screen printing > Screen Making
Under cutting & light scatter
RichardGreaves:
To me, correct exposure means cross-linking all the sensitizer (hardening the stencil), so it won't dissolve with water and (contrary to instinct) will reclaim easily. UV energy has to move all the way through the stencil from the bottom. We can't flip it like a steak.
Low UV energy doesn't penetrate well, and when the inside of the screen isn't hardened/cured completely it still dissolves with water. Multiple fluorescent lamps, designed low resolution, large areas like signs don't make halftones easy.
No matter what UV source you have, at the edges, UV energy strikes the stencil at an angle. In addition, light scatters when it hits at an angle and is beyond the control of your positive. Scattered, under cutting uncontrolled UV energy chokes some fine lines or halftone shapes.
Harmless plastisol is like salad dressing. We can get away with uncured stencils because there is no damage to be afraid of, but water-base or solvent ink attacks the raw inside and the screen breaks down. Of course, many printers are then furious when they cleanup with stronger than needed solvents that chemically bond the uncured stencil to the mesh making stencil removal impossible.
Calibration
If you make a positive with 30% dots and they get choked, and print a 20% dot on the shirt, you can anticipate and supply 40% dots on the positive, completely cure the stencil and print your goal of calibrated 30% final dots.
For the best results, you adjust for the conditions. If your dots are closing in, you are only doing half what must be done to get accurate prints with your underexposing shortcut.
Frog, can you make this a new thread?
ftembroidery:
Would I be correct in believing that it is actually impossible to overexpose a screen? (Allowing for undercutting as you stated in the first post and assuming the positive was 100% effective in blocking UV light)
squeezee:
Effectively, when it's cured, it's cured. There will be more under cutting though.
I did a TSF dummies guide:
http://www.t-shirtforums.com/t-shirt-articles/t108270.html
Northland:
--- Quote from: RichardGreaves on May 21, 2011, 12:44:53 AM ---To me, correct exposure means cross-linking all the sensitizer (hardening the stencil), so it won't dissolve with water and (contrary to instinct) will reclaim easily. UV energy has to move all the way through the stencil from the bottom. We can't flip it like a steak.
--- End quote ---
I've found this to be so true..... and when I attempted to getter thicker EOM's it was an even bigger problem.
.. retrofitted my fluorescent exposure unit (twelve 20W) with a metal halide source (three 400W) and now the penetration is great.
I know a single 1000W would be better, but for the time being I'm enjoying fully exposed screens using a 3 point that has the aprox. the same focal distance across the entire screen.
blue moon:
--- Quote from: squeezee on May 21, 2011, 06:27:54 AM ---Effectively, when it's cured, it's cured. There will be more under cutting though.
I did a TSF dummies guide:
http://www.t-shirtforums.com/t-shirt-articles/t108270.html
--- End quote ---
same article available here:
http://www.norwb.com/index.php?topic=84.0
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