I am going to be doing 2 color printing with a t head and the colors may come into contact a bit. I was
told I need to have a higher TAC in the first color. How do I add TAC? Or is it that you don't add TAC but you instead lower it in the second color? And will this also effect a slight shade of color change in the ink?
It is far more common to reduce tack than increase it and you should be able to get a tack reducer (or increaser) from your ink supplier. Start at a dozen drops if it is a liquid; a jellybean size if it is gel for an amount if ink to allow a quarter inch bead along the ink fountain.
As there is typically some contamination during the run the general rule is to put the darker color in the T Head. Contamination can be alleviated by running plates which are wider than the blanket.
If colors are going to come into contact with each other you should ensure you have trapping applied.
The edge of a plate always never runs clean and will lay ink at the plate edge on the blanket. This ink is not transferred to the printed sheet - it is outside the sheet area. The build up of ink will migrate to the other print unit - the T-Head will pick up the parent press color and vice versa.
Typically one would run 12 inch plate material and 11.5 inch blankets. I prefer 13 inch plates.
Thank you. So its okay for the t head ink to touch the press ink on the blanket as long as it has less TAC? And normally people add less TAC to the t head ink not more to the press ink?
I wouldn't say it is okay for the T-Head to touch the parent press ink but it is common. Ideally they would not. Run a few (hundred or thousand) jobs and have to stop and wash-up midway or twice during a run and it will make more sense.
Let's clarify Tack - It is Tack, not TAC. (TAC is reserved for Total Area Coverage in industry terminology). Tack affects the ability of subsequent inks to lay on top of a previous ink during a press run. It has no bearing on multiple passes - only ink laying on wet ink in the fractions of a second involved during multiple color work. You do not "add less tack"; you reduce the tack.
Before I compose a couple hundred words regarding overprinting vs. screen builds I'll mention this. A T-Head is only suitable for multiple color work where the colors either do not touch or merely touch. Overprinting one color over another will not produce the same result as the same set of plates overprinting either by two passes or a true two color machine. Tack is important for overprinting (and screen builds) but not overly important in colors touching (which ideally would be trapped).
Reduce the Tack in the T-Head a bit if necessary; it typically would not be.
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