Clarke Joe, Lynch Colleen C. M&R Professional Screen Printing Series
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11
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Screens
The next graph uses a nominal 1/16-inch off-contact distance and compares vari-
ous ink well sizes to the respective image tolerance. The range is from an
extremely close one-inch ink well to four and one half inches, which is also
impractical. Note that the Lpi listed is for reference and is far superior to the
one-eighth inch off-contact distance table above.
The graph in figure 1.22, includes both the 1/8-inch and the 1/16-inch off-con-
tact distances so that you can compare the inkwell and image tolerances for both.
Obviously the closer distance has appeal when shown in this context. Be wary of
being too close, the telltale signs are image stretch and poor ink transfer, particu-
larly in the center of the mesh. M&R equipment will allow you to run in the real
world as close as 0.020-inch with proper calibration but beware that closer is not
better for most cases. You will need to assess your inks, press calibration, tension
level, and squeegee parameters in addition to the frame size.
SSiizzee EExxcceeppttiioonnss
Note that there are several options for frame sizes on the Challenger and
Challenger II. The Challenger series presses are available with long stroke options
on some or all press heads. This means that a standard 22-inch stroke length
Challenger can be fitted to print a 30-inch or 36-inch stroke length. There are
opportunities and limitations to how you can configure the press. If you want a
longer stroke than standard, reduce the width of the image and frame. If you
need to maintain the width and increase the length, it is possible by restricting the
number of colors used. Customer service will be happy to assist you in under-
standing all possible configurations to make your Challenger series press even
more versatile.
1/16-inch (0.063) Off-Contact Distance
Ink Well
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Tolerance in Mils
1.9
1.3
0.9
0.7
0.6
0.5
0.4
0.4
Lpi Equivalent
89
133
x
x
x
x
x
x
Figure 1.20 This is similar to the table above but with an off-contact distance of 1/16th-inch (0.063). With this closer
distance the tolerance improves greatly. For example at a minimum inkwell of 1.5-inches the tolerance is 1.3 mils,
(0.0013) but do not be misled, there is more to the story. At this close off-contact distance it is difficult to develop contact
pressure between the blade and the mesh at the midpoint of the blade. Even the highest tension levels improve but do not
guarantee the contact pressure. You may want to review chapter five before you make this decision.
PAGE
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Chapter 1
Figure 1.21 This is sim-
ilar to the last chart; this
one compares inkwell and
image tolerance but this time
at an off-contact distance of
1/16th-inch (0.0016).
The curve shows that as the
ink well increases the toler-
ance improves. At approxi-
mately four inches the toler-
ance has reached diminishing
returns; a larger inkwell
only makes a marginal
improvement in tolerance.
Note that screen stretch due
to inkwell and off-contact
distance is 0.4 mils,
(0.0004) with this off-con-
tact setting. Move cautiously
though, off-contact is only
half of the issue, contact
pressure between the blade and the mesh does better with greater off-contact distances. Review chapter five for details on off-
contact inkwells and image tolerances.
At 1/16" Ink Well and Image Tolerance
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Ink well in Inches
Image tolerance in Mils
At 1/16-inch, as the inkwell increases in
size, the image tolerance improves.
Figure 1.22 For
your convenience we have
plotted both off-contact
distances on the same
chart. Note that the rel-
ative shape of the curves
is similar because they
are both bound by the
reality of improved tol-
erance with a larger
(minimum) inkwell.
However the closer off-
contact setting shows a
marked improvement at
any inkwell distance.
But as we have cau-
tioned you above, do not
jump to the conclusion
that you should set the
off-contact distance clos-
er. Review chapter five
for the details on setting
the proper distance. The first thing that this chart should tell you is that you need a full sized screen and the minimum
squeegee length for high tolerance work.
At 1/8" & 1/16" Ink Well and Image Tolerance
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Ink Well in Inches
Image Tolerance in Mils
At 1/8-inch off-contact distance
At 1/16-inch off-contact distance
PAGE
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Screens
FFrraammee SSttrreennggtthh AAnndd FFllaattnneessss
The frame you select should have the integrity to withstand your intended tension
level. You want the frame to resist weakness in two areas: torsion twisting and
internal deflection. Any twisting in the corners causes the frame to be racked or
not flat. This condition creates the same problem as a press that is not calibrated.
