At Graphic FX we mainly screen print custom t-shirts so these FAQ questions are answered with that process in mind and may not apply to other substrates
What is simulated process?
Simulated Process is another advanced technique that uses halftones of a few ink colors to represent the colors in the original design. This process differs from 4-color process in that the inks are solid opaque colors usually printed on dark colored shirts. Because the shirt colors are normally dark, simulated process generally requires the use of an underbase.
When working with simulated process, we start with a design created digitaly or one that is scanned into the computer. The image can range from something that is photo-realistic to a line drawing or illustration. Once in the computer at the correct size and resolution, we process the design through a special program that automates the difficult separation process. Currently, we use a plugin for Photoshop called T-seps (formerly FastFilms). We will end up with 7 or 11 color channels to work with. Now, we must adjust these chanels so that the final print on the shirt will match the original artwork. This is the part that takes skill and applied experience from our art department. Once the adjustments are finished, the separations are printed out as usual.
Simulated process requires relatively high mesh counts and screen frequencies. We generally use 305 mesh and 65 lpi seperations. Depending on the design, 3 to 9 or more ink colors may be needed to accurately reproduce all subtle color variations.
Simulated Process Separations
When printing simulated process, the underbase ( usually white ink ) is printed first and then flash cured to gel the ink. Then the rest of the colors are printed wet on wet on top of the underbase. Although sometimes 9 colors are used, most designs can be printed with only 5 or 6. This eagle design uses only five colors :
Index color is another separation and printing technique that uses a few ink colors to produce a full color print. Unlike 4-color and simulated process, index color does not use halftones. Rather, the design is broken up into tiny square areas of solid color. These squares are very small ( 200 per inch ) and when printed they blend together to trick the eye into seeing color shades and variations.
Index color works well on both white and colored garments but generally takes more ink colors to represent a design with lots of color variation. Index color has the added advantage that you can pick the exact ink colors to use. This way you can more accurately represent colors that are difficult to produce using a "process" method. It also requires high mesh counts to print the tiny squares properly.