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

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PAGE

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Figure 4.4

This shows the

relationship

between the size of

the ink well, the

off-contact distance

and the resulting

image tolerance.

Process-color toler-

ance can be com-

puted by the size

and structure of

the primary rosette.

Shown is a nomi-

nal 65 Lpi and

the needed combi-

nations of off-con-

tact distance and

ink well. Note at

a 2.5 inch ink

well, your off con-

tact distance must

be 0.082 or 82 mils. And as the distance decreases, the tolerance tightens. Closer means you must be even more accurate or

the value of being set closer is lost. Refer to the sidebar to assess your off-contact distance; if it is creating problems make the

necessary adjustments.

At 65 Lpi Tolerance

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Inkwell In Inches

Off-Contact Distance

Figure 4.5 This

illustrates the relation-

ship between off-con-

tact distance and ten-

sion levels. Initially

the ink well is fixed;

the squeegee is a given

length and the frame

is a given size. To

permit image toler-

ance, the off-contact

must be at the proper

height. To permit this

distance without screen

lag or image distor-

tion, the tension must

be at a specific level.

The graph shows that

you need about 26

N/cm² to permit an

82 mil off-contact set-

ting and register a 65

Lpi halftone with a 2.5 inch ink well but you should be cautious about two facets: ink transfer and image stretch in the

stroke direction.

Tension & Off-Contact Distance

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

10 15 20 25 30 35 40 45 50 55 60

Tension in N/cm^2

Off-Contact Distance in Inches

PAGE

. 10

of lift. Before the squeegee

begins the stroke, the screen

is at the initial off-contact dis-

tance. As the blade begins to

travel the screen lift is mini-

mal. And as the stroke con-

tinues the lift increases as

well. By the end of the

stroke, the lift is at a maxi-

mum distance.

It is the intention of screen

peel to eliminate positive

screen lag (poor snap-back)

and to equalize the contact

pressure between the blade

and the mesh at its ends and

midpoint. It offers the press

operator a rapid fix to a vari-

ety of problems. You may

find that increasing the peel

allows you to print with a

variety of shortcomings

including press settings,

screens and ink.

Following is a list of condi-

tions that can exist in every day production due to fundamental flaws in the

process; they may be solved with the peel adjustment.

You need more peel if 

1. Smaller well - needs contact pressure at midpoint. This includes a

smaller frame, longer squeegee blade and a longer stroke length.

2. Greater interface of blade to mesh - including a softer blade, more

force applied (higher line pressure) or a larger blade edge radius.

3. Higher ink tack - requires greater hydrostatic pressure and will be

noticed in the center of the screen. The ink will tend to hang-up in

the central areas.

Chapter 4

1. Review figure 4.4 to select off-contact based on

image tolerance.

2. Set the off-contact distance with a meter.

3. Adjust distance front and rear on each press head.

4. Always use the recommended screen size.

5. Use a squeegee 1 longer than the image width.

6. Review figure 4.5 for recommended tension level.

7. Set press to print with recommended screens and

blades.

8. Run squeegee pressure at a minimum.

9. If positive lagink tack and mesh geometry are

imbalanced.

10. If negative lagtension (and/or peel) are too high.

11. If image is oversizedoff-contact is too high.

12. If image is stretched (stroke direction) tension is

too low.

Determining Off-Contact Distance

How To

Instructions

PAGE

. 11

4. Imbalanced SS curves - will cause a tension loss during production and

this loss will affect registration and ink transfer.

5. Excess print speed - the blades on M&R presses can outpace many inks

ability to transfer through the mesh and force you to set a higher shear

rate with peel.

6. Lower off-contact - will increase the imbalance of blade and mesh con-

tact pressure between the ends of the blade and its midpoint.

7. Excess flood pressure - can cause a short ink (which regains its body

very fast) to transfer the screen inconsistently.

8. Twill weave - has the worst geometry for ink transfer, even at high ten-

sions, particularly noticeable with halftone printing.

9. Low fabric mass garments - which are the norm, do little to help pull

the ink from the screen mesh as the squeegee passes.

