Manufacture and Design of Prosthetics

Abstract

Additive manufacturing AM is an emerging technology that offers the ability to create parts without moulds, material wastage or labour. This offers a wide range of industries to rapidly produce both prototypes and end use products with extraordinary design freedom and a high level of customisation. The widely used AM methods are limited in the working materials which are most commonly high cost, heavy metals or low cost and performance plastics. Options for the AM of composite materials are limited, yet they are the material of choice for high performance applications. Modern composite materials are used to manufacture light-weight structures with high strength to weight ratios. The manufacture of composites is largely dependent on skilled labour, customised moulds and expensive tooling which are all manufacturing complexities that additive manufacturing eliminates. The additive manufacture of long fibre thermoset composites is an opportunity which is currently a topic of research at the University of Auckland. There are many industries which seek to benefit from long fibre 3d printing but the decision on the market entry point for such a technology is a multi-variant problem. Market research indicated that the prosthetics manufacturing industry to be a suitable market entry point. The manufacturing complexities associated with composite manufacture are exaggerated in this industry due to the need to meet the unique requirements of each patient and the New Zealand industry has expressed a strong desire to adopt new technology. The purpose of the project described in this thesis is to acquire knowledge about the requirements of prosthetic design and manufacture for the application of composite 3d printing. Interviews and work shadowing with the New Zealand Artificial Limb Service NZALS was conducted in order to understand the process behind manufacturing prosthetics. Laminate modelling and analysis followed by experimental validation showed the fibreglass laminates used by the NZALS to have an ultimate tensile stress between 50MPa and 60MPa. The strength and stiffness of the NZALS socket laminates was used determined the target volume fraction for printing to be 30% carbon fibre. Finite element analysis was used to further investigate the performance requirements of prosthetic sockets by modelling their structural behaviour. Test methods prescribed by ISO 10328 were used to develop a modelling methodology to analyse the response of a socket under load and the performance of laminates used. The model was validated using experimental data with predicted strain within 20% of the experimentally measured strain in critical areas of the socket. This model can now be used as a tool to work with the NZALS to further refine their socket lamination methods. The mechanical properties of socket laminates can now be quantified and compared such that more informed decisions can be made to improve current socket lamination practice. This modelling procedure also enables new materials created with 3d printing to be evaluated against current material systems to provide prosthetists and amputees more confidence in emerging digital manufacturing methods.