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| February 2006 issue of PRINTWEAR |
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From Art to Screens
Choosing methods and equipment
by Douglas Grigar, Master
Screen Printer
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(A version of this article originally appeared
in the February 2006 issue of PRINTWEAR)
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The screen printing
industry depends upon photo-reactive emulsions to make the majority of
printing stencils on screen mesh. The ease of use, price, time, and plethora
of photo-reactive products reflects the popularity of such stencil-making
procedures in the industry. Imaging photo-reactive stencils is the application
of one of three methods: projection (see Figure 1), light-passable
film or paper products called positives (see Figure 2), and direct-to-screen
methods (see Figure 3).
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[Figure 1: The projection method involves
projecting light to expose the photo-stencil.]
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[Figure 2: The light and dark areas
of pass-through positives (the most popular method in use) are pressed
directly to the emulsion for exposure.]
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[Figure 3: Direct methods involve UV-light-blocking
products applied to photo emulsion before exposure.]
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Projection exposure is now
quite rare and rather antiquated as an art-to-screen method. Super-fast-exposing
emulsions are needed and severe underexposure is almost required. The
only obvious benefit is the ability to use very small art projected onto
large screens to facilitate massive proportional enlargement. Use of projection
systems are almost unheard of in screen printing for garments because
of the inherent problems of underexposure.
Light-passable positives are the most popular
method to transfer the desired art to a screen stencil. Dark-printed and
open or clear areas on a translucent carrier sheet are pressed to the
emulsion, and UV light is blocked from hardening the emulsion underneath
the dark areas (via chemical linking), forming a stencil open area when
wash-developed.
Direct-to-screen – in relatively limited
use, owing mainly to the cost of the technology – will be discussed
in more detail below.
The Light and the Dark
D-max and D-min are terms
to used to identify ends of a scale of the relative light-blocking or
light-passing qualities of a product; simply, the terms are shortened
representations of maximum and minimum density. Screen printing uses these
terms to designate how dark and opaque the black areas are, and how clear
or translucent the open or clear parts of the positive are. D-max is density-maximum
and is used for the darkness of the opaque areas. D-min is density-minimum,
and is the relative measurement of open or clear areas. The D-max and
D-min of the positive taken alone is not quite as important as the contrast
from one to the next. The goal would be to have dense, dark blacks and
as clear as possible open areas (see Figure 4).
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[Figure 4: High contrast, where the
black areas are opaque and as dense as possible and the clear areas
are as clear as possible, helps in production of a good photo-stencil.]
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The Positive and Production Methods
As stated, the use
of positives is the most popular method to produce stencils. There are
four common methods or types of machinery for making a positive: a) laser
toner on vellum, b) inkjet on a clear film, c) thermal
imaging on a clear reactive base, and d) image-setter-exposed
silver film on a wet chemical processed base. Each of the methods
is progressively more expensive, but each is an investment in valuable
features. Low-end-laser-toner printers start in the sub-$1,000 range and
go up drastically for output over 8.5 inches wide. Laser-printer output
just larger than 12 inches will range in price from $2,000 to $5,000.
Inkjet printers with the necessary RIP (Raster Image Processor) start
in the price range of just over $1,000 for output over 12.5 inches. Inkjet
and RIP combination prices increase as the width of the output increases.
Thermal-imaging printers start in price at about $5,000 for output in
the 12-inch-wide range and increase in price as the width increases. Imagesetters
start at about $2,500 for 17-inch output but will need a film processor
and RIP station, adding about $4,500 to the price.
Toner on Translucent Base
Toner-based positives are most often printed
on a semi translucent product called vellum, or on a frosted heat-resistant
film. Several years ago one of the most popular methods of making positives
was using a toner laser printer (xerographic magnetic thermal transfer).
Slowly this method is loosing ground to superior and less expensive options.
While relatively inexpensive, the toner positive
suffers some major drawbacks that make is less than optimal for making
photo-stencils. Laser-positive film and vellum are designed to hold as
much toner in the black areas as possible, but a translucent base reduces
the available contrast. Toner-based positives are plagued by scattered
particles that are fused to the base (see Figure 5), as well as lower
D-max and higher D-min areas (see Figure 6), causing “burn-through”
(see Figure 7) and forcing the user to underexpose to get printable results.
Burn-through on toner positives is particularly troublesome in the centers
of larger black areas.
