3. Preparing Plus Emulsion & Manual Coating
Alkemi 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 is an important step in stencil processing. There are two main points to consider: the temperature and the humidity of the dryer.
Alkemi 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. As the drying temperatures increase, the effect becomes more severe.
One indicator of drying temperatures that are too high will be 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 emulsion / stencil failure. The reason why drying is so important to the stencil making process is very simple. In any direct system (Capillex capillary films or Plus emulsions) the diazo sensitiser is totally separate from the polymer chains in the emulsion before it is exposed.
During exposure, the diazo sensitiser forms links across the polymer chains in the emulsion. 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 resulting in a weaker stencil.
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.
Screens coated with Alkemi Plus emulsion 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 (Emulsion on Mesh or 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:
Be light tight
Have thermostatically controlled temperature, accurate to within + 1°C
Have good air movement
Filter the air
Exhaust the wet air and replace it with dry air
Allow the screens to be dried horizontally with the squeegee side facing up
When drying screens in a humid environment (climate conditions, sealed chamber or many screens drying at once) installing an inexpensive de-humidifier can dramatically improve the drying. Costing less than £200/€200/$300 these units can speed up production and prevent costly screens break down on the press.
Why is drying so important?
Some of the classic symptoms of insufficient drying are: fine detail may wash off during stencil preparation, there may be excessive pinholes, it may become sticky during printing, or it will breakdown on the press.
Unfortunately, one of the common side effects of breakdown on the press is that replacement stencils will be needed in a hurry. If under-drying was not a problem before, it certainly will be once the stencil making process starts to be rushed.
But what about temperature?
The only problem with increasing the temperature to improve drying is that above 35C the polymers in the emulsion can start to fuse which can make the stencil harder to washout and reduce the stencil resolution. Even higher temperatures can cause the Diazo to react and the emulsion will become completely insoluble.
As stated in the Pro Tip, one simple, low cost way to lower the humidity in a drying cabinet or drying room is to install a dehumidifier which can dramatically speed up emulsion or capillex capillary film drying.
In summary, insufficient screen drying is in the top three most common reasons for stencil failure, next to mesh preparation and under-exposure, and yet it is probably the least recognized. Drying a screen properly is straightforward, and will save you significant money in the long term by reducing the number of on-press breakdowns.