Heat transfer in rubber is likely to be significant from the first processing steps to the end use of the product. At the start, it is encountered in preparing, drying, and handling raw polymers; then in mixing and forming or molding compounds. In this stage, the thermoplastic nature of rubber is such that viscosity and other physical characteristics are especially temperature sensitive. Heat is the most important processing agent. The rate at which heat can get into and out of rubber, how expertly it can be transferred, affects the design of processing machinery and controls the speed of many mixing, extruding, and molding operations. In these processes, primary, turbulent heat transfer between rubber and steel and mass transfer in the rubber are complicated by frictional heat generation at the rubber-metal surfaces and large conversion of mechanical energy into internal heat. Convection heat transfer with air, steam, or fluids also usually occurs in processing rubber. Localized overheating at any stage can be disastrous for quality. Hence the basic heat problems in processing usually lie in rapidly and efficiently securing, and then maintaining, satisfactory temperature uniformity in a material with inherently poor heat transfer characteristics. Dispersing fillers in rubber requires large power inputs but the resulting temperature rise must not be so great as to cause prevulcanization or reduce unduly the efficiency of the mixing operation by too great reduction in plasticity and hence shearing stresses. On the other hand, Hahn has pointed out how advantageous such mechanical heating of rubber may be. A few minutes working on a mill may accomplish more in raising the temperature than hours of conduction heat flow. After rubber is mixed and formed, vulcanization ensues with flow of heat to raise the temperature throughout to the vulcanization range and thus activate the chemical reactions of vulcanization. At this stage, control of heat is exceedingly important for costs and quality. Any shortening of the vulcanizing cycle without detriment to quality provides opportunity to increase productivity of a large capital investiment. Finally, heat is one of the most destructive agents for finished rubber products. It presents a frontier for development of new rubbers and applications. External environmental heat imposes service limitations and in dynamic uses involving repeated, rapid deformations internal transformation of mechanical energy into heat may readily destroy thick rubber sections.