How can shrinkage be reduced?

For the further fabrication (thermoforming) of extruded sheets, sheet orientation and thereby shrinkage often have to be kept as low as possible. Shrinkage in machine direction always poses a problem, since it is higher. Shrinkage is mainly influenced by the speed of the take-off roll in relation to the speed of the last roll of the calender stack. The peripheral speed of the take-off roll should be 2-3 % lower than that of the last roll in the calender stack.

The roll temperatures also play an important part. The last roll of the calender stack should be hot enough for the sheet to just about adhere to it. If the roll is too hot, the melt sticks too strongly during detachment. The force required for detachment provokes shrinkage. If the roll is too cold, the sheet becomes wavy upon detachment.

The ideal roll temperatures depends on the roll diameter, the sheet thickness and the extrusion speed.

See sketch for recommended roll temperatures (typical values):

The following measurement can be performed to check shrinkage:
A sheet strip is placed on a tray sprinkled with talc for about 90min at 160°C. The change in dimension is subsequently measured. A shrinkage of c. 3% in a 3mm thick sheet can be termed good. Thinner sheets show higher shrinkage, and thick sheets less.

back