Study On Water-assisted Injection Molded Nylon-6 And Nylon-6/montmorillonite Nanocomposites

Water-assisted injection molding (WAIM) is a new technology for producing hollow or partially hollow products. With high pressure water-assisted melt filling, packing and cooling, WAIM can produce thinner parts with a better inner surface and with less shrinkage and warpage, but with a shorter cycle time. It can also reduce the material usage and the cost. WAIM technology has received extensive attention owing to its unique advantages. Up to the present, researches on WAIM have experienced the change from setting up WAIM hardware system and optimizing processing parameters, to analyzing and controlling the hierarchical structure of WAIM parts.Firstly, the effects of the processing parameters, including the short-shot size, melt temperature, water pressure and water injection delay time on the water penetration length, residual wall thickness, and wall thickness difference of WAIM PA6/MMT nanocomposite curved pipes were investigated through a single factor methodology. The results showed that the short-shot size had notable effect on water penetration length, which decreased with the increase of short shot size, melt temperature and water pressure and the decrease of water injection delay time. Moreover, the residual wall thickness and its difference at positions near the water inlet and near the end of water channel exhibited different changing trends with the processing parameters. The water penetration length and residual wall thickness of WAIM PA6 part were larger than those of WAIM PA6/MMT part. The residual wall thickness difference of WAIM PA6/MMT part was larger than that of WAIM PA6 part.Secondly, the computer simulation of cooling process in WAIM was carried out by using ANSYS. The results showed that at the beginning of cooling process, the skin layer cooled fastest than other layers. And the water channel layer cooled faster than the core layer. However, at the end of cooling process, the core layer cooled faster than the water channel layer.Finally, using the DSC and XRD measurements, the crystal structure and crystallization performance of the WAIM PA6 and PA6/MMT curved pipes were investigated, as well as the effect of processing parameters on the crystallinity in the water channel layer near the end of water channel. The results showed that theγcrystals preferentially grew near the skin layer owing to fast cooling and the ratio of theγ/(α+γ) phase was decreased first then increased with distance from the surface in PA6 and PA6/MMT parts. The crystallinity in the water channel layer near the end of water channel of the PA6 parts was investigated, the content ofγ-form decreased andα-form increased with increasing the melt temperature and water injection delay time. Moreover, the crystallinity distributed symmetrically across the residual wall of the WAIM PA6 parts, with those in water channel and skin layers lower and that in core layer higher. The differences of crystallinity among the skin, core, and water channel layers of the near the water inlet are larger than those near the end of water channel. Moreover, there were exothermic peaks on the DSC melting curves of the skin layer and water channel layer at both positions, which indicated the phenomenon of recrystallization.