goal of any inking system is to place
a uniform layer of ink across every dimension
of the printing plate. The Lithographic
process is unique in that it requires
the ink form rollers to pass in contact
with the nonimage areas of the plate without
transferring ink to them.
lithographic inking systems are made up
of three main sections:
Fountain and ball roller
ink fountain stores a quantity of ink
in a reservoir and feeds small quantities
of ink to the rest of the inking system
from the fountain roller. The ink distribution
rollers receive ink and work it into a
semiliquid state that is uniformly delivered
to the ink form rollers. A thin layer
of ink is then transferred to the image
portions of the lithographic plate by
the ink form rollers.
ink fountain holds a pool of ink and controls
the amount of ink that enters the inking
system. The most common type of fountain
consists of a metal blade that is held
in place near the fountain roller. The
gap between the blade and the fountain
roller can be controlled by adjusting
screw keys to vary the amount of ink on
the fountain roller. The printer adjusts
the keys in or out as the fountain roller
turns to obtain the desired quantity of
ink. In simple presses, the printer must
turn these screws by hand. The modern
state of the art press today, the adjusting
screws are moved by "servo" motors which
are controlled by the printer at a press
console. Thus the printer can make ink
adjustments electronically. If the printer
needs to increase or decrease ink in an
area of the plate (print), he need only
adjust the needed keys to allow more or
less ink flow through the blade.
ink distribution rollers spread the ink
out to a uniform layer before it is placed
on the plate. There are generally two
types of distribution rollers:
rotating distribution rollers rotate in
one direction. The oscillating rollers
rotate and move from side to side. Ink
is transferred to the ink distribution
rollers by a ductor roller. The ductor
is a movable roller that moves back and
forth between the ink fountain roller
and an ink distribution roller. As the
ductor contacts the fountain roller, both
turn and the ductor is inked. The ductor
then swings forward to contact a distribution
roller and transfers ink to it. The rate
of rotation of the ink fountain roller
and the gap between the fountain blade
and roller control the amount of ink added
to the distribution system. Any roller
on a press that contacts the plate is
known as a form roller. An ink roller
that touches the plate is an ink form,
a water roller that touches the plate
is a water form.
simple indication of the quality of a
printing press is the number of distribution
and form rollers. The greater the number
of distribution rollers, the more accurate
the control of ink uniformity. It is difficult
to ink large solid areas on a plate with
only one form roller. With three (generally
the maximum), it is relatively easy to
maintain consistent ink coverage of almost
any image area on the plate. Business
forms presses, which print very little
coverage, usually only have one or two
ink form rollers. Because of this, they
cannot print large solid or screen images.
Smaller less sophisticated presses also
have the same problem, however, many of
the newer presses today are being equipped
with larger better inking systems to meet
the growing print demands of the consumer.
whole battery of tests are run on every
ink batch produced.
is checked with an instrument called a
grindmeter which is basically a wedge
ground out of a stainless steel block.
This wedge is 1 or 2 thousands of one
inch deep at the deep end and graduates
up to 0. An ink sample is placed in the
trough and drawn down with a steel doctor
blade. The presence of large particles
will show up as scratches or specks in
the trough after the doctor blade has
been drawn down. Fineness of grind is
critical for several reasons. First of
all in order to gain the maximum efficiency
from a pigment, the most expensive ingredient,
it is essential that it be thoroughly
dispersed. Secondly, a coarse grind can
lead to premature plate wear or piling
on the plate, thus causing printing problems.
is more to formulating an ink than just
loading the maximum amount of pigment
in to it. The amount and type of pigment
will have a drastic effect on the body,
viscosity, flow characteristics, transfer
characteristics, and water pick-up properties
of the ink, in addition to the print density
and trapping characteristics on the print.
In process work shade is very important.
If the inks are not the proper shade,
the results from the press cannot be expected
to match the original artist copy or the
proofers preliminary print. Shade is checked
by a draw down versus the standard.
is a properly associated with the ability
of an ink to flow and form filaments.
Ink can be long or shore. Long ink flow
well and form long filaments. They are
undesirable especially on high speed presses
because thy have a tendency to fly or
mist. Short inks have the consistency
of butter with poor flow properties. They
have a tendency to pile on the rollers,
plate or blanket. Most satisfactory inks
are neither excessively long or short.
tack of an ink is the force required to
split a thin film of ink as it transfers
down or up the ink train from roller to
roller; roller to the plate; plate to
the blanket; and blanket to the stock.
The ink formulator will use an instrument
called an Inkometer to measure the tack
and will use this instrument to help him
adjust the final formulation of the ink.
