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

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PAGE

. 39

In the real world we moni-

tored a specific setting to ver-

ify that the pressure can not

only be reduced but in fact

can run very low at high pro-

duction speeds. The job was

120-line (per inch) four-color

process on black tees. A cus-

tom calibrated Challenger

press with 25 N/cm² screens,

the blade ran at least 24 inch-

es per second, 80º durometer

square sharp blades at 1.125-

inches unsupported height,

initial angle of 75° from verti-

cal, 0.082 off-contact dis-

tance and a 3-inch inkwell ran

at 775 pieces per hour with-

out incident. The underbase

white was flashed at 950° F.

with a panel height of 3/8-

inch. The part that may interest you is that the force on the squeegee was a

mere 30-PSI. The press calibration techniques, blade selection and settings as

well as screen and stencil specifications are all detailed in this manual. The actual

print is shown on page 3.18.

There are several areas where pressure is a significant factor in the printed result:

pressure on the ink, pressure on the mesh and pressure on the platen. M&R

presses control squeegee pressure pneumaticallywith air pressure. Since air is

elastic, it senses any variability in resistance from the mesh and the platen. This

makes the blade quite easy to set, it is forgiving but for the highest quality and

highest speed printing, you will need to properly set the screen, press and blade.

Detailed explanations of these three facets of pressure follow.

Pressure on the ink is a function of squeegee speed and the deflected angle of

the blade. This pressure serves to reduce the inks viscosity and inject ink into

the mesh openings. As the blade speed is increased the pressure on the ink is

higher and depending on the ink speed may alter the deposit or not but

nonetheless the pressure is increased. As the angle of the blade is lowered the

pressure on the ink increases rapidly once the ink transfer is total and the cover-

Press Calibration

Eliminate pressure on the platen by following these

steps:

1. Calibrate the press.

2. Set off-contact based on image tolerance.

3. Set the tension level based on off-contact distance.

4. Use ink with a low tack level.

5. Select a blade that shows no primary deflection.

6. Set pressure on Challenger II if available.

7. Angle blade to absorb additional pressure if needed.

8. Select ink with a low tack level.

9. Monitor results by observing for positive mesh lag.

Eliminating Pressure

On The Platen

How To

Instructions

PAGE

. 40

age sufficient, any more angle is detri-

mental. It will lead to excessive

deposits, positive mesh lag and loss of

edge acuity. You will find that for

high tack level inks it is necessary to

angle the blade just to get a propor-

tional drop in the viscosity and allow

the ink to transfer. The proper reme-

dy is not angle or blade pressure on

the ink but to fix the ink itself.

Pressure on the mesh, at the point that

the mesh contacts the platen, is a

function of the blade flexure, the off-

contact distance, and screen tension

and ink well size. Also known as con-

tact pressure, it serves to create a

consistent deposit both side to side,

over the stroke length and for succes-

sive prints. You will have troubles

with inconsistent contact pressure if

and when you have a high angle

blade, high tensioned mesh with a

small inkwell, printing on an uncali-

brated press.

Chapter 4

75° Squeegee Angle

Figure 4.33 This illustrates the squeegee angle on a

Challenger II press set at a 75° angle. If the angle is to be

adjusted lower, there are two constraints, do not encourage pos-

itive mesh lag, and do not trigger primary blade deflection.

Otherwise a lower angle will increase the shear rate on the ink,

reduce the pressure on the mesh and reduce the pressure on the

platen. Note: air pressure regulators and gauges are optional

on all M&R presses.

Position On Screen

Increased Pressure On Mesh

Figure 4.34 This illustrates the

increase in pressure between an un-deflect-

ed blade and the screen mesh. Even if the

tension is precisely consistent across the

surface of the mesh in its static state, the

increase is greater nearest the ends of the

blade and the edges of the mesh. We rec-

ommend a radiused blade end to buffer

the peaks of pressure increase but even

they will not eliminate the variance.

PAGE

. 41

You will have troubles with low contact pressure if and when you have high

squeegee force (pounds), a soft, tall blade, low screen tension and excessive off-

contact distance. This blade would fold over and the contact area would be

greater so the pressure (force over area) would be lessened. Ink transfer will be

poor in the central areas as well as at the edges of the image.

You will want to run high contact pressure and the best approach would be a 10°

to 15° near vertical angle 1.25-inch blade height maximum a 70° to 80° durome-

ter, 3/8 inch thick blade with a square, sharp edge, as large an inkwell as possible

on a sufficiently tensioned screen with proper off-contact on a calibrated press.

