How to choose the correct emulsion.
The question regarding which Photo Emulsion should I use is mystifying at times. The truth is that pricing should not necessarily be the only criteria that one should stick to. In recent years, One Part Emulsions have become the most popular way of making screens. They are made similarly by almost the same ingredients by different manufacturers. For water based and Discharge ink and for very long run, the best is Dual Cure Diazo with water resistance based emulsion.
Let us briefly examine what are Photosensitive Emulsions? Photo Emulsion is used to make a stencil for printing on a polyester mesh, put on a frame. There are five categories of photo Emulsion for screen printing industry, based on what type of ink will be used, the speed of exposure needed, edge definition, resolution and the longevity of the exposed Photo Emulsion.
There are few categories of emulsion:
- Diazo Emulsion (XENON – 903WR & 904RT)
- Dual Cure Emulsion (XENON- TEX, 902 QDC, QUANTA, RESOLVE)
- One part Photo polymer or SBQ emulsion
- Dual Cure One Part or Dual Cure SBQ Emulsion (XENON- PLASTOLFAST, NOVE, QXL, EDGE, QSX & QXL)
- ND technology, a new concept developed at Xenon Products, Inc. that combines all the above categories (XENON – 902 ND).
How does emulsion work?
Let us see what does photo emulsion do? And how does photo emulsion work? The simplest emulsion formulation was based on polyvinyl alcohol and reacting with Potassium Dichromate to sensitize it.
Now the Diazo based emulsion is in line with the same basic idea where you react Diazo with a hydroxyl group (Curing happens right there) that Polyvinyl Alcohol provides to the formulation through UV light and other light sources.
In the case of One Part emulsion, it is the SBQ monomer (Stilbazolium), attached to a hydroxyl group through grafting, that is being activated by the UV light. So when you expose the screen that has laser, inkjet or vellum film on top of it with the printed image (there will be black area and the image area, translucent portion of the film) on top of the dried emulsion, through the light source, the light would go through the open areas and the black ink on the film on rest of the screen would block the light; and thereby curing the emulsion wherever there is no black ink.
When you wash that screen the uncured emulsion on the image would wash out leaving the rest of the cured Emulsion intact- and you will have your image. Same happens with all the categories of Emulsion, except for the difference in the exposure time.
- Simple Diazo Emulsion is being produced from the early days. It is basically Polyvinyl Acetate, Polyvinyl Alcohol, and Plasticizer combination. Usually, they have longer exposure time. By changing the ratios, a product can be either water resistant or solvent resistant. One can tell by the solid content of emulsion, if Diazo Emulsion is solvent resistant, the solid contents should be around 27 to 34% In case of for water-resistant emulsions, and the solid contents should be within the range of 28 to 54%. The higher solid contents allow putting fewer coats of the emulsion on the screen.
- Dual Cure: When the monomers are added to the regular emulsion, you have your Dual Cure Emulsion. The term indicates that the emulsion is being cured in two separate ways; one is the Diazo reacting with the hydroxyl group and second is the polymerization of the monomer being activated by UV light- This happens simultaneously. Depending on the monomer, they provide faster speed of exposure, sharper edges, better resolution, enhanced water, and solvent resistance.
- One Part Emulsion: SBQ monomer replaced the use of Diazo. As screen makers understand that after adding the Diazo into emulsion, the pot life gets limited; four to six weeks, depending on the room temperature (may be extended by leaving the emulsion with Diazo in the refrigerator or cooler room)- It is recommended that one should use distilled water to mix the Diazo for a better pot life). Sometimes tap water also makes the Diazo into gel; as old pipes may have iron oxides or other impurities. The SBQ emulsion, not only offer faster exposure time, but also avoids the pot life issue. SBQ emulsion may be stored unopened for over one year. One of the issues with SBQ monomer is that it is water sensitive. So the use of One Part or SBQ emulsions with water-based ink may be achieved by adding Diazo to it, but you still do not get more than 1,500 prints at their best.
- Dual Cure SBQ Emulsion: Just like in Diazo Emulsion, once the monomers are added, it becomes Dual Cure One Part Emulsion. These types of an emulsion may be used for Plastisol, UV and solvent ink, depending on the formulation; but are not recommended for water-based inks, except for if Diazo was added to the SBQ emulsion. So the speed may be slowed down with Diazo, but they may be used and may yield 1,500 to 2,000 prints. We offer QXL, Edge, Plastofast, QSX (For faster exposure with a weaker light source, very solvent resistant) and Nova. The plain SBQ is not the best choice after most manufacturers have opted for Dual Cure One Part Emulsions.
- ND Technology: These are conventional emulsions, except that the curing agent is internally introduced into the product. We offer 902ND.
The exposure of emulsions is based on a few variables, such as the distance of light source to the fabric, the color of the mesh White or Dyed) and mesh count (30 to 420), humidity (55% relative humidity is idea- higher humidity requires longer exposure, from 1.2 to 1.5 times), thickness of dried emulsion coating. One should check with the supplier of the emulsion to get an optimum timing for exposure and the adjustment required fixing the problems and issues. If the emulsion is overexposed, the smaller line and image may not open. An underexposed emulsion may break down and reclaiming may get difficult.