Injection molding big parts and complex shapes with rigid PVC is probably the most demanding PVC process in terms of melt viscosities. PVC is a material with physical and chemical characteristics that make it process slightly more difficult in comparison with other thermoplastics. The high viscosity contributes to the development of a high amount of heat when the material is subjected to the shear applied by the screw during the plasticization. The thermal instability of the compound results from the breakdown either when it is heated at temperatures higher than admissible or when it remains for too long at high temperature. Hence evaluating the material behavior in terms of its thermal stability and melt viscosity is important for end-users of PVC compounds. This paper attempts to assist engineers who are faced with the task of screening compounds from various suppliers, prior to full-scale evaluation of the compound in production machines.