Why Emulsion needs to dry completely before exposure
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So many things can trip you up when screen printing and one of the most common mistakes that people make, is not leaving coated screens to dry for long enough.
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I always emphasise how important it is to make sure that your coated screens are completely dry before exposing, so I've decided to make a separate blog just to explain what actually happens when a coated screen hasn't been dried for long enough.​
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Prior to exposure the diazo sensitiser (seen below as the monkey) is totally separate from the polymer chains (green dots).
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During exposure the diazo sensitizer forms cross links between the reactive sites on the polymer chain which gives the emulsion its strength.
If the screen is exposed while there is still moisture (ie the emulsion has not been left long enough to completely dry) 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 most of the diazo will have reacted with water. The result is a softer stencil which will break down. If the screen is not dried thoroughly before it is exposed, then it will not be as tough as it should be.
This is why it's so important to leave your screens for at least 24 hours to dry (in humid or cold conditions leave them for even longer).
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Also remember - as already mentioned in my Photographic screen exposure blog, screens should be dried flat in a dark, warm, clean and dry environment.
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Signs of the emulsion not being dried for long enough
If you find that after exposure, while you wash out your exposed screen, that the emulsion is soft, rubbery or pulls off the screen, then this is a sign of the emulsion not having dried long enough before exposure. If you rub your screen with a cloth and the emulsion starts to come off the screen, it's kind of like washing a print that hasn't been cured properly.