10. Uncalibrated Press - including a crown or dish in an old platen, heads

too low, platens too high or a squeegee that is reaching for platens.

11. Lower tension screens - do not allow the proper transfer of ink at any

position, central or perimeter, on the printed image.

12. Coarser mesh - including thick threads, smaller openings, thicker fab-

ric, lower count and a higher flatness angle, need more pressure.

With an ability to on-site resolve these printing constraints, peel must sound like a

panacea for production. But if you look at the list, most of these twelve issues

are elements that can and should be controlled by prepress and the press opera-

tor. It is best for the long term to be proactive and calibrate and standardize your

production to eliminate the need for peel but peel will allow you to quickly react

to the situation be it quality or productivity.

Should you choose to over use peel to save a press run with conditions that are

beyond hope, there will be trade-offs in the form of: image reproportioning,

screen fatigue and bursting or ripping. Remember that peel is in addition to the

off-contact distance and therefore it will affect printed image tolerance. The

benchmark for peel is zero screen mesh lag refer to page 4.6 for details).

There are two independent adjustments for controlling peel: first is a global set-

tingpeel start and it is best used to remedy issues due to the following:

Press Calibration

PAGE

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1. Low off-contact

distance.

2. High ink tack.

3. Poor mesh geome-

try.

4. Low dynamic mesh

tension.

5. After flash ink tack.

Second is the rate of peel and

it is best used to remedy

issues due to the following:

1. Uncalibrated press.

2. Small ink well.

3. Low fabric mass

garments.

4. Platen deflection

due to excess pres-

sure.

5. Maximum stroke length.

6. Low squeegee pressure (large contact area).

We recommend that you begin with zero peel settings as they are very easy to

initiate and you dont want to over do initial peel or peel rate. If you are experi-

encing premature screen failure refer to page 1.45 and note the following: If the

screens burst in front, parallel to the blade, delay peel start. If the burst is in the

middle of the screen, retard the rate of peel.

Aligning the Press

There are a number of advantages to custom press calibration that far outweigh

the time and effort expended. A press that is out of calibration shares primary

responsible for:

1. Ripped screens are commonplace because each platen comes into

Chapter 4

1. Select the proper frame size to maximize the ink

well.

2. Use a properly sized squeegee.

3. Set the maximum off-contact setting for image toler-

ance.

4. This setting must provide sufficient ink transfer.

5. Use the indexers single point off-contact setting.

6. Use a plain weave, low elongation, thin thread screen

mesh.

7. Choose a screen mesh that is balanced in warp and

weft.

8. Tension screens to support zero lag and zero stretch.

9. Select and ink with a low tack level.

10. Set peel rate to zero.

11. Run several sample prints to warm the ink.

Minimizing Peel Rate

How To

Instructions

PAGE

. 13

print position at a different height and skewed, front to rear and side to

side.

2. Inconsistent flash curing is a given because a small change in distance

can have a significant change on watt density (refer to figure 5.10 for

details).

3. Inconsistent squeegee wear as the platens come into print at a variety

of positions, the blade deflection is inconsistent front to rear and side

to side. This wearing changes the edge of the blade and creates vari-

ance shirt to shirt and the next press run is even worse.

4. Inconsistent image color and sizing is a promise of an un-calibrated

press. There is no way to establish off-contact distance, screen tension

or squeegee pressure.

5. Loss of registration due to the isolated stress imposed by the extremes

of the squeegee pressure cause isolated loss of tension and therefore

image shift.

6. Long set up times result due to the variance of the platens. If one

screen is tilted front left and the platen is tilted right rear the size and

shape of the image will fluctuate. There is no way it will align properly.

7. Slow print speed will reduce the shear rate on the ink and minimize the

blurring and mottling but only at the cost of productivity.

The point of calibration is to insure that proper squeegee pressure is applied to

all points on every platen by every press head. This means that the screen mesh,

platens and carriage drives are parallel at all points and have a gap preset to a rea-

sonable distance.

