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

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PAGE

. 15

intimate contact with the film positive and light-undercutting will ensue. The

most blatant cause of a poor Rz is a stencil that is too thin. In such a case, the Rz

meter reads the undulations (hills and valleys) of the woven mesh, with the sten-

cil only minimally improving the readings. Once the proper EOM is achieved

(refer to page 2.13) a thicker coating can tend to become less flat. In theory, this

should not happen, but in the real-world this can

occur. It is generally due to the use of thicher

fabrics and/or round-edge coater blades that

leave thicker emuslison deposits with each pass.

Moisture gets trapped within the stencil and the

surface dries with a texture (not smooth). Refer

to page 2.14 for standards on EOM and Rz.

You may elect to use an Rz meter to establish

the actual flatness of the coating (see figure

2.10). The poor-man's Rz proof is to take a

credit card and lightly drag it across the surface

of a light table or the glass on a vacuum frame.

Then, as a comparison, drag the same card light-

ly on the press-ready emulsion surface. If you

feel the vibration or hear a buzzing, the surface

is not flat. A happy medium is to monitor your calibrated stencils with resolution

targets (refer to figure 2.17 on page 2.23 for details).

Note that a flat stencil on the impression side of the screen is fundamental to a

quality print. However, it can exaggerate the problem of after-flash tack. If this is

your situation, you will want to fix the ink (refer to page 3.18 for details) and not

resort to coarse fabrics with

high crimp angles and undu-

lated stencil coatings.

Drying The Stencil

The problem with excessive

moisture in the coating is

multi-faceted. First, you can't

expose the water out of the

coating; that must be done in

the drying stage. Second, the

Stencils

Figure 2.9 Majestech's Pocket Surf meter

allows you to determine the flatness of the

stencil on both the impression and the

squeegee sides of the screen.

Figure 2.10 Murakami shows a fine-line and space-configuration on a

350-screen mesh. These details are quickly lost if the screen has been pre-

exposed to extraneous light or if the washout is insufficient.

PAGE

. 16

coating won't expose proper-

ly; its cross-link density will

be too low and premature

breakdown is inevitable. The

soft stencil will invite solvent

and plasticizer penetration.

Third, during development

you may see scum or a cloudi-

ness clinging to the open

areas of the mesh. Fourth,

the developed coating will be

full of pinholes. Fifth, the

stencil may be very difficult to

reclaim, particularly after it is used on press.

The problem worsens as you realize that

humans cannot tell if and when the screen is

dry. The nominal amount of acceptable mois-

ture is 3 percent. A screen that has 10 percent

water content still feels dry, but will be a failure

on press with the flaws described above. A

stencil with three to four percent moisture con-

tent will perform extremely

well. It is the screens in the

area of five to nine percent

that we are most concerned

about. To gauge the perform-

ance of these, you will need a

moisture content meter (see

figure 2.11). The meter shown

ranges from three to twenty

moisture content and is

labeled with green for go, yel-

Chapter 2

1. Be sure your stencil has a consistent coating for

even drying..

2. Dry stencil in a warm, clean, low-humidity room or

cabinet.

3. Dry the stencil with the print-side down.

4. Use high-volume consistent air changes.

5. Time the drying period.

6. Use a moisture-content meter to verify the mois-

ture is less than four percent before exposure.

Drying The

Stencil Coating

How To

Instructions

Figure2.11 Majestech's Aqua-Check

meter will allow you to easily determine if the

moistuer content of the stencil is below 4%

and ready to expose.

Figure 2.12 Murakami shows this photomicrograph of a shadow

dot structure. These dots are "islands" and the first to sink in the case

of a screen loaded with moisture.

PAGE

. 17

low for caution and red to

stop your use of the wet

screen.

CCoonnddiittiioonnss AAnndd TTiimmee

FFaaccttoorrss

The obvious questions: Are

my screens dry enough and

how long should I wait? And

we would love to give you a

specific answer. Unfortunately

it would be just that, specific

and, therefore, not very help-

ful unless your conditions

matched our test parameters

exactly.

The fundamental conditions

for drying are the coated

weight of the emulsion, the

surface area exposed, the

vapor pressure of the coating,

the vapor pressure and tem-

perature of the surrounding

air, and the nature of the air

flow.

The coating weight is a com-

posite of the viscosity of the

emulsion, the edge of the

coater and the number and

location of passes applied to

the screen. The surface area

of the coating exposed to the

air is the second factor.

Therefore, a fine- mesh

screen (with lower surface

area) will dry faster than a

coarser-mesh screen.

Stencils

1. Guesstimate that the best exposure time is one

minute.

2. Double the time and run the test for two minutes

(one minute guess, times two = two minutes).

3. The results show the best target for detail is either

50 percent or 75 percent (shown in figure 2.13).

