Issue 48

T. Febra et alii, Frattura ed Integrità Strutturale, 48 (2019) 242-248; DOI: 10.3221/IGF-ESIS.48.25 243 Traditional overmolding is a two-stage sequential process in which a previously injected rigid polymer substrate is overmolded with a more flexible thermoplastic material. The final component is a single piece composed by two polymers with very different mechanical properties. The adhesion between the two materials is the key for a successful practice use, by which the component must be safe and the interface remain permanently bonded. Nowadays, polypropylene (PP) is a widely used material for structural purposes because of its mechanical properties, reasonable cost, easy processing and recyclability. Optimization procedures for overmolding parameters have been developed and published [1-4]. Recently, fibre reinforcement mats have been used as the substrate material for some applications where final stiffness can play important role. The overmolding process requires the merging of several characteristics to increase its functionality, including combinations of mechanical strength and touch feeling or grip ability, cushioning against vibration or impact, or esthetical factors like gloss. The rigid substrate provides the basic mechanical properties for structural purposes while the soft polymer cover adds the desired user comfort and product functionalities. The increasing use of overmolding components can be seen in applications such as automotive interiors, medical devices, telephone keypads, toothbrushes, shaving hardware, household appliances and hand tools [5–7]. Nowadays, polypropylene (PP) is replacing traditional engineering thermoplastics for structural purposes in automotive applications, due to its mechanical properties, reasonable cost, easy processing and recyclability. A low velocity impact event is one of the most dangerous loads on composite laminates, giving rise to different types of damages, including matrix cracking, fibre fracture, fibre-matrix debonding and delamination between different layers [8]. Internal delamination induces premature buckling of the structures with the consequent drop of compressive strength [9, 10] and also in less relevant of the tensile strength [11]. The response of composites subjected to single-impact loading is abundantly reported on scientific literature, also repaired composites [12, 13]. Andrew et al. [13] evaluated the residual compression after low-velocity impacts on GFRP composites repaired by Kevlar chopped short fibres/epoxy and found higher maximum impact load and lower contact time for repaired specimens relatively to the unrepaired ones. However, scarce information can be found about the performance of composites under repeated impacts. This subject was studied by Morais et al [14], concluding that stacking sequences and laminate thickness have a major influence on the composite’s response under repeated impacts. The structural performance under repeated impacts was studied by Cholakara et al [15] on Kevlar-fibre/epoxy composites, by Mouritz et al [16] and by Hosur et al [17] on glass reinforced laminates and by Wyrick and Adams [18] on the carbon/epoxy laminates. The parameters studied were the fibre mat type and the impact energy. The impact response was monitored in terms of the maximum load and displacement, dynamic impact modulus, absorbed and recovered energies and damage area. In addition to the study of the influence of the aforementioned parameters regarding simple impact response, this work also aims to compare the response of the material to simple impact and multi-impact with increasing energies. Reference Matrix Insert Mould Temp. (ºC) Processing Temp. (ºC) Injection pressure (Bar) MGFm Talc-filled PP Multiaxial glass fiber mesh 22 220 41 BGFn Talc-filled PP Biaxial glass fiber netting 22 220 41 Km Talc-filled PP Kevlar mat 22 220 41 Table 1 : Formulation and manufacture parameters of the composites. M ATERIALS AND TESTING he present work studied impact response using square 100x100 mm2 and 3mm nominal thickness specimens. The overmolding matrix was polypropylene (PP) filled by 20% in weight of talk, Hostacom TRC 352N Titan supplied by Yonde Basel. Three configurations with different insert fibre mats were manufactured, as is summarized in Table 1. In turn, the glass fibre inserts used were fiberglass multiaxial mesh fabric and biaxial fiberglass reinforcing mesh, supplied by HUESKER. The third insert was an Aramid Kevlar 49 mesh, supplied by Toray. The injection moulding process was carried out using a Sandreto 200 Ton machine, using the parameters indicated in Table 1. Fig. 1 shows the insert fibre mat placed in the mould.