Designing plastic parts with integrated hinges formed from the plastic itself is hardly a new idea. These so-called "living hinges" have been around for years on all sorts of packaging and consumer products. But now they show promise for automotive interiors. Engineers at Bayer Corp. have developed a concept for creating thermoplastic polyurethane (TPU) integrated hinges for armrest storage and other interior compartments.
TPU has a lot going for it in these applications. This elastomer family is well known for its high tear strength and dynamic flexural performance, particularly at low temperatures. Bayer has test data from non-automotive applications showing TPU living hinges lasting past one million cycles. And recently, the company introduced a family of aliphatic TPUs whose improved resistance to UV light now makes these elastomers well suited for colored automotive interior parts-not just basic black and dark gray as in the past.
The armrest and integrated hinge would be created during a two-shot molding process. The bottom and the lid would be produced in the first shot from a rigid plastic, such as ABS, while the second shot would add the TPU hinge that joins the two. Depending on the design of the part, the hinge would typically measure between 50 and 100 mils thick. To produce an even bigger bang for the two-shot buck, the second shot could also simultaneously cover the armrest substrate with the same TPU, imparting a soft-touch skin that might otherwise require a separate manufacturing step.
The tooling to perform such a complex overmolding job wouldn't come cheap, but Bayer engineers estimate that a 5-10% overall cost reduction would result from living hinges, in part because high automotive production volumes would minimize the impact of tool cost. The predicted savings would then come from the elimination of manufacturing and assembly steps as well as the cost of the mechanical hinges and their fasteners.
It's Alive: A living hinge, molded in a two-shot process from thermoplastic polyurethane along with the rigid armrest components, takes the place of a traditional mechanical hinge for an automotive armrest.