New screw design for cooling extruders

Heat transfer is a critical issue in most polymer extrusion operations. In plasticating extrusion, the objective is to add the right amount of heat to melt the polymer and to achieve the desired melt temperature. In some extrusion operations, however, the objective is to remove heat from the polymer. This is the case in tandem foam extrusion lines where the secondary extruder is used to cool down the mixture of polymer melt and blowing agent. Cooling extruders reduce the polymer melt temperature by a substantial amount, about 100 °C, to achieve a melt consistency that is conducive for foaming. Little information has been published about the design of cooling screws in the open literature. The present study discusses basic issues related to the design of cooling screws and presents a new screw geometry to achieve improved cooling. As the foam extrusion industry faces pressure to move from HCFC (hydrogenated chlorofluorocarbon) blowing agents to nitrogen and carbon dioxide (CO₂), the cooling capacity becomes more critical. CO₂ is less of a viscosity depressant than most HCFC blowing agents. As a result, with CO₂ more viscous heating occurs in the cooling extruder and more effective cooling is required to achieve the same reduction in melt temperature.