HP Indigo Digital Offset Color Technology

HP Indigo Digital Offset Color Technology
White paper
HP Indigo Digital
Offset Color
Table of contents
What is HP Indigo Digital Offset Color?.......................................2-3
HP Indigo ElectroInk......................................................................4
Thermal Offset..............................................................................5
Color switching.............................................................................5
The printing cycle.......................................................................6-7
Quality characteristics .............................................................8-11
Substrate compatibility.................................................................12
Productivity and versatility............................................................13
The family of HP Indigo digital presses..........................................15
The HP Indigo range of digital printing presses, based on its Digital
Offset Color technology and process, offers a unique combination of
best print quality in the industry, wide color gamut, substrate versatility,
speed, productivity, flexibility, and the ability to vary every printed copy.
HP Indigo printing inherently matches and at times, due to its digital local
image processing and ink coverage flexibilities, surpasses offset, and is
also the best replacement for photo silver halide applications.
This white paper describes the HP Indigo Digital Offset Color printing
process and its unique features. It also compares Digital Offset Color, or
liquid electrophotography (LEP), with dry toner electrophotography (DEP)
and inkjet technology-based digital printing presses, as well as with the
conventional, non-variable, offset lithography process that has
traditionally dominated the worldwide printing industry.
HP Indigo offers a wide range of digital presses for a variety of
applications, all based on the basic principles of its Digital Offset Color
Figure 1 HP Indigo 7600 Digital Press
What is Digital Offset Color?
Let’s break it down word by word:
Figure 2
Schematic comparison of color gamut with
4-, 6-, 7-, and 11-color HP Indigo ElectroInks
1. Digital
The printed image is created directly from digital data, avoiding the
use of any ”analog” intermediate media. It starts with digitally created
pages or print elements containing, for example, text, layouts or
images. Then, unlike conventional printing processes, there are no
intermediate pre-press processes between the digital document file and
the final print. No film, no imagesetters, no plates, no platesetters, no
photo-chemicals, and no other ”analog” elements. There is also no
press make-ready: no plate mounting, no registration adjustments, no
ink keys, and no waste. The HP Indigo process is fully digital from
image creation to printing. And, since it is a fully digital process, every
image can be a new one, enabling information to be varied as
2. Offset
Offset simply means that there is an intermediate cylinder that transfers
the ink image from its origin on the plate cylinder to the final substrate
(i.e. the paper, plastic or other material) for printing. In the printing
industry, the term offset is commonly used as a term for the lithographic
process. Indeed, modern lithographic (also called litho) presses do use
an offset process.
HP Indigo technology also uses offset printing, by the use of an offset
cylinder covered with a renewable rubbery blanket.
There are two main purposes of offsetting in printing presses. First, it
protects the surface of the printing plate from excessive wear due to
friction with the substrate as it is printed. Second, since the rubbery
blanket conforms to the local topography of the substrate, ink is
adhered both to the “peaks” and the ”valleys” of the substrate equally.
In other words, it acts as a kind of shock absorber and pressure pad,
ensuring good ink transfer from printing plate to the substrate.
Conventional offset presses can therefore print on a very wide range of
substrate surfaces and thickness, and are superior to non-offset
processes in this respect. The HP Indigo process uses offset for the
same reasons, thus making it capable of printing on a wide range of
A notable difference between conventional offset and HP Indigo digital
offset printing technology is that HP Indigo’s ink—ElectroInk—transfers
from the blanket to the substrate with virtually no ink splitting that
characterizes conventional offset printing systems. This enables the
creation and transfer of a different image each printing cycle.
A further difference is that HP Indigo ElectroInk is dried on the blanket
and the final image is transferred to the substrate in the form of a ready
dry film. This process does not depend on the final substrate. Thus,
there is almost no limitation to substrate flexibility, and all substrates are
printed with the same high HP Indigo quality and at the same speed.
Furthermore, since no drying is required the substrate does not undergo
any further stress. In comparison, other technologies require further
drying of the ink on the substrate (as in offset or inkjet technologies) or
fusing of toner on the substrate (as in DEP) and require the matching of
ink/toner and printing process with media.
4 colors (CMYK)
HP IndiChrome on-press (CMYKOV)
HP IndiChrome Plus (CMYKOVG)
HP IndiChrome off-press (CMYKOVG and reflex blue, bright yellow, rhodamine red,
and transparent)
The matching requirement, at times, affects productivity due to the
need, for example, to wait for drying or to wait for stabilization of the
heating mechanism. When an ink-process-media match does not exist,
some media has to be excluded.
The above difference results in superior versatility of LEP, leading to
productivity, by the ability to change data, color, and substrate at will
and with no time penalty.
