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About Trapping for Print

In commercial printing, the process of adding a slight overlap between adjacent areas of color to avoid gaps caused by registration errors is called trapping. Color trapping is necessary to compensate for poor color registration. Poor color registration occurs when the printing plates used to print each color are not aligned perfectly. Poor registration causes unintentional white slivers to appear between adjoining colors.

Trapping is accomplished by intentionally overlapping colors so that minor problems with alignment are not noticed. Many service bureaus prefer to create color trapping themselves by using a specialized trapping program. Most trapping today is done by software for automatic trapping or PDF trapping. Trapping for print is a critical part of the printing process!

With I-Trap™ you can trap your jobs in the print shop yourself!
Find out more about how customers use this powerful application here

Traditional Trapping

Traditionally trapping was done as an optical process by providing a slight distance between the image emulsion in the film and the actual plate material. In the US this was an intentional process due to use of negative film which places the emulsion layer of the film in direct contact with the plate material. This required the intentional insertion of an additional clear film between the negatives and the plate. This rasied the negative emulsion slightly away from the plate. During the exposure process this rasied negative allowed the exposing light to slightly shine under the edge of the emulsion. This "fattening" resulted in the traps.

In Europe and other countries it is normal that the plate making process was done with positive film rather than negative film. Using positive film results in the emulsion side not being in direct contact with the plate material and so the actual positive film does it own "fattening" during the exposure process.

It was often said in the past that "Trapping is an American Problem" which is far from the case. Now that most of the world is moving to CTP and film based plating is disappearing, nearly everyone needs trapping!

Over Print Trapping

Over print trapping in applications is achieved by different methods. Normally, portions of an object that are obscured by another object are not printed. However, if the top object is set to overprint, the obscured portions of any underlying objects print anyway, causing an overlap. This makes white gaps between different colors impossible. Overprinting works best when the top color is much darker than the underlying color, otherwise an undesirable third color might result (e.g., red over yellow might result in an orange object).

Depending on the color-trapping options you choose, overprinting can affect an objectís outline or its fill. This means that if an object with a red outline is set to overprint its outline only, then any portions of another object that are obscured by the first objectís outline are printed. This overlap creates a color trap.

A few of the types of trapping and over printing are at this link.

Other sources:

Hybrid Screening

David Lewis Blog


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