is there a such thing as over burning a screen and what happens when a screen is exposed longer than usual, and is there a remedy for washing out over exposed screens to retain the image or do i have to re-coat the screen and start over
is there a such thing as over burning a screen and what happens when a screen is exposed longer than usual, and is there a remedy for washing out over exposed screens to retain the image or do i have to re-coat the screen and start over
Yes there is such a thing as over exposing a screen. There is no remedy for it really, but you may try using a pressure washer with minimal pressure to try knocking out the stencil, but it is very likely you won't get a clean stencil or if any. Over exposing causes emulsion in the stencil to actually expose as well, causing it to be very difficult to blow out. You will have to reclaim the screen, and start over again.
how long is burn time for standard exposure unit...thanks for the response too!
There is no standard burn time. Too many variables come into play.
Type of exposure unit. type of bulbs, how old the bulbs are, type of emulsion, age of emulsion, type of film, vellum being used to burn art, screen mesh, etc.
What exactly are the issues you are having?
State What type of unit are you using, what type of bulbs, what type of emulsion and screen mesh
Crosslink the emulsion with sensitizer so it is durable and stays in the mesh. Crosslink all the sensitizer so when you want to reclaim the mesh and remove the stencil, it falls off the mesh with little effort because stencil remover attacks the crosslinks.
But if you expose long enough to do all that crosslinking, you may also have too much UV light undercutting your positive and bouncing around in the mesh so that your fines lines close up and ink won't transfer through the mesh.
It's not really over exposure that is the problem, it's that exposure can change the size of your image. You can prevent this with faster exposing stencils, stronger, single point, calibrated metal halide lamps so the work of exposure is done quickly, before the light scatter gets you!
You might want to get an emulsion calculator so you can target your precise exposure times. As everyone else has said, exposure times vary by emulsion type, light, how long it's been sitting and if any light has hit it before it was exposed.
I've been screen printing for a long time, used a ton of different emulsions and found that my diazo-core emulsion exposes best for 10 minutes on the dot. I then pre-wash the screen with a spray bottle of water and a damp cloth. I've found that lightly soaking and scrubbing the substrate (side that hits the shirt) side before I pressure wash it really helps get fine detail out.
over burning screen; is there a such thing 