Worse yet if you put such a frame into a press that is not calibrated: the only pre-
dictable result is too much downtime. There is no type of squeegee, off-contact
setting, peel rate or tension level that will solve this problem.
If you are using a roller frame you can and should be careful to flatten it prior to
tensioning. If your tensioning procedures cause the frame to rack again, contact
the manufacturer.
If you take the
racking out of a
frame that has
already been ten-
sioned, you are
creating isolated
stresses on the
mesh. This stress
will come back to
haunt you in the
form of a rip or
tension loss or
both.
If your frame
deflects internally
it will create a
pulsing action
during produc-
tion. It will be as
though you rapid-
ly took the mesh to a higher tension level and then released it, over and over
again. This may occur at the rate of 800 or 900 times per hour on a Challenger
or Gauntlet press and will typically jump as much as three to four N/cm². When
the frame has bowed inward it pulses right at the edge of the mesh and will
lead to a rip at the center of the longer frame wall. This jump from static (rest-
ing) tension to dynamic (printing) tension causes a proportional tension loss and
will have to be dealt with. There are a few options:
Figure 1.23 This illustrates some of the opportunities on a Challenger Press. There is a
standard screen width for each Challenger Press and it is listed in the operations manual.
However there are a variety of sizes that you can use if your image permits fewer colors or a
frame that is even more rectangular. If your image is long and narrow, for example a long
vertical stripe, you may be able to use a frame that is narrower side to side but longer than
standard front to rear. Also if you can run fewer colors you can utilize frames that are
wider side-to-side and longer front to rear than the standard. As you can see in the illustra-
tion, a frame that is only 28-inches long could be as wide as 32-inches, far over the stan-
dard. If you have a need to print oversized and have long stroke possibilities, call us at the
factory and let us help you engineer your printing.
PAGE
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Chapter 1
1. Find a more robust frame.
2. Reduce the level of tension.
3. Reduce the squeegee width and stroke.
4. Reduce the off-contact distance.
CCoonncclluussiioonn
Too often the decision is made to use a screen that is too small
to function properly on the press. The logic is that one can
save mesh, emulsion, exposure times and print speed. These
supposed savings are an illusion; ironically mesh dollars are
actually lost. The frame must be flat and not bow inward in
production or image quality and mesh life are immediately lost.
Remember that the press is your racecar, your way to make
money, do nothing to slow it down. Smaller frames will do just
that.
Tensioning
In this country, most tee shirt printers are using retensionable
roller frames for automatic production. The reason is that
most of the inks are very tacky and require a great deal of
force to transfer. Most of the frames are small for the image
size leaving very small inkwells. Finally too few of the presses
are regularly calibrated. With these conditions, retentioning can
be a blessing. But whatever your conditions or frame type,
there are certain obligations that the tensioning results should
meet or exceed. This is the focus of the following section.
GGooaallss
There are five fundamental goals that should be met in the tensioning process. Excluding any of
these can be the cause of severe problems on press.
1. Maintain a square mesh opening.
2. Avoid any areas of isolated stress.
3. Allow the off-contact distance to achieve image tolerance.
Typically the warp(threads run the
length of the bolt) allows a
greater percentage of elongation
than the weft. Weft (threads run
the width of the bolt) are lower
elongation and require more
force. This fact may lead the roller
frames.
Roller frames may have a tenden-
cy to twist while you are tension-
ing them. It seems as though you
need to be an octopus to hold
down the other side while you are
tightening the bolts. An easy solu-
tion to this dilemma is to mount a
U-shaped aluminum channel to
the end of your stretching table.
Remember to measure the highest
part of the frame when determin-
ing the size of the channel and
add room to line the channel with
foam to protect the mesh. Now
just slide the far end of the frame
you are tensioning into the chan-
nel. It will keep the frame flat no
matter how high you take the
tension.
K
K
eeping R
eeping R
oller
oller
F
F
rames Flat
rames Flat
PAGE
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. 29
Screens
4. Permit zero screen
lag.
5. Equalize contact
pressure between
blade and mesh.
MM
aaiinnttaaiinn SSqquuaarree OOppeenniinngg
It should be the goal of the
fabric weavers to offer a fab-
ric that can maintain a square
mesh opening. The primary
reason is that a distorted
shape weakens the mesh, cre-
ates misregistration, leads to
probable moiré, and wont
transfer ink as well.
