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Testing Exposure Light Distribution (return to top)
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SPTF conducted testing with radiometers to demonstrate how they can be used to measure exposure light distribution, and to show the effects some of the variables have on that distribution. Using a grid, researchers measured UV light output across the exposure surface of several exposure systems. Results presented in this report include:
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- A comparison of the two UV measurements and a complete description of how to render them in relative percentages for easy analysis
- A comparison of the light distribution from three different reflector designs
- The results of three different lamp distances on the light profile
- Results showing how light distribution uniformity was improved by fine tuning lamp position using a radiometer
- Stencil exposure effects from three different areas on the exposure glass that received different amounts of light.
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Several step-by-step procedures in the last section of this report detail how to utilize a radiometer to determine the acceptable exposure area, fine tune lamp position, monitor lamp life, check the integrator’s performance, and function as an independent integrator.
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Effects of Water Pressure and Chemical Treatment Combinations on Screen Reclaiming (return to top)
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SPTF conducted a study to evaluate the relative effectiveness of stencil reclamation with various combinations of cleaning/reclamation chemicals with three separate water pressures (a common garden hose with a spray nozzle; 1,000 psi from a pressure washer and 3,000 psi from a pressure washer). Three ink systems were used in the screen, and three different chemical treatments were used for de-coating. The report details SPTF’s conclusions that improving pressure and procedures will result in cleaner screens and therefore reduce mesh costs as well as the expenses and downtime associated with remaking screens.
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Effects of Drying Temperature on Screen Tension (return to top)
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SPTF designed and ran a simple test to determine the reaction of tension when a screen is exposed to various levels of screen drying temperatures typically used in the industry during screen processing. The experiment used four popular mesh counts, each processed using three different drying temperatures. Tension measurements were taken at each step of the screen making process to identify the effects drying temperature had on screen tension. Results showed that screen tension decreases when screens are dried at higher temperature. The report gives several production recommendations based on these results.
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Pulsed
Tensioning Project (return
to top)
With the advent of “Rapid Tensioning”,
screen stretching procedures will probably never be the
same. The hypo-thesis for this project was that just as
it was possible to shorten the screen tensioning procedure,
it should also be possible to reduce the time required
for the fabric to achieve full stabilization (i.e. little
or no additional tension loss over time).
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Three
approaches were investigated for their possible benefits
of tension improvements on mesh. Those included mesh
preloading, vibratory theory, and shock molecular alignment.
Check out what worked and what didn’t. The results
are interesting.
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Stencil
Thickness Effect on Ink Thickness (return
to top)
This SPTF study reveals some interesting facts about
how much ink actually gets added from different stencil
thicknesses, mesh, inks and tonal values. The variation
squeegee pressure has on the results was also explored.
Three dimensional graphs will be used to illustrate the
data. A practical way to estimate the amount of additional
ink from the stencil was developed from the results of
this experiment, and is incorporated in SPTF’s Ink
Calculator Software.
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Rz
Demonstration Screen Study (return
to top)
When printing detailed images, the stencil plays a
vital role in not only resolution and image duplication,
but also color control. The Foundation has documented the
effects different direct emulsion coating techniques have
on these characteristics, and put together a simple step
by step procedure for creating a test screen for optimizing
them in a shop. The difference between two coating sequences
can be very dramatic, and in house testing is the only
way a company will find out what works best under their
conditions. This has already helped a number of shops improve
their print quality, and has added a consistency to their
day to day work.
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Electronic
Thickness Gauge Evaluation (return
to top)
There are four applications for electronic thickness
gauges in the screen printing industry including measuring
fabric thickness, stencil thickness, dry ink deposit and
substrate thickness. SPTF determined that these instruments
are very accurate and consistent in measuring substrates
and dry ink deposits, but needed special procedures to
accurately work on fabric and stencil thickness. These
techniques have enabled printers to use these devices properly
in their process. Researchers have worked with several
manufacturers of these instruments to develop a screen
printing probe and to create electronic features to calculate
measurements useful to printers.
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Rapid
Tensioning (return
to top)
Several years ago, researchers completed a study to
determine the benefits of different tensioning methods.
