No printing process can be termed perfect. This is where trapping comes in. It compensates for paper stretching, mechanical shifts or dislocation of plates during printing. A color overlap is done to prevent any unprinted paper from getting visible in final printed product. Do note that the trapping being described is not linked with terms like dry trapping or wet trapping. The latter two describes behavior between paper or ink or between inks.
When multiple colors are printed on top of other, the registration precision has tolerance to a certain degree. The registration shifts, depending on the printing process, can be sufficiently big to cause clearly visible defects in the final printed result. Irregularities are easily noticeable in areas of paper which were not printed. These areas are termed “halos” or “flashes”. The latter takes the form of a gap of white which is seen between two comparatively darker color areas where the two should hopefully have met. The phenomena occurs when colors are made of dissimilar separations. These separations do not perfectly register on the colors border. The little area of paper continues to remain unprinted. This shows up in contrast as an extremely noticeable, bright stripe.
Reason for registration errors
There could be a number of reasons. The recording devices can be inaccurate. All recording devices are not capable of generating separations which can be termed consistent in all the dimensions. If separations of one page get exposed in the film's different positions, the separations generated may not match as planned.
The film can distort or stretch when it is being exposed and developed. Variations in humidity or temperature during film processing could result in registration errors. The separations must be exposed on to the film from the same batch when possible.
Film assembly inaccuracies
Any errors made during platemaking and also film assembly could result in mis-registration. This will happen even though the operator has taken maximum care to prevent such a result from happening. Inaccuracy may also result from the printing press's limited accuracy. Faults can happen due to the instability of stripping material or the film.
Time to trap
Trapping is needed when two colors abut or touch. Trapping is vital when the design has small serif type or fine line-work. On case two adjoining colors share common color, it is not necessary to do trapping. Simple trapping involves lighter color spreading into darker color. The image shape and size is determined by dominant darker color.