Impressions - January 2015 - 65

Screen Printing Technique

talking SHOP

Halftone Printing on Textiles
Use these tips and tricks to make the technical task of screen making
for halftone printing easier.
By James Ortolani Contributing Writer

creen printing halftones on textiles is not as daunting of a process
as some people make it out to be.
However, there are steps that must be followed to ensure success. Most garment
decorators are good at making screens
for spot-color jobs because the exposure
latitude is wide and screen making seems
pretty easy - until you try to expose that
ﬁrst halftone job. The following tips will
make screen making and printing halftones a little easier.
In the early years, process cameras were
used to convert a photo into a series of dots
to reproduce the shaded tones in the image. Today, this process is much easier by
using special raster image processor (RIP)
software, which converts images from vector form into a rasterized dot pattern.

SCREEN MAKING
CONSIDERATIONS
Screen Making with Film Positives: To
create ﬁlm positives for halftone printing, the rasterized image is sent from the
computer to a laser or inkjet printer and
printed onto clear transparency ﬁlms. A
common formula used to determine the
correct screen mesh count is to multiply
the dot line count by 4. For example: A
55-line dot multiplied by 4 results in a
220 mesh. In this case, round up to a 230mesh since it's a common mesh count in
this range.
CTS Inkjet Printers: With computerto-screen (CTS) equipment, you can bypass the ﬁlm-making stage altogether and
print the rasterized image directly from
the computer to the contact side of the
emulsion-coated screen. Not only does
this process eliminate creating expensive
ﬁlms, but it also eliminates the need to
store them. The main ink types used with
today's CTS systems include wax, waterbased inkjet toner and specially formulated UV-blocking formulations.
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Wax: The heated wax inkjet technology is a popular method for printing a
high-density image directly onto an emulsion-coated screen. Some CTS systems
that image with wax can reportedly print
1,200 dpi, allowing halftone frequencies
of up to 110 lines per inch (lpi).
Water-Based Inkjet: Some CTS systems use Epson piezo inkjet heads with
proprietary inks that have special UV
blockers added to increase performance

during the screen exposure stage. Inkjet
system manufacturers report that their
CTS systems can print 720 dpi, run on
standard PC software and use a standard
USB-2 port connection.

SCREEN EXPOSING AND COATING
Every screen-printing shop needs an
exposure calculator when attempting halftone printing. Basically a test ﬁlm positive composed of ﬁne detail, it helps to

High-tension
screens are best
for printing halftones
because they enable
better registration,
controlled off contact and excellent
"snap-off" behind
the squeegee.
Shown is a 23" x 31"
230 yellow mesh
panel stretched to
25 N/cm on a Newman Roller Frame.

January 2015 | Impressions

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Impressions - January 2015

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