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Exposure

Exposure is the process of transfering the positive image on the film to the stencil, and includes the washout of the image on the stencil. A UV light from a light source reacts with diazo or photopolymer elements contained in the photostencil emulsion, and causes a chemical cross-linking and hardening of the exposed areas of the stencil. When washed with water, these areas stay adhered to the mesh, and the areas blocked by the opaque image on the film, which has shielded the underlying stencil from the UV light, wash out. The end result should leave an image identical to the film positive image as open areas in the mesh, surrounded by a layer of hardened stencil adhered to the mesh.

Components of the exposure process are a light source, a vacuum frame, and a washout booth. An exposure calculator, timing device, and hot and cold water are also required. Indirect stencils require a developing tray and peroxide developer.

Light Source

The light source supplies UV spectrum light rays in the 330-440 nanometer range(as opposed to visible spectrum) which cause the photosensitisers in the emulsion to react. Light sources range from photofloods and blacklight tubes, which give off low amounts of UV light (and take longer to expose) through carbon arc (not used much due to fumes) to enclosed exotic bulbs such as metal halide, which use a transformer to jump the electricity wattage (3k,5k,7k, etc.) and ignite the gasses contained in the bulbs. Other light sources include the sun, and direct projection devices. The light sources can be run on a timer, or more commonly a light integrator, which reads the amount of UV light reaching the screen.

Some light sources are stand-alone units, able to be moved closer or farther depending on the size of the exposed piece. Others are mounted to the vacuum frame as a one piece unit.

Vacuum Frame

This consists of a piece of untempered glass, a sealed flexible rubber blanket, and a vacuum motor. When the vacuum motor is turned on it holds the film positive in direct contact with the stencil, and pressed tight to the glass. The UV light source is then aimed at the glass, passing through it and the transparent areas of the film, exposing the stencil.

Vacuum frames come as stand alone units mounted on a wall, or on wheels. For sizes up to about 4ft.x6ft., some manufacturers make combo units with the exposure lamp(s) built in. Glass in the exposure frame must be kept clean and free of dust or dirt which will start showing up unwanted on stencils.

Washout Booth

The washout booth is used for both the exposure and reclaiming of screens. It is constructed to contain water overspray, hold the screen at a comfortable working height, resist chemicals, and should have a backlit translucent panel to allow close inspection of the screen mesh during washout and reclaiming. The booth needs hot and cold water, and should have a pressure washer for reclaiming.

THE EXPOSURE

Before proceeding with an exposure, it is of utmost importance to ensure the following:

  • The screen and stencil are dry and cured.

  • The glass of the exposure unit, the screen, and the film positive are free of dirt or dust.

  • The light source, washout booth, and developing tray (for indirect) are ready for use.

  1. The first step is to orient the film to the screen, so that it can be easily printed. In some cases, the image will only fit in a certain position on the press, or needs to be angled, or needs specific clearances. These all need to be considered, and the film positioned accordingly. The positive can be taped to the screen, the glass, or laid in position on the glass. Ensure that the film is EMULSION SIDE UP, RIGHT READING.
  2. Next, lay the screen over the film, ensuring positioning is OK, and the frame does not obstruct the lid and blanket of the vacuum. Indirect stencils are placed emulsion side up. Most units have a cord to collect air to the vacuum pump, and this should be placed along the inside of the frame. Do not let this cover any image areas of the positive, as it can affect the final stencil image.
  3. The vacuum pump is then turned on, and once the blanket has sucked down, flip the exposure unit to face the light source. Before the light source is turned on, do a final inspection. If positives have shifted, or dust or dirt is visible in the image, fix the problem before proceeding.
  4. At this point, turn on the light source. Some units require pre-heating, and are equipped with shutter systems for the actual exposure. The timer or light integrator is turned on at the start of the exposure, and shuts the lamp off or closes the shutter at the end. Different stencils require different amounts of light for proper exposure, and this is determined using an exposure calculator.