This squeegee blade would not deflect until the mesh reached the platen and pro-

vide a minimum area of contact and therefore the highest pressure.

Pressure on the platen is a result of one or more problematic settings or conditions.

It causes excessive changeover to clean the platens or to remask. It causes the

loss of table adhesive due to the same ink build up. It wears out the screen and

causes bursting in the center of the mesh. It causes a loss of opacity on darks

and smearing and blurring when printing on a flashed under base.

If there is primary deflection where the blade deflects before it reaches the plat-

en, the contact area will most often be irregular. There will be more contact area

at the edges of the mesh. Finally this pressure imbalance will be remedied by the

continued folding of the blade even further on the platen. If the press is not cali-

brated, excessive platen force is guaranteed.

Challenger II has a pressure gauge so that you can prevent this occurrence. The

stock pressure on a Challenger at line pressure is 178 pounds. It is most often

Press Calibration

Side

Inkwell

Endpoints

Midpoint

Mesh

Figure 4.35 This illustrates the need to create pressure between the midpoint of the blade and the screen mesh often leads

to unwanted pressure on the platen. If there is primary deflection (before the blade and mesh have reached the platen) there

will be an inevitable need to deflect the blade further on the platen. This is necessary to make the contact edge of the blade

consistent across the surface of the mesh. Otherwise the deposit will be inconsistent.

PAGE

. 42

necessary and depends on your ink well size, tension level and

off-contact distance. Particularly with high tensioned mesh,

uncalibrated presses and small inkwells, the high force is neces-

sary to get the edge of the mesh into contact with the platen.

(The center of the mesh follows accordingly but the force is

needed at the end of the blade and the edge of the mesh.) By

applying the details in this manual you can dramatically reduce

the force on the blade and enjoy extended screen life, sharper

quality printing at higher speeds. Whenever you adjust pressure

or make any other squeegee adjustment, be sure that you have

checked the new position on a few platens particularly if your

system is not in calibration. A calibrated system includes:

screens of the same relative tension, blades of the same flex-

ure (height, length, shape and durometer) and a press that has

been set according to the instructions on page 4.15.

Squeegee blade pressure on the platens causes all sorts of

problems. First since the garments are mostly air space, it

causes seepage and puts ink on the platen where you will have

unnecessary downtime to clean it up. Simultaneously it ruins

the effect of the table adhesive which causes rejects and slows

productivity. Since it is driving the ink into the garment you

will see less opacity on dark garments.

More subtly it the major reason that screen mesh bursts in the

center of the screen. The cost of this downtime mid-run is

staggering. Meanwhile such a setting causes image stretch in

the print direction. You can witness smearing and blurring on

the far side of the image as the squeegee passes. Along with

these maladies you will find that overprint colors mottle and

your ink build-up is worse in the center of the screen. The

pressure from the blade should be no higher than the pressure

from the tension and off-contact at the edges of the mesh and

the end of the blade. Until you fix the problem you cant be

sure if it is the ink.

TThhee SSoolluuttiioonnNNoott TThhee PPrroobblleemm

Although the squeegee may not be the cause of all the prob-

lems that it is asked to resolve, it is the tool of the moment

on press, so we will help you adjust accordingly.

Chapter 4

LLooookk ffoorr tthhee ffoolllloowwiinngg cchhaarraacc-

tteerriissttiiccss wwhheenn sseelleeccttiinngg yyoouurr

ssqquueeeeggeeee bbllaaddee::

1. Polyurethane for resistance to

wear, ink and wash-up.

2. 70° to 80° durometer depend-

ing on ink pressure needed.

3. Square profile, 3/8-inch X 2-

inch, with a 1.25-inch unsup-

ported height..

4. Edge should match the mesh

used. The coarser the mesh the

larger the radius.

5. Aproximately 1.00-inch longer

than the maximune image

width.

IInn aaddddiittiioonn ffoorr pprroocceessss pprriinnttiinngg::

1. 80° durometer.

2. 3/8-inch X 2-inch with a 1.00-

inch unsupported height..

3. 10° to 15° angle

4. All blades the same height and

length.

5. Set all blades at same pressure

with optional air regulators..

6. 20-inches per second speed.

DDoo tthhee ffoolllloowwiinngg ttoo pprreeppaarree

yyoouurr ssqquueeeeggeeee bbllaaddee::

1. Sharpened before use for a

true edge.

