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Yet another exposure problem.

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Yet another exposure problem.

Postby Vagabundo » Sat Jul 31, 2010 6:24 am

Hi guys, I'm new to the community and I got here trying to find the answer to one of my perils, so sorry for making my first post a selfish one.

Here's the deal. My exposure unit is home made. It consist of 12 40 watts, 4ft. long, fluorescent unfiltered black light bulbs. On top of it there a 40in x 54in 1/4 thick completely clear glass (no treatments of any kind on the glass). The wood I used for the walls are 12 inch tall so I estimate that the bulbs are 7 to 8 inches far form the glass.

I use the Saatichem Textile PV photopolymer emulsion and I coat my 110 mesh screens one time on each side. Properly coated with a coater and properly dried in an horizontal rack.
I print my transparencies on an Epson Stylus 1400. I use standard inkjet transparencies but I print two copies of the design (in the best quality possible) and place them together to make a dense positive.

So... the dilemma is that I've done every step wedge imaginable to man. And it seem that my screens over expose if I give them more than 40 seconds of light. I then used 35 seconds and 30 seconds. When I try to wash out the design it doesnt matter if I use a pressure washer or a standard hose the design proves so hard to wash out it will sooner or later blow out of the screen. I did a step wedge and 5 seconds, 10 seconds and 15, wash out, 20 and 25 are under exposed, 30, 35, and 40 seem right and the rest over exposes. The thing is that it is a really small design about 3 inches wide 2 inches tall but when I expose a very big design 40 seconds prove just right. Is it possible that the emulsion will shoot a lot faster than 30 seconds even though the step wedge washes out? I'm kind of new to screen printing and coming from dual cure which exposed for me in 5 to 7 minutes seems kind of impossible that the emulsion will shoot at 15,20 or 25 seconds. Any thoughts or recommendations on this? Keep in mind that the positive is making the right contact with the emulsion. I'm applying over 40 to 50 pounds of pressure on top of the design.
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Re: Yet another exposure problem.

Postby d fleming » Mon Aug 02, 2010 9:23 am

Not familiar with the emulsion you are using but sounds like maybe time for some fresh stuff. HAve you checked the age of your emulsion? I have in the past gotten outdated material from the supplier so you have to check age when you get a bucket in to make sure your material is useable when you receive it. I use the same light setup for non-halftone work with ulano rlx and expose for 4.5 minutes using a single film printed on an epson 9500 no problem.
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