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Making Screens for Long-Run Production

(July 2002) posted on Mon Aug 05, 2002

Coudray walks through screen and stencil preparation, identifiying techniques for producing accurate and durable screens that will last beyond 100,000 impressions.

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By Mark A. Coudray

A stable mesh is critical. For this reason, we should avoid using new mesh on a long-run. Virgin mesh is simply not stable enough to make a satisfactory long-run screen. I prefer screens that have been cycled and retensioned at least four times. This allows for all cold flow and knuckle binding to stabilize. With a stable screen, the image will not change size on press, and the ink transfer will be consistent over the entire run.


Proper degreasing and neutralization of the cleansing agent on the mesh is also important. The thread surface must be in optimum condition to accept the emulsion. Improperly degreased screens or mesh with ghost images will lead to premature failure on press.


Choosing the right emulsion


For long-run performance, nothing beats high-solids direct emulsion. I prefer the dual-cure (photopolymer/diazo) variety. The dual-cures give the best combination of edge definition and complete exposure conversion. High-solids emulsions resist shrinking during drying, leaving ample resin to resist squeegee, flood, and substrate abrasion. Coating technique is very important. The idea is to create ideal emulsion-over-mesh (EOM) buildup. The exact EOM measurement will depend on substrate and ink type. Printers using UV inks may need only 8-12 microns, while a high-density textile printer may require 700 microns.


The higher the EOM requirement, the more likely you will need to employ a capillary/direct (also known as direct/indirect) stencil system. Regardless of whether you use direct emulsion or capillary film, it is important that the squeegee side of the screen be completely encapsulated with emulsion. The greater the encapsulation, the longer the screen will resist the constant abrasion of the squeegee and floodbar.


Drying and exposure will also have a huge bearing on the durability of your screen. Coated screens need to be completely dry before exposure. This means that the internal humidity of the dry emulsion should be the same as the relative humidity of the drying area (about 40%). If you dry screens in an area with humidity below 40%, you run the risk of brittle emulsion that will prematurely crack and breakdown on press. Humidity above 60% will interfere with the proper exposure, effectively canceling the formation of long molecular chains (one major source of emulsion toughness and strength).


Ensuring proper exposure



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