Hinge Design in plastic injection molding

A hinge is defined as a joined or flexible device on which a door, lid, or other swinging part turns. Two basic of hinges are used in plastic injected parts design, molded-in hinges and formed hinges. Molded hinges can be used for some, but not all, thermoplastic, such as PP or POM. Where hinges are needed thermoplastic materials that have excellent toughness, ductility and fatigue resistance, such as polyethylene (PE), polypropylene (PP), nylon 6, and nylon 66, Polyformaldehyde (POM) are used primarily. The molded-in hinge is used more widely since it can be molded by the standard injection molding process, as Fig 1.

Fig 1 Molded-in hinges

One of polymer property of molecular orientation greatly affects the durability and long-term performance of a hinge. Molecular orientation which transverse to center axis of a hinge will withstand many flexing cycles as show in Fig 2.

Fig 2 Hinge design

The design of the plastic hinge need to pay attention to several points. Radius of curvature part of the hinge using thin-wall as possible, easy to bend and overlap. Hinge section shape should be symmetrical. When the hinge turn, hinged parts of space should be set aside, which increases the size of hinge parts. Material in the molding process should be conducive to the formation and folding direction vertical orientation, which can improve the fatigue strength of plastic hinge. Mould design of gate location should be chosen not far away from the hinge and on the hinge the orientation in the injection molding process of location.

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