2. Ideally prepare an identical set

of back-up blades for your

press run.

Tips On Squeegee

Blade Selection

PAGE

. 43

1. Compensate for positive mesh lag. The real way to fix

the problem is to reduce the ink tack, increase the off-con-

tact distance or increase the screen tension but for the

moment, raise the blade slightly, make the angle more verti-

cal and increase the speed. Be cautious if your press is not

calibrated the blade may not print on all platens. The con-

tact area was too great and the ink is recovering its body

before the blade passes.

2. Correct for image stretch. The real way to fix the prob-

lem is to reduce the ink tack, increase the off-contact dis-

tance or increase screen tension but for the moment, put a

scrap shirt on the press and reduce the squeegee pressure.

If it brings the image closer to in register then raise the

blade further and set a more vertical angle and apply pres-

sure gingerly if needed. Be cautious if your press is not cali-

brated the blade may not print on all platens. The off-con-

tact was too low for the combination of squeegee force and

mesh tension.

3. Clear the ink from the mesh. This is a job for larger

mesh openings an off-contact distance check and lower ink

tack but in a bind: if the surface of the non image areas are

covered with a thick ink film after the stroke, you want a

more vertical angle. If the ink doesnt clear but the non-

image areas are clean, a lower angle and slightly more pres-

sure is the solution. If the ink hang-up is in the central

areas only there is nothing that the squeegee can do, you

must increase the off-contact distance.

4. Increase ink deposit. The best method is to select a mesh

or ink that will give you or the coverage or opacity that you

need but in a pinch: Angle the blade lower and leave the

pressure alone. If you need more pressure apply it sparingly

and gradually after a few prints.

5. Improve image detail. This is a job for the stencil but if

you need a cure now: be sure that the edge is sharp, an 80°

durometer blade works well for detail, take some pressure

off of the blade and bring the angle to approximately 10°.

Once a week or so you might

want to perform the following

procedure, so that you can verify

press calibration and to bring your

press heads back into a home

position.

Remove screens and flood bars

and start with clean platens

preferably covered with peel. Load

your standard size squeegee into

the first press head, raise the

platens and stop the blade at mid-

way through the print stroke.

You may have to raise the blade

so there is some clearance

between the blade and the platen.

Use a business card as a feeler

gauge sliding it left to right under

the blade and set the blade posi-

tion so that the drag on the card is

consistent from edge to edge.

If you are in doubt as to parallel

settings between the carriage and

the platen then position the blade

at both front and rear of the

stroke. Be sure that the drag on

the blade is consistent from side

to side at all three positions front,

middle and rear of the platen.

You can perform the same feeler

gauge test with a screen and flood

bar. This test will verify paral-

lelism between the carriage and

the screen holders or surface of

the mesh.

Press Calibration

Verification

PAGE

. 44

6. Higher blade speed. The

ink and the flatness of the

mesh are the two con-

straints to squeegee speed.

The blade can do its part

by minimizing the contact

area between blade and

mesh. Raise the blade,

make its angle more verti-

cal and apply as little pres-

sure as you can get away

with. Be cautious if your

press is not calibrated the

blade may not print on all

platens.

Following is a table that

shows the relative results to

be expected if a single factor

of the blade is changed as

described. It is based on the

change from that of an ideal

specification for example, if

the Change is Softer then it

is softer than the ideal

durometer. Changes to

Bevel or Square are com-

parative changes. There are

three columns listing the

Causes as an anticipated result

of the change. The third col-

umn lists the Needs that must

be in place for that Change to

occur. And what the change is

expected to permit is listed in

column four.

Chapter 4

Follow these steps to properly set the squeegee blade:

1. Set the angle adjustment at 75°.

2. Use a feeler gauge to set initial

position.

3. Adjust position with equal turns of

right and left adjustment knobs.

4. If the ink transfer is consistent but

inadequate, lower the blade angle

and increase the pressure.

5. If the screen mesh exhibits posi-

tive mesh lagraise the angle, and

you may need to decrease the

pressure.

6. Note the setting for future use.

7. If your press is calibrated, the first

area to clear the ink from the

screen will be near the edge of the

frame. If so, lower the squeegee

by equal turns on both sides. This

keeps the blade parallel to the

platen.

8. Cycle the press with scrap material

on the platens to check the deposit

as the ink warms, as well as the

deposit on several platens.

