I'm pretty new to the forum and really new to screen printing. I've been pretty lucky so far and have not run into too many issues, but this latest has really been stubborn.
What's been happening is that the middle of my screen is exposing great. I use an exposure guide and it's telling me I'm at the proper exposure and it washes out great with excellent detail. The problem is when it gets out to the outer edges of the screen near the frame, the image gets progressively overexposed until the outer 1" won't even wash out. I'm using an 18x24 230 screen and my design is about 14" wide.
I think I may know the answer to the problem, but I thought I might see what insight I could get here first. I've been using bricks to weigh down a piece of black foam core that I wedge into the squeegee side of the screen while exposing. I've been really careful about making sure my film is sandwiched flat so I've gone up to 12 bricks on the screen which I'm assuming is about 60lbs. My glass is pretty thick at 3/8", but I'm thinking this weight might be bending the glass. I can't see a bend, but I can't imagine how else I could be getting this extra exposure on the sides.
I'm going to try less weight tomorrow, but if anyone has any other ideas I could try it would be much appreciated.
Thanks for the replies everyone. I'm using Murakami emulsion, I'm not at my workspace to verify, but I want to say its SP 9500. Its possible the thickness is inconsistent, but I tried buring this same stencil 6 times yesterday and the overexposure areas were almost identical. And I did try dropping my time down to 90 seconds but then it was underexposed and washed out.
Here's another piece of th puzzle that I'm starting to think may be the culprit: it may be the foam core. I have it cut to exactly the inner frame size thinking that the snug fit would give it a good seal. Now I'm thinking the snug fit may be preventing a proper seal. I think I may want to switch to a piece of rubber and a slab of melamine or some combo like that to make sure I'm getting the screen evenly flat on the film.
If the centre image is exposing and the outside isn't (the image) but also the emulsion is not washing off, your exposure time is right, but your glass/film/screen emulsion is not in intimate contact. The light is getting under your film at the outside.
You still haven't said what lamp/distance.
If you are using a bulb, try doing it the other way around.....on the bottom, a foam pad (not foam core) thicker than your screens and smaller than the inside of your frame.
Put a piece of thin black clothe on it, no wrinkles.
Screen face down, so it is hanging on the foam.
Film face down on the screen
glass...3/8 glass should weight it down fine - you can see if the film is being compressed before you start.
lamp hanging above, 1.5 x the diagonal of the image/screen (if this didtance is the same as your setup now, the time will be the same.)
ah tubes.....OK. they are sending light up from all directions.
You can make a vac top for your unit with two hinges, a wood 2x2" frame, a piece of wetsuit rubber, and an old vacuum cleaner. Just sayin...
A better compression method...foam pad, slightly smaller than the inside of the frame and higher than the screen (if it is really soft foam , it could be 1/2" or 1" higher.
You could put a piece of wood on this, then wieght it, or build a top with hinges. load the film/screen/foam, then pull the top down and have a latch or two. this compresses the foam, which compresses the mesh to the film to the glass. Coronado Studios have this and they burn all sorts of fine detail.
Success! I cut a nice slab of melamine close to the size of the inner frame and cut down the edges of my foam core by 1/2". So I made a foam core, melamine, stack or bricks sandwich on top of my screen and it did the trick. I'm still going to investigate some of the other suggestions from this thread as well for future projects, but I just wanted to try something with the materials I had on hand first. I'd really like to move away from the foam core and use some sort of rubber material for the first layer.
Thanks again to everyone's insight here. It definitely helped me get to a temporary solution, and thinking about a more permanent one.