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Exposure Control Films/Scales
Exposure control films/scales are an outstanding tool to check the consistency of the exposure and stencil developing or washout. These scales are a small film-positive strip containing a range of graduated-neutral film densities, sometimes referred to as an emulsion hardness scale. One of the most common is the Stouffer 21 step transmission and step wedge, with densities from 0.05 to 3.07, which runs about $7 from www.stouffer.net. The testing done by SPTF used the "Quick Check" from KIWO, but this scale is no longer available (see Figure 5).

Once the ideal exposure time is determined with a regular exposure calculator, the control film is a quick visual check to ensure the exposure and washout occurred correctly on each screen. The small control film is placed on the vacuum frame, or screen, just outside the image area on each exposed screen. It's a good idea to leave the scale in the same place or vicinity on the glass for each subsequent exposure to avoid differences from light distribution. The first screen is washed out using normal production washout procedures, and the scale is read to determine the last "solid step" or hardness.

During development, some of the 21 steps will rinse off and some will remain attached. The last step that shows no sign of delaminating or ragged edges is the solid-step target for that emulsion and mesh coating combination. Exposing to a solid-step seven means seven steps have remained completely attached to the screen, while 14 steps fully or partially fell off (see Figures 8, 9 and 10).

The step's corresponding number can now be used as a reference for each subsequent screen with the same emulsion and mesh coating variables. The solid step-target number may be different with other conditions, so each unique mesh/emulsion and coating combination must be determined separately.

With this reference step in hand, each screen developed can be evaluated for a consistent solid step. If a different solid step emerges, it is an instant indication something changed, either with the exposure or washout. Unfortunately, the scales cannot tell you what has changed, so troubleshooting skills become important. But knowing that a change has occurred is definitely worth the time investment it takes to use the exposure scale.

Lamp deterioration, insufficient stencil drying, coating thickness changes, mesh count and diameter changes, lamp distance changes, emulsion changes and inconsistent washout are just a few issues that can change the scale results. SPTF's stencil troubleshooting guidelines can aid in the process of finding the problem source.

Exposure scales are an excellent quality control checkpoint, ensuring that each screen has been exposed and washed out to specification before it enters production. While exposure-related changes are more concerning, changes in washout techniques can produce surprisingly large variations in the scale and can be misleading if washout is not consistent. These scales are not only an indicator of exposure but of consistent washout practices, and are useful to ensure washout occurs to the same degree on each screen.

SPTF Washout Testing
SPTF completed testing on washout techniques, specifically, a comparison between pressure washing and conventional garden-hose pressure, washout time on each side and washout length. A total of six screens were tested, each having two distinct washout techniques applied to an identical test image that was exposed on the screen.

The screens were constructed from the same mesh and coating, which is used to print 85 lpi in SPTF's Four-Color Process Workshop. The frames measured 1-m-by-1.25-m (39-inches-by-49-inches), and had a 380/31 plain-weave mesh stretched to 26 N/cm. A high-quality, dual-cure emulsion was coated two-on-two on an automatic coater, with one face coat on each side, which was applied when the screens were dry.

Exposure times were determined with an exposure calculator and identical films containing tonal spectrums of 11 line counts and various text and line sizes were exposed side-by-side on each screen. An exposure control scale was put under each of the two film positives as well. A 5KW metal halide lamp at a fixed distance of 76 cm (30 inches) was used for exposure.

A matrix and description containing the washout conditions for each screen is shown in Chart 1. Since two images were exposed on each screen, a magnetic mask was used to block half of the screen from receiving water during the first washout procedure (see Figure 6).

Water temperature was between 21oC to 27oC (70oF to 80oF) for all of the procedures. The screens were dried, measured and printed under near-identical conditions to observe any differences. Indicators measured for each test included dot gain, stencil thickness, Rz, screen tension and exposure-control scale hard-step. The images also were magnified for comparison (see Figure 7).

Washout Test Results: Stencil Thickness
Stencil thickness was measured with a Fischer Deltascope using a screen printing probe. All six test screens showed only a minimal stencil thickness change. Average Emulsion Over Mesh (EOM) was measured at 3 microns +/- 0.5 microns. No significant trends were noticed.

