This article describes methods for limiting the occurrence of destructive patterns in screen printing.
By Wim Zoomer
Destructive moiré is an undesirable phenomenon for the screen printer. It can appear suddenly during screenmaking, even when procedures are standardized, and remain during printing. Beating destructive moiré entirely is unrealistic; instead, we must focus on minimizing it.
Undesirable moiré patterns appear when two regular grids are superimposed at one angle, or when the two grids have slightly different steps. One very practical example is an effect that occurs on television. We may observe a visible pattern of what appears to be moving dark and light bands caused by interference between the weave of clothing worn by people on TV and the pattern in the TV screen. When we look around critically, we conclude that moiré is everywhere around us.
One of the characteristics of moiré is its ability to magnify tiny shapes. Magnification by moiré occurs when viewing a chain-linked fence through another, identical, chain-linked fence. The detailed fence structure is visible even at a certain distance. Figure 1 depicts a similar situation created when one set of parallel, vertical lines is superimposed on another set of parallel, vertical lines.
The size of the created moiré pattern depends on the angle between the two superimposed patterns. Figure 2 shows a moiré effect created by the sun shining through the double-layered fabric back of an office chair. The light and dark bands, apparently projected upon the lines underneath, appear as if black lines are side by side in some areas and cover each other in other areas, thereby producing a light band.
Screen printing presents numerous opportunities in which regular grids are superimposed. A regular grid can be a halftone-dot pattern or a set of parallel lines. Other possibilities include:
• The fabric, commonly polyester, stretched to a certain tension level on a frame.
• The image can be, among other things, a single halftone pattern. This (AM) pattern is supposed to be a regular pattern. Another image may consist of multiple halftones with the colors positioned at different angles.
• Last, but not least: the substrate. The substrate is not necessarily smooth, but it can be a textured pattern or even a textile.
Interference between grid patterns can cause different forms of moiré:
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