Exposure technique and tips
Exposure Time: the exposure time depends on the emulsions also, but mostly the exposure time is varying from 10 Minutes - 15 Minutes
Dry the screen in a dark room at 90 degree angle with a fan on it. It should dry in about an hour.
You can also lay the screen flat on the ground when by stacking it on blocks from the frame.
Place your positive film transparency reversed on your screen where you want it to print.
Cover it with glass
Expose for nearly 12 minutes
Wash with cold water from a hose sprayer (small pressure washer is recommended).
When your design starts washing out, you can switch to a spray bottle filled with cold water and wash the detail part of your design out with a cold hard spray from a spray bottle. This may save it.
Further Exposure Tips to Closely Follow:
First of all, be sure that you are doing all parts of the emulsion and exposure process in a light safe environment with a yellow bug light or light safe bulb and NO outside or bright light what so ever. This includes mixing your emulsion, coating your screens, drying your screens, exposure, and washout.
Be sure that you are not coating your emulsion too thick. When using a scoop coater, also be sure that you take the black plastic or rubber application protector off the coater before applying the emulsion on the screen.
Be sure that your film is very opaque and dark. If you hold it up to a light and can see through it, you probably need to double print your film and stack the film on top of each other to achieve a more opaque image.
Insure that there is a positive contact between your screen mesh and your positive film. If the film is not pressed completely against your mesh then you will get light reflection between your positive screen film and your screen which will result in a blurry and not clear image.
Also be sure that you are exposing your screen for the correct amount of time which the light or exposure system you are using. If you are unclear on exposure times please feel free to ask us.
* If you have to take your screen outside to washout and you have no other options. Be sure that you take it outside in a very black trash bag and do not expose to light during movement. Also be sure that you are washing your screens out in a dark shaded area or at night if possible. If any bright light gets onto your screen during the washout process you might have problems washing them out completely.
About light source?
Many of us look for primary choices....
Point light source: The ideal light source is one where all of the light is collimated that is, all the light rays are parallel to each other. This lessons the chance of undercutting (light finding its way between the film positive and stencil material), as each ray of light travels perpendicular to the film and screen. If the rays come off the lamp at various angles, undercutting can result in lost detail and reduced sharpness. The best results come from combining a point light source with a well-designed reflector.
Multiple UV light source: All ultraviolet (UV) light sources are not the same, and light output should be matched to the requirements of the emulsion. Emulsions respond best to a specific range of UV light, and there is quite a difference between, for example, the light output of a metal-halide lamp designed to expose screens and that of a metal-halide lamp used to light a parking lot. While there are special UV fluorescent lamps that emit light waves in the right spectral range for stencil exposure, several such lamps are required to cover the dimensions of a screen. Since the light, therefore, comes from several directions, collimation is lost and undercutting becomes a problem.
The results produced by any multiple-light source are inferior to a quality metal-halide point light source. Some printers lump quartz-halogen into the general category of point light sources, but quartz-halogen is not a good light source for exposing screens, as it emits virtually no UV light, the most effective type of light for exposing emulsions. Mercury, the another type of light source, worked well in years past, but is not as good a match for today's emulsions.
Consistency and control
For a good quality light source to be effective, it needs to produce the same results on every exposure. One of the best devices to ensure consistently exposed screen is known as a light integrator. It has the ability to measure the actual quantity of light energy reaching the screen rather than simply the duration of the exposure and to adjust the exposure length in order to compensate for minor voltage fluctuations. It also compensates for aging lamps, which are prone to gradually declining output levels. The end result is that each screen will see the correct amount of light, each and every time.
Accomplishing an objective similar to that of a point light source, a good vacuum frame and blanket system is a critical element in holding fine detail by helping to eliminate undercutting. The vacuum is another item often absent from homemade or lower-end units. In an attempt to compensate for this missing features, some printers use weights to hold the film against the screen. However, this approach is no match for a good vacuum system. That kind of pressure and consistency is impossible to match with plywood and weights.
Param's good quality exposure unit is the critical element in supplying your presses with consistent, high quality screens. It not only reduces the number of screens that have to be remade, but also reduces the number of times you're forced to run an inferior job because there's simply not time to make another screen and still meet a deadline. It also reduces the number of customer complaints and lost business due to poor-quality prints. The sad fact is, if you start with a poor quality light source without a vacuum system, there will never be enough time to shoot a high-quality screen, because it can't be done.
Shops are getting what they may consider "good enough" screens every day from a variety of exposure units, both manufactured and homemade. The exposed screens seem adequate because, when someone keeps seeing the same screens produced the same way day after day, they have no other point of reference.
The bottom line is that Param's better exposure system will give you a consistenly higher quality product. As you grow your business by adding presses, by purchasing your first automatic press, or by taking on specialty work, take a look in the screen room and make sure you're turning out the best screens possible. The benefits of a high-quality exposure system may seem intangible, but they're real. A decision to invest in quality exposures is one you will never regret.
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