The measure ink tack and the stability
of ink tack an Inkometer is used.
ink formula must be balanced to obtain
the best combination of fast, low temperature
dry and good ink train stability. To check
the inks tack and tack stability, ink
measured on a inkometer is measured at
90?F at 1200 RPM. Some ink companies will
measure ink at 90?F 900 RPM. Thus a ink
measured at 1200 RPM with an ink tack
of 16 will have an ink tack of 13 when
measured at 900 RPM.
is a measurement of the flow characteristics
of a given ink and it determines the force
required to move an ink down the ink train
or to pump it through pipe lines. The
instrument that we commonly use is the
Laray or "falling rod" viscometer.This
instrument can be used to measure the
viscosity and the shortness ratio of an
ink. The shortness ratio enables the formulator
to measure the gel structure of an ink
and to predict the performance of the
ink in the fountain and on the ink train.
It also is a valuable tool to monitor
an ink from batch-to-batch to assure consistent
offset printing, the ability of the ink
to interact with the fountain solution
is critical; however, excess water pick-up
can lead to emulsification which can show
up on the print as loss of strength, enlarged
dot, dull finish or poor rub.
for Unground or Foreign Material
test is conducted by mixing a sample of
ink in a solvent and then filtering the
residue on a fine mesh screen. The screen
is then rinsed with solvent and dried.
An observation is made of the foreign
material. The operator must make a determination
whether to accept or reject the batch
based on the amount of this material.
the ink formula has been prepared in the
lab and tested for proper physical properties,
it can then be run through a battery of
tests to determine its printability and
performance after printing.
are three properties of ink that control
the ease and quality of image transfer:
quality (Coldset only)
- is used to accurately describe the "body"
of ink. Some inks are heavy (offset and
letterpress) and some are light (flexographic
and gravure inks). Viscosity, or resistance
to flow, can be measured and is a term
universally accepted in the printing industry.
- is actually the "stickiness". Tack must
be controlled during the printing process
in order to transfer images and deliver
the sheet through the press. Tack can
cause paper (especially coated paper)
to stick to the blanket of an offset press.
Ink that is excessively tacky may also
pick the surface of the paper and cause
misfeeding. Remember that to pick means
to lift or tear small pieces of the paper뭩
surface. Tack will increase as one color
is printed over another as it starts drying.
When printing multicolor and process color
(black, cyan, magenta and yellow) work,
decrease the amount of tack on successive
runs. The first printing unit should have
the highest tack. Each successive printing
unit should be printed with ink of less
tack. In this manner the ink is "trapped"
or transferred from the printing unit
(image on the blanket) to the paper or
ink on the paper.
example, if we are printing black, cyan,
magenta and yellow the black ink will
contain the highest tack and yellow the
lowest. As we first print black (highest
tack) the ink begins to dry as soon as
the ink is transferred from the blanket
to substrate (paper). As the ink begins
to dry, the tack increases. At the second
printing unit, the cyan ink (which has
a slightly lower tack) is transferred
to the substrate and possibility onto
the just printed black ink. Since the
tack of the black ink is higher than the
tack of the cyan ink, the cyan is "pulled"
or "trapped" onto the higher tack black.
As the cyan/black dries, it increases
in tack which traps the magenta and so
forth. This ability to "trap" is important.
You will hear many printers and paper
people talk about "backtrap" mottle or
"backtrapping" problems. Some of this
pheonomen is caused by the interaction
of paper and ink.
Quality - The final, and extremely
important, property of ink is its drying
quality. With sheetfed inks, there are
two stages in the drying process.
should instantly set or stick to the paper.
When ink on the press sheet is set, it
can be handled without smearing. If ink
does not set as it is stacked in the delivery
side of a press, the image will transfer
to the bottom of the next sheet. This
transfer of wet ink from sheet o sheet
is called setoff.
needs to harden. When ink has hardened,
the vehicle (solvent) has completely solidified
on the paper surface and will not transfer.
The time it takes for liquid ink to harden
to a solid state in called the drying
are three methods for coldset ink drying:
sheetfed inks contain a drying oil (soy,
linseed, cottonseed, etc.) which is hardened
by a chemical process called oxidation.
To oxidize is to combine oxygen with the
drying oil which changes the vehicle of
the ink from a liquid to a solid.
an ink is printed on an absorbent substrate,
drying results from a physical process
called penetration. When ink dries by
penetration, most of the vehicle is absorbed
into the substrate. The ink vehicle is
not changed to a solid state in this drying
process. Inks that rely heavily on drying
by penetration are not popular because
the ink never hardens. Handling work printed
with penetrating-drying ink usually results
in ink transfer to the hands. Business
forms and newsprint inks rely on this
form of ink drying.
coldset inks dry by evaporation. Resinous
and other film-forming solutions in the
in vehicle pass off a vapor during the
drying process. Drying by evaporation
is much like drying by penetration. The
volatile solutions disappear (by evaporating
instead of penetrating), leaving an ink
film on the surface of the substrate.
sheetfed inks use a combination of oxidation
and penetration to accomplish drying.