4. The results show my best target for color is 75

percent.

5. Select 75 percent as the factor.

6. Take the two minutes test time, multiple it by 75

percent, the calibrated exposure time is one and

one-half minutes or one minute, 30 seconds.

Exposure Calibration

How To

Instructions

Figure 2.13 Autotype's Exposure Calculator allows as few as a

single test exposure to pinpoint your proper production exposure time or

units. Upon close examination, you can see that the filtered areas look

bleached out. This indicates under exposure.

PAGE

. 18

The evaporation of the emulsion is a function of the vapor pressure of the coat-

ing as well as the vapor pressure of the surrounding air. The vapor pressure of

the emulsion is that state of equilibrium between the liquid and vapor. As tem-

perature increases the vapor pressure increases in direct proportion.

As an example, you can increase the temperature of water until it finally boils at a

pressure of one atmosphere and 212° F. In your drying room or cabinet, the

ambient air has its own vapor pressure and it may or not cooperate. The greater

the difference in the vapor pressure of the ambient air and the vapor pressure of

the coating, the faster the coating will evaporate.

Be cautious if you are using warm air to accelerate the drying of your coated

screens. Of course it will be very effective, but heat can partially expose the

emulsion on its own and, with too much heat (try to stay below 110° F), the shelf

life of the screens is in jeopardy.

Airflow may seem so simple, but you must realize that it is extremely complex

and not at all intuitive. For our purposes, we will not attempt to explain or define

this aspect; rather, we will try to alert you to the existence of its subtleties. The

dry time of a screen will be affected by whether the air is impinging (directed at)

or flowing parallel to the surface of the screen. For this and the reasons stated

above, it would be meaningless to isolate a standard dry time for a coating.

AAddddiittiioonnaall CCoonnssiiddeerraattiioonnss

The room or drying cabinet should be as dust-free as possible and in, the typical

T-shirt plant, this could be quite a challenge. Clean-room environments are

beyond the scope of this book, but they are advantageous. Short of that, you

may invest in washable indoor/outdoor carpet at the entrance to the room, along

with tack tape for the floor to remove any dust and dirt from the bottom of your

shoes. An air cleaner in the room, alongside your dehumidifier, will help

immensely. Finally, at the auto-body or hardware store, pick up a supply of tack

rags. These are gauze cloths coated with shellac and solvent. They are dust mag-

nets that can be used to clean the screen and not leave anything behind.

Be forewarned that screen emulsions are not totally insensitive to all yellow lights,

particularly if the lights are high intensity (formulas relating to this topic may be

found on page 9.1 of the appendix). If they wash out great when the screens

are fresh, but fight washout once the screen sits in the darkroom for a time,

then your lights may be the problem.

To test, use black electrical tape, applied to both sides of the screen, forming an

Chapter 2

PAGE

. 19

X-shape. Let the coated screen sit as close to the lamp for as long as is nor-

malseveral hours is minimum and several days is typical. Then take the unex-

posed screen to development and remove the black tape. If you have a stray light

problem, the X will washout more easily than the surrounding emulsion.

HHuummiiddiittyy AAnndd MMooiissttuurree CCoonnttrrooll

There are a few tools that will allow you to test and determine the time required

for drying. The first purchase you should make is a thermometer and hygrometer

for the drying room. These will show that a certain range of temperature and rel-

ative humidity can have a serious affect on the dry time. The rate of evaporation

is directly proportional to an increase in temperature and a decrease in relative

humidity. A moisture-content meter (refer to page ___for details) can be a life-

saver since no one can tell by sight or touch whether a screen is dry. It is possible

to monitor airflow, but unless you are building a drying cabinet, the other recom-

mended gauges should suffice.

Exposing The Stencil

Stencil exposure is critical for two aspects of quality production. First is image

accuracy. If the screen is over-exposed, detail is lost, the size of the image is

altered and registering one screen to another or color accuracy can be a problem.

Second, if the screen is under-exposed, pinholes and premature breakdown on

press is assured.

Before you try to establish

and control proper exposure

you may want to review ten-

sioning methods (refer to

page 1.43), coating (refer to

page 2.14) drying (refer to

page 2.16)and frame size

(refer to page 1.21).

CCaalliibbrraattiioonn

There are two steps to stencil-

exposure control: calibration

and monitoring. Calibration is

the stage prior to production

that accurately determines the

Stencils

Figure2.14 Shown is the Autotype Exposure Calculator to help cali-

brate your screen exposures. This film positive effectively runs five exposure

tests in one exposure. Complete instructions are included with the test film.

PAGE

. 20

proper exposure based on a test sample of your normal conditions. Although

there are several methods of calibration, we will detail the use of an exposure cal-

culator by Autotype. These are available at a nominal charge from your supplier

or provider of stencil products.

