Positive pressure or contact between your screen and film is extremely important in creating a crisp image on your screen. If your positive film and positive image are not pressed extremely tight against your screen, light will work its way around the edge of your image and expose a soft line instead of a crisp line. Without positive pressure you can throw the idea of a high quality screen out the window! Positive pressure can be achieved one of two ways. The easiest (and most convenient) is a vacuum exposure unit. The vacuum blanket sucks the screen down to the glass with about 60-70 lbs. of pressure and sandwiches the film between the glass and the screen creating the prefect amount of positive pressure.
You can also create positive pressure by using dense two inch foam and placing weight on top of the foam or compressing the foam with a compression lid. As you put pressure via weight or compression on the foam, the foam distributes this weight and presses the screen mesh into the glass which sandwiches the film in between creating the needed amount of positive pressure. You want to use between 50-60 lbs. of pressure or weight using this method. If you use less, you will not put enough pressure on the screen in order to create good positive contact.
Vacuum Frame and/or Positive Pressure:
1. Open latch, raise top (vacuum and compression lids).
2. Position transparency on glass, right reading - optimally taped to screen.
3. Place screen over transparency, film emulsion to screen emulsion.
4. Lower top, close latch or place foam and weights (open top units).
Dust and scratches on the vacuum-frame and open glass units can easily appear as defects in your stencil. One of the easiest ways to avoid this type of problem is to keep the glass clean and exercise care when placing items on the glass. Clean the glass on one side first thing in the morning and then throughout the day as needed. Clean both sides at least once a week. Use a soft cloth and a quality glass cleaning product.
The second precaution is to use care whenever you lay a screen onto the glass. Metal screen frames can easily scratch the glass, especially if you slide them around while positioning them for exposure. Once a scratch is introduced to the surface of the glass, it will be reproduced on any stencil that is laid over that particular area of the glass.