The Morphology And Structure Of Isotatic Polypropylene Parts Molded By Water-assisted Injection Molding

Water-assisted injection molding, an innovative molding process, has been recently developed on the basis of conventional injection molding and gas-assisted injection molding. Water-assisted injection molding has unique advantages in molding the thick wall, tubular and large flat panel parts, and it has attracted wide interest by many researchers in recent years. However, much of the available literatures in this field focused on the mathematic simulation of water penetration, the relationship of macro quality and molding process, the water channel and water needle design, etc. However, the study on crystalline morphology of water-assisted injection molding parts is rare. In view of this, in this paper, more attentation is paid to the morphology, structure and formation mechanism of the isotactic polypropylene parts molded by water-assisted injection molding and the effects ofβnucleating agent, using polarizing microscope, small angle X-ray scattering and wide angle X-ray diffraction instruments and so on. Through the differences study of physical fields between water-assisted injection molding and conventional injection molding, we clarify the influence factors of crystalline morphology and orientation behavior. This study can provide theoretical and experimential guidelines for improvement of the mechanical properties by controlling the morphology and structure during water-assisted injection molding process. The main results are as follows:(1) Crystalline morphology and orientation behavior of the water-assisted injection molding parts were studied, it is found that these parts can be divided into three layers:skin layer, core layer and water channel layer, according to the different morphology and structure from the mold wall to water channel surface. Due to the water penetration, more shish-kebab structures appeared in the skin layer and water channel layer, furthermore higher orientation degree of the lamellae is achieved in these fields.(2) Through the study on crystalline morphology and orientation behavior of isotactic polypropylene, with different molecular weight, molded by water-assisted injection molding, we find that more shish-kebab structures appear in the surface layer and water channel layer of the isotactic polypropylene parts with lower molecular weight.(3) Hierarchical structure of isotactic polypropylene parts containingβnucleating agent were parpared by water-assisted injection molding, we find thatβnucleating agent can significantly influence the hierarchical structure of water-assisted injection molding parts. With the lowerβnucleating agent, the oriented region distributes over a much broader range. However, the boundary of the oriented region and isotropic region disappear with the higher content ofβnucleating agent, and the size of the spherulites becomes very small.(4) Through the study on shish-kebab of water-assisted injection molding parts with different loadings of nucleating agent, we find thatβnucleating agent can improve the content of shish-kebab. As the content ofβnucleating agent increasedto 1%, shish-kebab distributes over a much broader range and reachs to the core layer. In addition, the crystallinity, content of theβ-form crystal and orientation degree are found to be increase with the increase ofβnucleating agent.