I just got an older Atlas Vacu-Lite table and wondering if anyone has experience with them and what exposure times might be, roughly. I'm using 195 mesh (white and yellow) with Ulano QTX. I would just start running test strips but QTX doesn't change color until you start to hose it out.
1. you can get an exposure calculator from a few sources. it will allow you to calc a proper time.
2. you can also do it with a couple of test screens and some good film, with fine detail on it
3. Yellow and white mesh will have two different times
4. you need to ensure your coating on your screen is even and consistenet - any thick and thin areas will give you a false reading/bad exposure. you will probably want to coat 2 + 2 (2 on each side with your sharp side of your coater.) If it seems really thick, then do a skim coat on the bottom to remove excess (don't tip the coater just use the edge to skimm off the excess)
5. Do 4 tests to start, at 1, 2, 3, 4 minutes. If you are clever, get a piece of rubylith or black bristol board/cover weight paper and cut a 1/4 of it away, enough to fit your test film. you can then flop this around for your 4 exposures, masking the unexposed/exposed areas as you do each time.
6. Look at your exposure after a good washout.
- if it falls off, it's underexpsed. if your 4 minutes fall off, then do your next screen at 5,6,7,8. All that should bee too long on that unit, unless your bulbs are toast.
-if it didn't wash out, it's overexposed
- what you want is a solid strong stencil with no pinholes, and all the image opening showing a sharp stencil edge and no closing in of the image.
-if the image is filling in (compared to the film) you are overexposed, back it off a bit.
Keep adjusting time until you get it. Good emulsions will have an exposure latitude of a minute or two. white is usually about 1/3 faster exposure, but it will also not reproduce as fine a detail on your stencil.
Exposure calculators are cheap. After being give one by the friendly folks I was purchasing films from, I have been able to dial in perfect exposure after one test. The next time you order supplies, throw one in your cart. The extra 7 bucks and some change are worth it. On print day, I always throw it on my first screen (one with not too much detail) to make sure everything is exposed correctly and adjust for variation for the following screens that contain finer detail.