This article describes methods for limiting the occurrence of destructive patterns in screen printing.
By Wim Zoomer
• Primary moiré is image interference between two or more halftone patterns at different angles. They clash during printing, because the respective angles create an undesirable wavy interference pattern.
• Secondary moiré occurs when halftone angles or line counts (rulings) clash with the weave of the fabric. Careful mesh selection can help minimize the effects of secondary moiré.
• Tertiary moiré occurs when a halftone is printed onto a texture, such as textile. The texture pattern clashes with the halftone dots.
• Local moiré only occurs in certain tonal value areas of the image, whereas primary, secondary, and tertiary moiré predominantly occur throughout the complete image.
Secondary moiré, in particular, presents the greatest challenges to screen printers. Let’s discuss the major causes in hopes of developing a better understanding of the moiré phenomenon.
Fabric selection always depends on the printed image’s requirements. In general, a finer fabric allows us to print finer halftones. A rule of thumb is that the fabric mesh count is F (= 3 to 5) times finer than the halftone screen. The ration between the fabric mesh count and halftone screen ruling should not be an integer, allowing a decrease of the moiré pattern frequency to make moiré less visible.
Mesh count (lines/in.) = F x Halftone screen ruling (lines/in.)
Fabric-selection tables show us the thread grades S, M, T, and HD. Grade HD refers to a robust and strong thread for heavy-duty print jobs. A large-diameter thread superimposes more halftone dots than a thin thread and, therefore, causes more moiré, as shown in Figure 3. Furthermore, large-diameter HD threads impede ink flow significantly more than the thinner S threads. This regular restraint of ink flow is visible as moiré.
Stretching mesh to its optimum level is an underestimated and often overlooked activity. Inaccurately stretched mesh may create local moiré patterns. Locally, the ratio between the mesh and the halftone ruling differ and are no longer optimum.
Dyed fabric, as opposed to white mesh, reduces undercutting effects substantially during exposure. This, in turn, reduces loss of the tonal range, which reduces the risk of moiré.
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