The function of fountain solution in lithography

The  function  of fountain  solution in  lithography
The function of
fountain solution
in lithography
FUJI HUNT PHOTOGRAPHIC CHEMICALS, INC.
HEADQUARTERS
40 Boroline Road
Allendale, NJ 07401
Tel: 201.995.2200
Fax: 201.995.2299
www.fujihuntusa.com
MANUFACTURING FACILITY
50 Industrial Loop North
Orange Park, FL 32073
Tel: 904.264.3500
Fax: 904.278.9697
www.anchorlith.com
INTRODUCTION
CONTENTS
We all view lithography as such an everyday phenomenon that
perhaps you have not carefully thought about the role that
water/fountain solution plays.
The Functions
Supply a Film of Water
Desensitize the Background
Cleaning
Rapid Spreading and Dampening Throughput
Lubrication
Emulsification Control
Summary
2
2
3
3
5
5
6
The Chemistry
Introduction
Ingredients
Desensitizing the Background
Cleaning the Background During Press Starts
Water Leveling
7
7
8
9
10
Water Flow
A Uniform Film
Dynamic Surface Tension
11
12
Water Throughput
Alcohol
14
Lubrication
16
Piling and Linting
Piling
Linting
17
18
Emulsification
Emulsification of Fountain Solution into Ink
19
Summary
20
You might say that the answer is obvious, "Keep ink off the background
areas while the press is printing and protect the plate when the press
stops."
We'll respond with, "You are correct, but that is far from the complete
story." Lithography is a complicated and often mysterious process,
at least whenever it's not working well. This process is both physical
and chemical in nature which means that a lot is going on. Later you
will grasp both the chemical and physical aspects in greater detail.
We hope that you find this book to be interesting and helpful in your
daily work. If you have any questions or comments, please feel free
to call or write to us at:
Technical Staff
Fuji Hunt Photographic Chemicals, Inc.
50 Industrial Loop North
Orange Park, Florida 32073
800.354.2300 or 904.264.3500
www.fujihuntusa.com | www.anchorlith.com
Printed in USA 2/03.
Copyright © 1993-2003. All rights reserved. No portion of this
booklet may be used or reproduced in whole or part without our
written consent.
1
The Functions
The Functions
Let's begin with two simple observations; turn down the water and ink
starts to print in the non-image, turn up the water and the ink starts
to leave the image. You are seeing competition of both ink and water
for image and background. When the process is working well it is said
that the ink and water are in balance. Each surface is carrying a thin
film of primarily the intended material. Notice that even though the
two surfaces are very different, both can accept ink or water.
Think of the background as pulling out on its water drop, causing it to
spread into a thin layer. The background surface is literally pulling
outward with a kind of capillary action.
Supply a Film of Water
If the plate is allowed to run too dry, ink will begin to wet the
background and print and you will have what is called scumming. This
occurs because there is not enough water to coat the plate surface
completely, and the bare aluminum accepts ink.
Thus, function number one is to provide an adequate film of water.
Desensitize the Background
There are important differences between the image and background
surfaces which explain the physical process that makes lithography
work.
Here you see two water drops. On the left the water spreads across
the water-loving (Hydrophilic) background while on the Hydrophobic
(water-hating) image, the water beads.
2
It is critical for fountain solution to maintain this hydrophilic surface of
the background. However, the image is similar to a freshly waxed car.
The water does not "wet" the paint (or image in this case) but only
beads up, and the fountain solution must not convert the image to a
hydrophilic nature (we call this blinding of the plate).
Cleaning
The next example is not quite so simple. Ink up a dry plate, and we
see that the ink sticks well everywhere; it "wets" both the image and
the background. If it's a good plate and you gently rub the background
with a wet pad, the ink will lift off and be displaced by water. The same
capillary forces that caused the water to spread to a thin film will draw
the water under the ink and "float" it off. If we had used fountain
solution rather than plain water, the surface would be "cleaned" more
efficiently. This cleaning process is critical during press starts.
