Finding the right emulsion can be tricky, but is also vital to the screen printing process. Emulsion is used to create your stencil, meaning without it, you have nothing to screen print. It is photosensitive no matter what form it is purchased in, the options being direct emulsion (liquid) or films, so be sure to handle in yellow safelight conditions. Deciding which type is best for you comes down to understanding the subcategories along with the presented options.
What type of liquid emulsion is out there?
A liquid emulsion is a direct emulsion applied to your screen with a scoop coater. You have three choices in liquid emulsions, and all three print plastisol ink. There are pure photopolymer emulsions, diazo emulsions, and dual-cure emulsions. Diazo and Dual-Cure emulsions are both two part emulsions: they require the screen printer to mix the diazo sensitizer into the emulsion base. Pure Photopolymer emulsions are the only one part liquid emulsion.
Most screen printers will recommend a pure photopolymer direct emulsion for quite a few reasons. To use this type of emulsion, no mixing is required and it can be used straight out of the container. In regards to the screen printing process, this emulsion does the work for you while staying consistent and lasting longer, meaning that you have to purchase it less as well. However, the one thing you have to watch out for with this emulsion is how fast it exposes. While it is great in terms of getting the job done, this short range of exposure latitude means if you don't have your correct exposure times down you can very easily overexpose your screen and be left without a usable stencil. With that in mind, please note that pure photopolymers tend to be able to create a thicker stencil (though any can create a thick stencil if coated enough times). When it comes to your work environment, this emulsion is one that is least affected by its surroundings, giving it a shelf life of anywhere from one to two years. It is the most light sensitive of the three (making it faster acting), but also the most expensive.
Ulano QTX Pure Photopolymer Direct Emulsion.
If you're interested in the cheapest version, you will want to go for a diazo emulsion. However, you need to be aware that this type of liquid emulsion is the most affected by its environment and takes the longest to burn. You must have a very strong light source to prevent underexposure and be extra careful of where it is stored. Even with its downsides, it is still recommended to beginning screen printers because of its price and the educational use of two part emulsions: it changes color as you mix the two parts to create the emulsion to affirm that the sensitizer has reacted completely. It also can be very forgiving during exposing times as even if it takes you longer depending on the humidity, you may still have a good result. Unfortunately, even with this exposure forgiveness, you often risk the possibility of underexposing your screen, which can make other parts of the process more difficult. You have the option to create a solvent or water resistant emulsion with diazo, but you can only choose one or the other.
The last option is certainly a unique one. Dual-Cure emulsion's purpose is to combine the benefits of both a photopolymer and a diazo emulsion. The effect is created by adding diazo to a photopolymer base. Much like the diazo, the contents must be stirred together to create the emulsion, meaning it also has the value of color change. Dual-cure is forgiving like the diazo as well, but leaves less opportunity to underexpose. The main reason this one is not normally chosen over the photopolymer is the mixing process and the fact that even with the better properties of the diazo, it still is not as long lasting as the photopolymer with having a shelf life of only six to eight weeks. It will also break down if the run gets to be too long, but the run would have to be in the thousands for this to affect you. However, it is a good option in regards to working with both solvent and water based inks.
Note on Burning: It is important to realize the burning times differ between these three emulsions. For example, in a decent environment with a good light source, if a photopolymer is burning a basic design at 1 minute, then a dual-cure will burn the same design at 2 minutes and a diazo at 5 minutes.
What are emulsion films?
Capillary film is emulsion (any of the three represented above) dried onto a sheet by the manufacturers and then sold by quantity and thickness. By already being set, you're guaranteed a fast, smooth, even coat. Using film allows for more versatility as you can screen print on coarser materials such as twills, piques, and corduroy and depending on the level of thickness you are using to screen print, it is argued to be possible to print white ink on black shirts without flashing, more than doubling your production. One downside to capillary film is it’s only really as good as the emulsion dried on it, but the main issue is the cost. If you're screen printing smaller images, using an entire sheet seems wasteful, but also unavoidable. In regards to buying the film in quantities, it can also be argued as a waste if you're doing a short run even with large images. To avoid other printing difficulties, it is important to note that there have been issues with applying the film to mesh. In order to reduce this problem, you need to make sure to wet down the screen before application with either water, or for better application consistency, a wetting agent. A way to avoid this is an indirect-direct method, but it requires you to re-coat thicker film with liquid emulsion, upping your cost that much more. However, Texsource does offer Chromaline's Direct/Indirect Stencil System which eliminates some of the cost of this process.
In the end, which is better?
Arguably, the photopolymer direct emulsion and the capillary film are on the same level, especially when the film has dried photopolymer on it. The difference being that when exposed properly, the pure photopolymer direct emulsion will last a long time and its shelf life is longer as well. When that is considered, along with the overall cost, the smarter move may be to go with the photopolymer. In regards to finding a financially easier route, a dual-cure would be your next option eliminating both the film and photopolymer for their cost. It is a sturdy emulsion yet allows for errors while still having a decent shelf life. A diazo emulsion would be a last resort for some, or a first for a startup company without the funds for a better functioning emulsion. It has a very low shelf life (4-6 weeks) which can get lower depending on the storing environment.
With this information at hand, you always have the option to contact your local sales representative to figure out the best emulsion for you. Texsource offers many different types and colors, so it will always come down to your individual shop and preference.