Clarke Joe, Lynch Colleen C. M&R Professional Screen Printing Series

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presses that characterize the tee shirt printer, higher durometer (80°) blades are

very popular particularly for highly detailed printing such as four-color process.

In the case of white ink (all else being held constant) a softer 70° deflects more

readily and applies more pressure to the ink, which increases deposit and there-

fore opacity.

Age, use, sharpening and labeling all have a serious impact on the durometer of

the blade. The first three factors cause an increase in the durometer of the blade

versus its original manufactured and aged hardness. The last factor, labeling

includes variability from batch to batch and any initial aging after the measure-

ment was taken. Some of the manufacturers have a tolerance of three up and two

down. That means that your 70° durometer blade may be between 68° and 73°

hardness. Without someone, you or your supplier measuring the blade prior to

use, there is no way to tell the precise durometer.

Length  is the measure of the blade from end to end. A typical tee shirt blade

is 16 inches to 20 inches in length. This facet of the blade is grossly overlooked

and it affects downtime more than any other aspect of the blade. The blade is

never too short or the entire image would not print at the edges, so it is always

too long, it is simply a matter of how much too long. To buffer the affect on the

mesh you will want to radius the ends with a grinding wheel. The goal of this

exercise is to increase the area of the blade in contact with the edges of the

mesh.

The length should be a nominal ½ inch longer than the width of the image.

Shorter becomes impractical and gets dangerously close to the edges of the

image. A longer blade eats up the ink well, forces more blade pressure, prints less

consistently and wears out the mesh sooner. To accommodate a longer blade can

Press Calibration

Length

Radiused

Ends

Figure 4.25 This shows the front view of an M&R Squeegee with blade. The length is indicated on the illustration and

note that the edges are radiused. This is a cost effective step to buffer the tension increase at the end of the blade, nearest the

edges of the mesh. The worst case is if the blade is a short height, high durometer its sharp ends will rapidly rip the mesh.

The length affects the ink well, which in most tee shirt printing applications is too small to begin with.

PAGE

. 30

be impractical. The options are to increase frame size or to increase tension and

reduce the off-contact distance. For real world usage, standardize a length for the

largest image you will run on a given frame size, then calibrate for those condi-

tions. When you use a shorter blade, the benefits will be greater both in produc-

tion as well as in quality. It can be used with less pressure and will print more

consistently from end to mid-point.

Thickness  the standard

blade for the tee shirt printer

is 3/8 inch (0.375) thick.

This dimension is more evo-

lutionary than

revolutionaryit just evolved

as user friendly. For example

at half the thickness, a 3/16

inch blade would have to be

extremely hard or unrealisti-

cally short to resist buckling under a normal load. Such a blade would be nearly

impossible to set on a multicolor press and by now the 3/8 inch size is the de

facto standard. A considerably thicker blade would use more polyurethane and at

a given durometer and unsupported height could be deemed inflexible. In addi-

tion a thicker blade would not fit all the holders in the market place. And a blade

that is simply thicker presents no advantages to warrant a new holder. Even the

laminated blades conform to the 3/8-inch profile.

Height  most of the blades in the tee shirt printing industry are cut or cast to

2-inch height. With the Gauntlet or Challenger squeegee holders, you are clamp-

ing ¾ inch in the aluminum holder. If your original height differs, consult figure

4.29 on page 4.35. Note that there is no assurance that a blade inserted into a

holder is precisely true and straight; it should be sharpened in the holder to be so.

Of course as the blade is sharpened the overall height is altered but this aspect is

covered in Unsupported

Height (refer to figure

6.27).

Shape  (illustration)

there are a variety of

shapes intended to help

solve some problems

unique to your printing

particulars. You should

Chapter 4

Figure 4.26 This

figure shows the thick-

ness of the blade.

Thickness in most

graphics applications is

0.375 or 3/8-inch. A

thicker dimension will

change the flexure of

the blade thicker will

flex less.

Blade

Thickness

Figure 4.27 This illustrates

the height and the unsupported

height of the blade. Most of

the stock cuts are available in

2-inch original height. The

standard unsupported height in

the M&R holder is 1.25-inch-

es. A greater unsupported

height causes the blade to flex

more (refer to figure 4.29 for

details.)

Blade

Height

Unsupported

Height

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realize that in order to truly take advantage of the shape and edge of the blade,

that your press and screens must be accurately calibrated.

