport systems is well known, and the principles disclosed herein can be readily applied to a variety of physical configurations persons skilled in the an. The drive electronics 561 consist of two head driver circuits and one computer interface circuit. The two head driver circuits provide synchronized data and control signals for the two heads 563
and 564. The head 563 prints on one side of the paper 560. The head 564 prints on the other side of the paper 560.
The paper is supplied in continuous rolls, and the paper transport is performed by a series of rollers 562. After one side
of the paper is printed by head 563, the paper is dried and turned over by the rollers so that the other side can be printed
by head 564. This is required if gravity is the principle force that moves the ink drops from the head to the paper, but
may not be necessary if the ink drops are accelerated by a strong electrical or magnetic field. After printing each side,
the paper moves through a forced air drying region, which may use heated air to accelerate drying. This allows the
physical size of the printing module 574 to be minimized. The paper is then cut into sheets by the automatic paper cutter
569.
[0168] Gravity feed of the ink is a convenient way to obtain a stable and accurate ink pressure at the heads. Gravity
feed allows the ink to be replenished without interrupting the print cycle. The ink reservoirs 572 can contain an automatic level maintaining system, which may consist of a master reservoir 578 which is connected to a reservoir 579. The
ink level in the reservoir 579 is regulated by a mechanism which may be a float valve, or may be an electrical level sensor which controls an electromechanical valve. The level of ink in the reservoir 579 is adjusted such that the ink pressure caused by the difference in height between the head and the ink level is the optimum operating pressure for the
head. The ink flowing to the master reservoirs 578 can be piped from a central reservoir which feeds all of the printing
modules in a print shop. In this manner, no manual filling of the ink reservoirs of the individual print modules is required.
Four ink reservoirs are shown in figure 9. The number of ink reservoirs required depends upon the number of ink colors
to be printed. Seven ink reservoirs are required for CC'MM'YKK' printing.
[0169] To maintain the correct pressure, the ink level in the reservoir must be a specific height above the printing surface of the heads. The two heads 563 and 564 are set at the same height, so a single set of reservoirs 573 supply the
heads by gravity feed.
[0170] The paper 560 can be supplied on rolls 575. As paper rolls of substantial length may be very heavy, there may
be difficulty in changing the paper rolls. This can be alleviated by supplying the paper rolls in a sturdy frame 576, which
may include caster wheels attached to the frame. The modular printing system can be arranged so that the frame 576
of the paper roll 575 is at floor level. When the paper roll is empty, the frame is simply wheeled out of the printing module. A full paper roll is then wheeled into the printing module, and the paper is 'threaded' through the printing mechanism. The entire operation can be completed in a few minutes, without requiring fork lift trucks or other equipment
[0171 ] A fault indicator light 596 indicates when the printing module 574 requires human attention. This attention may
be required to replace the paper roll when empty, or to correct a fault. A human operator can also stop the machine by
pressing the pause button 598. When the printing module stops due to an internally detected condition, or through
pressing the pause button, printing and paper transport stops. However, the conveyor belt 571 does not stop. This is
important to maintain fault tolerant operation, as discussed later in this document.
[0172] In many cases, multi-sheet documents must be printed. To achieve this a number of digital color printing press
modules can be used to maintain document printing rates at 60 copies per minute. For example, if a 100 page color
magazine is to be printed, 25 printing modules can be used. Each module prints four pages simultaneously in one second. The printed sheets 570 are transported on a conveyor belt 571 , with each module adding one sheet to each stack.
[0173] Figure 10 shows three adjacent digital color printing modules 574 on a high volume printing line. The printing
modules 574 are supplied with paper from rolls 575. The printing modules print the pages, which automatically fall in
stacks 570 on the conveyor belt 571 . The last machine on the conveyor belt can be an automatic binding machine. It is
not necessary to have just one line of printing modules. The printing modules can be arranged to suit the collation and
binding process. For example, many books and thick magazines are bound as a plurality of groups of 32 pages (eight
25
EP 0 767 740 B1
sheets), which are then glued into a cover. This binding method can be accommodated by operating a number of short
lines each containing eight printing modules.
[0174] This modular approach to high volume printing has many advantages, including:
1) The entry cost for a printer is low, as a single printing module can be used. Even a single printing module is capable of 360 A4 pages per minute.
