This article describes methods for limiting the occurrence of destructive patterns in screen printing.
By Wim Zoomer
We can approximate the risk of the appearance of moiré before the preparation of the stencil. Put the frame with the tensioned mesh on a light table and put the right side of the film positive on the print side of the mesh. Any visible moiré can be reduced by rotating the film positive across the fabric. Mark the new position and expose the image after applying the emulsion.
Screen printing halftones causes dot gain. The amount of dot gain is equally distributed along the circumference of the dot. This phenomenon is visible when the dots just start to touch each other. The so-called double-symmetry dot gain may cause a large jump in tonal value and may be visible as moiré. A relatively high ink deposit reinforces this effect. Moiré is reduced by choosing a dot shape that differs from the square shape of the mesh openings. Round dots or elliptical dots can resolve the issue. The advantage of an elliptical dot shape is that, due to the different distance to the surrounding dots, it produces an area with a tonal-value increase without a sudden, unwanted tonal jump. Another solution is to use different dot sizes in different tonal areas to avoid the tonal jump.
Screen printing halftones within a tonal range of 15-85% is generally feasible. However, the requirement is that the smallest printable negative dot must be larger than the sum of one mesh opening plus two thread diameters. After all, the stencil requires sufficient support to adhere to the mesh.
On the other hand, the finest positive halftone dots are printable when the dots are above a mesh opening. Halftone dots do not appear on the substrate when the dots are superimposed on single threads or crossing mesh threads. This kind of moiré, caused by regularly leaving out dots, is strongest between tonal values of 40-60%.
Halftone screen angle
If we want to avoid primary moiré, the angle between two halftones must be 15° or a multiple thereof. In theory, we can print four colors at 0°, 15°, 30°, 45°, 60°, etc. Obviously, we should check the film positives for the appearance of film-to-film moiré. The screen used in the screen-printing process requires a compromise on the printer’s part to the eventual moiré effect caused by the four colors and the mesh fabric.
Yellow, the least visible color, is basically placed at the most visible angle, 0° or 90°. Next, a combination of theory and experience establishes the following sets of halftone screen angles depicted in Figure 4. To avoid printing halftones parallel to the threads and, as a result, the creation of harmful moiré, consider the practice of offsetting the screen angles. The screen angles represented in sets 2, 3, and 4 in Figure 4 are also suitable for single halftones, duotones, and more.
A thick stencil can cause moiré when printing a four-color-process graphic—especially in the dark tones, where excessive build-up of several ink deposits causes irregular relief and, as a result, deformation of the dot shape. This deformation is visible as a moiré pattern. A minimum, but sufficient, emulsion over mesh (EOM) thickness prevents many issues. A thin and smooth stencil results in good contact between stencil and substrate and, therefore, produces a crisp printed dot.
Substrates designed with surface textured can increase the chances of creating harmful moiré patterns—woven textiles, for example. We can predict and minimize the occurrence of moiré by mounting sets of film positives to the substrate. Rotating the film positives to the left or right side will reveal the problematic areas. Using the optimum angle between film positive and substrate reduces moiré between the print image and substrate.
Moiré is not necessarily supposed to be our enemy. The effect can also be helpful in identifying materials. Collect all sorts of mesh fabrics, cut samples no larger than 4 x 4 in., and mark the samples accordingly. To identify an unknown fabric, we simply put the standard sample on the unknown mesh fabric. A typical moiré pattern will form when we rotate the standard sample onto the fabric. The moiré pattern is characteristic for the two matching regular structures.
Wim Zoomer (email@example.com) is owner of Nijmegen, Netherlands-based Technical Language, a consulting and communication business that focuses on flatbed and reel-to-reel rotary screen printing and other printing processes. He has written numerous articles for international screen-printing, art, and glass-processing magazines and is frequently called on to translate technical documents, manuals, books, advertisements, and other materials. He is also the author of the book, “Printing Flat Glass,” as well as several case studies that appear online. He holds a degree in chemical engineering. You can visit his Website at www.technicallanguage.eu.
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