White card board Drying: Equipment Selection

The difficulty with white card board is not heat sensitivity but stiffness and deformation. As the web dries it loses both moisture and stiffness; while it is still wet it cannot carry its own weight, and coming out of the dryer it folds, takes a crease and curls at the edges. The advantage of a flash airflow line is that the material is carried by the hot air the whole way, with a small drop height and little mechanical action on the sheet, provided the conveying upstream is steady and the web never enters folded or doubled. Temperature follows the grade ceiling, and white card is more forgiving than coated stock; air volume and velocity are the other variable.

The difficulty with white cardboard is not heat sensitivity but stiffness and deformation. As the web dries it loses both moisture and stiffness; while it is still wet it cannot carry its own weight, and coming out of the dryer it folds, takes a crease and curls at the edges. The advantage of a flash airflow line is that the material is carried by the hot air the whole way, with a small drop height and little mechanical action on the sheet, provided the conveying upstream is steady and the web never enters folded or doubled. The inlet temperature follows the grade ceiling, and white cardboard is more forgiving than coated stock; air volume and velocity are the other variable, because too high a velocity lifts an already formed sheet and makes the edges shrink unevenly. What to confirm at selection is the adjustable range of the air volume, not simply whether it can dry. Moisture has to be even, with the cross-direction spread kept within one percentage point, otherwise curl and dimensional instability are hard to avoid.

Why does white card fold and crease after drying?
Because wet sheet cannot carry its own weight. The remedy has three layers: steady conveying upstream, so the web never enters doubled or folded; a drying rate that is not too fast, since thin paper loses stiffness faster as it loses moisture; and flattening and cooling at the discharge end, so the sheet does not land on itself.

For white card, is air volume or temperature the thing to control?
Temperature decides whether the sheet is cooked and whether the edges shrink too far; air volume decides whether the water can actually be carried away evenly. White card is more forgiving on temperature, and too high a velocity lifts an already formed sheet and makes the edges shrink unevenly, so the adjustable range of the air volume matters more than simply whether it can dry.

Why does the cross-direction size of white card vary?
Uneven cross-direction shrinkage, which in the end is uneven moisture across the web. Beyond the machine itself, the conveying has to keep cross-direction tension even, and the discharge end needs a conditioning step so the paper rebalances at a constant humidity.