The results showed that the common practice of staged tensioning
provided little benefit over a 5 minute rapid method as
far as mesh stabilization. This improved tensioning technique
has not only saved numerous companies time, but has simplified
training, reduced bottlenecks, and increased through-put.
Several mesh manufacturers are now supporting our conclusions
with their own research. We believe this will become the
standard approved practice for the industry in the future.
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Physical
Changes in Polyester Mesh During Tensioning (return
to top)
To facilitate making wise mesh selection decisions
for screen printing applications, SPTF analyzed physical
changes experienced by polyester mesh when tensioned from
free to working tension levels. Based entirely upon original
research conducted, the exhaustive study features over
60 easy to understand graphs and charts with extensive
technical information on polyester fabrics. Topics addressed
include: What are the differences between the manufacturer’s
specifications and the actual values for screen printing
fabrics? Which physical attributes of mesh change most
(or least) as a result of tensioning? Do similar fabrics
exhibit similar physical characteristics at the same tension
levels? How predictable and repeatable are the physical
changes in a tensioned fabric?
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Four
Color Process: The Effects of Changing Mesh Count (return
to top)
The bulletin covering this study deals with the importance;
measurement and quality control procedures for mesh count
when printing high-resolution images. The reason these
steps are needed is that mesh count changes, in some cases
dramatically, during the tensioning process. This means
that the mesh count labeled on the fabric will not be the
mesh count you end up with on the tensioned screen. In
addition, there are unusual occasions when a fabric may
be miss-marked directly on the bolt itself. Incoming quality
control measures outlined in the bulletin will help you
catch these errors before they ever get to the press and
cause costly problems.
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Capillary
Application Variable Evaluation (return
to top)
The goal of the project was to determine the major
factors effecting the degree that a capillary film extends
into the mesh during application and drying. By identifying
and understanding the factors in capillary application
that cause the greatest degree of variance of the final
stencil thickness under a given set of conditions, controls
can then be implemented in those critical areas so stencil
thickness can be repeated accurately time after time.
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Five
variables were selected for the study included mesh
count, squeegee pressure, time of water run off, temperature
of wet screen, and manufacturer of capillary film (identical
thicknesses were used). The range of stencil thicknesses
produced with the sixteen combinations tested had a
high of 17.2 microns and a low of 10 microns. This
is an indication that some control and consistency
during capillary film application is necessary to produce
a repeatable stencil thickness for screen printing.
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For
a copy of an SPTF Update story detailing this study
contact sptf@sgia.org
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Transfer
Differences between Plain and Twill Weave Mesh (return
to top)
In this SPTF study, “ink transfer factor” was
used to characterize the differences in plain weave and
twill weave mesh. “Ink transfer factor” is
a ratio of the ink transferred from the screen to the substrate
from the total ink available in the mesh cells. Comparisons
between transfer factors of plain weave and twill weave
mesh indicate that plain weave transfers approximately
80% of its available ink, while twill only transfers 60%.
The transfer factor ratio is an indicator of the ink rheology,
surface tension of the screen and the mechanical structure
of the screen itself.
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Mesh
Modulus Study (return
to top)
All materials obey certain laws of elastic behavior
when a force is applied. Polyester material is no exception
to this rule. However, Polyester screen mesh is a woven
construction that has the ability to temporarily absorb
and redirect the stretching force. This mechanism produces
an effect that is both linear and exponential depending
on the amount of tension applied to the screen. Learn how
these little understood facts can affect the way a screen
ultimately responds to the various methods used to generate
a stable printing platform.
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Elongation
vs. Mesh Dimensions (return
to top)
Having found that mesh dimensions change significantly
during tensioning, a limited study was performed to determine
if elongation could be used to predict the changes that
would occur to a mesh. No consistent pattern could be determined.
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Bolt
to Bolt Variation of Polyester Mesh (return
to top)
How much does ink deposit
have to change to create an unacceptable color
shift? Does mesh differ significantly from
bolt to bolt from the same manufacturer? Is
it important to do incoming quality control
checks on mesh? How much do differences in
mesh dimensions influence the ink deposit from
a screen? This study brings light to all of
these questions. The results will surprise
you.
