A better bond in twin-sheet forming.

With an eye to potential applications in auto fuel tanks and appliances, Portage Casting & Mold Co., a thermoforming toolmaker in Portage, Wis., is developing innovative tooling that greatly improves the bond in twin-sheet thermoforming. In this process, two sheets of plastic are simultaneously thermoformed and welded together to make a hollow part. The big advantage of twin-sheet forming over blow molding is that it uses less expensive tooling and machinery than blow molding, and can use multilayer sheet to make barrier parts with potentially more control of layer thicknesses than by coex blow molding. However, a perennial problem for twin-sheet forming has been the reliability of the weld.

What Portage devised is aluminum-alloy molds with inserts of a very high-heat copper alloy in the pinchoff area. The alloy's thermal conductivity is 40% greater than that of aluminum and more than six times that of steel. Portage licenses this use of the alloy from Ampco Metals in Milwaukee. (CIRCLE 129)

Portage also built a separate rheostat heater into the perimeter of the tool so heat could be kept on the pinchoff while the rest of the mold cools below the glass-transition temperature of the formed sheet. This high-heat alloy mold insert can seal at temperatures up to 1200 F, while the mold proper heats only to 600 F, says Portage president John Griep. Tooling had to be adjusted in the pinchoff area for excess plastic, so Portage removed metal from the mold to make room for a reinforcing bead of material on the inside of the seam.

Portage developed twin-sheet expertise over several years through various strategic partnerships, including a development project with B.F. Goodrich Co. in Cleveland for a twin-sheet refrigerator door (which won a thermoforming award last year from the SPE for intricacy of tooling and design). Recently, Portage teamed with Hughes Plastics Co. of Redwood City, Calif., to prepare twin-sheet processing data to present at the SPE ANTEC in Detroit.

Their research used a double-shuttle pressure former made by Gulden Machinery, Vancouver, Wash., adapted to twin-sheet by Hughes. Ongoing work on twin-sheet forming of a fuel tank involves Portage and an undisclosed automotive OEM.