The principle of the exposure calculator is the use of several highly detailed,

measurable images. Behind these is a series of progressively darker filters, from

clear to very dark (see figure 2.14). The filters relate to each other in that they

transmit incrementally decreasing amounts of light. The first transmits all of the

light and is marked 1.0. The second transmits seventy percent of the light, and

is marked 0.70; the third, half of the light; fourth, thirty-three percent and

fifth, one-quarter.

Start with two screens of identical mesh and stencil, and prepare to expose the

screens with the calibration target. Guesstimate your exposure in units or real

time, and then double it only for this test (see how-to on page 2.17). Expose at

the double time, then develop

the screen as normal and view

the results while the image is

still wet. One image will look

better than others. You are

looking for two things: the

cleanest detail in the star tar-

gets at the center of the scale,

and a background color that

resemble the color of the

Factor 1 scale (see figure

2.16).

Of the five factors, if your

test is not one of the middle three, you will want to run the test again. The rea-

son is simple: if your test indicates the best scale is at either end, you can't

determine if more or less exposure would be better.

Using an exposure calculator you will determine the correct exposure time for the

mesh count/color, type of emulsion and coating procedure used during the cali-

bration. The exposure time will allow a highly detailed screen with excellent dura-

bility on press. If any of these factors change, you should re-calibrate for the new

or different conditions.

There is a method of exposure calibration that is very precise. That is to use a

radiometer with its probe behind the exposing screen. It works quite well with

Chapter 2

Figure 2.15 Autotype's Exposure Calculator allows as few as a single

test exposure to pinpoint your proper production exposure time or units.

PAGE

. 21

diazo type coatings, as the diazo bleaches out as the crosslinking progresses. On

the other hand, pure photo-polymers do not bleach at all and there is no plateau

upon which to establish sufficient exposures. If you are going to evaluate dual-

cure coatings, approach cautiously.

CCaauuttiioonnss

You will want to resist the temptation to under-expose in order to hold detail. If

you underexpose a screen, much of the coating washes down the drain and the

result is a pinhole-ridden, thinner stencil that will only appear to hold superior

detail. Pinholes are always in the perfect center of the mesh opening which tends

to be the thinnest part of the coating. Unfortunately, this water-eroded coating is

not robust enough to act as a gasket for the transfer of ink.

MMoonniittoorriinngg

Once you have calibrated (above) your proper exposure times for each specific

mesh and stencil, you will want to monitor the results to see if and how much

they vary from the standard. There are two very inexpensive yet very accurate

film-positive tests that can ensure the accuracy of every screen you shoot.

Immediately after calibration, select a prepared screen identical to the one that

has just been calibrated.

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The first film-positive is a 21-step, transparent gray scale

(shown in figure 2.16). It is a numbered gradient of 21 sepa-

rate densities of gray filters. Step 1 is clear and step 21 is the

most dense or opaque black. These steps are mathematically

stepped, their densities related to

each other. Step 1 transmits (relative

to the scale) all the light. Step 21

transmits only 0.1 percent of the

light. Each step is approximately 15

percent more or less dense than its

neighbor.

For example, you calibrate a screen

and stencil and the very next screen

washes out to a solid step 9 (see figure 2.17). Step 9 will

forevermore be the proper step for screen development

for the calibrated conditions. Lets say that things run for

several weeks and then, to celebrate, you buy a coating

Stencils

Figure 2.16

This is a

Stoffer Scales

21-step trans-

parent resolu-

tion guide. It is

a film positive

that can be

used over and

over again to

gauge consistent

stencil expo-

sure.

Figure 2.17

This is a Stoffer

21step transpar-

ent gray scale

imaged on a

screen. This scale

is processed to a

solid step num-

ber nine.

PAGE

. 22

trough that looks about the same as the old one. But its edge and length are dif-

ferent and it deposits a thicker coating of emulsion. It is not apparent to the

naked eye and the screen goes for exposure and development. When the screen is

developed, it turns out to be a solid step 8, exactly one step from the standard

step 9.

The step 8

screen is under-

exposed

because of its

thicker coating;

you should

refer to the

look-up table

on page 2.43

and increase the

exposure by

approximately

43 percent (the

same as divid-

ing it by 0.70).

If you do so,

the next screen

will be back to

a solid step 9 on

the scale. The look-up table will assist you in maneuvering on a scale, without

having to reshoot the exposure calculator. But beware, this is only accurate when

you are one or two steps off on the 21-step grayscale.

You may think a 15 percent change is not very much and you would be cor-

rect. Such a variance can rarely be detected on a press. And that is just the point,

no one will be able to tell that there is variability in the screen exposure, the

results will be that accurate. But now the screen department will have evidence of

accuracy and the press operators won't have to wonder.

