we have run into an issue recently where after washing out the screen following exposure the stencil looks good (even with some small, 10 pt type) but there are a lot of ~tiny~ pinholes in the screen (you can only really see them when you hold the screen up to light). when we print through the screen, there is definitely ink passing through these problem areas when any sort of real pressure is applied to the squeegee. we think we have narrowed it down to the screen being slightly underexposed, but i was wondering if anyone knew of anything else that might be causing this??
we are using 200 mesh screens and Ulano TZ water-resistant emulsion with oiled positives and an overhead exposure unit.
Could be dust? Maybe if your emulsion pot has got a little contaminated. Is it fresh, or have you been using it for a while? I've noticed this as well, as I get furtther down the pot , Pinholes begin to appear. If they're in open areas, a litlle bit of tape on the underside covers them. In more detailed areas, maybe touch up with a little emulsion and re-expose some?
there are many reasons for pinholes. most of which have nothing to do with the emulsion.
here are some reasons...
improper coating technique
screen not degreased properly
oil from fingers on screen
screen not completely dry
improperly sensitized emulsion
emulsion not fully dry before exposure
ive always used ulano (qtx), if it sucks whats better?
ive always just done one coat of emulsion instead of 1 on each side like everyone always says. it saves emulsion and im cheap. ive never had any problems that i could contribute to that. i barely get any pinholes too, when i do i just use a blockout pen. blockout pens rule.
here's another thing to check for...you say you are using oiled transparency, and it's a bit underexposed.
1. Look at your screen under a bright light after your emulsion coating has dried...if you have pinholes at this stage, you need to coat the screen agian on the bottom wet on dry. the second coat will fill the holes. However, if there are no holes at this stage, and there are no dust flecks all over your stencil or glass, then try the next two steps:
2. If there are no holes before the exposure, but there are after, check where they are. If they are only in the area where the transparency was, then you need to make better positives, or burn longer, or both. if you see a definate discoloration of the stencil in the area of the transparency, this is because it is not letting the UV ligth through. two words... good film. try it you'll like it.
3. If the holes are where the transparency was, and also on the stencil where there was no transparency, then you are underexposed. Increase your time by half and see what happens.