Most of the presses in the field have been adjusted to treat symptoms and have

not been returned to a zero or home position. If this is the case with your press

you may be fighting some of the symptoms on a daily basis:

Carriage drive not parallel to the platen:

1. Squeegee pressure inadequate front or rear

2. Screens tend to rip near one end of the stroke.

Carriage drive not parallel to the mesh:

1. Registration problems in stroke direction.

Press Calibration

PAGE

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2. Screens tend to rip near one end of the stroke.

3. Excessive ink deposit near one end of stroke.

4. Cant set floodbar without ripping mesh.

Mesh not parallel to the platen:

1. Registration problems in both directions.

2. Image smearing is commonplace.

3. Need to overdo squeegee pressure.

4. Mesh bursts as well as rips.

Until parallelism is established between the three planes, there is no reason to try

to fine tune any of your process including: squeegee parameters (edge, angle,

durometer, speed), mesh parameters (opening, thread diameter, tension), off-con-

tact distance, peel rate and so on. The reason is that you will be trying to hit a

moving target.

Imagine that the press is perfect and I drop one platen 1/8 inch but maintain its

level. To get that platen to print, you will reach with the squeegee and as a

result you will run too much pressure on all other platens (by 1/8 inch displace-

Chapter 4

Four Parallel Planes

The carriage drive must be parallel to

the squeegee edge, which

must be parallel to the

mesh, which must

be parallel to

the platen.

Figure 4.6 This graphically represents the four planes that need to be parallel for proper imaging. The carriage rails

must be parallel to the squeegee blade, the screen mesh, and the platen. The platen is the most critical since it will be under

all other print heads.

PAGE

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ment). If I took only one press head and raised it 1/8 inch main-

taining level it would have to reach all but one of the platens

by 1/8 inch but one of them would be ¼ inch away. That head

would rip screens and wouldnt print very well.

Imagine if I randomized your heads, carriage drives and platens.

One up and angled back, one down angled side to side and no

two the same? This describes a press that has been continually

adjusted and never reset to a home position. If you have a need

to improve the quality of your images, need to reduce rejection

or need to print faster, then it may be time for you to character-

ize and then custom calibrate your press.

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You will need the following tools to calibrate the press:

1. One perfectly flat tensioned screen frame.

2. One perfectly flat open screen frame.

3. Shim stock to match your off-contact distance.

4. Or an off-contact meter.

5. At least one torpedo level (two is preferable).

6. Two 9/16-inch and two 3/4-inch open-end wrenches.

7. Two 15/16-inch open-end wrenches.

8. Two 1 1/8-inch open-end wrenches.

DDoouubbllee-cchheecckk tthhee cceenntteerr sshhaafftt..

1. Find a perfectly flat platen- no crown or dish.

Press Calibration

1. Check weekly with the flood

bar test for press head and

platen calibration.

2. If the press has been tampered

with it will need recalibration.

3. If the press is not returned to

home position.

4. If the press is not performing

properly:

a. Screens rip prematurely.

b. Ink build-up is a problem.

c. Ink deposit is inconsis-

tantfrom point-to-point

on the shirt and shirt-to-

shirt on the press run.

d. Registration is lost during

a press run.

5. If your conditions are radically

changed.

6. If you need to constantly

adjust squeegee blades, angles,

pressures and speeds to get a

quality print.

7. Once every three months.

Neither the Gauntlet nor the

Challenger drift. If they

appear to, either your calibration

procedure is ineffective or

someone is changing the

adjustments.

Knowing When To

Calibrate The Press

Figure 4.7

This shows a

level placed

diagonally on

the platen

and the user

looks for a

gap in the

middle of the

level, indicat-

ing a dish or

if the platen is crowned, the level will rock end to end. We are not looking for level at this time.

For your number one platen it is imperative that the surface be perfectly flat. Use the level at this

angle and then on the opposite diagonal. If your press is old, and excessive heat is typical of

your shop, you may want to resurface one platen or order a new platen from the factory, either

way it must be flat.

PAGE

. 16

2. Mark the platen and the press arm #1; always return this

platen to this arm.

3. Level the platen front to rear and side-to-side.

4. With level centered front to rear, rotate platen to 12, 3, 6

and 9 oclock.

5. Check level at all four directions for plumb of the center

shaft.

