Stencil Exposure Testing
Ulano One-Step & Transmission Grayscale
1. Tape positive to the screen.
You don’t need any special equipment to make an Exposure Step Test, just a coated screen, a typical positive with a variety of line thickness and some Ulano Rubylith™ masking film to block UV light. You are going to make 5 exposures by blocking portions of the positive as you expose it
Ulano 1-Step Exposure Calculator
Exposure calculators simulate 5 exposures at one time with a filter. The filters vary the amount of UV light that gets to the stencil in one exposure so you don't have to take the screen in and out of the vacuum frame every time you want to move the mask. The result should be identical to the time consuming manual step test.
Older style exposure calculators attached the filters directly to a camera film positive but this usually doesn't represent your actual conditions if you use laser or inkjet positives. Large calibrated filters like the Ulano One-Step can be placed directly over your laser or ink jet test positive for the best results.
1. Tape your positive to a typical coated screen. You should coat all the screens in that mesh count the same so the exposure time can be the same. You will not move this positive during this test.
2. Vacuum and expose to UV light.
2. Place the coated screen with the positive taped on it into your vacuum frame and expose for half the time you estimate as the estimated exposure time for this combination of light source and stencil. For example: I think this screen will take 4 minutes to expose, so my initial exposure will be 2 minutes. Remember to use a rope to let air escape from inside the screen.
3. Turn off the vacuum and turn over the screen.
4. Tape Masking film over the positive.
5. Move the Masking Film and expose again.
Ulano has excellent training videos for Direct Emulsion and Capillary Film, including sections on how to make an exposure test.
4. Tape a piece of masking film so it covers about 1/5th of the positive. Put it back in the vacuum frame and expose for 1 more minute.
For the third exposure, move the mask so it covers 2/5ths of the positive and expose for 25% of the approximate exposure time.
For the fourth exposure, move the mask so it covers 3/5ths of the positive and expose for 25% of the approximate exposure time.
For the last exposure, cover 4/5ths of the positive and expose for 25% of the approximate exposure time.
The exposure times can be varied if you desire but the goal is two exposures above and below the approximate amount of UV light.
Stencils with diazo sensitizer will have a color change because the yellowish diazo will actually be used up and the stencil will return to the color is was before you sensitized it. For the best durability, look at the section where you don’t see any color change. This means the stencil reached it's saturation point, and more light just doesn't change it any more. Don’t waste time exposing it if it won't change any more.
Process the stencil in the usual manner and look for signs of under exposure on the squeegee side of the stencil, where the stencil will wash away because it didn't get enough light to change it so it won't dissolve with water.
When the stencil is dry, make a print with it. Compare it to your positive and look for the best resolution. Decide if there is a difference between the section where the color stops changing and the best resolution. Here is where your judgment is required. If they are both in the same section, you now have proof of the best exposure. If not, you will need to sacrifice durability or fine lines or change emulsion.
Write the exposure for this emulsion and mesh count in a safe place and remember that different mesh counts will have different stencil thickness and different exposure times.
Close-up of filters over your positive
Transmission Gray Scale
Once you know your optimum exposure, use a small transmission gray scale on every screen you expose, so you will know if your conditions have changed. A transmission gray scale is a small film positive with darker and darker filters next to each other. When you have one on the stencil as you expose you will get a simulation of many different exposures to the stencil.
When you wash out the stencil, areas that didn’t get enough exposure will dissolve with water and go down the drain.
Using the exposure you determined was best with your larger exposure test - you can determine how many steps of the gray scale will resist washout and stay in the screen.
Usually there will be a step that washes away a little between an open area that dissolved and the first step that survived. Count the steps and document that this stencil and mesh count should get a solid step 4 (this is only an example, your results will vary).
When you put a gray scale on every screen, you will notice when it washes out differently and you will know your lamp or stencil is different and you have to change your exposure.