Designing With Plastics

Some degree of compromise is almost always necessary in designing plastic parts. Arriving at the best compromise usually requires satisfying the mechanical, thermal, and electrical requirements of the part, utilizing the most economical resin or compound that will perform satisfactorily and be attractive, and choosing a manufacturing process compatible with the part design and material choice. Setting realistic requirements for each of these areas is of utmost importance. Probably no plastic will provide 100% of the requirements for the desired performance, appearance, processibility, and price. Selecting the best qualified material is not based simply on comparing numbers on published data sheets; such values can be grossly misleading. For example, choosing the most economical material for a part by comparing the cost per pound of various plastics is a mistake. Some plastics weigh twice as much per cubic inch as others and so would require twice as much to fill a given cavity and cost twice as much to ship. A more meaningful comparison is cost per cubic inch. But since most expensive plastics are far stronger than the cheaper ones, cost/strength values should be analyzed as well. Paying more per pound or per cubic inch is often more economical if less material can do the job. There is an attractive simplicity in deciding on the material with the highest ASTM test values as listed in manufacturers' data sheets. Unfortunately, this method seldom results in the best choice. First, the choice of any material should be based on the best combination of properties, not only on one property. An ideal material will have a value for each strength property just sufficient to perform properly and safely in a given application and no higher. The best material choice usually represents a trade-off among satisfactory properties, ease of processing, and cost. It is seldom