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Curling of paper and board 

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 200
0C.  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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