Particularly given the tee shirt printers coarse meshes, high tensions, small ink

wells and multi-colored presses the beveled and chiseled edges wear so fast that

you may witness a change in shape even a short press run. In principle they offer

advantages but in the real world they are troublesome and too inconsistent.

The bull-nosed or total radius blade prints with lower pressure due to its greater

contact area but at virtually any initial angle. It has a place with coarser screen

mesh and low yield stress inks (refer to figure 1.12 for details). It helps the mesh

last forever but all coarser fabrics are durable so this is not necessarily an advan-

tage. For the mainstream tee shirt printer who has evolved to finer detail and

screen mesh, this shape has little utility.

The square shape is our recommendation as it has enough material at the print

edge to last a long time, it allows a variance in deposit with a proportional change

in the initial angle and it can be sharp or dull as needed for the widest range of

screen mesh at top printing speeds.

Construction  of squeegee blades fall into two

catagories; composites and laminates. Laminated

blades are those that are composed of two or

more different durometer materials may have

some practical applications for the tee shirt print-

er. If you compare for example a 60/90/60 to a

straight 70 blade, the laminate will have superior

durability and resistance to swelling. However

there is no magic flexibility or super-durometer

factor with these blades. In fact their spring rate

could be simulated with a single durometer blade.

Cast blade edges  create a durable edge, which

is nice until you sharpen it, unfortunately to have

a true edge even a new blade should be sharp-

ened. You dont want to mistake convenience for

quality or productivity.

Composite blades  have a higher durometer blade that fits into the holder

and extends near the substrate while the lower portion is softer and more flexible.

The concept is that you can print with the edge of the blade, at the highest pres-

sure without the blade buckling at the ends or on the bed. There are a few con-

Press Calibration

Figure 4.28 Shows the Short Cut

squeegee sharpener from Encore

Engineering. The shape of blade that we

recommend is discussed on page 4.30 and

is quite simple. It is a square edge with an

edge for finer meshes and the other edge for

coarser meshes. There are times when a

specific shape can assist in solving a prob-

lem but the blade is not often the cause of

the problem so lets not make it the solu-

tion.

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. 32

siderations if you should choose to use this blade. First it

should be kept sharpened and once it has worn too low it

should be discarded. It should not be inverted top to bottom

in the holder as it will abuse the screens and be difficult to set

on press. Second unless you can put a slight radius on one

edge, you will probably find that it is best on light colored gar-

ments. Third it is made to run well on calibrated presses, the

worse your press is the less you will like the composite blade.

Composition  not all polyurethane is created equal and

there are a variety of grades, casting methods, aging processes

and cutting methods used to manufacture the material. These

variables affect its solvent resistance as well as ability to sharp-

en and hold a sharp edge through abrasion of rough meshes.

The knitted tee shirt hides a lot of flaws but remember that

when you are printing on an underbase, you are now a vinyl

sheet printer and the edge is crucial and its results are obvious.

It is wise to shop for the highest quality blade available; it will

pay for itself many times over.

None of us have troubles with the easy work but when the

image becomes very demanding we hope that you have devel-

oped proper habits or frustration goes up and profits go

down. This is never truer than when at a glance on the press

you see the following for a four-color process image: A short,

hard blade on the black ink to force it through the screen. A

softer blade folded in half to get the white through the thick

coarse mesh. Since the yellow is first we ran a shorter squeegee

length, about 10-inches. A long stroke is used on all but the

one color of the image and a nearly flat blade to pamper the

magenta ink-or the image looks to reddish.

If the blades are different heights, the pressure on the mesh is

radically different. If the angles change, the pressure on the

inks differ as well and you may be forced to run an excessive

off-contact distance. The black screen will stretch and the yel-

low wont and there is no way to calibrate a press for these set-

tings. You will see color variance from area to area and certain-

ly from shirt to shirt. You will need to constantly fiddle with

the settings because none of these extremes is conducive to

consistent printing. Finally if you see registration problems, it

If you have an adequate inventory

of squeegees, the configuration is

an option; you wont be forced to

run the wrong blade. What con-

stitutes the wrong blade is one of

extremes used in a conventional

setting. If you are a typical tee

shirt printer you will want to

avoid the extremes.

