Curl in paper

1. The dominant curl axis originates from a wet end condition.

— constant slice conditions must be maintained, with the least fiber orientation consistent with other desired paper properties and good running.

— the natural curl induced in the making at the wet end can then be balanced off later in the system..

2. High sheet density produces a greater curl tendency.

— limit refining and carefully select pulps, e.g., keep away from pulps that need extensive refining.

— using more broke in the furnish will usually reduce curl.

3. Wet pressing affects curl to a minor degree tending to produce curl towards the side from which water is removed last.

— some degree of compensation for curl towards the wire side can be obtained by increasing reverse pressing and decreasing straight-through pressing.

4. Passing the web through a size press can relieve many built-in strains in the sheet, even if only water is used.

5. The last section of dryers should have separately controllable top and bottom dryers.

— if the sheet curls up, the bottom cylinders should be hotter (the sheet curls away from the hotter cylinders).

— if the sheet curls down, the top cylinders should be hotter.

6. The use of a sweat dryer and a damp dryer felt will, to some extent, relieve curl to the wire side.

7. MG papers should have glazed surface on the top side, if it is possible to design for this condition.