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

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PAGE

. 35

Developing The

Stencil

The process of developing

(or washing out) the stencil

receives little attention, but it

can be a subtle cause of many

imaging problems. The idea is

to apply water to both sides

of the stencil and allow the

unexposed areas to act like a

sponge and suck the water up.

This leaves the coating very

fragile and, with a light pres-

sure, the softened areas dis-

solve and wash down the

drain. For this to happen con-

sistently may require some

planning and preparation.

Use a backlit sink for

washout; to save remake time

with those areas that you just

can't see without illumination.

A water-temperature mixer

keeps the water at ± ½° F for

consistent and reliable results.

The temperature should be a

consistent 125°. A power

washer (not the force used for

reclaiming) mixes air from

your compressor with water

to keep shrinkage to a

minimum.

Most of the developing

should be done from the

squeegee side of the screen to

protect the shoulder of the

image on the impression side.

Stencils

1. Use a backlighted sink.

2. Use consistent 125° water temperature.

3. Use an air/water mixing gun.

4. With a high volume, low pressure mist:

a. Spray the impression side thoroughly.

b. Spray the squeegee side thoroughly.

c. Repeat in 30 seconds.

d. Wait an additional 30 seconds.

5. With firm pressure and lower volume;

a. Work from the squeegee side.

b. Maintain a 6-8" distance from the screen.

c. Move continually from top to bottom.

d. Neither favor nor ignore gray scales and reso-

lution targets.

e. Gently spray impression-side only if needed.

6. When complete, rinse with cold water.

7. Vacuum excess water from squeegee side.

8. Vacuum excess water from impression side.

9. Inspect for any residual coating in image areas.

10. If found, reapply firm pressure to that area and

continue as above.

11. Process multiples for effiency.

12. If you have difficulty, review the section on storage

and screen room lighting

Developing The Stencil

How To

Instructions

PAGE

. 36

If you find you must resort to blasting the coating off of the

screen, something is wrong. This is a particular problem if the

emulsion is under-exposed (refer to page 2.24 for details). The

screen may have been exposed to light or heat, or could just be

too old to work well. In any case, blasting is not the best

answer.

TToouucchh-UUpp

If you subscribe to the recommendations in this manual, you

will have little need for screen touch-up (also known as spot-

ting, filling, blocking, blocking out, pinholing and many other

names). The purpose of touch-up is to seal the ink from acci-

dently printing or seeping through and ruining garments. It is

not intended to be a repair station for poor films or screen

processing.

The greatest reason for excessive touch-up is a stencil that is

too thin. There are a variety of reasons that this might occur.

First is that the coating applied was too thin (refer to page

2.14). Second that the screen was under-exposed (refer to page

2.33).

Dust, dirt, static electric-charge, coarse mesh, thick threads,

coaters that are too large, coaters that are damaged, and bad

emulsion are all blamed for pinholes. Of course, all of these

can be culprits, but are most often secondary to a thin and

under-exposed coating.

Since the purpose of touch-up is not that of a repair station

for poor processing, there should be a go/no-go point as a

standard for passing or rejecting screens that need mummi-

fied. That point is 60 seconds for Challenger and Gauntlet

screens. If you cannot touch-up a screen in 60 seconds, some-

thing is wrong.

If you average one screen in 60 seconds (hopefully and realis-

tically less) then one person can touch-up over 400 in a shift.

Work to make your touch-up process more of a final inspec-

tion step. If you concede that this is reasonable, then you will

want to initiate one of the procedures outlined in the sidebar

on page 2.34.

Chapter 2

If Touch-up takes more than

60 seconds per screen:

1. Record the number and times

on a sheet of paper.

2. Record the reason why (films,

coating, exposure, etc).

3. Set the screens aside for

hourly inspection.

4. Return the screens to the

responsible party.

5. Notify your supervisor.

To ease the usage, application,

and removal of perimeter tape:

1. Calibrate your press and use

minimum squeegee pressure.

2. Use a proper-sized coating

trough.

3. Thoroughly dry the screen

prior to exposure; this per-

mits complete and proper

exposure.

4. Thoroughly expose the

screen, being sure to cure

the emulsion on the squeegee

side.

5. Use permanent blockout on

the perimeter.

6. Use a quality tape intended

for the job.

