Screen printing, or silk screening, is an age-old process to print on just about any material. With new machinery it is even possible to print on cylinders which, with traditional screen printing methods, would be impossible. The original silk used for a screen has been replaced today with a nylon material that has a very, very tight mesh. It does still have the feel and appearance of silk.
The screen material is stretched over a frame, often made of wood. It is a lot like a screen window that you would have in your house, although the screen mesh is much tighter, meaning the weave is much closer together. The screen is coated on both sides with a product called "emulsion". Emulsion is a photo sensitive material that will coat the nylon mesh. When it is exposed to light the emulsion hardens and bonds with the screen mesh. Nothing happens to the emulsion where light is blocked from reaching it and it is later washed away.
The process involves three basic steps. One is to prepare your artwork, image or text and get that made into a piece of film. This film is clear with the artwork or text being black. Second is to lay this film on top of your screen and expose it to a light source for a pre-determined amount of time. Third you will lay your screen onto the material to be printed on and drag ink over it with a squeegee.
Let's say we want to print the letters "ABC". We would start by creating a piece of film that would have the three letters on it, probably right in the center. The three letters would be black on this piece of film and the rest would be transparent. This piece of film is laid flat onto the screen and a piece of glass is laid over it to keep everything from moving. When we expose this to light the clear areas on the film will harden the emulsion and make it permanent on the screen. After the exposure the screen is washed with running water and the parts of the emulsion that were not exposed to light will wash away. In this scenario we will have our letters "ABC" on our screen where ink will pass through and print to whatever material is below it because the unexposed emulsion will wash away.
If we wanted to print each of the letters in a different color, let's say red, white and blue, we would need a separate screen for each of the three letters, but we would have to have each letter in the correct position so that they didn't print on top of each other or out of position. The easiest way to do that with our "ABC" scenario is to set up the three separate screens and then block the "B" and the "C" with paper or a tape and do the exposure of just the "A". We'll call this our red screen. We'll block the "A" and the "C" off our next screen and call that our white screen. Now we'll block the "A" and "B" and call this our blue screen. Now we have three separate screens, each in a separate frame and each with part of our "ABC" image. We could make three separate pieces of film as well, each one with just one letter, but that takes extra materials.
When printing this "ABC" image we will print the "A" screen, then the "B" screen and finally the "C" screen to come up with our three color image. Care must be taken to register your frame in the exact same spot each time so that all text and images on the screen show up in the correct spots on the finished product.
To print the screen is laid down on the material that you want to print. Everything must be held securely in place so the frame, nor the material moves at all. The color of ink you want to use is then applied to the screen above the image. If we are printing the "A" then that screen is placed over the material and red ink is placed onto the screen. We will then take a squeegee and pull the red ink down over the letter "A" and then back up again to go over it a second time. This will have printed the letter "A" in red ink onto whatever you are printing on, a t-shirt, poster or whatever. Repeat this process with the other two letters and ink colors to finish the product.
Screen printing is one of the best ways to print onto cloth and paper among many other things. Modern equipment allows you to mass produce products like t-shirts. A typical t-shirt printer will be set up with stations and several people will operate the machine at the same time, each printing one color. The Screen printing machine would then index, or revolve, to move the t-shirt into the next position and print that color.