2) The maximum capability of a single printing line is high, as 86,000 copies of a color document (for example, a
magazine) of any length can be printed per day, when using one printing module per document sheet
3) Maintenance requirements are very low.
4) There is almost no down-time required to change the images on the pages being printed.
5) Service is simple, with replaceable units.
6) The development and manufacturing cost can be amortized over a large number of small modules.
7) The printing system can be made fault tolerant, with operation of the printing line automatically restored within
one second of detection of a module fault.
System-level fault tolerance in modular printing systems
[0175] Reliability of large printing systems can be very important, as the printing industry often operates 24 hours a
day, and on short deadlines. A modular printing system which comprises many printing modules, each with complex
digital circuitry and paper movement mechanical systems, generally could be expected to have a lower reliability than
a single large mechanical offset press. For modular direct digital printing to succeed commercially, it is essential that
system reliability approach or exceed that of current mechanical offset presses. This can be achieved through the
implementation of system -level fault tolerance.
[0176] The present invention provides a method and apparatus for restoring operation in a modular digital color printing press prior to the correction of the fault causing operation of one module to fail has been invented. One preferred
embodiment of such system comprises:
(a) the provision of at least one additional spare printing module to the number of printing modules required for the
printing task in the absence of a fault, the spare printing module being the most downstream of the printing modules
for which faults are to be compensated;
(b) transfer of the data representing the page or sheet to be printed to a downstream printing module after detection
of a fault in a faulty printing module;
(c) transfer of the data in the downstream printing module to the printing module downstream of the printing module
prior to or substantially simultaneously to the transfer of data into the downstream printing module from the faulty
printing module;
(d) The repeat of step (c) for subsequent downstream printing modules, the last printing module for which data is
transferred into being the spare printing module;
(e) the discontinuation of printing by the faulty printing module; and
(f) the continuation of printing by other printing modules, including the spare printing module.
[0177] The system may also include a method of restoring normal operation in a modular digital color printing press
after the correction of the fault comprising:
(g) transfer of the data representing the page or sheet to be printed from the spare printing module to the printing
module directly upstream of the spare printing module after the fault in the faulty printing module has been corrected;
(h) transfer of the data in the upstream printing module to the printing module upstream of the printing module prior
to or substantially simultaneously to the transfer of data into the upstream printing module from the spare printing
module;
(i) The repeat of step (h) for subsequent upstream printing modules, the last printing module for which data is transferred into being the previously faulty printing module;
G) the discontinuation of printing by the spare printing module; and
(k) the continuation of printing by other printing modules, including the previously faulty printing module.
[0178] Figure 11(a) shows a printing 'assembly line' which uses eight printing modules 574 to print a thirty two page
(eight sheet) document. A ninth module 574 is provided as a spare in accordance with the approach of the present
invention. The printed sheets are transferred from one module to the next by means of a modular conveyor belt 571 .
Each active printing module adds one sheet to the paper stack, so eight active modules will create a stack eight sheets
26
EP 0 767 740 B1
high. Such a system is capable of printing a thirty two page full color document every second.
[0179] Figure 11(b) shows the consequences of a fault in the printing module which is printing sheet 5. In a system
which does not use fault tolerance, the entire printing line must be stopped until the fault is corrected. The fault may be
any event which prevents the printing of the sheet of the document such as running out of paper or ink, or a mechanical
or electronic fault. It is clear that as the number of printing modules in use increases, the mean time between failures
(MTBF) decreases. The cost of downtime also increases, as more printing modules are idle while the fault is repaired.
If the printing module takes one hour to repair or replace, the printing assembly line will be inoperative for a time that
would otherwise be sufficient to print 3,600 copies of the document
[0180] Figure 11(c) shows a solution to this problem. As soon as the fault is detected, the digital image data describing
sheet 5 is transferred to the printing module which was printing sheet 6. Simultaneously, the data describing sheet 6 is
transferred to the printing module which was printing sheet 7, and the data describing sheet 7 is transferred to the printing module which was printing sheet 8. The data for sheet 8 is transferred to a spare printing module at the end of the
printing line. If this data transfer can occur in less than the time required to print a sheet, the line can continue printing
without stopping, and without any wastage of printed copies.
[0181] If more than one spare printing module is included at the end of the printing 'assembly line', then more than
one simultaneous fault can be accommodated without productivity loss.
