Living hinge

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A living hinge on the lid of a Tic Tacs box

A living hinge is a thin flexible hinge (flexure bearing) made from plastic (rather than cloth, leather, or some other substance) that joins two rigid plastic parts together and is typically manufactured in an injection molding operation that creates all three parts at one time as a single part. A "living hinge is a thin strip molded into a plastic part to create a line along which the part can bend. Properly designed and executed, it can be closed and opened over the life of the part with little or no loss of function. [...] The best resins for parts with living hinges are polyethylene and polypropylene" due to their excellent fatigue resistance.^[1]^[2]

A thinned section of the plastic bends to allow movement. The minimal friction and very little wear in such a hinge makes it useful in the design of microelectromechanical systems, and the low cost and ease of manufacturing makes them quite common in disposable packaging. These can flex more than a million cycles without failure.^[3]

[edit] History

A few years after the introduction of polypropylene (PP) in 1957, "engineers at Enjay (now ExxonMobil) noticed an unusual phenomenon while studying pigment dispersion in very thin-walled color chips. Below a certain thickness, the PP molecules oriented in the direction of flow. Bending perpendicular to this orientation resulted in a stronger part that did not break with repeated flexing. Bob Munns, who worked at Enjay at the time, coined the term living hinge and the name stuck. The living hinge was introduced to the industry as a hinged recipe box at the 1963 NPE. Over the next 40 years, creative design engineers used living hinges in thousands of applications ranging from dispensing closures with hinged caps to automobile gas pedals and carrying cases. The current trend for parts consolidation and assembly minimization has created a renewed interest in integrally molded hinges."^[4]