Screen Printing Inks
There are two parts to screen printing ink. The first is the pigment. This is what gives the ink its color. A
blue colored ink would have a different pigment than a yellow colored ink. The other part of the ink is the vehicle.
This is what carries the ink to where it needs to be. Two different colored
inks can use the same vehicle. Early inks were very similar to regular paint.
Today there are a variety of different inks for the many different applications
of screen printing. For example, there would be a special ink for making a
billboard, because it is going to exposed to harsh elements (sun, rain, snow,
and cold). The inks used on billboards would be different than those used to
make posters. There are even inks that their main purpose is to prevent
“ something from being ate away. ” This is used in electronics to make circuit
boards (see fig. 12). A circuit board is made up of copper paths. Before it has
paths, it is one sheet of copper on a base support (fiberglass). In order to
make the paths in the copper, a protective ink (etching resist) is screen printed
on. Then the circuit board is exposed to acids. The acid removes bare copper,
but leaves areas coated with protective ink. These areas become the paths to conduct electricity. The chemical agent
does not dissolve the fiber glass. Figure 12 is a posterization positive for making a circuit board. Can you
understand how the image in black will later be the copper pathways for electricity? A screen will be made from
this and then the etching resist will pass through to the circuit board in the shape of the image. Then, once we
expose the board to acids, the etching resist will protect those areas.
The amazing thing about inks is that almost anything that will
mix with the ink can be added. For example, sometimes glass is ground
up to look like crystals and added to ink. This is used to print some
billboards. The outcome is billboards that reflect car lights at night. This
way even an unlighted billboard, advertises to people that are driving
after dark. Other ink additives change the properties of the ink. For
example, a drier material can be added to make the ink dry faster. You
will learn in a future reading that drying time of ink is an important
factor in screen printing. Another important property of ink is its
viscosity. Viscosity simply means resistance to flow. The higher the
viscosity, the higher the resistance to flow. For example, an egg yolk
(thicker) would have a higher viscosity than water (thinner). You want
the ink to have the optimal viscosity. This means not too thick or not too
thin. Imagine that on the second day you had been trying to print with an
ink the thickness (viscosity) of water. The water would have been all over the place? It would have also saturated
your paper. So we want the ink to be thick, so we can control it. However, if it is too thick, it may not fit through the
screen openings or mesh. The finer the mesh, the thinner the ink has to be (see fig. 13). When you are printing
with the ink, notice the viscosity. If the viscosity is too high, we can add a thinner. In normal talk, the ink should
“flow like honey. ”
The ink that we will use in class is called plastisol ink. It is the most popular ink in the industry because
you can practically print on anything with it. Once the plastisol ink is printed on a shirt, it must be cured or else the
ink would “wash out ” in the wash. Proper curing of the ink is based upon time and temperature. The temperature
must approximately 335 degrees in order to properly cure. You can test to see if it is properly cured by stretching
the print. If the print cracks and does not retract, the ink is probably undercured. The temperature may need
adjusted or the belt speed on the dryer should be slowed down.