Sheet forming of flax reinforced polypropylene composites using vacuum assisted oven consolidation (VAOC)

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Fibre reinforced plastics are widely used in different industry sectors since the past half century. Considerable research has been carried out on natural fibres as reinforcements because of their light weight, good mechanical properties, biodegradability, sustainability and higher stiffness per unit mass compared to glass fibres. This paper develops a knowledge base for the manufacture of vacuum assisted fibre reinforced thermoplastic composites inside a simple oven from sheets of the fibre and the thermoplastic. The compaction behaviour of the reinforcements used was investigated and used to target a range of achievable fibre volume fractions to manufacture the composites. The reinforcements were also characterized on the basis of their transverse permeability to quantify the ease of polymer flow through the reinforcements. An experimental setup was designed to measure the relevant processing parameters during the experiment. Thickness, pressure and temperature of the composites were monitored with applied vacuum pressure and oven temperature during the experiments. The mould temperature inside the oven was first increased to the melting temperature of the thermoplastic, held at that temperature for a certain time period and then cooled subsequently. The application of compaction pressure and the holding time was varied to study its effect on consolidation quality. The achieved fibre volume fraction of the manufactured composites was calculated by first using the average thickness of the composites and then using the density of the composite. The cross-section of the laminates was examined under an optimal microscope to evaluate the quality of consolidation. Tensile and short beam shear tests were performed to assess structural integrity, strength and stiffness of the composites. The results show that this manufacturing technique can produce composites at expected fibre volume fractions under vacuum. The cooling rate, applied temperature, pressure and sheet layup were found to have considerable effects on the fibre volume fraction. It was also found that the mechanical properties of natural fibre reinforced thermoplastic composites (NFRTS) do not change appreciably even after modifying the process so as to reduce the manufacturing time.