Washing out the problems related to stencil processing

This month William Shorter explores and explains the more complex side of stencil processing. Even though these are relatively straightforward processes, there are still a number of key factors that you need to get right in order to produce a good screen.

Washout booth

Screens should be developed in a dedicated washout booth that is separate from the screen cleaning booth. This will prevent potential contamination of the stencil from the decorating chemicals used to strip the stencil.

The washout booth should be positioned in a yellow safelight area to prevent the screens fogging while waiting to be developed, but should be fitted with a white backlight to enable effective screen inspection during washout.

Water temperature and pressure

The water used for the washout should be filtered to prevent particles in the water supply from becoming embedded in the soft emulsion surface. The optimum water temperature for developing direct stencils is 15 to 30°C; too cold and it will slow down the washout and too hot may cause the emulsion to soften and swell.

The washout pressure should be quite strong (4-6 BAR) and with a good spray pattern. The object is to quickly dissolve and rinse away the unexposed emulsion without softening or damaging the stencil. For manual washout it is good practice to wet down the squeegee side first and then conduct most of the washout from the print side of the screen, as that is where the bulk of the stencil is.

If you are tempted to use a high pressure gun (HPG) to wash out the stencil, select a diffuse spray pattern and make sure that it is held at least 0.5m from the stencil. Never use the gun from the squeegee side as it will blow the stencil off the mesh.

For small screen development for ultra high definition applications, a compressed air accelerated washout can be used with great effect, as this opens up the fine detail with minimal risk of damage to very fine lines/tracks.

Automated washout

For processing large screens in a busy shop, an automated washout system can help significantly. These tower developers wash out the screen from both sides and often reuse the developing water for the first part of the washout to minimise water consumption.

Stencil washout faults

Stencil breakdown – If screens break down during washout, this is often a sign of gross underexposure and/or insufficient drying of the stencil prior to exposure. Breakdown can also be caused by too hot water temperature, or too high water pressure.

When developing ultra thick stencils (>100 microns) avoid the temptation to pre-soak the screens, as this will soften the stencil leading to breakdown during washout. It is best to use a strong water spray from the print side only as this will give the best result.

Scumming – This can be seen as a transparent glossy residue in the open areas of the dried stencil and is caused by incomplete washout from the squeegee side (also a sign of underexposure), or possibly emulsion build up in re-circulated washout water supply.

Summary

Every stage in the process of screen making is important and contains variables that may affect the final output. A basic understanding of all elements of screen making and a methodical approach to fault finding will pay dividends in the long run.