Screen Printing/Common Problems
Over-exposure causes a loss of detail in your image area. When photo emulsion is exposed to UV light, it polymerizes or will crosslink, making it more difficult to wash out. As UV light reaches the photo emulsion it becomes crosslinked, rendering it difficult or impossible to wash out during the development stage. This over-exposure happens when photo emulsion stencil making when the exposure is longer than it should be and the UV light begins to creep around the edges of the positive areas of your art, thus decreasing fine lines or completely obliterating them. Extreme over-exposure can "burn" through the opaque areas of the film positive rendering the stencil a solid block of hardened emulsion.
Common Causes of Over-Exposure
An excess of UV light reaching the stencil causes over-exposure. This is often caused by the use of film positives that are not made of optically clear film. The use of semi-opaque positives such as vellum materials will require a longer exposure time, and thus the UV light may penetrate the opaque areas of the film positives much quicker. One should always be more concerned with the minimum density, i.e. the clear areas, of the film positive than the maximum density or solid areas of the film positive. The contrast between the clear and opaque area of the film positive must be extreme for the film positive / UV light / photo emulsion combination to work well.
Wide spread over-exposure is likely to happen if your positive material is not transparent enough to allow the UV light to harden the photo emulsion before the UV light has penetrated the maximum density areas of the film positive. Clear film positives allow 91-percent of the UV light to reach the stencil, where vellum allows only 54.5-percent of the light to pass. This amounts to a 36.5% difference in light transmission, thus a 5 minute exposure using film would increase to 6.825 minutes using vellum positive. This two minute difference in exposure can cause the UV light to completely pass through the opaque areas of the film positive.
Loss of Detail
All light will bend and thus an over-exposed stencil will result in the light bending around the edges of the details in the film positive. Thin lines will be decreased or be completely obliterated. Utilizing the correct exposure time will lessen the loss. Also, assuring that details are sufficient for the mesh count used will achieve this goal. The finest detail that one should attempt to print with a specific mesh count will be equal to the width of one mesh opening and two threads. The size of mesh openings and thread width, stated in microns, is readily available from all mesh manufacturers.
If you made the film positive by hand double siding you should use a clear film positive with an opaque ink. If you printed or copied directly onto the transparency, try increasing the darkness of the image. It is possible to use two copies sandwiched together, as long as the clarity of the minimum density areas are not affected. Reread the section on making positives for more information.
Widespread over-exposure can also happen if you allow the photo emulsion to receive too much exposure. Light will pass through the maximum density areas of the film positive given enough time, thus exposing the emulsion. Reread the section on exposing the screen for more information.
Similarly, UV light that reaches the photo emulsion drying or storage stage could expose the emulsion. Screens may be coated in ordinary room light, but should be stored in an area that is devoid of UV light. Ordinary room light will cause the photo emulsion to harden in a matter of minutes, thus the stencil should be stored in a light-safe environment applying the emulsion to the screen. Reread "Drying the Screen" for more information.
Local over-exposure is evident when small sections of your pattern won't wash out, but everything around it does. This probably just means that there's a flaw in your positive that let the light through in that spot, and you need to go back and touch up the positive for next time.
Unfortunately, there's not a whole lot you can do once you've over exposed a screen except deal with the imperfection (in the case of local over-exposure) or reclaim the screen and try again with the above things in mind. Don't give up right away though, over-exposure occurs by degrees, and just because it's harder than usual to wash out your pattern doesn't mean it's hopeless. Keep scrubbing and if it doesn't seem to be changing at all after a while, then you can probably assume it's over-exposed.