7. Use a tape that strips clean

(without residue) and quickly

(without ripping).

encil Finishing Tips

PAGE

. 37

TTaappiinngg tthhee PPeerriimmeetteerr

The taping process has sadly evolved into a mummification

process, and the reasons are only sort of legitimate. The press

operators know from experience that the stencil will break

down prematurely, so they use an abundance of tape to pre-

vent leaks. It is affective but should not be necessary.

First, the stencil must be properly coated, dried and exposed

(refer to pages 2.14, 2.16 and 2.33 respectively). Such a stencil

will not break down rapidly. Secondly, use a quality tape

intended for the purpose. Cheap masking tape will cost you

a fortune in leaks and time to strip it from the screen.

A foot of quality tape costs a nominal 8¢, which is the cost of

one minute of time for a $5-per-hour employee. But your

press can earn you 8¢ per second (at 40¢ per shirt and a 60-

dozen-per-hour cycle rate). All breakdowns take more than a

minute to fix and will ruin a few shirts in the process.

The key to economy is to use the tape for the purpose it is

intended. There should be no need to tape the whole screen

and never to tape both sides of a production screen. If you

think you are taping for strength you may be mistaken. The

tape restricts the normal flexing of the screen and does noth-

ing to stabilize tension. Next thing you know, you need more

squeegee pressure to transfer the ink

...

check mate.

Reclaiming The Stencil

This process often takes a variety of inks and chemistries and

shoots them all over a roomno wonder it seems to be a messy

job. Use the appropriate hand and eye protection for your

safety. Since the primary focus of this manualand the pri-

mary goal of a pressis to run efficiently without stopping,

you may want to adopt the following procedures.

It is easier to find unskilled labor than semi-skilled or skilled,

and we assume that some of your most skilled operators are

press operators and paid accordingly. If so, all you really want

them spending their time on (workload permitting) is printing

and the accompanying paperwork. Any distraction can be a

Stencils

To make reclamation as easy as

possible:

1. Supply hot water at the pre-

scribed pressure and volume.

2. Use the prescribed reclaiming

chemistries.

3. Thoroughly clean and dry

screens before applying

reclaimer.

4. Avoid a coating trough that is

too long.

5. Avoid coating softened

corners.

6. Thoroughly dry screens prior

to exposure.

7. Thoroughly expose the

coating.

8. Avoid solvent based "screen

openers" on press.

Tips T

o Ease

eclaiming

PAGE

. 38

costly error in labor utiliza-

tion. So it is our recommen-

dation that the press opera-

tors do not shut the press

down to clean screens, ink

and blades: but they should be

cleaned immediately and thor-

oughly before sent to recla-

mation.

Once the screen is thoroughly

cleaned with commercially

available cleaners, which rinse

fasr and completely, it should

be allowed to dry. When dry,

reclaiming chemistry should

be applied to both sides and

allowed to penetrate the sten-

cil before proceeding. Since

there is a waiting period of a

few minutes, you may want to

spray several screens at once

and allow the chemistry to do

its work while you are

reclaiming the previous batch.

(Note adding reclaimer to wet

screens only dilutes its effec-

tivenessparticularly one

that is wet with screen wash).

The water temperature used to reclaim screens should be hot150ºand flow at

a nominal 2-4 gallons per minute. You will want to be sure your hot water sup-

ply can deliver this quantity of water. Take the number of screens per day and

multiply that by the time per screen. Then presume that you may want to do

them without stopping. For example, 50 screens per day at our recommended

three minutes per screen equals 150° minutes at 2-4 gallons per minute of

intermittent water at 150° F or hotter.

The initial pressure of the power washer is best suited around 3000 psi and de-

rated by changing the orifice to output 2-4 gallons per minute at or near 1200

psi. High-pressure air does not remove the stencil, high-pressure water does.

Chapter 2

1. Remove residual ink.

2. Remove tape and tape adhesive completely from

the screen.

3. Use a screen wash that is compatible with reclaim-

ing chemistry.

4. Mist reclaimer on both sides of the screen.

5. Lay screen flat impression aide up.

6. Wait three to five minutes.

7. Use hot water140° plus.

8. Use high volume watertwo to four gallons per

minute.

