When exposing a screen, exposure time varies depending on the type of light, whether there is reflector, distance from the screen, bulb wattage, etc. And if you don’t expose a screen long enough, the image gets washed away. If you expose a screen too long, you will start to lose fine detail of the image.
What kind of light to use? I currently use a 250 watt halogen work light with the UV filter glass, and safety cage removed (you assume all risks if you remove your glass and/or cage from your halogen light). In the past, I have used a 75 watt incandescent bulb in a reflector with a clamp. Whatever you use, make sure you have some way of clamping it, or mounting it above the frame for an extended period of time.
Before you expose real work, you want to run a test to find the best exposure time. First take the light you have, and place it above a frame. You want to place it high enough that the whole screen gets equally exposed, but not so high that the screens will take forever to expose. If you use a spot light, it might focus too much light on the center of the screen, and not as much on the corners. This would lead to uneven exposure.
Once you have the light and placement figured out, we need to figure out some rough times. In the Speedball manual they give some times for a 150W clear incandescent bulb:
Screen size, bulb height, exposure time
8×10, 12 inches, 45 minutes
10×14, 12 inches, 45 minutes
12×18, 15 inches, 74 minutes
16×20, 17 inches, 92 minutes
18×20, 17 inches, 92 minutes
The higher the bulb is above the screen, the longer it needs to be exposed. If the wattage of the bulb is lower, you need to exposure it longer.
When I used the 75 watt incandescent bulb at 17 inches, I had to adjust the time. Since the wattage was half of the 150 watt recommended bulb, I simply doubled the exposure time. This worked fine.
When I started using a 250 watt halogen light at 23 inches, I needed to refigure the exposure time. Since the 250 watt halogen light was brighter than a 150 watt incandescent light. A halogen bulb puts out more light than an equivalent wattage incadescent bulb. So the exposure time would need to drop. And since I was raising the light higher above the screen, the exposure time would need to increase.
So not knowing the best time for exposure, I had to experiment. I didn’t want to expose screen after screen until I got it right.
This is what I did:
I coated a screen the usual way. I printed a transparency with numbers from 10 to 90 in increments of 5. With the light set up, I placed the screen under the light to be exposed. I had a stop watch going. After 5 minutes, I covered the 5 with an opaque piece of paper. After 10 minutes, I also covered the 10. I did this until all of the numbers were covered. Then I washed out the screen. The lowest numbers that were distinctly printed gave me the correct exposure time. The numbers below 40 all got washed away. 50 looked okay, but 60 looked better. So I used 60 minutes as an exposure time for my 250 watt halogen light (without UV filter glass) at 23 inches. With experience, I have since lowered the exposure time to 45-50 minutes.
What kind of emulsion are you using? Do you have a favorite brand that is most effective with your light and timiing?
Comment by Marissa — May 14, 2007 @ 8:40 pm
I use Diazo Photo Emulsion by Speedball. I buy it in the larger 26.2 ounce jars. But there are less expensive smaller-sized jars available. Between coating screens, I store it in the refridgerator.
Comment by Rob — May 14, 2007 @ 10:47 pm
[…] not sure… suggest doing test patches at same distance, like 3 feet away for two minutes, ten minutes,… http://www.screen-printing-t-shirts.com/…; says…Screen size, bulb height, exposure time 8×10, 12 inches, 45 minutes 10×14, 12 inches, 45 minutes 12×18, 15 inches, 74 minutes 16×20, 17 inches, 92 minutes = 1&1/2hrs 18×20, 17 inches, 92 minutes […]
Pingback by The Printers Blog » Photo emulsion exposure time? — June 19, 2007 @ 2:43 pm
I just did my first screen with Speeball’s Diazo kit. I used direct sunlight (no haze, clouds, etc.) for about 50 seconds. It was slightly underexposed because the wind moved my positive (plexi I had was larger than the screen) but I had my timer set for 60 seconds. I think 60 would have been a little better. Anyway, it worked! I have a screen drying as I type this. I will be cutting a piece of plexi for the frame I have and make a backboard with black felt that will fit it as well, then I can fix everything in place next time.
Comment by Ken — August 21, 2007 @ 8:24 pm
Sunlight is usable, BUT unpredictable. The times you used today, may not work later. There are too many variables such as season, air polution, clouds, time of day, etc. The time needed at noon, may be less than what is needed at 6pm. Just be prepared for some screens that don’t turn out as well as you would have liked.
Comment by Rob — August 22, 2007 @ 7:08 am
Using the Diazo photo emulsion, I took a cue from the instructions that came with the kit and picked up a BBA 250 watt photoflood. I exposed a 10×14″ screen for ten minutes with the bulb twelve inches away. The image washed out almost perfectly, save for a couple of segments of some very thin lines. I’m assuming I could have exposed it slightly longer. In any case, I picked up the the photoflood for less than ten dollars at a camera shop. Seems to do the job nicely.
Comment by Phil — January 6, 2008 @ 6:05 am