Today's KNOWLEDGE Share: Integral Hinge Material Characteristics

Today's KNOWLEDGE Share:

Integral Hinge Material Characteristics:
I have worked on a number of projects, both material selection and failure analysis, involving an integral hinge. An integral hinge, also known as a living hinge, is a thin flexible web that connects two relatively rigid wall sections. The living hinge is formed when partially oriented plastic is drawn cold and flexed for the first time. Because of its molecular structure and flexural fatigue properties, polypropylene is widely used in integral hinge applications. Well oriented polypropylene webs are considered to have virtually unlimited fold endurance, assuming appropriate part design, resin, and molding conditions are utilized.

When reviewing the commercially available polypropylene resins, it is helpful to consider which compositional characteristics maximize the material’s utility for a living hinge application.

Typically, fillers and reinforcements compounded into polypropylene will negatively impact the performance of the integral hinge. Fillers and reinforcements reduce the elongation at yield of the material, which accounts for this reduction in performance.

A superior molded integral hinge relies on the ability to freeze polymer orientation during molding prior to cold drawing. A number of polymer variables affect the amount of frozen orientation.

Molecular Weight: High molecular weight is desirable in a living hinge application. However, lower melt flow rate resins can be more difficult to mold properly.

Molecular Weight Distribution: A polypropylene resin with a broad molecular weight distribution results in superior integral hinge performance. Longer polymer chains cannot relax as easily as short chains.

Nucleation: Nucleation enhances hinge quality by accelerating the process of freezing the polypropylene orientation. However, it is important to note that if there is melt flow hesitation, nucleation will reduce the performance of the integral hinge. It is very important that the material, and molding parameters be balanced to produce an optimally functional integral hinge.

Polymer Type: Given their higher tensile strength, polypropylene homopolymers generally produce better integral hinges. Homopolymers are characterized by relatively high stiffness, even at elevated temperatures. However, they exhibit brittle behavior at temperatures below 5 °C (41 °F). Polypropylene random copolymers exhibit excellent transparency, and offer reduced stress whitening within the hinge. They generally exhibit cold temperature performance that is better than homopolymers. Polypropylene heterophasic copolymers (block copolymers or impact copolymers) are characterized by good impact properties down to temperatures below freezing.

Source:The Madison group
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