screen printing > DIY - From master engineered marvels to cobbled together jury-rigged or Jerry-built junk!
My DIY LED "Expansion" Board
IntegrityShirts:
--- Quote from: blue moon on June 18, 2014, 12:06:18 PM ---that's funny, as I was looking for that article this morning to reference it and could not find it. It shows the curves for both the penetration and sensitivity unlike the other graphs out there.
In a quick recap it states the light should be 360-390 for SBQ and 390-420 for Diazo.
I think 365-380-410 would probably be the best to cover all the possibilities. According to the article, you should be golden with the Diazo!
Yes, the measuring portion of the UV meter is rather small, about the size of one LED, so you should be able to dial it in rather nicely!
pierre
--- End quote ---
Yeah I kinda went into this with that article as a basis for my LED choice, not so sure that was a good idea or whether it is truly accurate across all diazo/photopolymer emulsions.
After some more testing of exposure times vs. emulsion thickness, I'm not so sure these LED's are the best solution. They just don't seem bright enough to fully penetrate the emulsion in a timely manner. By timely I mean faster than my 5k watt Olec. If the emulsion is a thicker coating it is PAINFULLY slower than the Olec to get a full exposure. You can see the outline of the film pretty well which tells me not nearly enough light got through to the squeegee side.
I knew going into this project that the LED choice was going to be the hurdle, as the rest is easy wiring. Think I'll request a couple samples from different manufacturers to do some more testing, which will delay a true verdict for a while.
IntegrityShirts:
UPDATE with new LEDs and power supply!
I opted to order some more chinese 395nm LED strips and power them at opposite ends and power EACH row individually to avoid voltage drop on the thin strip material. Then, to further avoid voltage loss, I bought an adjustable power supply to pump these up to 13.8V to make sure they weren't thirsting for voltage.
The results? Well, pretty much the same: 150S with a 1/2 coat round edge Aquasol HV is right at 40 seconds for a good exposure.
Is it a good exposure? Yes, on Aquasol HV, it's a good solid exposure similar to 40 light units on the Olec 5k.
Is there some undercutting? Yes, a little. I need to move the LEDs closer to the glass but have to clearance some of the wiring loom at the ends to get them up higher which I think will reduce the undercutting.
Are these the best LEDs? NO. I'm not sure you can reliably get chinese UV LEDs that perform as indicated. It's a crap shoot.
ebscreen:
I have the same DMM.
Barring more sophisticated test equipment have you thought of using the photocell from you Olec integrator
to compare output? Obviously the two light sources would be measured differently but you'd at least have a comparison.
Northland:
Nice.... well done. If the exposure time is the same, I'd be tempted to dial back the voltage.
When I turned up the voltage on mine, they got pretty warm & that's the number one cause of failure (overheating of the internal diode joint).
Gilligan:
--- Quote from: blue moon on June 18, 2014, 08:42:53 AM ---from what I can tell, PhotoPolymer emulsions are really liking 340 or so for exposure, so chances are your times would be significantly lower with lower wavelength. See chart from Ulano below:
pierre
--- End quote ---
Am I reading this wrong or wouldn't the 350nm be just the perfect wavelength for everything? (if you had to pick just one) It hits SBQ at 87%, Fe+++(whatever that is) at 52% (not great but WAY better than 5% that the 405nm hits it at), Diazo at 93% and Diazo/AC (whatever that is) at 70% (again, better than 45% that the 405 hits it at).
If you had to pick ONE wavelength what would it be?
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