Coating a screen with emulsion is actually fairly simple, but choosing the right emulsion to use is not as easy. The type and amount of EOM (Emulsion Over Mesh) dictates the exposure time and amount of detail that can be held within a design. There are many different types of emulsions that every screen print chemical manufacturer offers and it is important to understand when each of them are used. There are essentially two primary types of emulsion, Photo Polymer & Dual Cure, with each available in their own different types. The big difference between “Photopolymer” and “Dual Cure” emulsion is that the latter must be mixed with a chemical called diazo.
Photopolymer emulsion is used straight out of the bucket and typically has a longer shelf life than dual cure emulsions. Because Dual Cure emulsion is mixed with Diazo it can hold more detail within a design and the exposure time has more latitude, which makes burning a screen much easier. Higher mesh screens (160 and higher) typically work better when coated with dual cure emulsion as a screen with a high mesh count can’t print detail without emulsion that allows it. Dual cure emulsion takes around twice the amount of time to burn a design onto a screen compared to photopolymer emulsion.
Outside of just looking at emulsion in terms of “PhotoPolymer” and “Dual Cure” it is important to realize that both come in different versions to be used with different types of inks. To keep it simple, it is important to understand that water breaks down most emulsion and plastisol ink does not contain water so the ink does not break down most emulsions. However, specialty inks like discharge and waterbase often require a special type of emulsion so the ink does not break down the emulsion as it is being printed on the press.
Coating screens with emulsion is not the only way a screen can be prepared to be burned with a design and then printed. “Capillary Films” are a substitute for using emulsion out of a bucket, and are essentially a sheet of emulsion that can be laid onto a screen and applied using a small amount of water and a squeegee and then must be dried like emulsion coated screens. Capillary films allow for perfect consistent amount of emulsion across the entire screen and thus the same ink deposit.