Sizing decides how paper behaves when it meets liquid – ink on a writing sheet, glue on a corrugator, rain on a carton. Get it wrong and you hear about it from customers fast: feathering, poor glue bond, cartons that go soft in the godown.
Two ways to size
Internal (rosin) sizing
Fortified rosin is added at the wet end and fixed onto fibres with alum. It sizes the whole sheet, inside and out. It is economical and well understood, and works best in acidic to mildly acidic systems.
Surface sizing
Surface size chemicals, often applied with oxidised starch at the size press, treat the face of the sheet. They improve print quality, surface strength and resistance to ink and water where it matters most – on the surface.
Which one is right?
- Writing & printing paper: usually both – internal sizing for the body, surface sizing for print quality.
- Kraft and liner: internal sizing for water resistance; surface sizing if print or Cobb targets are tight.
- Duplex board: surface sizing on the top layer improves printability on cartons.
- Tissue: little or no sizing – absorbency is the goal.
Benefits of getting sizing right
- Sharper print and less feathering.
- Better glue bond on corrugators.
- Cartons that hold strength in humid weather.
- Fewer returns for “absorbency” complaints.
Common problems and fixes
Sizing drops suddenly
Check pH and alum dosing first. Rosin sizing loses efficiency if pH drifts up.
Deposits on the machine
Excess alum or poorly emulsified rosin causes sticky deposits. Balance alum to rosin and keep the system clean.
Good Cobb in the lab, poor results at the converter
Sizing can develop over a few days after production. Test reels again after storage before changing the programme.
Tips
- Keep alum and rosin dosing proportional – one without the other wastes both.
- Watch the effect of recycled fibre; contaminants can consume sizing.
- Run surface sizing trials on one reel set before full changeover.
Facing sizing issues? Tell us your grade, pH and current chemicals – we will suggest a sizing programme that fits. Ask our team →