3. Color
As it simply sounds, HP Indigo technology enables digital printing in full
color. However, unlike conventional offset litho color presses, which
require one complete printing unit per color, HP Indigo presses print
multiple colors for each single pass of the substrate through the press. As
described above, in Digital Offset Color technology all the ink transfers
from the blanket to the substrate. The HP Indigo digital press transfers
with each rotation of the press cylinders, on the single set of blanket and
imaging plate, not only a different image but also a different ink. HP
calls the method or configuration “on-the-fly color switching.” This is all
done without physical interaction between the different color separations.
Printing with 5, 6 or 7 colors, in addition to CMYK, the flexibility of
adding pre-mixed HP IndiChrome spot colors, and the capability to vary
the number of impressions per single color, offers major enhancements in
color quality, range, fidelity, and luminosity (see figure 2).
In summary, HP combines digital, offset, and color into a powerful
printing process. The three core technologies of the HP Indigo Digital
Offset Color process are:
•ElectroInk – HP Indigo’s liquid ink
•Thermal offset transfer
•On-the-fly color switching
Figure 3
Digital Offset Color core technologies.
•Substrate versatility
•No Fusing
•No Drying
•Color/speed versatility
HP Indigo
•High speed
•Efficient use of material
•Negligible media interaction
•Multi-layer (hit) flexibility
•Extreme saturation possible
Superior image quality
•Gloss uniformity
•Color saturation
HP Indigo ElectroInk
All HP Indigo digital presses use ElectroInk, HP Indigo’s unique liquid ink.
ElectroInk contains electrically charged ink particles, dispersed in liquid.
Similar to DEP, ElectroInk enables digital printing based on the
application of strictly controlled electrical fields to move charged color
particles. This control enables accurate placement of the printing
material. However, unlike DEP, ElectroInk enables the use of very small
particle size, down to 1-2 microns. These small particles dispersed in the
liquid carrier allow for higher resolution, uniform gloss, sharp image
edges, and very thin image layers.
The thin image layer closely follows the surface topography of the paper.
This gives a highly uniform finish, complementing the paper and resulting
in a similar texture both on the image and on the non-image areas.
For the two other digital technologies, DEP and inkjet, the situation is less
favorable. In DEP, the particle size cannot be made too small, as
particles then become airborne, and uncontrollable. Therefore, higher
printing speeds require larger particle sizes leading to adverse impact on
print quality attributes such as color performance, gloss uniformity, and
sharpness. Inkjet technology, while still laying a thin layer of ink on the
media, suffers from the inherent inaccuracy of the ink jetting. Since the
ink must strongly interact and absorb into the media, some of the pigment
or dye sinks below the surface rendering it less effective and reducing
saturation (or alternatively increasing cost). Part of the absorption follows
paper fibers leading ink away from the original droplet thus breaking the
edge of the drop in a less controlled and random manner (”wicking”).
These effects result in reduced control of the final product and with it loss
of sharpness and resolution.
HP Indigo ElectroInk is available in an increasing range of colors,
•Standard CMYK (cyan, magenta, yellow, black) process colors.
•HP IndiChrome wide-gamut 6- and 7- color sets. These incorporate
orange and violet inks for the 6 colors and also green for 7 colors to
extend the color reproduction capabilities far beyond the range
possible with CMYK inks only.
•HP IndiChrome spot colors – mixed from a set of base inks, matching
spot colors including most of the PANTONE® color range.
•White ink gives opaque backing for labels and flexible packaging
and enables high-value applications for commercial printing.
•Light cyan, light magenta, and light black for photo prints, competing
with silver halide quality.
The capability to increase the number of ink layers to more than four,
which is natural in offset and in ElectroInk, is very difficult, if at all
possible, when using the thick layers of DEP or, for inkjet, when
conflicting with the drying requirements of fast heavy coverage printing.
This results in a print quality versatility only offered by LEP.
ElectroInk is supplied in a concentrated form that is loaded into the press
in cartridges in a ”clean hands” operation. Inside the press it is fed into
ink supply tanks, diluted with oil and combined with a charging control
fluid, to form a fluid mixture of carrier liquid and colorant particles ready
for printing. The mixing is done under accurate automated control,
resulting in a stable ink with nearly constant physical traits, leading to
consistent prints. Furthermore, ElectroInk incorporates pigments which are
the same as offset; this supports both the price and the availability of the
final ink product.
To summarize, ElectroInk enables high quality, wide and accurate color
gamut, sharp images, and color with gloss closely matching the media,
similar to and at times surpassing conventional offset printing, and
exceeding the quality achieved by competing digital printing
Thermal Offset
Color switching
The HP Thermal Offset process uses a heated blanket causing the
specially shaped pigment-carrying particles within the ElectroInk to melt
and blend into a smooth film. When it contacts the cooler substrate, the
ElectroInk strongly adheres to the substrate, immediately solidifies and
transfers with almost no change in dimension or shape. Since the image
is completely defined on the blanket, issues such as ink media interaction
or ink-ink interaction which are common in nearly all other printing
methods are virtually nonexistent in LEP.