Rapid Spreading and Dampening Throughput
The fourth and fifth functions promote the water spreading on the
background and rollers; fountain solution helps the water flow evenly
and smoothly through the dampening rollers onto the plate. It is
critical to achieve a thin, uniform film of water across the plate surface.
This must happen quicklyin the time that the plate cylinder rotates
from the dampening form roller to the blanket. On modern
high-speed web presses, this time is very short (0.05 seconds at 1500
fpm).
3
The Functions
The dampening rollers always impart nonuniformities similar to
the ridges that a paint brush leaves before the paint has had a
chance to "flow" out. This same "flowing out" must occur on
the plate surface.
Additives are used to promote this rapid spreading and flow
across the plate surface. They are called wetting agents or
surface active agents (surfactants). These materials, when
dissolved in your fountain solution, reduce the surface tension
of the water. Look at how they work.
A vector diagram from physics makes this easy to understand.
When a water drop is sitting on a surface there are two
important forces (represented by the arrows) acting on the water.
The Functions
On the left, observe the proper balance of forces; while on the
right, the fountain solution is trying to bead up on the plate
background. This plate may tend to be sensitive to ink.
Lubrication
The sixth function is lubrication and anti-piling. Fountain
con-centrates should contain lubricants and "release" agents for
the blanket. The film of water carried on the plate background
retards friction wear, thus increasing plate life. This reduction of
friction prevents the ink train and plate surface from over
heating. It has been shown that paper lint and dried ink resin
do build up on the blanket, and this is called piling or linting. The
release agents help to prevent the blanket from becoming
"sticky" thereby picking excessive paper linters.
Emulsification Control
Force 1 - Surface Energy of the Plate Pulls Out
Force 2 - Surface Tension of the Water Pulls In
The balance of these two forces determines how easily the
drop spreads. Fountain solution has a dual role affecting both
forces. The surfactants (wetting agents) reduce the surface
tension thus making the inward forces smaller. The
desensitizers maintain the hydrophilic surface, thus increasing
the outward forces. The net result is that fountain solution,
when it's working well, promotes good spreading by increasing
the outward forces and minimizing the inward forces.
4
The final function is to help prevent excessive emulsification of
water into the ink (formation of a water-in-oil emulsion). The
ink maker designs his ink to print correctly after it has taken on
some water. This "emulsified" water is essential to proper ink
transfer and required to maintain open reverses.
It is also critical to not promote the formation of oil-in-water
emulsions. This "oil-in-water" condition is what happens when
the detergent in your washer takes oily soil off the clothing. The
fountain solution must not break down the ink or it will pass
into the water. If tiny ink or pigment particles are floating
around in the water they can stain the background or print as
a light tone.
5
The Functions
The Chemistry
Summary
Introduction
The seven functions can be summarized as follows:
The fountain solution's assignments have been listed. By
studying the chemistry and physics of fountain solution, you will
better understand how the tasks are accomplished.
1.
2.
3.
4.
5.
6.
7.
Keep ink off the background with a film of water.
Maintain the hydrophilic nature of the background.
Quickly clean ink off the background during press starts.
Promote fast spreading of water over the plate surface.
Help the water flow evenly through the dampening rollers.
Lubricate the plate and blanket.
Control emulsification of ink and water.
You can now imagine that to accomplish these tasks is much
more complicated than just providing water to the background
of the plate. In rare instances, water might work; but most
printers would find it difficult to work with.
In the next sections you’ll examine what chemicals are used
and see in greater detail how fountain solution actually works.
6
Most American fountain solutions are formulated in a
somewhat similar manner; however, this is not to imply that
they all perform equally well. There are many ways to
accomplish the functions and some of these are much more
efficient than others. The following are the major classes of
ingredients used in fountain solutions:
Ingredients
•
•
•
•
•
•
•
•
•
•
•
•
Water soluble gums.
A pH buffer system.
Desensitizing salts.
Acids or their salts.
Wetting agents (also called surfactants).
Solvents.
Non-piling or lubricating additives.
Emulsion control agents.
Viscosity builders.
Biocides (fungus, bacteria, and mold control agents).
Defoamers.
