Drying PLUS emulsions
PLUS emulsions require thorough drying after coating in order to give the strongest stencil. The way the screen is dried can also affect the finished result and therefore should be considered an important feature of stencil processing. There are two main points to consider (i) the temperature and (ii) the humidity of the dryer.
PLUS emulsions can be hardened by heat as well as being hardened by UV light. Temperatures higher than 40ºC (104ºF) can start to have a hardening effect and as the drying temperatures increase the effect becomes more severe.
The first sign of too high a temperature drying, will be seen as a slow, difficult washout and a loss of resolution. In severe cases the stencil will be completely insoluble. If your busy production schedule means that you have to dry at 45ºC (113ºF) or higher, you must make sure that the stencils are removed as soon as they are dry.
Insufficient drying is one of the biggest causes of stencil failure. The reason why drying is so important is very simple. In any direct system (Capillex or direct emulsions) the diazo sensitiser is totally separate from the polymer chains before it is exposed.
During exposure the diazo sensitiser forms links across the polymer chains. This is depicted by the legs of the octopus. The more links you can make, the harder the stencil will be, If the screen is exposed whilst it is still damp, the diazo will react with the water molecules rather than the polymer.
This means that even if you give the screen a full exposure (that is when all the diazo has reacted) a large portion of the diazo will have reacted with water. The result is a softer stencil which will break down very quickly on the press.
PLUS screens should be dried horizontally with the squeegee side up to allow the emulsion to settle on the print side of the screen. If the screen is dried vertically, there is a chance that the emulsion could run before it has started to dry which can cause 'curtaining' or a higher stencil thickness (EOM) towards the bottom of the frame.
To tell when the screen is dry, it should feel dry to the touch with a non sticky surface. However a more accurate way is to use a moisture meter that is accurate below 5%.
A good drying cabinet will have the following features: (i) be light tight, (ii) have thermostatically controlled temperature, accurate to within + 1°C, (iii) have good air movement, (iv) filter the air, (v) exhaust the wet air and top up with dry air and (vi) allow the screens to be dried horizontally with the squeegee side uppermost.