As the HP Indigo LEP technology employs only contact transfers and
since the HP Indigo ElectroInk does not tend to become airborne as DEP
toner, the result is a process which can inherently be much faster than the
DEP process and provide much higher quality than inkjet at the same
speed. Currently the process speed, or image generation speed, is up to
2.35 m/sec or 462 ft/min with the release of the new HP Indigo
10000 Digital Press platform. This inherent speed is combined with the
fact that HP Indigo’s Digital Offset Color printing technology enables the
printing of all color separations by a single engine. After one color
separation has been created and printed, the next one (usually a
different color) is created and printed on the same engine. This is
possible since the blanket completely transfers the previous image, and
none of the image stays on the blanket. Single engine printing has
several obvious advantages, including compactness, lower cost of
hardware, and better mechanical accuracy, but beyond that it offers the
flexibility to balance quality with color content and color accuracy on the
same press and even within the same run. An example of the flexibility to
balance between speed and quality can be given by the two-engine HP
Indigo W7250 Digital Press. It can produce 480 duplex monochrome
letter size pages per minute, which amounts to 960 letter-size page sides
per minute. With Enhanced Productivity Mode (EPM), even higher speed
can be achieved using CMY printing technology. Using the EPM
technology this press can print 33% faster, delivering 320 pages per
minute. If choosing to print higher quality 4-color pages the speed will be
reduced by a factor of four to a still impressive 240 four-color page sides
per minute. Going to even higher quality the printer may chose to
employ 5, 6 or 7 colors and possibly add multiple passes of certain
colors for added opacity and special effects. In this case the output
speed will be reduced accordingly, but the value of the page increased
The result is an image with a true offset look and feel, accurately
replicating the gloss and texture of the substrate on which it was placed.
Since the fusing and drying is done on the blanket and the transfer is by
contact, in contrast with DEP printing, there is little need for environmental
control of the media prior to printing or to expose the media to extreme
heat which limits the media types and may warp the media. LEP does
not require the image to be dried on the media, as in offset printing and
in inkjet printing. The print is effectively dry as soon as it leaves the press,
eliminating the risk of ink set-off marking other copies. Thus, print finishing
may be performed immediately. This is a major benefit over conventional
lithography which requires either assisted drying systems, or a natural
drying time of several hours, before any print finishing processes can be
In summary, HP Indigo’s Digital Offset Color technology enables the look
and feel of conventional offset printing, the ability to print on a virtually
limitless substrate range, and the capability of immediate drying, which
enables duplex printing or finishing with no waiting period.
To summarize, the color switching technology offers an optimal balance
between speed and enhanced quality and at the same time minimizes
press footprint and cost of hardware.
Figure 4
Various applications printed on an HP Indigo digital press
Step-by-step description of the HP Indigo
Digital Offset Color printing process
The negatively charged particles are directed by electric voltage toward
the PIP while the positive charged particles are attracted to the charging
device and neutralized.
The charge roller is more efficient in creating and directing charged
particles toward the PIP. Thus, it requires less space, and less handling
than the older scorotron systems. It also creates almost no undesired
ozone, thus reducing the need to replace ozone filters. In order to
maintain the process stability, the voltages applied to control the transfer
of the charges to the PIP are routinely automatically calibrated to
accommodate for changes in the photoconductor’s discharging level.
The printing cycle
The HP printing engine performs the following operations sequentially:
1. Electrostatic charging of the electrophotographic Photo Imaging Plate
(PIP) which is mounted on the imaging cylinder.
2. Exposure of the PIP by a scanned array of laser diodes. These lasers
are controlled by the raster image processor which converts
instructions from a digital file into ”power” instructions for the lasers.
3. Image development performed by the Binary Ink Developer (BID) units.
4. Discharging the PIP in preparation for transfer.
5. Transfer of the inked image to the blanket cylinder.
6. Heating of the inked image carried by the blanket forming the final
image in form of a thin tacky film.
7. Complete transfer of the final image film to the substrate held by the
impression cylinder.
8. Removal of any residual ink and electrical charge from the PIP and
cooling of the PIP after engagement with the hot blanket.
These operations repeat themselves for every color separation in the
image. They are described below in more detail:
1. PIP charging
The first step in LEP is the deposition of a uniform static electric charge on
the photoconductor. This is achieved either by a charge roller or, for
older press models, by scorotrons. In both techniques charged particles
(atoms, molecules and free electrons) are produced by a glow discharge
effect (i.e. the ionization of the air) through the application of highvoltage.
2. PIP exposure
As the PIP cylinder continues to rotate, it passes the imaging unit where
as many as 32 laser beams in parallel expose the image area,
dissipating (neutralizing) the charge in those areas. When the exposed
PIP rotates toward the next station it is carrying a latent image in the form
of an invisible electrostatic charge pattern conforming to the image to be
3. Image development
Inking is performed by the Binary Ink Developer (BID) units, one for each
ink. The BID units prepare a thin film of highly electrically charged
ElectroInk on their roller surface.
During printing the appropriate BID roller engages with the PIP cylinder.
