Emulsion Selection & Coating Techniques For Coarse Mesh Counts
Question
We use a high-solids content emulsion and are very happy with the results on our 158 and 110 mesh. We have problems, however, with our 61 mesh and we can't coat our 24 mesh screens at all because the emulsion drips right through. Do we need to find an emulsion with an even higher solids content?
Solution
Not necessarily. You made the right choice in deciding to use a high-solids content emulsion, since you need high solids in order to cover your coarse mesh without too much shrinkage. However, the first thing we need to do is make a distinction between solids content and viscosity, or thickness.
Emulsion manufacturers are able to control solids content and viscosity independently of each other. High solids does not always mean high viscosity. In fact, many high-solids content emulsions are supposed to be low in viscosity because they were designed primarily for producing high quality stencils on fine mesh counts, where the flow properties of the emulsion are more critical. Obviously in your case, where you are using coarse mesh with a high percentage of open area, high viscosity along with the high solids content is an essential feature if you are going to avoid having problems with the mesh counts at the lower end of your range.
If you are using a diazo or dual-cure emulsion, you could try simply adding less water to the sensitizer when you mix it. If the emulsion is already too thin as supplied, or if you are using a pure photopolymer type that requires no mixing, then you may have to switch to an emulsion with a higher viscosity specification.
The second thing you need to address is ensuring that your coating method is optimized for the type of mesh you are using. As you go from 158 down to 24 mesh, the percentage open area of the mesh increases from 32% up to 55%. This, in conjunction with the fact that the openings are so much bigger, means that for the coarser mesh you really have to reduce the number of coats you apply. If you don't, too much emulsion will be transferred onto the fabric, and even the thickest emulsion will sag and drip if you lay it on too heavily.
For the 158 mesh screens, two coats on the print side, followed by two or maybe even three on the squeegee side, will produce a nice smooth finish to the dried emulsion. For the 110 mesh, two coats on each side should be sufficient. The last two coats should always be on the squeegee side to push the emulsion back where it belongs on the print side of the screen.
For the 61 mesh, cut back to only one coat on the squeegee side. And remember, these screens should always be dried flat with the print side underneath. By now you should have the emulsion onto the print side of the screen, and you want to make sure it stays there. Emulsion on the squeegee side of the mesh is more difficult to harden properly on exposure, and even when it is hardened, is more likely to suffer from abrasion and wear by the squeegee during printing. Thus causing pinholes and breakdown.
By the time you get to the 24 mesh, you need to adopt a different technique. In this case, apply one slow coat to the print side. Then, turn the screen and apply one slow coat to the squeegee side. Immediately after this, use your coater in the horizontal, untilted position to remove the excess emulsion from the print side of the screen by applying a scrape stroke. This will leave enough emulsion on the screen, but should eliminate the possibility for drips to form while the screen dries.
Note: one important point to remember is that the coarser the mesh, the slower you should coat in order to minimize bubble formation. With the coarser mesh, for instance 61, and particularly with 24 mesh, there is a tendency for air bubbles to get trapped in the mesh openings. As you probably know already, every trapped bubble is a potential pinhole.