Trapping is one of the simplest and yet one of the most complex concepts to be mastered by desktop publishers and graphic designers. Basically, trapping is the creation of an overlap between colours that are adjacent to each other on a printed sheet to overcome "misregistration" between the colours. Misregistration occurs when the two colours are not lined up perfectly, which, given the imperfect nature of paper stocks, printing equipment, and the humans operating it, occurs to at least a slight degree on most print jobs.
You have likely seen white edges on text caused by misregistration on a number of occasions. Because misregistration happens to at least a tiny degree on most print jobs, a useful way of avoiding the effects of it is to "trap" the colours. In this case, we could either "spread" the blue text by adding a slight blue outline to it or "choke" it by allowing the red to encroach the text area slightly. Better yet, if you're running state of the art desktop publishing software, you can tell it to "autotrap" when creating separations and let your software worry about the details. Unfortunately, even the best software can't address all circumstances, so it's best to understand some of the issues around trapping.
Some of the main issues involving trapping include:
The type of press used
If you are having a job run on a low quality press, no amount of trapping will save your job if the colours must be in tight register. Be sure to ask your printer what level of registration their presses are capable of.
The type of plate used
Traditional metal plates are the best (and most costly) solution to ensure tight registration. When comparing print quotes, make sure that your quotes are based on the same type of plates. Unfortunately, its not uncommon for some printers to quote jobs on the basis of cheaper "plastic" plates, only to increase their price after winning the job because it requires metal plates. Using metal plates can add 30% or more to the cost of a print job, but they're the only way to ensure truly great registration (with or without trapping).
Some colours lend themselves to effective trapping while other colours are problematic. For instance, if you have black text on a yellow background, you may have a challenge. If even the slightest bit of black mixes with the yellow ink on the press, you may risk contaminating the yellow ink and having a "muddy" yellow.
The shop you are dealing with
Different shops set different standards (based on their equipment and operators) for the amount of overlap allowed when trapping. Before doing any manual trapping or setting autotrapping in your print jobs, be sure to check how much trap is allowable.
Some shops use software that will automatically generate trapped separations when printing to film. This can be a good or bad, depending on the type of software used and the type of job you are having run. If your print shop is handling the trapping of your job, make sure to review the process closely to make sure it is complimentary to the job you are running.
Graphics and logos generated in such vector based software such as Adobe Illustrator or CorelDraw can be saved to EPS format, which will allow for "overprints" and trapping of outlines set in the files. Tiff and JPEG files, however, have absolutely no features that allow for trapping to be set when files are created in software such as Adobe PhotoShop.
Are you confused by trapping? You're in good company! Effective trapping is one of the most difficult areas to master in printing. That said, it's well worth the effort to start a dialogue about trapping with your printer and make an effort to at least understand the issues involved.
We strive to be as accurate and current with our information as possible. Due to the infinite number of scenarios that occur in print & desktop publishing, we can not guarantee that the above information will be correct in all situations.