The electrical fields between the PIP and the BID result in attracting the ink
paste to the image area and repelling it from the non-image areas,
shearing the ink film accurately and instantaneously.
The result is the replication of the electrical latent image with a clean
and sharp inked image.
Figure 5
HP Indigo digital press printing cycle
1. Charging station
2. Laser exposure
3. Binary Ink Developer units (BIDs)
4. Pre-transfer erase unit (PTE)
5. First transfer (PIP to blanket)
6. Blanket heating
7. Second transfer (blanket to substrate)
8. Photoconductor cleaning station
4. Pre-transfer erase
Just before the image is transferred a set of diodes illuminate the PIP. The
illumination causes a homogeneous conductivity across the PIP leading to
dissipation of the charges still existing on the background. This enables a
clean transfer of the image in the next stage avoiding the background
charges from sparking to the blanket and damaging the image and, in
time, the PIP and blanket.
5. First transfer
The PIP then rotates into contact with the electrically charged blanket on
the transfer cylinder, and the ink layer is electrically transferred to the
6. Film formation (blanket heating)
Following the inked image, on the rotating and heated blanket, the
ElectroInk is being heated both from the blanket and, in newer model
presses, from an external heat source. This causes the ink particles to
partially melt and blend together. At the same time most of the carrier oil
is evaporated, to be collected and reused as part of fresh ink in the
tanks. The result is a ready finished image in form of a hot, nearly dry,
tacky plastic film.
7. Second transfer
As the ink comes into contact with the substrate, which is significantly
below the melting temperature of the particles, the ink film solidifies, sticks
to it, and completely peels off from the blanket, ensuring 100% transfer
from blanket to substrate. The blanket is therefore clean and ready to
accept the next impression with its new ink layer.
The second transfer method differs according to press models: The HP
Indigo digital press sheetfed models use the Multi-Shot Color imaging
sequence. In Multi-Shot Color the substrate stays on the impression
cylinder for several rotations of the press cylinders as it receives each
separation from the blanket one after the other. As the final separation is
printed, the substrate is either moved for duplex printing or delivered to
the output tray. HP Indigo webfed presses employ a One Shot Color
process, as it is not possible to wrap the material around the impression
cylinder for multiple passes. In this case, the PIP cylinder rotates several
times, transferring a succession of separations and building them up on
the blanket, before they are transferred to the substrate all in the same
impression pass.
8. Cleaning station
Returning to the PIP, after transferring the image to the blanket, it rotates
past a cleaning station which removes any residual ink and cools the PIP
from heat transferred during contact with the hot blanket. At this point this
part of the PIP surface has made a complete rotation and can be
recharged ready for the next image.
As mentioned before, HP Indigo presses print multiple colors from the
same offset blanket. The cycle repeats itself for each color separation
and the only difference between the cycles is in the ink application and
the image data corresponding to the printed color separation.
Digital Offset Color advantages
After describing the technology and the process, let’s touch on the
advantages which stem from them.
Figure 6
Comparison between dense lines
Quality characteristics
1. Edge sharpness and definition
Figures 8 and 9 display microscope images of dots printed with HP
Indigo ElectroInk, conventional offset technology, and DEP technology.
All prints have been calibrated to produce the same final color value.
Viewed at high magnification, ElectroInk forms much sharper features
than any other printing method. For small dots, the conventional offset
creates “smudges” due to the ink splitting, while DEP looks like a pile of
powder with regions flattened by the fusing and uncontrolled dust
between the dots.
The sharpness of ElectroInk is particularly noticeable at the edges of
halftone dots, or fine type characters (see figure 6 and figure 7).
Noticeable is the contamination-free background of ElectroInk images.
This is first due to the small size of the ink particles and also to the way
ElectroInk particles are transformed when they are on the press. As
opposed to both DEP and inkjet, ElectroInk is not transported by flight
though air, but rather a thin ink paste is placed on the PIP and “cut” to
the correct size. This produces very accurate edges and almost no
background contamination. In the printing process ElectroInk paste is
transferred by contact to the heated blanket, because the ink particles
melt and blend, the strong surface tension of the liquefied heated ink,
while becoming a film, facilitates the formation of a sharp, clean edge.
When transferred from the blanket to the final printing substrate (paper
or plastic), the heated plastic film cools down to form a thin colored
plastic layer on the substrate surface. When printing on paper, the
cooled ElectroInk does not soak (or wick) into the paper fibers. Thus,
printed dots, linework, and text stay sharp and well defined on the
surface of the paper.
HP Indigo ElectroInk
Figure 7
Comparison of text printed with HP Indigo digital offset
(left) with text printed by DEP (right)
Like ElectroInk, DEP toners don’t penetrate the paper either, but they do
suffer from large particle size and stray toner particles scattered outside
the image edges, leading to poor edge definition no matter what
substrate is used.
Comparison of text printed with HP Indigo digital offset (left)
with text printed by industrial inkjet press (right)
Figure 8
Compare ElectroInk (left), offset (middle) and
DEP (right).
