Impressions - June/July 2019 - 37

This is the original
football player image in
Adobe Photoshop before
the separation stage.

SCREEN-COATING TIPS
Screen coating should be done by using the scoop coater's dull side,
with the first coat of emulsion being applied to the screen's back
side. The last emusion application is coated on the inside (squeegee
side) of the screen and rack dried with the screen's back side facing
downward.
This coating method will provide an adequate emulsion over mesh
(EOM) ratio, which is critical for ensuring good stencil bridging over
the mesh's cross sections. EOM also affects the amount of ink deposited during the printing process.

however, the only time you may run into issues if you change
the Red PMS 032 to a bright red and the image is a person with
flesh tones.This could alter the image quite a bit.
"In the software, you have buttons that can be used to automatically merge channels, such as the purple, turquoise and
gray," he adds. "I don't immediately merge channels. Instead, I
look at each channel one by one to determine first which ones
don't have any information so they can automatically be deleted.
Once I see how many I have left, then I determine how many
additional channels I need to delete, and I'll usually select the
gray and merge with my highlight white."
Clement says red, blue and yellow should be kept in designs
since they are primary colors and can't be created by mixing
other colors.Then, it comes down to secondary colors like purple,
green and turquoise.
"I will select the info on the purple channel, for instance, and
add it to the red and blue," he says. "You can do it manually in
Photoshop or use the drop-down menu in the software. The
green will go on the yellow and blue, if needed; I usually do this
in Photoshop.The turquoise usually goes on the blue. However,
with all three of these, I look at them individually and see where
the information is actually falling in the design. For example, I
may find the turquoise color actually is more green than blue
and I may be able to add it to the green. If you are able to keep
the green, or add it to the blue and yellow, you will now need to
merge the green as well."
Screen Selection
The most common frames used for simulated-process printing are
retensionable aluminum and static aluminum stretch-and-glue
frames.Yellow mesh in the 230-305 mesh-count range should
be used, stretched to 25 N/cm or higher.This particular mesh
color is ideal for halftone printing because it reduces the amount
of light scatter that causes undercutting during screen exposure.
A common formula used to select the correct mesh count for
printing halftones is to take the dot line count and multiply by
four, then round up to the next mesh count available. For example,
a 55-line dot count multiplied by four equals 220; rounding up

yields a 230 mesh or the next available mesh count.
Halftones and RIP Software
A common dot size to use for textile screen printing is a 55-line
elliptical dot with the angles set to 61 degrees for simulated-process
work. Most simulated-process jobs are designed to be printed
with a white underbase and flashed.Then, all subsequent colors
are printed wet-on-wet on top.
After the image has been color separated, the next step is to
output the separations onto film or send them to a CTS unit
prior to screen exposure.With either method, you must convert
the image into a rasterized halftone image with raster image
processor (RIP) software.
In basic terms, the RIP converts the information from art files
into halftones and communicates this data to the inkjet printer
or CTS for output. There are several RIP software packages
available, including AccuRIP,Wasatch, FastRIP and more.
Film Output and Exposure
The film separations can be output with an inkjet printer onto
polyester film - sheets or rolls - and pre-aligned on the screens
with a commercial pin-registration system.
The next step is to expose the screen. It's critical to first use
an exposure calculator to properly dial in the correct time when
exposing halftones. Several types of exposure calculators are
available, including grayscale 21-step, filter-type and step-wedge
exposure calculators. Older, traditional exposure units use a metal
halide bulb that can expose screens in the two- to three-minute
range, whereas newer LED exposure units on the market can
expose as quickly as eight to 50 seconds.
Using an exposure calculator will save you from the painful
exercise of exposing multiple screens to get a good exposure.
Setting Up the Press
"The software always separates in the same order: 1. Base White,
2. Black, 3. Red PMS 032, 4. Blue PMS 300, 5. Pantone Purple,
JUNE/JULY 2019

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Impressions - June/July 2019

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