Platemaking using Screen Printing Exposure Unit

You will be working blind. Both offset exposure units with glass-faced vacuum contact frames and photopolymer exposure units with krene-covered vacuum drawers allow you to see whether there is complete contact between plate and film. In the case of krene, the material can be rubbed for better contact, and glass shows Newton’s Rings to indicate contact, reveal dust or hair, etc.
If you can’t inspect the work for contact before exposure, you could waste a lot of time and material making bad plates.

A screen printing unit releases a lot less of the required UV energy than a platemaker as well but you could give it a shot. Make sure to pick up a Stouffer step-wedge test (they sell them at boxcar and most screen printing suppliers) to tell if you are giving the plate enough UV energy.

Moe- your screen unit should work just fine. you’ll probably have to adjust your exposures to compensate for the different output of the bulbs, but other than that it should be a piece of cake.

As far as contact goes: if you are having difficulty keeping the negative and plate in good contact, add a piece of 1/8 closed cell foam to the bed. That should easily fix the problem.

Thanks for the advice everyone!

I understand that I won’t be able to see how good the contact is, but I have the same issue with screen films, and the detail I get is still pretty fine. I’ll just have to take extra precautions with cleaning the glass before I place the film and plate down, and I’ll definitely look into the Stouffer step-wedge test.

If that doesn’t work and I experience the wasted time from bad plates as described by parallel_imp, I can certainly build a top-down exposure unit. I just wanted to know why everyone was using the top-down instead of bottom-up, and didn’t want to build something new if I could use what I already have.

Thanks again! And if I’m not too lazy, I’ll post my results so you all can see if I end up making a fool of myself or looking like a DIY genius ;)

To add my own experience, I burn screens using a 1000w metal halide bulb. It’s fairly strong but not really ideal, even for screens. To hold a solid step 7 (necessary for screens) I burn for about 15 minutes. PP should burn to a solid step 18 and even at a one hour exposure I was only holding about a 13 or 14. If you’ve got a blacklight set-up going with multiple fluorescent tubes you may get away with a 30 min exposure, that’s what a friend of mine does. The real trick seems to be in hardening after the fact, though. Let me know if you find a good way to harden them once post-exposed. All we’ve tried is toaster oven at low head (too hot) and a heat gun (inconsistent)

i put my plates back in the exposure unit twice as long as the main exposure.

You don’t harden the plate after post-exposing. Post-exposing IS what hardens the plate, but is only done after drying. After washout, you dry, then cool, then post-expose.
You need moving heated air to carry the moisture away from the plates, not simple radiant heat. A toaster oven won’t do that unless you add moving air, which may burn out one or more elements—I know that from experience. Left in a still, warm oven, solids will get lenticular cracks.
Look for a heater-fan at the hardware store. I used to use a coffee can (opened at both ends) to contain and direct the heat from the fan, then put small plates inside the can. For larger plates I put them in front of the forced-air central heater here in the shop.
Also, note that different brands and grades of plate are exposed to different steps on the Stoufffer scale (check mfr. specs), and those steps are to determine the baseline exposure. A given negative may need more (fine line) or less (solid or reverse).

you mean to say that i’ve been doing it wrong since the 80’s, i didn’t see that the plate was any different. no wonder i’m not happy with my plates. Dick G.

Well, if you look at a plate right after washing, the counters of the type may look very shallow because some partially-exposed photopolymer has taken on moisture but hasn’t washed away (on all-yellow plates this may not be at all obvious). During the drying process, the counters recede to a useable depth for printing (or not printing). Then you need to let the plate cool before post-exposure or it may be slightly swollen. Post-exposure fixes the whole process. Bear in mind also that the plates are sensitive to relative humidity, and plates can change durometer with changes to moisture in the air.

This is true and not true.

There is a “relative reverse relief depth” that is quite similar for all letterpress configured plates, so yes, there is a certain counter depth, not matter what the thickness of plate, so called “deep relief” (moulding plates) or whatever.

But photopolymer plates do not swell during washout or retain water in the matrix. This is sort of a wide-spread superstition They are, after all, plastic. We’d be in a hell of a lot of trouble if they actually did do that (they might look that way when they are wet, but hell, what doesn’t?).

But yes, it is best (somewhat) if they are brought to room temperature before post-exposure, AND… before initial exposure (much more of a concern during the winter months).

Gerald
http://BielerPress.blogspot.com

Gerald, jargon aside, what I said about counters I have personally observed on HX plates; it is a lot easier to see with the visible layer of the HX than on the all-yellow MS and DS. I’m not saying they swelled in washout, rather there can be residue in the counters that is reduced in the drying process. Perhaps if you are using the sponge roller (or compressed air) after washout you will not see this.
I should add that this is something I didn’t notice during my many years of hand-washout, where I tend to over-develop. But now with brush-type processor in shop, and at another job with pile-type processor, the counter differences before and after drying were evident.
The other point about cooling before hardening is a manufacturer’s instruction. I would assume that is because things generally expand when they are heated. The photopolymer layer can certainly contract, which is what makes plates with large solids curl.

Have concerns about negative and plate not making good contact, use Spacebags and a vacuum to get the air tight seal you are looking for with a transparent barrier to verify.