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The curling of paper and board occurs for three reasons;
structural differences between the two sides of the sheet,
which may be intended, as in the case of label paper;
detrimental factors in the making of the paper; or during
the converting of the material.
The cause of the curl in the latter case is less
complex and for this reason it is discussed first.
Curl
occurring during converting
The types of curl discussed under this heading relate to
satisfactory paper and board, which is flat when, delivered
and remains so when exposed to the range of atmosphere
normally occurring in unconditioned working premises
throughout the year.
Press
damping curl
This is the curl that develops in lithographic printing if
the paper is subjected to excessive damping, a situation
that might occur when there is little ink coverage needing
more than normal damping.
With alcohol damping this type of curl is less of a
risk, for with the better wetting of the plate less water is
present. Initially,
the side that is wetted expands excessively in the papers
cross direction and the curl is towards the undampened side
of the sheet with the axis of the curl parallel to the
papers machine direction.
When the sheet dries out the final curl is then
towards the dampened side, the curl reversing because of
shrinkage occurring after dried-in strains in the dampened
side of the paper have been released.
With the press damping curl,
the axis of the curl is always parallel to the paper machine
direction because fibers expand more in their width than
they do in their length and fibers in machine made paper are
mainly oriented with the latter dimension in the machine
direction. Paper
that is delivered dry is more prone to this type of curl.
To eliminate the curl requires the dampening of the reverse
side which might happen when the printing the second side,
or alternatively printing wet with a plain plate, but both
these remedies require the paper to be flat enough to feed.
(The fact that paper curls when wetted on one side has one
advantage, it presents a useful way of determining the
machine direction of the paper.)
Tail-end
hook (back-edge curl)
This is the situation when only the back-edge of the sheet
is curled away from the printed side when leaving the
printing nips an offset-litho sheet-fed press. This is a mechanical curl put in by the press, which impedes
the delivery of the sheet and produces a poor knock-up in
the stack and prevents satisfactory feeding for any further
pass through the press.
Factors adversely influencing the problem are, a
large sheet, grammage below 90 g/m2,
smooth paper, heavy ink coverage at back-edge of the sheet,
too narrow a plain margin at the back-edge of the sheet,
high tack ink, excessive printing pressure, fast running and
soft blankets.
The problem is directly related to the stiffness of the
paper and those containing a high filler content, be it
coating or loading in the base sheet, are most liable to
give the problem.
Cures, apart from changing the
press conditions as indicated in the causes mentioned above,
are change immediately to a hard blanket (no further action
may then be necessary). If only the back-edge of the sheet
has heavy ink coverage. Consider making it the gripper edge,
and for repetition work use ‘short grain’ paper, but
only if it goes through the press once for each side
printed. This last cure is not for four-color work on two and singles
color presses.
Ink
and varnish shrinkage curl
This is not a common type of curl, but it can happen with
heavy ink coverage or varnishing on one side of the sheet,
or more likely with combined ink or varnish film shrinks
when dry. The
shrinkage curls the paper in its stiff direction, the cross
direction, giving a curl towards the inked or varnished
side, with the axis of the curl parallel to the papers
machine direction.
Laminating
curl
There are two types of curl occurring during laminating of
substrates that do not have an inherent curl, although in
one case the curl is related to the property of one
substrate.
The first type of curl is the
tension curl, which results from laminating two substrates
of the same or varying grades, with different web tensions
for the two reels on the laminator.
This results in a curl towards the substrates of the
same or varying grades, with different web tensions for the
two reels on the laminator.
This results in a curl towards the substrate that had
the highest tension and the axis of the curl is then
parallel to the laminates cross direction.
The second cause of curl is with the laminating of
two paper substrates that have a significantly different
equilibrium relative humidity.
When a laminated, particularly with a water content
adhesive, the two papers will eventually reach equilibrium
with each other and one will expand and the other shrink.
Whether or not they curl, and the degree of curl,
depends on the moisture dimensional stability of the two
materials and their relative strength of pull on each other.
When a curl occurs, the axis of the curl will be
parallel to the paper machine direction and away from the
original dry paper.
High
temperature curl
The curl due to subjecting paper to high temperature is
associated with a few specialist papers and most common in
this category are copier papers and laser copier papers, in
particular, where fusing occurs at 2000C.
Such papers with too high a moisture content are most
prone to curl, but since it is a structural curl, it is more
related to two-sidedness of the paper and the axis of the
curl is parallel to the papers machine direction.
For example, a difference in the fines (very small
fibers) and filler content, plus the orientation of the
fibers on the two sides of the sheet can be factors in the
curling. For this reason the better quality laser copier papers are
made on a twin-wire machine to overcome two-sidedness.
When sheeting on a heat-set
web offset press, curl can develop for the same reason as
for laser copier paper, because the paper is subjected to a
similar high temperature.
Again, the axis of the curl will be parallel to the
paper machine direction and towards the wire side, but there
is a difference because remedial methods can be tried on a
heat-set web offset press to overcome the problem.
First, reduce the temperature
of the chill rolls if they are too high and if this is not
successful, dampen one side of the paper, the wire side,
with the silicon applicator, or with a spare unit if one is
available.
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