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

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PAGE

. 25

threads that are too thick. Under-exposure appears to be a viable solution to

holding finer (highlight) details, the circle around the register mark, the serifs on

small type faces. . .and those specs that you thought would wash out.

Several things occur with an under-exposed screen: much of the coating that you

applied washes down the drain because, like the portions that did not get exposed

to light, none of the screen got enough exposure, and is still water soluble. There

is a likelihood of this scum filling some of the image areas.

Fine details like highlight dots do not plug but they will print poorly because of

the development-thinned stencil. Perhaps most consequential is the fact that an

under-exposed screen will show an abundance of pinholes (refer to page ___)

and breakdown prematurely on the press.

After the press run, the under-exposed stencil will continue to haunt you. The

soft, porous squeegee side of the emulsion goes into production and friction,

flash heat and plasticizer (the main component of the plastisol ink) terrorize your

stencil. The coating absorbs the plasticizer and, under the flash, it quickly

crosslinks the coating and bonds it thoroughly to the mesh. Now you will have a

difficult time reclaiming.

Finally, the bulbs begin burning out as soon as they are first used. The problem is

that they will remain bright to the human eye even after the actinic portion of

the energy is nearly gone. So the bright light will require an increased time and, if

your system has a light intergrator, it will compensate for the most part. Still use

the scales, though to monitor (refer to page 2.21 for details)

OOvveerr-EExxppoosseedd

With a flat coating (refer to page 2.14), adequate lamp distance and proper vacu-

um on the exposure frame, the consequences of over-exposure are relatively min-

imal. Because of the conditions typical in the T-shirt shop, over-exposure threat-

ens a significant loss of detail.

Once you achieve proper vacuum, the optimal light distance and a flat coating,

you can calibrate your exposures and your worry of over-exposure (or under-

exposure) is over.

ooiissttuurree CCoonntteenntt

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

for drying. The first purchase you should consider is a thermometer and hygrom-

eter for the drying room. These will show that certain temperatures and relative

humidity can have a serious effect on the dry time. The rate of evaporation is

Stencils

PAGE

. 26

directly proportional to an increase in the temperature and a

decrease in relative humidity. A moisture-content meter (refer

to page 2.16 for details) can be a lifesaver since no one can tell

by sight or touch whether a stencil is dry. It is possible to

monitor airflow but, unless you are building a drying cabinet,

the recommended gauges should suffice.

CCoonnttaacctt VVaaccuuuumm

Retensionable frames are quite popular in the textile-printing

industry; however, many of the frames in use are racked (twist-

ed), no longer laying flat. This condition causes vacuum frame

glass to break prematurely. The use of tempered glass is not

recommended as a solution, because it can obstruct the short-

er wavelengths of light that expose the emulsions.

One solution is to find a system that keeps the frame flat dur-

ing stretching. This can be accomplished by attaching a U-

shaped channel of metal to the stretching table. Line the inside

of the channel with foam to protect the mesh. Be sure that the

channel is large enough to slide your frame into with the foam

padding. When you begin tensioning, slide the far end of the

frame into this channel, it will keep the frame from twisting.

It is also wise to seek a balance between breaking the glass and

a proper vacuum. If you lower the vacuum too much you will

see: (1) a loss of detail, (2) a shift in tonal range (halftones

only), and (3) a change in image size. You may want to discuss

proper settings with the manufacturer of the vacuum frame.

NuArc units are shipped with a preset, but adjustable, calibra-

tion of 15 inches of vacuum. This number is based on actual

testing of images with resolution common to the screen-print-

ing industry.

It is important to note that vacuum is affected by humidity and

altitude. For example, on a rainy spring day in Atlantic City it

will be easier to achieve 15 inches of vacuum, than in Denver

on a typically dry day.

PPoosstt HHaarrddeenniinngg

Post hardening is an old trick that suggests you under-expose

in an attempt to hold detail; then after image development,

1. Integrate through the screen,

only if all screens are virtually

identical.

2. Don't force the limits of the

vacuum blanket.

3. If the glass is hot, take a

break.

4. Don't put a fan in the box of

a self-contained point-light

source.

5. Clean the ink off the frames

thoroughly before exposure.

6. Keep the glass cleanbe

sure that the glass cleaner

does not leave behind a UV

resistent film.

7. Be sure screens are not

racked before positioning.

8. Maintain the oil level in your

vacuum pump.

9. Do not replace vacumn

frame glass with tempered

glass.

10. Lower than 26 psi (on the

vacuum gauge) will sacrifice

resolution.