If the blade is soft below 70°

durometer then it should be

shimmed or short. If it is long it

will simply fold over and force

you to run excessive off-contact

distance and eventually your mesh

will burst in the center. While it is

printing it will buckle under the

load of the air cylinders and ten-

sioned mesh and your prints will

be inconsistent and platen deflec-

tion will be very high. You will

want to run such a blade very near

vertical and it is a delicate balance.

If the blade is hard, over 80°

durometer then you will not want

it to sharpen it too far. The differ-

ence in deflection from a 1.25-

inch to a 1.00-inch blade is a 2:1

ratio. If your blades are getting

stubbier, watch out for image

stretch and you will definitely

want to do two things: run the

shorter ones as a group on a par-

ticular imagedont mix them up.

Second you will want to run them

at a more vertical angle. Be sure

that the angle is not so steep that

it causes positive mesh lag.

Chapter 4

Blade Height and

Def

lection

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. 33

is not the press, rather the way it is set-up. If you have ever seen this, read about

squeegees.

PPrreeppaarriinngg TToo PPrriinntt

Once you have purchased the blade you will insert it, sharpen it for a true edge

and put it into the press. You will set the initial angle and the skew if possible. It

is important that you are aware of how these properties interact and that is the

goal of this following segment.

Unsupported Height  as the graph below shows, the change in the unsup-

ported height has a dramatic affect on the deflection of the blade. If you are

using a grinding wheel sharpener, small amounts can be removed without a

severe change in the height. Pay particular attention if you use a slicing squeegee

sharpener, even a 1/16 inch (0.063) slice has a dramatic impact on the deflection

of the blade. As the blade becomes shorter, all things held constant you may see

the following:

1. Increased screen stretch.

2. Greater fabric fatigue and loss.

3. Reduction in ink deposit.

4. Sharper detail.

5. Greater variance platen to platen.

6. A need to run a shallower angle.

When printing a highly technical job such as four color process, be sure at least

that the blades are true and of the same unsupported height and durometer. And

they should be the same length, as this will insure the same ink well and squeegee

pressure. Also if you are not sharpening your blades, the odds are that you never

do have a true edge. The polyurethane never fits quite perfectly and needs to be

dressed to have a consistent height. Change in height is one of the main reasons

that you have to incessantly adjust the angle of the blade. As it gets shorter, there

is reason to run the blade at a shallower angle so that it has a sufficient contact

area and the pressure is not too excessive.

True Edge  a brand new blade should have a true edge but there is no sure

way to insert a blade into any squeegee holder and expect it to be straight. Once

again look at figure 4.29 and you will see that as the unsupported height changes,

the percentage of deflection changes dramatically. And you must use enough

pressure to cause the hills and valleys of an untrue blade to conform to the print-

ing surface.

Press Calibration

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. 34

Once you insert the blade and gently tighten it in

position, we recommend a grinding wheel to true

the edge. (photo) A few passes of the wheel and

the edge is straight and true. It is this step that

makes the press calibration, mesh tensioning con-

trols and squeegee settings worthwhile. If you have

purchased a molded edge blade your intention is to

keep from sharpening but you may find it impracti-

cal to insert the blade so that it is true without

sharpening.

Initial Angle  If you align the pivoting bracket

of the Challenger or Gauntlet press vertically, it is

easy to set repeatable and to check for consistency

(see figure 4.31). This setting puts the blade at an

angle of attack of approximately 12°. This angle is

to be seen as the baseline setting, it can work well

for most printing from opaque whites to the finest process color.

You may need to decrease the angle if the fit of ink and mesh is poor. If the ink

Chapter 4

Height & Deflection

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

100% 88% 78% 68% 59% 51% 44% 37% 31% 26% 22% 18% 14% 11% 9% 6%

Percentage Deflection

Unsupported Height

Figure 4.29 Shows the relationship between the deflections of a blade that starts out at 1.25 inch unsupported

height and the relative deflection as the blade is sharpened or sliced. For example if you are slicing 1/16-inch (0.063)

increments the new blade will flex 86% of the original. If you slice another 1/16 inch the blade will only deflect 73%

as much as the original. If you thought it didnt matter, it is the same as saying that the durometer doesnt matter.

Figure 4.30 This is from Encore

Engineering and shows the Diamond

Maxi squeegee sharpener. We prefer the

grinding wheel because it takes off only a

small amount of blade per pass. Also it

fits our recommendation of different radii

on opposed sides of the blade. This tech-

nique allows you to quick adapt to fit the

mesh you are using and regulate its pres-

sure drop (refer to page 4.37 for details).

