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

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PAGE

. 5

duction. A "faster" reacting coating will be more sensitive to

time (or exposure units) and to thickness of the coating. With

the faster coatings, particularly the "pure photopolymer" coat-

ings, your conditions should be closely calibrated to insure suc-

cess. (Refer to page 2.19 for calibration and monitoring).

Reclaimability should not be counted as crucial as those facets

of the product that slow down productivity (transparency for

example, see side bar page 2.3). The cost of off-press func-

tions should not be weighed as high as on-press functions.

Nevertheless for reclaimability you may trade water resistance,

but the bulk of the process is in the hands of your exposure

and drying (refer to pages 2.19 and 2.25). Insufficient drying

and/or under exposure can make the job of reclamation very

difficult. (Note that some coatings are more difficult than oth-

ers to reclaim, so check with your supplier on a starting point,

then be sure you don't make the condition worse).

Multiple wet coats and coat-dry-coat are cross grain to the

desires of both the emulsion suppliers and screen room per-

sonnel. Some of the emulsion suppliers don't want to have to

say that the best results are achieved with the coat-dry-coat

method: its not what the printer wants to hear. And the

screen-room personnel are under pressure to get the screen to

the production floor: coat-dry-coat takes more time. However,

for the best real-world outcome, coat-dry-coat produces supe-

rior results. Even the premium products shrink toward the

knuckles of the mesh; depending on your conditions and qual-

ity standards this shrinkage may not bother you but will be

affected by mesh geometry, coating thickness, type of coating

and vacuum conditions. With coat-dry-coat, you allow the

emulsion to shrink which forms an irregular surface; then you

return after drying, probably with a thinner blade, to put a

thin, flat finish coat on the surface, that fills in the valleys

caused by shrinkage.

Transparency of the dried emulsion is crucial for image regis-

tration on the T-shirt press. The best solution is to use the

patented M&R Tri-Loc system so that any need to see through

the screen is minimal. Otherwise, dyed emulsion, which is

often the complimentary color to the screen mesh, will offer

Stencils

1. The tension level of

the screen.

2. The edge of the coater.

3. How much force you

want to apply.

4. If you have permanent

blockout around the

perimeter.

5. If you wipe up the drips of

coating near the corners of

the frame.

6. The maximum image size.

ariables Ef

ecting

Selection of Coat

Length

PAGE

. 6

Chapter 2

the best resolution and latitude of exposure in trade for the

ability to see through it with ease. Before you might under-rate

this aspect, look to page 6.7 to see the value of one minute of

downtime.

Coating The Stencil

When you are ready to have flawlessly consistent screens, color

after color, job after job and year after year, you are ready for a

Digikote, digitally controlled screen coater (see figure 2.1).

There are a variety of coaters on the market and, though they

all look similar, Digikote has solved the one problem that

exists with all other coating machines: how to apply pressure

consistently (see figure 2.2).

Electronic drivers on each side of the screen apply consistent

pressure on the face of the screen regardless of your airflow,

Round-edge coaters:

v allow a faster coating speed

than sharp.

v bridge coarser (160 and

below) fabrics.

v create less turbulence (air

bubbles) on coarser fabrics.

v apply more coating with

fewer passes.

v allow a wider range of [oper-

ator] force.

v require more [operator] force.

v should be slightly shorter

than sharp.

v are less sensitive to the angle

(coater to screen) of attack.

Sharp-edge coaters:

v work better on higher tension

levels.

v more accurately meter the

coating onto the mesh.

v are definitely best for face

coating.

v are generally better for finer

(160 and up) mesh counts.

ound vs Shar

Figure2.1 Frontal view photograph of the Digikote automatic screen coater.

PAGE

. 7

age of the machine or characteristics of the screen. No other coater can make

that claim. All T-shirt printers print too close to the edge of the screen; it is a

requirement of multi-color

press screens. Now you can

have a stencil that is the same

thickness at the edge as in the

center. Digikote is fully pro-

grammable to coat the front

and/or the back of the

screen.

Digikote comes with two

coating blade edgesa .050"

and .080" (one and two mm).

The .050" is typically used on

finer mesh counts of 230

and up, while the .080" is

used for 230 mesh counts and

down.The .050" lays down a thinner deposit and results in a smoother surface.

The .080" lays down a heavier deposit, which creates a rougher surface. For the

ultimate application you can use the .080" first and finish with the .050" (refer to

figure 2.5).

CCooaatteerr LLeennggtthh

The proper selection of a

coater length depends on a

few variables. If your tension

is low, below a nominal 20

N/cm² (Newtons per-square-

centimeter), then a shorter

coater length is recommend-

ed. If you are using a round-

edge coater a shorter length is

also in order. The longer the

coating trough, the more

force you will have to apply

to get an accurate stencil

even more force if the edge is

rounded. If you have perma-

nent blockout on the perimeter

Stencils

Figure 2.2 Electronic pressure sensor from the Digikote.

