How to Coat Screens with Emulsion•
In screen printing, coating screens is the key way to improve print quality, minimize print mishaps, and separate yourself from the competition. A properly coated screen alleviates many of the issues that can arise in your shop, and with some practice is a very easy thing to do right.
What You Need
To properly coat screens, you will need a scoop coater, also known as a coating trough, and a screen with good tension. If your screen’s tension is too low, you will run into issues with areas of the screen having too much emulsion and other areas not having enough. You will also need a quality emulsion.
As a rule of thumb, your scoop coater should be at least four inches smaller than the shorter dimension of your screen - so a 20"x24" screen should be coated with a 16" scoop coater. Because the screen tension is much higher the closer it is to the frame, you can get uneven coating results if your coater is too large. Too small, and you will have to compensate for the lack of emulsion with extra tape to cover the rest of the screen area.
How to Coat
Fill your scoop coater with more than enough emulsion to coat the number of screens you plan on coating. If you don’t have enough emulsion, it won’t flow properly onto the screen and, again, you can run into issues.
The first coat goes on the garment/substrate side of the screen. Hold the screen vertically, with the bottom edge resting firmly on a stable surface. Start near the bottom of the substrate/garment side of the screen, and with slow and consistent speed and a fair amount of pressure, tip the round edge of the scoop coater so that the edges of the coater are touching the screen. Tip the screen so that the emulsion in the coater begins to flow onto the mesh, and draw the coater slowly and consistently upward. When you get to the top, lean the screen back so that the excess emulsion flows back into the coater. Flip the screen to the other side (the side the squeegee will be hitting) and coat the same way. If necessary, coat again on the squeegee side of the screen.
You will know you have coated your screen with enough emulsion when it appears glassy on the substrate side. If it looks kind of matte, put another coat of emulsion on the squeegee side. KIWO has a number of videos that show how best to coat a screen and what to look for. Regardless of how many coats you apply, just be sure to coat the same mesh count the same way each time.
To dry your screen, place it so it is substrate-side-down, allowing the emulsion to fall downward. This will ensure that you have enough of a gasket to provide good ink laydown when you print.
There are a lot of theories about the “best” way to coat a screen, but ultimately there are three core truths: (1) almost always use the round edge of the scoop coater, (2) always make your last coat be on the squeegee side of the screen, and (3) always dry it substrate-side down.
Pro Tip: For very coarse screens such as 25, 40, or 60 TPI mesh counts, the sharp edge of the coater might be appropriate for coating because the rounded side may deposit too much emulsion. The other uses for the sharp edge are to scrape off an “oops” when you coat or to put a face coat on the substrate side after the first emulsion coat has fully dried.
Why a Properly Coated Screen is Important
When exposed and washed out correctly, a properly coated screen is going to create a good “gasket” for your ink. A gasket is the space that is created by the emulsion between your substrate and the screen mesh, and is the space where your ink will be printing. If your gasket is too thin, you won’t get good coverage; if it is too thick, you will get a lot of ink spreading.
You will also see improvements in screen quality and longevity. A properly coated screen will have enough emulsion that it won’t break down from the repeated stress of the squeegee hitting and sliding over the screen. Oftentimes a properly coated and exposed screen can withstand 30,000 or more impressions.
Finally, you will see improved print quality. When your screen coating process in on point, the other dominoes will fall into place – your screen will be easier to properly expose, wash out, and reclaim.