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Tips for Optimum Screen Exposure

(December 2012) posted on Tue Feb 12, 2013

This guide examines stencil-material characteristics, issues related to light sources, and methods for testing you can use each time you create a stencil.


By Screen Printing's Solution Sourcebook

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Light-sensitive chemistry
Two types of light-sensitive chemicals are used in direct photostencils. Diazo and dual-cure types can be grouped together, as the diazo sensitizer in both that primarily determines exposure length and degree of latitude in exposure time. One-part photopolymer emulsions and films use SBQ sensitizer, which is designed to react much faster than diazo when exposed to the right kind of lamp. Diazo and photopolymer stencil materials differ in wavelength, which is why we see a somewhat complicated relationship in their relative photographic speeds.

Resolution
Too many factors affect image resolution to use it as a guide for determining exposure time. For example, filled-in detail doesn’t necessarily mean the stencil is overexposed. A rip in the exposure blanket or poor vacuum caused by a leaking seal can hinder image resolution, even at a fraction of the correct exposure time. Incompatible combinations of screenmaking materials, such as coating white mesh with photopolymer emulsion and then exposing with a fluorescent tube, can also lead to low resolution.

Testing
Optimum exposure time is determined by assessments of a stencil’s depth of cure. A proper cure means the stencil is completely cured through its full thickness. Various techniques allow effective evaluation of depth of cure. One popular method is the exposure calculator. It uses a series of increasingly darkened, neutral-density filters, overlaid on a repeating design. It allows multiple exposures of 100%, 70%, 50%, 33%, and 25% to be simulated in one step.

After exposing and processing a test screen with an exposure calculator, the finished stencil must be evaluated by the color-change method, not for resolution. For example, yellow diazo sensitizer shows up as a strong, yellow undertone where residual, unused diazo remains. Correct exposure is determined as the time taken for the yellow diazo sensitizer to be bleached out completely. In testing, no yellow undertone should be seen on one of the middle sections of the calculator image. Complete exposure is only indicated when the color remains unchanged for two successive steps. This type of calculator works very well with diazo stencils.

Two separate color changes often happen simultaneously with dual-cure stencils, with the extra dual-cure component causing a fainter, but more persistent, color change. This trick is to determine when, exactly, the diazo part stopped changing color.


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