Unfortunately the tension
meter never really knows if
the opening is square, because
it is not designed or intended
to pick up the difference.
Below are details on percent-
age of elongation as a tech-
nique to be used in addition
to reading tension with a meter. There is an accurate method of testing to find
the final actual (tensioned) mesh count in either direction and that is a TPI
(thread per inch) mesh counter (refer to page 1.43 for photo). By laying the test
film on the tensioned fabric, you can evaluate whether or not your opening is
square. Simply backlight the screen as on a light table and lay the film on the
impression side. Rotate the film until interference fringes are precisely horizon-
tal. This moiré pattern will allow you to identify the specific mesh count.
AAvvooiidd AArreeaass OOff IIssoollaatteedd SSttrreessss
As elongation increased to achieve higher and higher tension levels, the need to
reduce stress in the corners became more obvious. This technique called soften-
ing, keeps the mesh from ripping but can be used to improve overall tension
levels and make them more consistent. Use a tension meter at an angle and slide
it from the corner to the center of the mesh. (see figure 1.38 on page 1.44).
Watch for any spikes in the reading. If the tension level of the mesh is higher
Choose one that:
1. Is as wide as the press will permit.
2. Is as long in the stroke direction as the press will
permit.
3. Is as robust as the frame design permits and the ten-
sion level requires.
4. Does not twist at its corners.
5. Does not bow inward.
6. Holds the mesh securely.
7. Maintains a square mesh opening at any tension
level.
8. Does not create isolated stress.
9. Permits off-contact for image tolerance.
10. Prints with zero mesh lag.
11. Helps equalize blade/mesh contact pressure.
Selecting The Proper Frame
How To
Instructions
PAGE
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Chapter 1
near the corner than it is in the center, that area will be the first to rip. (Refer to
page ___ for details). Further, if there are high stress areas then there are also
low stress areas. These are the ones that lose tension during the production run.
AAllllooww TThhee OOffff-CCoonnttaacctt DDiissttaannccee TToo AAcchhiieevvee IImm
aaggee TToolleerraannccee
The tension level of the mesh must be sufficient to allow an off-contact distance
that accommodates the tolerance of the images that you print (refer to page 1.49
for complete details). The off-contact distance is constrained by the minimum ink
well, screen tension and image tolerance. The first two are internal issues; the
third will be obvious to your customer. Tension and ink well should accommo-
date the tolerances that your customer needs. Tension must be high enough to
prevent image stretch and yet low enough to prevent reproportioning and fabric
degradation.
1. Square Opening-
Ideal Weave
2. Rectangular Opening-
Imbalanced Weave
3. Rectangular Opening-
Imbalanced Elongation
Figure 1.24 This drawing illustrates three possible mesh openings and counts. The far left illustration shows a mesh with
a square opening. This one would have the same mesh count for warp and weft and would by definition be ideal. The second
drawing shows a rectangular opening as woven. Its opening is not square so the count differs between warp and weft It will be
very difficult to tension this mesh properly. The third drawing shows a tensioned version of the rectangular weave it is unlike-
ly that you can achieve a moiré free screen that will not lose tension on press with this mesh.
Frame
l 10 N/CM
2
l 20 N/CM
2
8 25 N/CM
2
Isolated Stress
l 22 N/CM
2
l 20 N/CM
2
Isolated Stress Tension Loss &
Ultimate Burst at 25 N/CM
2
Figure 1.25 This drawing illustrates the problem of
isolated stress. Lets say that at the corner of the frame
the tension level is 10 N/cm² because the corner was
softened to prevent ripping during tensioning. Then we
check the tension a little further from the corner and get
a 20 N/cm² reading. Further from the corner again
and we peak at 25 N/cm² and still further we return
to 20 N/cm². This screen will most assuredly rip at the
25 N/cm² line. Once the isolated stress exists the ratio
of stress to strain (stretch to tension) diminishes. The
solution is to progressively soften the corners as described
on page 1.35. Drawing compliments of Screen
Graphics Magazine.
PAGE
11
. 31
PPeerrmm
iitt ZZeerroo SSccrreeeenn MM
eesshh LLaagg
Due to the off-contact distance, after the squeegee passes, the screen should
snap back at the same rate as the squeegee speed at the midpoint of the stroke.
