Plate Cracking
The predominant cause of plate cracking is plate flexing during
the run. Plates will not crack if they are mounted to a perfect fit on
the cylinder and if no uneven or unusual stress is applied to them.
Of course, those conditions do not always exist within the real
world of web offset printing. Many factors contribute to a less-than-
snug fit resulting in the “pre-stress” of the aluminum substrate.
Most plates manufactured for the offset web market are textured, or
grained, in order to achieve and maintain an appropriate equilibrium
of ink and water (balance). This is a “non-directional” topography
of the surface. This should not be confused with “aluminum
directionality.” All alloys of aluminum, by nature of their manufacturing
process, are comprised of a uniform physical molecular
structure.
For most web presses applicable plates should be ordered so
that the aluminum directionality runs around the cylinder. By viewing
the reverse side of a one-sided plate you can see this directionality,
which appears as indentations or scratches in the aluminum
substrate. With magnification or by unaided eye, there
should be no problem discerning grain direction on any lithographic
plate.
The press is often blamed for cracking the plate when, in reality, it
is the repeated pressure/no pressure action of the form rollers and
blanket cylinder that causes the plate to flex and then crack. However,
it should be noted that the plate, bender, packing material,
mounting procedure and gauge are often other major contributing
factors that can result in cracking. Aluminum memory, otherwise known
as spring-back, plays an integral role in the prevention
of plate cracking. Thinner gauges of plates have more
spring-back than heavier gauges.
It is important to remember that the engineering design of the plate
bender corresponds directly to the gauge of plate in which it was
meant to bend. In many cases the press specifications call for a
0,012 gauge (0,30 mm) plate, however a 0,008 gauge (0,20 mm)
plate is substituted and packed accordingly onto the plate cylinder.
Aluminum spring back in this case can prevent the plate from
seating correctly onto the cylinder/lock-up mechanism (rod reel).
