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| September 2006 issue of PRINTWEAR |
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How
to Color Separate for Screen Printing
by Douglas Grigar, Master
Screen Printer
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(A version of this article originally appeared
in the September 2006 issue of PRINTWEAR.)
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I am often surprised when talking with
new printers about how many are mystified at the front end of the printing
process: producing art and color separations. Often the reaction from
new printers to separations is akin to an apprenticeship in the ancient
art of alchemy. While separations are at their most basic quite simple,
more advanced graphics and application are limitless in complexity.
A complete overview of this process and just one of the common graphics
programs would easily taken ten times the space available in this entire
issue of Printwear. We will focus, therefore, on the basics
and some points that are most commonly missed by or unknown to screen
printers.
One of the most critical operations involved
in screen printing is the correct planning and execution of graphics
and separations for multi-color prints on garments. But don’t
let this challenge prevent you from jumping right into the graphics
programs used most commonly for this procedure. (For a better understanding
of the two major types of programs used, see Printwear, February
2005, for two articles about pixel and vector programs.)
Simple at the Base Level
Screen printing is a simple ink-or-no-ink
proposition. Each color has to have a separate screen, which is part
of the reason for the term “separation.” As each separate
screen needs a positive form of the art to expose, each color will have
an art “film positive.” As printing uses ink-or-no-ink to
produce an image, anything that needs a less than full strength version
of the color in the screen will need to have the stencil on the screen
to break the color area into small dots of various sizes or in various
patterns to create the illusion that there is a graduation in tone of
the color. Those are the simplest of the basics.
Understanding Fundamental Issues
Four challenges are part of making separations:
understanding the basics, generation of the graphics themselves, the
requirements of the garments and screens, and the production of the
separations in a form to be exposed on screens for a stencil.
To make successful separations in a screen
printing shop the first challenge must be met first. Screen printing
has some unique properties that make separations more difficult and
more graphic-planning intensive. Generally, screen printing on garments
lends itself best to the application of “spot colors” (independent,
separately chosen colors) on light and dark garments (see Figure 1).
While successful continuous-tone (or process color) printing on garments
is not uncommon, the details are beyond the scope of this article.
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[Figure 1: When separating for a spot-color
design, solid areas of ink/no ink are the method of choice. If the design
has graduated color, small dots of various sizes (called halftones)
are required.]
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The first of the obstacles for screen printing
graphics is choosing among the art programs available in our market.
Most graphics programs are geared for use in printing methods that differ
greatly from screen printing (see Figure 2). While screen printing transfers
ink from a well in the screen frame into and past holes in the screen
mesh/stencil and onto the substrate, other methods of printing such
as offset, flexography, gravure, or letterpress take thin layers of
ink on raised or recessed areas of a printing plate, or from a plate
to a flexible roller, and onto the substrate.
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[Figure 2: Screen printing methodology
(left) differs drastically from those of other printing disciplines
(right), of which one of the greatest benefits is the ability to deposit
ink as much as 100 times thicker than other printing methods.]
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One advantage screen printing has over the
other processes is the very high volume of ink that can be deposited,
many times thicker (often hundreds of times thicker) than any other process,
giving screen printing the ability to print extremely thick and opaque
layers, such as is required for printing white ink on dark shirts.
Because of issues caused by opaque
inks, mesh, stencil thickness, raised inks on the substrate, and screen-stencil
contact with wet ink, graphics production has to take into account multiple
physical aspects (not present in other printing methods) that affect the
final print. To sum up all of the production issues involved in screen
printing on a particular garment, the garment properties themselves must
be taken into account when planning and producing the graphics for a given
job.
Generating the Graphics
Producing the graphics for screen printing
with the available programs on the market is typically an exercise in
how to modify a file in a program made for other printing methods
into a form that will work well screen printing on garments. Multiple
spot colors with unusual layers and techniques is the norm rather than
the exception with screen printing art. While other printing methods often
stop adding colors after they reach four (as in four-color process) it
is not uncommon to see six-, eight-, and ten-color print garments. Graphics
programs can produce and work with multiple spot colors, but this requires
that the artist plan in advance the colors needed and consider each color’s
physical properties and requirements for each garment. The artist must
make sure all color areas in the electronic drawing are using and have
assigned to them specific spot colors. Forgetting any part of a drawing
and not assigning it one of the specific spot colors will produce a separated
positive with required areas missing (see Figure 3).
