Many applications require Gar-Dur® UHMW plastic rod, bar or sheet to conform to a specified contour surface or to bend around corners. Shaping thin
(<1/8" thick) sheet over large (<10") diameter contours can be accomplished simply by forcing the sheet against the surface and mechanically fastening them together. To form more difficult contours, heavier cross sections, or sharp edges, however, the sheet must be heated and shaped.
Unlike most thermoplastics, UHMW plastic does not become a liquid when it is heated above its "melting point". Because of its high melt strength, it can be handled and shaped above its crystalline melting temperature of 265° F (129° C). Although UHMW plastic can also be shaped below the crystalline melting point, this causes residual stress in the part.
Optimum forming temperatures for Gar-Dur® plastic sheet, rod or bar are 280 to 300° F (138 to 149° C). At these temperatures, the plastic is translucent and newsprint can be easily read through thin sections.
Methods for heating Gar-Dur® plastic include air-circulating ovens, conventional ovens (similar to those in the home), heat lamps, radiant heating panels and oil baths. The most reliable ones are the air-circulating oven and the oil bath, since these heat most uniformly. With other methods, it is difficult to control temperature accurately and uniformly. Colors can also affect radiant energy heating.
A conventional fryer can be used as an oil bath for heating small parts. Most units for use in homes will hold temperatures with sufficient accuracy for the forming processes. Recommended heating oils are glycerin, silicone oil and commercial automobile antifreeze (ethylene glycol).
Forming is a simple process. After being heated to the recommended temperature, Gar-Dur® plastic sheet, rod or bar is forced to conform to a contoured surface and is then held in place. It is important to clamp the workpiece to the contoured surface until it is completely cooled.
Use a less thermally conductive material, such as wood, for the molding form. If a metal surface is used for a molding form, the metal may draw heat from the Gar-Dur® plastic piece before the forming process is complete. If it becomes too cold, it will be difficult to form and will have residual built-in stresses.
High residual stresses occur because of rapid cooling of the formed part. This is most likely when forming thick (<1/2") parts. Residual stress is minimized if the formed part is cooled slowly by covering it with an insulated cover or blanket.