How To Match A Direct Emulsion's Print Quality Performance With That Achieved By Capillary Film
Question
We recently switched from capillary film to high-solids direct emulsion, applied on an automatic coating machine, and are very pleased with the quality of our stencils on 230 and 305 mesh for solvent-based printing. However, we suffer from poorer print quality on our 380 mesh UV screens, most of which involve reverse printing of very fine detail.
Regardless of how we coat these screens, we are unable to match the stencil thickness achieved with capillary film. Is this the cause of our problems?
Solution
As you have discovered, reverse printing of fine detail is probably the most highly demanding and critical test for the edge definition of a stencil. In order to match the quality produced from a film-based stencil when using a direct emulsion, even a high-solids emulsion combined with a coating machine, particular attention has to be paid to optimizing certain parameters.
The fact that you are unable to match the stencil build-up obtained with capillary film when using direct emulsion is related to your problem, but is not itself the cause.
To cut a long story short, the root cause of your poor print quality when running UV can be traced back to the small percentage of open area present in the mesh used, (which is why you chose that mesh in the first place, in order to minimize the ink deposit).
The use of an automatic coating machine does enable you to transfer an even and repeatable amount of emulsion onto the mesh every time you coat a screen, and you are correct to use a high solids content emulsion to minimize shrinkage on drying. However, the problem with the very fine mesh used for printing UV is that it not only minimizes ink deposit, it also impedes the transfer of emulsion through the fabric when using the two-sided, wet-on-wet coating procedures that normally yield satisfactory stencils with more open meshes. You have obviously picked up this effect when monitoring your stencil thicknesses and compared them to what was obtained with capillary film.
The poor print quality is caused not by the lack of stencil build-up, but by insufficient smoothing over of the knuckles, which are formed during weaving of the mesh. The result being that during printing, the stencil will not provide the gasket seal required to prevent ink bleeding under non-image areas. In technical terms, the stencil Rz value is too high. Refer to "What Is A Stencil Rz Value?" tech tip.
Compounding the effect of the thin emulsion coating is the fact that the finer UV mesh probably has bigger knuckles to cover in the first place. That's because there is a good chance that with the mesh you are using, the same diameter thread was used to produce both 305 and 380 mesh. Below we have illustrated what happens, by comparing 305 and 380 mesh woven with 34 micron threads, a commonly used thread diameter for these mesh counts.
When the threads are packed more closely together, not only is the percentage of open area reduced from roughly 30% with the 305 mesh, down to around 15% with the 380 mesh (which is good for minimizing ink deposit), but the knuckles formed by the dense weaving structure are much more prominent and difficult to cover.
For the 305 mesh, you might expect to coat a high-solids content emulsion, say, two coats on the print side, with three coats on the squeegee side, and achieve something like a 6-7 micron build-up with a nice smooth finish. With the same procedure, the 380 mesh on the other hand will only have a 1-2 micron stencil thickness, and will not print with the same quality as a film-based stencil.
Attempting to build up a thicker and smoother coating by applying additional coats to the squeegee side during wet-on-wet coating, is a futile task with mesh designed for UV printing. The mesh is not designed to allow the emulsion to pass through freely, and even if you apply an additional three or four coats to the squeegee side, you will be lucky to add a few microns to your stencil thickness. Meanwhile, the emulsion in the scoop coater on the print side of the screen is busy skinning over.
The surest way to guarantee the best print quality when producing this type of stencil is to seal the fabric with a few coats on each side, and then dry it, prior to polishing up the print side of the screen with an additional coat. With this method, you will still not match the stencil thickness obtained with capillary film, as the additional coat applied after drying will add but a micron to the overall thickness. You should, however, be able to match the print quality. Plus the fact that now that you are working with a thinner high-definition stencil, there may even be some benefits in terms of controlling ink deposit (and mileage).