Page 152 - Graphic Design and Print Production Fundamentals

5.4 Trapping

Wayne Collins

Trapping can be a very complex procedure in pre-imaging software for certain imaging technologies.
It is an electronic file treatment that must be performed to help solve registration issues on certain
kinds of press technologies. Generally, if a substrate has to move from one colour unit to another in the
imaging process, the registration of one colour to another will not be perfect. That mis-registration must
be compensated for by overlapping abutting colours. As soon as two colours touch in any two graphic
elements we must create a third graphic element that contains both colours and overlaps the colours
along the abutment line. That third element is called a trap line and can be generated many different
ways that we will review.

Electrophotography

First let’s look at the differences between the four most common imaging technologies and determine
where and why we need to generate these trap lines. Electrophotography, or toner-based digital printers,
generally use only process colours. Each time an electrostatic drum turns, it receives an electrical charge
to attract the toner colour it is receiving. The drum keeps turning until all colours of all toners are on
the drum, and then all colours are transferred to the substrate at one time. There is no chance for mis-
registration between the cyan, magenta, yellow, and black toners as they are imaged at the resolution
of the raster generated by the RIP, and the placement of the electronic charge for each colour can be
adjusted until it is perfect, which makes it stable from image to image.

Lithography

Let’s compare electrophotography to the lithographic print process. In lithography, a printing plate is
generated for each colour and mounted on a plate cylinder. The plates are registered by manually turning
wrenches to hold plate clamps, so the plate-mounting procedure can generate registration errors. Each
colour printing unit is loaded with a separate ink, a plate that has been imaged to receive that ink, and
a blanket that offsets the image from the plate before it transfers it from the blanket to the substrate.
This is another mechanical transfer point that can cause registration errors. Most high-end lithographic
presses have servo motors and cameras that work together to adjust for mechanical registration errors
as the press runs. The substrate must travel from one printing unit to the next, and it is here that most
registration errors occur. There are slight differences in the substrate thickness, stability, lead (or gripper)
edge, and a different rate of absorbing ink and water that cause slight mis-registration. Also, consider
that most sheet-fed litho presses are imaging around 10,000 sheets per hour, and we are only talking
about movements of one-thousandth of an inch. On most graphic pages, however, the naked eye can see
a mis-registration of one-thousandth of an inch, so the process must be compensated for. The solution
is generating trap lines to a standard for lithography of three one-thousandths of an inch. This trap line
allowance in abutting colours allows for mis-registrations of two-thousandths of an inch that will not
show on the final page.

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