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The Search for the Supreme Separation Method

(August 2007) posted on Mon Aug 20, 2007

Explore common color-separation techniques, the ideal situations in which to use each one, and how to combine the methods for optimum results on press.

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By Thomas Trimingham

The screen-printing industry still has yet to make the most of this method of separation. I would guess that the reason for this is twofold: Most printers simply don’t know how to create these separations well, and index has a bad reputation for creating grainy prints that lack subtlety and need too many colors. You will regularly require six to eight colors at the minimum to create decent index prints. It isn’t uncommon for an index print to bump up the colors into the 12-14 range for painting and complex design recreation. This can scare a lot of printers away from the method entirely.

The truth about index separations and the printing of these designs is that the process is so simple it can become quite addictive, especially for a printer who commonly separates by way of simulated process. I’ve worked with companies that print nothing but index seps, and they no longer use the other processes because they can’t deal with all of the downtime. That is the huge advantage of index separations for screen printing. Printers can save tons of time on the separation side, as it’s typical to separate an index set of posi¬tives in fewer than 10 minutes, even for a very complex design that requires some customization. Index separations shine even brighter on the printing side of the process.

Index separations were originally developed to convert images into fewer colors by using a unique color table to replace a much larger gamut. The computer averages the colors and forces them together in a ratio of pixels similar to the way they are displayed on screen. Each dot in an index separation is actually one square pixel, and the colors fit next to each other like a checker-board. The big advantage of this in screen printing is a reduction in dot gain. Very opaque colors are printed next to each other, not on top of one another as they are in process-color printing. Index separations have many other advantages in comparison to traditional halftones (Figure 4).


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