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. 35

is too thick and tacky, if the mesh geometry restricts the ink flow, if the blade is

very rigid or if the tension is too high for the quality of press calibration you may

find that you need to run a lower angle. The benchmark for decreasing the angle

for existing press conditions is poor transfer or inconsistent prints in sequence.

You may need to increase the angle if the flexure of the blade is too high. That

includes any combination of excessive unsupported height, low durometer or

excessive force (which should not stem from a pneumatic press). The benchmark

for an increase for existing press conditions is positive mesh lag (refer to page 4.6

for details).

This aspect of the squeegee settings is fast and easy to adjust but should not be

overdone. It can be used as a correct all for a variety of problems in screen and

ink areas. Remember that if the ink or screens are broken and you choose to fix

them with the squeegee, there is always a trade off. It may be in speed, screen life,

print quality but the price will be paid. Better to use the angle adjustment to get

through the job on press and then address the real problem.

Skew  this is a feature popular on our graphics presses that serves to alter the

pressure drop of the mesh as the squeegee passes over. The blade runs substan-

tially parallel and perpendicular to the screen mesh which is stretched straight

on the frame. As the squeegee passes over the mesh it hits the parallel threads

like your tires pass over a speed bump. But now address the speed bump from an

angle; the passage is mild in comparison. This change in the pressure drop (refer

to page 1.14 for details) allows you to adjust the amount of ink without altering

image registration or squeegee pressure. It is omitted from tee shirt presses due

Press Calibration

90° Angle

75° Angle

Figure 4.31 This llustrates a side view of

a squeegee and floodbar and their relative

positions. The floodbar is set at a 90° angle

and the squeegee is set at a 75° angle. The

only reason you should ever move the angle of

the floodbar is to alter the size of the ink well

(between blades) or to accommodate small

screens (which we never recommendrefer to

page 1.22 for details). This squeegee (from a

Challenger II press) is shown at an initial

75° angle and it serves as a good standard.

As the angle of the blade is reduced, the pres-

sure on the ink increases, the pressure on the

mesh and the platen drop.

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to size restrictions and the degree of skewing required having a significant affect

on the image.

OOnn PPrreessss PPeerrffoorrmmaannccee

Deflected Angle  because the blade is flexible under the load of the press, it

bends when it hits the rigid surface. This bending occurs lower, closer to the plat-

en with a more pliable blade. The softer blade creates more of an arc and the

harder blade creates more of an angle which occurs closer to the squeegee hold-

er. Further under a fixed amount of force as the initial angle is set lower, the

deflected angle becomes incrementally lower as a factor of the initial angle. For

example if a 75° initial angle causes the blade to deflect to 65° there is a 10° dif-

ference. Now take the initial angle to 70° and the deflected angle may become

55° or 15° less than the initial angle. As the initial angle drops the difference

becomes greater.

It is this final (deflected) angle that applies pressure on the ink. A blade that is

soft must run at a more vertical angle or it will buckle under the load of the air

cylinders. The deflected angle of a more rigid blade will tend to more closely

approximate the initial angle setting. The purpose of the angle is to apply enough

pressure to the ink to inject it into the mesh openings. If the angle is too high the

blade wont apply sufficient force to the ink and a lower angle wont allow zero

mesh lag.

Chapter 4

Figure 4.32 Shows the no-tools adjustment of

the squeegee and floodbar angles. If you need a

lot more inkwell and have an adequate screen

frame size refer to page 1.14 for details. Note

that a change in angle alters the relative pressure

of the blades on the mesh. Before you adjust

angle, first estimate the impact that the change

will have on the pressure of the blade on the

mesh and compensate for it. Then make your

angular adjustment and recheck the pressure. For

example if you want to raise your squeegee angle

because you have witnessed positive mesh lag

(refer to page 4.6 for details.) first raise the

blade. Then adjust the angle and then balance the

pressure. This approach to squeegee and floodbar

settings will save you a lot of ripped screen mesh.

Squeegee Folldbar Angle Adjustment

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. 37

Edge  If you are not accustomed to sharpening your blades you should take

advantage of a better blade for control of coverage and image accuracy.

Sharpening is the term used but perhaps dressing is more accurate. Some sharp-

eners allow you to select the edge that you want on the blade on one side or

another. The knit shirt is very forgiving but if you print and flash you have just

become a vinyl sheet printer. Technically the flaws occur when printing directly

on the garment but if you cant see them then who cares? but printing on the

plastisol underbase is a different story.