9. Use high-pressure water1200 psi at two to four

gpm (gallons-per-minute).

10. Work from the squeegee side.

11. Sustain a four to five inch distance from wand-to-

screen.

12. If you have difficulty, review the section on proper

exposure (page 2.19).

13. If you have residual haze review the ink and wash-

up sections.

Reclaiming A Screen

How To

Instructions

PAGE

. 39

Many of the bargain pressure washers claim high-pressure

but don't output much water.

The active ingredient in reclaiming chemistry is sodium meta-

periodate. It is normally used at 3 percent concentration (high-

er concentrations come out of solution readily and the SmP

falls to the bottom of the pail). It is available in liquid or crys-

tal form. If you are mixing your own to save dollars, be sure

that such is really the case. Overor underuse of the crys-

tals could cost you far more in raw materials or conversion.

For standard-sized Challenger and Gauntlet screens, you will

want to allow 2-3 minutes per screenif the conditions here

are followed, that number is very attainable. (Note that it will

require less time if you have used a permanent blockout on

the perimeter of your screens (refer to page 2.13 for details).

If you find you cannot attain this rate and you have the proper

chemistry, water pressure and flow rate, read pages 2.37

through 2.38 on screen exposure. It may be that you are cross-

linking the coating on press due to under-exposure. For com-

parison, test-reclaim a screen that has not been used on press

and see if it is marginally easier.

HHaazzee RReemmoovveerr

It is often necessary to remove ink haze that should have been

removed in the wash-up phase prior to reclamation (refer to

page 2.40 for the details). Poor housekeeping habits in this

area will cost you money.

The distinct disadvantage in the use of haze remover is that it

shortens the life of the screen mesh. Continued and repetitive

exposure to caustics will cause enough erosion that the fabric

will rip or burst prematurely. And the cost of this is extreme if

it rips on press.

If you can't seem to get rid of the ink haze, consider the color

ink with which you are having a problem. Ink haze is a result

of the inability of your wash-up to disolve the vehicle in the

ink. Specifically, blues and blacks are the most notorious colors

for plastisol haze. The reasonthese pigment particles are

Stencils

1. Calibrate the press for mini-

mal mesh abrasion.

2. Sufficiently tension screens to

reduce abrasion.

3. Minimize squeegee pressure.

4. Ensure thorough exposure.

5. Use adequate pressure and

water flow for reclaiming.

6. Use a proper wash-up chem-

istry applied as directed.

7. Clean the screens immediate-

ly after use.

Minimizing The Need

or Haze R

emo

PAGE

. 40

lightweight and high in oil absorption. These properties cause the ink manufac-

turer to use different (polymeric) plasticizers, which are insoluble in many con-

ventional wash-ups. Second, the dye in some of the very bright red inks, which

origniallyattached to the pigment, leaves the pigment during printing and

becomes attached to the mesh. The stain caused by this process may not restrict

particle passage, but certainly will effect exposure time in the affected areas.

Finally, is the example of white inks, which use polymeric plasticizers, however

the particle size is infinitely smaller. The problem it invades all the nooks and

crannies of the mesh; resulting in the worst of all three types of ink haze. If you

have hazeno matter what you have triedget your screen chemistry and to

ink supplier and ask them to find an equitable solution.

Real-World Results

While it is difficult to provide exact specification for your shop (due to the vari-

ance in equipment, products, etc.), we furnish a starting point through real-word

examples offered as a guideline. Our tests were conducted at Kiwos laboratory.

They provided lab personnel and the specified emulsion. Dynamesh provided the

specified screen mesh. Screens and stencils were prepare to our specifications,

measurements taken on all typical parameters of the stencilthe results can be

found in figure 2.30. This test gives you actual case studies, so that you can gauge

the performance of your stencils. Nothing was done to force the results; no tricks

or unusual techniques were used. These stencils are just as you might process in

your plant.

PPrreeppaarreedd TTeesstt-SSccrreeeennss

Column One lists the nominal mesh count in inches. Column Two the thread

diameter in microns. Column Three lists the initial tension in Newton's per

square centimeter. Column Four is the method of coating used. For example if it

reads: 2 imp-3 sqg.-Dry-1 imp, the screen was coated two passes on its impres-

Chapter 2

N/cm²

Method

Edge

Type

Dry

Moisture

EOM

80S

71m

1 Imp. - 1 Sqg.

Rnd.

