engineering fundamentals Living Hinge
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Living Hinge
Living hinges are thin sections of plastic that connect two segments of a part to keep them together and allow the part to be opened and closed. Typically these are used in containers that are used in high volume applications such as toolboxes, fish tackle boxes, CD boxes etc.

The materials used to make a living hinge are usually a very flexible plastic such as polypropylene and polyethylene. These can flex more than a million cycles without failure.

Besides meeting the design guidelines, the hinges have to be processed properly. The molecules have to be oriented along the hinge line for the hinge to have acceptable life.

As molded the fibers of the plastic are somewhat random in orientation. In order to orient the fibers to aid in prolonging the hinge life, some or all of the following practices should be followed:

  • The gate location should be such as to allow the plastic to flow across the hinge for maximum strength.

  • As the part comes out of the mold, it needs to be flexed a minimum of 2 times while it is still hot, for optimum strength

  • Coining is often done to give the hinge, enhanced properties. The coining process compresses the hinge to a pre-determined thickness. The strain induced is greater than the yield stress of the plastic. This will plastically deform the hinge (i.e. place it outside the elastic range into the plastic range). The amount of coining (compression) should be less than the ultimate stress, to keep the hinge from fracturing.

  • The finished thickness after coining should be from 0.25 to 0.5 mm (0.010 to 0.020 inch). This keeps the stress in the outer fibers from exceeding the yield strength when being flexed.

    This process can also be done by heating the hinge or the coining tool to a temperature below the glass transition tempertature of the plastic. This allows for easier coining and somewhat enhanced properties, as the plastic "flow" easier when being heated.

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