No. 34 - Gas-Assist Molding

An estimated 15% to 20% of U.S. injection molders use gas assist to core out thick sections, reduce sinks and warpage, and mold larger parts at lower pressures. Four milestones marking the development of gas-assist molding started in 1971 with a patent application filed by German inventor Ernest Mohrbach. He described internal gas injection to form a void within a molding. In a first test, the patented process was applied to a structural-foam shoe heel as a means to reduce cycle time and improve the surface. The gas pushed the melt against the mold, collapsing the foam but enhancing cooling.

In 1973, Asahi Dow in Japan filed a more detailed patent involving internal gas injection. Asahi Dow was seeking ways to injection mold hollow parts that could compete with blow molding. It also sought to achieve a better surface than structural foam. Its patent proposed a device to compress the gas and inject it together with or just after the melt using the same injection nozzle.

In 1975, Ernest Friedrich at Rohm GmbH in Germany patented a gas-assist method for making one-piece hollow acrylic blocks to compete with hollow glass blocks used in construction. The Friedrich patent was the first to discuss venting gas pressure from a molded article (by sprue break or piercing the part). U.S. patents for the Rohm technique were granted in 1978.

In 1976, Union Carbide filed for a patent on a new “web” molding process that injected gas to hollow out substantially all of a part and leave only a random pattern of supporting ribs between two skins. Johnson Controls (now Uniloy Milacron) purchased the patent and commercialized structural web molding in 1982.

Around 1980, James Hendry, a leader in the field of structural foam since the mid-’50s, was working at Keystone Manufacturing (KMMCO) of Detroit to produce foam parts with smooth surfaces. First, he injected gas between the part and mold wall. Then he put the gas inside the part (a printer base and cover) using gas pins separate from the melt nozzle. KMMCO tried the technique with other molds designed for structural foam, adding channels that directed the gas to hollow out thick ribs and bosses. This produced smooth surfaces and eliminated sinks. KMMCO called the process Smooth Surface Technology (SST).

In 1984, Hendry, now a consultant, introduced the SST process to a British structural-foam molder, Peerless Foam Molding. Peerless started a new company called Cinpres to market the process. Cinpres did not obtain a license to the SST process since KMMCO was by then bankrupt. Cinpres and Battenfeld teamed up to introduce the process commercially in 1985.

In the U.S., Nitrojection Ltd. used gas-assist patents from its parent company, Sajar Plastics, to bring the process into major appliances in 1987. In 1986, custom molder Detroit Plastic Molding (DPM), acquired the expired U.S. patents on the Rohm method, and applied the process to automotive moldings. It found early success in producing a chrome-plated plastic part for Volvo of Sweden, reducing warpage, scrap rates, and cycle times. DPM used the method successfully again in 1988 to mold a car-door component of 35% glass-filled PBT for General Motors. For that job, DPM advanced the method of using spillover cavities.

As late as 1989, only four U.S. molders were known to be using gas assist: DPM, Sajar, Cascade Engineering, and Encore Systems. In 1989, DPM created a new company called Gain for gas-assist process licensing. Gain acquired the KMMCO SST patents in 1992.

Patented by Asahi, external gas injection—forcing gas between the part and mold wall to reduce sinks and speed cooling—is an alternate use of gas assist. It has been marketed since 1992 by U.K. firms Gas Injection Ltd. and Cinpres (now merged). Alliance Gas Systems in the U.S. applied gas injection to compression molding of thermoplastic and thermoset composites in 2001.