During corrugated board production, moisture control of board is critical. Excessively high moisture will make finished boards soft, limp, and prone to box collapse after forming. If moisture content is too low, the boards become hard and brittle, and the finished boxes tend to crack.

Controlling board moisture is relatively complex, which is mainly determined by the following two factors.

I. Base Paper Moisture

The moisture of incoming base paper shall be strictly controlled. According to current standards, the moisture of base paper at delivery shall be (8±2)%. A separate warehouse is recommended for base paper storage, with temperature maintained at (18±3)°C and relative humidity at (35±5)%. Base paper must not be stored outdoors; otherwise, uneven moisture will occur with inconsistency between paper edges and the paper middle section.

II. Temperature

Temperature is one of the decisive factors affecting corrugated board quality. It can adjust base paper moisture and heat up the base paper, serving as an essential condition for adhesive curing. Therefore, multiple preheaters and flat dry cylinders are installed on the board production line to adjust the moisture content of base paper and single-faced board, cure adhesive and achieve bonding.

Four Key Points for Effective Moisture Control

1.If the board moisture is too high and preheater roll temperature fails to meet requirements, increase boiler steam pressure to supply saturated steam. Normally, steam pressure shall be maintained at 1.1MPa±0.2MPa.

2.If moisture content is low, adjust paper guide rolls to reduce preheating area. Meanwhile, fully utilize the spray device (dampening roll) on the single facer to spray base paper and raise its moisture to meet specifications.

3.Speed up the single facer before base paper roll change to accumulate more paper on the bridge. Upon receiving the signal, the combining machine shall reduce speed, and resume speed after the single facer completes paper splicing. In case of emergency shutdown due to equipment failure, lift the conveying felt to prevent excessive moisture loss of the board staying on flat dry cylinders.

4.For the single facer, properly set the gap between glue applicator roll and doctor roll. Normally, the gap is 0.2mm–0.25mm. When the machine is hot, the gap between glue applicator roll and main corrugating roll shall be 0.02mm larger than the thickness of the corrugating medium. For the double backer, the gap between doctor roll and glue applicator roll is also 0.2mm–0.25mm. The gap between pressure roll and glue applicator roll shall be 0.05mm–0.1mm smaller than the height of the passing single-faced board. The gap at both ends of the two rolls must be uniform and can be measured with a feeler gauge.