| In this work,some key issues in injection molding for fiber reinforced polymer composites(FRPC)are studied intensively to obtain the relevant theoretical knowledge and guide to improve the quality of the molded parts.The main study works of this paper are summarized as follows:1.The experimental equipment of FRPC injection molding was established and a bit complex injection mold was designed and manufactured.Thereafter,the relevant experimental studies were carried out.Based on the comprehensive consideration of the residual stress field,temperature field,and the non-uniform anisotropic thermal and mechanical properties formed in injection molding process,a three-dimensional finite element model was developed to predict the total warpage deflection of FRPC injection-molded parts by coupling the injection molding flow with the thermal-stress analysis.Prediction and experimental results revealed that the trends of both predicted and experimental warpage deflection were quite consistent and showed a dome-like deformed shape,which validated the prediction model proposed.2.Focusing on the core issues of the filling and fiber orientation in injection molding process,and based on the theory of viscous fluid dynamics,the basic control equations of melt flow were established for FRPC injection molding.Aimed at the feature of the injection molding of long fiber reinforced polymer composites(LFRPC),the regularities of fiber distribution were studied,and emphatically the mechanism of fiber distribution was analyzed and explored through the main processing parameters.Based on the homogenization theory of the mechanical properties of composites,a nonlinear elastoplastic constitutive model of LFRPC was developed,the discrepancy of the warpage deflection results from different constitutive models was studied by the coupled finite element analysis method,and the main effect of fiber parameters on warpage deflection was analyzed.The results showed that:(1)the injection time is the most significant effect on the fiber orientation,the longest and shortest fibers coexist around the gate,and fibers near the wall are often a bit longer.with increasing the injection time or the decrease of mold temperature,fiber orientation and fiber length were decreased,with the increase of melt temperature,fiber orientation decreases,but the fiber length is decreased firstly and then increased,with the increase of injection pressure,the fiber length decreases,and the effect of holding pressure on the fiber orientation is related to the transformation of velocity/pressure.The above results are consistent with the literature.(2)the warpage deflections show a dome-like shape based on both the elastic and elastoplastic constitutive model,and the former is about 18% smaller than the latter in warpage deflection,the simulation results are consistent with the theoretical analysis.With the increase of the initial fiber length,aspect ratio and fiber content,all the maximum amount of warpage deflections decrease.3.Based on finite element mothed and comprehensively considering the wall-slip velocity and the effect of pressure on viscosity,a three-dimensional numerical model was established for the thin-walled injection molding flow,the numerical results were verified by comparing with the experimental results,the effect of wall slip on the velocity distribution during filling was investigated,the effect of wall slip and pressure dependence of viscosity on melt filling were analyzed and obtained the influence mechanism theoretically.Comparison between the experimental and numerical results showed that to better predict the thin-wall injection molding process need to consider wall slip and pressure dependence of viscosity simultaneously,while it can affect the thin-wall injection molding process to different degrees if ignoring the wall slip and pressure dependence of viscosity.The above studies are helpful to better understand the theory of thin-wall injection molding.4.On the basis of the validation of the three-dimensional numerical model of thin-walled injection molding,the effects of wall slip and pressure dependence of viscosity on fiber orientation and warpage deflection of FRPC injection molding were studied.The results from different simplified models were analyzed,and the influencing mechanism was explored and revealed.The effects of the main processing parameters on the fiber orientation and maximum warpage deflection were investigated.The results showed that:(1)By experimental validation,the three-dimensional numerical simulation with considering wall slip and pressure effect on viscosity can better capture the fiber orientation through part thickness,while it can affect the results of fiber orientation in varying degrees if ignoring the wall slip and pressure dependence of viscosity.The calculated fiber orientation in the core layer based on the conventional Hele-Shaw thin-wall approximation is greatly different from the actual distribution.(2)The injection rate has the overwhelming influence on fiber orientation,and then the melt temperature,the influence of the mold temperature,holding pressure and holding time on the fiber orientation is small.(3)With the injection rate,melt temperature and mold temperature increase,all the maximum amount of warpage deflection increase,while the effect of injection rate reduces with a certain value.With the increasing of injection pressure,the maximum warpage decreases.The holding time has small effect on the maximum amount of warpage deflection. |
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