Setting The Squeegee Blade

How To

Instructions

PAGE

. 45

Press Calibration

Figure 4.36 This table lists the relative change in the blade parameter, what that relative change may cause, what it will

need to accommodate the change and what the change permits. For example if the change is to a lower blade angle. It will

cause more pressure on the ink and may trigger positive mesh lag. To accommodate this change you may need to increase the

off-contact distance or the tension. It will accommodate higher ink tack and press irregularities. We do not want to encourage

you to use the blade as a permanent solution, it is most profitable to fix the problem and not to treat the symptom, but in a

crunch, the above table shows the relative results of such actions.

Change

Causes

Needs

Permits

Softer

Primary deflection

Excess force on platen

Closer off-contact

Vertical angling

Press irregularities

Shorter blade

Harder

Lower contact area

Center-to-edge variance

Higher static tension

Higher ink tack

Shorter (H)

Less deflection

Closer off-contact

Finer detail

Taller

Positioning problems

Consistent press

Varying platen height

Shorter (L)

More linear force

Mesh fatigue

Higher start position

More off-contact

Tighter image

tolerance

Longer

Ink transfer imbalance

Increased contact area

Closer off-contact

Consistent press

Wider image

Square

Less deflection

More precise press

Use of higher tension

mesh

Bevel

More deflection

Conformity

Sharpened continually

More precise setting

Center to edge

consistency

Round edge

Greater deposit

Reduced pressure drop

Low yield stress ink

Any angle

Sharp edge

Lesser deposit

Greater pressure drop

Precision setting

Lower angle

Higher resolution

Lower angle

More pressure on ink

Positive mesh lag

Higher off-contact

Higher tension

Higher ink tack

Press irregularities

Raise angle

Less deposit

Image stretch

Lower pressure

Low yield stress ink

Edge acuity /

resolution

Faster

Positive mesh lag

Flatter mesh

Higher mesh flow rate

Increased cycle rate

Slower

Negative mesh lag

Flashing problems

Thin fabric

High ink tack

Hi pressure

Blade deflection

Mesh burst

Lower blade flexure

Higher print tension

Higher ink tack

Lo pressure

Increased productivity

Low yield stress ink

Screen life

productivity

PAGE

. 46

AAnngguullaarr AAddjjuussttmm

eenntt

On the Challenger and

Gauntlets the plate that locks

in the initial angle of the

blade should be set at a home

position of 90°. This gives an

initial angle of attack of

approximately 12° and should

do a reasonable job for most

of your work providing that

the blade is true and dressed

to comply with the surface of

the mesh.

The proper angle is high (ver-

tical) enough so that the blade

does not deflect on the mesh

alone. It should not deflect

until it has reached the platen

and then only a controlled

amount. The angle should be low (horizontal) enough to apply sufficient pres-

sure on the ink and to buffer excess pressure on the platen.

Harder blades (lower flexure) can run at lower angles than softer blades (higher

flexure). Lowering the angle of a blade that is rigid does less to increase the

pressure on the mesh or the platen but it does increase the pressure on the ink.

SSppeeeedd

The speed of the blade is infinitely adjustable on all M&R presses and the con-

ditions that we have spelled out in this manual, if met, will allow you to run at

top production speeds.

1. Tack level of the ink.

2. Flow rate and Flatness of the screen mesh.

3. Pressure drop of the stencil.

4. Printing tension (static tension and off-contact distance).

5. Pressure on the ink.

6. Color of the ink.

Chapter 4

Follow these steps to properly select and set the

squeegee blade:

1. 70°  80° durometer, square pro-

file.

2. 3/8-inch X 2-inch X 1.25-inch

unsupported height.

3. Radius edgebased on mesh

count.

4. Blade must be sharpened to be

true.

5. 15° to 25° degree angle.

6. Fast (20 ips) flood speed.

7. 15-inches per second speed.

Selection And Setting

Blades For White

How To

Instructions

PAGE

. 47

PPrraaccttiiccaall

SSppeeeedd SSeettttiinnggss

It is to the color of the ink that we address the speed of both the squeegee and

flood bar (refer to figure 4.38 for details). Plastisols are conversationally half liq-

uid plasticizer and half PVS resin with some pigment added. The size and shape

of the particles as well as their loading in the ink cause some to print well at high

speeds and others to prefer slow speeds. But we have two blades and the flood

bar can be used to pamper or push the ink to a higher cycle rate. The table below

lists generic colors typical to the industry and blade profiles recommended.