This condition is called zero screen lag . Negative lag occurs with excessive ten-
sion, peel rates or off-contact distances. Positive lag (snap back slower than the
blade speed) occurs with the opposite settings and is far more obvious and equal-
ly destructive (see figure 1.26). If you have the proper tension for your press con-
ditions you will automatically get zero screen lag. If you have positive lag raise the
off contact, negative lag lower the off contact. This is a temporary fix only, to be
used when you are on the press. To eliminate the problem next time you tension
screens (press condition remaining constant) for positive lag, increase tension lev-
els, for negative lag, decrease tension levels.
EEqquuaalliizzee CCoonnttaacctt PPrreessssuurree BBeettwweeeenn BBllaaddee AAnndd MM
eesshh
It is not in the purview of the tension level to singularly reach the targeted cpn-
tact pressure. The inkwell must be large enough to allow equal pressure between
Screens
a. Positive Lag
b. Zero Lag
c. Negative Lag
Figure 1.26 illustrates the range of
lag possible just by varying the off-
contact distance. The top drawing
shows the frame too close to the
platen and the result is positive
mesh lagthe screen will tend to
stick to the platen and not snap
back. It will be particularly notice-
able in the central areas as mesh
marks or will force you to use thin
ink. The middle drawing shows the
ideal, as the stroke continues the
speed at which the screen lifts is
precisely the same as the squeegee
speedthe condition is zero mesh
lag. The bottom drawing shows
negative mesh lag. It is due to over-
tension for the off-contact distance.
It will be obvious by ripped and
burst screens.
PAGE
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. 32
the blade and mesh across the
entire surface of the blade.
Unfortunately this size
inkwell is an anomaly it
doesnt happen too often. It is
worst when the tension
extremetoo high or too low.
LLooww EElloonnggaattiioonn SSccrreeeenn
FFaabbrriicc
With the introduction of
low elongation screen fab-
ric, all of the rules in the
print shop changed. Low
elongation is the generic
name for mesh that has been
manufactured to have a high-
er modulus of elasticity. That
means that with less stretch,
you get higher tension and
supposedly less tension loss
prior to and on press.
You should be aware that
low elongation is not a specific entity; it is only a tendency for a screen mesh
to develop more tension with less elongation. There are some fabrics for exam-
ple, that develop high tension with 2% elongation. While other manufacturers of
the same count may not achieve that tension until 5%. At first blush, the first
one seems better, more for less, right? But it may not be so in the real world.
The faster the fabric develops tension the more likely it is to suffer from two
issues: First it may have an imbalanced SS after tensioning because of a smaller
window of opportunity. Second that it may tend to lose that tension faster on
press. Yes with retensionable frames, you can just pull it back up but the only
thing that gets satisfied with this doctoring is the tension meter. The mesh
opening will become erratic leading to progressive moiré and radial moiré. You
will want to compare the rate that the mesh develops tension to the variance of
the warp and weft SS curve (refer to figure 1.15 and page 1.16 for complete
details).
Chapter 1
To check for isolated stress follow these steps:
1. Tension your screen as normal.
2. Put the meter in the corner, facing
outward.
3. Angle the meter approximately
45°.
4. Slide the meter slowly toward the
center of the frame.
5. Tap the meter periodically if nec-
essary.
6. Observe for any high points in the
tension.
7. If so these are areas of isolated
stress.
8. They should be relieved before
you go to press
.
Checking For Isolated Stress
How To
Instructions
PAGE
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. 33
LLEE VVeerrssuuss CCoonnvveennttiioonnaall FFaabbrriicc
Because the operator can achieve tension with less elongation,
even if the SS curve is not balanced, it will perform better
than conventional on press. There is less need to stage tension-
ing, (relax, tension, relax and so on) due to less elasticity of the
fabric. The mesh manufacturers as well as the SPTF have stud-
ied rapid tensioning and the findings indicate minute advan-
tages in retention of the initial tension levels with staged ten-
sioning.
The rule of thumb is that you will lose about 20 percent of
the original tension in the first 72 hours and then the fabric
will stabilize. With less elasticity you will want to run closer
off-contact distances (refer to figure 1.39 on page 1.50 for
details) and in doing so, you will see even more stable tension
on press. Finally, the need to retention is reduced for all of the
above reasons. All in all low elongation fabric is the only way
to go but two caveats: You will still need a fabric that has a
balanced SS-curve and it should be properly handled on press.
