Determining Stencil Moisture Content
Question
How can I tell when my screen is dry enough to expose?
Solution
Number one, you can't tell by looking at it. Number two, unless it is very damp, you can't tell by feeling it either. You may get some indication when you have trouble peeling the artwork back off the screen, or when the emulsion sticks to the inside of the glass in your exposure frame. But by then, it's too late.
Your best bet is a contact moisture meter. This inexpensive electronic device will tell you exactly how much percentage moisture is being retained in your coated and dried screens by measuring directly on the surface of the emulsion.
As you know, you can definitely eliminate a lot of costly stencil breakdown problems if you ensure that your coated screens are thoroughly dried prior to exposure. As far as emulsion goes, the drier the better, in order to obtain a tough stencil.
There are some guidelines you can follow when measuring the moisture content of your screens to make sure that during exposure most of the sensitizer is reacting with emulsion and not water. If the moisture content of the coated screen is less than 4%, you should always get a nice tough stencil, assuming you are using the correct exposure. Between 4% and 6%, you will notice a deterioration in wet stencil strength during washout, and you will also have more pinholes. This is most critical for emulsions that use a low diazo level in order to get a shorter exposure time. Above 6% moisture content, most emulsions will produce soft stencils regardless of exposure time, and you run a real risk of breakdown on press. In this case, depending on your run length, you may or may not have a problem when printing plastisol. However, you will have a problem if you try to print water-based inks. As coated screens become more damp, stencils will be progressively softer. Eventually you reach the stage where they suffer spray damage during washout, regardless of how long you expose your screens.
Once you are able to measure the moisture content of your screen, how should you proceed if it isn't dry enough? Typically, if the area where you dry your screens is affected by the weather to such a degree that it affects the way the emulsion dries, then simply leaving the screens to dry longer is not going to make any difference. The emulsion will have reached equilibrium with the conditions around it. At that point, all you can do is wait for the weather to change before your screens will dry out any more (unless you take steps to reduce the relative humidity in the area where your screens are drying).
Preferably you need a drying chamber in order to dry screens properly. At the least, you really need to have a separate, dedicated drying room. There is no point in putting a de-humidifier in the corner of a room and expecting to see a big improvement if doors (assuming you have them) are open constantly, and someone is using a pressure washer to de-haze screens in the next room.
The temperature in the drying chamber or dedicated drying room should be approximately 100°-110°F. Coated screens placed in this environment will dry faster, and will dry down to a much lower moisture content, as long as you use a fan to pull out the damp air and keep the relative humidity as low as possible.
Raising temperature is by far the most efficient way of reducing relative humidity and maximizing drying capacity. After all, how many hair dryers work on the principle of de-humidification?