How To Reduce Pinholes
Cleanliness/Quality Control
One of the main causes of pinholes is specks of dirt, lint fibers, adhesive residue from tape, or any other kind of contamination that can get stuck on glass surfaces in the exposure frame, or to the artwork, or even the coated screen itself. Airborne contamination needs to be avoided during coating, drying and storage. This is important particularly while the emulsion surface is still wet.
Maintaining good housekeeping procedures should prevent any build-up of debris on the floors, drying racks, etc., which could be disturbed and then land on coated screens. Exposure frame glass should be kept spotless and positives should be cleaned and inspected before use if necessary. Artwork which is badly scratched will cause pinholes, and should therefore be re-made. Likewise, deep scratches on the glass inside the vacuum frame will cause pinholes in your screens. If the glass is badly scratched, it should be replaced, or at least reversed so that the scratches do not cast a sharp shadow onto the emulsion during exposure.
Emulsion Handling
Bubbles in the emulsion that transfer onto the mesh during coating are a major cause of pinholes. Diazo and dual-cure emulsions need to be stirred during sensitizing with a broad flat paddle until a smooth and even consistency is obtained. If you beat the emulsion with a stick, then it will take ten times longer to enable the trapped air bubbles to escape. The smaller the bubble, the longer it takes to rise to the top and pop. If the emulsion does contain fine air bubbles, then they may not cause a problem when coating fine mesh, since the mesh openings are too small to hold a bubble, but they will cause a problem with your lower mesh counts.
Even if you start with an emulsion free of bubbles, you can still expect to run into pinhole problems with your coarse mesh counts if you coat too fast. The turbulence generated in the emulsion as the scoop coater rides over the large woven knuckles of lower mesh counts is a great way to make foam. Scoop coaters with a sharp edge increase the likelihood of this occurrence due to the higher rate of shear on the emulsion.
Note: when coating a large number of coarse mesh screens, pour off the emulsion in the scoop coater (and let it recover) as soon as it starts to retain a lot of bubbles. Start coating again with fresh emulsion and you will have a lot less pinholes. (Refer to "What Are Optimum Drying Conditions" tech tip.)
Exposure
Optimum exposure, or rather avoidance of gross under exposure, is the third area that requires attention. The first thing to avoid is batch exposing screens of very different mesh counts. For example, a direct emulsion stencil on a 61 mesh needs twice the exposure compared to 110, and roughly three times when compared to 158 mesh. If you do batch expose, then only combine screens with a narrow range of mesh counts, and don't mix white mesh with dyed mesh screens. With a dyed fabric, even if the mesh count is the same, it still needs an approximately 50% longer exposure time to enable the emulsion to harden properly.
Also, avoid coating screens which have lost tension. The thick patch of emulsion that builds up in the middle of the screen is not only difficult to dry properly, it will also not expose correctly and will cause premature breakdown on press.
Finally, do some exposure tests to check how near or far you are from optimum exposure. You may use an exposure calculator or digital radiometer. (Refer to "Determining Optimum Exposure" tech tip.)