A study of decompression after cold isostatic compaction of ceramics
There are a number of possible mechanisms which may be responsible for the development of cracks in isostatically pressed components including the progressive detachment of the elastomeric bag which results in the development of tensile stresses within the compact, and through the development of high internal air pressure. The investigation of these issues is exceptionally complex to perform experimentally. Therefore the approach taken within this thesis has been to develop simulations which model as fully as possible the pressing process, both through the compaction phase and the decompression process. The simulations described in the thesis, indicate that the elastomer bag will detach progressively with the interfacial condition between the powder and the pressing tool, the property of the elastomeric bag and the property of the mandrel having a significant impact on the tensile stresses developed within the component. These simulations indicate that low interfacial friction between the powder and the bag and a compliant elastomeric bag are likely to reduce the probability of tensile stresses developed within the compact. The simulations performed also examine the role of air within the compaction process, and demonstrate that due to the closed nature of the isostatic pressing system there is little which can be modified to significantly alter the tensile stresses developed as a result of air pressure within the compact. Some tactics, such as holding periods, can help alleviate the likelihood of cracking in some components, but this approach is not guaranteed to prevent cracking in components and can in some cases increase the probability of cracking.