[doelike]

A family member took it upon themselves to make me an exposure unit -- so far, so good, but I'm having a ton of problems with it and hoping someone can give me an idea of WHAT is wrong.

The box is about 30" x 24" and just over eight inches tall. There are two T8 ballasts (?? I'm not sure if my terminology right here -- I'm new to all of this.) with a total of 4 24" BL tubes. They are 18w each. The lights are evenly spaced and approximately 5 inches away from the glass.

I have spent the last few days experimenting with exposure time between 1 and 5 minutes. 1 minute had the best wash-out, but it was a sort of weird, bubbly film that you sort of have to rub off rather than simply washing out. Like, the whole unexposed area would just float off the screen. Really, it's done this with 1, 2, 3, 4, and 5 minute exposures, but 1 minute gave me the quickest wash-out before the entire screen started to wash out. (So, basically, it was under exposed.)

Now, here's a few more details that might be useful for diagnosing the problem --

-- The glass is, again, approx. 5" away from the lights. It is 1/8" thick and has no UV protective coating. I have no other details about the glass.

-- Originally, the unit had 4 regular florescent tubes. 30 watts, if I recall correctly. I was able to expose a screen with these bulbs in about 10 minutes but clearly these aren't the sort of bulbs I needed, so they were replaced with the 18w BL tubes.

-- With all attempts, including attempts made with the original 30w florescent bulbs, I lose small details. No half-tones, no registration marks, no crisp lines. All fine details are lost.

-- I am using SaatiChem Grafic PU emulsion and have used both inkjet vellum and transparency film both single and doubled up.

-- I am using a thick, black fabric and 20lbs of weight over the screen during exposure.

I have literally been at this for DAYS and I have wasted a lot of time and money trying to get this to work. The unit, itself, cost the aforementioned relative approximately $150 so it would be a huge waste to just get rid of the thing. Making adjustments? Sure. Going back to my old exposure method? A huge waste.

I've searched through forums and looking at photos and reading posts of similar nature, I can only "guess" at what might be wrong -- do I need higher wattage on the BL tubes? Do I need more of them? Does the glass need to be closer to the lights?

I know you guys are the experts. Please, please, please help me. I don't know WHAT I'm doing wrong and I'm starting to get incredibly discouraged.

 

[Dkenzie1]

If you are you the regular Black light tubes that could be part of your problem. The black light that you need is UNFILTERED. they look like normal tubes. other factors could be old emulsion, screen not degrease properly and about 20 other things. I use standard bulbs but I run 6 20 watters and they are 3" away from the glass. expose a 110 mesh coated 2 1 in about 13 min. if possible I would up the wattage on the tubes. could be your screen is not dry when you coat it. Humidity plays a huge factor in exposing screens. we try to keep the humidity below 50%. also what is the solid content of your emulsion? the higher the better results. Hope this helps a little.

 

[Dtag1971]

Agreed on the top comment. I'm jst going to tell you what I use maybe this will help. My light source is a 500 watt photo flood bulb 18" away from the glass. Exposure time is 5 minutes with saatichem pv emulsion. 110 to 125 mesh counts so far. coated 1/1 with the rounded side of the scoop coater. You can use the sharp side, will lay down a thinner coat of emulsion. On my old exposure unit I used grow lights 3 of them evenly spaced about 3" inches away from the glass....can't remember the exposre time. With the diffused light source you get some undercutting around halftones. Or at least that's what I hear. All my designs so far have been solid blocks of copy and simple art with NO halftone work in the design. On the degreaser....I always degrease my screen no matter what!! Little more time for this step but no screen failure yet . So I'll keep on doing it. I hope this helps feel free tp PM me. Good luck.

 

[doelike]

Thank you both for the replies so far, let me see if I can add any additional info that might identify the problem --

-- The BL tubes are unfiltered as far as I can tell. They're white and glow a bit purple when on. They're not the purple-coated tubes that I'd picture when I say "blacklight." I did a lot of research (here, in fact) on the lightsource when I originally replaced them. These are the tubes I purchased: Actinic BL 18W (F18T8/BL) - 18 Watt, 24 Inch T8 Black Light Fluorescent Bulb | Bulbs.com Are you guys seeing something I might have overlooked, perhaps?

-- The emulsion I purchased was opened on 1/21 according to the label I put on it. Could it be "old" without being opened? It's kept in cool, dry, dark place. I have no idea about a "solid content," unfortunately. Would the SaatiChem website tell me this, maybe? I'm currently at home so I can't check the label.

-- I work in an indoor studio that is always cool. No humidity as far as I can tell.

-- I'm not sure of the mesh count, because, again I'm not at my studio, but I do know one screen I've been experimenting with is 110 and the other is 130. Same results with both. I also degrease my screens each time I clean them/remove the emulsion.

-- I've used both sides of the scoop coater. Same results regardless of how thick/thin the coating of emulsion is.

-- I have tested both large, solid art work and larger halftones. Same results with both.


I've honestly done this so many times, hence the frustration. Would the wattage on the BL tubes change anything? I tried to get the highest wattage I could find, but because of the size of the fixtures they go in, I'm pretty limited, apparently. Most bulbs come in a T12, not a T8.

 

[ScreenFoo]

Are you absolutely sure that the screens you coat are not getting partially exposed before you get film on them?

And I don't mean to doubt you, but I can guarantee you that you will get different results switching edges on your scoop coater if you're still pulling the same number of coats the same way--if you're alternating them when you coat, you will likely have problems figuring out an exposure time.

 

[doelike]

I should say "similar" results, not "the same (exact) results." No variations are changing the results for the positive, it's basically the same general result -- it either doesn't wash out fully or I expose it for a minute and it's underexposed enough that the entire screen begins to wash out.

Honestly, when I first started screen printing I did this in a dim garage using a 250w photo flood light bulb with a tin pan around the bulb for 10 minutes and I have gotten better results with that than I have with this exposure unit. (Although, I wasn't doing halftones at the time, but even a big, solid silhouette produced similar results.)

 

[PrimalScreen]

Try letting the emulsion dry a little longer before you expose it ...or try using some fresh emulsion. I've had emulsion get old and it will give you very unconsistant fits.

 

[PrimalScreen]

Back when I used a homemade exposure unit I found that Ulano Proclaim emulsion was a very easy, durable, and exposed very well with all sorts of different lighting and screen coating. But that is my opinion =)