[0182] This principle can be applied to other types of modular printing presses which do not use other printing heads.
[0183] This system can be implemented without requiring any additional hardware to be incorporated in the printing
modules 574. However, such a minimum implementation is not necessarily desirable. For example, data transfer for
fault tolerance can be achieved by re-transmitting the data from the raster image processor 551 to each of the printing
modules where the data must be altered. This data is transmitted over the high speed data link 578 in the same manner
as when the data is initially transmitted to the modules upon setup for the printing run. If each printing module 574 prints
four A4 pages at 600 dpi in 7 colors, then 116 MBytes of image data must be transferred for each module for which the
data is to be changed. In a printing line with 8 active printing modules, this means that 928 MBytes must be transferred
across the data link 578. If the data link 578 is an FDDI connection with a maximum data rate of 100 Mbps, then at least
84 seconds would be required to transmit the data. In practice, the data would take a much longer time to transmit over
FDDI. If the data was stored on a conventional hard disk drive with an avenge sustained data access time of 1 MByte
per second installed in the raster image processor 551 , then it would take a minimum of 928 seconds to access this
data and transmit it to the printing modules. This time may be comparable to the mean time to repair (MTTR) of a typical
fault in a printing module. In this case, no advantage is gained by incorporating fault tolerance in the production line.
[0184] An alternative to storing the data on a hard disk drive, is to store it in semiconductor memory in the raster
image processor 551 . In this example, 928 MBytes of semiconductor memory would be required in addition to the normal operating requirements of the raster image processor. This approach can speed the recovery of the system, but is
expensive. It is also inflexible, as more memory is required if the number of printing modules in the printing line is
greater than eight
[0185] To benefit from the fault tolerance method described herein, the time taken to re-load the data to the printing
modules should be substantially less than the MTTR. Ideally, it should also be less than the time taken to print one
sheet. If this is achieved, the printing line can continue operating when a fault is detected without stopping the conveyor
belt 571 and without printing any incomplete copies of the document
[0186] This requirement can be met by providing bi-directional data transfer links between successive printing modules 574. As successive printing modules in the printing line will typically be physically adjacent, the high speed bi-directional data links can be simply provided by short point-to-point parallel connections. The data transfer rate required is
116 MBytes per second. This can readily be provided by a 32 bit parallel cable operating at 29 MHz. High reliability can
be achieved by using ECL balanced line drivers into twisted pair shielded cables over distances in excess of five meters.
This will be adequate for direct connection between printing modules in typical printing line configurations. Such connections can be constructed using well known commercially available technology. For example, the parallel digital television standard for broadcast television production uses 8 bit parallel cables using balanced line eel drivers, operating
at 27 MHz. This technology can readily be operated at 29 MHz, and the data bus width can easily be extended to 32
bits. This technology is uni-directional. Bi-directional operation can be established by providing cables in both directions. Data communications between adjacent modules can also be established using more recent technologies, with
much fewer connections.
[0187] Figure 12(a) shows a bi-directional data transfer cable 599 connecting adjacent pairs of printing modules 574.
This shows a system configured to simultaneously print eight sheets of a document, utilizing a total of eight active printing modules and one spare module. In this example, the module printing sheet four has failed. In many cases, failure
can be automatically detected. Such cases include running out of paper or ink, paper jams, or failure of various portions
of the circuitry which may be automatically tested on a continual basis. The printing unit also can have a pause button
598 (fig 10) which causes the appropriate module to stop printing. This can be activated at any stage by a human operator if a fault which is not automatically detected occurs. It can also be activated for any other reason that it is required
27
EP 0 767 740 B1
that a module stop printing, for example; for regular maintenance, adjustment or calibration. Automatic detection of a
fault or a human command for the module to stop printing result in the same sequence of subsequent actions, so are
treated identically.
[0188] Figure 12(b) shows page image data being transferred via the point-to-point data links 599. If the data is transferred completely synchronously and simultaneously between all of the modules, no additional memory storage capacity beyond that normally required for the printing module 574 is required. If the data is completely transferred within the
time taken to print a sheet, printing can proceed uninterrupted. Once data has been transferred to 'downstream' printing
modules and printing resumes, the fault in the printing module can be repaired without causing a line stoppage. The
entire printing module electronics 561 or paper roll 575 can even be replaced without stopping the printing process.
