Sunday, March 20, 2011

Screen Printing : The Step Test

I also discovered this thing called the step wedge test on youtube. It is a method where you can determine the exact exposure time for the type of emulsion and the type of lamps/bulbs that you use. In the test you burn onto the emulsion an image on your film positive (a print out of a pattern, i.e. a series of lines, dots or text) like the regular burning process. Only you gradually expose portions of the emulsion by moving a piece of opaque paper (placed under the screen to block light) every a constant number of seconds. So as you do this process the emulsion is exposed in different lengths of time.

For example, if you repeat it 6 times every 20 seconds, the the first area that you exposed is exposed for 2 minutes, the next is 1 min 40 secs, next is 1 min 20 secs, next is 1 min, then 40 secs, and the last for 20 seconds.

When you remove the film positive and wash the screen with pressured water, you will observe that some areas will be completely washed off (under exposed) while others areas will produce a stencil (good exposure) and other areas will not wash off at all (over exposed). Take note of the exposure time of each area. Look for the area that has perfectly copied the pattern from the film - the exposure time used on that area will be the the correct exposure time for everything that you used on the test (type of emulsion; material of film, the quality of the black printer ink; the type, number and wattage of the lamps, and also the distance of the lamps from the screen).

I hope I explained it clearly here.

Anyways, I designed my own pattern for the step wedge test. I'll have it printed on tracing paper and do the test maybe after a couple of days. It's a Saturday night and I want to put this project and all the disappointments aside. I wasn't a worry-free birthday tomorrow.

1 comment:

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