Porosity

Note: The more open and porous the structure, the greater the porosity or air permeability.
1. Increased refining will close up the sheet and reduce the porosity.
— increase refining consistency.
— check refiner tackle – material of construction, pattern, etc.
2. Change furnish if other tests permit.
— more hardwood will yield paper with a closer formation.
3. Improved formation will reduce the air permeability of the sheet.
— temperature affects the drainage rate that affects formation.
— adjust jet/wire ratio.
— adjust foil locations and blade angles to control formation.
4. Adjusting vacuum in suction boxes will change permeability within limits.
5. Check and adjust draws.
— slack draws allow more shrinkage that helps close the structure.
6. Check dandy operation.
— running wet under dandy will close up the sheet.
7. Increase solids or pick-up at size press.
— use film formers in size press solution.
8. Check calender operation.
— increase number of nips and nip pressure, 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.
See Pinholes, reducing.