Thought I'd post a follow up since I've now done my complete test and it worked like a charm. Probably could have just created my own exposure test, but this one does the trick pretty well and saves me the time of having to create one.

35 second exposure time. Woohoo!

 

Pwhite20, What type of light source and wattage, and were you using film or emulsion? I will be using the Ulano E-Z Film. Thanks for any feedback you can provide.

 

...or try this http://www.t-shirtforums.com/t-shirt-articles/t106506.html
I suspect that calculating exposure to a vellum is more critical because vellum has a lower Optical Density and allows some burn-through.

 

That is what I did also. Easy and accurate.

 

Yes, but the object of an exposure is to crosslink the exposed bits of the screen to be insoluble whilst leaving the masked bits soluble.
If you have a solid mask with perfect contact it's easy, anything above the minimum will work. Once it's cross linked it won't become more crosslinked and the mask will obsure the open areas.
If you have an imperfect mask, that lets through a proportion of light, you will be gradually crosslinking the open areas and exposure time becomes important. If you use a vellum with a density of 2.3, which was typical when I was working on it, 5% of the light gets through, plus vellum is matted to scatter light and increases undercutting. The exposure is crucial and the exposure latitude, the difference between the minimum and maximum exposure times, is small.
With vellum it might be more important to work out the exposure time with the vellum than with an idealised mask. It's late and I'm getting confused with the maths, (and cabernet sauvignon).

 

Slightly off topic... how do you use an exposure calculator under a vacuum lid? You can't move the paper down. Maybe I'm just not understanding this.

 

I usually tape film to my screen when I do these, that way it stays exactly in the same place so I can move the blockout object. Otherwise, I normally tape films to glass. Hope that helps?

 

dd2, I'm using a 1000W metal halide bulb with emulsion. I'm not sure what kind of exposure times you'll need for the Ulano E-Z Film, but I'd be curious to find out what you end up at after you've tested it. I've never used the E-Z film myself, but have thought about it several times for convenience reasons, and may just give it a try one of these days.

 

I ended up at 5 minutes on a home made lightbox with 15 watt /18 inch black lights. I was way over exposing the image thinking that my low wattage needed to be compensated with more time. Did the step test I believe you suggested and it worked like a champ. Thanks for your help and postings!!

 

Hi, can anyone think of a reason that this cannot be printed onto a normal positive instead of vellum?

dont know if it would affect the guide as it would be more transparent? I just assumed that they said to use Vellum because that is what they sell

 

Should not be any difference for the exposure calculator. I would just make sure the film is applicable for your printer.

 

Gentlemen. It's just a repeated test pattern positive to used when making a manual stepped exposure. Scott's is ready for printing, but any positive will work.

 

I meant, what do you use to block the light in steps if you have it under a vacuum lid?

 

I don't have a vacuum lid, however, even if I did, what I would do is tape the film to the bottom of the screen, I use a 3 mil garbage bag, cut to a managable size, then lay the bag piece flat covering all parts of the step you don't want to expose, do you first 30, then open your lid, then slide the piece to expose more, expose, and repeat. I hope that helps? If that doesn't seem to work with a vacuum lid, then I better stop talking now

 

I've used the shirt backers--the cardboard they put in the back of stencil number packs is good too (and slippery), they have worked on both of the vac units I've used. You have to give it a pretty good tug though.

It's also easy to mark your steps along the edge on something light colored.

 

I'll be giving this procedure a practice today. Will report back with results.

 

i know this is an old post, but i just built a new exposure unit this weekend and was hoping to get under 2 min exposure times. Tried burning a bunch of screens in a range from 1 to 2 min with no luck. Did this test and my time was 34 seconds. Burned another screen at 34 seconds and it turned out great. I am pumped!!1