What is a Screen Coater?
Coating a screen with emulsion is not terribly hard, but also easy to mess up. Coating a screen revolves around pouring emulsion into a “screen coater” that is essentially an aluminum trough. The screen should be in some type of screen holder so that both hands of the person coating the screen can be holding the coater. Coating a screen with one hand is possible, but is much harder to get as good of results when using two hands. Screen coaters come in a range of widths for different types of screens. The closer the width of the coater is to the width of the screen, the less screen tape has to be used. 17 Inch wide coaters are typically used with 20×31 screens whereas a 19 inch wide coater is idea for a 23×31.
How to Coat a Screen
Every screen coater has two sides with one side having a sharper edge while the other has a round edge. The round edge of a screen coater will lay down around twice as much emulsion as the sharp edge. The amount of emulsion that is necessary on the screen comes down to the color and type of ink being used, mesh count, and other variables. A screen is coated by first dragging the coater vertically up the print side (flat) of the screen. The screen must then be flipped around so the squeegee side is facing the person coating the screen, and the screen should be flipped 180 degrees. By flipping the screen 180 degrees the emulsion is coated the opposite direction as it was on the print side. Coating the inside part of a screen is harder than the outside as you have to deal with the frame getting in your way at the top and bottom of the screen.
Once a screen has been coated it must dry, preferably in a screen drying cabinet with heat. A screen dryer will drastically reduce the dry time, and also prevent potential problems with the emulsion not drying evenly. Screens should always be stored in a “bookshelf” fashion after they have been coated. Bookshelf refers to the print side facing down and the squeegee side facing up. This is very important because during the drying process gravity pulls on the emulsion, which we want to be on the print side of the screen. The print side of the screen needs to be where the ink is sits as it will determine the total ink deposit for the screen.
Understanding Ink Deposit
The amount of emulsion that is coated onto a screen will determine the “ink deposit” when the screen is on the press and being used. The higher the ink deposit the more ink is laid down onto the product being printed. To increase the ink deposit on the screen it is typically coated using the round side and sometimes multiple times on one side. Puff ink for example sometimes requires a very thick ink deposit so a screen printer might do what is called a 2-1 coat in which the print side is coated twice before being flipped around to coat the squeegee side. Multiple coats will allow for a thicker amount of emulsion and thus a thicker ink deposit. Keep in mind that every time a variable is changed, like the side of the coater used to coat the screen or amount of coats, the exposure time changes.