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[Figure 3: Forgetting any part of a
drawing and not assigning it one of the specific spot colors will produce
a separated positive with required areas missing.]
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Understanding some graphic-production
terms such as butt, trap, and choke, and how to assign those properties
to each area of a drawing as necessary, is also imperative to generating
garment graphics.
The butt separation describes where
two colors meet when properly aligned, come to the edge of one another,
stop, and do not overlap (see Figure 4). The butt separation is the most
common in screen printing, first because that is the default setting when
making a drawing in a vector program, and also because a butt is most
effective in preventing edge smearing while printing too closely registered
colors, especially when one of the colors is flashed.
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[Figure 4: Butt registration –
the most common with spot-color designs – means (theoretically)
that one color begins at exactly the same point the previous ended,
with no overlap.]
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The trap separation is when two
colors meet when aligned, and the edges overlap one another (see Figure
5). The trap is effective in helping production align colors faster because
small amounts of misalignment are hidden by the overlap. Trapping a separation
most often requires that the printer flash the first-down color to prevent
smearing. One possible visually disturbing aspect of a trap when printing
plastisol is that the underlying color can make the edges of overlap have
a distinctive “shine,” as the top color is sitting on a smoother
flat surface of flashed ink.
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[Figure 5: The trapping technique provides
for a slight overlap between successive colors, lending considerable
forgiveness to a registration tolerance.]
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The choke is used when an underlay
color is needed or desired to produce a brighter print such as a white
base color under a light color that would fade if printed directly on
a dark garment. The choke is when one separation is directly under a second
separation when aligned on top of another, and the edges of the underlay
or base print are evenly smaller on all edges so the color on top will
overlap slightly and cover the edges of the base color (see Figure 6).
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[Figure 6: The choke is used when an
underlay color is required – typically to “neutralize”
a shirt color that is darker than the final ink color – but must
be completely covered by the overprinted color.]
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| Garment and Screen Requirements
Garments will demand a particular thickness
of each color to print effectively; judging the need for each garment
is a balancing act of thickness of ink coverage and detail needed for
the particular design.
Generally speaking, the higher the need for
design detail, the higher the mesh count must be (more threads per inch,
therefore finer threads); the corollary is that some detail may have to
be sacrificed for a thicker deposit of ink. Darker garments, for example,
often demand a thicker layer of base colors to provide an opaque layer
of coverage.
Garments with open weave or large threads
or texture will also require that the graphics be made with a lack of
small detail, as heavy texture on a garment will often prevent small details
from printing accurately.
Screen mesh has a limited range of detail
available that it can produce faithfully, and changes in mesh count are
necessary when ink layer thickness or changes in detail are required.
Mentally working in reverse is often the best trick used by graphic designers
for dealing with the demands of the garment, screen mesh, and detail.
By thinking of the demands of the garment first, then working backwards
in the process to screen mesh, stencil properties, film-positive parameters,
and finally, adding all of the factors into the generation of the graphics
in the chosen program is often the most successful method to initiate
art production.
Producing the Separations in Positive
Form
All of the available graphic programs have
an imbedded separation program included for users to print directly from
the program itself. The separation programs can automatically produce
a separate print for each color with the areas to print on black or clear
film (or vellum), along with any target marks needed for accurate registration.
The printing parameters for separations are
customizable by the user as long as a printer capable of understanding
and printing the commands dictated by the separation program is available.
For screen-stencil exposing with photo-reactive emulsions, graphics program
settings will have to be changed to “right reading” positive,
emulsion-up, along with the frequency of the line count or LPI (lines
per inch) and line/dot angle, to match the mesh that is necessary for
the given color. (A detailed discussion of what the “PI” terms
mean may be found in the August 2004 issue of Printwear, page
100.)
The best method for learning how to effectively
use the separation functionality of your graphics program is to use it
frequently and constantly push yourself into more challenging and difficult
work, realizing that even the best artists in the industry learned with
one hand on the mouse and the other holding the user’s manual.
Further explanations and details about choosing
the printing parameters and matching them to the mesh will be covered
in a future article.
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