Abuse on press with squeegee pressure and off contact
extremes can fatigue and destroy even the best LE meshes.
TTeennssiioonniinngg IInnssttrruuccttiioonnss
Building a screen is the foundation of your screen print
process. You cannot correct the shortcomings with the stencil
or on the press. This is the time to be sure you have a good
system in place and follow it, even when you are under the gun
to get production out the back door. Any corners cut in this
process will come back to haunt you during the press run.
Review the section on the cost of press downtime (see
page6.35) if you have any doubts.
FFaabbrriicc OOrriieennttaattiioonn
There are a lot of fish stories about which direction warp and
weft should be orientated. Here is the real world truth: If the
fabric is SS balanced with one and a half percent or less vari-
ance, (refer to page 1.29 for details) and the frame is square,
then it would never matter. The practice of orienting the weft
threads parallel to the stroke direction was to hedge ones bet
with a fabric that was imbalanced. You would have additional
WWiitthh llooww eelloonnggaattiioonn ffaabbrriicc yyoouu
sshhoouulldd eexxppeecctt::
v Higher tension with lower
elongation.
v Less need to stage
tensioning.
v More tension stability overall.
v Less tension loss on press,
with proper press conditions.
v Less need to retention.
YYoouu mmuusstt iinnssiisstt uuppoonn::
v Consistent mesh counts for
warp and weft.
v Balanced SS-curved before
and after elongation.
Screens
Lo
Lo
w Elongation
w Elongation
PAGE
11
. 34
stretch in the warp direction, so to stretch it parallel to the
squeegee direction insured more imbalance in the mesh open-
ings and excessive stretch in the image.
We highly recommend that you use a balanced fabric and as
for orientation, there are two avenues open: If you are a high
tension advocate, real world mesh will more often than not
have greater elongation in the warp direction. So you will want
to run the warp parallel to the long sides of a Challenger or
Gauntlet frame. (see figure 1.29). The reason is that the short
dimension of a nominal automatic tee shirt frame resists
deflection, about three times as much as the long side. For the
same reason, you will want to tension the warp first.
If you run at reasonable tension levels 25 to 35 N/cm², and
you select balanced fabric, run the way that will give you the
greatest yield from the bolt. But under no circumstances
should you ever swap orientations. Once you pick an orienta-
tion, stick with it, any other antics and you are simply asking
for trouble and inconsistency.
TTeennssiioonniinngg TTiimm
ee
With the introduction and near universal acceptance of low
elongation fabric the need for all the staging required for con-
ventional fabric no longer exists. You will find that you reach
diminishing returns for waiting beyond the time it takes to ten-
sion. Low elongation fabric will lose a nominal 20 percent of
its tension level in the first 72 hours. (This percentage will vary
according to manufacturer mesh specifications and user proce-
dures).
Two options will allow you to rapidly tension low elongation
fabric tee-shirt screens. First, stage them in groups of five to
ten. Progressively tension these in stages; initial, second and
third stage for final tension. This gives the filament time to
equalize and prevents a shock to the system. Second, you can
accelerate the relaxing process by applying a weight to the cen-
ter of the screens. A steel plate ¼ thick by the image dimen-
sions can be laid upon the screen between tensioning. This
weight applies far less force than the squeegee blade and it will
cause the initial drop in tension to occur in far less time.
If you are prone to running mesh
on a bias we can probably save
you a lot of mesh money. Refer to
Chapter Two on stencils and you
may find all of your detail needs
are met by proper stencil meth-
ods, not by the mesh count or
usually its orientation. If you do
need to bias as in the case of fine-
ly woven goods with a series of
thin straight opaque lines, angle
the mesh to the *Alpha as shown
on page 1.19. You will want to
insure that your fabric has close to
the same count for both warp and
weft. This square mesh geometry
allows you to run lines (or rows of
dots) with a minimum of interfer-
ence. Further, look at the flatness
of the fabric that you are using.
The stencil can be far flatter and
hold far better detail if the mesh
starts out flat. Finally, look into
the thickness of thread that you
are using and select the thinnest
one for the count that you need.
*The Alpha angle ranges between
68 and 74 degrees, depending on
the mesh count, thread diameter
and percent of elongation.
Chapter 1
Mesh On A Bias
Mesh On A Bias