Frustrated

[n_folch]

Not really having someone with me to coach me with their years of experience, is frustrating. I have so far exposed 4 screen, and have had no final good result . On the positive side, I do feel like I understand better thanks to my mistakes, and the process seems less intimidating the more I do it.

Without a teacher to guide me, my theories on why my screens are not exposing properly often feel like guesses.

I am going to attempt to explain.... I'll stick only to my last two attempt.

I'm not sure what the wattage of my light box is. What I do know is that it was built with solar plates in mind. Solar plates, for those who don't know, are copper plates that have a layer of photo-polymer emulsion on the top. The plate is used for both relief and intaglio plate making. So, I know my light box is strong enough to do the job.

My last two screens have two problems in common. Both had areas that were underexposed, while other areas are over exposed.

I have three possible theories:

1. the emulsion was not put on in an even coat.
2. the screen is too close to the light source, creating areas of underexposure.
3. problems with design.

reason to theory #1 - my scoop coater is one inch smaller than the printing area needed, forcing me to go over each scree side with the emulsion more than once. I've ordered the correct size coater, should arrive in a few days.

reason for theory #2 - light source opening is 10.25" x 14", inside screen area measures around 7" x 12" (outside/frame 10" x 14") - frame is resting right attop the lights - 5" or 6" from the bulbs.

reason for theory #3 - my designs is composed of lines of varying thicknesses... thin lines are over exposing, thick lines exposing well (no problems)


It should be noted that a Stouffer guide was used in every exposure attempted. The guide was always placed at the "top" of the screen. In this place it indicated underexposure (exposing to a solid 6.5) - yet the "bottom" area of the screen seemed overexposed.

Any ideas, thoughts, votes, or comments are welcome...

Thanks!

[d fleming]

Some questions
Is your exposure unit a vaccum frame?
What are you using for art?
What type of emulsion are you using?
Have you performed a step wedge test to determine proper exposure time?

[n_folch]

I don't have a vacuum frame. I have been using a heavy stack of books to ensure contact between screen and negative.
My artwork is a drawing I re-worked on illustrator, and then printed on a transparency.
Emulsion used - S.E.X. Emulsion (Super EXposing Photopolymer Emulsion) from Performance Screen Supplies (http://www.performancescreen.com/tech-s ... ctions-use)

Yes, I am using the step wedge guide (Stouffer Guide) to determine exposure - the "step wedge guide" was used in every exposure attempted. The guide was always placed at the "top" of the screen. In this place it indicated underexposure (exposing to a solid 6.5) - yet the "bottom" area of the screen seemed overexposed.

[d fleming]

For screenprinting purposes a step wedge test is done by exposing the screen in three or four increments of time by covering portions of the screen from light source and revealing more as exposure continues. This should in one or two tries narrow down the optimum exposure for your situation. If your emulsion is fast exposing than you have less margin for error and open yourself to a dead screen from accidental roomlight or sunlight during washout. The stoufer guide is mainly a platemakers' tool for paper printers. The last time I used one was many years ago in a darkroom making films on a graphics camera to judge the opacity of my shots as I processed them. The search function on the site is a great tool for researching questions that have been answered on this forum many times before and can be a very good resource for you. What are you trying to print on a 7x14 screen? Small even by textile standards. Just curious. I'm guessing you either have not reached the right exposure time yet, are possibly trying to shoot too fine a line for mesh being used, have a positive that is not opaque enough, or have some undercutting going on. My first bet would be exposure time. Keep trying and keep us posted on your experience as you go. I'll help where I can.

[n_folch]

Ah, I apologize if this is just one of those questions that people ask really often... I had done a search through the forum for "uneven exposure", but I hadn't really felt like I had found anything that applied to my situation... I was just trying to figure out why my screen was exposing unevenly... over exposed in some areas, and underexposed in others. My apologies for getting the two exposure testing methods confused. The people I bought the screens from suggested the Stouffer guide, and I bought it assuming it was the best way to determine exposure. I'll try the step wedge next time). In any case, I don't think figuring out exposure time is really my problem... yet.

My current problem is uneven exposure... which is why I thought it might be my emulsion application or my light source being too small, and not spreading evenly over my screen.

I hadn't considered my positive not being opaque enough. I'll look into this as well. Thanks for pointing that out!

My image fits in a printing area of 5.5 x 8.5 ... I don't need to print any larger than that for now, and figured a 7 x 14 screen would be sufficient. I'll be printing on paper, and so I'm using a 230 mesh. I could have gone with a larger screen, but it was more expensive... plus, I already had the light box, which I had been using it to expose solar plates. The light box is small (10.25" x 14"), and therefore figured that a 7 x 14 screen would was just perfect.

Thanks for all the suggestions...

[d fleming]

14 is good but 7 may not leave you enough well area to print with any ease considering your print size. Try a different light source just for s&g. Maybe a sunlight shot? Hey no problem with newb questions, you posted in the newb forum right? I was just pointing out what a great tool the search function is here. You can get some inexpensive unfiltered black light tubes from a lighting supply house and try a larger exposure box. Won't be able to produce quality halftones with that like you would a halide but it's a start. Good luck.