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A living hinge, sometimes called a flexure bearing or flex bearing, is a flexible segment of material, usually made from some type of plastic, that joins two rigid surfaces. The hinge is flexible, allowing it to bend. This type of hinge is most often used to join a lid to a container in disposable packaging.
While similar hinges can be made from materials such as fabric or leather, the term "living hinge" applies specifically to those made of some type of plastic. Such hinges are usually injection-molded, meaning that the plastic is injected into a hard mold, then removed when set. Injection molding is quick and inexpensive, and the amount of material needed for a living hinge is relatively minute, so such hinges generally are an inexpensive option.
Despite the low cost, such hinges are unexpectedly strong and can last far longer than might be expected. Standard hinges generate friction between moving parts, leading to wear-out. Formed from a single piece of plastic, living hinges do not physically interact with any other part, so no friction is generated, increasing the life of the hinge.
Different types of plastics are used for different scenarios. Polyethylene and polypropylene resins are popular materials, as they have a low fatigue rate. This means that they can be bent many times without cracking or failing. Hinges that are unlikely to be flexed more than a few times are sometimes made from less fatigue-resistant materials, such as nylon resin.
Disposable packaging is the most common use for living hinges. Such hinges can be seen on the packages for items ranging from cleaning supplies to baby wipes to food stuffs. Carrying cases made of plastic, such as soap containers or pencil boxes, also frequently use living hinges. Another common use, perhaps less apparent to the general public, is in the manufacture of microelectromechanical systems such as the circuitry used in common electronic devices, including telephones, computers and televisions.
A living hinge capable of flexing several thousand times before failure is known as a fully elastic hinge. Fully plastic hinges can flex only a handful of times. Elastic plastic hinges, however, can flex hundreds of times or more.
Designing a living hinge that will last as long as needed requires careful calculation and a solid knowledge of physics. The angle to which the hinge must open, along with the elasticity of the plastic fibers used, are critical factors in hinge life. Design engineers must understand precisely how much stress will be placed on the hinge in order to determine the correct thickness and material for the need.
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