[0189] Figure 12(c) shows operation immediately after a faulty unit has been repaired or otherwise put back into operation. Data is transferred back to the original printing modules via the bi-directional data links 599. After restoration of
the printing process, all of the copies of the document which were at, or downstream of, the faulty print module at the
time of restoration should be removed from the printing line, as they will be incorrectly collated. In this example, there
will be six such copies. These copies can either be discarded or manually collated.
[0190] The conveyor belt 571 of the faulty module must continue to operate while the module is being repaired or
replenished with paper or ink. As a result, the system is not tolerant of faults in the conveyor belt. However, the conveyor
belt is a simple mechanical mechanism, which can readily be constructed to have a very high MTBF More significant
than conveyor belt failure, however, is that modules cannot be replaced while the system is operating. An alternative
system where the conveyor belt is separate from the printing modules 574 is possible, and will solve this problem. However, the advantages of an integrated modular conveyor belt outweighs the disadvantage of not being able to exchange
entire modules while they are operating.
[0191] Figure 13 shows a simplified block diagram of a single digital color printing module 574 which incorporates
direct data connections to adjacent printing modules. There are two data interfaces 599 which must be able to operate
simultaneously. When initialized for a new print run, a computer interface 551 accepts data from a raster image processor via a high speed data link. This data is stored in the bi-level page memory of the appropriate print head, page memory and driver module 550.
[0192] When a fault is detected, a message is transmitted to downstream printing modules. This messages may be
in the form of a change of state of a single signal, or may be a sequence of digital codes, or other signaling method.
Data in the bi-level page memories contained in the head memory and driver modules 550 is then transferred to the
high speed data interface 590. This data is transferred to the downstream printing module via the downstream data link
599. The downstream data link of a module is equivalent to the upstream data link of the module directly downstream
from it.
[0193] When a fault is identified in an upstream printing module, a signal indicating this will be received. This signal
is passed on to downstream printing modules. Data from the upstream printing module will be received by the high
speed data interface 590 via the upstream data link 599. This data is stored in the bi-level page memory. Prior to storing
received data in a memory location, the data at that location is read and sent to the downstream printing module via the
computer interface and downstream data link 599. The total data transfer rate to and from the bi-level page memories
of a printing module is 232 MBytes per second, sustained for one second. Care must be taken in the design of the data
link circuitry not to overwrite the contents of the bi-level page memory before the contents are transmitted to the downstream printing module. This can be achieved by operating the upstream and downstream data links in a completely
synchronous manner, and operating the bi-level page memory in alternating read-write cycle. Alternatively, FIFO's may
be incorporated into the data link circuitry and the data transfers may be operated slightly asynchronously. However,
this technique is substantially more complicated and is not recommended.
[0194] When the fault in a printing module has been corrected, the operator presses a go button 597 which returns
the module to service. When this occurs, the repaired module sends a signal to the downstream printing module. Data
is then received via the downstream data link and stored in the bi-level page memories.
[0195] When a signal is received from an upstream printing module indicating that an upstream module has been
restored to operation, this signal is passed on to downstream printing modules. Data from the downstream printing
module will be received by the high speed data interface 590 via the downstream data link 599. This data is stored in
the bi-level page memory. Prior to storing received data in a memory location, the data at that location is read and sent
to the upstream printing module via the high speed data interface 590 and upstream data link 599.
[0196] As all printing modules downstream from (and including) the faulty printing module transfer data simultaneously, all of the data transfers required for the entire printing line can be completed in one second.
[0197] Figure 14 is a perspective drawing of a row of eight modular digital printing presses 574.
[01 98] The pause button 598 and go button 597 should be large and conveniently positioned so that a human operator
can quickly access them. An indicator light 596 shows when a particular module requires human attention. This light is
positioned on top of the printing module 574 so that it is visible from a distance, even when there are many rows of printing modules.
28
EP 0 767 740 B1
[0199] The door to the printing module 574 can be in three sections which can be independently opened. The lowest
door section 593 allows access to the paper roll 575. If the printing module 574 includes an automatic paper feeding
system, then this door may be the only required access when changing paper rolls. The middle door section 592 allows
access to the paper path and print heads. This door is ventilated and includes the paper drying fans. For operator convenience, the airflow should be from the front of the machine to the back. The top door section 591 provides access to
the electronics, conveyor belt, and ink reservoirs.
