Excellence in stencilmaking yields screens that stand up to production wear and produce detailed, high-quality graphics. Find out how to regulate the instabilities in the process that can lead to less than desirable results.
By Ross Balfour
A screen-printing stencil has to perform four functions. Two are important for any type of screen printing, since the stencil must first reproduce the image that is to be printed and be resistant to abrasion and chemical at-tack. The last two functions are particularly important for high-quality line or halftone printing, because the stencil can help to control the amount of ink that is printed. The stencil is also responsible for controlling image accutance, more commonly referred to as print-edge definition.
Clearly, the stencil plays a critical role in the overall success of a screen-printing job. This article will review the variables associated with stencils and describe how to minimize them and remedy the harmful effects they may impart to your work.
Rz value and stencil profile
The two parameters that affect print quality are Rz value and stencil profile. Rz value regulates edge definition, and the stencil profile contributes to ink deposit (Figure 1). You can measure and control both, regardless of which type of stencil system you plan to use.
Stencil profile is used, along with the screen mesh chosen, to control ink deposit. A thick stencil is beneficial for certain applications; for other applications it is advantageous to minimize the stencil buildup. Rz or surface roughness of the finished stencil controls the print’s edge definition. For most types of printing, an Rz value of 10 microns or less will result in good edge quality. For highly demanding printing, such as small, reversed text, or high-line-count halftones, a value closer to 5 microns is necessary. Below 5 microns, if the stencil becomes too glossy, then ink splattering or cobwebbing can occur when printing on glossy substrates.
Capillary film stencil systems are manufactured in different thickness grades, each designed for optimum performance on a narrow range of mesh counts. Best results are obtained by selecting the correct grade for the mesh count being used. Excess water is removed from the mesh during processing with a light squeegee action. Pressure is not required, and would in fact lead to detrimental results as the film could become overdissolved. If the correct capillary film thickness is used, the water that remains is sufficient to absorb half to two-thirds of the original emulsion layer into the mesh. What remains comprises the stencil profile and controls the Rz value (Figure 2).
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