Chapter 2

TTiippss FFoorr SSccrreeeennss

AAnndd EEqquuiippmmeenntt

PAGE

. 27

expose the screen again for a longer period of time. If you are so inclined, con-

sult the manufacturer of the emulsion you are using to see if it has much post-

hardening efficiency. Many emulsions do not. Our test case emulsion offered a 30

percent improvementbut only at 600 percent post hardening time. You must

investigate on a per-case basis.

If the humidity level is high and the moisture content of the coating is over 4%,

your screen is not ready to be exposed. There is no amount of exposure time or

post hardening which will correct for excessive moisture. Refer to page 2.15 for

more details.

LLaammpp PPaarraammeetteerrss

There are pros and cons to both independent and integrated light sources. The

independent system can offer the most flexibility as well as the highest potential

quality. It is imperative that the position of the lamp be controlled to optimize

either the productivity or the image quality. Further, without integration, an inde-

pendent lamp is problematic at best.

The two types of self-contained exposure units are point-source and multiple-

tube light sources.

Multiple UV fluorescent tubes, self-contained unit (see figure 2.18):

1. are not readily available with light integrators on all tubes;

2. may build up heat and cross-link the stencil, prior to develop-

ment;

3. are often designed so that the light energy undercuts the image;

4. do not often peak at the optimal wavelengths of today's coat-

ings;

5. may work extremely fast or handle multiples of screens.

Point-light source, self-contained exposure units (see figure 2.19):

1. are best when integrated;

2. may build up heat and cross-link the stencil, prior to develop-

ment;

3. do not allow variable light-to-frame distancescheck center to

edge light falloff;

Stencils

PAGE

. 28

Chapter 2

1. Place lamp at a distance from the screen, equal to

the screens diagonal measurement.

2. Use two identical screens, ready to expose.

3. Use two factory calibrated 21-step gray scales and

two resolution guides.

4. Place one of each type of scale, centered on one

screen.

5. Place one of each at the extreme corner.

a. Corner of first screen if exposing one at a

time.

b. Corner of second screen if exposing multiples

at a time.

6. Load the screen(s) into the vacuum frame.

a. 1st set of scales directly in front of the lamp.

b. 2nd set of scales at the extreme corner of the

vacuum frame.

7. Be certain that you have a minimum of 15 inches

of vacuum pressure.

8. Expose, develop, dry and inspect.

9. Interpreting results:

a. Two sets of scales are identical.

i. And the quality excellent.

ii. Lamp may be moved closer to permit

reduce exposure time (page 2.31).

iii. Use graph in figure 2.25 to estimate

results of lamp movement.

b. Center scales indicate more exposure than cor-

ners.

i. Screen lacks durability and resolution.

ii. Lamp must be moved further away.

iii. And exposure time must be increased.

iv. Use graph in figure 2.25 to estimate

results of lamp movement.

Determining Lamp

Distance

How To

Instructions

Figure 2.18 NuArc's First Light is a

multiple tube light source.

Figure 2.19 NuArc's freestanding Metal

Halide exposure lamp.

Figure 2.20 NuArc's freestanding Metal

Halide exposure lamp.

PAGE

. 29

4. have metal halide bulbs that peak at desirable wavelengths;

5. multiple-up configerations limited by the physical design.

Independent-light source and vacuum frames(see figure 2.20 and 2.21):

1. best if integrated;

2. no residual heat buildup;

3. position lamp for bal-

ance in speed and pro-

ductivity ;

4. have metal halide bulbs

that peak at desirable

wavelengths;

5. multiple-up configera-

tions limited only by the

size of the vacuum frame.

LLaamm

pp IInntteeggrraattiioonn

The section below details the results of moving an independent light source clos-

er or further from the screen. The principles for center-to-edge light falloff may

be applied to self-contained point-light sources as well.

If you have a single-point light source, typically a bulb with a reflector behind it,

you can and should integrate the lamp. Integration is another form of monitoring

the exposure of a screen and should be used in tandem with the scales described

above. An integrator has three fundamental components: (1) a sensor to detect

the energy from the lamp, (2) an adjustable digital meter to determine the energy

level, time and again, (3) a switch to turn the lamp on and off.

An integrator compensates for a wide range of variables in the exposure process.

Some of these, critical to quality screen making, are:

1. altered lamp

distance;

2. energy loss from voltage or lamp life (at the prescribed or preset wave-

length);

3. energy falloff if the sensor is mounted on the vacuum frame.

Stencils

Figure 2.21 The SPVF from NuArc is available in a range

of sizes to serve your needs as a companion piece to a freestanding

light source..

