Care and feeding of living hinges

Polypropylene resins are uniquely suited for one-piece integral or living hinges. Due to its toughness and ductility, polypropylene (PP) is uniquely suited for a fastening and joining concept that can't easily be reproduced with other engineering thermoplastics: the integral or living hinge. Molded-in hinges aren't new, but designers under the gun to consolidate parts and eliminate assembly steps are taking a closer look at molded-in hinges for everything from glove-box bins and tackle boxes to CD cases and electrical connectors. Living hinges can be molded from any PP resin including homopolymers ( single monomer), copolymers (mixed monomer), impact copolymers ( toughness enhanced), and random (nonordered monomers). And while injection molding is the manufacturing option of choice, other processes, such as extrusion, blow molding, and stamping, can also form hinges that flex more than a million cycles without failure. There are two basic types of integral hinges — molded-in and die formed. Molded-in hinges are the most widely used because they can be made with conventional injection-molding techniques. Key to designing a living hinge that survives the long haul, however, requires proper polymer processing during molding. Optimum hinge performance, including high strength and good fatigue endurance, comes when PP molecules are oriented perpendicular to the center axis point of the hinge. The mold must be designed so the polymer flows across, and not along, the hinge length, and preferably at high velocities and melt temperatures. Filling too slowly, using low melt temperatures, and nonuniform flow through the hinge may cause premature hinge failure. It is also important that the polymer's flow front crosses the thin section of the mold instantaneously. Gate locations must also provide balanced mold fill. Designers should be aware that a substantial pressure drop occurs as the flow front crosses the hinge. This increases the polymer's shrink rate and