Figure 2.3 The width of the coater should be larger than the maxi-

mum image width yet leave ample room between the coaters ends and

frame for consistent coatings center-to-edge.

PAGE

. 8

of the screen, the coater

should not pass over the

blockout. As the length

increases you will spend more

time contending with runs

and puddles in the corners of

your retensionable screen.

To be practical, the coater

should be two inches longer

than your maximum image

size (see figure 2.3). The limit

of the coater length is predi-

cated upon the consistency of

the coating from the very

edge to the center. You may

not care if the margins

[edges] are properly coated,

but consider this: as the

coater gets closer to the edge,

you need more force to create

equal pressure across the sur-

face of the screen. If the

pressure is unequal, so will be

the coatingthicker in the

center. Resolution is lost and most of the coating that you paid for will wash

down the drain during development wash-out, due to underexposure. The con-

sistency of the coating can be assessed by a gray scale and resolution target test.

(Refer to page 2.21 and 2.23 for details). Mount one of each of the scales to the

middle of the screen and one at the edge. Expose as normal, develop the

images, then compare the two scales. The solid step for the center should match

the solid step for the edge.

Chapter 2

1. Length must be 1" greater than the maximum

image width.

2. Length must be 2" (or more) shorter than the

frame I.D. (interior dimension).

3. Gauntlet = 17" to 18"

4. Challenger = 20" to 21"

5. The edge of the coater must:

a. Fineness of mesh thread and count.

b. Resolution goals.

c. Viscosity of emulsion coating.

d. Applied force, screen tension, coater length

and frame I.D.

e. Intended number of passes.

f. Intended dry time.

g. Coating speed.

Selecting a Coater

How To

Instructions

Coating

Method

Edge

Type

EOM

Step

Pos.

Res.

Neg.

Res.

80/71

1 Imp. - 1 Sqg.

Rnd.

Dual

16.2

25.0

8.5

8.0

150/45

1 Imp. - 1 Sqg.

Rnd.

Dual

9.5

14.2

3.5

3.0

230/40

2 Imp. - 1 Sqg.

Rnd.

Dual

10.4

6.6

3.0

2.5

305/35

2 Imp. - 2 Sqg.

Dry - 1 Imp.

Rnd.

Sharp

Dual

8.3

5.8

2.5

Figure 2.4 Test Screen Look-Up-Table

PAGE

. 9

On a Challenger screen, the coater should be a maximum of

22" wide and, on a Gauntlet, 18" wide. If at all possible, use of

a narrower coater can save you dollars. It is our recommenda-

tion that you never try to coat to the very edge of the frame.

CCooaatteerr EEddggee((ss))

In the battle of the blades edges, there is room for both round

and sharpit just depends what you want to accomplish (see

figure 2.5). The best of all worlds is, at times, to apply the ini-

tial coats with a round-edge blade, then to finish with a sharp-

edge. To decide, see the coating tables, (figure 2.4) for details

on how we coated the four test fabrics. The four test fabrics

were choosen to allow a wide range of printing applications

and all four offer excellant ink transfer and coverage. Or you

can use the infor-

mation contained in

this section to

decide for yourself,

if and when to use

each type of coater

blade.

The Digikote offers

two standard blade

edges, a 50-mil

sharp and an 80-mil

round. Available

coaters in the field

range from a 20-

mil sharp edge to a

100-mil round edge.

The general fea-

tures of round and sharp blade edges can be found in the

sidebar on page 2.6.

Round edges are more forgivingand can have its time and

placebut, for the highest resolution imaging, while a sharp

will be more demanding the results are worth the effort.

LLoossss ooff HHiigghhlliigghhtt DDoottss

v Add more impression-side

coats.

v Over-exposed.

v Inadaquate vacuum.

LLoossss ooff SShhaaddooww DDoottss

v Add more impression-side

coats.

v Development too

aggressive.

v Under-exposed.

LLoossss ooff BBootthh

HHiigghhlliigghhtt && SShhaaddooww DDoottss

v Add more impression-side

coats.

PPiinnhhoolleess ((cceenntteerr ooff mmeesshh ooppeenn-

iinnggnnoott dduusstt rreellaatteedd))

v Coating too thin.

v Severe under-exposure.

v Moisture content too high

during exposure.

SSmmeeaarriinngg oorr bblluurrrriinngg dduurriinngg

pprreessss rruunn..

v Add more impression-side

coats.

v Related to press settings or

poor calibration.

IInnkk PPiilliinnggdduuee ttoo ttoooo mmuucchh iinnkk

vvoolluummee iinn ddeettaaiillss..

v Add more squeegee-side

coats.

v Sorely under-exposed.

v Should see scumming during

washout.

