Impressions - January 2015 - 66

Screen Printing Technique

Here, a computer-to-screen (CTS) machine
is imaging a 230 yellow mesh with an exposure
calculator test pattern.

EXPOSURE

TIPS

Veteran screen printer Richard
Greaves offers the following tips
to keep in mind when it comes to
exposure:

determine the optimal exposure time for
the emulsion, mesh and stencil materials
being used. There are two main kinds of
exposure calculators: the step wedge and
ﬁlter type.
It's important to periodically repeat the
exposure calculator test because exposure
times will increase as the light source gets
weaker. The most accurate way to compensate for weak bulbs and power ﬂuctuations
is to invest in an exposure unit with a light
integrator. This device uses an optic photocell to collect and measure how many units
of light are delivered to the screen surface
during exposure. This greatly helps to control exposure times.
Halftone printing can get pretty technical. Before the advent of YouTube,
where screen printers can watch videos
to answer technical questions, industry
veterans and experts like Richard Greaves
and Mark Coudray were printing halftones, and here they share their decades
worth of knowledge.
"When it comes to stencil considerations
for exposing halftones, there are dual-cure
stencils that use both diazo and photopolymer sensitizers," Greaves says. "They can be
formulated with high solids content, which
means better resolution, exposure latitude
and coating properties than conventional

* The best stencil is a completely cured stencil, so that
it easily removes and resists
water or chemicals.
* Overexposure doesn't choke
your halftone dots and ﬁne
lines, undercutting does. The
inescapable laws of physics
explain that even a 10-inchwide single-lamp light
source always will undercut
your positive at some sort of
angle - more at the edges
than the center. Multiple UV
sources are much worse.
* At an angle, UV energy
always will go under your
positive and cure the
image areas you wanted
to dissolve.
66

Impressions | January 2015

pure photopolymer or diazo stencils. But
they do take longer to expose. Dual-cure
direct emulsions are typically 50%-60%
water, and the solids that are left behind
when you dry a stencil shrink and take on
the texture of the mesh that rarely leaves
an even ﬁlm on the bottom of the mesh to
control your image."
Greaves adds that direct ﬁlms are superior because they have a solids content near
90%, meaning they're not liquid. "You stick
them to wet mesh like a postage stamp," he
explains. "This also means stencil preparation is much faster because you remove the
drying step after degreasing and only application water needs to evaporate, compared
to the 50%-60% water in direct emulsion.
Direct ﬁlms also deliver stencil consistency
- even from inexperienced screen makers
- unlike direct liquid emulsions."
Greaves says a strong vacuum on your
exposure unit and the use of dyed mesh
can help reduce the choking effect by absorbing scattered UV energy and pressing
the positive tightly against the stencil.
"The untrained screen maker, who
has been told the stencil has been overexposed, will react by reducing exposure
- which means the stencil will be 'medium rare,' not 'well done,'" he adds. "For a
plastisol ink print, this usually isn't a problem because plastisol never hurts a stencil.
But water-based or solvent inks and stencil
removers will attack the uncured stencil
on the inside where the ink and squeegee
will be pressed down hard and rubbed like
an eraser. The only way to compensate for
undercutting is to make dots larger on the
positive, so that you get the size you want
on the ﬁnal print and have a completely
cured, durable stencil. This is like sighting
on a riﬂe for accuracy."

SCREEN PRINTING HALFTONES

To determine if your squeegee is sharp
enough, lightly drag your ﬁnger across the edge
of the blade. If you can feel the individual ridges
of your ﬁngerprint, it's sharp enough.

For screen printing halftones, I prefer
to use an elliptical dot in the 45-line count
range exposed on 230 yellow mesh tensioned at 20-25 N/cm. Also, I print with
a dual- or triple-durometer, straight-edge
squeegee. I keep off-contact distance minimal to allow the screen mesh to snap off
right behind the squeegee during the print
stroke. Too much off contact will cause
excessive ink on the substrate and the
dots will expand in size. I use Great Dane
Graphics software and typically select a
45-line halftone at 61-degree angles using
an elliptical dot shape. (My ﬁlm positives
impressionsmag.com

http://www.impressionsmag.com

Impressions - January 2015

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