Shrinkage
Because of the orientation imparted during manufacture, acrylic sheet shrinks slightly when heated to thermoforming temperatures.

Original dimensions won't change in fabrication operations not requiring heat or if the acrylic sheet is heated while clamped in a frame. However, unrestrained sheet heated to thermoforming temperature changes dimensionally. Continuously manufactured sheet changes by up to 3 percent maximum shrinkage in the manufacturing direction and 0.5 percent maximum width increase (transverse direction). Manufacturing direction can be determined from the sheet label or print on the masking. The lines of print are perpendicular to the direction of manufacture. Cell cast sheet will shrink up to 1.5 percent in both directions.

Measure the shrinkage in a preliminary test. Then, determine the size of material required to compensate for shrinkage before cutting any blanks.

Predrying
Predrying acrylic sheet is rarely necessary. Keep the sheet wrapped until used. To prevent blistering, dry high-water content sheet in a forced-circulation drying or vacuum oven before heating. Drying time depends on water content and material thickness. Twenty-four hours at 176 degrees F (80 C) dries most sheets.

To reduce length of the forming cycle, pre-dry the sheet in a spare oven and transfer it directly into the forming oven at 176 degrees F (80 C) after the dring period.

For more information, contact Mike at Ultra Acrylics, Inc.