Plastic product design filled material molding

Molded plastics parts in many ways seem analogous to castings since the mold can be any shape. There are a number of limitations on the part because of the fact that the mold is filled with plastics material and it represents, in addition to the form for the product, the flow path for the plastics material. One factor that is affected is the wall thickness of the part. The viscous melts produced by many plastics require that a minimum wall thickness be used based on the ability to fill the mold rather than on the function of the part. For example, rarely is any injection molded part of any size designed with a wall thickness less than 0.025 to 0.030 inch. This is particularly true if the part has a fairly large surface area because the plastics will not flow to properly fill the part. In addition, as will be seen in Chapter 10, flow conditions can severely affect the properties of the material. 

ISO 3167 specifically deals with the production of multipurpose test specimens. It requires the use of a balanced, two-cavity mold with a defined gate design to prepare test specimens used to measure properties of plastics materials. Two identical cavities with large gates and a balanced runner system promotes filling and packing consistently and nniform orientation, minimizes shear effects, and produces uniform specimen. Figure 20-2 illustrates differences between old style, multicavity, unbalanced family mold and ISO test specimens (6). 

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