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

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PAGE

. 45

each screen and to compile

data on them, be sure that

you take tension readings on

five points (see figure 1.37 on

page 1.44). These include the

center and the four corners of

the maximum image area.

Also note the tension range,

which is the highest tension

of the five points and the

lowest.

You will want to use the

meter to check for isolated

stress areas. (see figure 1.38).

These often occur with reten-

sionable frames just inside the

final point of corner soften-

ing. (refer to page 1.35 for

details on progressive corner

softening). The isolated stress

areas must be eliminated to

insure stable tension on the

press. Of course you will

want to strive to achieve equi-

librium and the same tension

over the surface of the mesh.

However even with this con-

dition, problems can ensue

and will never be detected by

the tension meter because the

trouble is that the meter has

no idea of how much the mesh has been elongated.

Troubleshooting Screen Life

It will be very helpful to identify where your screens blow out so that you can

identify the more relevant question of why they blow out. There are two distinct

and separate elements of total screen failure: those that burst and those that rip.

Screens

1. Use LE, plain woven, dyed, mono-filament

polyester.

2. Get a SS curve (or data) from your supplier.

3. Run the warp in the long direction.

4. Insert the mesh into the frame.

5. Use figure 1.31 for elongation for

Challenger and Gauntlet screens.

6. Mark off the recommended distance.

7. Soften the corners progressively.

8. Stretch in two stages, half from each end of

the frame.

9. Stretch half the distance warp first.

10. Stretch the weft, half the distance.

11. Record tension and percentage elongation.

12. Complete the warp.

13. Complete the weft.

14. Use the meter to check for isolated stress.

15. Adjust softening procedure if needed.

16. Check tension at the five points.

17. Record tension and total elongation.

General Tensioning Procedures

How To

Instructions

PAGE

. 46

MMeesshh BBuurrssttiinngg

The mesh will be prone to burst in the center of the screen from external stress-

es not from the elongation or its resulting tension. The center where it is most

difficult to transfer energy is subjected to the highest wear from the shearing

effect of the blade. This is the point of least change from static to dynamic ten-

sion. Eventually the blade erodes the fabric and finally it bursts. Your images will

mis-register due to stretch in the stroke direction. You may observe the harp

like strings of mesh that break loose of the bonded knuckle. The tear is a lacera-

tion or rough cut and there are remedies and they are listed below.

1. Reduce the squeegee force.

2. Raise the off-contact distance.

3. Increase the size of the inkwell.

4. Calibrate the press.

MMeesshh RRiippppiinngg

The mesh is prone to rip near the frame wall due to both internally and externally

applied stresses. The internal stresses are imposed during the tensioning process;

the highest stresses are nearest the point where the mesh is being pulled.

Secondly is the very obvious abrasion from the ends of the squeegee blade,

hopefully not from the flood bar. It is at the ends of the blade, near the edge of

the mesh that the contact pressure between blade and mesh are at the highest.

Mesh elasticity is at its lowest and blade pressure may be at its highest.

Misregistration will be due to over-sizing the original image (refer to page 4.4 for

details). The tear in this case is a slice and looks cleanly cut as with a knife. The

remedies for ripping mesh follow:

1. Reduce the off-contact distance.

2. Radius both ends of the squeegee blades.

3. Increase the size of the inkwell.

4. Center the squeegee blade.

5. Calibrate the press.

If you are experiencing both ripping and bursting mesh the most probable cause

is that your press is out of calibration (refer to page 4.15 for complete details). It

is probable that you have press heads that are out. The higher ones are causing

Chapter 1

PAGE

. 47

the mesh to rip and the lower

ones are causing the mesh to

burst. If your press is in cali-

bration then the dual prob-

lems are due to erratic screen

tension. You will want your

range of tensions of screens

on any one set-up to be more

closely related in other words,

dont run high and low ten-

sion extremes on any given

press run. Be sure that your

squeegee blades are sharp and

true and centered in the

screen.

