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Emulsions and Choosing the Best Product


In screen printing, once we have the proper screen at the proper tension, our next critical step is the emulsion we use to coat the screen. This emulsion will be a light sensitive liquid that reacts with ultraviolet (UV) light. There are a few options (and many opinions) available to you.

Diazo
Diazo sensitized emulsion is less common today, but is still available. You will hear this term within the industry, so it’s good to be familiar. The primary drawback to a diazo emulsion is the fact this product contains more water than later emulsions. This high water content can result in shrinkage of the emulsion film on the screen during the drying process. This shrinkage may cause pinhole issues during production.

Pure Photopolymer
Also know as fast-burning or premixed emulsion, pure photopolymer is called “one pot” since no mixing is required. This product comes premixed and ready to immediately go onto your screen. Added to the ease of use, this emulsion exposes much more quickly than traditional two-part emulsion systems.

For most of us, especially new printers, the speed of exposure has little impact on our productivity. The procedure for exposing screens usually involves our washing out and drying one screen while another is being exposed. In this scenario, super fast exposure times measured in seconds won’t matter, since it takes us much longer to wash and dry our already exposed screens.

In addition, this emulsion is much more expensive than two-part emulsions and more importantly for us, much less forgiving during the exposure process. This simply means we can easily – too easily – overexpose a pure photopolymer screen, causing difficulty during washout. Two-part emulsions give us more latitude… more room for error. And new printers can always use a little more room for error.

It seems that more and more screen printers will advise new printers to go the pure photopolymer route because of the fast exposure times. But most of us will find little real benefit from this quick exposure. This advice apparently has been picked up from suppliers and sales reps at trade shows.

Dual Cure
Dual cure emulsion is actually a hybrid product. It is a diazo-sensitized pure photopolymer. Compared with diazo emulsion, this product has less water so the problem of emulsion film shrinkage is eliminated. This combination product is extremely durable through repeated prints and can be used for short run water based printing as well as traditional plastisol.

This product comes in quarts, gallons and beyond. Each container comes with a bottle of powdered sensitizer, enough for the quantity of emulsion you have ordered. Warm water is added to the bottle of sensitizer and shaken. Try not to get the sensitizer liquid on your hands as it will leave a brown/yellow stain on your skin.

Once the sensitizer and water is mixed completed, pour the contents into the glue base. Mix thoroughly with a wooden spoon or paint stir. The sensitizer will cause the emulsion base to change color so you can tell if you have mixed the product well. After mixing, allow the mixture to set for 2-3 hours to allow bubbles to dissipate. This product is thick, so it takes time for all the bubbles from mixing to make it to the top.
“If I’m really in a bind, can I use this product immediately after mixing?” Yes, but bubbles may cause thin areas of emulsion on your screen. Under most circumstances this will not be a problem, but with long runs you may begin to see pinholes opening up. So allow the 2-3 hours whenever possible. Storing your mixed emulsion in the refrigerator will extend the life of the product.

Emulsion is less expensive if you buy larger containers. But don’t buy more than you need and try to mix part now, and the rest later. Measuring out portions of the sensitizer, water and the base product is very difficult and may cause more trouble than the money you saved. Order the size containers that you need.

You will notice that some emulsions are blue, and some are brown, and some are pink, and so on. These different colors are only a function of the different brands of product. Emulsions are dyed these different colors so that it is easier after exposure and during washout for you to see what portions have been washed out properly on your screen and what portions have not.

Terry Combs is a 30+ year veteran of the screen printing industry. He is an industry teacher and consultant through the website TerryCombs.com, offering hands on and online classes. And, he is the owner of the screen printing supply company, GarmentDecoratingSupply.com.​
 

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Thanks Terry for your input on emulsions. A few other notes:

For high production shops with multiple automatics, an SBQ fast exposing emulsion is often the difference in having jobs ready for press, or not. Shops that shoot 50-200 screens a day often have their exposure units constantly filled with screens. Some SBQ emulsions shoot much better than others, with little loss of details even when exposed +/- 10%, maintain image quality and resist difficult inks like discharge without breaking down.

Another SBQ difference: shelf life. SBQ pure photopolymers can last 1 year or more without refrigeration and can withstand temps to 100 degrees without any 'dark hardening' of the SBQ unlike the diazo found in dual cures and diazo emulsion. So SBQ's shelf life is a good thing for smaller printers using only a gallon every three or four months.

Refrigeration: Ok it does extend a diazo emulsion's shelf life, but at a cost to product quality. Refrigeration also increases viscosity. When used right out of the refrigerator the emulsion coating can be too thick and affect exposures, just warm it up before coating, but one more thing: As the bucket becomes less than full the air inside condenses when put back in a cold refrigerator and this water can run into the emulsion, especially as it gets 1/4 or less full, and weaken the emulsion strength and resolution. Storing any emulsion in a refregerator is not recommended. Keeping diazo emulsion stored below 80 is recommended to prevent dark hardening.

Dual Cures - like you point out they combine some of the best qualities of both. Ease of use, good details are all hallmarks of these emulsions. But when it comes to certain ink types like discharge and water base inks they may not have the durability performance of an emulsion designed specifically for these inks.

Ink type is one other major factor in emulsions and the emulsion purchased should be chosen based on the solvent or water resistancy and the type of printing done. An emulsion is either water resitant or solvent resistant, or somewhere in between. An emulsion that can withstand the ink system used by the shop should be the starting point. If you can't print the job cost doesn't matter. A stopped automatic press costs the printer 50 prints in five minutes to fix a poorly performing emulsion. So that ten dollar savings in a cheaper emulsion can cost dearly at the end of the day if the press is down for any significant length of time.

Alan Buffington
 
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