Coating Screens in Your Shop

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How do I coat a screen with liquid emulsion?

Seems like it should be a fairly simple step one, step two, step three response. But ask any 10 printers how they coat their screens, and you might very well receive 10 different answers . Let’s take some of the mystery out of this process.

First, let’s assume we are using either dual cure or pure photopolymer emulsion. Our approach will be finding a method that is both efficient and effective. Some recommend a multiple coat method – one side, both sides, or coat-dry-coat. This multi-coat approach can be attributed to a couple of reasons. The screen printer (or the person who taught him) may have had experience with older diazo emulsion and struggled with shrinkage of the emulsion on the mesh causing pinholes. Or the printer has struggled with pinholes even using dual cure or pure photopolymer, and has fixed the problem by applying more emulsion as a type of band-aid approach.

Today, modern emulsions offer good coverage without the shrinkage experienced with earlier diazo emulsions. And if the issue is pinholes, proper degreasing and protecting the screen from contaminants is the real fix, not applying more emulsion to the problem. Extra emulsion means more material cost, longer coating and drying times, and more difficulty reclaiming the screens after production.

In this article, I’ll recommend you coat screens with just one pass on the print side and one pass on the ink side (watch the video). Just another opinion from a screen printer out there in the world? This method is based on years of production and being fast, efficient and effective. Coating one pass on each side of your screen should give you a quick and perfectly effective screen, and ready for a full production run.

1.) Pour your emulsion of choice into a scoop coater with at least 1” clearance from the inside of the screen frame on each side. (If the inside width of your screen is 18”, the scoop coater should be no more than 16” wide.) The sharp side of the squeegee will go against the screen mesh.

2.) Stand the screen up on a table and hold with your left hand (if you’re right handed). With your right hand (or vice versa) pick up the loaded scoop coater from the center, take care not to spill emulsion out of either end. The operator in the video is left handed, by the way, so don’t be confused.

3.) On the print side of the screen, place the scoop coater against the scoop coater toward the mesh until a bead of emulsion touches the fabric across the entire length of the scoop coater. Do not roll the scoop coater over so far that the flat plastic sides of the scoop coater rest against the mesh. Rolling too far forward may cause the scoop coater blade to pull away from mesh during coating and result in a heavy emulsion lay down or emulsion spilling out onto the mesh.

4.) Pull the scoop coater up the screen slowly, allowing the thousands of tiny openings in the mesh to be filled with emulsion. (Experienced printers who come to my classes always seem coat the screen as fast as they can pull the scoop coater up the fabric.)

5.) About 1” before the inside edge of the frame, stop, roll the scoop coater back to allow emulsion to pour back in, then slide the scoop coater up and then away from the mesh.

6.) Turn the screen around and repeat.

Any excess emulsion on the screen, on the sides of your coating stroke, can be carded off with a small square of cardboard or plastic. This will speed drying of the screen and prevent emulsion from dripping down onto another screen during the drying process.

Now, dry the screen in a horizontal position either in a professional or homemade drying rack with the print side down (just as the screen is placed in your printer). Print side down will give you a heavier deposit of emulsion on the print side of the screen so that you have a nice smooth surface touching the shirt being printed.

If you make a mistake in coating, it’s okay to clear any excess emulsion from the coated area with the sharp edge of the scoop coater. Screen coating is a very forgiving process, so your repaired coating job may not look perfect, but it will work perfectly well.

When you are finished coating your screens, pour the excess back into the emulsion container for reuse later. Wash out the scoop coater immediately with water and allow it to dry upside down on a towel. Coated screens will last about two weeks in a dark room or light-tight container.

Coating is one of the processes in screen printing that tends to frighten new participants. (I’ve witnessed many a shaking hand during this part of class.) Just remember that proper preparation and a simple inside and outside stroke will result in a quick, easy and perfectly functional screen, ready to load on your press.

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