Stencil-Surface Profile Rz
Average Rz was 9.11 microns +/- 1.0 micron for all the stencils. Pressure-washed screens had a slightly higher Rz on average than garden-hose screens.

Screen Tension
Using a pressure washer for development did not cause any drastic drop in screen tension. The screens had been allowed to sit and stabilize - for 24 hours after gluing - before they were processed. The normal tension drop seen between cut away and 24 hours occurred before testing.

Exposure Scale Hard Steps
Exposure scales were placed on each side of the tested screens to demonstrate how washout variations and underexposure changed the apparent-hard step. In Figure 8, both scales were equally exposed on identical emulsion coatings. However, they show two distinct hard-steps because of the washout. The top scale, 1.2A, was developed using a garden-hose technique with conventional side-to-side concentration and duration, while the bottom scale, 1.1A, was pressure-washed. The screen is properly exposed in both cases, but the referenced hard step is evidently dependent on the washout practice.

The scales shown in Figure 9 show the effects of underexposure when using the same washout procedures. Both the pressured-washed pair, 2.1A and 2.2A, and the garden-hose pair, 2.1B and 2.2B, show the hard-step changing. But because washout was the same, we can determine it was an exposure difference instead of stencil development variation. On closer examination, the underexposed stencil, which was washed with the pressure washer, was missing some small shadow dots. The pressure and weak stencil allowed them to be washed off. The fully exposed stencil did not show this effect.

In Figure 10, results from a short and long washout time are compared. The top two scales, 3.1A and 3.1B, show development after using a pressure washer for two different lengths of time. The short washout was only 15 seconds on each side while the long one was one minute. Note the hard-step is different even though the exposure was the same. The bottom two scales, 3.2A and 3.2B, were done with a garden hose for 45 seconds each side on the short washout, and 3 minutes each side for the long washout. Again, we see a drastic hard-step difference between the high- and low-pressure groups.

Print Results
As they say, the proof is in the print - so two groups of magnified areas on the prints are shown in Figures 11 and 12. The 10 percent and 50 percent area on the 90 lpi tonal scale are pictured, as well as 4-point serif text. Measured dot percent values (Murray-Davies method) also are indicated under each respective photo.

In Figure 11, the garden-hose print clearly has less detail opening up in the dots and text than the pressure-washed prints. You'll also notice the two pressure-washed prints are slightly different because of the amount of washout that occurred on each screen side. For the conventional method, most of the washout occurred on the substrate side as opposed to the half-and-half method, in which equal time was spent on both sides. The dots are more defined and accurate when using a pressure washer for equal time on both screen sides.

The short versus long washout print results are shown in Figure 12. When comparing the garden-hose short- and long-methods, the longer washout duration produced improved dot accuracy. Improvements also are noticeable when extending the washout time of the pressure washer. Without a doubt, longer washouts are needed to remove the emulsion from the halftone areas. Of the four print samples, the last screen that was pressure-washed on both sides for a longer period of equal time produced the most accurate rendition of the film.

For a more complete look at the tonal range results, two graphs containing the 90 lpi measured dot area are illustrated in Figures 13 and 14. Figure 13 compares the underexposed with the correct exposure results for both garden-hose and pressure-washer development. The underexposed screen washed out with the garden hose shows improvements in the highlight dots when compared with the fully exposed screen that was washed out using the same methods. However, the fully exposed, pressure-washed screen matches the results on the garden-hose underexposed screen in the highlight areas. Here we can provide some proof that fully exposing a screen can match an underexposed screen in tonal range when a pressure washer is used for development.

In Figure 14, longer washout times are compared with short times. Again, the longer washout times open up more highlight detail than shorter times for both washout methods. So regardless of the washout pressure option used, extending the time of washout is definitely a good practice.

