Ask yourself. How many times have I had an emulsion stencil breakdown prematurely during printing? If you can identify one or more occasions when this has occured, then this 'How to Screen Print' guide on 'Correct Drying of Emulsions' is what you've been looking for.
Is your screen printing emulsion dry?
Drying of screen printing emulsions is more critical than people realise, especially with dual cure and single cure diazo emulsions. The following guide takes you through the process of drying the emulsion and how incorrect drying can effect the durability of your stencil.
Screen Printing 101 Hints & Tips: Correct Drying Of Emulsions
After you have degreased and coated your screen you must then dry it horizontally with the print side down.
All drying cabinets for direct stencil systems should have a maximum temperature of 40°C. Ideal conditions are 30-35°C with low relative humidity. The reason why drying is so important is very simple. In any direct stencil system, prior to exposure the diazo sensitiser (as represented by the Octopus in the picture below) is totally separate from the polymer chains.
During exposure, however, the diazo sensitiser becomes excited and it forms cross links between the reactive sites on the polymer chain. This is depicted by the legs of the Octopus which are now joined together to form a matrix which gives the emulsion its strength.
If the screen is exposed while there is still residual moisture in the system, the diazo will react preferentially with the water molecules rather than with polymer. This means that even if you have given the screen full exposure (that is you have used up all the diazo sensitiser) a large proportion of the diazo will have reacted with water.
The result is a softer stencil which will break down. This is why in the summer months when the humidity increases in certain areas of the world, screen printers experience more screens breaking down during printing. If the screen is not dried thoroughly before it is exposed, then it will not be as tough as it should be.
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Name: William Shorter
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