How to under expose a screen?

[Gilligan]

Almost a serious question.

Older MSP 3140 with a new bulb (not that it matters really).

We did a new step wedge test (desperately needed), we took a chromaline blue coated fairly thick, if memory serves me 50% EOM, on 150-S yellow mesh.  We started at 15 light units and went 4 units up each time for 10 test.

Started developing it and couldn't hardly get blue to come off on a paper towel rubbing it back and forth with some pressure.  ZERO slime.

It was technically under exposed in my opinion until step 4 or 5 (27-31 Light Units) as it didn't hold well below and wasn't washing out as well above.  This is developing with COLD water (40 degrees outside) and a cheap pressure washer (it's on the list).

So, I don't really have a real question but does this seem right?  Shouldn't I have gotten some slime on there?

[alan802]

shirt side or squeegee side?  Should have had some definite slime on the squeegee side, maybe not the shirt side.

[Gilligan]

Squeegee side.

I agree, I thought for sure I'd have some slime.

[alan802]

Ok, now there could be several things at work here.  Is there any possibility that the squeegee side could have slightly been pre-exposed to UV light?  Maybe your light source is doing a better job than you thought and you can dial back your exposure times a good bit, or perhaps there is something wrong somewhere along the chain of events.

[Gilligan]

Possibly pre exposed... the screen did sit out on the unit with the lid up for a pretty long time.

But we have almost ZERO UV as the weather is retarded out here and we have rain forecasted till next year it seems.  Didn't even attempt to post expose outside like I typically do.  40 degrees, wet and gray.

[Orion]

If your 3140 has a 1250 watt bulb, I too would think that there was some light contamination as Alan suggested, especially with a pure photopolymer emulsion.

[Gilligan]

If your 3140 has a 1250 watt bulb, I too would think that there was some light contamination as Alan suggested, especially with a pure photopolymer emulsion.

Please elaborate?

If you mean spill because it's the older style and doesn't go all the way up the sides then that's not the case as they stay in a light tight cabinet around a wall before coming out to be exposed.

[Orion]

Pure Photopolymers expose very fast but with the exposure times that you used and a 1250 watt bulb, I would think the screen would have exhibited some signs of underexposure, such as delamination and "slime" on the squeegee side. That leads me to believe that there was some light contamination prior to exposure.

[GraphicDisorder]

Walk around in open light with it like we do, it will help