Dyes.
7
The Chemistry
The Chemistry
Desensitizing the Background
Plates are often treated during manufacturing with these same
salts following the anodizing to impart a durable, corrosionresistant surface.
Recall the discussion of the hydrophilic nature of the
background. When the plates are manufactured, the aluminum
surface is treated to be both durable and water-loving. As the
plate wears and is constantly exposed to ink and dirt, this
treatment must be renewed. Several ingredients contribute to
this renewable desensitizing:
Water soluble, film-forming gumsgum arabic has been
the choice of lithographers for many years. This is the dried
sap of the acacia tree that grows in parts of Africa. This gum is
very water soluble and has a high affinity for aluminum metal.
It bonds best at pHs near four. Other water soluble polymers
such as larch gum, starches, CMC, PVP, some acrylics and
others have also found use as plate desensitizers. After each
application of fountain solution, a small amount of gum adheres
to the background providing a protective film. When the press
is stopped for any length of time, it is very important that this
gum film is adequate to protect the plate from oxidation or
handling.
Desensitizing saltsseveral inorganic salts aggressively react
with aluminum metal to form hydrophilic compounds. These
salts all contain what are called strong polar bonds
which attract water (hydrophilic, again). Examples are
silicates (Si-O4) and phosphates (P-O4). Notice that
both contain oxygen atoms (O); these polar oxygens will
stick up from the plate surface and draw water.
8
Cleanersin a secondary role are acids, solvents and wetting
agents. These act as cleaners and tend to remove any
accumulated oily soil from the plate surface. If left to build up,
this soil will attract ink and cause sensitivity. Acids, like
phosphoric, are used for metal cleaning to "brighten" the
surface. This is where the term "etch" came from. A very thin
layer of metal, only a few molecules thick, is dissolved off the
plate exposing a fresh surface.
Cleaning the Background During Press Starts
Commonly, ink will get on the dry background when the press
stops. Recall that ink will easily "wet" the non-image if it is dry.
When the dampeners come back on, it's essential that the
fountain solution displace this ink quickly. Minimizing waste is
the name of this game! If conditions are right, the ink will
almost instantly return to the ink forms, and the first sheet will
then be clean. If the ink passes into the fountain circulator and
forms an "oil-in-water" emulsion, the suspended ink can cause
toning problems.
The sequence of events during a press shutdown and restart
goes something like this:
1. The thin film of fountain solution dries on the plate.
2. Ink may get on the background after shutdown if the
press is inched.
3. During restart, fountain solution is applied to the water
form roller wetting the background.
4. The dried gum film should dissolve and lift off any ink or
dirt.
9
The Chemistry
5. The desensitizing ingredients will reestablish the
hydrophilic layer on the background, and the plate will
be printing clean.
A failure at any point will slow the restart and cause more
spoilage. To sum up restarts:
•
•
•
•
There must be a thick enough film of gum from
the evaporating fountain solution.
The gum film must be readily soluble when the water
returns.
The wetting agents should help remove any oily soil.
The desensitizing salts must reestablish the hydrophilic
surface which will strongly attract water.
Water Leveling
Printing is a very dynamic process, especially on newer highspeed web presses. Conditions at the important printing nips
are very different from a beaker sitting in the laboratory. This
is the main reason that so-called "models" of press conditions
often fail to duplicate working results. An example of this was
the "first generation" alcohol substitutes that looked promising
in the lab but worked very poorly on press. Their designers
failed to consider the dynamic requirements.
The dampening assignment is deceptively simple; rapidly lay
down a thin, uniform film of water across the plate surface.
Press engineers have worked hard to mechanically improve
over the original conventional ductor design of the dampening
train. They have tried sleeves, sprays, brushes, slip nips,
oscillators, etc. to help provide a more uniform water film.
10
Water Flow
A Uniform Film
Two requirements must be met to obtain a uniform film:
1. At the instant the water form roller and the plate split,
the ratio of wettest to driest areas must be a minimum
across the plate.
2. Any surfactant in the fountain solution or alcohol
substitute must be capable of acting very quickly to
allow for the maximum leveling that is possible.
