Curl is the tendency of paper or paperboard to curl or roll up, generally in the cross-machine direction, due to changes in relative humidity (RH). This is often referred to as “CD curl”. Machine direction curl (MD) is often called “roll set curl”.
Curl tendency can be measured using a variety of sample sizes and sample orientations. We measure curl at various relative humidities, but normally samples are tested at 50% RH and 20 % RH. Our normal sample size is 1 inch x 6 inches, with the long dimension in the cross direction for measuring CD curl. We use multiple samples, placing the test strips on a flat surface and measuring the distance from the surface to the corners of the strips.