An influence of nanofiller size on the joining strength of injection overmolded component by insert of nanofiller-filled thermoplastics films at interface

Injection overmolding process is promising process for fabrication of thermoplastic composites which has excellent mechanical properties and complex shapes. Concretely, the continuous fiber-reinforced thermoplastic composites are fabricated by stamping process as substrate. Followed by the melt polymer is overmolded onto the substrate by injection molding for fabrication the ribs and bosses. Therefore, in this process, joining strength between two materials is a major factor in the mechanical strength of a product. As the new approach to improve the joining strength, nanofiber-filled thermoplastic film is inserted between the substrate and overmolded polymer for reinforcing the interface directly in nanoscale. In our previous works, we revealed that multi-walled carbon nanotubes (MWCNTs) filled polypropylene (PP) films could improve the interlaminar shear strength. However, it is still unclear cause why interlaminar shear strength increased by adding the films. In this study, to clarify the influence of nanofiller size on the adhesion strength, two kinds of nanofillers, CNTs and vapor grown carbon fibers (VGCFs) were used. As a quantitative evaluation method of adhesive strength, a tensile overlay shear strength test was performed. As a result, it improved the joining strength at CNT contents of 0.5wt% of CNTs film the best. CT scan images confirmed that CNT tangled with polymer chains. This indicated that entanglement between CNTs and injection resin has effect on the adhesion. By diffraction scanning calorimetry (DSC) data, the crystallinity of nanofiller-filled thermoplastic film was lower than PP film and it was considered that addition of nanofiller distracted crystallization PP. Therefore, although the crystallinity of substrate surface decreased by adding nanofibers, CNT contents of 0.5wt% was the best contents for joining strength. Consequently, the relationship between crystallinity and joining strength requires detailed investigation, such as focusing on crystallization behavior during cooling.