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| April 2005 issue of PRINTWEAR |
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Exposure
Systems…Exposed!
The major features that make up exposure systems
by Douglas Grigar, Master
Screen Printer
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(A version of this article originally appeared
in the April 2005 issue of PRINTWEAR.)
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The screen printing
exposure system is the central star around which the entire screen printing
system revolves. The choices of exposure units are as important as, if
not more than, the choice of what press is going to facilitate production.
Choosing from the vast array of products
available can be overwhelming, as can be the additional possibility
of producing a home-built unit from available products and kits.
Rather than recommending one type of screen
printing exposure system over another, it will be more useful to dissect
the components and their characteristics into smaller groups of facts
to consider.
The Glass
Surprisingly, the selection of glass used
in any screen printing exposure unit plays a large part in the effectiveness
of the unit. With few exceptions, most exposing units use standard plate
or “float glass” that, due to the manufacturing process,
is remarkably smooth, consistently flat, and of even thickness across
the entire surface.
There are two commonly available types
of glass used in exposure units: an inexpensive standard float glass
and a specialty clear glass made from low-iron-content materials. Iron
in standard glass gives it the characteristic blue-greenish tint; unfortunately,
this inexpensive glass hinders light transmission. Six millimeters or
a quarter-inch thickness of standard glass will block 38% of ultraviolet
light transmission. Optically clear glass is simply float glass with
a reduced iron content. Low-iron glass increases the transmission of
ultraviolet light considerably, with a six-millimeter thickness blocking
only 14% of the UV light traveling into and through the glass. Low-iron-content
glass is typically four to ten times more expensive than standard float
glass.
Inclusion is a term used to describe
areas of particulate contamination in glass products. One type of inclusion
is the “stone,” which is an area incased in the glass product
composed of nickel metal sulfide contaminates; “stone-free”
glass is a term used to describe a single section of glass product with
little or no inclusion of nickel metal sulfide. Current glass-manufacturing
standards have lowered the occurrence of nickel metal sulfide contamination
to less than 1%, even in the standard-production float glass products.
Contrary to popular misconception, tempering
glass has no effect on the color, chemical composition, or light transmission
characteristics of float glass products. Tempered glass is three-and-one-half
to four times stronger than standard glass of the same thickness, but
compressive strength, hardness, thermal conductivity, light transmittance,
stiffness, and expansion characteristics are all unchanged by heat tempering.
Tempered glass not only adds strength to the product, but when broken,
will crumble into small fragments reducing the danger posed by large,
sharp, and broken shards. One downside of the tempering process is the
formation of gentle waves on the surface caused by the rollers used
in the tempering procedure. Such subtle variations can undermine the
critical film-positive-to-emulsion contact during exposure.
Intimate Contact
A sometimes overlooked aspect of screen
printing exposure systems is the method used to assure complete contact
between the film positive and the coated screen while exposing a stencil.
Intimate contact between positive and art is absolutely necessary for
clean, crisp lines and detail resolution.
There are two popular methods to accomplish
positive-to-stencil contact. One is the compression of the emulsion
and the art to the glass with a foam pad or block, with pressure or
weight pinching the art-and-emulsion-sandwich. The second method is
to remove all of the air from a sealed frame where a flexible “blanket”
is drawn to the glass tightly by removing the air with a vacuum. It
is best to choose a vacuum blanket of a non-reflective color (typically
black) to prevent light from bouncing back into the stencil.
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[A weighted pad is used in the compression
method to pinch positives in between the glass and emulsion. Vacuum-blanket
frames draw a flexible sheet to the glass, holding the positive and
mesh tightly, and significantly improving the sandwich’s intimacy.]
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can be gained by the blanket-and-vacuum type frames over the compression
method. The vacuum frames can gain from 15% to 40% additional detail resolution
over the compression method, regardless of the chosen light source.
The type of vacuum used in a blanket-and-vacuum
frame also plays a part in quality control, with higher-power vacuum pumps
creating vastly tighter contact than rotary vacuum blower units can provide.
High-power vacuum pumps and deep-draw flexible blankets provide the best
contact and highest image resolution.