Figure 9
Comparison between color dots printed with
HP Indigo ElectroInk (left) and DEP (right)
2. Optical density, dot gain color and image consistency
Optical density—the amount of light which the ink absorbs, or its
darkness is defined by the thickness of the ink on the media and the
concentration of light absorbing material (pigment in our case) it
contains. Dot gain is the difference in dimension between the dots and
lines as they were designed in the original digital file and their final
dimension on the final print. In combination, these two elements are the
basis for print stability. For HP Indigo digital presses, both optical
density and dot gain are periodically monitored and press parameters
are automatically adjusted to minimize fluctuation. The result is color
and linework that are consistent and predictable over time and
between presses. HP Indigo presses automatically adjust the optical
density (i.e. appearance in terms of lightness or darkness) and dot size
so that they always appear the same, copy after copy, regardless of
small changes in press parameters and even in uncontrolled changes in
media batch. On top of this HP Indigo presses have built-in dot gain
compensation which corrects the exposed dot size and thickness so that
it prints to the color profile with the media included in the equation.
With a conventional offset lithographic press, there are wider
fluctuations during a run caused by factors that include: fluctuating ink
and water temperature; water/ink balance and their tendency to
emulsify; plate and blanket wear; and atmospheric humidity altering the
absorbency of the paper. Neither automatic nor manual adjustment can
completely iron these out, because there is always a time lag between
the problem appearing and the adjustment taking effect, during which
many copies are printed. With HP Indigo presses, there are fewer
operating variables, and the optical density of the printed image can
be electronically set by the operator within a wide operational range.
The capability of printing to a specific color profile is a necessary
requirement in order to achieve print consistency. A special capability
of the HP Indigo presses is the ability to print linework and fonts with
nearly zero dot gain. This is achieved by combining a per-pixel
variable laser power capability together with the previously discussed
inherent sharpness and accuracy of the ElectroInk process. The
combined result of accurate color and linework is a print which is
consistent from print to print and press to press.
Just as importantly, the parameters which created the print, such as
optical density, color profiles and screen, are kept for repeat runs in
future, meaning that a repeat job will be identical to the original
regardless of the press or media batch. This is difficult to achieve with
lithography, which depends to a great extent on operator skill.
Figure 10
Gloss comparison for various printing systems
3. Imaging flexibility
HP Indigo presses have been created to offer, by design, full flexibility.
The setting of crucial elements such as optical density, color curve, screen
and even dot gain may be done before the RIP (Raster Image
Processing), but may also be done just before printing by a touch of the
keyboard or touch screen and the image will be reprinted with the new
required conditions, without re-RIP or time lag. Since the printed ink layers
are thin and do not interact, there is little need to change color profile
when changing screen. This allows quick proofing cycles when proofing
is required, in turn shortening time and minimizing waste of media.
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Obviously most of these capabilities are lacking with lithography, but
even DEP is limited by the fact that the thick toner layers strongly
interact (or trap) each other and thus any change in screen, optical
density, dot gain or color curve will require a new RIP to receive a
reasonable color output.
Paper gloss %
Paper roughness
Figure 11
HP Indigo ElectroInk complements the topography of the media
Paper covered
with HP Indigo ElectroInk
4. Image gloss
Many people believe that the key to the quality of offset litho printing is
its glossy appearance. This isn’t quite true—what is important is the
uniformity of the gloss and matching with the media. For instance, many
magazines and company brochures have varnished front covers to give
a high quality feel, but either gloss or matte varnishes may be used. In
addition, some of the most expensive looking substrates have intentional
textures which should not be glossed over.
The uniformity of the finish is the key issue rather than just its surface
gloss. HP Indigo ElectroInk gives a highly uniform finish complementing
that of the underlying substrate—whether the substrate is a high gloss
coated paper or a rough matte paper. This is very different from DEP
printers and copiers, which produce images having the same gloss
irrespective of the paper they are printed on, and thus suffer from
variable gloss levels between shadow areas (i.e. solid toner coverage)
and highlight areas (with little or no toner coverage).
ElectroInk images match the gloss of the underlying printing substrate,
from rough to dull to high gloss, just like conventional offset prints. Paper
stocks have a typical surface roughness ranging from about 1 to 10
microns in height. The ElectroInk layer is only about 1 micron thick, and
therefore it follows the ”hills and valleys” of the substrate surface texture,
rather than filling them in. The result is that there are no large variations in
gloss between the inked image areas and the bare paper substrate (see
figure 11).
Even the finest DEP color toner is limited to a particle size no lower than
5 to 9 microns, otherwise it is too small to be controllable and forms a
powder cloud or dust. Since powder toner particles are so large, they
create thick images—which cannot replicate the surface roughness of the
paper. Powder toner images therefore have their own unique gloss which
contrasts with the gloss of the paper. This gloss non-uniformity is
perceived as poor quality printing. Plotting the gloss characteristics of the
various digital and lithographic processes on a graph demonstrates that
ElectroInk has practically ideal reflective characteristics, nearly matching
the paper gloss for all but the very smoothest coated surfaces. DEP toners
plot as straight horizontal lines on the graph, indicating they have their
own gloss no matter what the substrate.