Printed documents exit the system via the conveyor belt 571 . This conveyor belt can feed the documents
[0200]
directly into a binding machine.
[0201 ] A human outline 595 shows the approximate scale of the system.
[0202] The foregoing describes one embodiment of the present invention. Modifications, obvious to those skilled in
the art, can be made thereto without departing from the scope of the invention.
29
cr 0 767 740 B1
A p p e n d i x
LIFT
s
head
A
type A4-6-800
This is a six color print head for A4 size printing. The print head is fixed, and is the full width of the A4
paper. Resolution is 800 dpi bi-level for high quality color output.
10
15
20
25
30
35
B»s>c specifications
Resolution
Print head length
Print head width
Ink colors
Page size
Print area width
Print area length
Page printing time
Pages per minute
Basic IC process
Bitmap memory requirement
Pixel spacing
Pixels per line
Lines per page
Pixels per page
Drops P« page
Average data rate
Ejection energy per drop
Energy to print full black page
Recording medium speed
800 dpi
215 mm
8 mm
6
A4
210mm
297 mm
1.3 seconds
37 ppm
1.5 u\m CMOS
44.3 MBytes
31.8 ^m
6,624
9,354
61 ,960,896
247,843,584
32.9 MBytes/sec
977 nJ
242 J
22.0 cm/sec
Yield and cost
40
45
50
Number of chips per head ~—. 1
Wafer size
300 mm (12")
22
Chips per wafer
Prim head chip area
17.2 cm2
Yield without fault tolerance
0.34%
Yield with fault tolerance
89%
Functional print heads per month
195,998
Print head assembly cost
S10
$17
Factory overhead per print head
Wafer cost per print head
Approx. total print bead cost
$3 1
$58
Derivation
Specification
Width of print area, plus 5 mm
Derived from physical and layout constraints of head
CC'MM'YK
Specification
Pixels per line /Resolution
Total length of active printing
Derived from scans, lines per page and dot printing rate
60/(120% of print time in seconds)
Recommendation
Bitmap memory required for one scan (cannot pause)
Reciprocal of resolution
Active nozzles / Number of colors
Scan distance *resolution
Pixels per line • lines per page
Pixels per page • simultaneous ink colors
Pixels per second • ink colors / 8 MBit*
Energy applied to heater infinite element simulations
Drop ejection energy • drops per page
Irresolution • actuation period times phases)
Derivation
Recommendation
Recommendation
From chip size and recommended wafer size
Chip width * length
Vsing Murphy's method defect density = / per cm3
Seefault tolerant yield calculations (D=l/cni, CF-2)
Assuming 10,000 wafer starts per month
Estimate
Based on SI 20m. cost for refurbished 1.5 \un Fab line
amortised over 5 years, plus SI 6m. PA., operating cost
Based on materials cost of $600 per wafer
Sum of print head assembly, overhead and wafer costs
EP 0 767 740 B1
A p p e n d i x
5
LIFT
A
head
( c o n t ' d . )
type
Nozzle and actuation specifications
10
Nozzle radius
Active nozzles per head
Redundant nozzles per bead
Total nozzles per bead
Drop rate per nozzle
Heater radius
20
25
79,488
6,944 Hz
Active plus redundant nozzles
l/( heater active period *number of phases)
10.S p.m
From nozzle geometry and radius
4,968
39,744
2.3 p.Qm
Heater resistance—— 1,5 17 Ci
6.0 mA
Average beater pulse current
Settling time petween pulses
Clock pulses per line
Dock frequency
Drive transistor on resistance
30
39,744
Specification
From page width, resolution and colors
Actuation phases *nozzles per phase
Same as active nozzles for 100% redundancy
Heater thin film resistivity
Heater active period
Average bead drive voltage
Drop selection temperature
Heater peak temperature
For heater formed from TaAl
From heater dimensions and resistivity
From heater power and resistance
18 (is
126 lis
From finite element simulations
Active period *(actuation phases-] )
5,678
39.4 MHz
Assuming multiple clocks and no transfer register
From clock pulses per line, and lines per second
From recommended device geometry
Heater current *(heater +drive transistor resistance)
56 Q
9.4 V
50 °C
Temperature at which critical surface tension is reached
From finite element simulations
120 °C
Ink specifications
Basic ink carrier
Surfactant
40
Max. energy for self cooling
45
50
Derivation
Water
Specification
Suggested method of achieving temperature threshold
