Plastic Molding Review and Mechanical Design Guidelines

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Designing With Plastic Design Considerations

If the product consists of more than one component, determine if the components can be combined into a single molded part, eliminating extra materials, molds, molding operations, and assembly procedures? Can integrating them into the molded part eliminate hardware or other components? Caution should be exercised in that some cases, trying to design multiple features into a single component can result in a complex part that results in the tooling to be more expensive or the part too difficult or unreliable to mold. This is especially true when the mold must open in many directions to eject the molded part.

Selecting Material

There are a wide variety of injection moldable thermoplastic materials. Often, material the used is selected based on experience one that the molder or material supplier recommends. Unfortunately, proper material selection can be a complex process. The plastic designer should consider the following criteria.

Operating Temperature Requirements

Determine the normal and extreme operating temperature requirements of the component, which you are designing. The plastic, which, you select, should have the capability to withstand the normal, as well as extreme, operating temperature required of the product. Be sure to check shipping temperatures, sources of internal heat, and any assembly or finishing operations that may expose the component to extreme temperatures. Mechanical and electrical properties usually depend on temperature.

Stress Levels

Consider the operating load and stress levels that, the component will be subject to. The operating stress is a significant consideration in many applications. Elevated stresses on a component tend to decrease the components resistance to the operating conditions.  Stresses incurred during snap fits, molding operations, forming or assembly

Operating Environment

Every substance, solid, liquid, or gas that can come in contact with the plastic part over its expected life span should be considered for chemical compatibility. If the compatibility information is not obvious from the product literature, the designer should consult the material supplier and/or request or conduct specific testing if necessary. Some plastics are highly susceptible to erosion, or similar damage when exposed to particular substances.


Check to ensure that the proposed material lends itself to the expected assembly operations. For example, certain classes of materials are difficult to solvent bond, while others may not work well with ultrasonic methods or thermal processes


Determine whether plastic resin can be easily molded into the proposed product with the desired final appearance. If not, can the part be easily and economically finished?


To determine the economics of using a particular resin, consider the cost per pound in the applicable color and purchase volume. In addition, look at the specific gravity and typical molding cycle times of the particular resin grade, since these affect the final molded part cost. The cost per unit volume of a particular plastic resin (or any material) is as follows:

Cost/in.3 =0.0361 x Specific Gravity x Resin Cost/Pound

The material cost of a part is obtained by multiplying the cost/cubic inch by the part volume. Finally, a very rough part cost estimate is obtained by doubling the material cost of the part.


Check to see that the chosen resin will be readily available in sufficient quantities when needed for production.

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