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| Posted on 6/16/08 at 07:50 AM | |
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  The great thing about a Stouffer 21 Step Transmission Gray Scale is that you can use math to calculate an exposure correction and it only costs from US$6.00 to US$12.00 and should last a lifetime. The 21 Step is a real photographic positive with 21 steps of opacity that with one exposure, simulates 21 different exposures. Much faster than than a manual step test where you have to move a piece of cardboard or Rubylith to block UV energy. Before an Exposure Calculator You have to find an acceptable exposure so the stencil survives the print run before you can use an exposure calculator (that brackets that exposure with 2 steps under and 2 steps over exposure), so you can judge the fineness of line you can print. The minimum exposure to cross link emulsion so water won't dissolve it and it holds in the mesh, is a Solid Step 7. More exposure will make the stencil more durable (harder), but light scatter in the mesh and diffused light from a longer exposure, can under cut the positive, and choke the final image size. Compensate The problem shows up with halftones and fine lines under 0.030". It may turn out you have to make a line 0.040" to that you actually print a line 0.030" and you have couquered Mother Nature's attempt to frustrate you.  Don't under expose, so you can hang on to fine lines that close up, because of undercutting, because you will have a stencil that is not completely cross linked and will break down on the press. You must outwit Mother Nature and calibrate all halftones and fine lines to compensate for this choking or stencil failure. Add Known Controls to your Positive Add simple lines of a known width to your positive and use that positive to MEASURE the result on your final screen print.  Large block letters - Who cares? Exposure Math Each step to increase or decrease exposure has a value of 1.414. If you need to move more than one step, you have to multiply the steps. 1 step 1.414 so one step you should increase 40% 2 steps 1.414 * 1.414= 1.99 so double exposure 3 steps 1.414 * 1.414 * 1.414 = 2.827 3x exposure. For visual feedback like a speedometer, I want you to put a 21 Step on every screen you expose for the rest of your life. [Edited on 11/3/08 by richardgreaves] ____________________ Richard Greaves, ASPT Ulano Corp. New York Technical Services 718-943-1338 direct | |
  
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