Effect of the interdiffusion at the polymer/polymer interface on the flexural properties of over-moulded short glass fibre/glass fabric reinforced PA6 composites

The injection over-moulding of 30wt% short glass fibre reinforced PA6 (SGF from Solvay Engineering Plastics) onto consolidated unbalanced (87/13) 70 wt% glass fabric reinforced PA6 (Continuous Fibre Reinforced Thermoplastic (CFRT) from Solvay Engineering Plastics) was investigated with an objective of optimising the flexural and interlaminar shearing of the consolidated structure. Among the processing parameters, the temperature of the fabric before injection and the over-moulded melt temperature associated to the mould temperature (cooling rate of the part) were revealed as the main parameters governing the mechanical properties of the part. Moreover, the flexural modulus and the apparent interlaminar shear strength fall down critically in the main direction (chain direction of the fabric) at a CFRT temperature of 150 degrees C. The effect of the SGF/CFRT interface was quantified in term of quadratic distance of diffusion through the interface. First, the 1D cooling of the complex was simulated according to the heat transfer module of COMSOL Multiphysics (R) in order to determinate the variation of the temperature field during the cooling stage. The calculations were achieved with an initial CFRT temperature of 23, 100, 150 and 200 degrees C, the mould and SGF melt temperatures were kept constant. The diffusion theory has then been applied to calculate the variation of the auto-diffusion coefficient through the thickness during the complex cooling, the diffusion is supposed occurring only at a temperature above the PA6 crystallisation temperature (185 degrees C). The calculation of the quadratic distance of diffusion through the thickness confirmed the mechanical results. Under a CFRT temperature of 150 degrees C, the ability to the molecular diffusion at the interface becomes non-existent. The melt temperature of the SGF PA6 should be sufficient to melt the CFRT PA6 interface, the time of diffusion directed by both the CFRT and mould temperatures (cooling rate) has to be long enough to allow the molecular diffusion from one material to the other.