Although some printers prefer to create their own separations, it’s always good to understand the process. Be sure to communicate with your printer as they will specify their requirements, and will often give you tips for avoiding potential issues in the process.

There are three artwork techniques commonly used for type of printing: Trap, Knockout and Overprint.

The Trap technique is when the bottom color “bleeds” under the top color, ensuring no gaps are left from inaccurate printing of the second color (when the so-called “registration” between the colors is “off”).
Screen printing is not always an exact printing technique; for this reason the Knockout technique is rarely used, as it relies on printing a color precisely in a gap left on the bottom color.

The third technique, Overprint, is the easiest to achieve as the top color prints directly on top of the bottom color; often this produces a new color, as the top ink color is not always opaque.

Why is Trapping needed?
Generally speaking, the registration precision when printing several colors one on top the other has a certain degree of tolerance. Depending on the printing process used, registration shifts can be large enough to cause visible defects in the final printed result. One of the most conspicuous irregularities are areas of unprinted paper, called "flashes" or "halos".

Why is Trapping needed?
Generally speaking, the registration precision when printing several colors one on top the other has a certain degree of tolerance. Depending on the printing process used, registration shifts can be large enough to cause visible defects in the final printed result. One of the most conspicuous irregularities are areas of unprinted paper, called "flashes" or "halos".