Page 153 - Graphic Design and Print Production Fundamentals

Graphic Design 141
Inkjet

Inkjet is the next imaging technology we must assess and compare. The print heads on all inkjet
machines are mounted on the same unit travelling on the same track. Each ink is transferred one after the
other and the substrate does not move after receiving each colour. It is like electrophotography in that
mis-registration between print heads can be adjusted electronically, and once in register remain stable for
multiple imaging runs on the same substrate. If the substrate is changed between imaging, the operator
must recalibrate to bring all colours into registration, and ensure the placement of abutting colours is
perfect and no compensation is needed. As a result, no trapping will be needed for most inkjet imaging
processes.

Flexography

Flexography is the fourth imaging technology we need to assess. This technology has the most points
where mis-registration can occur. The printed image must be raised on the plate to receive ink from an
anilox roller that can deliver a metered amount of ink. The computer graphic must be reduced (or flexed)
in only one direction around the plate cylinder. A separate printing plate is developed for each colour
and mounted on a colour unit that includes an ink bath, anilox roller, doctor blade, and a plate cylinder.
The substrate travels from one print unit to the next on a continuous web that is under variable amounts
of tension. If a graphic has a lot of white space around it, the substrate can be pushed into the blank space
and cause distortion and instability in the shape and pressure of the raised inked image on the substrate.
Flexography is used to image the widest range of substrates, from plastic films to heavy corrugated
cardboard. This process absolutely needs trap lines generated between abutting colours. Standard traps
for some kinds of presses can be up to one point (1/72 of an inch, almost five times our standard litho
trap). Graphic technicians need to pay particular attention to the colour, size, and shape of the trap lines
as much as to the graphic elements. In most packaging manufacturing plants, there are pre-imaging
operators that specialize in creating just the right trapping.

Let’s examine some of the ways these traps can be generated. The simplest way is for a graphic designer
to recognize that he or she is designing a logo for a package that will be imaged on a flexographic
press that needs one-point trap lines generated for all abutting colours. The designer isolates the graphic
shapes that touch and creates overprinting strokes on those graphic elements that contain all colours from
both elements. That doesn’t even sound simple! (And it’s not.) It becomes even more complicated when
the graphic is scaled to many different sizes on the same package or used on many different packages.
So most designers do not pay attention to creating trap lines on the graphics they create and leave it
to the manufacturer to create trapping for the specific documents on the specific presses they will be
reproduced on.

There is specialized software that analyzes a document, determines where abutting colours are, and
generates the tiny graphic lines as a final layer on top of the original graphic. This is done before the
document goes to the RIP so it is raster-image processed at the same time as the rest of the document.
Most RIPs process PDF files these days, and there are specialized plug-ins for Adobe Acrobat that will
analyze a document, generate trap lines, and let an operator examine and edit the thicknesses, shape,
and colour of those lines. It takes a skilled operator to examine the extra trap lines and determine if they
are appropriate for the press they are going to be printed on. Press operators also need to determine the