From finite element simulations
Black ink density at 60°C
Ink drop volume * ink density
.. 1-Hexadecanol
Ink drop volume-...-.. 9 pi
1.030 g/cm'
Ink density
9.3 ng
Ink drop mass
Ink specific heat capacity
Total ink per color per page
Maximum ink flow rate per color
Full black ink coverage
Ejection ink surface tension
Ink pressure
Derivation
Specification
Number of actuation phases™. 8
Nozzles per phase
15
10 pm
A4-6-800
4.2 J/Kg/°C
Ink carrier characteristic
Ink drop heat capacity *temperature increase
Drops per page per color *drop volume
1,164 nJ/drop
0.56 ml
Ink per color per page / page print time
Ink drop volume *colors *drops per square metre
0.4 1 ml/sec
35.7 ml/m5
38.5 mN/m
Surface tension required for ejection
2 *Ejection ink surface tension / nozzle radius
7.7 kPa
Ink column height to achieve ink pressure
Ink column height—.. 763 mm
31
EP 0 767 740 B1
Claims
1. A multi-sheet document digital printing system comprising a plurality of digital printer modules (574), each including:
(a) means for supporting and feeding a print medium (560) from a supply station (576) trough a print path and
from a print path outlet;
(b) means (563, 564) for printing upon said medium (560) during its movement trough said print path (562); and
(c) sheet conveyor means (571) for transporting sheets from said print path outlet along a module transport
segment to a module egress, characterized by said modules being interconnected in a serial array wherein the
module egress of upstream modules (574) are coupled to the sheet conveyor means (571) of the adjacent
downstream modules (574) at the print path outlet of the downstream modules so that sheets fed from the print
path outlet of downstream modules are aligned with sheets from upstream modules to form a stack (570) of
print sheets build up upon the coupled sheet conveyor means (571) as the stack (570) passes along the transport segments, from the first module to the last module.
2.
The printing system defined in claim 1 wherein each module comprises a lower section housing for housing the
present medium supply, an intermediate section housing, said printing means and an upper section housing said
conveyor means.
3.
The printing system defined in claim 2 wherein said supply station includes means (576) for supporting a roll (575)
of continuous web material (560) on a removable frame and said modules each further include means (569) for cutting such medium into sheets prior to exiting the print path.
4.
The printing system defined in claim 1 further comprising a plurality of digital page memories (505) each respectively associated with a respective printer module (574) and control means (551) for loading such page memories
(505) with page data and synchronizing the digital printing of successive print pages in each module (574) with the
feed of sheets by said conveyor means.
5.
The printing system defined in claim 1 wherein each printing module comprises at least first and second printing
heads (563, 564) spaced along said path for respectively printing on first and second sides of the print medium
(560).
6.
The printing system defined in claim 5 wherein said module print paths are configured so each printing head prints
downwardly and print media is manipulated during feed to achieve side reversal.
7.
The printing system of claim 1 characterized by the printing modules being adapted to be cascaded to achieve a
higher total printing rate.
8.
A printing system as claimed in claim 3 wherein said removable frame includes wheels mounted on the underside.
9.
The printing system defined in claim 1 wherein each module (574) has:
(a) means for connecting to a raster image processing computer to receive data for producing a plurality of digitally halftoned binary page images;
(b) a plurality of digital page memories for storing such binary page images;
(c) a plurality of liquid ink printing heads; and
(d) an ink reservoir and ink pressure regulation system which maintains ink flow to said heads.
10. The printing system defined in claim 9 where the printing heads are fixed at the same height.
11. The printing system defined in claim 10 where a single ink reservoir for each color supplies all of the printing heads.
Patentanspruche
1.