TTeennssiioonn CCoouunntteerr MMeeaassuurreess

To negate whatever qualities

the fabric manufacturer and

you put into the screen mesh,

its tension and life expectancy

use the following list of mesh

killers:

1. Uncalibrated press.

2. Imbalanced SS curve.

3. Minimal ink well.

4. More squeegee force.

5. Rigid blades.

6. Higher off-contact.

7. Double stroke.

8. More peel.

9. Non LE mesh.

10. Any flood pressure.

Screens

1. Calibrate your press regularly.

2. Use the standard recommended screen sizes.

3. Use a squeegee blade 1 longer than your image

width.

4. Set squeegee pressure at a minimum level to com-

pletely transfer ink.

5. Use a blade setting that accommodates your press

conditions.

6. Run the off-contact distance as low as ink transfer

permits.

7. Do not make a habit of double stroke - fix the

problem.

8. Always try to run at zero peel, use only when

absolutely necessary.

9. Always run low elongation screen mesh.

10. Do not flood as a preprint stroke.

11. Use ink with low tack level.

12. Use mesh with balanced SS curves.

13. Progressively soften the corners of each screen.

14. Establish tension levels by using elongation

techniques.

15. Check for isolated stress areas with the meter.

16. Check all five-tension spots.

17. Beware of super-heated platens and their affect on

the screens.

18. Seal the perimeter with permanent blockout.

19. Use a Mylar® tear resistant tape on all corners.

20. Protect the groove of the locking strip with

cloth tape.

Protecting Tensioned Screens

How To

Instructions

PAGE

. 48

11. High ink tack.

12. Improper tensioning.

13. Excessively hot platens.

14. Unsealed perimeter.

15. Poorly taped corners.

16. Unprotected locking grooves

TTeennssiioonn LLiimmiittss

Prior to low elongation fabrics, and particularly in the US, we

were preoccupied with higher and higher tension levels. There

are a few reasons; in this country the tee shirt has achieved an

all time high for popularity. The baby boomers and level of

affluence created an unprecedented demand and therefore

colossal automatic and manual machine sales. Most of these

machines were not maintained very well and were sorely out of

calibration. All of the machines (by necessity) are designed

with small inkwells and the printer has a lot of screens. The

ink of choice in the US is plastisols and most of them are very

tacky, sticky and difficult to transfer through the mesh. The

tees usually range from 15% to 25% fabric mass. Take highly

elastic fabric, uncalibrated presses, small inkwells with an abun-

dance of screens, tacky inks and a porous substrate and the

advantages of higher tension screens are obvious.

These screens took the form of retensionable frames and the

dominance of the roller frame in the tee-shirt industry was

unprecedented. Without low -fabrics there is a need for con-

tinuous take-up (tightening the fabric) and a lot of it. Some of

the low modulus (opposite of low elongation) required elonga-

tion percentages of 10% to develop reasonable tension levels.

And once you got there, even when you incurred the least bit

of stress on the mesh, the tension level was lost. This made a

continuous take-up retensionable frame a very logical method.

Now that low elongation fabrics are available, the utility of the

retensionable frame has changed.

An insufficient tension level

would create the following prob-

lems:

1. Require excessive off-contact

distance.

2. Allows a positive mesh lag

behind squeegee stroke.

3. Causes image stretch in the

stroke direction.

The minimum tension level of

the mesh has to meet several cri-

teria:

1. It must permit an off-contact

distance below image tolerance.

2. It must resist image stretch at

the above off-contact distance.

3. It must resist positive lag dur-

ing the print stroke.

The ideal tension level would

meet or exceed the following cri-

teria:

1. Permit minimal off-contact

distance.

2. Permit zero mesh lag where

snap-back equals squeegee

speed.

3. Resist image stretch in the

stroke direction

(Continued next page.)