Heatset inks are far different from sheetfed
inks. They dry by evaporation of the main
solvents and setting the ink by the chill
rollers. See the web offset section for
a description of ink drying.
assist in ink drying, additives can be
added to the ink. These additives are
"catalysts" which accelerate the oxidation
process. The two main ink dryers used
- a very powerful surface dryer,. Violet
in color until oxidized where it will
turn brown. This dryer tends to discolor
whites. It is readily soluble in organic
acids, so it may be affected by the fountain
solution. Cobalt acetate can be dissolved
and introduced into the fountain as a
- is a "through-put" dryer with less vigorous
catalytic action than cobalt. It is brown
in color, has less effect on whites than
cobalt and is unlikely to leach out by
are ways to calculate the amount of driers
used in a ink. See Table 1 and 2 below for
recommendations are based on percent metal
on vehicle solids
calculate the quantity of various driers
required to be added to an ink formulation
it is necessary to know, a) vehicle solids,
b) percentage of metal available in the
driers, and, c) required percentage of
metal on vehicle solids
quantities of driers is then calculated
from the following formula:
Drier required (b) = Vehicle solids (lb)
X % Metal required / % Metal in drier
||Drier Based on vehicle solids
||0.01 to 0.1% Metal
||0.02 to 0.0% Metal
||0.35 to 0.5% Metal
||0.10 to 0.3% Metal
||0.10 to 0.3% Metal
||0.10 to 0.3% Metal
||0.10 to 0.15% Metal
||0.20 to 0.3% Metal
|* Shepherd Brand Drier
blend of the two drying additives, colbalt
and mangenese for example, is usuallydded
to allow for rapid surface drying (colbalt,
to prevent set-off) and quickly harden
the ink (mangenese, throughput drying).
The printer will use a blend of these
to accomplish this feat. The problem is,
however, that there is a "window of opportunity"
for dryers. To much will plastize the
ink, and too little will slow drying.
Generally, less than 1% (by weight) is
used. There are today available other
newer types of dryer additives that will
enhance the inks drying ability even more.
While most people think coated papers
are more prone to drying problems, uncoated
papers many times experiences greater
drying problems. This is due to the fact
that coatings are made to absorb the oils
in the ink. Uncoated papers made with
a very hard or tight surface will not
absorb the oils easily resulting in slow
ink drying. Care must be taken when printing
these type of sheets. Drying can be easily
accomplished by taking the precautions
ahead of time. The printing, if printing
on a tight uncoated sheet, should contact
his ink supplier for recommendations on
altering the ink. In most cases, a paper
sample should be supplied to the ink company
for this purpose. Uncoated papers vary
greatly in their surface characteristics.
Proper care in ink selection should be
done. Newsprint, for example, is not Strathmore
Elements. Ink will react completely different
with these two types of paper!
mentioned earlier, sheetfed or coldset
inks dry mainly by oxidation and absorption.
Many business forms ink, however, dry
only by absorption. Oxidation is the process
in which oxygen crosslinks with the oils
and varnishes in the ink to form a solid.
Absorption is the process of the solvent
penetrating into the paper allowing the
oxygen to interact with the oils and or
Setting - is the removal of the solvent
from the ink film by absorption into the
stock. This raises the viscosity of the
ink and starts the surface drying. Setting
the surface allows for backside printing,
Hardening (drying) - or total ink
drying is accomplished over a longer period
of time (up to 24 hours).
will separate from the ink and bleed into
the coating or paper leaving the varnish
on the surface to cross link with oxygen
and harded. At first the surface of the
ink will harden. This is known as setting.
oxygen cross-links with the help from
driers, the ink over time will harden
all the way through. This is known as
total ink drying Solvents will separate
from the ink and bleed into the coating
or paper leaving the varnish on the surface
to cross-link with oxygen and harden.
At first the surface of the ink will harden.
This is known as setting
of the main drying catalysts used to accelerate
the ink drying process contain Cobalt
helps accelerate the surface drying
speeds the internal drying.
best recommendation one can give when
printing a new type of paper is for the
printer to consult with their ink supplier
as to the proper amount as well as the
maximum and minimum to use. Ink suppliers
will take a paper sample and adjust the
ink for drying properties or at least
make recommendations to the printer on
how to handle the drying process.