Understanding Your [Liquid] Emulsion Options

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Diazo Emulsion Systems

Diazo emulsions actually replaced bichromate emulsion systems years ago. The advantages diazo has over bichromates:

1.) It is much safer to use because it is Mercury free (heavy metal)

2.) Has a much longer shelf life. Bichromate emulsions have a shelf life of up to 8 hours.
Which means it must be mixed every day. When diazo emulsions were introduced, shelf life was increased up to 6 weeks. This made it much easier to have active emulsion ready to coat. Screens could now be stored for weeks, or longer, before exposing.

Diazo emulsion is a proven stencil choice. It will image most everything thrown at it; it’s tough against conventional solvent based inks, as well as abrasion resistant and it reclaims easily.

But diazo emulsions do have their problems. One of the big complaints is that it stains the mesh. This is very true, however, a properly exposed screen will lessen the staining. Also, reclaiming soon after the screen has been used will also reduce staining. Another problem is that it has limitations on image quality such as fuzzy edges and fill-in. Again, true. But, in most printing applications it will perform adequately.

The main components, without giving away the formula, in diazo based emulsion are; poly vinyl alcohol, poly vinyl acetate. The diazo is what is added to the emulsion by the customer.

Dual Cure Systems

Basically, the name says it all; it is a system that uses two sensitizers. Typically, the second sensitizer is a diazo added by the customer. It has distinct advantages over a straight diazo emulsion. Such as...

1.) Higher solids content, but not always. Typical dual cure emulsions will be in the range of 30 to 45% solids.

2.) Wide exposure latitude. These emulsion systems are very forgiving on establishing an acceptable stencil.

3.) High resolution. It is possible to achieve 50 micron lines. (A human hair is about 75 microns).

4.) Wide variety of solvent resistance.

By using two different sensitizers in one emulsion, more bonds are formed to give better stencil strength and imaging. The emulsion without the diazo added is ultra violet reactive by itself, but the diazo must be added in order to create a stencil. Dual cure emulsions can, but not always, be used with water based and solvent based inks.

Ever wonder what is in a dual cure emulsion? They have polyvinyl alcohol, polyvinyl acetate but they also have expensive components like urethane acrylates, photo initiators, and diazo. These components will yield quicker build-up, faster exposures, and higher resolutions than a straight diazo emulsion system. They are also more difficult to manufacture and in turn cost more to buy. Active solids are pricey.

Pure Photopolymer Emulsion Systems

This emulsion is also called “one pot”. There is no need to add a diazo sensitizer. All you have to do is pop the lid and coat the screens. You don’t have to worry about the shelf life like you do with diazo. Typically, it will be possible to make stencils up to two years from the date of manufacture. Exposure times are much shorter than the other emulsions. Many times exposure can be counted in seconds versus minutes. You have to be careful of the speed. Because it exposes so fast also means that the exposure latitude (exposure forgiveness) is much shorter than a diazo sensitized emulsion. It is very important to zero in on the correct exposure.

Pure photopolymer emulsions can be coated to a very thick E.O.M. (emulsion over mesh) easily because the solids content can be up to 50% solids. This means fewer coats to achieve thick stencils. Another added benefit to this fast exposing emulsion is that there will be fewer pinholes as compared to other systems that use diazo.

A typical pure photopolymer emulsion system contains photoactive polyvinyl alcohol, polyvinyl acetate, urethane acrylate monomers and oligomers and photo initiators.

There you go three types of emulsions...but which one should you use?

Let’s compare the pros and cons:


  • Inexpensive
  • Very good resolution
  • Excellent durability
  • Lower solids
  • Shelf life: 4 to 6 weeks

Dual Cure

  • Moderately priced
  • Excellent resolution
  • Very good durability
  • Moderate solids content
  • Shelf life: 4 to 6 weeks

Pure Photopolymer

  • High priced
  • Very good resolution
  • High solids
  • Good durability
  • Shelf life: 1 to 2 years

The things to think about that should dictate what emulsion system to use is “What are you trying to print?” “What is the resolution that is needed?” and the most important question to be asked is “What type of ink is to be used?” All three systems have a product that will work for your application. When in doubt, ask your distributor.

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