PAGE

. 30

A seemingly minor change in the distance between the lamp and vacuum frame

can have a significant affect on its exposure time. The integrator (and the sensor

mounted on the vacuum frame) automatically compensates for any change in this

distance (refer to page 2.31 for graphs that relate to this topic).

LLaamm

pp DDiissttaannccee

As a rule of thumb, your independent

light source should be the distance of the

diagonal of the frame or frames that you

intend to expose at one time. For exam-

ple, if your screens are 26" x 35" and you

are doing two at a time in the vacuum

frame, your lamp should be a minimal 62"

from the surface of the vacuum frame

(see figure 2.22). This ensures that the

light intensity at the edges of the screens

is 70 percent of the amount in the center

of the glass. Keep in mind that the

increase will force a longer exposure time.

LLiigghhtt FFaallllooffff ((cceenntteerr-ttoo-eeddggee))

There is a law of physics that describes the relationship between the amount of

light directly in front of the lamp versus the amount in the corners (or any other

point).

That law dictates that, if you place the lamp at the distance of the diagonal meas-

urement of the frame, you are assured of 100 percent of the light in the center

(directly in front of the lamp) and 70 percent of that amount in the corner of

the frame.

Move the lamp farther and the ratio of center-to-edge light improves but the

exposure will need to be increased. Conversely a closer lamp needs less exposure

but will leave the corners of the image even more under-exposed.

There is a tendency to think that more is better. That is not often the case here.

Figure 2.27 on page 2.34 shows the consequences if one or multiple screens are

Chapter 2

Figure 2.22 Move closerexposure time decreases,

but the consistency from center, light-to-edge light is com-

promised. The minimum distance is the diagonal of the

frame(s) resulting in a ratio of center-to-edge light of

70 percent.

New

Exposure

Time

Old

Exposure

Time

New distance

Old Distance

PAGE

. 31

exposed at one time. There is one real advantage to exposing more screens at

once; the time for the vacuum drawing down is less, as a percentage of the total

time. But even for a vacuum frame that would hold four Challenger frames, the

vacuum draw-down time should be less than one minute.

Stencils

Time Change With Distance Change

50%

100%

150%

200%

250%

1.5 1.6 1.8 2.0 2.2 2.4 2.7 3.0 3.3 3.7 4.1 4.5 5.0 5.6 6.2

Time in Minutes

(Based on 3 min. @ 100% Distance)

Percent Distance Change

Time and Distance Percentages

50%

100%

150%

200%

250%

70%

73%

77%

81%

86%

90%

95%

100%

105%

111%

117%

123%

129%

136%

144%

Percentage Distance Changes

Percentage Time Change

Figure 2.23 As you

change the lamp distance

(with a freestanding sys-

tem) the required time of

exposure changes.. The

graph is based on a three

minute standard at 100

percent distance. If you

double the distance, mul-

tiply the exposure by four.

Halve the distance and

quarter the exposure.

Figure 2.24 This

graph shows both the

exposure time and lamp

distance as a percentage.

PAGE

. 32

You will find two charts on page 2.34: one for Gauntlet

screens and one for Challenger screens. Column One lists

the number of standard-sized screens being exposed at a

time. Column Two lists the overall size of those screens

loaded with no space in between. Column Three is the

preferred lamp distance, which is the same as the diagonal

of the total frame size. Column Four lists a nominal

exposure that can be used as a multiplier for your particu-

lar exposure time or units (see example immediately fol-

lowing). This number increases as the lamp distance

increases. Column Five lists the percentage of light in the

corner of the frame. In our example, it is always 70 per-

cent since we placed the lamp at a distance equal to the

diagonal of the frame. Column Six is the multiple-frame

percentage of the single-frame time. By multiplying this

number by your specific time for one frame you can easily

select an optimum number of frames at a safe lamp dis-

tance.

EExxaammppllee

After you have calibrated your exposure levels (refer to

page 2.19) you have determined that the proper time is

two minutes for a 23" x 31" Gauntlet screen, with a lamp

Chapter 2

Results of Changing the Lamp Distance

20%

40%

60%

80%

100%

120%

3.0 2.3 1.9 1.5 1.2 1.0 0.8 0.7 0.5 0.4 0.3

Ratio of Lamp Distance to Frame Diagonal

% Edge - Center Light

200%

400%

600%

800%

1000%

Greater Than 1 Less Than 1

% Change in Time

Figure 2.25 Edge Light

versus Time Adjust This graph

will allow you to model the

results of both time and resolu-

tion if you were to move the

lamp distance to the screen.