Digitales Drucksystem fur eine Vielzahl von Blattern mit einer Anzahl digitaler Druckermodule (574), von denen
jedes
32
EP 0 767 740 B1
(a) Mittel zum Haltern und Transportieren eines Druckmediums (560) von einer Vorratsstation (576) entlang
einer Druckbahn zu einem Druckbahnausgang,
(b) Mittel (563, 564) zum Drucken auf das Druckmedium (560), wahrend dieses sich entlang der Druckbahn
(562) bewegt, und
(c) eine Blattfordereinrichtung (571) umfaBt, die Blatter vom Druckbahnausgang entlang eines Modultransportsegments zu einem Modulauslass transportiert, dadurch gekennzeichnet, dass die Module in einer seriellen
Anordnung miteinander verbunden sind, wobei der Modulauslass vorgeschalteter Module (574) mit der Blattfordereinrichtung (571) benachbarter nachgeschalteter Module (574) am Druckbahnausgang der nachgeschalteten Module derart gekoppelt ist, dass Blatter, die vom Druckbahnausgang der nachgeschalteten
Module gefordert werden, mit Blattern vorgeschalteter Module ausgerichtet sind und mit diesen auf der gekoppelten Blattfordereinrichtung (571) einen Stapel (570) aus Druckblattern bilden, wahrend der Stapel (570) sich
entlang der Transportsegmente vom ersten zum letzten Modul bewegt.
2.
Digitales Drucksystem nach Anspruch 1, dadurch gekennzeichnet, dass jedes Modul einen unteren Gehauseabschnitt zum Aufnehmen des vorhandenen Druckmediumvorrats, einen Zwischenabschnitt fur die Druckmittel und
einen die Blattfordereinrichtung aufnehmenden oberen Gehauseabschnitt aufweist.
3.
Digitales Drucksystem nach Anspruch 2, dadurch gekennzeichnet, dass die Vorratsstation ein Mittel (576) zum
Lagern einer Rolle (575) aus einem Endlosband (560) eines bahnformigen Materials auf einem abnehmbaren
Rahmen aufweist und dass jedes Modul ein Mittel (569) zum Schneiden des Mediums in Blatter umfaBt, ehe das
Medium die Druckbahn verlaBt.
4.
Digitales Drucksystem nach Anspruch 1, dadurch gekennzeichnet, dass eine Vielzahl digitaler Seitenspeicher
(505) vorgesehen ist, von denen jeder einem entsprechenden Druckermodul (574) und Steuermitteln (551) zugeordnet ist, urn die Seitenspeicher (505) mit Seitendaten zu laden und das digitale Drucken aufeinanderfolgender
Druckseiten in jedem Modul (574) mit dem mittels der Blattfordereinrichtung erfolgenden Transport der Blatter zu
synchronisieren.
5.
Digitales Drucksystem nach Anspruch 1, dadurch gekennzeichnet, dass jedes Druckermodul mindestens einen
ersten und einen zweiten, entlang der Druckbahn voneinander beabstandeten Druckkopf (563, 564) aufweist, urn
jeweils die erste und zweite Seite des Druckmediums (560) zu drucken.
6.
Digitales Drucksystem nach Anspruch 5, dadurch gekennzeichnet, dass die Moduldruckbahnen derart ausgebildet
sind, dass jeder Druckkopf nach unten druckt und das Druckmedium wahrend seines Transports derart manipuliert
wird, dass eine Seitenumkehrung erfolgt.
7.
Digitales Drucksystem nach Anspruch 1, dadurch gekennzeichnet, dass die Druckmodule hintereinandergeschaltet sind, urn eine hohere Druckgeschwindigkeit zu erzielen.
8.
Digitales Drucksystem nach Anspruch 3, dadurch gekennzeichnet, dass der abnehmbare Rahmen auf der Unterseite montierte Rader aufweist.
9.
Digitales Drucksystem nach Anspruch 1, dadurch gekennzeichnet, dass jedes Modul (574) folgende Komponenten
aufweist:
(a) Mittel fur den AnschluB an einen Rasterbildbearbeitungscomputer, urn Daten zu empfangen, die der Erzeugung einer Vielzahl digital gerasterter, binarer Seitenbilder dienen,
(b) eine Vielzahl digitaler Seitenspeicher zum Speichern der binaren Seitenbilder,
(c) eine Vielzahl von mit Flussigtinte arbeitenden Druckkopfen, und
(d) einen Tintenvorrat sowie ein Tintendruckregulierungssystem, das den Tintenstrom zu den Druckkopfen
aufrechterhalt.
10. Digitales Drucksystem nach Anspruch 9, dadurch gekennzeichnet, dass die Druckkopfe alle auf der gleichen Hohe
angeordnet sind.
