forming temperatures

FORMING TEMPERATURES

Chemcast® GP sheets should be heated to temperatures of 290°F. to 340°F. for Chemcast® GP II, G, and 55; 265°F. to 315°F. These are sheet temperatures. Oven temperatures should be approximately 10% higher than the difference between sheet and room temperatures. The formability of hot Chemcast® GP sheets changes little within these temperature ranges. Higher temperatures may cause a decrease in the tear resistance of the sheet and may also impair its physical properties without visible change in appearance and before bubbling occurs on the surface.

Also, if the Chemcast® GP is too hot, the surface is softer and more apt to pick up fingerprints, glove marks, specks of dirt and imperfections (called "markoff") from the form. These marks may require extensive polishing to remove.

Thinner material must be heated to temperatures higher than those used for thick stock because of the more rapid rate of cooling of the thin material.

In some cases, the sheet may be heated above the required minimum forming temperatures, removed from the oven, and the surfaces allowed to cool slightly before they come in contact with the mold. In this procedure the center of the sheet remains hot enough that excessive stress is avoided, but the surfaces are sufficiently cool to minimize markoff.

Note: Never use an air blast as this will chill local areas of the heated sheet.

The temperatures recommended are intended as a guide in establishing proper forming conditions. Variations for the different methods are based on average time requirements for handling the Chemcast® GP between the oven and the forming apparatus and for clamping the sheet and completing the forming operation.

Sufficient time should be allowed for the sheet to be heated throughout to forming temperature. This time will depend on oven temperature, air velocity in the oven and thickness of Chemcast® GP. In general, at least one minute heating time should be allowed for each one hundredth inch of thickness of the Chemcast® GP sheet, when both surfaces are exposed to the circulating hot air, e.g., a sheet .250" thick should be heated for 25 minutes.

Rather than control the oven temperature for each different thickness of sheet, it is general practice to control the oven at the highest temperature required for any Chemcast® GP being heated, then control the temperature of the sheet by the length of time it is kept in the oven.

Forming Temperatures and Cycles