If you were asked to teach a class of your own subject. What would you teach ?

[jmowreader]

158. Trapping

Not trapping animals, but a different kind of trapping altogether.

When you print in color on a printing press, all the colors of ink that you use are applied in different places on the press. They're called units. All four to 12 units on the press - if you have more than six, you generally have a mechanism called a perfector between two units so you can print both sides of the sheet in one pass through the press - have to put the ink in exactly the same place on the sheet. We call this registration. Because the press is a mechanical device one or more of the units can drift just slightly out of register, and when this happens you can get little white lines between colored objects on the sheet. This looks terrible. "Slightly out of register" is measured in ten-thousandths of an inch.

To end the problem of little white lines, you make objects just a little bigger than the holes they are going in; this way, when the press goes out of register you don't get little white lines. This is trapping. One way to think of it is to consider a manhole cover: the cover is bigger than the hole so it won't fall in. That's trapping, but printers use ink instead of cast-iron discs.

Trapping is one of the black arts of printing because no two traps are alike. When I was still doing trapping - you don't trap a newspaper because you don't want to take the time to run a trapping routine on it, and you don't have to trap on an HP Indigo digital press because that system automatically adjusts register - I had four big presses and they all needed a different amount of trap. That's not the only thing you have to consider: the kind of paper you use, the kind of ink, the amount of water in your dampening system, the kind of fountain solution concentrate you use and the conductivity of the fountain solution, and even the time of day or the time of year can affect your traps. If you use too much it looks like everything has an outline around it, and if you don't use enough you may as well not have used any. You also need to look at the colors you're trapping: a red object sitting on a yellow background uses a different style of trap (this is a choke - you reduce the size of the hole) than a yellow object on a blue background (this is a spread - you expand the object), and black type on a background uses a completely different style of trap called an overprint where you don't knock the background out from under the type...unless the type is really big and then you do a choke because the background color will change the type color if the background is left under the type.

I think I'd buy a robe and pointy hat to wear to teach this because this is like Hogwarts School of Prepress, Witchcraft and Wizardry. But seriously, I have never seen such crappy traps as I have since people stopped burning plates from film. Absolutely NOTHING is trapped right these days.