5. Color gamut
HP Indigo ElectroInk primary colors are similar to the ones defined in
ISO 12647-2. This similarity allows the transfer of jobs between offset
presses and Indigo digital presses with either minimal or no color
transformations. The conformity with offset, in turn, enables the
combination of digital and offset pages and the possibility to replace
long offset runs (and the accompanying storage requirements) with a few
short digital runs. The nature of the ElectroInk process earned HP Indigo
digital presses GRACoL proofing and Fogra production certification,
confirming their ability to create accurate color prints and work with well
defined color standards for color printing.
HP Indigo printed material can withstand considerable handling activities
immediately, unlike conventional offset lithography, not equipped with
assisted drying, which requires a drying period of several hours before
further processes, such as cutting or folding, can be performed. Print
drying is considered by GATF (The Graphic Arts Technical Foundation)
as the number one problem of conventional lithographic printing,
responsible for many image artifacts and defects, as well as print
operation issues.
Both DEP and inkjet also produce ready to handle products, nevertheless
the extreme heat needed for DEP to fuse on the paper or the requirement
to absorb the ink of inkjet into the paper and then to dry it on the paper
strongly limit the media selection for these technologies.
ElectroInk is easily calibrated for out-of-standard thickness and dot gain
supporting the use of different color profiles and removing the need to
change ink per application. Therefore ElectroInk can use one CMYK
color set for all applications, unlike lithography which requires many
different CMYK ink formulations.
7. Lightfastness
The encapsulation of the pigment sub-particles within the ElectroInk
plastic resin helps preserve the chemical properties of the pigments
against oxidation and humidity effects, especially under strong ultraviolet daylight conditions. This means that color durability of printed
images, either in the form of fading or deepening, is superior
compared to conventional offset printing.
Beyond the four process colors (CMYK), HP Indigo digital presses may
utilize up to three additional colors in the HP IndiChrome set (CMYK,
orange, violet) or IndiChrome Plus set (CMYK, orange, violet and green)
increasing available color gamut.
For very discerning applications which require accurate spot color, the
unique HP Indigo Ink Mixing System (IMS) provides great flexibility to
customers wishing to match specific PANTONE® spot colors. The system
allows users to mix, on site, a wide range of special colors from a set of
11 base inks, with fully automated software guiding the user through the
measurement, analysis, and mixing steps.
Photo consumers are also seeking alternatives to traditional photo printing
with silver halide chemicals. In new testing* conducted by Wilhelm
Imaging Research (WIR), HP Indigo prints exceeded or matched the
display image permanence ratings of the best-rated silver halide product.
The WIR testing also validated the album permanence of HP Indigo
photobook pages, which received the highest possible dark permanence
rating of > 200 years. In comparison, the WIR gave all the silver halide
photos a much lower album/dark storage rating of > 100 years.
6. Instantaneous image drying
Because ElectroInk solidifies as soon as it transfers to the substrate, the
finished print emerges dry from the HP Indigo printing press. Further
image hardness is acquired in the first few hours post printing.
Figure 12
HP Indigo print permanence
maximum longevity
compared with silver halide
* Results published on March 7, 2011
HP Indigo Album Permanence
Silver Halide
HP Indigo Display
Fujicolor Crystal
200 years
Substrate compatibility
1. Variety
HP Indigo’s Digital Offset Color process is compatible with a wider
variety of substrate types, surfaces and thickness than any other digital
printing process. These include paper, card stock, plastic, film,
paperboard, and metals. Only one formulation of HP Indigo ElectroInk
is needed to print on any stock that the press can handle. This means
that HP Indigo press users can rapidly switch between substrates
without having to worry about changing inks. With conventional offset
printing, different inks for papers and non-absorbing plastic films are
required; the latter usually need UV polymerizable inks. When printing
on paper stocks with different absorbencies, it may be necessary to
adjust the viscosity of the ink by means of thinning or thickening agents,
or even use specially formulated inks. Particularly absorbent papers can
also increase offset ink consumption by up to about 50%, which
contrasts with ElectroInk where consumption has almost no dependence
on the substrate properties.
DEP technology is heavily dependent upon the electrostatic properties
of the paper substrate, and small changes in the environmental relative
humidity may result in noticeable variations in print quality. Moreover,
the high fusing temperature needed for DEP puts serious limitations on
the choice of coated paper stock or plastic films that can be printed.
Inkjet suffers from even a stronger limitation due to the requirement to
absorb the ink into the media. Plastics are out of the question unless
dealing with UV inks or coating the media with a coating that will, in
effect, absorb the ink.
The flexibility in media is what led HP Indigo industrial presses to be
the leading source for digitally printed labels and thus are also
replacing flexography in certain high-end applications.
