Maybe you’ve never heard of “slush molding”. It’s defined as a type of casting in which a liquid resin is poured into a hot, hollow mold where a skin forms. Excess slush is drained off, and the molded product is removed. Slush molding was developed long ago to inexpensively produce skins for automotive instrument panels and door trim from polyvinyl chloride. It works great for applications that require relatively intricate designs. As auto designs have become more detailed, the need for slush molding has boomed.
Unlike most thermoplastics processes, slush molding works with the plastic material in powder form, avoiding expensive tooling and machinery. The problem: PVC. It becomes brittle with aging. Increased use of hidden passenger airbag designs requires ductile materials at very low temperatures. Also; according to European and other environmental authorities, PVC creates disposal problems. There are also problems when volatile organic compounds are emitted, particularly when PVC is exposed to bright sunshine through a car windshield. McDonough Baumgart Design, an environmental consulting firm, urges clients to avoid PVC in designs. Replacing PVC in many applications, such as medical tubing, however can be difficult because of its lower cost and functionality, ranging from chemical resistance to electrical properties.
I learned of a fascinating alternate, however, while judging the Society of Plastics Engineers Innovation competition. There were a handful of excellent entries. Engineers at General Motors and systems supplier Inteva have developed a low-cost polyolefin blended material for thin skins that can be processed on the same slush molding equipment used for vinyl and other compositions. The first application for slush molded products from TPO/TPE (thermoplastic polyolefin/thermoplastic elastomer) is the 2009 Saab 97X. TPO itself is a blend of polypropylene and pololefinic rubber.
“The formulation was developed to meet the required combination of both a very low melt viscosity for the slush process, and the necessary ductility for cold temperature deployment,” says Tom Ellis of Inteva. Some of the magic comes from a unique additive to ensure the blend is a free-flowing powder. “The process requires excellent powder flow,” says Ellis. What is the additive? It’s not a fluorinated material. That’s all I could get from Ellis and his presentation partner from GM. The new blend is said to have a melt viscosity very similar to PVC – and without the environmental issues. The new blend also reduces weight 20 percent. The new resin is called STP 747 NA TPO/TPE.
A patent search shows that a few companies have been working on this technology, including Mitsuboshi Belting, Kraton, Delphi, DuPont, and Zeon Kasei.
Winners of the SPE competition will be announced Nov. 20.