This article highlights some of the most important variables in the process and offers tips for achieving consistency.
Controlling a stencil’s exposure is key to imaging properties, durability (mechanical toughness, flexibility, adhesion/mesh penetration), and full molecular cross-linking, which affects chemical resistance of the ink on the front end and ease of stencil removal on the back end of the workflow. One industry school of thought suggests that aiming for a consistent washout increment on a grayscale is the best way to control exposure. This is a subtle tool, however, and is reliable only if stencil thickness is absolutely consistent—and if washout water temperature and pressure are closely controlled.
I’ve found it more reliable in evaluating optimal exposure to make a step-wedge test and look for the first exposure increment where the stencil material reaches its full color density and the outline of the artwork can no longer be seen. With direct emulsions and capillary films, the stencil maker can increase exposure time above that increment, as long as that does not reduce details that are needed for the job. This ensures cross-linking of the stencil material for durability and mechanical resistance, solvent resistance, and easy stencil removal—a fully cured stencil is less vulnerable to being locked in by solvents.
With indirect films, the first maximum density increment is usually very close to optimal and cannot be increased by much at all without risking over hardening of the stencil and resultant poor adhesion. Control the exposure of any stencil material not exposed by digital direct exposure by using a light integrator with a cell filter to compensate for any shifts in spectral output as the bulb ages.
The deliberate underexposure of direct stencils to keep details from closing up is a recipe for disaster and heartbreak. A complement of emulsion on the squeegee side of the mesh that should be hardened to hold onto the mesh will be washed away. Post exposure will harden what emulsion remains, but less emulsion remains, so there’s less to harden, less to hold onto the mesh, and the resultant stencil is weaker and more vulnerable to mechanical damage.
Successful screen printers should try to leave nothing to chance, and that includes several things we’ve not yet mentioned that affect print quality and consistency: ink rheology, floodbar control, and squeegee durometer, angle, and pressure.
I once heard Joe Ulano say, “The reason most people and most companies are average is that, somewhere along the line, they stop paying attention to detail.” Paying attention to detail—that really does seem like the key to standardizing the production of stencils and exceeding customer expectations.
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Donald Marsden has been associated with Ulano in a wide range of activities since 1972. He has lectured and published technical articles in the US and abroad. He is a member and chairman emeritus of the Academy of Screen Printing Technology and is a graduate of Princeton University.
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