Written by Master Screen Printer, Douglas Grigar
Screen printing emulsion is just one of many variables that confront screen printers each day. In the battle to keep the consumer wardrobe adequately stocked with t-shirts we explore the role of screen printing emulsion as it relates to getting ink onto a shirt in an efficient and professional way.
As part of the photostencil phase of the production cycle, screen printing emulsion is the light-sensitive liquid or capillary-direct films that you coat or adhere to the degreased screen fabric prior to taping your film positives in place. Screen printing emulsion comes in a variety of formulations, and your selection should be based on the features needed for the final application.
To decide which screen printing emulsion to choose for your job, you need to understand the properties of each type. Once this is clear, it is easy to determine what you need to choose based on the particular job you need to print.
Screen Printing Emulsion: The Science Behind It
Most screen printing emulsion contain two chemical polymers: polyvinyl acetate, a water-resistant polymer often referred to as PVAC; and polyvinyl alcohol (PVOH), a solvent-resistant polymer. Some screen printing emulsions have both solvent and water-resistant features and can offer a more universal approach to the screen printers’ needs. However, long print runs call for a screen printing emulsion formulated according to the ink manufacturers’ recommendation.
All emulsions shrink when dry (see Figure 1) and will follow the threads of the mesh.
This creates divots in the surface that can break and cause pinholes if the dried emulsion is too thin to bridge the mesh holes. Low-solids-content emulsions shrink excessively and are sometimes prone to this problem. Therefore, look for emulsions containing high solids content – more than 35% or 40%.
Look for screen printing emulsion containing high solids content – more than 35% or 40%.
Be aware that some lower-quality emulsions use inert filters to boost the solids content ratings. However, introducing these inert solids into an emulsion is akin to adding contamination to it. The additives also lengthen the screen’s exposure time and cause interference with the chemical linking that takes place during exposure.
The Big Three Screen Printing Emulsion Types
The three main screen printing emulsion types represent two basic chemical technologies:
- Light-reactive benzene diazonium (diazo)
- Styryl Basolium Quaternary salt (SBQ) based emulsions
1. Diazo Screen Printing Emulsion
Diazo-sensitized screen printing emulsion is mixed with raw polymers. When dried on the screen mesh, they make a photo-reactive product capable of reproducing a mid-range level of stencil definition. When exposed to ultra-violet light, diazo links with the polymers to make a solid and strong stencil block (see Figure 2). Diazo screen printing emulsion typically have the slowest exposure times.
2. Pure Photopolymer Emulsion
In the ‘80s, when pre-sensitized screen printing emulsion using SBQ chemicals entered the industrial market, a new screen printing technology was introduced. Often called pure photopolymers or single pot emulsions, SBQ-sensitized emulsions use photo-reactive molecules and are pre-bonded to the polymer solids that link to each other when exposed to UV light (see Figure 3).
Pure photopolymer screen printing emulsion is quick to expose. The bonds formed in SBQ emulsions are not quite as strong as the bonds made with diazo, but they are much faster to form. This high solids-content formulation creates good bridging capabilities.
3. Diazo Dual Cure Emulsion
The newest choice in emulsion is a hybrid mixture of diazo and SBQ-sensitized emulsions. Dual-cure emulsions are popular because many of the best features of both types of emulsions are available in these types of mixtures. While partially pre-sensitized, dual-care emulsions need to have the diazo component mixed before use, in the same way as diazo-only emulsions. As expected, the linking of polymers in dual-care formulations shares characteristics of both SBQ and diazo emulsions. They offer pre-bonded linkable molecules, and free-floating diazo photo-sensitive properties together in one product (see Figure 4).
Dual-care emulsions often have excellent bridging and detail resolution. They are slower to expose than a pure photopolymer, but are considerably faster than a diazo-only mixture.
Screen printers sometimes assume that capillary film is different form the other three emulsions (mentioned above). Actually, capillary film is any of the three choices of emulsions pre-sensitized, and dried onto a film base that is ready for application by the end user. Any of the three types of emulsions are available in capillary film form and continue to have the same exposure characteristics.
While more expensive, capillary film is an excellent choice for new shop owners, special applications, or jobs needing the most accurate thickness of emulsion. Special applications such as super-thick stencils almost demand a capillary film for its exacting thickness. Many printers find that the ease of application (no mess!) and the time saved using capillary film more than makes up for the higher-per-inch coverage cost.
The main difference between emulsion applied in liquid form and as capillary film is that liquid emulsions form a total encapsulation of the mesh threads. Capillary films, however, only draw into the mesh about half way up the thread diameter.
In the past, capillary films suffered from a bad reputation because they were assumed to have a short print life. But with proper preparation of the mesh and a good application procedure, they can easily make thousands of impressions without stencil breakdown.
To benefit from the best features of capillary film and the encapsulation-holding-strength of liquid emulsion, combine liquid emulsion (as a kind of adhesive) with capillary film. Attaching this combination to your mesh creates a combination stencil. It is the most expensive stencil option but holds the features of both types of emulsion products.
The best choice in emulsions is based on specific shop needs. To determine the best match, list the desired features of each emulsion for your print jobs.