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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

Until computer-to-screen (CTS) imaging technology becomes prevalent and CTS equipment prices begin falling, the majority of us will continue to rely on the vacuum-frame exposure process when producing stencils for high-resolution printing. Stencil exposure on a vacuum table is a critical step in the prepress workflow, and one that is almost universally neglected. This month, I would like to spend some well-deserved time outlining important factors to be aware of when working with vacuum-frame exposure units. In particular, I would like to focus on issues surrounding the condition and maintenance of the glass exposure surface.


The primary factors we're concerned with in regard to the glass exposure table are its cleanliness and surface condition. Cleanliness is the first and most obvious issue to consider. Only a clean glass surface will allow us to fully expose stencil images without pinholes or other defects. When pinholes occur, we not only risk material waste, we lose production time while we stop the presses to make stencil repairs.


Dirt on the vacuum glass can be a real problem, since there are so many sources of airborne debris and so many ways it can find its way to our exposure units. Making the matter even more difficult is the fact that the constant release of high-intensity UV light from the units facilitates the development of static charges on the glass surface. This static can draw dirt out of the air and onto the glass like a magnet.


Keep it clean for pristine screens


How clean is clean enough for exposure glass? It might sound like a ridiculous question, but judging from the condition of exposure units in hundreds of shops I have visited over the years, it is one that needs to be addressed. The first thing to note is that "clean" is a relative term that depends on the operation to which it is being applied.


I once visited a printing operation that was renowned for screen printing extremely high-resolution halftones (150 lines/in. and up). When I arrived at the plant, I expected to see printers and prepress technicians wearing white lab coats and performing their tasks in sterile, hospital-like conditions. As it turns out, my expectations were contrary to what I found, especially in the screenmaking area.



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