Press Calibration

Squeegee And Floodbar Controls

Figure 4.37 This

shows the squeegee (and

floodbar) speed controls.

The squeegee had a

direct impact on produc-

tivity (refer to page 6.7

for detail.). The flood-

bar only has an indirect

affect on productivity by

allowing the squeegee to

run at optimal speeds.

The floodbar should

pamper, not preprint,

the plastisol.

Color

Flood

Gap

Flood

Speed

Squeegee

Blade Angle

Squeegee

Blade Speed

Squeegee

Speed Limit

White

Fast

Low

Medium

Mesh flow rate

Standard Opaque

Fast

Low

Medium

Loss of opacity

Opaque Fluorescent

Medium

Low

Medium

Mesh flow rate

Metallic

Far

Slow

Low

Fast

Mesh opening

Fluorescents

Far

Medium

Low

Slow

Ink tack

High Density

Fast

Low

Medium

Ink tack

Process Colors

Far

Slow

High

Fast

Dot Gain

Black

Far

Slow

High

Fast

Ink separation

Blues

Far

Slow

High

Fast

Ink separation

Figure 4.38 This table lists several colors in column one that perform quite differently based on their respective pigment

types and loadings. Flood Gap is the space between the flood bar and the mesh. Flood Speed is the relative speed, which nei-

ther hampers nor boosts production but allows the fastest squeegee speed for top production. Squeegee Blade Angle is the

used to apply pressure to the ink to inject it through the mesh openings. Squeegee blade speed is based on all other settings

as indicated. And the Squeegee Speed Limit is the typical reason that you cant run faster assuming that the ink is a

constant.

PAGE

. 48

Registration

Often times misregistration is due to a variety of causes exclu-

sive of the press and there are only two stages where registra-

tion is easy to check for accuracy: the films and the final print.

Since you can check the film positives we are not going to deal

with artwork or film output rather we will go to those areas of

registration that are more elusive. Some of these are the

responsibility of the press; most causes of misregistration are

not.

M&R presses are designed to fit three high tolerance case

hardened cam follower bearings into three machined U blocks.

The fit is glove tight and will last indefinitely. This tried and

true method of registration has allowed our customers to excel

as the highest quality printers in the industry. Our field techni-

cians handle the installations and the registration is set to less

than ± 0.001 at the bearing.

Note: If damage has been done to your press or if the air sup-

ply is contaminated or erratic, the press can begin to run

rough. It is then that the registration can be in jeopardy and

M&R service should be contacted immediately.

TTrrii-LLoocc RRaappiidd RReeggiissttrraattiioonn SSyysstteemm

Still this level of repeatability did not offer all that the printers

needed so we designed the Tri-Loc Rapid Registration System.

Press repeatability eliminated registration error as a cause of

interruptive downtime, but setup time needed to be addressed.

The Tri-Loc makes setup registration simple enough for all

your staff members. No longer do you have to rely on one,

key individual to tweak that image into alignment.

The Tri-Loc system registers the films to the screen and the

screens to each other on the press. It is a total system registra-

tion package. There are only four components to the system:

an exposure registration frame, an on-press alignment platen,

pin bars, and carrier film sheets. With these few simple tools

and no modifications to your screens, your downtime can be

cut to shreds.

Image inconsistency is due to a

historical flaw in the screen-print-

ing process; the blade cannot

apply equal pressure from the

mid-point to the end of the blade

on a screen with a small inkwell

without exhibiting severe deflec-

tion. Even if your screen is per-

fectly tensioned or extremely high

tension, the rateof increase of

tension as the screen is deflected

is higher at the ends of the

squeegee blade.

To print properly the pressure

between the blade and the mesh

(at all points on the mesh) must

be greater than the pressure

between the blade and the platen.

Otherwise the ink cannot be

injected through the mesh proper-

ly the transfer will be incomplete

and inconsistent. Higher tension

and harder blades mitigate the

problem but they do not resolve it

and reduced off-contact can actu-

ally worsen the condition. The

reason is that rate of increase in

tension is greater at the edges of

the mesh nearest the frame while

the lowest rate of increase is dead

center of the mesh.

The key to a consistent ink

deposit is to have consistent pres-

sure across the length of the blade

between both the blade and mesh

as well as minimal but consistent

between the blade and the platen.

It is unlikely that you will be able

to set the blade perfectly as long

as you are stuck with a smaller ink

well. But these are the reasons

that the settings are so critical and

at times may appear so

randomized.

Chapter 4

The Classic Flaw

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