Orthotic insoles are used for numerous applications; they can be prescribed to treat medical conditions such as rheumatoid arthritis and to maintain the health of the feet of diabetic patients. Orthotic devices are also extensively used in sporting activities and can be used for improving skeletal function, thus enhancing the biomechanical performance of the user and subsequently providing a more economical gait. This paper focuses on the manufacture of sports insoles and provides a methodology for the design and manufacture of a personalised symptom-specific sports (3S) insole. The framework includes the biomechanical assessment methods required for the effective prescription of a personalised insole. The requirements of a functional insole should relate not only to the geometry and condition of the foot but also the application in which it will be used. Different sports are played using specialised footwear, on varying surfaces and using diverse movements and so require an alternative design with regards to the geometry and materials used. Thus novel manufacturing methods are required and two examples are described, namely the cryogenic machining of soft foamed polymers to achieve suitable impact attenuation and the autoclaving of a carbon fibre composite material to produce a slim, rigid design.
The traditional method for producing polymer-based products is the use of moulding technologies such as injection moulding. CNC machining methods are predominantly used for metal part manufacture. The use of CNC machining methods for direct machining of polymers has been discussed in previous studies, particularly for hard polymers such as polypropylene. The CNC machining of soft elastomers, such as ethylene vinyl acetate (EVA), even at significantly reduced temperatures presents a number of challenges, with one being the formation of a machining phenomenon termed adiabatic shear bands, which can lead to increased part surface roughness and reduced part quality. The adiabatic shear band is an area on a chip where the ductile properties of the material being machined have been exceeded and the heat generated does not have sufficient time to be removed. This can lead to permanent material damage resulting in reduced fatigue resistance. The adiabatic shear formation has the potential to be even more evident with the machining of elastomers, leading to rapid material degradation, poor surface finish characteristics, and reduced material attenuation. In this paper the state of the art in cryogenic manufacturing is described and the concept of cryogenic CNC machining of elastomers is discussed. The experimental work consists of machining EVA and Neoprene utilizing the described cryogenic CNC machining facility. Sample chips are taken from the experimental testing and analysed using a scanning electron microscope to illustrate the adiabatic shear band formation in some cases. It is observed that in order to reduce this effect, correct machining parameters corresponding to the elasticity values need to be used and the glass transition temperature needs to be maintained.
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