In the area of beverage packaging, the barrier properties (to gases, water, organic vapors/flavors) are parameters of great importance. For poly(ethylene terephtalate) (PET), largely used in bottling, the improvement of these barrier properties upon biaxial orientation is a direct result of both the level of crystallinity as well as the orientation of the material. However, even with higher crystallinity and orientation for some applications, the PET barrier properties are not adequately efficient and thus multilayer polymer materials must be used to make the bottles. During plastic-forming processes, the semifinished multilayer thermoplastic products (such as a preform) must be heated to a rubberlike state. Experiments on PET reveal that temperatures either below or close to the glass transition (T-g) are essential to achieve principally oriented glassy structure and that the crystalline structure could be enhanced as well by thermal or high-drawing treatments. Because of the great differences in the stretching ratio inside the preform and the thermomechanical behavior of the materials, a good stretching process requires that the material have a nonuniform temperature profile. A model is developed to calculate temperature distribution in multilayer preforms. This model is a useful tool for understanding the heating stage of thermoplastic products and is suitable to control and optimize industrial processes.