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Study
on the Breaking Points of Mesh (return
to top)
Does mesh count and thread diameter determines the
breaking point of woven fabric? What is the relationship
of “Specific Cross Section” to the ultimate
breaking point of the mesh? What type of screen offers
a strength advantage? These and other questions can be
answered by a review of the data generated in this interesting
SPTF designed study.
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The
Effect of Rz on Textile Printing (return
to top)
A short study was done to determine if reducing Rz on
screens used in textile applications produced print definition
improvement when printing halftones. A test screen was made
with six different coating combinations and printed. Results
showed that face coating did reduce dot gain.
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Deflection
vs. Tension (return
to top)
Does screen deflection continue to decrease in a predictable
way at increasing tension levels? The result of this study
suggests a repeatable shift in the way mesh responds to
deflective force at specific tension levels. Typically
one would expect a steady linear decrease in screen deflection
if the force remains the same and the tension is gradually
increased. For each screen mesh tested data suggests a
definite turning point in this “normal” trend
that may affect the performance of the screen on press.
The trend change usually occurs between 22 - 32 Newtons
per centimeter and may be described as a flattening of
the deflection curve. It is theorized that this change
is the result of the tensioning force being applied directly
to the polyester material itself instead of the combined
effect of the material and the structure of the mesh. Make
up your own mind by reviewing the graphs and data.
Cubic
Volume of Mesh Cell vs. Tension (return
to top)
Researchers have documented the change in the cubic
volume of mesh cells as a screen is tensioned to various
levels. The cell volume changes as a direct result of the
changes in some of the mesh dimensions during tensioning.
This change in cubic volume has a direct bearing on the
wet ink thickness produced from the mesh.
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Multi-Material Tester (return to top)
Using an over the counter experimenter’s kit and internally designed measurement probes, researchers have demonstrated the ability of this technology (“Charge Transfer Sensing”) to measure: Degree of Stencil Cure, Moisture Content of the Stencil, Moisture Content of the substrate, Degree of UV Cure, Off-Contact Distance and Mesh Count or Ratio of Solid to Open Area. Through its unique approach to signal processing “Charge Transfer Sensing” provides a way to detect very small changes in almost any material. Since screen printers are very much involved in using “materials” it only makes sense to develop new and better ways to measure those materials.
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For a detailed analysis of the project and the possibilities it brings to the industry, follow the link to:
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Squeegee (return to top)
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Ink (return
to top)
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Ink
Deposit on 27 mesh counts with 2 inks (return
to top)
In a continuing effort to expand the Foundation’s
data base of ink deposit from various fabrics, a project
was completed measuring the wet ink deposit on 27 different
mesh counts ranging from 60 to 460 mesh count with two
different inks. This includes 14 plain weave mesh counts,
meshes to test thread diameter differences, and 5 twill
weave meshes. Testing was completed using rapid tensioning
and screens were printed under identical conditions. Results
from this study have verified that SPTF’s ink estimation
formula is superior to other calculations used in the industry.
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Wet
Ink Thickness Formula (return
to top)
SPTF has developed a method of estimating wet ink deposit
from various meshes that has proven much more accurate
than currently used formulas. Improving the printer’s
ability to predict ink deposit from a mesh improves ink
consumption estimates and mesh selection choices. The project
is extensive, founded on years of SPTF research and represents
the best possible information available today on the subject.
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Ink
Calculation Software (return
to top)
A prototype user-friendly computer program has been
developed to estimate both ink deposit thickness and the
quantity of ink for a job using SPTF’s ink estimation
for-mula. This program is the most comprehensive and accurate
ink estimating system currently available to the screen
printing industry. This is the accumulation and application
of years of inter-related research projects and is yet
another example of how consistent fundamental investigation
into the inter workings of the process will continue to
produce practical solutions to real world problems.
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Color
Matching: Sample Preparation Technique Evaluation (return
to top)
One of the first projects undertaken looked at the
accuracy and repeatability of various off-line ink sampling
methods including wire wound draw down bars, open gates,
and manual screen printing. One of these systems performed
very poorly with screen printing inks. The others were
found to need some control to get consistent results. Many
times these systems are used for testing a color match,
but if the ink deposit is not the same as what will be
produced on the production press, it ultimately will not
match on the finished product. Making this connection will
help printers reduce their press down time by doing things
right the first time.
