Are you printing the optimum halftone line count for your screens? Find out how to test the highest resolution that your screens and screenmaking procedures can deliver.
Poor Dmax on a film positive causes the smaller dots to disappear and the more open areas to be gritty and inconsistent in the final screen. A light film positive will produce dots in the screen that look ripped on the edges or oblong in shape, even if the screen looks fine to the plain eye. The emulsion has to work harder as a result of exposure, and it takes additional pressure and time to rinse out the screen.
Don’t be fooled by expensive equipment or new processes. Always check your positives with a loupe. In the case of a computer-to-screen machine, check the printed surface of the screen. I’ve seen several newer systems perform with worse quality than vellum upon close inspection, so don’t blindly trust that you’re getting the best results without seeing for yourself.
Checking the screens
Always use films that are in good shape to create the best screens possible. A great screen print comes from quality screens with well resolved halftone dots. Proper exposure is key. Undercutting and underexposure are serious issues you should work to avoid.
Undercutting is a problem that’s typically caused by poor vacuum seal during exposure, allowing the emulsion on the film to come away from the surface of the coated screen. The light then flows around the dots to some degree and causes poor exposures and bad edge quality on the final screens.
Underexposure is usually caused by the exposure unit’s bulb being too old or the improper calculation of actual exposure time. You can resolve these concerns by using a manufacturerprovided exposure calculator—available from most emulsion companies—and offsetting the age of the exposure unit’s bulb life in this calculation to account for declining effectiveness of the bulb over time. [More information about calculating exposure time can be found in Ross Balfour’s article, “In Pursuit of the Perfect Stencil,” beginning on page 82 of this issue.]
Apart from exposure issues, frequency (or moiré) presents the largest problem that printers face in creating screens. This issue can be a relatively simple one to fix, but it can still cause a lot of headaches. Frequency is a surprisingly common problem for even experienced printers to have on a recurring basis.
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