Paper Curl, Dimensional Stability, Effects of Shrinkage and Xerographic Paper

PAPER CURL, DIMENSIONAL STABILITY,
EFFECTS OF SHRINKAGE AND XEROGRAPHIC PAPER

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CURL BASICS (Read more in article Curl Basics, below )

Curl is effect of differential dimensional changes between layers in a sheet.
These are produced by different moisture expansions (usually from differences in fiber orientation); the release of differing amounts of internal strain; or from bending.
Xerographic (copier) curl is usually toward the print at moistures below 5-5.5% and away from the print at higher moistures for 75gsm papers.
Balancing amount of hydrophilic binder applied to each surface should reduce curl.
Adjustment of jet versus wire speed can reduce the difference in wire versus felt side fiber orientation.
A reduction in inherent moisture expansion of furnish should be helpful in reducing curl (see below).

DIMENSIONAL STABILITY (Read more in article Aspects of Dimensional Stability, below)

Inherent dimensional stability of a furnish or pulp needs to be measured on free dried sheets. If they are oriented sheets, a calculation of geometric mean expansion should be compared.
Factors in stability include fibril angle and degree of refining, additives, and filler.
Paper machine factors include fiber orientation, wet pressing, wet straining and sheet shrinkage.

SHEET SHRINKAGE IN DRYING

Shrinkage is perhaps the most important process effect on the mechanical properties of paper. Recently, Wahlstrom and Fellers showed that tensile stiffness, tensile strength, compressive strength and breaking strain are very closely correlate with sheet shrinkage or wet straining. A most likely explanation is that shrinkage affects micro-compressions, which in turn affects fiber properties, which affect sheet properties. Read more at "Effects of Paper Shrinkage" and "Property Relationships", below.

XEROGRAPHIC (copy) PAPER

Paper qualities important in xerographic papers include stiffness, smoothness, electrical resistivity, friction coefficient, curl, moisture, sheet flatness, millwork, porosity and composition. The usual basis weight of xerographic paper is 20 pounds (17" x 22" 500 sheets) or 75 grams/sq. meter. Good manufacturing control should keep weight within 0.5-0.75 pounds of nominal 99% of the time. The choice of a caliper specification depends on factors such as the type of fuser roll used in the copy machine. There may be no single set of paper properties that are optimum for all xerographic machines (including ‘laser’ printers). Read more at Summary of Xerographic Paper Properties, below.

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Charles (Chuck) Green, retired
Publisher, CPPN Electronic Library
Consulting: Curl, Xerographic Paper, Paper Physics
23 Maryvale Drive, Webster, NY 14580
585-265-9734

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