Cockles

1. Usually result from poorly formed sheet.
(a) level the basis weight and caliper profiles across the web.
— check jet-to-wire ratio.
— if rushing, can have poor formation and uneven drying.
(b) reset sheet on wire, eliminate streaks.
(c) increase headbox velocity to reduce flocculation
— adjust shake stroke and/or speed, if applicable.
(d) check dry line.
— if sheet is set too quickly can get uneven drying and shrinkage with resultant cockles.
(e) check wire tension.
(f) lower headbox consistency, if possible.
— raise slice to maintain head.
2. Non-uniform shrinkage in dryer section.
(a) reduce refining to free up stock.
(b) clean or change worn or filled dryer felts.
c) for edge cockle:
— reduce air volume at edges to prevent over drying.
— reduce press loading to raise moisture at edges.
(d) sheet must not be dried too rapidly in early stages, use proper temperature gradient.
(e) wet sheet (high moisture) at reel will tend to cockle.
(f) check for slack draws and tighten where needed.
(g) check and clean dryers and dryer doctors.
(h) ensure correct alignment of dryer cylinders.
3. Change or regrind couch rolls.
4. Check for worn press rolls and correct at earliest opportunity.
— make certain press loading is uniform front and back.
5. Check size press operation for uneven distribution or pick-up which promotes uneven shrinkage in after dryers.
6. Check size press for roll slippage, plugged nozzles or improper nozzle positioning.
See also Blistering or cockling.