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| June 2005 issue of PRINTWEAR |
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Coating
Screens for Maximum Effect
by Douglas Grigar, Master
Screen Printer
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(A version of this article originally appeared
in the June 2005 issue of PRINTWEAR.)
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While any stencil that can hang onto its
mesh and make a successful print could qualify as a “money maker,”
does this mean it is “good enough”? In many ways, the screen-printing
industry is saddled with a “good-enough-is-good-enough”
mind set. Many of us seem to suffer from a resistance to take a logical
and critical look at long-standing methods, asking whether, while they
may work, they work well enough? It is important to understand
that, while common methods may produce useable results, it is often
a simple change that can produce greater consistency, improved product
performance and money-saving efficiencies.
The facts to be explored in this article
revolve around three parts: first, the reasons behind better performance
given proper levels of emulsion over mesh (EOM); second, a method for
coating wet-on-wet to attempt to produce a significant EOM; and third,
comparing this method to standard procedures.
Introducing a method of coating screens
using the “wet-on-wet” procedure, while additionally increasing
the emulsion-layer thickness (in a quick and friendly production-friendly
manner) is my primary goal. Clearly, face-coating will produce a flatter
smoother-surfaced stencil than the typical “Wet-on-wet”
method. It is the extra time, labor, and supplies, though, that make
the application of an additional face-coat of emulsion on a previously
dried emulsion layer unpopular; so it typically doesn’t get done.
Because wet-on-wet coating is most often the rule – and we will
not be discussion face-coating here – this article will focus
on the facts and results of wet-on-wet coating procedures, in an attempt
to render such procedures as effective as possible.
Inescapable Basic Variables
Speed, angle, pressure, coating-trough
fullness, and coating-trough edge are the basic variables we have to
deal with when coating a screen with a coating-trough (see Figure 1).
We have to take into consideration that from each person coating screens
to the next we should expect to see measurable changes, some quite pronounced.
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[Figure 1: Speed, angle, pressure,
and coaster fill-level all effect the flow and amount of emulsion transferred
to the mesh of a screen.]
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The travel speed of the coaster will change
the amount of liquid emulsion transferred to and into the mesh. As a
coater increases the speed, the amount of emulsion deposited will decrease
and, unfortunately, the amount of bubbles formed in the coating trough
will increase.
The angle of the coating trough will also
vary the amount of liquid emulsion: increasing or decreasing the angle
at which the coater is held to the mesh will affect the transfer of
emulsion.
The pressure that is applied to the coating
trough against the mesh will also change the emulsion transferred along
with the consistency of horizontal distribution of the liquid. Too little
pressure can cause heavy uneven streaks of emulsion to form in the center,
while too much pressure can result in damage to the screen and mesh.
The fill-level of emulsion in the coater
also will affect with the amount transferred to the mesh, resulting
in a measurable change in transfer of emulsion from 5% to 25% depending
on the fill-level difference. Fill-level changes will have the same
drastic effect on layer thickness if you are using an expensive coating
machine as it will when coating manually.
The coating-trough edge profile will also
change the emulsion-transfer amount with the rounded edge transferring
double or more the flow of emulsion into the mesh than the sharp edge
(see Figure 2). A rounded edge will transfer emulsion faster, translating
to less time for higher value of emulsion deposited.
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[Figure 2: The coating-trough edge
changes the amount of emulsion transfer. With a rounded edge often doubling
(or more) the flow of emulsion into the mesh fabric, the rounded edge
attains the “glisten” goal in fewer strokes.]
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While discussing these coating
variables along would fill an article, my objective is to point out why
even small changes in coating procedure will affect the final product,
and why drastic changes in product, from screen maker to screen maker,
is common.
Emulsion Over Mesh
Emulsion over mesh is simply the measure
of how much the emulsion layer extends beyond the threads of the mesh,
and represented as a percentage of the total thickness, threads and all.
Too little EOM and the threads and their profile have a detrimental effect
on edge definitional and dot formation (see Figure 3).
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[Figure 3: EOM is a percentage of the
total thickness of the stencil. Low EOM will follow the thread profile
while proper EOM levels help left the threads from the stencil edge,
forming sharper prints.]
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As ink travels through the mesh, it will
often take a path under the divots formed in the face of the emulsion
by the thread profile. If there is not a “wall” of emulsion
layer, past the threads, the ink literally squirts out from the edges.
Too little EOM will also allow the threads of the mesh to come into direct
contact with the substrate and form an unwanted temporary “stencil,”
blocking ink from forming a full, even dot or sharp line (see Figure 4).
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[Figure 4: The three panels show how
dots will print with less interference from the mesh as the EOM layer
increases. Note how the low EOM on the left lets the mesh sit directly
on the substrate and you can see where the mesh thread knuckles form
almost stencil-like blockages inside the dots.]
