The thumbnail version:
- Textile screen printers have been known to “wing it” with some technical aspects.
- Stencil thickness is one of those aspects.
The full version:
The textile screen printing industry is still notorious for “winging it” in some of the more technically tricky aspects of screen printing. Some shops are very particular about technical precision but many are not. For instance, how many shops own a stencil thickness gauge? How many just slap on a one-on-one coating of emulsion and hope for the best? Do you?
Why would anyone need a stencil thickness gauge? Well, for a start, how do you know whether or not a screen is appropriately coated with emulsion for that particular mesh count if you cannot measure it? And secondly, you could be having print issues from time-to-time and not know that they are caused by a too-thin or too-thick coating of emulsion.
The emulsion over mesh (EOM) ratio is used to determine the appropriate thickness of a coating of emulsion relative to the thickness of the mesh. The idea is to give the stencil sufficient shoulder to bridge the mesh threads properly and provide a good gasket with the substrate. Anything more or anything less than the appropriate EOM can result in print problems. Without the ability to measure the EOM how would you know whether a print problem is stemming from an inappropriate EOM rather than the ink, the squeegee pressure, the substrate, and all the other usual suspects who get blamed first?
Experts will tell you that for most textile applications (high density printing being and exception) an EOM of 20% is recommended. That would mean that a 110 thread/inch mesh with a 80-micron thread diameter would print well with approximately 23 microns of emulsion. How can anyone know whether or not they have applied an EOM of 20% with a one-on-one method and no way of measuring it?
If the coat of emulsion is too thin, these things can happen:
- smearing in the direction of the squeegee stroke;
- dot gain in the shadow areas;
- dot loss in the highlight areas;
- excess ink build-up on the substrate side of the screen;
- saw-toothing in the print;
- and fine details closing due to over-exposure if the exposure times were set for a thicker stencil.
If the coat of emulsion is too thick, these things can happen:
- difficulty in drying;
- poor ink release from the stencil, or too much ink deposit in cases where the ink does release from the stencil;
- dot loss or gain as a result of the previous problem;
- pinholes and possible premature stencil breakdown resulting from under-exposure, if exposure times were set for a thinner stencil.
So, eliminate stencil coating as a suspect in print problems – get a stencil thickness gauge and use it to check every screen before it goes for shooting. Considering the time that can be spent trying to solve print problems and the consequences of those problems in terms of rejects and returns, a few hundred dollars for a stencil thickness gauge seems to be a bargain.
Any one of the Canada-wide Stanley’s branches can help you with questions about stencil thickness gauges. Call them: Cambridge 1 877 205 9218; Calgary 1 800 661 1553, Edmonton 1 888 424 7446; Richmond 604 873 2451.