NPE News Wrap-Up: Injection Molding

Stress-free production of large car windows

Battenfeld used a special injection / compression process with a hinged tool to mold 11-lb PC car hatchback windows without inducing stresses in the parts.

Probably the most dramatic demonstration at the show was stress-free molding of large polycarbonate car windows at Battenfeld’s booth. The application used a new PC material developed by Exatec LLC of Wixom, Mich., the joint venture of GE Plastics and Bayer Corp. (see PT, July ’03, p. 62). The key to the process was an unusual injection/compression technology dubbed IMPmore (In-Mold Pressing), which teamed a Battenfeld HM hydraulic press with tooling from Summerer Technologies of Germany and a Synventive hot-runner system. Battenfeld molded a 4-mm-thick tailgate window weighing 11 lb and measuring 11 sq ft, or about 1 sq meter, on a 2200-ton press. Normally a part this size takes a 4000- or 4500-ton machine. A smaller press was made possible by the need for low-pressure injection to avoid stresses and thickness variations with the viscous PC material.

Battenfeld contributed a special barrel and screw design. Summerer came up with special tool that has a tilting top half. The tilting action is designed to pool the injected melt in the top half of the mold under low pressure. Then the top of the mold tilts back into its fully closed position—like a book cover closing—which squeezes the melt down into the rest of the cavity under low pressure. A row of retractable cylinders in the bottom of tool extend out to touch the other mold half, providing counterpressure in the tool during the initial injection phase when the melt is pooled at the top. The cylinders retract to a flush position during the compression stage.