1. You must first establish

proper exposure times for

example 2 minutes.

2. You must be using a mov-

able lamp.

3. It is based on a ratio of

1.0 where lamp distance

equals frame diagonal.

4. This is a 100%, 2-minute

exposure with 72% corner

light.

5. Measure your ratio of

frame diagonal to lamp dis-

tance.

6. Lets say it is one

Challenger frame 44" diag-

onal.

7. At a lamp to glass distance

of 53".

8. Your ratio is 1.20 (53

divided by 44).

9. Find the closest ratio on the

horizontal axis of the

graph.

10. Look upward from 1.20

on the horizontal axis to

the series 1 curve.

11. This is the percentage of

edge to center light or 80%.

12. Drop to the lower series 2

curve.

13. This is the percentage of

exposure change or 160%.

14. 2 minutes times 160%

equals 3.2 minutes or 3

min. 12 sec.

15. To improve edge lighting

from 72% to 80% of the

center, you will need to

increase exposure from 2

minutes to 3.2 minutes for

one Challenger frame.

PAGE

. 33

distance at the frames diago-

nal dimension,or 38". If you

were to load two frames of

the same size, your exposure

would be 209 percent of the

2 minutes or 4.18 minutes (4

min. 11 sec.) at a distance of

56 inches lamp-to-glass, the

falloff would be identical to

that with the single screen.

Note that the per-frame expo-

sure would be slightly longer

but this would be offset by

the vacuum draw-down time.

GGaauunnttlleett CCoonncclluussiioonn

As you can see from the table,

the lowest screen-exposure

time is one frame per. But

when you consider vacuum

draw-down time, the others

can become a bit more equi-

table (particularly if your

blanket needs to be repaired

or replaced). Given the capacity, two or four would be agreeable as long as the

frames are not too close to the edge of the vacuum beading. Avoid the odd-

shaped three-frame configeration, as the per-screen time is prohibitive.

Stencils

1. Use an integrated light source.

2. Measure and standardize lamp-to-glass distance.

3. Calibrate exposures for each set of mesh and coat-

ing specifics.

a. Measure and record EOM.

b. Measure and record R

c. Measure and record stencil thickness.

d. Use a calibration test film.

4. Monitor each exposure.

a. 21-step gray scale.

b. Transparent resolution guide.

5. Expose same meshes and coatings per exposure.

6. Adjust times, units and lamp placement based on

developed scales.

Exposing A Screen

How To

Instructions

Gauntlet Per Screen Exposure

100

105

110

115

38 55 70 77

1234

Distance / Number of Frames

Per Screen Exposure

Figure 2.26 When you expose

multiples of screens with a freestand-

ing lamp, you have the opportunity to

move the lamp for higher productivity

or higher quality, but the two direc-

tions are opposite. The graph shows

that a multiple of three Gauntlet

screens should be avoided for optimal

results.

PAGE

. 34

CChhaalllleennggeerr CCoonncclluussiioonn

As is the case with Gauntlet screens, three screens should be avoided. Refer to

the table and chart immediately above (figure 2.24 and 2.25) for the details on the

Challenger screens. Two up is satisfactory and four is preferable if your vacuum

frame capacity is sufficient. Be cautious not to get too close to the edge of the

vacuum blanket beading. Doing so will only stretch the components and cause

leaks. The vacuum will take too long to draw-down resulting in no combination

of screens that will expedite your screen exposure.

Chapter 2

Challenger

Screens

Overall

Screen Dim.

Lamp

Distance

Center Units

(Based on 100%)

Corner

Units

Exposure

Per Frame

23 x 31

100%

70%

100

46 x 31

209%

70%

105

46 x 54

339%

70%

113

46 x 62

412%

70%

103

Figure 2.29 The table lists the data used for per screen exposure time in the graph above.

Challenger Per Screen Exposure

100

105

110

115

120

43 62 80 87

1234

Distance / Number of Frames

Per Screen Exposure

Figure 2.28 With an exposure unit

that will accommodate up to four screens,

four Challenger screens would be the

most productive quantity to shoot. Three

would be the lowest productivity per

screen to hold the same quality of center

to edge lighting.

Gauntlet

Screens

Overall

Screen Dim.

Lamp

Distance

Center Units

(Based on 100%)

Corner

Units

Exposure

Per Frame

23 x 31

100%

70%

100

46 x 31

209%

70%

105

46 x 54

339%

70%

113

46 x 62

412%

70%

103

Figure 2.27 The table lists the data used for per screen exposure time in the graph above.

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