7. Wash Out & Dry Stencil
Washout (Developing the Exposed Stencil)
A best practice for screen printers is to develop screens in a dedicated washout booth that is separate from the screen cleaning booth. This will prevent potential contamination of the stencil from the de-coating chemicals used to strip the stencil.
The washout booth should be positioned in a yellow safelight area to prevent the screens ‘fogging’ whilst 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 15-30°C. If the water used in washout is too cold 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 emulsion 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.
When correctly dried and exposed, Alkemi Plus emulsion stencils are very tough and will withstand a number of washout conditions.
For plus direct emulsions, wet both sides of stencil with cool water. Follow with a sharp washout using a strong spray of cool water - washing from the print side until image clears, then finish on the squeegee side. As a rule, Capillex stencils benefit from a ‘short-sharp' washout using a strong spray, predominantly from the print side of the screen.
If the water pressure is too low to achieve a strong spray, then a high pressure gun can be used, ideally set with a diffused spray and held at a minimum distance of 80cms (~31.5in) from the stencil.
As high pressure guns are available in many different powers it is important to test the jet on a non-printing area before processing a production screen.
If the screen is small enough to be handled easily, the ideal washout process is to turn the screen part way through washout so the unexposed emulsion is rinsed clean from the squeegee side as well as from the print side. The bulk of the washout should take place on the print side.
Although tempting, pre-soaking a small screen prior to washout may cause problems due to the softening effects that soaking can have, especially on thicker stencils that have been under exposed.