Capillex CP offers unique benefits when printing high definition four colour halftones with UV inks
Screen printing is all about reproducing what was on the positive as accurately as possible.
In halftone printing this is where all four colours are printing cleanly, with dots that are the same size and shape as on the positive. The photo-micrograph (to the right) of a halftone print shows how it should be
This problem is commonly known as 'Flooding'. We know that this is caused by too rough a stencil surface i.e. too high an Rz allowing the ink to flow out under the stencil edges. The diagram (to the right) shows how the uneven stencil surface on the print side, shown in magenta, allows the ink to spread out under the surface to cause the sawtoothed dot.
The industry solution is to flatten off the surface by multi-coating a direct emulsion, or to use a conventional Capillary film. But both these remedies can cause another problem, especially when using conventional UV curing inks.
This problem is commonly known as 'Skipping' This is caused by too thick a stencil i.e. too high an EOM.
Skipping is caused by high ink build on the print. It is usually only seen on the third and fourth colours of a four colour set when using conventional UV inks.
The normal remedy for skipping is to increase the squeegee pressure.
This however leads to stacking. The image above shows how by using a higher squeegee pressure the Magenta ink has been forced down into contact with the substrate resulting in large dot gain. To show the level of dot gain the Magenta positive has been overlayed on the print in register and then photographed. The outline of the magenta dots on the positive have been ringed in white and it can clearly be seen how the magenta ink has 'slumped' down to the right hand side of the magenta positive. This print would look very Magenta heavy due to the dot gain.
Increasing the squeegee pressure was not the answer as this is treating the symptom not the cause.
What is not commonly recognised is that very small changes in stencil thickness (EOM) will have a very big affect on the height of a halftone dot. This is not just theory as the prints below show the effect that a two micron difference in EOM can have on the print.