Stencils

encil

oubleshoo

ting

Figure 2.5 M&R will install a Delrin® knife-edge with

the coating trough, you specifya 50 mm sharp edge or an

80 mm round edge. This engineering plastic is virtually inde-

structible and is manufactured to the precise edge required for

coating emulsions.

PAGE

. 10

CCooaattiinngg TTeecchhnniiqquueess..

To gauge the potential of

your coating methods, use

either a meter to measure

both EOM (emulsion over

mesh) and Rz(flatness of the

emulsion surface), (refer to

pages 2.13 through 2.14) or

for a lesser investment (and

less quantification), use the

gray scales and resolution tar-

gets described on pages 2.21

through 2.23.

Proper coating techniques are

a delicate balance of the

emulsion parameters, ambient

conditions, exposure capabili-

ties, the mesh geometry and

tension, the particulars of the

coater blade, the Digikote set-

tings and operator influence.

We will assume that you have

quality film positives, a well-

prepared screen (refer to

chapter one), and that your

press is calibrated (refer to

chapter four). Use the trou-

bleshooting list in the sidebar

(found on page 2.9) to assist

you in adjusingt your coating

methods.

HHooww TToo CCooaatt WW

iitthh AA DDiiggiikkoottee

AAuuttoomm

aattiicc CCooaatteerr

Automatic stencil coating has

been made affordable by

M&R with the introduction

of the Digikote. For decades

Chapter 2

1. Select the proper length coater (page 2.7).

2. Use the prescribed coater edge (page 2.9).

3. Use a coating rack or support the screen at a

slight angle.

4. Fill the coater to the same point frequently.

5. Coat the impression side first.

6. Center the coater near the bottom of the

screen .

7. Apply firm pressure from left to right and

throughout the stroke.

8. Slow and steady maintaining contact, bring the

coater near the top.

9. Do not alter the angle of the coater.

10. Make additional passes as required (page 2.14).

11. * Turn the screentop to bottom(vertically) and

coat the squeegee side last.

12. Wipe the edge of the coater with a damp soft

cloth.

13. Refill the coater with emulsion frequently: do

not wait until almost empty.

14. Dry print side down.

15. Apply additional coats after drying if necessary

(refer to the how-to onstructions on page 2.14).

* Although it is recommended to flip screens vertically

during manual coating, the size of the T-shirt screen

rarely demands it. Flipping screen is done to compen-

sate for inconsistent pressure from side to side or bot-

tom to the top. Large format screens tend to show

more variance in pressure (deposit) as the operator

reaches the top of the screen. It gets increasingly harder

to maintain even pressure as the operator gets closer to

shoulder height. If you are experiencing a thickness

variance in your screens, you may want to flip your

screen, if notyou may choose to eliminate this step.

Manual Stencil Coating

How To

Instructions

PAGE

. 11

the most sophisticated print-

ers have known that to get a

quality print, you need a qual-

ity stencil. But until the

Digikote, automatic screen

coating was affordable only to

the largest plants.

The Digikote T2 accommo-

dates one or two screens at a

time with no variance in the coating (see figure 2.6). The T2 is intended for high

throughput plantsover 100 screens per day. Like all Digikote units it is fully

programmable for virtually any viscosity, profile or application of liquid emul-

sion coatings. Operating the Digikote is straightforeword; refer to the sidebar

above for instructions.

Your coating will be flawlessly consistent, dry times will be more predictable,

Stencils

1.. Turn the Digikote on (see manual for specifica-

tions).

2. Place a round coater(s) in the back of the Digikote.

3. Place a sharp coater(s) in the front of the Digikote.

4. Insert the frame into the self-centering clamps.

5. The impression side of the mesh should be facing

the operator.

6. Lower the top clamps to near the frame.

7. Push the frame-locking button, located on the cen-

ter of the top rail of the machine. The button

rigidly locks the frame into position.

8. Set the speed, begin at 5 ips (inches per second).

9. Set the pressure, beginning at ___ units.

10. Set the number of coats, front and back.

11. Store this procedure in memory.

12. Fill the coating troughs to the same level and refill

often to keep the level consistent.

13. Refer to table 2.31on page 2.42 for mileage.

14. Measure the results as described in this

chapter.

15. Push the start button to begin coating.

16. When done, release frame from holder.

Automatic Stencil Coating

How To

Instructions

Figure 2.6 Start, stop, frame

lock and releaseall program-

ming controls are conveniently

located on the eye-level control

panel, on the right hand side of

the Digikote.

PAGE

. 12

exposures will be more consistenttherefore press break-

downs will be reduced. Still, T-shirt screens offer some unusual

challenges for the stencil maker: Please refer to the sidebar left

for further details.

