Hey all, my first post. I'm working on some screens that will have a printed area of 18x28". I've been doing screens that are small-scale (under 10x10"). I've been using a 150W clear incandescent bulb.
Obviously, using one bulb on such a large area at that same distance is going to over-burn a big centered hot spot. What I'd like to do is increase the distance until I can visually see the light disperse, which looks like about 30" or so.
I'm currently setting up a test screen to measure as best I can for exposure times, but I'm not sure if my results will tell me anything and I'm hoping to get cooking on this pretty soon. Any advice about this increase in distance?
Here's the poster, black and red printed on yellow paper...
I think the lamp distance is supposed to be around 1.5 times the diagonal measurement of your image. So if your image is 12" from top left to bottom right, your light should be at least 18" away (for a 'pointed' light source). I'm sure there are other factors to consider, but that's a good starting point. As for the increase in time, just do an exposure test.
I use a 250W lamp at around 24" for most of my stuff, and the times range from 15 minutes to 25+ depending on mesh count, mesh color, etc.
Hmm, good suggestions. I'm definitely going to raise the amount of light.
What i did for this was a test screen at 17 inches (150W, 1h:20m), with the light directed at the top half, and I turned the light halfway through, so it focused on the bottom half. Then when my time was up, I did a few 10-extra-minutes tests, by covering up the image by another inch or two every ten minutes. The extra time was because I wasn't convinced that the indirect light was having a substantial effect. But it turns out, it was.
I ended up at 17 inches, 1hr 20m, and rotating the light at 40 minutes. And I was pretty happy with how the screens came out. The emulsion was spread a bit thick (and drippy) on the first one and bit smoother on the second one.
Now here's where my story gets interesting, stupid, comical etc. I have to also disclose that this whole trip was completely experimental. I don't really know anyone local who is doing this, and I'm basically swinging a baseball bat in the dark.
I did everything wrong. I used old curtains, not silkscreen. For the black, I used housepaint with about 10% floetrol and not any kind of a proper screening ink (it got really gummy and gross). I used a couple different kinds of paper, with different surfaces, just to see what each would do. And with these piss-poor tools, I chose to attempt a good-sized two-color piece. My philosophy is, if I'm going to screw up, might as well screw up early, and big. Figure out what's ok to skimp on, what's better to spend the money on.
That said, the thing that completely tripped me up was my own stupid noodling. I had pieced together about 8 sheets of actetate per screen. A bit of light had burned through the less opaque areas of my acetate, resulting in a little bit of blocky (from the overlap of the acetates) haze on the clear parts of the screen. I was concerned about that blockiness showing up in the print.
What the hell. I'm a painter, I got my screen cleaner and a small paintbrush and cleaned up a few of the larger areas, them some of the medium areas...then calamity. Literally, on the word 'Calamity'. I don't know what I was thinking, but at one point, the 'MIT' just disappeared. So-so-so stupid. Dummy, this isn't like an acrylic painting you can just wander around the canvas touching up this and that. Now those letters are just gone. I got my screenblock stuff, laid the acetate under the screen for guidance and tried to touch it up, recreating the letters as best as I could. Which wasn't too horribly awful. I let that dry overnight, went home to brag about how I was the stupidest moron alive.
Next morning, the screen block didn't really hold up to a lot of passes and started leaking at about my seventh print. And it was leaking that gummy black I described earlier. Rather than go nuts trying to salvage any more, I called it a day, with lessons learned. I ended up with six prints that look ok. And a lot of lessons learned about paper, curtains, exposure, inks, and patience. I may re-burn that screen this week onto a real silkscreen (the curtain was a bit too low of a mesh for the details on my black screen, but it did ok on my less-detailed red screen), and finish my print run (about 30 pieces total was the plan).
The good thing is, since no one around here is doing silkscreen posters, my friends were pretty happy with just having the six. I'll take and post a picture of the final print when I get a chance.
Well... Here's one of the final prints. You can see the issues with the lost letters. Again, the red was designed to be rough, so that if it came out sloppy (which it did in places), it wouldn't ruin the overall piece. This isn't one of the 'better' prints, it's about where the screenblock started breaking down. The other issue I was having was that the ink was getting sloppy at the top of the red plate, and what I discovered was that the small bits of board I was using to align the paper was a bit thicker than the paper.
big props to making it all work even though you used all the wrong materials. that is awesome!
for large exposures i have used the sun. 7 minutes. i spray painted a piece of cardboard black (i had flat laying around) so that i would not get the emulsion exposed from random reflections just like you do with a light source.
it worked great. fine details like text were tight.