I cannot guess how many times this question, or variations silkscreen imaging questions have been asked. We always send the readers in the direction of "Spot Colors" in Photoshop primarily because CMYK (process color) is not appropriate for flat-color printing and requires a huge investment in equipment.
But indulge me for just a moment with this brief explanation of screen printing (serigraphy) because some of our readers today may not be familiar with this printing process. If you are familiar with the process, jump on over to the separations tutorial
serigraphy: It is not known for sure how or when serigraphy or screen printing was first invented, but art historians like Anthony F. Janson trace this art form as far back as 4,000 years in China, and thereafter throughout Asia and Japan. Silk fabric, being a primary product in those cultures, had the unique qualities of being able to be stretched very tightly, and of being somewhat resistant to staining. Someone discovered that they could stretch single-thread silk fabric very tightly across a frame and inks would flow freely through the fabric onto a substrate like other fabric or paper. They established techniques to block the flow of ink in certain areas of the silk, while allowing the ink to freely flow through open areas. Thus they could block the screens with ornate designs and print those designs repeatedly. The art of serigraphy was born. (Here, you can learn more about serigraphy)
Today's Flat Color Screenprinting:
At right, I've drawn a very stylized diagram of how very basic screen printing works. This is a simple two-color print we'll be using in our tutorial today.
ONE: At the top of the stack we see the Film. This is a film positive, which is clear with the exception of the image to be printed which is opaque. (black) There are other methods, like painting directly onto the silk with a "hold-out" solution, but for this tutorial we're using a film, or vellum printed on a computer.
TWO: Next, a photo-sensitive material called "photo mask" or "photo stencil", which behaves much like photo film, is exposed to strong light using the film positive as a mask. In this process the light-exposed areas of the photo-sensitive "gel" emulsion becomes hardened. The emulsion areas of the stencil which are protected by the opaque areas of the film remain unexposed and thus soft. In the "development" stage of the process, the exposed areas of the stencil remain hardened, while the unexposed areas of the stencil soften and wash away, forming the image areas where the inks will pass through.
THREE: The "photo stencil" is embedded (either before or after exposure) into fabric stretched very tightly across the printing frame. After the stencil dries, the "Screen Frame" is pressed against the substrate (a shirt in this scenario) and ink poured onto the surface of screen is dragged across the design using a squeegee. The results is ink deposited on the shirt. And that's how screen printing works.
Now, there are many, many variations and technical modifications to this process but this is the basic way it works. Even though it's such a simple process, it can reproduce an amazing amount of detail when done correctly. Some of the Chinese designs produced thousands of years ago demonstrate this primitive process with incredible beauty. I, personally, have used this exact scenario to run tens of thousands of prints.
Since this is not a screen printing tutorial, let's move on to making the films and separations. If you want to get into screen printing for a hobby or a business, I recommend Roni Henning's excellent orientation into the process.
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from the Editor:
I was delighted that day back in 1989 when Peggy Killburn called to ask if I could handle one more speaker in my "Great Graphics Tips & Tricks" session scheduled for the 1990 Macworld Expo. "Yes" was my response to her request to add Russell Brown to my panel. After all, we loved Adobe's young "Illustrator" program, and were quite anxious to try out their upcoming new product called "Photoshop." After seeing his demo, I was convinced Photoshop would be big. So the next month we added "Photoshop Tips & Tricks" to our regular DTG Magazine uploads to Compuserve, GEnie and AOL. The rest is history.
I only regret that I didn't trademark the name.