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| Suggested Procedure: The
“Glisten” Method
As the desired result is to increase the
deposit of emulsion to raise the EOM of the stencil, we want to make sure
we include the procedures and supplies that will help attain this goal.
A good quality emulsion with a solids content of over 35-40% is the best
choice: any less and the shrinkage when dry will be excessive. Increasing
the emulsion layer will also increase the exposure time; depending on
your prior coating method, this could double or more the exposure time
needed.
Using the rounded edge of the coating-trough
will be necessary to cut down on the number of strokes needed to gain
the volume of emulsion desired. Switching from the sharp to the rounded
edge can be a shock to the user as the emulsion deposit increases drastically.
Practice will be required.
Coating screens with this procedure depends
on a particular order of application, and the user will always start the
application on the substrate or shirt-side and always end on the squeegee-side.
The “glisten” method starts with coating strokes onto the
print-(shirt) side until the opposite side (squeegee) shines or glistens,
from where the name for the procedure originates.
Your screen must be clean, dehazed and degreased
for this procedure. Heavy mesh fabric haze will cause the emulsion to
flow differently and not glisten on the squeegee side evenly; it will
have the look of wet cloth but not glisten in all areas. Look for signs
of shininess outside of areas where haze has altered the ways the mesh
changes the emulsion travel.
Coating is now concluded on the screen’s
squeegee side. Regardless of the number of strokes on the substrate side,
the amount of emulsion deposited is always controlled by the final strokes
on the squeegee side pushing emulsion to the print side while metering
the total emulsion by returning excess back into the coater.
Squeegee side last, always. Coating the squeegee
side last pushes the emulsion from the squeegee side to the print side
where the EOM will have the most positive effect.
Dry your stencil squeegee side up, always.
Gravity will pull the emulsion down and, when dried with the squeegee
side up, the emulsion will dry with the thickest layer pulled to the substrate
side.
Interesting Side Notes
The glisten procedure allows for an easy
fix if the coating is botched: simply scrape off the emulsion from both
sides and start at the beginning, coating for a shine; a fall-back point
every time.
Higher mesh counts begin to “self-meter”
emulsion levels as the constriction of flow by the smaller openings helps
even out flow and make the coating more consistent; in other words, the
higher the mesh where detail demands are higher, coating with this method
become even more consistent. Fixes using the fall-back point described
above will have less than a 4% change regardless of how much emulsion
was coated on the substrate side.
The Comparisons
The most common screen-coating procedure
is to use a coating trough with a sharp edge to apply a particular number
of application strokes on one or both sides of a screen. While the typical
spot-color “Joe’s Bar & Grill,” printed with a 150
– 160 mesh on light shirts may not necessarily suffer enough from
the lack of sharp stencil profile (or at least most customers may never
notice it), it will suffer. And, as the degree of job tolerance
gets closer and closer, it will suffer more until, eventually,
the customer will notice.
The common 1/1 – one coat on each side
– with sharp edge will guarantee the coating of dried emulsion over
the mesh threads will be less than 2%; that is less than half of the recommended
5% for small-dot resolution and one-tenth the recommended 20% for general
printing (see Figure 5).
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[Figure 5: 1/1 sharp-edge coating will
produce less than 2% EOM, one-tenth the recommended level of 20% for
general printing.]
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| Often, printers will point out
that, when they coat sharp-edge 1/1, the screens show fewer flaws…which
could only mean such screens are “better.” What this is really
pointing to is that a thicker layer of emulsion is darker and will show
flaws and contamination in the mesh from lack of complete and effective
cleaning. Hiding flaws does not eradicate them, and thinking that hiding
them is effective and helpful is not logical.
A side-by-side look: The micro-photographs shown in Figure
6 represent 1/1 with a sharp edge, against the glisten method of stroke-to-a-shine
and one stroke on the squeegee side with a rounded edge.
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[Figure 6: The top pair of micro-photographs
shows the difference in dots between sharp-edge 1/1 coating and round-edge
“glisten/1” coating on 150-tpi mesh. The middle pair shows
the same, on 150-tpi mesh, while the bottom shows the contrast with
fine lines on 150-tpi mesh. Clearly, the “glisten/1” method
produces superior results.]
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| The results are quite clear where
the 1/1 sharp just covered the threads, the rounded edge with the glisten
method gained a considerable thickness of emulsion over the threads giving
us much sharper lines and dots. All photos are of stencils that are fully
exposed and developed with a 1,000-psi pressure washer at 30-36 inches.
Never “Good Enough”
While no one can argue that any job that
is “good enough” to get paid for may not be considered a business
success (because the order made money). But saddling the print shop with
“good enough” procedures will only translate into problems
when the need for precision is necessary in order to successfully finish
a project. The introduction of a production-friendly method to increase
quality will help any shop tackle more demanding jobs in the future with
less downtime attempting to make changes specific to a new task.
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