Material and process problems in the manufacture of coolant reservoir tanks

The programme was concerned with the effects of processing variables on the structure and properties of coolant reservoir tanks. The work was concentrated on the extrusion blow moulding process and on propylene polymers, following an earlier similar research in the Institute on high density polyethylene. We have worked with a company which specialises in technical blow mouldings; the work has dealt with coolant tanks which are required for high temperature, high pressure service in the presence of antifreeze. It soon became apparent that the application made severe demands on polypropylene in that failure was possible by a variety of mechanisms, including environmental stress cracking and ductile blow-out failure. Additionally, the shaping process by extrusion blow moulding was not being carried out in the optimum way, either with respect to process economics or to product properties. Investigation of the surface texture of corn mercial tanks revealed that shrinkage from the mould occurred frequently due to inadequate holdon pressure; this loss of contact gives poor surface finish and retarded cooling. This phase of the research was supported by a programme carried out on the Bradford University equipment and by providing appropriate thermal data for the Bradford mathematical model for cooling. To reduce the cooling cycle time, the efficacy of internal cooling techniques, including forced air cooling and liquid carbon dioxide, was studied. Further, the possibility of shaping polypropylene in the supercooled region was investigated and a double extruder system was developed to examine the shear viscosity of supercooled polymers. The main concern has been failure caused by stress cracking and environmental stress cracking, especially of development grades of propylene polymers. The structures of various ethylene-modified polypropylenes have been elucidated, as have the relationship of structure' to processing history and its relevance to the severe stress cracking encountered. A new method for determining ethylene content of ethylene- propylene systems has been developed, as this is one of the important characteristics of ethylene modified polypropylene. The available processes of manufacturing the coolant reservoir tanks have been considered; in particular, competing methods of manufacture of extrusion blow moulding and injection moulding/welding have been compared. The design of coolant tanks, in particular the wall thickness required, has been examined. Stress analysis, supported by tensile failure and relaxation data, has been carried out to provide background to the failures encountered in practice. Finally, possible new candidate material for the coolant tank application, polypropylene-linear low density polyethylene (ethylene-octene copolymer) blends have been investigated.