Process Simulation And Hierarchical Polymer Blend Part Structure In Water-assisted Injection Molding

Water-assisted injection molding (WAIM) is a technique for producing hollow orpartially hollow polymer parts. In recent years, WAIM is attracting more and more attentionfrom both industry and academia because it has sevral advantages, such as materials saving,short cycle time, slight part residual stress and smooth part inner surface. In WAIM, thehigh-pressure water is injected into the core of polymer melt, pushing the melt to fill the moldcavity rapidly, followed by cooling the melt directly. This results in unique flow andtemperature fields in WAIM, and so leads the hierarchical structure in the WAIM parts.However, the formation mechanism of hierarchical structure of WAIM parts was notunderstanded clearly because the complex flow fields were not revealed. Consequently, in thisdissertation, the melt filling and cooling processes in WAIM were simulated, and then theformation mechanism of hierarchical structure of WAIM polypropylene/acrylonitrile-stryenecopolymer (PP/SAN) blend parts was investigated based on the relevant simulated results.Mathematical models were established for the three processes in WAIM, namely, shortshot, high-pressure water penetration and cooling. The simulated results showed that the meltfilling process propelled by the high-pressure water was much shorter than the short shotprocess and could be divided into three stages, namely, initial filling, fast filling, and terminalfilling stages.The flow fields, including the velocity, elongational rate and shear rate fields, in WAIMwere analyzed for the first time. At a specific time in water penetration process, that is0.24safter the beginning of water injection, the shear rate distribute uniquely in the melt. The shearrate distribution in the unpenetrated zone was similar with that in the conventional injectionmolding, whereas the shear rate in the penetrated zone was nearly zero. The high-pressurewater had a strong elongational effect on the polymer melt around the water front. In thewhole water penetration process, the shear rate of the melt throuth the whole thickness wasrelative higher and increased along the flow direction in the penetrated zone. Moreover, theelongational rate of the melt near the water channel was the highest and decreased towards the mold wall. The simulated results of the temperature field in the cooling process showed thatthe cooling rate in the skin layer was the highest followed by the channel layer and then thecore layer.The hierarchical structure in the WAIM PP/SAN blend parts was observed by scanningelectron microscope and polarized optical microscope. It was found that SAN fiber wasdominant across the whole thickness in the blend parts. The SAN fibers at different layers,namely, skin, core and channel layers, was formed at different stages during the WAIM.Interestingly, the high shear rate could cause the formation of transcrystalline structuresbetween the interface of PP matrix and SAN fiber.