No. 13 - Structural Foam

Until the mid-1960s, thermoplastic molding processes had no presence in large, flat, structural parts such as pallets or furniture that were then dominated by metal, wood, and thermoset fiberglass composites. Injection molding machines at the time did not have enough muscle to pack out large or thick parts, raising the probability of post-mold warpage and sink marks on surfaces opposite internal ribs or bosses. Imaginative molders in the 1930s had tried to solve the problem by adding sodium bicarbonate—ordinary baking soda—to the polymer melt to expand the polymer, however, the results were negligible.

The breakthrough came in 1963, when Richard (Dick) Angel, a researcher at the Bound Brook, N.J., laboratory of Union Carbide developed a process to foam parts by injecting nitrogen gas into the barrel and allowing it to be mixed into the melt by the screw. The new method, patented by Union Carbide in 1966, produced a rigid part with a dense skin and cellular core.

This method could produce complex parts with wall thicknesses from 0.125 to 0.5 in. without sink marks, while reducing part weight 10% to 30%. It also allowed production of stiffer parts at the same weight along with resin savings and parts consolidation. The low-pressure process permitted the use of less expensive aluminum tooling. However, molders had to pay Union Carbide $25,000 for a license, plus a royalty of up to 2¢/lb on products.

The first materials used were polyethylenes, polystyrenes, and ABS. Structural foaming of EVA, PP, ionomer, PVC, and nylons was not far behind. Extensive R&D by GE Plastics helped usher polycarbonate, PBT, and Noryl PPO-based alloys into structural foam. The process was initially highly successful in business machines but lost much of that market as a result of the drastic downsizing of electronic equipment made possible by the microprocessor. More lasting applications have included pallets, crates, shipping containers, and waste receptacles.

Carbide licensed the technology but did not offer a machine. Four equipment makers (Waldron-Hartig, Williams-White, Kohler General, and Sterling Extruder) were the first to build machines according to guidelines offered by Union Carbide. In 1969, Springfield Cast Products (now Uniloy Milacron) of Springfield, Mass., is said to have built the first commercial horizontal press designed expressly for structural-foam molding. (It went to Kusan in Clinton, Mass.)

By 1968, there were a variety of chemical blowing processes competing with the Carbide patent. Allied Chemical (now Honeywell) licensed a method of tumble blending a dry chemical blowing agent with resin pellets and then injection molding the foam in a conventional injection press. The Engelit process from Phillips Petroleum (based on a 1964 French patent by T.P. Engle) used pre-compounded pellet concentrate of a blowing agent tumble blended with resin. And Marbon Chemical (later Borg-Warner Chemical and then GE Plastics) introduced the first ready-to-mold preblended compounds of resin (ABS) and blowing agent.

Union Carbide pioneered the “short-shot” or “low-pressure” structural-foam process, in which foam is allowed to expand and fill the mold. However, in 1968 USM Corp. at its Beverly, Mass., Research Center, was developing a “high-pressure” process—later marketed by USM’s Farrel Co. Div.—in which the mold was initially packed full, and then the mold expanded to permit foaming at the core of the part. This approach was designed to prevent post-blowing and also produced a smoother skin, but it greatly limited part-design flexibility and added to mold cost.

Multi-nozzle injection enabled multiple molds to be filled at the same time or in sequence. Today, as many as 40 2-lb parts are molded at once using multiple nozzles. Rotary multi-station machines with one injector filling several molds emerged from suppliers such as Hettinga, Wilmington Machinery, and Presma in Italy.

Other innovations included foam-core sandwich molding (the ICI process), which involved simultaneous coinjection of solid skins and a foamed core. Battenfeld introduced coinjection machines for this process under an ICI license in the 1970s. Another smooth-skin process that arrived in the 1970s was gas counterpressure molding, in which the mold is pressurized with gas before injection.

Industry sources say structural foam molding had an unintended side effect of spurring the growth of the plastic painting market. Foam parts often require painting to hide the characteristic swirl effect or “splay.”