Are you printing the optimum halftone line count for your screens? Find out how to test the highest resolution that your screens and screenmaking procedures can deliver.
Most frequency issues with halftone dots in screen printing are caused by an interaction between the dot pattern on the film and overlapping areas of the screen mesh (typically called the knuckles), where warp and weft threads overlap each other. One of the primary ways to resolve this issue quickly is in mesh selection. Always have a ratio of at least three threads per dot. For example, 32-line/in. halftone on a film needs to have at least a 110-thread/in. mesh (32 x 3 = 96). Where this typically causes printers trouble is in the screen for the underbase white. You want to get the brightest white underbase possible, so you may opt for a 150- to 160-thread/ in. mesh with a common halftone line count for simulated process, such as 55 lines/in. Consider that 55 x 3 = 165 and you’ll discover that your choices will create some bad frequency patterns because there are not enough threads per dot and the halftone resolution is very close to the ratio of threads. The dots get lined up with the knuckles of the mesh and create that ugly feathering effect that wreaks havoc with a nice gradient (Figure 2).
So what’s the solution? Multiple factors are at work in this case, but I have found that lowering the mesh count is not always the right choice. The main reason that a 156-thread/in. mesh puts down more ink on a shirt as an underbase than a 180-thread/in. mesh is that the threads in the 156-thread/in. mesh are much greater in diameter and therefore trap more emulsion in a layer. Screen-printing geeks know this as EOM, or emulsion over mesh.
A common misconception is that larger mesh openings will always lead to a thicker, more opaque ink layer. Ratio of the size of the opening to the size of the mesh thread dictates ink flow, but does not necessarily lead to a thicker, opaque underbase. A better solution is to use a higher mesh count that has the proper ratio of threads per dot and attempt to improve the EOM by thickening the layer of emulsion. An extra face coat of emulsion gives a significant increase in underbase brightness without the frequency problems that will result from using a lower mesh count.
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