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Through the Looking Glass

(May 2002) posted on Thu Jun 13, 2002

Learn how and why dirt affects exposure glass and how to keep it clean.

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By Mark A. Coudray

When it comes to tape adhesive, one of the most annoying situations is the adhesive that sticks to the blanket, and is subsequently transferred to the glass. The same situation happens with ink and emulsion. To combat this, I have seen printers cover the printing frame with 6-10 mil black polyethylene plastic before the vacuum blanket is lowered and vacuum pressure applied. This plastic, typically used as garden groundcover, is available at just about any hardware store. It acts as an insulator between the glass and the blanket. Hardly anything will stick to the plastic. And as the plastic ages, we simply cut a new piece and replace. This is an excellent--and inexpensive--way of keeping the vacuum blanket clean and in good condition.


Another very helpful approach is to protect all areas of the vacuum glass that are not being used for exposure. Most printers work with only one or two standard frame sizes. In such situations, there's an easy way to add years to the life of the exposure-table glass. Simply take long strips of clear polyester film (0.004-0.007 in. thick) and tape the strips to the glass where the printing frame edges will rest. This way you can slide your frames on the polyester instead of the glass. Sliding metal frames on the glass is a sure way to scratch it. And once scratched, the glass will become much harder to clean. In the worst cases, large scratches can impact exposure and make it necessary to completely replace the glass.




Clean vacuum-frame glass is about more than just achieving good exposure. When the glass is dirty, most printers will simply increase their exposure to burn out the dirt. This may be fine if we have a short-run job with coarse detail. But if there are fine lines or halftones, we cannot use this option.


Increasing the exposure will close up fine lines and plug small halftone dots. Worse, the particles of dirt on the glass will create weak spots in the screen. Invariably, these spots lead to stencil breakdown on the press. I know of few things more frustrating than constantly having to stop the press to patch a problem pinhole. It seems the more you patch, the more that show up. But when you account for the reduced productivity over the coarse of a shift that results from these frequent pinhole breaks, the extra time it takes to clean the glass will seem insignificant.





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