Author Topic: Under cutting & light scatter  (Read 4676 times)

Offline RichardGreaves

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Under cutting & light scatter
« 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.

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?


 
« Last Edit: May 21, 2011, 12:58:51 AM by RichardGreaves »
Screen printing since 1979 - SGIA Academy Member
ex Stretch Devices General Manager ex Lawson Supply Director
ex Screen Printing columnist 1985-1995  ex Printwear Technical Editor 1995-1999
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Offline ftembroidery

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Re: Under cutting & light scatter
« Reply #1 on: May 21, 2011, 01:36:02 AM »
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)
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Offline squeezee

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Re: Under cutting & light scatter
« Reply #2 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
imagesetters for screenprinting  A Troll-free zone :-)

Offline Northland

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Re: Under cutting & light scatter
« Reply #3 on: May 21, 2011, 06:57:58 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.

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.

Offline blue moon

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Re: Under cutting & light scatter
« Reply #4 on: May 21, 2011, 09:59:29 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


same article available here:
http://www.norwb.com/index.php?topic=84.0

Yes, we've won our share of awards, and yes, I've tested stuff and read the scientific papers, but ultimately take everything I say with more than just a grain of salt! So if you are looking for trouble, just do as I say or even better, do something I said years ago!

Offline squeezee

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Re: Under cutting & light scatter
« Reply #5 on: May 21, 2011, 02:11:07 PM »
Ahhh yes, I remember  ;)  One forgets so easily.
imagesetters for screenprinting  A Troll-free zone :-)

Offline DouglasGrigar

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Re: Under cutting & light scatter
« Reply #6 on: May 22, 2011, 12:33:41 AM »
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)

There are extremes in everything...

One item that helps greatly with this is the use of dyed mesh - and contrary to common belief the increase in exposure time is often not excessive. Un-dyed or white mesh aggravated this problem with the increase in refraction of actinic light and its travel along the white threads. (the ?emulsion socks(c)? of thread growth show this.)
When there are no standards, you must make them!