11. Digitales Drucksystem nach Anspruch 10, dadurch gekennzeichnet, dass die Druckkopfe alle von einem einzelnen
Tintenvorrat fur jede Farbe versorgt werden.
33
EP 0 767 740 B1
Revendications
1. Systeme d'impression numerique de documents a feuilles multiples comprenant une pluralite de modules d'imprimante numerique (574), comprenant chacun :
(a) un moyen destine a supporter et a charger un support d'impression (560) provenant d'un poste d'alimentation (576) au travers d'un trajet d'impression et depuis un orifice de sortie de trajet d'impression,
(b) un moyen (563, 564) destine a imprimer sur ledit support (560) durant son deplacement au travers dudit
trajet d'impression (562), et
(c) un moyen de transporteur de feuilles (571) destine a transporter des feuilles depuis ledit orifice de sortie de
trajet d'impression le long d'un segment de transport de module vers une issue de module, caracterise en ce
que lesdits modules sont interconnects suivant un ensemble en serie dans lequel les issues de modules des
modules en amont (574) sont reliees au moyen de transporteur de feuilles (571) des modules en aval adjacents (574) au niveau de I'orif ice de sortie de trajet d'impression des modules en aval de sorte que des feuilles
chargees a partir de I'orifice de sortie de trajet d'impression des modules en aval soient alignees avec des
feuilles provenant des modules en amont af in de former une pile (570) de feuilles d'impression accumulees sur
le moyen de transporteur de feuilles (571) relie lorsque la pile (570) passe le long des segments de transport,
du premier module jusqu'au dernier module.
2.
Systeme d'impression selon la revendication 1, dans lequel chaque module comprend un logement de section inferieure destine a loger la presente alimentation en supports, un logement de section intermediate, ledit moyen
d'impression et une section superieure logeant ledit moyen de transporteur.
3.
Systeme d'impression selon la revendication 2, dans lequel ledit poste d'alimentation comprend un moyen (576)
destine a supporter un rouleau (575) d'un materiau en bande continue (560) sur un chassis amovible et lesdits
modules comprennent en outre chacun un moyen (569) destine a couper un tel support en feuilles avant de sortir
du trajet d'impression.
4.
Systeme d'impression selon la revendication 1, comprenant en outre une pluralite de memoires de pages numeriques (505), chacune etant respectivement associee a un module d'imprimante respectif (574) et un moyen de commande (551) destine a charger de telles memoires de pages (505) avec des donnees de pages et a synchroniser
I'impression numerique de pages d'impression successives dans chaque module (574) avec I'alimentation des
feuilles par ledit moyen de transporteur.
5.
Systeme d'impression selon la revendication 1, dans lequel chaque module d'impression comprend au moins une
premiere et une seconde tetes d'impression (563, 564) espacees le long dudit trajet af in d'imprimer respectivement
sur des premiere et seconde faces du support d'impression (560).
6.
Systeme d'impression selon la revendication 5, dans lequel lesdits trajets d'impression des modules sont configures de maniere a ce que chaque tete d'impression imprime vers le bas et que le support d'impression soit manipule
durant le chargement pour obtenir une inversion de face.
7.
Systeme d'impression selon la revendication 1, caracterise en ce que les modules d'impression sont congus pour
etre mis en cascade af in d'obtenir un debit d'impression total plus eleve.
8.
Systeme d'impression selon la revendication 3, dans lequel ledit chassis amovible comprend des roues montees
sur la face inferieure.
9.
Systeme d'impression selon la revendication 1, dans lequel chaque module (574) comporte :
(a) un moyen destine a relier un ordinateur de traitement d'image par echantillonnage de fagon a recevoir des
donnees afin de produire une pluralite d'images de pages binaires tramees numeriquement,
(b) une pluralite de memoires de pages numeriques destinees a memoriser de telles images de pages binaires,
(c) une pluralite de tetes d'impression a encre liquide, et
(d) un reservoir d'encre et un systeme de regulation de pression d'encre qui entretient la circulation de I'encre
vers lesdites tetes.
34
EP 0 767 740 B1
10. Systeme d'impression selon la revendication 9, dans lequel les tetes d'impression sont fixees a la meme hauteur.
11. Systeme d'impression selon la revendication 10, dans lequel un seul reservoir d'encre pour chaque couleur alimente la totalite des tetes d'impression.
35
EP 0 767 740 B1
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58
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