Beware that warped, bowed frames can flatten
under vacuum pressure while exposing, which will cause the stencil to
stretch out of registration when the pressure is removed and the screen
returns to its warped configuration. Warped frames and sharp projections
can also cause glass to shatter when high-power vacuum pumps press the
irregularities into the glass.
Emulsion Sensitivity
All photo-reactive emulsions are sensitive
to a specific ultraviolet wavelength range. All of the three types of
emulsions available on the market, regardless of manufacturer, are primarily
sensitive to ultraviolet light in the 320 to 420 nanometer range. Photopolymers
are most sensitive to UV light in the lower 320 to 370 nanometer wavelengths,
while the diazo products are most reactive in the higher ranges from 370
to 420 nanometers. While all of the emulsion products have a favored specific
range of sensitivity, any UV radiant waves in the 320 to 420 nanometer
spectrum will expose any of the three emulsion types. Mismatched lights
will lead to higher exposure times. The spectral targeting range of emulsion
(available from the manufacturers) will be most important when dealing
with the light source choices detailed later in this article.
Two Configuration Choices
There are two basic types of light sources for screen printing exposure
systems: the single-source (or “point”) light system
made of a single focused light and its reflector, and the multi-source
light system made up of an array of fluorescent tubes or multiple placements
or separate bulbs of any type (see Figure).
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Combination or booster systems
are often referred to as “single source with an additional booster
light,” but by strict definition the “combination” set-up
is simply a multiple-source unit. Many combination units make use of a quartz-halogen
(or more properly, a tungsten-halogen) lamp with one or more booster lamps
of unfiltered fluorescent black lights. As it happens, the booster lamps
provide the majority of the exposure light in the desired ultraviolet spectrum.
The most striking differences in the light sources of screen printing exposure
systems are the detail resolution that can be provided, and the time needed
to gain a full exposure.
Simply put, a single-source exposure system (with all other factors equal)
will provide 7% to 20% improved resolution over a multi-source system. The
single-source system provides a more even, collimated (light rays
in more of a focused, thin-wedge, fan formation) spread of light, providing
a sharper edge definition due to the more perpendicular angle of light penetration.
Multi-source systems create more undercutting (light passing through
the positive at an angle) because of the multiple angles from which the
light emanates.
Focusing the Light
Just as a flashlight will cast more parallel rays when focused into a
tight beam, so will a single-source exposure light. Too close, and a tight
“hot spot” will form in the center, and too far (while providing
a more parallel set of light waves) will extend the exposure time exponentially.
Thus, the industry-recognized focusing distance is 1.5 times the diagonal
measurement of the screen itself, or the diagonal of the entire containment
frame if exposing multiple screens. One-and-one-half of that diagonal
measurement should be considered the minimum to prevent the light from
overexposing the center of the target and risking the possibility of the
outside edges suffering underexposure (see Figure).
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[The red arrows show the diagonal measurement,
while the yellow arrows show the distance from the glass to the bulb
- the latter being 1.5 times the former. The blue dotted line shows
the exposure area if too close to the bulb and the “hot spot”
shown in dotted red formed by the ill-focused light.]
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Available Light Sources
There are various light sources pressed into the role of exposing stencils.
The most common choices are quartz-halogen, fluorescent black-light tubes,
mercury-vapor, and metal-halide.
Because emulsions are most sensitive to a particular wavelength portion
of the spectrum, the most common lights used for exposure need to be compared
in terms of light-output spectrum, relative speed of exposure, and suitability
of purpose.
The first light source from the list is the quartz-halogen (see
Figure). This lamp is often considered a simple, generational improvement
over standard incandescent lamps, providing 15% additional light output.
Quartz-halogen lamps release 79.7% of their output in infrared (heat), 20%
in visible light, and only 0.3% in the ultraviolet spectrum. This lack of
UV light output is one reason for the extended exposure times when using
this light source. Quartz lamps lose only 10% of their total output for
their entire working life cycle, the point being that quartz lamps can be
run literally until burnout without much change in usability.
