Warping, curl and twist

The flatness and dimensional stability of paper and board materials relate to their ability to remain flat under the effects of humidity changes. The shape of paper and board is defined by curl and twist.

Curl is defined as the inverse of the radius of curvature of the sheet, and is positive when the printed side is convex.

Curl in paper and board can cause problems in a variety of printing and packaging processes. This can range from misregister of multiple colour print, to problems at the in-feed of photocopiers, cartonning and tray erection machines.

Curl in paper and board can take a number of forms. The three main types of curl are referred to as mechanical curl, structural curl and moisture curl.

• Mechanical curl occurs due to stresses applied to the material which leave it distorted.
• Structural curl results from the material being non-homogenous in the thickness direction.
• Moisture curl occurs due to variations in the moisture content of the material from one side to the other.

The effect of humidity changes on the flatness is used to assess the suitability of papers and boards for specific applications and international markets.

Pira's testing is performed to British and International standard BS ISO 11556, BS ISO 14968 and ASTM standard D4825.