Question

We use a very high mesh count (380) and high solids content emulsion, which we coat to produce a ten-micron stencil thickness for a sharp image. Everything looks great, but we still run into problems when printing four-color process. Instead of nice round dots, the image is made up of irregular shapes, and some dots are missing altogether.

We know this causes problems with controlling neutral colors and flesh tones, and suspect it contributes to moiré, so what can we do to make a better stencil?

Solution

It sounds like you are already making very high quality stencils, and they are probably ideal for fine line or even single-color halftone printing.

The problem is your ink deposit is way too high for four-color process printing, and you are suffering from some of the same problems that printers using UV-cured inks have had to tackle. Remember, plastisol inks are 100% "solids" too, and the stencil/mesh combination you describe will probably lay down an ink deposit of around 20 microns. This won't cause a problem with the first color down, and probably not the second either. The problems usually arise when trying to lay down small dots of the third or fourth colors. The ink deposit that has already been built up prevents the stencil from contacting the substrate evenly. In areas where the two don't touch, ink will not be pulled cleanly from the screen once the squeegee passes. Instead of a flat substrate, you are trying to print on something with a topography which resembles a microscopic version of Manhattan. The piled up dots of the first few colors produce a high-rise barrier between the screen and the substrate.

The particular 380 mesh you mention, is produced with a twill weave configuration where threads are insertedd into every second space in the weave. This means that although it is very fine when measured by the number of threads per inch, there is still a lot of space for the ink to pass through. In comparison, plain weave mesh, where threads are inserted into every space in the weave, has a much lower percentage of open area, as well as a lower fabric thickness. This combined effect of the smaller openings with the thinner fabric would reduce your ink deposit tremendously, as the comparison shows.

A further benefit of using plain weave mesh is that you can achieve good edge definition and sharp dot reproduction with a much lower stencil thickness. Instead of having to build up a ten-micron stencil in order to smooth over the long knuckles formed in the twill weave fabric, you could get away with a five micron stencil thickness on plain weave mesh. This will shave even more precious microns off your ink deposit, and minimize the build-up that occurs and causes problems with your later colors. Simply lowering stencil thickness on the mesh you are using just now is not the answer. While you will print less ink, your halftones will suffer poor edge definition. Star-shaped dots are a major cause of problems with tonal balance and moiré, the very problems we are trying to control.

On a different note, another technique that would also help is the use of positives that have been separated with gray component replacement. With these types of separations, in areas of the design where yellow, magenta and cyan are all present, the density of the first three positives can be reduced. The difference is then made up at the end by printing with a heavier black than would be used with normal separations. The benefit of this method is that early ink build-up is minimized. This, in conjunction with the right mesh and stencil combination, will provide you the biggest window of operation for all the other parameters you have to control, once you get your screens to press.