2. Lamination and over-varnishing
Stock printed with ElectroInk is compatible with standard coating
processes such as lamination or varnishing. Lamination of thin plastic
films over the printed stock can be done in the conventional way, using
a variety of solvent-born, water-based, UV-based or solvent-free
A varnish coating, either UV or water-based, can also be used and
ElectroInk plastic resin withstands a large variety of chemical solvents.
No significant image degradation occurs when using most of the
standard coating materials.
HP Indigo Digital Offset Color technology enables printing on a vast array of substrates.
Productivity and versatility
The design considerations of the HP Indigo digital printing system and
the accompanying end-to-end solutions maximize productivity and
versatility. HP Indigo technology is highly scalable, with the HP Indigo
10000 platform increasing format up to 29 x 20 inches while
retaining all of Indigo’s technology processes and high print quality. In
what follows we discuss some of these design choices.
The HP Indigo ElectroInk process is inherently faster than any DEP
process by nearly an order of magnitude. As discussed in the color
switching section, the speed is converted to flexibility of balancing
between productivity vs. color gamut. The press owner can choose,
even within the same run, to produce monochrome pages at up to 960
8.5 x 11 inch pages per minute (on the Indigo W7250) even up to
high-value pages with 7 colors at the cost of reduced productivity. HP
Indigo provides further color flexibility with the option to order readyprepared spot colors or to use the HP Indigo Ink Mixing System (IMS)
at the client site.
The thermal transfer, which is intrinsic to the technology, provides nearly
unlimited versatility in substrate choice with no speed impact.
Flexibility is also achieved by a wide choice of Digital Front Ends, each
designed to match the application needs in order to create the most
adequate printing environment. Plus, at the finishing end, the HP Indigo
press owner can choose between a variety of inline, near-line or offline
finishing solutions increasing the productivity of the full end-to-end print
The robust design of the HP Indigo printing environment is aimed
toward a 24/7 production flow. Critical process elements are
automatically controlled by the press and reduce operator intervention.
This self-management begins with the real time control of press status
parameters and print quality and continues with the HP Indigo Print
Care system which, in effect, places the knowledge of HP Indigo with
the press operator on the production floor.
Why is the HP Indigo digital printing process unique?
The HP Indigo Digital Offset Color printing process is the only variable
imaging printing technology that can equal or exceed the quality, color
range, and substrate compatibility of conventional offset lithographic
printing, as well as flexographic printing, and is also the best choice
for replacing silver halide technology photo printing.
While competing with offset printing for print quality, the digital
technology offers important benefits both economical and
environmental. Since each product may be targeted at a final recipient,
waste is reduced and warehousing is minimized as pages may be
printed just in time and on demand. Moreover, the HP Indigo digital
process offers cost effectiveness and is environmentally friendly as it
both reduces waste and eliminates the use of hazardous chemical
Apart from the HP Indigo Digital Offset Color process, the other main
variable imaging digital color press technologies are the DEP process
and inkjet presses. The limitations of dry toner printing have been
detailed above, but to recap briefly: fine detail and acceptable colors
can only be achieved with very small pigmented particles. Dry toner
particles have to be above a critical size of at least 5 to 9 microns
otherwise they form a dust cloud and cannot be controlled in the press.
Some DEP processes can produce high gloss toner images, but they
cannot match the gloss of the substrate surface, thereby producing high
“gloss contrast” which is perceived as poor quality. HP Indigo’s liquid
ElectroInk uses oil to bind and distribute its pigment-carrying particles,
which are about 1 micron in size, and consequently capable of
creating much finer detail and thinner printed films. DEP presses, with
their inherently limited process speed, require multiple printing units
where the only flexibility is the decision of whether a unit engages the
media or not. Therefore an important consideration when making
comparisons between these and the single unit HP Indigo presses is
simplicity. HP Indigo’s color switching technology enables single station
printing presses, which results in flexibility, more compact presses, less
parts to maintain, and less potential for things to go wrong.
For inkjet technology, the comparison parameters are different. Slow
high-quality proofing devices based on inkjet technology exist for some
years now. Nevertheless, inkjet technology requires both that the ink
absorbs into the media and that vast amounts of liquid are evaporated
from the media after the ink was placed. This pretty much narrows the
substrate range and the image coverage of inkjet high speed devices.
When combined with the inherent inaccuracy of the jetting and
wicking the result is a technology which, though very adequate for
certain applications, is less versatile and less flexible and cannot
compete for overall quality or for overall media range with the HP
Indigo Digital Offset Color process.
To summarize, the HP Indigo digital printing process offers a unique
combination of quality, versatility, and productivity unmatched by any
other existing digital technology. The key considerations for
organizations investing in digital color printing are quality, speed, total
costs (fixed and variable), versatility, and product range, and though
for some niche markets other solutions may be equally advantageous,
HP technology places it as a leading contender in all these respects.
