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Stencil Washout Techniques, Part II
By Dawn M. Hohl, Screen Printing Consultant, Uncommon Enterprises LLC
Exposure Control Films/Scales
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
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-Surface Profile Rz
Exposure Scale Hard Steps
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.
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:
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.
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.
Bottom Line: Be Consistent
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.
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. email@example.com
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.
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