An intimate understanding of the T-shirt printing business

allowed us to design the Digikote to adapt to all of its particu-

lar needs and more. Digikote can be automatically pro-

grammed to handle your needs:

1. One round and one sharp coater blade come

with every Digikote.

2. Infinite speed control accommodates the

coarsest mesh counts.

3. Only Digikote can adapt to the deflection of

a textile screen.

4. Only Digikote is sensitive to the gamut of

screen tensions.

v Meshes range from 20 tpi

(threads-per-inch to 460 tpi,

however, T-sirt printers tend

to use relatively coarse mesh

counts.

v The image is very close to the

edges of the frame.

v Industry tension levels vary

up to over 50 N/cm².

v The quantities of [multicolor

press] screens per day is very

high.

v The wear on multi-color

screens is significant.

v No other process prints wet-

on-wet with plastisols.

v No other process must deal

with the heat created by flash

units over the platens.

Chapter 2

Obstacles Of The T

Shir

t Screen Mak

Figure 2.7 The Digikote model T-2 can hold two screens and nearly double your productivi-

ty without sacrificing coating accuracy or consistency.

PAGE

. 13

5. Programmable controls allow highest

throughput.

6. Sufficient and repeatable EOM eliminates

breakdowns.

7. Proper R

improves print quality.

8. Consistent coating weight allows consistent

and thorough exposure.

PPeerriimm

eetteerr CCooaattiinngg..

You may contact your supplier for a permanent blockout,

which can be applied to the perimeter of the low-elongation

mesh screen. The blockout seals the ink in and saves you time

and money on processing chemistry. Often, as much as 40 per-

cent of the screen can be stabilized. That could mean a sav-

ings of 40 percent in chemistry and the associated time for

using that chemistry. For example, the amount of reclaiming

solution is reduced, as is the time of reclaiming the smaller

area. If you choose to seal the perimeter, be mindful that it

must be a thin layer on the impression side or it will adversely

affect the coating thickness.

EEOOMM ((eemmuullssiioonn oovveerr mmeesshh))

nndd RR

((sstteenncciill ffllaattnneessss))

IImmppoorrttaannccee

These two measurements should be read in tandem and con-

sidered as a pair of data points. If you considering them singu-

larly, it can be very misleading. Refer to the table on page 2.14

for standards on both EOM coating thickness, as well as R

flatness.

EEOOMM

The basis for this measurement is to produce a stencil coating

to overcome the irregular surface of the screen mesh. Once it

has done so and the stencil has formed a gasket or relief on

the underside of the screen, it is usually thick enough. There

are always exceptions such as high-cut-pile terry towels and

high-density fine-detail printing, but for the majority of T-shirt

printing, the thick enough policy is sound advice. You may

1. A smaller well requires more

pressure for coating

consistency.

2. Coating too close to the

edges can harm the mesh.

3. Keep the coating trough

filled to a consistent level.

4. Begin at five ips blade speed.

5. Higher mesh counts allow

higher speeds.

6. Mesh counts that have a flat-

ter mesh surface allow higher

speeds.

7. If intermittent drying, use a

round-edged coater first.

8. Use a round-edge coater on

the squeegee and sharp on

the impression-side.

Stencils

Tips On Using The

Digik

PAGE

. 14

choose to use a thickness

gauge to meter the thickness

of the coating (see figure 2.8).

An inadequate coating thick-

ness leads to pinholes, very

poor image detail, poor expo-

sure latitude and premature

breakdown on press. If the

coating is too thick, the

dosage of exposure to get the

screen properly exposed

(refer to page 2.19) is exces-

sive. This leads to light diffusion and a loss in image accuracy. Further, such a

coating will begin to lose its flatness (due to a reduced ability to evacuate the

moisture from the screen)effecting the R

level, as well as its ability to transfer

ink (due to the long tunnel created by the too-thick stencil). The EOM reading

should be used in conjunction with a flatness or R

reading.

While you are achieving a coating that is just thick enough, it should be moni-

tored for flatness. If the impression side of the stencil is not flat, it will not hold

accurate detail. Even under the best vacuum, the coated screen will not develop

Chapter 2

1. Record your coating procedures (per mesh type).

a. Edge radius.

b. Passes per side.

c. Dryingposition and number of times.

d. Dryingtimes and environmental conditions.

2. Measure and record EOM.

3. Measure and record R

both sides.

4. Expose test screens with gray scales and resolution

targets.

5. Develop, dry and inspect.

6. Compare results to the tables on pages 2.40 and

2.41.

7. Interpreting results of scales:

a. Loss at positive and negative ends of scales

improve R

b. Generally low resolution of scalesimprove

EOM.

Determining Coating

Properties

How To

Instructions

Figure2.8 Majestech's TQM-ETG elec-

tronic stencil thickness gauge is designed

specifically to measure the stencil thickness in

microns. This is the best method of gauging

EOM.

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