RReessoolluuttiioonn TTaarrggeett

Resolution is the ability of the stencil system to accurately reproduce a minimum

image.

A resolution target is a transparent film-positive with divergent scaled lines and

spaces. This particular scale has divergent lines that range from 1/10 of an inch

Chapter 2

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IIff yyoouu  TThheenn yyoouu sshhoouulldd 

need to increase by one step, divide time or units by 0.70.

need to decrease by one step, multiply time or units by 0.70.

need to incr

ease by two steps, divide time or units by 0.50.

need to decr

ease by two steps, multiply time or units by 0.50.

need to incr

ease by three steps, re-calibrate.

need to decr

ease by three steps, re-calibrate.

PAGE

. 23

at one end to 1/10,000 of an inch at their tips. One side is composed of black

lines and clear spaces; the other has clear lines and black spaces (see figure 2.17).

Your limits of resolution are many fold but will all be evidenced by a filling in of

the lines and spaces or by a moiré pattern. The loss of lines and spaces is detailed

immediately below.

This scale can act as a crosscheck to the gray

scale (described immediately above). By inspect-

ing the limits of the lines (highlights) and spaces

(shadows) and comparing the two under a loupe,

you can establish the degree of exposure balance.

If the screen is properly exposed the two sides,

positive and negative of this scale should be simi-

lar. For example, if the scale shows a 5-mil (0.005") line on the one side, then the

other side should have a space resolution approximately equal to the 5-mil line.

(figure 2.17) The two sides of the scale need to be similar, but the space or nega-

tive side of the scale is generally easier to resolve than the line or positive side.

Stencils

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PPrroobblleemm DDeessccrriippttiioonn

Over-exposed: Highlight loss, shadows clearer but print poorly.

Under

-exposed: Shadow loss, highlights clearer but print poorly.

Coating too thin: No exposure latitude, poor detail, star-like moiré

in image.

Coating too thick: Difficult to develop, tips of lines washout.

Mesh too coar

se: Abrupt stoppage of lines, very short range.

Thread too thick: Star-like moiré patterns and jagged edges at finer

ends of lines.

Stencil too w

et: Erosion causes coating to become thin; targets

print poorly.

Insufficient vacuum: Loss in highlights, gain in shadows, printed tar-

gets looks like short tonal range.

Note: It is critical that you wash out these scales just like you do the balance of the

image; do not focus on or baby their development. You may want to turn to the side-

bar on page 2.35 to review our recommended procedures for development.

Figure 2.17 " is a Stoffer Scales trans-

parent resolution guide. It is a film positive

and can be used over and over again to

gauge the resolution level of the screen.

PAGE

. 24

Remember that the resolution should be comparable between the positive [lines]

and the negative [spaces] on the scale.

Its most affective test is to gauge the stencil or coating thickness as a benchmark

of detail capability. Once your detail surpasses the level of the mesh, (and such is

the case in virtually all halftone printing) to sustain it now you must rely on the

stencil. A coating that is too thin (the typical case) does not rise above the surface

of the mesh and any detail will be lost as there is no intimate contact between

stencil and positive. A coating that is too thick will require excessive exposure,

and light diffusion will cause image edges to deteriorate in the print. The resolu-

tion target will help youdetermine edge acuity (the definition at the edge of an

image). On a coating that is too thick or too thin, the quality of the lines will be

distorted. Thin coatings will leave a ragged edged, saw-toothed line. Excessively

thick coatings will leave an edge that has been exposed by diffused light so the

size is altered, lines get smaller and spaces(between the lines) get larger.

EExxppoossuurree VVaarriiaannccee

In this chapter we will discuss many of the causes of exposure variancelow

EOM and R

, inadequate vacuum, lamp undercutting and non-integrated light

sources. This segment deals with two of the results, under-exposure (the univer-

sally prevalent symptom) and over-exposure. Further, it deals with moisture con-

tent, a very common but unforeseen cause of under-exposure and stencil break-

down. Finally, it discusses the all-too-often senseless results of post-hardening, or

virtual exposure.

Proper lamp placement, adequate vacuum, balance of lamp and coating, and con-

trolled output are the variables that are detailed in these pages. We have also rec-

ommended low-cost target films to calibrate and monitor screen exposure.

Exposure is critical even where there is a minimum of detailif a screen is

under-exposed, and most are, reclamation will be back-breaking. If there is a

modicum of detail in the image, watch out, exposure can mean the difference in

pass or fail for the image.

UUnnddeerr-EExxppoosseedd

This is most common because the most common method of inspection in the

screen room is to hold a screen up to the light and, with the unaided eye, make a

call on the quality of the exposed and developed stencil.

Under-exposure is a knee-jerk reaction to a variety of problems: insufficient vacu-

um, poor R

value, low EOM and a mesh that is too coarse, twill woven or has

Chapter 2

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