Browsing by Author "Mahmoodi, Mehdi"

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    Electrical, Thermal, and Machining Behaviour of Injection Moulded Polymeric CNT Nanocomposites
    (2013-10-02) Mahmoodi, Mehdi; Park, Simon
    Carbon nanotubes (CNTs) are promising additives for thermoplastics, resulting from their superior electrical, thermal and mechanical properties. Due to the desirable properties of CNT/polymer composites and their wide application in technological devices, these materials have attracted a great deal of attention from both academia and industry. A considerable amount of research has been devoted to the processing of CNT-filled nanocomposites, but only a few investigations have focussed on the injection moulding of these nanocomposites. This research was aimed at the study of the flow-induced alignment of CNT/polymer nanocomposites through the injection moulding process. We focussed on the understanding of the alignment of multi-walled carbon nanotubes (MWCNTs) in a thermoplastic matrix and the investigation of the alignment’s effect on the electrical, thermal and machining characteristics of the injection moulded nanocomposites. The nanocomposites were first prepared with a melt mixing technique (i.e. twin screw extrusion), and they were then injection moulded under various processing conditions and mould geometries. High aspect ratio nanotubes could be partially aligned in the parallel-to-flow direction, resulting from the in-plane shear flow exerted on the polymeric melt in the injection cavity. It can be concluded that the volume resistivity of the moulded samples could be varied up to 7 orders of magnitude by changing the processing conditions and gate type in the injection moulding process. The electromagnetic interference shielding effectiveness (EMI SE) of the moulded composites was studied by considering the alignment of the MWCNTs. The EMI SE decreased with an increase in the alignment of the injection moulded MWCNTs in the polymer matrix. Anisotropic thermal conductivity was observed for the injection moulded nanocomposites. It was shown that thermal conductivity can be enhanced by aligning the nanotubes in the parallel-to-flow direction. The post-processing of injection moulded CNT nanocomposites, such as their machinability, plays an important role in their economic viability. The addition of CNTs to polymers can improve the machinability of the polymers, mostly due to the high thermal conductivity of these nano particulates. However, there are some challenges in the machining of CNT-filled nanocomposites, such as the absence of comprehensive knowledge of the material properties, especially at the high strain rates encountered in machining. In this study, the micro mechanical machining of injection moulded MWCNT-filled nanocomposites was examined, and some of their machining characteristics were compared with those of pristine polymers. In addition, the effect of the nanotube alignment on the cutting forces was experimentally investigated, and a mechanistic micro-milling force model was used to predict the cutting forces. It was found that the CNT alignment and concentrations influence the micro cutting forces, and the force model was verified with the experimental milling forces. The machinability of the CNT nanocomposites was improved over that of pure polymer, due to the enhanced thermal conductivity and mechanical characteristics. In addition, better surface quality and dimensional stability of the machined slots was observed for CNT-based nanocomposites. From the results obtained in this study, one can determine the optimal processing conditions for the injection moulding of CNT-filled thermoplastics. It can be concluded that the electrical and thermal conductivities of these nanocomposites strongly depend on the alignment of the nanotubes within a polymer matrix, which can be controlled by selecting proper processing conditions and mould geometry. These parameters can be selected based on the desired application of the product. In addition, this research work provides a fundamental understanding of the material removal behaviour of CNT-filled composites, which is very important in the design of these novel materials for specific engineering applications.