We are often asked for guidance with screen printing and the most common request is for help with exposing screens and the problems encountered. Benjamin Franklin is quoted as saying “By failing to prepare, you are preparing to fail” – I don’t know how good he was at screen printing but it sounds like he had some experience! One of the most common problems is, when using a new light source, to assume the exposure times will be the same as the guidance/previous lamps. A successful Photo Emulsion Stencil will depend on many things: the wattage of the lamp, the timings, the distance from the screen and the thickness of the emulsion.
To start with you must ensure that your screen is clean and dry, any grease (even from fingers) can cause problems with the emulsion adhering correctly. The best products we have found for cleaning screens prior to coating are Screen Cleaner or Speedball Speed Clean. Either product should remove any trace of grease, make sure you wash the cleaner thoroughly off the screens and then leave the screen to dry fully.
The next step is to coat your screen. I would advise coating the screens in a room with subdued lighting – no need for a safe light but subdued light does allow you a little more time to coat your screen. If you are using new emulsion, it is best to mix it an hour or so in advance so that all the bubbles can disperse. You can watch a video showing how to mix up your emulsion here. When coating your screen the best tool to use is a coating trough, failing that, I have often used an old credit card. The aim of coating screens is to fill all of the holes in the weave of the mesh, not to deposit a couple of mm on top of the mesh. Make sure your emulsion coat is even and that you coat and remove excess emulsion from both the inside and outside of the screen. It is also important to wipe off any blobs around the edge. Dry the screen horizontally, printing side up somewhere dark. It is good practice to put some wedges (blocks) under the frame to allow the air to circulate. It is possible to speed this process up by using a fan (on cold never warm) but I have experienced more problems when drying this way.
When the screen is completely dry you are ready to expose your screen. The next crucial part is the positive. This should be printed on acetate and be very dark so that light cannot pass through the dark areas, you can test for this by placing the positive on a lightbox or holding up to a window. We have packs of Inkjet Screen Film available or can print Film Positives for you.
Some exposure units have lights below, others from above. Whichever unit you have, you must ensure tight contact between the positive and the screen so that travelling light cannot spoil your screen. If your exposure unit is a home build that has a light from above, use a piece of black foam rubber under the screen with the printing side of your screen facing up. Place your positive on top of the screen and then secure and emulate a vacuum by placing a sheet of glass on top of the acetate.
As previously discussed, the timings will depend on your lamp and the distance between the screen and the lamp. As a basic rule, the lamp should be at least the distance of the longest side of your screen.
The best way to work out timings is to do a graduated exposure (a bit like black and white photography). Use a piece of opaque black card and place it over your screen allowing about 1/5 of the screen to be exposed. I would recommend using a 250w bulb as a minimum. If you are using an A4 screen I would start with an 8 minute exposure then move the card to about 2/5 for a further 2 minutes (10 minutes), then move the card to 3/5 for a further 2 minutes (12 minutes), then move to 4/5 for a further 2 minutes (14 minutes) and then remove the card fully for a last 2 minutes (16 minutes). Turn off the light and look at your screen. Where the positive has been the emulsion should be a lime colour. The rest of the screen should be darker. Using a shower hose, pour some warm water over both sides of the screen and then with a more forceful jet (a pressure washer is good at this stage) blast your screen. You can also use a soft sponge to help to remove the unexposed emulsion. The emulsion should react differently depending on the length of time that it was exposed. Using this method should determine the optimum time. Where the emulsion washes out entirely (both the positive and negative image) it is underexposed, where the positive image is very hard to wash out it is overexposed.
Now you have your optimum time, you are prepared to expose your real screen.