Emulsion is determined primarily by the type of ink you use. Some emulsions are solvent-resistant or water-resistant, while others are both. Some water-based inks contain solvents in them, and these are referred to as co-solvent inks. Other factors that determine the best emulsion for your printing needs include:
Length of run/number of impressions: Some inks have very aggressive solvents in their makeup, and in some cases contain coarse particles within the inks such as glitter and shimmer inks. In this case, an emulsion with high solvent and abrasion resistance is required. Many shops use very aggressive press washes and screen washes, so an emulsion with very high solvent resistance is best.
Type of ink deposit: Some customers want a print that has a very thick 3D deposit, which is also called a “high-density” print. When this is called for, a direct emulsion that is capable of building a thick stencil on the underside of the screen with multiple wet-on-wet coatings is a good choice. There are also laminating emulsions available to bond thin stencil films of precise thicknesses to produce a thick stencil. There are also emulsions with higher solids content and higher viscosities that assist in building a thick stencil.
Screen exposure system: There are many exposure systems used by screen printers. These include:
- Fluorescent tubes
- Single-point mercury vapor/metal halide lamps
- LED systems
- CTS systems
When weaker or slower exposure systems are used, it helps to have an emulsion with a faster exposure speed like an SBQ or one-part emulsion, which are outlined in the types of emulsions below.
Level of detail required: Very fine lines and higher range four-color process printing (65 line and higher) need an emulsion capable of maintaining high resolution. Fast-exposing emulsions can be too fast and have very narrow exposure latitudes, so an emulsion with a wide exposure latitude is preferable.
There are four popular types of direct emulsion on the market today: diazo, diazo-photopolymer, SBQ, and SBQ-photopolymer.
Diazo emulsions: These emulsions consist of two parts-the base unsensitized emulsion and a diazo sensitizer (powder or syrup). The sensitizer needs to be dissolved with water and mixed with the emulsion under yellow safelight. Once mixed, this type of emulsion has a shelf life of four to six weeks, has an excellent image resolution, and requires longer exposure times in minutes. These emulsions can either be water-resistant or solvent-resistant.
Diazo dual-cure emulsions: These emulsions by name employ two sensitizers. The emulsion itself is a UV curable resin, and it is mixed with a bottle of diazo sensitizer to improve ink resistance. These emulsions can exhibit both solvent- and water-resistant properties. Many new water-based inks are co-solvent types of ink, as are some new high-solids acrylic inks. These emulsions, while slower than SBQ single- and dual-cure type emulsions, are much faster than Diazo emulsions and exhibit a very wide exposure latitude, as well as a high level of image resolution. Once sensitized, the shelf life is four to six weeks.
SBQ emulsions: SBQ emulsions come in single- or dual-cure versions. In this definition this is the single-cure pre-sensitized SBQ emulsion, nothing else is added and it’s used straight from the bucket. Single-cure SBQ emulsions typically exhibit very fast exposures, only taking seconds to expose a screen versus minutes. Screens can overexpose if the exposure time is too long. There are newer SBQs that are designed to expand the exposure latitude by a bit more. In the pot or usable, the shelf life is one to two years.
SBQ dual-cure emulsions: These emulsions use the SBQ emulsion along with a diazo sensitizer added to the emulsion. These emulsions have better resolution, wider exposure latitude, better image resolution, and are more solvent resistant than single-part SBQ emulsions. Once sensitized, the shelf life is four to six weeks.
In all the emulsions described here, they are used under yellow safelight while coating the screen and drying.
There are many variations of the types listed above. Your emulsion supplier can assist you in choosing what emulsion is best suited for your shop.