Chapter 1

Tips For Establishing

Your

Proper Tension Level

PAGE

. 49

There are two camps of thought: First is the camp that allows

the mesh manufacturer to spin and weave mesh to perform

properly on press. It has a large mesh opening and a flat sur-

face with a thin thread and is low elongation. It is engineered

and manufactured for stability and ink transfer. The second

camp says that you want to buy the strongest mesh available

and tension it to a plateau to make it stable, flat and transfer

ink properly. There are few in the business that have the con-

trols in place to do the latter so we recommend that you let

the mesh makers do the engineering. Use the proper tension

level to achieve your goals, not as a cure all for all the other ail-

ments of the screen printing process.

The most appropriate cliche is If the lights go out  dont

fix the plumbing. Particularly in the US, we have used screen

tension to attempt to solve a variety of problems rather than

accurately identifying or solving the problems themselves. We

have been plumbers groping in the darkness.

It is true that excessively low-tension levels lead to misregister

and poor ink transfer at a slow rate. But excessively high-ten-

sion levels, set in an attempt to resolve non-tension issues cre-

ate other problems: screen bursting is one, buckling squeegee

blades is another. In many conditions high tension actually cre-

ates an imbalance in the amount of ink transferred. Look to

press calibration, ink tack, mesh geometry, sharp and true

squeegee blades and proper frame size as their own culprits,

youll find it an illuminating experience and a lot cheaper in the

long run.

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Of course there is a relationship between tension and the

amount of off-contact distance but that relationship relies

upon more than just the two facets. The criteria for tension as

it relates to off-contact distance is screen stretch. In other

words, once you have set the off-contact distance to accom-

modate your image tolerance (refer to figure 4.4 and figure 4.5

on page 4.9) then your tension level must allow you to print

without image stretch in the stroke direction. You will need

higher tension for a given off-contact distance if the image

The maximum tension level of

the mesh has to meet several

criteria:

1. Does not lead to image

enlargement.

2. Does not cause premature

fatigue.

3. Does not cause negative lag

during the print stroke.

An excessive tension levels create

the following problems:

1. Forces impractical off-contact

distances.

2. Or results in image

enlargement.

3. Creates negative mesh lag.

4. Causes premature fatigue or

bursting.

Screens

Tips For Establishing

Your

Proper Tension Level

Continued

PAGE

. 50

stretches in the print stroke direction. The table below gives you a starting point

for tension and off-contact distances.

Begin by establishing your image tolerances and then use the tables shown on

page 4.9 to determine the maximum off-contact distances. Once you have deter-

mined the off-contact distance, you can set the tension specific to that off-con-

tact distance. Use the graph below for reference.

Chapter 1

Tension Levels and Off-Contact Distances

Tension N/cm²

Oc Inches

.375

.250

.160

.100

.069

.055

.042

.035

.026

.019

.015

*Oc Fractional

1/4

1/6

1/1

1/2

1/3

1/5

1/6

*Fractional equivalents are approximate and are listed for reference sake only. You should use

a meter for press calibration and it is most likely that the meter will be scaled in mils or

thousandths of an inch.

Figure 1.38 This table lists the tension level and off-contact distance combinations. This list is for reference and a starting

point only. Refer to Chapter five for specific details on setting off-contact distances and then establishing the proper tension to

support the off-contact. As the table shows, a 25 N/cm² screen will be needed for an off-contact distance of 0.100 or

1/10th-inch.

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

Figure 1.39 This

graphs the relation-

ship between tension

and off-contact dis-

tances. Off-contact is

the first variable to be

set on press. Tension

along with squeegee

settings must support

the proper off-contact

distance. Details are

located in Chapter

Five.

Chapter

STENCILS

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PAGE

. 3

Stencils

The role of the stencil is multi-faceted. It is to adhere to the

mesh and resist reasonable abrasion from the squeegee and be

enough elastic to allow proper off-contact distances. The sten-

cil must be flat enough on the impression side to create a seal

to the film positive during exposure, as well as provide an ade-

quate print shoulder for accurate line reproduction in print

production. It should be sufficient on the squeegee side to

meter the proper volume of ink for fine-detail printing. The

color should be transparent enough to encourage thorough

exposure [on the inside of the screen] and yet it should be

dyed to filter extraneous light rays.

