Curling or flatness
The curling of sheet materials depends generally on temperature and conditioning state. So multilayer materials made of materials having different coefficients of thermal expansion or unequal expansion/shrinkage due to deviating hygroscopic tendency may tend to curl caused by a kind of bi-metall effect. In addition to that excessive mechanical stress in the processing of rolls can lead ot curling.
Standard Tests
flatness: flatness temperature
Specific Norm: DIN 55403
This method is useful for determining the flatness temperature of multi-layer films, which are produced by usage of individual layers with different coefficients of thermal expansion. This can cause a tendency to curl in consquence of temperature variations ("bimetaleffect").
flatness: circle method
Specific Norm: DIN 55403
A circle cut with a diameter of 113 mm is stored, the direction of curling and the edge distance in [mm] is measured. The measurements are carried out in standard climate at 23°C and 50% relative humidity and are well suitable for material comparisons. Measurements under varying climatic conditions are possible for an additional charge, please contact us.
flatness: cross section method
Specific Norm: DIN 55403
When materials with strong curling tendency have to be tested this method differentiates better than the methodologically similar circle method. A diagonal cross cut is made in the film, and the sample is stored. The direction of curling and the distance between the tips of the cross cut are recorded in [mm]. The measurements are carried out in standard climate at 23°C and 50% relative humidity and are well suitable for material comparisons. Measurements under varying climatic conditions are possible for an additional charge, please contact us.