Dual

1.0

< 4%

16.2

25.0

150S

45m

1 Imp. - 1 Sqg.

Rnd.

Dual

1.0

< 4%

9.5

14.2

230S

40m

2 Imp. - 1 Sqg.

Rnd.

Dual

0.5

< 4%

10.4

6.6

305T

35m

2 Imp. - 2 Sqg.

Rnd.

Dual

0.3

< 4%

8.3

5.8

Dry - 1 Imp.

Sharp

Figure 2.30 The table shows the real-world results of our four test screens, coated, exposed, dried and developed.

Although your screens, methods and results may vary, the data serves as a guideline for your stencil methods.

PAGE

. 41

sion side, turned and given three passes on the squeegee side, dried and given one

more passes on the impression side. Column Five is the edge of the coater

round or sharp. Column Six is the type of emulsion used. Column Seven is the

time it was given to dry in decimal hours. That is, 0.5 hours is equal to thirty min-

utes. Column Eight is the moisture content as a percentage after the given dry

time. Column Nine is the R

flatness of the stencil. And Column Ten is the

emulsion over mesh. These two were measured after the stated exposure time.

Figure 2.31 shows the results of the exposure test on the four test-screens.

Column One gives the mesh count in inches. Column Two is the thread diameter

in microns. Column Three is the type of lamp used including the wattage. For

example a 5kW MH is a 5000-watt metal halide lamp. Column Four is the dis-

tance from the lamp to the vacuum frame glass during exposure. Column Five is

the exposure units. Column Six is the exposure time. Column Seven is the weight

of one square inch of coated mesh (the weight of the mesh is not included in

this number). This will allow you to compute the cost of the product coated to

this level. Column Eight is the step on a 21 step transparent gray scale. Column

Nine and Ten are the positive and negative resolution in mils.

TThhee CCoosstt OOff TThhee CCooaattiinngg

The table shown in figure 2.32 gives the weight of the mesh alone on the four

Stencils

Mesh

Count

Fabric

g/in²

1 Coat

g/in²

Pct.

Gain

1 + 1

g/in²

Pct.

Gain

1 + 2

g/in²

Pct.

Gain

1 + 3

g/in²

Pct.

Gain

80 S

4.0 g

.14

3.5%

.25

6.2%

.35

7.9%

.45

9.9%

150 S

4.0 g

.09

2.2%

.16

4.0%

.23

5.7%

.29

7.2%

230 S

4.1 g

.06

1.4%

.09

2.1%

.12

2.9%

.15

3.6%

305 T

4.0 g

.04

1.0%

.06

1.5%

.08

2.0%

.10

2.5%

Figure 2.32 The table shows the resulting weight of coating applied for single and multiple coats of emulsion on our

four test screens.

Lamp

Distance

Units

Time

g/in²

Step

Pos. Res.

Neg. Res.

80S

71m

8kW

50

150

150 sec.

0.2

8.5

8.0

150S

45m

8kW

50

75 sec.

0.2

3.5

3.0

230S

40m

8kW

50

55 sec.

0.1

3.0

2.5

305T

35m

8kW

50

45 sec.

0.1

2.5

2.0

Figure 2.31 This table shows our test screens and in grams per square inch the amount of coating that was applied

and then the resulting EOM and Rz for this application.

PAGE

. 42

test screens. We then applied several coats of emulsion, weighed the mesh after

each coating was dried and recorded the increase in weight (coating only weight).

The coating used has 50-percent-solids and so the dried coating is approximately

half of the wet coating.

Column One lists the four test screen mesh counts. Column Two lists the weight

of the mesh in grams per-square-inch. Column Three is the beginning of the

application of the coats, one pass only and lists the weight of that coating.

Column Four lists the weight in Column Three as a percentage. Column Five lists

the results of 1-plus-1-coats and so on, up to and including 1-plus-3-coats.

Highlights from the table are plotted in figure 2.33 .

Specifically, an 80S mesh weighs 4.0 grams per-square-inch uncoated. If you

apply one pass with a round-edge coater, you are adding 0.14 grams per-square-

inch. This is equal to a 3.5 percent increase over the weight of the mesh. Look at

the end of the row with 1-plus-3-coats of emulsion on the uncoated screen. An

addition of 0.45 grams of emulsion per-square-inch is the result, which equates

to a 9.9 percent increase over the uncoated mesh.

