PLASTICS DESIGN CRITERIA

For these reasons design is spoken of as having to be appropriate to the materials of its construction, its methods of manufacture, and the stresses involved. Where all these aspects can be closely interwoven, plastics are able to solve design problems efficiently in ways that are economically advantageous. 

The equations and calculations presented here are based on the development work of many people over the centuries in simplifying the testing and evaluation of all types of materials. Realistic approaches are thus available for comparing different materials and providing practical and useful guidelines for designing a whole range of products. 

Materials selection depends on a wide variety and range of factors, including cost, technical suitability, safety, energy requirements in production and service, quality, the ability to be manufactured in the required quantities and to be satisfactorily assembled and finished, the influence of the service environment, recycling and waste recoverability, its estimated lifetime, and many other factors, which are often interrelated. All these elements must be considered before deciding that the use of a particular material makes good sense. 

Caution in using such nonmetallic materials as plastics, composites, wood, and glass is proper in view of the penalties that may be incurred if parts fail in service. However, 

Obviously, the lower densities of plastics allow them to be used in thicker sections than metals, which can have a significant influence on panel stiffness and strength. For example, assume that the four panels have equal weights and therefore different thicknesses (r). When the panels are loaded in flexure, their stiffnesses depend on Et ) and their strength on af) where E and a are the material s modulus and its strength. For panels of equal weight it follows that their relative stiffnesses are governed by Els ) and 

---

PLASTICS DESIGN CRITERIA 161 Table 3-4. Rate of Strength Decay for Wood and Thermoplastics per Decade of Time Under Sustained Stress ... 

---