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[Quartz
lamps provide a low proportion of light in the spectrum where emulsions
are most sensitive. The two blue arrows mark the part of the spectrum
most desired for exposure. Quartz bulbs have a very consistent output
for their entire working cycles.]
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The second light source is the much-maligned
fluorescent-tube light, known for its problems with undercutting,
caused by wide banks of multiple tubes (see Figure). Fluorescent black-light
tubes cast off a majority of their light in the desired spectrum and produce
little heat. Fluorescent black lights, while emitting considerable proportions
of their light in the correct range for emulsion exposure, will still
have extended exposure times due to the lower power range of operation.
High-volume screen-exposure demands often cannot be met by the relatively
slow exposure times of fluorescent systems. Fluorescent black lights suffer
from cathode decay and drop about 25% ultraviolet output in 600 to 800
hours of operation (about six months) and should be replaced before light
level drops excessively or darkens completely. The lights will continue
to emit visible light long after most of the UV light has diminished,
so a calendar date for replacement should be noted.
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[Unfiltered fluorescent black-light
tubes emit a large portion of the light in the desired spectrum, but
suffer from low volume output. Black-light tubes decay over time and
should be replaced by the 800-hour mark where they start to drop in
output.]
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The third option is the mercury-vapor
or medium-pressure mercury arc lamp, commonly used in single-source
screen printing exposure systems (see Figure). Mercury-vapor lamps emit
11% to 24% of their light in the desired spectrum, but take considerable
time to “warm up” to full light output, often taking longer
than 10 minutes. The ultraviolet life of the mercury-vapor lamp is about
800 hours of operation. The clear casing of this lamp slowly discolors
(called “clouding”) and eventually becomes opaque to UV light,
choking off the desired output.
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[Mercury-vapor lamps provide a higher
power output of light in the desired spectrum. Such lamps lose light
output quickly then drop off almost entirely when the casing clouds
and chokes off the light in the desired spectrum.]
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The fourth light source, the metal-halide
lamp, is the most favored single-source lamp for emulsion exposure
(see Figure). The metal-halide lamp is similar to mercury-vapor lamps
but has additional metals added to the active matrix within the casing
(often called “doping”) to emit more light in specific areas
of the UV spectrum. Often, specific metal-halide lamps will cast more
than 30% of their light in the desired UV spectrum. As with mercury-vapor
lamps, metal-halides have a UV life of 700 to 1,000 hours, but will cast
off visible light long after the UV output is reduced to unacceptable
levels.
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[Metal-halide lamps can provide the
largest and most powerful output of light in the desired spectrum. Such
lamps suffer from a similar “choke off” point as mercury-vapor
lamps, but provide many hours of desired light with quite short exposure
times. Hundreds more screens can be exposed in a metal-halide lamp’s
lifetime when compared to low-power exposure lamps.]
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Diminishing Returns
Just as with all moves into technically advanced options, each level of
increased performance typically becomes smaller and significantly higher
in cost. And simply doubling the power of the cost does not equate to half
the exposure time or double the available dot resolution.
In testing the exposure-unit features, doubling the wattage of exposure
lamps will often only net a 40% drop in exposure times. Exposure times will
drop from minutes to seconds when switching from fluorescent-tube multi-source
units to metal-halide single-source units, but the available resolution
will only rise about 17%.
Shops have to consider whether a 5%, 10%, or 20% rise in performance is
worth double the price. Such decisions must take into consideration concerns,
features, and needs to help with educated equipment choices.
If a shop only needs simple spot-color prints that require only a few screens
per day, economy in price may be a valid option. When high performance,
maximum stencil resolution, or high volumes of screen exposures per day
are mandatory, economy in time and predictability are paramount. Often,
economy is found in time and performance rather than initial price.
Product choices such as the need for a light-sensing integrator must be
weighed against budget demands and long-term loss of work hours following
changes in bulb performance. Often, maintenance issues where more time is
taken to follow changes, cost more than features such as integrated timing
sensors.
Screen printing revolves around the stencil on the screen. Screen printing
exposure units are the central fulcrum against which all production demands
are leveraged. Choices in this area should be taken very seriously indeed.
(graphics by Douglas Grigar)
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