This plate is going to be
very tricky to run, because
the dampening system has
laid down a film of widely
varying thickness. This film
will never be able to flow
out in time.
This plate, however, has
taken a fairly even charge of
water and will be easy to
run.
The plate must be dampened sufficiently to prevent scumming
in the driest area. If the film is uniform, the overall amount of
water reaching the plate will be a minimum; and ink/water
balance will be easiest to obtain.
11
Water Flow
Water Flow
Dynamic Surface Tension
Dynamic Surface Tension
Any surfactants that are in the fountain solution must be
capable of acting very quickly. This is called good "Dynamic
Surface Tension Reduction." When a water film is split (at the
nips, for example) these surfactant molecules react by traveling
(migrating) to a water surface. To be effective, the molecules
must line up with their hydrophilic ends pointed towards the
surface. They have no effect while inside the body of the liquid.
The strength and concentration of the surfactant controls how
much the surface tension will be reduced, but of greater
importance the "migration speed of the molecules" controls
how quickly the reduction or rapid wetting will occur on press.
Observe on the left how the molecules have lined up quickly
with their hydrophilic ends pointed to the fresh surface. On
the right most of the molecules are still floating around
randomly, and there will be little surface tension reduction of
this film, because the molecules were too slow to reach the
new surface.
12
continued
Observe how both solutions reach a surface tension of 30, but
that in the first graph (Surfactant II) it takes so long that at
faster press speeds this will not be effective. Surfactant I is
effective after only 0.05 seconds and will work well at highest
speeds.
13
Water ThroughPut
Water ThroughPut
The other important aspect of dampening flow is what we call
"Water Throughput." This term describes how easily the
fountain solution will move from the water pan through the
dampening train and finally onto the plate. This is of primary
importance with the continuous flow/alcohol type dampening
systems. Alcohol's main function is to promote higher
"throughput" and level water films. Both surfactants and
viscosity builders in fountain concentrates will increase water
flow and thus imitate the performance of alcohol.
Alcohol
Increasing the viscosity implies making the solution "thicker"
and that seems contrary to the common sense notion of
"making the water wetter or thinner" so that it will pass the
metering nip more easily. However, as the viscosity increases,
the water film at the rubber roller surface will be dragged
through the nip more effectively. Think of it as trying to wipe
off a film of 90 weight gear oil versus paint thinner.
Alcohol
Let's briefly examine alcohol and why it is used so frequently.
Common sense tells you that alcohol makes the "water
wetter" and improves dampening flow. The following facts will
help to explain why alcohol is so popular and seems to solve a
myriad of press problems:
•
•
•
•
•
14
Alcohol is a weak wetting agent.
Alcohol evaporates from the ink train quickly, leaving no
residue.
The evaporation helps cool the ink train.
The viscosity of water increases as IPA is added (up to
about 25% alcohol concentration by volume).
Alcohol is used at high percentages compared to other
fountain ingredients.
continued
Alcohol gives only medium surface tension, but, because the
molecules are small, it’s dynamic properties are exceptionally
good. Recall the migration diagramthere are 100 to 200
times as many alcohol molecules near a fresh surface than the
surfactant molecules in an alcohol substitute. What this means
is that alcohol, because of its high use level, will give very fast
surface tension reduction. Successful alcohol substitutes mimic
the surface and viscosity behavior of alcohol closely.
15
Lubrication
Piling and Linting
Running a thin film of fountain solution over the plate surface,
blankets, and rollers prevents excess friction, heat, and wear.
Several additives such as alcohol substitutes, polymers, and
glycols are used in fountain solutions to enhance the
lubricating effect of water.
Piling
Without adequate lubrication between the plates and the form
rollers there would be tremendous friction and rapid plate
wear. We must also prevent the blankets from becoming too
tacky (sticky) thus pulling off paper fibers, which is known as
picking.
Recall the water-in-oil emulsion formed when the dry ink
picks up fountain solution. As the plate passes under the forms
some water is squeezed back out of the ink providing a
cushioning effect between the roller and the plate. If the forms
are not turning at exactly plate speed (which they usually won't
be), the ink/water cushion provides slip preventing friction
wear.
