How to Guide - Coating PLUS emulsions
Modern coating machines are now classed as a standard piece of equipment in most busy screen making facilities and are essential if the screen is larger than 1m². A coating machine will basically replicate what a good operator can achieve, but with total control and consistency.
It is important that a coating machine is aligned correctly as misalignment will lead to inconsistent coatings and rapid wear of the drive gears.
When setting up a coating machine ensure that the coating speed is slow enough to achieve full mesh encapsulation, this is especially important when coating coarse mesh counts.
Attention must also be given to maintaining a consistent emulsion depth in the trough as differences in coating thickness will occur if there are significant differences in emulsion depth in the trough from screen to screen. This is especially important if matched screens are being produced for high quality four colour halftone printing.
The following video shows the basic steps for coating quality screens on a coating machine.
When coating manually it is very important that the screen is held firmly. A simple coating stand can help significantly. The screen should be held vertically, angled slightly away from the operator.
Depending on the size and shape of the coating trough it can be held either at the ends or by the body. Ensure that the trough is level and even pressure is applied across its length. Try to keep a constant coating angle for the coating trough lip. It is important that the trough does not ride over the edge of the frame or onto a high build adhesive overlay otherwise this will lead to thick edges.
It is important not to have the screen too high as this could affect the trough lip angle in the upper part of the screen. Typically the top of the screen should be no higher than your chest.
Ensure that there is no loose clothing (or hair) which could come into contact with the wet emulsion or that the screen is positioned too low for comfort.
The picture on the left shows how some simple modifications to coating technique can dramatically improve the consistency and quality of the coating.
Measuring stencil profile (EOM)
The ability to measure stencil profile is critical for consistent screen printing, as even a +1µ difference in stencil profile can give a noticeable difference in wet ink deposit. A hand held coating thickness measuring device is simple to use on Polyester mesh and will give an instant reading.
Measuring stencil roughness (Rz)
A rough stencil (high Rz) will give poor print quality, so the ability to measure Rz is very useful. There are many portable Rz metres available today that will give you an instant measure of your stencil’s surface roughness.