All of these parameters are operator dependent: that is, they

can range from bad to good solely based on the way that you

use them. If your preoccupation is with the lowest price per-

gallon, you are probably creating problems with quality and

downtime that are easy to resolve, but costly nonetheless.

EEmmuullssiioonn TTyyppeess

There are three basic types of stencil emulsions: diazo, dual-

cure and pure photopolymer.

The diazo is the traditional coating; relatively low cost, long

exposures and temperamental in many aspects due in part to

moisture sensitivity.

Pure photopolymer is at the other end of the spectrum from

the diazo. It offers lightening fast exposure, is somewhat unaf-

fected by relative humidity and darn expensive. It is well suited

for those with small or weak light sources or those with high

quantity and reasonable demands. If you plan to use a pure

photopolymer extra caution should be taken in calibrating

exposure. The latitude is very tight, which can easily lead to

over or under exposure.

Dual-cure is generically a blend of diazo and pure photopoly-

mer. It is most often the best choice for performance. It can

be faster than diazo, but have user-friendly latitude. Its the

best resolution in the game and yet is relatively easy to reclaim.

The visual transparency of the

completed stencil is critical to the

press operator. In a perfect world,

you could align the image with a

Tri-Loc and lock the image in

placeyou would never need to

move it again. For those of you in

the real world, there may be times

that it just doesn't work that

wayyou need to see through the

stencil to the image below. In

most shops a minute of press

time is worth $5.00 to $20.00 dol-

larsdon't do anything that will

waste a minute.You may want to

rely on visual registration if...

v you don't have a Tri-Loc.

v the screen frame is racked.

v the screen frame slips.

v the mesh fatigues.

v the original art really

doesn't line up.

v your press is out of

calibration.

Stencils

encil T

ransparency

and

egistration

For long life it tends to be

unsurpassed. If you are not

sure what to use, err on the

side of dual-cure emulsions.

Each of the coatings has

specifications and parameters

that are highlighted in the few

paragraphs below. The infor-

mation is to enable you to

choose the best product, not

to pigeon-hole you into a spe-

cific label or performance

parameter.

CCooaattiinngg PPaarraammeetteerrss

High solids content is the first

parameter that you should

look at to attain higher edge quality, fewer pinholes and longer life on the press.

Not curiously, these higher-solid coatings cost more, since less of the product is

water.

Bridging, the ability of the emulsion to "bridge" over the open area of the

mesh, was a hot topic long ago when both the solids content and print stan-

dards were dangerously low. It is, of course, constrained by the percent of solids

(price) but is much more reliant on the particulars of the fabric and on the

methods of application used. (Refer to pages 2.10 and 2.14 for details of fabrics,

coating and exposure methods). The best bridging will come from multiple

coats and, in fact, from coat-dry-coat techniques.

Resolution is a term that is bandied around by all of us to cover a multitude

of virtues. Many elements of the coating chemistry (such as resin particle size),

account for its ability to hold detail. Unfortunately, the reason for poor detail

many times is a function of usage: improper coating methods, drying, exposure

and development conditions. Very few printers actually tap into the resolution

capabilities of any of the emulsions. Remember that it is resolution on the print

that is critical, not resolution potential in the can. Here again the optimal resolu-

tion occurs when the stencil is the flattest. This is accomplished through a bal-

ance of coat-dry-coat technique.

Exposure speed can be beneficial but be sure that it is not hurting you in pro-

Chapter 2

Look for an emulsion with:

v High solids content.

v Transparency to ease registration.

v Durability on the press to resist pinholes and

breakdown.

v Wide exposure latitude to be production friendly.

v Fast drying to increase throughput.

v Easy reclaimability for labor economy and screen

life.

v Fast exposure, specifically on your light source.

And most of all, don't let price interfere with

productivity.

Selecting an Emulsion

How To

Instructions

PAGE

. 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