I'm currently in the process of perfecting my ability to print process color. So far things have been going well. It has taken at least 6 sets of 5 screens to get me to where i'm at now and expect to go though more screens in the future. I'm glad to share whatever knowledge i have.
Probably the most critical aspect if process color is the exposure. I'm using a vacuum frame and a used commercial exposure setup. Only with a great exposure setup is it possible to make great halftones.
Nearly equally as critical is having a good positive to burn into the screen. I use a postscript laser printer and print on kimodesk transparencies. Inkjets can work just as well.
For ink i use Union ink plastisol CMY inks, but generally use a good spot black ink for the K. For white underbase i'm liking unions cotton white.\
For screens i use 305 mesh for the CMYK and 110 mesh for the white underbase and the optional clear overprint.
For output i use adobe photoshop and print out using adobe default screens for 55 lpi. Adobe screens are slightly different in angle and line value than the traditional angles.
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Separate from the mechanical aspects of printing process color, there is also a calibration process. For this i use adobe photoshop's transfer function in the print dialog. These are the magic numbers which makes everything work correctly.
In me case, i'm calibrating my transfer to allow a double squeegee action for each color. Rather than depending on a single squeegee action to lay down a 100% perfect image, i use 2 actions which when averaged together form the final image.
Calibrated double stoke to eliminate snowflakes
Registration is the easy part. Either you have a press which can be registered and hold register or you don't. Process color is no more demanding that a multi color spot job. All of the registration marks need to stack over each other. The amount of trap on the white is probably the most significant registration point.
Printing on transfer paper is coming up soon on my list. From my limited experience, i believe that a transfer should come out looking better than a direct print, since a direct print is effected by surface defects, where a transfer is printed on a perfectly flat piece of paper which is pressed over the surface defects. When screen printing over a bump, the bump effects the color, but i would think that if a perfect image were pressed over the bump, the result should just be a bump.
Anyone have a recommendation on transfer paper???
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Personally, i believe Adobe Photoshop is essential to the production process. If you don't have it, buy it. If you can't afford it, buy an academic copy. Photoshop is the simplest way to make the white channel.
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Postscript printer or rip. While not 100% requires, process color is already hard enough, no need to make it a lot harder.
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From the amount of fear process color printing evokes in many screen printers leads me to believe that there should be a market to screen printed process color. I also believe that there are good reason why screen printers are afraid of process color. Every step in the process must be correct, otherwise the final result will be incorrect. There is at least a dozen things which can be done wrong and effect the final result. This dozen things gets multiplied further by 5 or 6 for the number of screens which need to be correct.
I'm currently working my way though the adobe illustrator sample director. Following that i'll move onto photoshop and corel.
fred