Wet end impact on twist curl

The curl of paper or board is caused by anisotropy of the sheet surfaces. It is directly proportional to dimensional change two-sidedness and inversely proportional to sheet thickness. If fibre orientation angles on surfaces differ sufficiently from each other and from the machine direction, twist curl can occur. Main curl is controlled with right drying strategy and with orienting one of the surfaces, normally the back layer, so that cross-machine direction dimensional change in different sides of the sheet is correct. Twist curl is minimised by matching the dominant layer slice-opening profile to the corresponding flow-profile to produce a straight fibre orientation profile. Matching can be done with the help of an optimisation programme. Adjustments can sometimes be adequate to get rid of twist curl problems.