You may want to prepare your blades with one sharp edge and the other dull for

controlled ink deposits on the widest range of mesh counts at highest blade

speeds. In the interest of making an invulnerable blade, some manufacturers have

overlooked the need to dress the blade. The solvent resistance can be so high that

sharpening causes the blade to fracture or crumble. Such blades are not needed

for plastisols printing and their benefits area outweighed by their resistance to

sharpening.

A sharp blade is recommended for 160 meshes (per inch) and higher. It offers

highest pressure on the mesh and permits image control and the application of

ink pressure by angling. For the highest resolution and edge acuity a sharp (and

straight) edge is imperative. You will find that this edge more so than the others,

requires a calibrated press. AS the counts get higher say in the 420 and 460 area,

blade sharpness becomes even more critical.

A dull blade is recommended for 85 to 150 mesh counts. The sharp edge either

creates a serious pressure drop or forces too much contact area between the

blade and the mesh. The radius blade edge will hydroplane and fail to clear the

mesh on this range of counts.

A radius blade is recommended for 85 meshes and lower will allow you to reduce

the pressure drop characteristic of the screen mesh without running at a skewed

angle. This in turn will give you a greater ink deposit and we presume that

deposit is why you are on such a coarse mesh. Further the radius will permit

higher speeds at lower pressures. Angling this blade will increase pressure on the

ink but in reasonable doses angling will have little affect on the pressure on the

mesh. You will find that with a low yield stress ink (refer to page 3.14) you can

run much faster with this type of edge. As the edge distance increases the time of

pressure applied to the ink increases proportionally at a fixed velocity. And addi-

tional time is required on these coarser mesh counts.

Flexure  is the facet most people want to know when they say durometer.

Press Calibration

PAGE

. 38

This is the tendency of the blade to buckle or deflect under a given force. It is a

composite of the durometer, the physical dimensions, the edge and the shape of

the blade. A 3/8 inch thick, shorter than 1.25 inches, high durometer (over 75°)

blade with a square shape and a radius edge would have the lowest tendency to

flex.

There are three distinct possibilities of blade flex that should be understood; first

the blade is high flexure (it flexes very easily) this one will buckle at the ends and

deflect to a lesser extent at the mid point. Second is the other extreme, a low

flexure (very rigid) blade that does not deflect against the mesh or on the platen.

Third is a medium flexure blade, which does not deflect against the mesh but

deflects a controllable amount as the mesh reaches the platen. This third possibil-

ity is our goal.

It is incumbent upon the print engineer to select the proper ingredients that will

provide the proper flexure for the productivity, press conditions and image needs.

Our instructions are here within and our recommendations are below.

Buckling  is a result of the mesh overpowering the blade and is typical in set-

ups where the screen tension is very high. Instead of simply deflecting, the blade

buckles at its ends near the perimeter of the frame. Buckling causes a greater

contact area at the ends of the blade than in the center. The effect of buckling

must be negated in order to have a print that is consistent edge to edge.

Unfortunately the result is that the blade is deflected evenly on the press bed and

this factor is death on screen life. This is why you must be certain that the blade

is centered left to right on the frame. There should be neither buckling nor

deflection as the blade pushes the mesh to the substrate.

Pressure  there has been a lot of confusion as to the cause and effect of pres-

sure on the squeegee blade. First of all the press applies force in pounds pressure

is that force applied to a specific area. If the pounds were applied to a square

inch then a force of 178 pounds would be equal to 178 psi (pounds per square

inch). But if the area was doubled to two square inches then 178 pounds of force

would be distributed over twice the area and the pressure would be 89 psi.

When the blade deflects, the contact area is increased and since the area goes

upthe pressure goes down. It is far preferable to run a more rigid blade at a

lower angle (if necessary for high ink tack), than to run a more flexible blade

with excessive deflection. On press, there are only three pressure possibilities: too

little (which never happens or the ink wont go through), perfect (please!), and

too much (which is always the casebest avoid too much squeegee pressure).

Chapter 4

Clarke Joe, Lynch Colleen C. M&amp;R Professional Screen Printing Series

Подождите немного. Документ загружается.

Clarke Joe, Lynch Colleen C. M&R Professional Screen Printing Series