Research On Cellular Structure And Warpage Of Microcellular Injection Molded Parts

Microcellular injection molding（MIM）technology using supercritical fluid as physical foaming agent can lower the clamping force and reduce the weight and warpage of the parts.In this dissertation,the filling flow of polypropylene（PP）melt/supercritical nitrogen（Sc-N₂）solution,cell diameter and warpage in the MIM of box-shaped PP parts were numerically simulated.On the basis of the numerical simulation,the influences of injection speed and Sc-N₂ content on the cellular structure and warpage of the MIM box-shaped PP parts were investigated;The microcellular injection-compression molding（MICM）was used to improve the cellular structure and minimize the warpage of the PP samples.The gapwise distributions of the shear rate,temperature,viscosity and pressure of the PP melt/Sc-N₂ solution,as well as the cell diameter and warpage of the box-shaped PP parts were obtained by using orthogonal experiment design and numerical simulation.The effects of six main processing parameters（injection speed,shot size,Sc-N₂ content,melt temperature,mold temperature and cooling time）on the cell diameter and warpage of the parts were investigated.The results showed that injection speed had a significant effect on the cell diameter and warpage of the parts,and Sc-N₂ content had a certain influence on the warpage of the parts.The cell diameter and warpage of the parts can be reduced by raising injection speed（200²90 mm/s）and decreasing Sc-N₂ content（0.2⁰.6 wt%）.The effects of two injection speeds（250 and 290 mm/s）and Sc-N₂ contents（0.3 wt%and 0.6 wt%）on the cellular structure and warpage of the PP box-shaped parts were quantitatively investigated.According to the simulated gapwise distributions of shear rate,temperature,viscosity and pressure of PP melt/Sc-N₂ solution at the last moment of the filling stage in the molding of the parts,the formation mechanisms of the cellular structure and skin layer within the walls of the parts were analyzed in detail.The effect of the injection speed on the tensile strengthes of the parts was also investigated.It was demonstrated that microcellular cells with smaller diameters（less than 90μm）were developed at the core layer of the box-shaped parts at higher injection speeds.It should be noted that the tiny cell area with diameters less than 20μm and more regular shape appeared near the skin layer of the box-shaped parts.This is due to the synthetic effects of higher shear stress combined with lower melt temperature.Along the filling direction,an exponential relationship was found between density and mean diameter of the cells within box-shaped parts sidewall,and the skin layer thickness increased in a nearly linear manner.The results demonstrated that more uniform and compact cellular structure as well as thinner unfoamed skin layer is beneficial to lowering the warpage of MIM parts.The cuboid PP samples were fabricated by MICM.The effects of melt temperature,injection speed and injection compression parameters（compression distance,compression speed and compression force）on the cellular structure and warpage of the samples were investigated.The results showed that the cellular structure in MICM samples was more uniform,densely packed and the shrinkage uniformity was better under appropriate processing parameters.The melt temperature,compression distance and compression force had more significant influence on the warpage of the samples,whereas the injection speed and compression speed had less effect on the warpage of the samples.This dissertation provides a guidance for improving the cellular structure and lowering the warpage of the MIM parts.