Injection Moulding of Long-glass-fibre-reinforced Poly(ethylene Terephthalate): Influence of Processing Conditions on Flexural and Impact Strengths

Abstract

This paper aims to identify the main parameters that influence the mechanical properties of long-glass-fibre-reinforced poly(ethylene terephtalate) parts in order to optimise the potential of these composites. A Taguchi design of experiments (DOE) was used for this purpose. The shape of the injection-moulded specimen was representative of the complexity of industrial parts (presence of sharp frontal and tangential steps). The mechanical properties (bending and Charpy impact tests) were measured at different locations in the part in order to highlight anisotropy. Variance analysis has shown that holding pressure, injection speed, and mould temperature are the most effective processing parameters. It has also shown that the optimised parameter sets that lead to the highest flexural strength on the one hand and the highest impact strength on the other hand are different, the polymer melt and mould temperatures being opposite. Finally, microstructure analysis has shown a fibre content gradient along the flow axis, which is greatly amplified by the presence of geometrical accidents (frontal and tangential steps) including the reorientation of fibres in the flow direction at the end of the parts.

Injection moulding of long glass fibre reinforced poly(ethylene terephtalate): Influence of carbon black and nucleating agents on impact properties

“…This decrease is about 50% for flow direction (FD) samples, and about 35% for transverse direction (TD) samples. As there is a fibre accumulation in the core layer due to fibre segregation phenomena reported previously [11,17], the effect of the matrix brittleness is higher for the flow direction samples (most of the fibres are aligned in the impact direction (Figure 3a)). …”

“…Lower filling time (0.2 s measured in front of each pressure sensors) was recorded for the black coloured compound R1, that is to say lower pressure loss, suggesting higher fluidity. As a direct consequence, this viscosity diminution decreases the shear stresses in the melt flow and thus the previously noticed [11,17] fibre segregation occurring during cavity filling. Thanks to its good thermal conductivity, carbon black induces a quicker cooling of the polymer in the cavity (Figure 8), thus shorter freezing times that decrease the holding phase efficiency.…”

“…In this context, as long glass fibre/PET compounds were rarely studied, the optimisation of long glass fibre/poly(ethylene terephtalate) injection moulding still needs dedicated research efforts. The effect of injection moulding conditions and the existence of fibre segregation phenomena along the flow path and through the thickness were reported in a previous paper [17]. In the present paper, the influence of different additives (carbon black and nucleating agents) on the Charpy impact properties of long glass fibre/PET injection-mouldings is investigated.…”

“…A Taguchi Design of Experiments (DOE) [34] was used in a previous study in order to identify the main process parameters responsible for anisotropy and heterogeneity of the moulded part, and then to optimise the injection moulding process [17]. The moulding conditions resulting from this DOE used to manufacture the parts are summarised in Table 2.…”

“…low mould temperature and low injection speed) lead to impact strength increase [22]. Cilleruelo et al [17] confirmed that it is possible to optimise the impact strength of 30 wt% long fibre reinforced poly(ethylene terephtalate) (PET) by properly setting the two above-mentioned main influencing parameters, however at opposite setting levels compared to those recommended to optimise flexural properties [11,15]. Beyond the factors mentioned previously, the incorporation of additives in the thermoplastic matrices (pigments/dyes, nucleating agents, etc.)…”