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| December 2004 issue of PRINTWEAR |
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Mesh-Selection
Primer
A matter of variables
by Douglas Grigar, Master
Screen Printer
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(A version of this article originally appeared
in the December 2004 issue of PRINTWEAR.)
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“What mesh do I need to use?”
is one of those misleadingly “simple” questions that misleads
because it touches on only one part of a whole collection of variables.
Most printers think about the thread count, measured linearly and referred
to as “TPI” or threads per inch (see “What the ‘PI’
Terms Mean…” Printwear, August 2004) and don’t
realize that weave configuration and thread diameter are just as important
variables in the equation as thread count.
More Than Just the Over/Under Issue
Monofilament mesh is produced in two weave
types - plain weave, where each single thread passes over one thread
and then under the next, and twill weave, where threads pass over or
under more than one thread at a time. The important difference in weave
types revolves around the detail-holding ability and the printed results
of each type.
With all other factors equal, plain weave
will more accurately hold small details, hold sharper edges, print sharper
dots, and produce less dot gain (see Figure 1).
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[Figure 1: A comparison of twill and
plain weave mesh and their printed results. Note how the twill weave
on the bottom increases dot gain and darkens the final print.]
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Twill weave mesh prints star-like patterns
as ink passes under larger gaps (often called divots) made by longer
and larger arches of thread. The printed results of plain versus twill
weave shown in Figure 2 leave little doubt that weave configuration
has a huge impact on the printed product.
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[Figure 2: Twill weave printed dots
on the left and plain weave dots on the right. (Original image courtesy
of Saati.)]
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A Modest Defense of
Older Terminology
My favorite resurrection of older standard
terms is the use of the generic-letter designation of S (for the thinnest-diameter
threads), T (for medium-diameter threads), and HD (for the thickest of
thread diameters) for identifying mesh-thread thickness within
a given mesh count (as shown in Figure 3). The older generic-letter
designations for thread thickness are valid and useful for explaining
the effects of mesh thread diameter in the printing process. The terminology
may be old but is quite easy to understand. A secondary benefit is that
S, T, and HD is a concept easily transferred from the various mesh counts
and brands, rather than memorizing long lists of specific diameters.
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[Figure 3: From left to right, S, T,
and HD show the progressive thickness of thread diameters in a single
mesh count. (Original image courtesy of Saati.)]
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Nay-sayers will claim that such older terms
be discarded and their use could only be the result of clinging to old
screen printing resources. Nonsense, I say. Using modern research to form
educational approaches should not discount older standard terms that can
promote conceptual understanding. Simple regurgitation of specific details
without a basic understanding of the underlying concepts hinders the process
of understanding.
All Threads Are Not
the Same
There are several primary manufacturers and
many secondary sources of mesh for screen printing, and none
of them produce the exact same mesh; each and every manufacturer has its
own standards for thread count, weave type, quality control, physical
properties, and exact dimensions.
The inconsistencies of cross-brand mesh standards
makes generic terms useful when talking about mesh in conceptual terms.
When attempting to apply conceptually general mesh concepts to issues
such as ink flow, ink deposit, and stencil-image resolution, the logical
step is to make an exact choice based on product offerings. When moving
from concept to application, take into consideration the fluctuations
from just one brand and its specifications, as well as the possibility
of performance change when dealing with two or more brands and their contrasting
physical specifications. In the simplest of terms, we’re talking
about the fact that one brand of mesh marked as 230-tpi with 40-micron
thread diameter will most likely not perform in the exact same way as
a competitor’s brand with similar specifications.
Cross-brand inconsistency does not suggest
a lack of quality, but rather the result of standard variations from one
manufacturer of mesh to the next. The most consistent results would come
from standardization within each plant based on a particular brand, weave,
color, and thread-diameter preference for each mesh count used.
Threads: Big issue or small?
The thickness of the threads plays a part
in several aspects of how ink is printed, the detail that can be printed,
and the level of tension that can be applied to the mesh. It is the thickness
of the threads that changes the level of tension that can be applied to
the mesh, in that the thicker the individual threads in a mesh count,
the greater the tension that can be applied to the mesh.
In a given mesh count the thickness of the
threads also affects the flow of ink due to the actual space the threads
occupy. Larger threads constrict and lengthen the openings that the ink
travels through to the substrate (see Figure 4). Just as water prefers
to travel the path of least resistance, ink will also flow faster and
with greater volume into open areas with less restriction. Heavy or thick
threads have a reputation for difficult printing when used in some applications
because of the constriction of the open areas in the mesh.
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[Figure 4: From top to bottom HD, T,
and S threads showing the change in open areas and a theoretical ink
volume that could pass onto the substrate.]
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While the higher tension that is available
with heavy threads could provide higher print speeds and other benefits,
the cost is loss of resolution, ink flow, and ink transfer. Claiming that
the higher tensions available with the thickest HD threads is always a
better choice than the larger open areas and higher detail capacity (but
lower tension) of S threads would be irresponsible and simplistic. Choice
of thread thickness (like all the choices we have to make) is a balancing
act where all of the variables such as dot resolution, ink deposit, and
ink volume have to be taken into account for an educated choice.
Giving Under Pressure
Mesh flexes under squeegee pressure and will
return back to a flat platen after the squeegee passes. It is this property
that renders screen printing capable of printing with inks and deposit
thickness that other printing methods could never achieve. Higher tensions
reduce the off-contact distance needed to produce the proper snap-off
for clear, clean prints. The flexible nature of mesh will always overcome
even the highest mesh tensions. When bridging large spans of space, squeegee
pressure can flex the mesh enough to touch the substrate. When squeegee
pressure flexes the mesh to the substrate, thickness of the mesh is all
that is left to control the depth of the ink deposit (see Figure 5). The
concave nature of the ink deposit can be lessened to some extent by high
tension, but must be taken into consideration when large blocks of open
area are printed.
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[Figure 5: Large open areas of mesh
will have the centers pushed to the substrate leaving only the mesh
to control the thickness of the print in the center.]
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Common Compromises
Most suppliers choose to stock only
one of the mesh thicknesses available, and base that choice on inventory
and sales concerns. Stock that revolves fastest - that is, “middle-of-the-road”
mesh thread with the largest appeal - will determine what gets stocked,
rather than an individual printing plant’s performance issues. Suppliers
are often making an effort to come to a performance compromise while considering
sales volume and stocking concerns.
Buying mesh based strictly on mesh count
is ignoring many factors that will affect the printed product. With every
change in each of our mesh variables we can expect secondary effects all
the way down our process chain. If consistency in art-production formulas
and repeatability in ink deposit from job to job is important, a standardization
of mesh choices (and proper labeling and identification) will become paramount.
Thus, choosing a specific mesh not stocked by a supplier may force a printer
to change suppliers, or to custom order desired mesh choices.
The Bargain Trap
Sale prices on bargain mesh glued to a batch
of inexpensive wood frames could prove disastrous. That bargain mesh could
be made from unpopular old stock of the thickest diameter thread, different
in color, and in a twill weave, creating a selection of screens that will
never perform the same way as carefully selected products. Bargain-basement
screens for pennies on the dollar could prove significantly more
expensive that careful choices when production has to grind the screens
incapable of printing the desired results. No printer can expect optimum
performance and resolution for process, simulated process, or any other
critical job when using a mélange of mesh.
Mesh selection is one of the most critical
choices in our process. A basic but thorough understanding of the general
concepts is needed to advance any plant’s capabilities.
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