Recommended Stencil Development Practices:
1. Use an exposure calculator to determine the correct exposure for each mesh count/thread diameter, emulsion and coating combination used. Expose longer and develop harder to produce an expanded tonal range.
2. Use an exposure-control scale on each exposed screen. Determine the resultant hard-step after developing the first properly exposed screen with the production washout procedure. Document the results and post them by the washout tank.
3. Wash out the screen as soon as possible after exposure. If immediate washout is not possible, screens should remain in a dark room or under yellow safe lights until they can be developed.
4. Use cold to lukewarm water, 15°C to 32°C (60°F to 90°F). Do not use hot water to washout stencils. A clean, backlit booth is preferred.
5. Completely flood both sides with low-pressure water to begin softening the unexposed areas of the stencil.
6. Direct emulsion: Starting on the squeegee side, wash out the stencil using a pressure washer (2,000 psi or lower with a fan-shaped spray nozzle) and holding the tip at a distance of 30 cm to 45 cm (12 inches to 18 inches) from the screen surface. Start the pressure-washer off the stencil area, and then move it in a consistent rate and pattern across the screen for a predetermined number of minutes or repetitions. Another option for timing is to spray the squeegee side until it begins to open up, and then move to the substrate side to complete the washout. If the screen is intentionally underexposed or a soft SBQ or low-quality emulsion is used, do not use a pressure-washer. A hard fan spray on a garden hose can be used instead to safeguard the more sensitive stencil.

Repeat the same washout process on the substrate side. Ultimately, it should take one to three minutes on each screen side (depending on the screen size). A timer can be used to ensure a consistent time is spent on each side. Observe the exposure-control scale and make sure the correct hard-step appears.

Capillary Film: For properly adhered and exposed capillary film, a pressure washer also can be used, but only from the substrate side. Only street-pressure spray should be used on the squeegee side, where the stencil is more vulnerable.
7. Spray both sides of the screen with a final low-pressure wash to remove any blow-back debris from the sink that may have landed on the screen, especially if reclaiming occurs in the same sink. A final rinse also will help remove any residual emulsion scum.
8. Examine the stencil edges and highlight dots with a 30-50X magnifier to determine if they are clear. A 10X loupe is simply not strong enough to see any stencil residual still remaining. For even greater control and verification, a new device called the ScreenReader can measure dot area on the actual developed stencil to verify dot size present in the screen, (see Figure 15). The instrument is literally a screen densitometer, while also able to measure film, CTS images and litho plates. The ability to measure the stencil is the ultimate verification of exposure and washout consistency before a screen enters production. The ScreenReader is available through Ripware Ltd. (www.ripware.co.uk) and KIWO.

9. Use a screen vacuum to remove water residue on properly exposed stencils. Water marks can be caused by hard water deposits. Removing the water will reduce this possibility as well as speed drying. You can use a wet/dry vacuum for this purpose, but get a suction head designed to safely contact screens, (see Figure 16). Rhino Tech and SaatiPrint both offer one. If a screen vacuum is not available, blank newsprint can be used to blot the water. If the newsprint sticks at all to the screen's squeegee side and shows emulsion color, the stencil is most likely underexposed. Use caution here because the stencil will swell after washout and, therefore, be sensitive until it thoroughly dries.
10. Create a written washout procedure - chances are you don't have one in place. Train screen making employees on exactly what is expected during the stencil development, and, more importantly, why it is so significant.

Bottom Line: Be Consistent
The washout method is part of the stencil process and will impact image accuracy as shown. Four-color process color control can be compromised if the washout is erratic. The bottom line is: Repeatable procedure should be implemented for developing screens, particularly if they carry halftone or fine-line detail. Whatever washout viewpoint you hold, be consistent in how the task is performed across shifts and by various employees. Automatic equipment can help, but settings must still be set consistently for repeatable results.

Hopefully the information presented here has provided a new perspective on developing a stencil. Take the time to address this unsung variable and enjoy improved print quality and a more repeatable process in the future.

References
"The Benefits of High-Pressure Washes and High-Tension Screens," Mark Coudray, Screen Printing, July 2006. "Automatic Stencil Developing," Peter Kiddell and Carol Burnside, Screen Printing, May 2002. Special thanks to KIWO for sharing testing conclusions on washout with SPTF and the author.

Dawn Hohl is a Technical Trainer and Consultant on screen printing. Her industry experience includes 18 years with SGIA, overseeing screen printing workshops, developing training resources and conducting process-related research. She is a member of the Academy of Screen Printing Technology. uellc@hughes.net

This article appeared in the SGIA Journal, 2nd Quarter 2008 Issue and is reprinted with permission. Copyright 2008 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.