Prediction of part performance and processing requiresmodels that characterize the response of plastics to thermal, mechanical, and electromagnetic stimuli. In mechanics, or engineering science, the relationships between imposed stimuli and the resulting response are called constitutive equations. Because of the complex response of plastics – both in the solid and liquid phases – constitutive equations for plastics are more complex than those for metals and ceramics. Simple models for the mechanical behavior of plastics are discussed in Chapter 4.