Air permeability

1. Increased refining will close up the sheet and reduce the porosity.
— increased refining consistency has effect similar to added refining.
2. Control sheet drainage on forming fabric.
— reduce vacuum in vacuum boxes, if possible.
— change foil blade angles or foil locations to control formation.
3. Improved formation will reduce the air permeability of the sheet.
— adjust jet/wire ratio.
4. Temperature affects the drainage rate that can affect formation.
5. Check and adjust draws.
— slack draws allow more shrinkage that helps to close up the sheet structure.
6. Check dandy operation.
— running wet under the dandy will close up the structure.
7. Increase solids or pick-up at size press.
— consider use of film formers in size press solution.
8. Check calender operation.
— increase number of nips and loading, if possible, to reduce permeability.
9. Increased temperature of calender stack helps close up the sheet.
10. Higher moisture levels into stack greatly reduce the porosity.
11. Higher loading at wet presses will close the sheet so it is less porous.
— eliminate any build-up on press rolls.
— check condition of felts for finish, wear, and openness.
See Pinholes, reducing.