The family of HP Indigo
digital presses
Since its introduction in 1993, Digital Offset Color has grown from a
technology seen primarily as one needing to create its own markets to
the leading and benchmark technology for digital color printing.
More importantly, printers using this technology are creating new and
very lucrative printing businesses, moving printing away from a
commodity, to a high-value business.
Today it is perceived as an offset replacement for short and medium
runs and in general the preferred replacement for silver halide
applications. Equally, it is transforming industrial printing processes in
areas such as labels and packaging and thus is also replacing
HP Indigo’s commercial and labels and packaging printing product
lines offer customers the product best suited to their unique
applications, print volume, and budget requirements. All are based on
HP Indigo Digital Offset Color technology, offering its unique
HP Indigo 10000 Digital Press—A 29 inch format press that produces any commercial job—with Indigo quality, substrate versatility, and production
flexibility. The format of the press enables highly efficient imposition of jobs and lower cost per copy, pushing the breakeven point vs. offset. Printing
3450 sheets per hour (4/0) and 4600 in Enhanced Productivity Mode (EPM), the press is capable of producing over two million sheets per month.
The press is an easy fit for offset print service providers, printing large volumes of high-value pages and a wide variety of applications.
HP Indigo 7600 Digital Press—The fastest digital 13 x 19 inch format sheetfed press in the market, the press combines proven quality,
versatility, and breakthrough productivity with new intelligent automation that maximizes uptime. New special effect capabilities—including
raised print and textured effects—expand opportunities to deliver high-value applications. Using EPM the press prints 160 color pages per
minute for four million color pages per month with higher profitability.
HP Indigo 5600 Digital Press—delivers unmatched digital offset and photo quality with high versatility and productivity. Seamlessly switching
between jobs and media types—with a new added ability to print on synthetics—the press is ideal for PSPs with a diverse job mix. Capable
of printing well over two million color pages or five million monochrome pages per month, the press is the leading choice for medium volume
printers. Using EPM, the press prints color jobs at a speed of 90 ppm. It also reaches a monochrome speed of 272 ppm.
HP Indigo 3550 Digital Press—offers HP Indigo’s exceptional print quality and versatility at a low initial investment, making it the ideal solution
for launching digital production. The press fulfills broad digital production requirements with its solid productivity and ability to print with true
spot inks for corporate brand color matching. Throughput reaches 68 ppm in full color and 136 ppm monochrome, regardless of media type.
HP Indigo W7250 Digital Press—provides high-volume production using its unique combination of quality, productivity, and competitive
total cost of ownership (TCO). With 60 installations worldwide, this application-focused press platform prints on any media, delivering 320
ppm in full color with EPM and higher speeds in 1 and 2-color printing.
HP Indigo WS6000p Digital Press—A high-quality simplex, continuous feed press with a 38 inch print length, the press serves photo, book
covers and any other simplex applications demanding no-compromise print quality and a production line approach.
HP Indigo WS6600 Digital Press—is the most cost-effective, high-quality printing solution for high-volume labels and packaging production
including cartons and flexible packages. Breaking digital productivity records with its printing speed of 130 linear feet per minute in color
the press significantly pushes the crossover point vs. conventional processes making the vast majority of jobs more profitable when printed
digitally. While enabling production of labels and packaging with the greatest shelf impact, the HP Indigo WS6600 Digital Press offers
lower operational costs, improved supply chain efficiency, and shorter turnaround time.
HP Indigo WS4600 Digital Press—A “no compromises” entry-level solution that delivers high productivity and the finest quality, the press
provides converters with all the tools they need to produce dozens of jobs per day with seamless job switching. It prints up to 50 feet per
minute in 4-color mode, 70 feet in Enhanced Productivity Mode and up to 100 feet in 1- or 2-color mode.
North America
Hewlett-Packard Company
5555 Windward Pkwy
Alpharetta, GA 30004
Tel: +1 800 289 5986
Europe, Middle East, and Africa
Hewlett-Packard Española S.L
Cami de Can Graells, 1 – 21
08174 Sant Cugat del Valles
Tel: +34 902 027 020
Fax: +34 935 82 1 400
Asia Pacific
Hewlett-Packard Company
138 Depot Road
Singapore 109683
Tel: +65 6727 0777
Fax: +65 6276 3160
Latin America
Hewlett-Packard Company
5200 Blue Lagoon Drive
Suite 950
Miami, FL 33126
Tel: +305 267 4220
Fax: +305 265 5550
To learn more, visit www.hp.com/go/graphic-arts
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without
notice. The only warranties for HP products and services are set forth in the express warranty statements accompanying such
products and services. Nothing herein should be construed as constituting an additional warranty. HP shall not be liable for technical
or editorial errors or omissions contained herein.
4AA1-5248ENW. March 2012.
Hewlett-Packard Company
Kiryat Weizmann
P.O. Box 150
Rehovot 76101
Tel: +972 8 938 1818
Fax: +972 8 938 1338
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