| External Gas-Assisted Injection Molding(EGAIM)is an innovational technology that developed on the basis of Gas-Assisted Injection Molding(GAIM).It has a wide application prospect in the field of injection moulding of thin-walled parts,with numerous advantages,such as reducing clamping force,shortening moulding cycle,reducing sink mark and warpage and so forth.Compared with Conventional Injection Molding(CIM),EGAIM technology adds gas injection,which increases the process parameters affecting EGAIM and makes the process control more difficult.At present,there are relatively few studies on EGAIM,and many key issues remain to be solved.In this work,some key issues in EGAIM are studied intensively,in order to obtain the relevant theoretical knowledge and experimental experience and guide to improve the surface quality of EGAIM parts.The main research contents are summarized as follows:1)Based on the three basic governing equations of fluid mechanics and Cross-WLF constitutive model,the mathematical models of filling process,external gas injection and holding process were established respectively with reasonable assumption and simplicities of thin-walled model according to the characteristics of the EGAIM.The gas was used as incompressible Newtonian fluid.The compressibility of polymer melt was considered in the gas holding stage.The finite volume method was adopted to solve the mathematical models,and the volume of fluid(VOF)method with artificial compression term was used to track the free interfaces.The relationship between solidified polymer and polymer melt in the solid-liquid coexisting zone were described based on Darcy's law during gas holding process.A new solver,based on the open source OpenFOAM on Linux system,which could be used to describe the process of EGAIM was developed.2)The causes and evolution mechanism of sink mark of EGAIM were studied intensively.Based on the established mathematical model of EGAIM filling process,a numerical simulation program was developed.The two-dimensional melt filling process in a thin-walled cavity with different rib thickness was simulated.The pressure field,velocity field and temperature field of polymer melt in the cavity after filling were obtained,which provided the initial physical field data for the subsequent analysis.The simulation analysis results of gas filling showed that there should be a certain delay after the melt filling,so as not to blow through the surface of the melt.3)Based on the mathematical model of external gas holding process,the numerical simulation program was developed.The temperature,density,pressure,velocity field distribution and solidification for polymer melt during CIM under non-pressure and EGAIM were simulated and compared.The results showed that the pressure field distribution of polymer melt in the mold cavity was uniform under gas holding.With the time increased,the temperature of the polymer melt in the mold cavity decreased,among which the temperature of inner polymer melt decreased comparatively rapid in the area with thinner ribs,the solidification rate of the melt was higher,the solidification layer became thicker,so as the density of the solidification layer was correspondingly larger.The solidification layer which closing to the rib was extruded,resulting in deformation and concave the central area of ribs connecting with the main cavity.The simulation results provided theoretical support for the experimental study on how to reduce the sink mark of EGAIM parts.4)Based on the polymer viscoelastic theory,the mathematical models of stress-strain between the gas holding stage and the natural cooling stage after demoulding by EGAIM were constructed,and a coupled finite element prediction model of warpage deformation of EGAIM parts was proposed.The comparison between numerical simulation and experimental results showed that the warpage of EGAIM parts decreased gradually from center to edge,forming an "arch" structure,and the change along the melt flow direction was less than that along the vertical flow direction,which validated the prediction model proposed.EGAIM technology with lower gas pressure could achieve the holding effect of CIM under relatively high pressure.5)The EGAIM experimental study was carried out based on the developed experimental platform.The effects of melt temperature,mold temperature,gas injection delay time,gas injection pressure and gas injection time on shrinkage,sink mark and warpage of EGAIM parts were investigated.Through the comparison and analysis of the experimental results and numerical simulation results of warpage of EGAIM parts,it was found that the simulation results were qualitatively consistent with the experimental results,which verified the accuracy and reliability of the prediction model of warpage deformation of EGAIM parts established in this paper.Some reasonable advice about process optimization of EGAIM was proposed based on the simulation and experimental conclusions. |
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