Screen Printing - August/September 2017 - 30
this shirt features a four-color
print using three HD inks and a
color blend. the print was built
up gradually, with about 15
layers of each color. note how
sharp the edges of the print
remained despite the ink
thickness and multiple passes.
apply a double coat of the same emulsion to the inside of
the screen using a round-edge scoop coater to ensure a good
bond between the film and emulsion. You can dry the screen
between the two steps if you like, but it isn't necessary. But
be sure the screen is dry before exposing it, remembering
that you are creating a very thick stencil. With a heated drying
cabinet, depending on the settings and the humidity in the
screenroom, it should take about an hour. It will take several
hours without a heated cabinet, and you might consider letting the screen dry overnight.
Avoid using weak light sources such as fluorescent tube
units to expose thick-film screens. You might be able to get
such an exposure unit to work, but the exposure times will
be very long and the edge definition will probably not be as
good. I prefer using metal-halide lamps in the range of 3000 to
6000 watts, allowing 90 seconds for each 100 microns of film
thickness; this means a 400-micron film would take 6 minutes
to expose. I've had mixed experiences using LED exposure
units for this application, with results that have varied widely
depending on the manufacturer. My advice is to talk to your
stencil system supplier and test.
After exposing the screen, I like to let it soak in a dip tank
filled with water for about 5 minutes. This isn't necessary,
but will make it easier to wash out the screen. Then I use a
1000-psi pressure washer with a V-shaped nozzle from about a
foot away. Wash only the side that has the film attached to it.
After washing is complete, dry the screen thoroughly. I prefer
vacuuming my screens dry.
Ink selectIon and job setup
There are several different types of HD formulations - inks, gels,
and bases. HD printing is most commonly done with plastisol,
but over the past few years, several water-based ink lines have
been introduced that are also designed to achieve raised effects.
HD inks can be used straight out of the can and produce
extremely sharp, well-defined edges. The most common colors are black and white; they can be mixed with other colors
to create secondary colors. After curing, inks should retain
their sharpness and have a matte finish.
HD gels are similar to inks and are available as a clear;
they are also available with metallic or other particles in
them. (See the silver gel shown in Figure 1.) When printed,
they may or may not have dimension and sharp edges,
depending on the gel you use. When fully cured, they should
be glossy with rounded edges (see Figure 2).
HD bases usually require toner or ink to be added for
color. So many HD bases are available that it's difficult to give
a blanket description of what they do. Some rise like puff
inks while others have a very hard finish that won't flatten
when foil is applied to them through a transfer machine, a
popular use for HD bases (see Figure 3).
Since these are all three-dimensional inks, you should
set your off-contact distance higher than you would for a
standard print. If you are printing multiple HD colors in a job,
set each screen a little higher than the previous one to avoid
the ink not clearing the screen. For jobs that involve standard
plastisols with HD effects, print the HD colors last.