LIVING HINGE

Here are some guidelines to mold living hinges with polypropylene (other plastics are similar but may require their own specific dimensions dependent upon their particular viscoelastic behavior) ... 

Fig. 3-37 Example of an extrusion blow molded container with a living hinge.

Fig. 3-38 A molded living hinge in the as-molded position (a) and a flexed position (b).

Reduction of the number of product components, usually achieved by combining functions. Typical examples include box and cover assemblies joined with live hinges, the use of self-tapping screws to eliminate nuts, and staking of molded plastic bosses instead of metal rivets or screws. 

Do not use pigmented caps on natural HDPE bottles make caps and fitments on HDPE bottles compatible with HDPE (except living hinge applications). 

Hard grades of thermoplastic elastomers such as thermoplastic vulcanizates find applications as living hinges (useful in electronic products, toys, tools, and cases). 

Living Hinge, Coextruded hinge, Petro Plastics Llomar, Weatherable polyester film, Courtaulds... 

Tres, P.A. (1995) Living Hinges. In Designing Plastic Parts for Assembly, (ed. P.A. Tres), Hanser, Munich, p. 140. 

Keywords homopolymer, block copolymer, hinge, living hinge, plastic hinge, hinge geometry, process parameter, morphology, oriented layers, stacked lamellae, molecular orientation, hard elasticity, deformation mechanism, transmission electron microscopy (TEM). 

Properties of injection-molded parts can be unique. Given a metal or plastic s typical properties, injection molding imparts some of its own. Because of flow patterns, molecular orientation, and other factors, molded part properties are often different than those of the base material. An example of the benefits of orientation is the living hinge , the flexible plastic hinge foimd on spice, shampoo, and dental floss containers. Properties of injection-molded parts can be flexible or stiff, soft or hard, tough or brittle, clear or opaque, specific chemically resistant and flame retardant. Parts can be tinted, colored, plated with varions metals, and recycled. 

Standard of living hinges on the ability to educate a large number of sufficiently innovative engineers. 

The designer can use conventional plastics that are available in sheet form, in I-beams, or other forms, as is common with many other materials. Although diis approach with plastics has its place, the real advantage with plastics lies in the ability to process them to fit the design, particularly when it comes to complex shapes. Two or more parts— including mechanical and electrical connections, living hinges, colors, and snap fits— can be combined into one part (see Chapter 11). 

Figure 5-42. Using a combination of snap fit, living hinge, and mechanical fastener capabilities in molded polypropylene makes possible the storing and shipping of a large chassis grid in a flat form. For assembly it converts to a cagelike inner housing fiiat holds separate units, for cost savings.

Polyallomers mold as easily as PPs, yet they have better dynamic flexural fatigue resistance, as in living hinges, particularly at low temperatures. Their flow and moldability is better than that of linear PE and has a superior softening point as well as greater hardness, stress-crack resistance, and mold shrinkage. Polyallomers are superior to rubber-modified PPs in color, clarity, moldability, electrical properties, and blush resistance when bent or stretched. 

PP is widely known for its application in the integral living hinges that are used in all types of applications (see Chapter 11). PP s excellent fatigue resistance is utilized in molding these integral hinges. 

Some examples of a few of the thousands of successful living hinge applications are shown in Figure 11-96. Others include dual-flap cap closures, lifting cap tabs, diaphragm valve flex hinges, and so on. 

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