We have always approached screen exposure with idea that you expose as long as possible without losing detail. We recently got the "Blackmax Ink System" which is a highly opaque black ink that is loaded into all 8 slots of our Epson. When you output your film, it pulls equally from all eight cartridges.
Our films with this system are crazy opaque, allowing us to expose for much, much longer without any loss of detail. My assumption is that we should go with it, expose longer, and have stronger screens as a result.
My question is- Are there any draw backs to super long exposures where no detail is lost? Can emulsion become too brittle?
We use a nice vacuum uv exposure unit. Our exposures used to be around 3 mins. (Varying for eom, mesh selection, and artwork) No washout issues, and no breakdown on longer runs. We had what I thought then were very opaque films. No problems with 55-60 line halftones.
With this system it seems we could expose for twice what we are used to without losing any detail.
There is a certain lattitude in exposure, but I believe the fundamental (of photopolymer at least) is that it cross-links in a chemical reaction to UV energy. Once you've got it cross-linked, it won't break down. So if you are already fully exposing your emulsion, longer exposures will do nothing.
There's no way to 'over-expose' emulsion, this is just a term used to mean detail filling in—it's more like 'exposing the parts you don't want to because of creep, film density etc.'
I think Paul is correct - once the emulsion has hardened all the way through, it won't get 'better'. It's cooked.
the check is always to wash on the side away from the light - once it has washed, is it slimey? if it is, it is still underexposed.
the damage from overexposure in your case (where there is no danger of the light burning through the film) is loss of detail - closing in due to undercutting, of fine lines and halftones. You have more risk with a flourescent bulb unit due to light going all directions.
In addition, to paper I do a lot of textile work with discharge ink. Discharge will breakdown emulsion like nobody's business, so we are always looking for ways to improve the durability of our emulsion.
I figure, having the ability to expose longer can't hurt, but it doesn't sound like it will help much either.
If your ink is breaking down your emulsion, exposure won't help I don't think. Discharge is a very aggressive ink, and if you were printing with underexposed emulsion (a stencil in which small amounts of the emulsion hadn't been crosslinked) I don't think you would notice your stencil breaking down, I think you would notice it would be impossible to reclaim, as the discharge chemistry is mixing with that underexposed emulsion before it cross-links to turn it into some sort of reclaimer-resistant cancerous stencil. This is the case with all inks (and why it's easier to reclaim a properly exposed stencil) but would be more noticeable with discharge.
If your properly-exposed stencil is breaking down under discharge, it might be time to shop around for a different emulsion. I have only done short-run (200 pieces max I think) discharge stuff, and when I bought the ink my sales rep sent over a very specific emulsion for the ink. I had no problems there.
We never have reclaim issues. We were never underexposing. Always 7 to 8 steps on the 21 step.
When it comes to discharge ink and emulsion, we have tried many different emulsions (6 or 7)with varying results. We have recently switched again to CCI DCM-LV. We like it the best so far, and the CCI rep said we should have no problem with it and discharge ink. We haven't had any disasters with it, but I do feel like I can see the emulsion staring to breakdown on longer runs 200+.
The one I used was Image Mate DZ 343. Never done a run larger than 200, though, so I really don't know how it stacks up. I'm also not sure of differences in ink/discharge agent formulations. I was using a waterbased discharge made by the Lancer Group.