Chapter 2

100

150

200

250

Number of Screens

1 1+1 1+2 1+3

Coating Method

Screens Per Gallon

80S

150S

230S

305S

Figure 2.31 The graph shows the number of our four test screens that can be expected from the four coating

methods used. Of course the single coat would yield the most screens but only at the sacrifice of resolution and press

performance.

PAGE

. 43

This data should not be construed as a limit for the number of coating you apply

to your screens. That decision should be based on resolution and press perform-

ance. It is intended to help you see the results of multiple coats of emulsion

applied to a given mesh count.

The table shown as figure 2.32 is an extension of the data in figure 2.30. It uses

the same four screen meshes and lists incremental amounts of coating weight to

each of the screens. It then estimates the number of screens that could be coated

in that manner from a gallon of 50% solids emulsion.

Specifically, 305 T mesh, prepared with 1-plus-2 coats would have a coating

weight of 0.35 grams per-square-inch. Such a coating weight would allow you to

coat a 17" x 24" area of mesh for a yield of 108 screens-per-gallon.

Stencils

Mesh

Count

Fabric

g/in²

1 Coat

g/in²

Pct.

Gain

1 + 1

g/in²

Pct.

Gain

1 + 2

g/in²

Pct.

Gain

1 + 3

g/in²

Pct.

Gain

80 S

4.0 g

.14

3.5%

.25

6.2%

.35

7.9%

.45

9.9%

150 S

4.0 g

.09

2.2%

.16

4.0%

.23

5.7%

.29

7.2%

230 S

4.1 g

.06

1.4%

.09

2.1%

.12

2.9%

.15

3.6%

305 T

4.0 g

.04

1.0%

.06

1.5%

.08

2.0%

.10

2.5%

Figure 2.32 A 26" x 35" OD Challenger screen has a nominal coated area of 17" x 24" or approx-

imately 400 square inches, coated with a round-edge coater, at 50 percent solids.

100

150

200

250

Quantity of

Screens

80 S 150 S 230 S 305 T

Screen Mesh Counts

Screen Coating Estimator

1+1

1+2

1+3

Figure 2.33 The Screen Coating Estimator allows you to gauge the number of screens that you can get

from a US gallon of emulsion for our four test screens under the coating methods described herein.

PAGE

. 44

If you alter the conditions, obviously you will change the

resulting estimate of screens-per-gallon. A sharper blade, more

passes, intermittent drying, a coarser mesh-count than speci-

fied, a larger coated area or a thicker thread diameter will all

consume more coating.

To calculate a 21" x 31" OD Gauntlet screen with an nominal

coated area of 15" x 22", or approximately 330 square inches,

coated as in figure 2.32, multiply the number of Challenger

screens (listed in the same table) by 1.21the result is the

total number of Gauntlet screens that can be coated with a

gallon of emulsion. So you would get 107, 305 T screens from

a gallon coated 1-plus-3 wet-on-wet coats as applied in the

table.

To estimate a cost-per-screen take the number of screens and

divide it by the cost-per-gallon. For example, if you coat 88-

305T screens with a 1-plus-3 method, as described in figure

2.32, and your cost is $75 per-gallon, your cost per -creen is

approximately 85 cents. At a glance you can see that your

screen cost goes up, if you put additional coats of emulsion

on the screenshardly a surprise. But you must realize that

with the additional coats, there is a return on your investment.

If for no other reason then press downtime, you should use

the additional coats, they are a bargain regardless of your

image. Refer to page 2.13 for R

and EOM results and use

these to monitor your progress.

Chapter 2

1. Fewer pinholesstecil bridg-

ing is improved by the thicker

coating

2. Longer press lifestencil

elatisity is maintained and the

thicker stencil is more

resistent to squeege abrasion.

3. Sharper printing at higher

resolution is assured because

along with proper EOM,

comes an improved stencil

flatness.

4. Less downtime due to blur-

ring and dot gain because the

stencil gasket acts as a bar-

rier to the ink filling non-

image areas.

Benef

its Of Pr

oper

Coating Thic

kness

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