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Taking Control of Screen Coating

(May 2009) posted on Wed May 20, 2009

The stencils you use for printing are only as good as the coating procedures you follow to produce them. Here you


By KIWO Technical Support Department

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Fine detail can be taken to mean any artwork in which the finest elements are equivalent to the width of two threads and two mesh openings of the particular mesh fabric being used. You can test this concept by laying your film positive right on the screen and looking through a 50x microscope to see how many mesh openings and threads your finest design elements span. If your smallest dots or lines are not much larger than two threads and two mesh openings, you have fine detail. With such details, if you do not have enough EOM, the threads will create a dam around open areas in the same way that a stencil does. In halftone printing, such low EOM leads to dot loss as the highlight dot is cut into pieces by the threads (Figure 2). (Figure Reference: With insufficient EOM, the mesh in open areas of the screen can make direct contact with the substrate during printing. As a result, the knuckles of the mesh serve as dams that prevent ink from spreading through the entire open area. The result is dot loss and poor dot reproduction.)

Stencils with low EOM also likely suffer from high Rz values. The incomplete gasket caused by high Rz allows the ink to push beyond the image edge and leak into the valleys of the stencil. This results in dot gain. At the same time, the low EOM leads to dot loss and sawtoothing. High Rz and low EOM are both the result of poor coating techniques. However, these conditions can be eliminated by observing the screen-coating pointers we’ll consider in the next section.

All stencils must have some EOM for proper printing. But, a stencil can be too thick as well. For many printing inks, stencils perform well with an EOM that is between 10-20% of the mesh thickness. For finest detail printing, including four-color process, the EOM should not exceed 10% of the mesh thickness. If the stencil becomes thicker, problems such as ink piling, skipping, and skewing can occur when printing with UV ink. Moreover, highlights may not open up, and ink transfer becomes difficult. For excellent printing on most substrates, the stencil will also need a low Rz value (approx. 6 microns if possible).

 

The coating process


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