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Originally Posted by blueskies_tees |  | | | | | | | | | We have a 1000 watt quartz lamp and we were advised to expose it for 6 minutes from 23 inches by the people we brought it from.
This didn't seem to work and it was really under exposed, and after trial and error we ended up exposing it for up to 20 minutes.
This does work, even though on some occasions the emulsion still washes out slightly.
The biggest problem comes when I push the ink through the screen, the design only comes out slightly, as if there's something stopping the design coming through the screen even though the image is clearly washed out.
Before this we had a 500 watt bulb and were told to wash it out for 5 minutes but were not given a specific distance and had exactly the same problem.
We're getting quite fed up of this consantly happening as it's meaning we have to wash out the screens again and is wasting so much ink and emulsion. | |  | |  | |
Who did you buy the equipment from and told you the initial exposure time?
Lamp Distance
The standard distance calculation is to measure the diagonal size of your image and multiply that by 1.5. An image with 20 inches from the NW corner to the SE corner you would put the lamp at 30 inches.
Variables
Every lamp ages, all emulsion ages. Every emulsion has a different speed and that speed is effected by the coating thickness and the amount of moisture left in the coating.
The fundamental skill every screen maker has to learn is how to make an exposure test. For durability and the minimum exposure to hold the stencil in the mesh, I use a 21 Step Transmission Gray Scale. It simulates 21 exposures with one actual exposure. A starting point is to have a solid Step 7. When you expose the stencil so that that Step 7 doesn't wash out, you know you have a minimum exposure.
The longer the exposure, the more your positive can be undercut by light and 'choke' fine lines. This always happens and the way to calibrate is to measure your fine lines and make them fatter so when they get choked, they are the final size you wanted.
For fine line resolution, every emulsion company make a filtered exposure calculator that simulates 2 under exposures and 2 over exposures over your optimum exposure. It brackets what you think is the best exposure so you can have 5 or 10 simulations to judge the resolution you can get from your combination of mesh, stencil and lamp. If I was at your shop today, I would have to make a test.
Exposure Videos on the web
I have a videos on the Ulano web site that you can download and watch over and over:
Video Training for Screen Makers from Ulano Exposure Test
Stencils are easy. If it washes out, or peels off the mesh, it wasn't exposed to enough UV-A energy. Unexposed stencil should wash out easily.
If the
image is hard to wash out, it was exposed to heat or UV energy and has started to cross link, which makes it resist washout. If UV energy leaks through the the positive, it can prevent the stencil from washing out in the image area.
The next time you expose, test to see if your positive is blocking UV energy by attaching a 'dime' to the stencil. A dime will not let
any light pass through it.
Stepped Exposure Test
All exposure questions are answered with a FREE stepped exposure test which is essential to any screen maker. You do a series of 'stepped' exposures on the same screen & positive, then JUDGE the proper amount of exposure.
When you get tired of wasting time guessing about exposure, call your supplier and buy an exposure calculator ($35), and 21 step transmission gray scale ($10). Properly used, they will last a life time.
Homework Exposure or Emulsion?????
Exposure Time 
