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Ensuring Uniform Vacuum Draw in Screen Exposure

(June 2002) posted on Mon Jul 01, 2002

Discover the consequences of poor vacuum drawdown, and learn how to test the completeness and consistency of vacuum on your exposure unit.

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By Mark A. Coudray

Once you have a film that adheres correctly, make your exposure based on previous run exposure-time tests that you've run. Wash out the screen and look at the halftone areas under a 10x or higher magnification loop. I like to use a 50x microscope. You should see cavities and craters where the very fine 1% and 2% dots washed out, but the mesh thread will be blocking the openings. Carefully determine at what line count all of the dots are visible and printable. This will usually be in the 3-5% range, depending on the mesh count and thread diameter. You now have a benchmark to which you can compare your vacuum-exposure results.


Attach the same positive (after you have cleaned off the adhesive) to a coated screen, place it on the exposure unit, and draw vacuum on the blanket. Make your exposure and wash the screen out as before. Now compare this screen with the test screen you exposed earlier. If the screen you just developed looks the same as the one on which the film was applied with spray adhesive, chances are very good that your vacuum is adequate. If you have missing dots or the dots resolve at a different tonal range, you are experiencing undercutting and halation.


Other problem areas


Inconsistent vacuum pressure can also result from vacuum leaks. These can occur due to broken or damaged gasket strips, which surround the vacuum blanket and are often attached with some kind of rubber cement. The gasket can lose adhesion, and consequently, compromise the seal. Examine the entire length of the gasket to make sure that there are no nicks or other problems that could result in an incomplete seal.



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