16
The rollers are covered by a film of both ink and water. This
provides the vital slip needed to prevent wear.
One definition of piling is  a build up of ink (or ink
components) and paper linters, generally on the blanket. This
is a major concern for web printers because of the waste
generated each time that the blankets must be cleaned. As
these materials build and the blanket thickness increases in
local areas, print quality will deteriorate.You will generally see
this build up in the non-image area of the blanket, and it tends
to be worst at the trailing edge of solids.
When the piling becomes thicker, the
trailing edge of the image will begin
to lift off the blanket and stop
printing. Piling is a very good indicator
of fountain solution, ink, and paper
compatibility. On good stock, the
blankets may only require cleaning
every 200,000 impressions. The image does not usually pile,
because fresh ink is constantly being transferred from plate to
blanket to paper. This constant transfer tends to prevent any
significant accumulation of foreign material on the blanket.
There are several contributing factors that influence the rate of
piling:
1. The amount of water being carried on the plate/blanket.
Running too dry generally increases the rate of piling.
2. The "speed" of the inks. Inks formulated with faster oils
may tend to dry out and pile rapidly.
3. The lubricating ability of the fountain solution.
4. The type of plate used (smooth grain plates usually pile
less).
5. Paper surfaceloose fiber may be pulled off and added
to the accumulated ink resin.
17
Piling and Linting
Emulsification
Linting
Emulsification of Fountain Solution into Ink
Linting is caused by stock with loose surface fibers or by
excessive tack of the ink or blanket. Paper fibers are pulled off
the sheet and then build up on the blanket. "Release Agents"
are often included in fountain solutions or alcohol substitutes
to decrease the "tackiness" of the blanket surface and reduce
the tendency to pull the fibers off the sheet. Typical non-piling
additives are made from glycols that will tend to keep the
blanket moist.
Ink must take on water to work correctly. This statement may
seem contrary to intuitive thinking; however, the water film
that is always covering the ink on the rollers must have some
place to go. Part of the water passes into the ink, improving
the flow and transfer characteristics. Of concern is how much
water and exactly how is the water distributed while in the ink.
The term Emulsion means a two phase system of liquids that
do not chemically combine or dissolve into each other. If you
looked at the ink on the forms under a microscope, you would
see droplets of water suspended in the ink.
Some printers will run paper that has loose surface fibers
through a dummy unit before laying down the first color. Some
of the loose surface fibers will be pulled off and stick on the
dummy blanket reducing linting in subsequent units.
Ink I. will print smoothly with a fine grain structure. Ink II. will
be snow-flaky or print with an "orange peel" look. Whenever
ink is squeezed at the printing nips, these large (loosely held)
water droplets will come back out of the ink and show up as
voids in the image.
If the ink is too dry (low water pickup) or the water too
tightly held, there will be problems with scumming or filling in
of reverses. The water to keep the reverses or trailing edges
of solids desensitized must come from the ink. Picture the
water being compressed into the ink during the form roller
squeeze and then popping back out after the image has passed
the form roller.
18
The fountain solution should not encourage the ink to emulsify
into the water phase.
19
sUMMARY
We hope that you now have a better understanding of some of
the chemical and physical aspects of the dampening process.
Many man-years of research and development have gone into
improving inks and fountain solutions. There are still no "ideal"
combinations of products or equipment, because water and ink
are both antagonists and partners in the lithographic process.
Think in terms of a "whole system" or "window of effective
operation" as printers and suppliers our joint assignment is to
increase the latitude or "window" of the process.
Combinations with wide latitude are easier to use and will yield
consistently superior results.
A better understanding of dampening chemistry will help you
diagnose and then correct any problems that occur from time
to time. Carefully controlling and monitoring your dampening
chemistry will prevent problems caused by "over or under use"
and will provide important data in the event of trouble.
Finally, encourage your suppliers to work together to help you
obtain that "optimum" system to match your needs.
20
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