6. If shaft appears out, use a torpedo level on the center shaft.

7. Check all four sides of the shaft for plumb.

8. If it reads similarly, the indexer may need to be leveled.

Please notify M&R at this time.

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9. Clean all platens.

Chapter 4

Follow these steps to simplify

press calibration:

1. The threads of the bolts for

platen height adjustment as

well as off-contact distance are

16 tpi. That means that one

complete turn of the knob or

nut on the bolt will move the

part 1/16th of an inch or 60

mils (0.060). So if you need to

adjust a platen or a screen

holder 60 thousandths, simply

turn the nut or knob from 12

oclock until 12 oclock again 

one complete revolution, you

will have moved 60 mils.

2. If you have a relatively high

quality bubble level, it may be

capable of much more accura-

cy than you think. The bubble

is to be centered between the

lines right? Not specifically, the

bubble is to be centered

between the lines on the

backside of the tube. Look

closely there are rings that run

completely around the tube

containing the bubble. For you

to center the bubble between

the back side of the rings, you

will have to be directly in front

of the level. The degree of

error is negligible if you use

this proper technique.

3. When you have set the platen

or screen holders and it is time

to tighten them in position,

you want to do just that  dont

alter the position. The tech-

nique is simple. For the platens,

hand tighten gently and then

use two open end wrenches

and simultaneously turn them

to a center point. With the off-

contact knobs, turn each inde-

Tips For

Press Calibration

Figure 4.8 This shows the torpedo level running in the front to rear direc-

tion in 6.8a and side to side in figure 6.8b. Both directions must be level at

this time.

PAGE

. 17

10. Apply peel to the surface of all cleaned platens.

11. Remove all dirty screens and blades from the press.

12. Be certain that the screen holders are clean and free of

tape etc.

Press Calibration

pendently gently until snug and

then turn both knobs at the

same time to the center point.

4. You will want to use an off-

contact meter to get your press

calibrated accurately. Coins,

washers and the touch and go

method do not work as well.

But if you have all of the

speed, quality and mesh life

you want, it may be good

enough for your purposes.

5. When you are calibrating your

press, work with a friend and

so that you can stay friends, use

common terminology: front,

read, side, left, right, tighten

etc. And remember: righty -

tighty, lefty - loosey.

7. You will need two 9/16 and

one ¾ open end wrenches.

You will need a 30 standard

shop bubble level and one or

two torpedo levels. To hold the

level and off-contact gauge you

will need two C clamps with

a 6 throat.

8. Once you calibrate your press,

particularly if it is the first time

you will want to enjoy a sharp

squeegee blade. Use a squeegee

sharpener that holds the blade

during the sharpening and

never use a new blade until it is

sharpened.

Tips For

Press Calibration

Continued

Figure 4.9 This is a reminder

that all of the platens must be

cleaned and freeof adhesive and lint

before you attempt to calibrate the

press.

Figure 4.10a and 4.10b These photos show the operator applying pallet

peel and using a razor knife to trim the excess material. When complete any

excess can be folded underneath the platen so there are no adhesive areas or

loose corners to be caught by the shirt.

PAGE

. 18

13. Tighten the platen locks below each platen.

14. Pneumatic

Presses:

Disconnect the

airlines to the

stroke cylinder.

15. AC presses:

Disconnect the

electrical or dis-

able the electric

in the MTA.

16. Be sure that the carriages glide smoothly over the platenspallet peel helps.

SSeett CCaarrrriiaaggee TToo PPllaatteenn PPoossiittiioonn

17. Possibilities (first insure platens are level):

Chapter 4

Figure 4.11 This shows the front and rear screen holders and that they are both clear of any debris,

tape or adhesive. Anything sitting on the screen holder will skew the results.

Screen Holders

Figure 4.12 This shows the yellow locking handles located on the underside of each

platen. Be sure that these are tightly clamped and holding the platen securely.

Figure 4.13 At the point of

installation, leveling the carriage is

done automatically but if you are

calibrating, you must be certain

that the carriage (which drives the

course of the squeegee and floodbar)

runs parallel to the plane of the

mesh as well as the plane of the

platen. Whether your press has

rails or drill rods, a magnetic bub-

ble level works well.

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