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[Figure 5: Toner scatter is where particles
that bounce out of place are thermally fixed in undesired positions.]
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[Figure 6: This is a magnified view
of the toner black area - showing lack of complete coverage; note that
the vellum forms a milky background that is less than clear.]
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[Figure 7: The final result of lowered
contrast of high D-min and low D-max, where UV light “burn through”
caused slightly exposed emulsion to clog what should be open areas.]
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Inkjet on “Special” Treated
Film
The popularity of the large-format inkjet printer for
making positives is directly related to the usability and quality of the
positives and the relatively inexpensive nature of the equipment and supplies.
The highest-quality approach to inkjet positives is three-sided, in that
it is a combination of “special” film with an absorbent coating,
specific ink products, and the use of RIP software. A downside to the
inkjet printer output is the ink splatter near the ends of the dots (see
Figure 8) and small lines. Inkjet ink spreads outward into the absorbent
layer (called wicking), causing slight dot gain and rounding
of the edges (see Figure 9). When using “non-water-resistant”
films there is the possibility of smearing or removing the ink with wet
fingers or drops of water. Speed of production is an issue with inkjet
printers as well, as they are relatively slow to print large positives.
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[Figure 8: Inkjet dots showing the
splatter of the ink on the film.]
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[Figure 9: Wicking of the ink into
the absorbent layer on the film causes rounding of the edges as the
ink spreads outwards.]
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Thermal Image on Reactive Film
Thermal imaging for film production may well be the future
for many printers with its high speed of output and resolution quality
that approaches wet processed film products without the chemicals. Thermal
positives also have the benefit of low D-min and very high D-max, providing
excellent contrast and sharp edges for quality photo-stencil results (see
Figure 10). Thermal-imaged film continues to be reactive though, and exposure
to high heat will darken the film. Age also darkens thermal film giving
it a yellow-brownish tint. Manufacturer testing has shown this tinting
to be a reaction in the base film and not the reactive layer. Edges and
dots on thermal film can have a telltale “ghost haze” where
heat partially activated the film’s darkening properties (see Figure
11).
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[Figure 10: Note the sharp point and
edge, along with the high contrast of this magnified image of thermal
output.]
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[Figure 11: Note the “ghost haze”
on the small dots of this thermal image.]
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Imagesetters
Wet-processed film from a RIP-driven high-end
imagesetter is considered the pinnacle of quality positives, where sharp
clear lines and smooth hard-edged dots are the standard. High resolution,
often over 2,000-dpi, is also standard for imagesetters. The highest D-max
and lowest D-min numbers encountered with positives along with high processing
speed is the justification for the higher maintenance, supply, and initial
purchase price. The consistent maintenance issues and use of noxious chemicals,
as well as the high price of consumables, cause many screen printers to
pass on wet-processed film. There is concern about the cost and availability
of silver-based film in some regions along with the possible eventual
obsolescence of this technology.
Direct-to-Screen
Other types of printing have adopted what
is called DTP, or direct-to-plate technology. The equivalent
in screen printing is the DTS, or direct-to-screen system. With
it, a UV-blocking product is applied to the face of the screen for exposure,
or a device exposes the screen directly. This may well be the future for
high-end quality screen production. So far, a wax-applying inkjet system
is the most expensive of the systems reviewed, but also provides the best
D-min to D-max ratio, as there is no film base at all. However, DTS will
image stencils far beyond the capacity of film positives by having the
layer of ink directly on the stencil following the divots and knuckles
caused by the mesh.
In the article, “Exposure Systems…Exposed”
(Printwear, April 2005), we found that the intimate contact of
the film to emulsion caused by a powerful vacuum had the largest proportional
increase in detail; a direct-to-screen system creates the most intimate
contact possible to the emulsion. DTS systems do not use standard vacuum
table stands to expose, but need only the exposing light. The wax-ink-based
DTS inkjet system will also provide a benefit lost with standard inkjets
by not wicking into the emulsion or splattering as seen with standard
inkjets (as shown in Figure 8).
DTS systems are the most expensive compared
to positive systems, and often take a daily production of 30 to 50 screens
to justify the initial purchase. The return on investment for a DTS however,
is most often less than one year.
DTS systems will continue to grow in popularity,
as will the lower-priced high-quality inkjet film positives, filling two
great needs in the industry. The future will only bring more improvements
in technology, presenting a need to make educated and informed choices
for each shop.
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