Abstract
In recent years, polymeric materials are being used at an increasing rate in the biomedical industry. In particular, Ultra-High-Molecular Weight Polyethylene (UHMWPE), a thermoplastic polymer characterized by high toughness, good chemical stability and self-lubricating properties, is an ideal candidate for the manufacture of bearing implants used in hip or knee replacements. Nevertheless, it is difficult to achieve a good level of surface finish when turning it, because of its high instability at increasing temperature. In the present study, cryogenic machining was applied instead of dry cutting to machine a biomedical grade UHMWPE at different cutting speeds. The surface finish was assessed in terms of surface roughness, crystallinity degree and hardness in correspondence of the surface. To correlate machinability results with the UHMWPE mechanical behaviour, uniaxial tensile tests were performed in a wide range of temperatures. The obtained results showed that the application of cryogenic machining was an efficient mean to increase the surface finish: in fact, smoother and harder surfaces were obtained regardless of the adopted cutting parameters.
Highlights
Machining of polymers becomes mandatory when the production volumes do not justify the cost of injection molds or extrusion dies, or when high dimensional accuracy of the part is strictly required [1]
The only study about cryogenic cooling of Ultra-High- Molecular Weight Polyethylene (UHMWPE) [2] was about the effect of the workpiece pre-cooling on the cutting forces, chip formation and surface quality during machining: even if lower cutting temperatures favored the formation of smoother surfaces, such improvements were of lower impact compared to those provided by the tool rake angle variation and tool wear evolution
Even if turning within the rubbery region may lead to unsatisfactory surface finish, as a consequence of the material too high ductility [11], cryogenic cooling helped in preserving the material mechanical strength, assuring better machining performances
Summary
Machining of polymers becomes mandatory when the production volumes do not justify the cost of injection molds or extrusion dies, or when high dimensional accuracy of the part is strictly required [1]. Cryogenic cooling can represent a strategy to increase the UHMWPE machinability, since it helps in preserving the polymer mechanical strength during cutting, besides being an environmentally friendly medium. On the basis of this literature review, cryogenic cooling appears to be an attractive alternative to dry cutting, but its effect on surface integrity must to be further evaluated, especially when applied to UHMWPE. The present work aims at investigating the role of cryogenic cooling in affecting the surface integrity of UHMWPE when machined for biomedical applications. To this purpose, turning tests were carried out at three different levels of cutting speed, namely 200 m/min, 300 m/min and 400 m/min. Since the available experimental equipment did not allow cooling below -100 °C, the DSC analysis could not provide information about the UHMWPE glass transition temperature (Tg), whose value was assumed from literature equal to -110 °C [7]
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