The Morphology, Structure And Properties Of The Parts Molded By Gas-assisted Injection Molding

Gas-assisted injection molding (GAIM), an innovative molding process, has been recently developed on the basis of conventional injection molding (CIM). Such technology has been widely applied in practical production. However, with respect to the substantial problems involved in this process, most literatures mainly focused on the mathematic simulation of gas penetration, the effect of the gas channel design on the mechanical properties and so on. In view of this, in this article, through mathematic simulation, the similarity and difference between the GAIM and CIM process are expatiated. At the same time, more attentation is paid for the morphology, structure and formation mechanism of the neat semi-crystalline polymer and its blend, glass fiber reinforced polymer molded by GAIM. Such studies would provide theoretical and experimential guideline for improvement of mechanical properties by controlling the morphology and structure during GAIM process. The main results are as follows:(1) Employing mathematic method, various physical fields during the melt filling of CIM process, the short shot and gas penetration of GAIM process were studied. Owing to the gas penetration of GAIM process, the flow rate and shear rate is about 3 order larger than those in the CIM process. The physical fields in the short shot of GAIM process are approximately similar to the corresponding ones in CIM process. Shorter processing recycle is obtained during GAIM process because of the rapid cooling rate.(2) Studies of the influence of processing parameters on the penetration and residual thickness of GAIM part suggest that delay time and melt temperature have notable influence on them. Short-shot size and gas pressure have little influence on the residual thickness, however, short-shot size can markedly influence the penetration length.(3) The crystal morphology, orientated structure and mechanical properties of HDPE and iPP molded by GAIM and CIM were studied. The orientated crystalline such as shish kebab, shish are preferably formed in the GAIM parts, whereas spherulite dominates the CIM part. For such orientated structure, the mechanical properties of the GAIM part are generally improved. Gas pressure slightly influences the mechanical properties.(4) The hiberarchy structure of polycarbonate (PC)/polyethylene (PE) blend molded by GAIM and CIM was investigated. The CIM and GAIM parts exhibit different hiberarchy structure: PC phase is deformed obviously in the skin of CIM part and the core zone of the GAIM part respectively. Gas penetration is responsible for such difference. Though PC and HDPE are extremely immiscible, the transcrystalline of HDPE can be unexpectly formed on the PC fibrils of the GAIM part. The local stress field formed in the interface between PC and HDPE phase during the PC phase deformation rather than the macro-stress field. This is the fundmental precursor for the formation of transcrystalline. Gas penetration is benefical for the improvement of the tensile strength, but damages the impact strength.(5) GF orientation, transcrystalline and mechanical properties of the GAIM and CIM parts were studied. Obvious orientation in the skin of CIM part is observed, whereas GF highly orientate in the core zone of the GAIM part. The secondary shear stress brought by gas penetration is responsible for such phenomena. Though the poor adhesion between GF and iPP, the transcrystalline of iPP can be unexpectly formed on the GF in the GAIM part. As that mentioned above, the local stress field formed in the interface between GF and iPP during the GF orientation and that induced by the velocity difference between GF and matrix rather than the macro-stress field is the fundmental precursor for the formation of transcrystalline.