The general design of injection-molding heating chambers has become established after gradual development through several years of experience. Some of the details of design which are directly concerned with the plastic-material characteristics have been examined to a limited extent. Very little engineering-design information has been available for a careful consideration of these details. Heat-transfer and pressure losses in the heating chamber are interrelated and the geometry of the passages through which the plastic moves can be arranged for optimum heat-transfer rate with minimum pressure loss. The available theoretical, experimental, and practical information concerning the behavior of plastic materials, under high pressures, in granular and fluid conditions is considered. This information is combined into a systematic approach to the geometrical problems of the design of injection-molding heating chambers.