Improper stencil exposure is one of the foremost problems occurring in screen printing companies today. Incorrect or inconsistent exposure can create costly problems in production including stencil breakdown on press, excessive pinholes, sawtoothing, loss of detail, inaccurate image reproduction and reclaiming difficulty.
While there are quite a few exposure variables to consider and control, perhaps the greatest tool to ensure proper exposure is an exposure calculator. An exposure calculator simulates between 5 to 10 different exposures with incremental neutral density filters, all on the same piece of film.
The filters limit the amount of UV light that hits the stencil in progressive degrees, so a single test can provide multiple exposures to evaluate. For instance, a calculator may have six exposures at 100%, 75%, 60%, 50%, 33% and 25% of the original exposure. These side-by-side exposures are then judged based on color change, depending on the chemistry of emulsion used.
Successful screen making requires the use of this tool on a regular basis. Each and every stencil/mesh combination must be tested. Exposure times vary with stencil thickness, stencil brand/type, mesh count, lamp type and distance. Do not assume that a 2:2 coating on a 230/48 mesh will have the same exposure as the emulsion coated 2:2 on a 380/31. Each combination will have a different result.
Here's a procedure for using a neutral density type exposure calculator.
How to use a Neutral Density Exposure Calculator
Coat and dry a test screen. Create a chart to record the screen's parameters including mesh specifications (count, diameter, color and type), screen tension, emulsion type, coating technique, stencil thickness, type and power of lamp, and distance of lamp.
Tape the exposure calculator onto the substrate side of the screen with the film emulsion toward the stencil.
Determine an estimated correct exposure time by either consulting the emulsion specification sheet, calling the emulsion manufacturer or calculating the average exposure time currently used in production.
Expose the screen at double the estimated correct exposure time.
Example: Estimated Exposure Time = 60 seconds
Test exposure time = 60 x 2 = 120 seconds
Wash out the image area using the same technique as in normal production.
For SBQ or 'Pure' Polymer emulsions, evaluate the squeegee side of the screen directly following washout while wet. Look for the step that shows no change in coloring to the next longer exposed section. The emulsion will appear a milky white color in the areas where full exposure is not present.
Dry the screen to evaluate color change for Diazo or Dual-Cure emulsions. Place the screen on a light table and evaluate the steps for a yellowish color. The step showing no change in coloring to the next longer exposed section is fully exposed.
Evaluate the resolution of the stencil, keeping in mind the limitations of the mesh count and thread diameter to carry the image. Optimal exposure based on resolution is typically a bit shorter exposure than the step selected based on color change. Some stencil resistance is compromised to gain resolution if this step is chosen. If more resistance is needed use the step chosen in step 6 or 7.
Multiply the selected step's factor (decimal indicated on the film) by the test exposure time to determine the correct exposure.
Test exposure time x selected factor = optimum exposure time
If the selected step is at the lowest or highest factor section of the calculator, the test exposure time used was too short or too long. A new test must be made with a more appropriate test exposure time according to the results seen in the first test.
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