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Sample
Preparation Principles (return
to top)
Material sampling and testing must be an integral part
of any effort to control and improve a process and its
resulting product. In this report a basic discussion of
laboratory sampling and testing concepts will initially
be covered with a detailed explanation of the purposes
and critical characteristics of an ink sample, and the
selection of a proper substrate for testing.
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Polyester
Mesh Capability Study (return
to top)
Polyester screen mesh is one of the single most significant
influencing factors on ink deposit in the screen printing
process. The ability of polyester fabric to produce a uniform,
controlled and repeatable deposit thickness is important
to the screen printer interested in producing high-quality
products. It becomes critical when attempting to control
such properties as color in appearance, electronic values
and weatherability of that printed product. Four the aspects
of the research on polyester mesh are examined in this
study: UV ink shrinkage, repeatability and process control
of polyester, ink deposit trends, and a comparison of hand
screen printing in computer controlled screen printing
on polyester fabric.
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Wet and Dry Ink Measurement Method Evaluation (return
to top)
An extensive study was completed evaluating different
methods of measuring both wet and dry ink deposit. Many
instruments and devices were tested enabling SPTF to make
recommendations to the industry. In this report SPTF reports
along the repeatability of screen printing; the operation
and reliability of low-cost wet deposit measuring devices;
measurement procedures to best suited for these devices;
and the effects of different substrates on the instrument’s
accuracy and reliability.
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Screen Printer’s Tack Tester (return to top)
A prototype screen printer’s tack tester has been designed, tooled, assembled and is undergoing capability testing at SPTF. Using an ink system, substrate and mesh of choice, the tack tester can generate a number to define how the three interact when ink transfer takes place. It is hypothesized that a small change in any one of the variables will result in a measurable change in the “ apparent tack “. Once the testing procedure has been optimized, and the combination effects modeled, end users will have a versatile instrument capable of alerting printers to any change in the raw materials of the process; enable manufactures to test their products for consistent, optimum performance; and provide a means for quickly trouble-shooting a problem by a simple process of elimination.
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Ink Transfer Theory Development (return to top)
Researchers have conceptualized the forces involved in ink transfer in screen printing and have generated some theoretical drawings illustrating these concepts.
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Press (return
to top)
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Off-Contact
Gauge (return
to top)
The first electronic off-contact gauge has been developed
from the research efforts of the SPTF. The probe can easily
be moved anywhere on the screen, and measurements taken
instantly. Off-contact adjustments can be made while getting
feedback measurements from the gauge every two seconds
without moving the probe. Using this gauge will both reduce
press set-up time and ensure consistent off-contact distances.
Consistent off-contact distances will improve registration,
prevent screen blowouts and lower press down time.
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Curing/Drying (return
to top)
UV Ink Shrinkage Study
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Two
experiments were conducted to determine two different
characteristics of the shrinkage of UV inks. First,
the effect of time was looked at to determine if the
UV coating continued to shrink. Secondly, a progressive
set of samples was made to determine if the degree
of cure changed the shrinkage rate of the ink. The
study was designed to shed light on when the most appropriate
time to measure dry ink deposit is for UV curable inks,
and if shrinkage levels off when the ink is fully cured,
or whether over-curing causes further shrinkage.
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For
a copy of the short summary of this project contact sptf@sgia.org.
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Portable Squeegee Speed Monitor (return to top)
SPTF has conceptualized the design for a working prototype squeegee speed monitor to control squeegee speed on any press. This low cost design could be mounted on any press and would solve the problem of being able to set up squeegee speed consistently. Belt speed could also be monitored with this device. We have located and tested components that could be used to assemble such a device and believe the goal is achievable with minimal cost. This could be another low cost tool to monitor the screen printing process.
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Dynamic Squeegee Pressure Gauge (return to top)
The goals of this project were to identify the variables that affect the dynamic squeegee pressure (pressure the squeegee applies to the substrate during the printing process) and then relate those forces to the print quality. A prototype measurement system was developed to measure, for the first time, the actual squeegee pressure at the substrate. SPTF subsequently identified a measurement device that measures this variable.
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