Study On Mechanism And Optimization Of Penetration And Fiber Orientation In Short-Fiber-Reinforced Polymer Water-assisted Injection Molding

In recent years,water-assisted injection molding(WAIM)has received more and more attention due to its characteristics of thin residual wall thickness(RWT),short molding cycle and good economy,etc.Using the short-fiber-reinforced polymer(SFRP)as raw material in WAIM,the mechanical properties of the plastic parts can be significantly improved.However,due to the addition of short fibers,the process of WAIM has become more complicated.For the complexity of process of SFRP WAIM,the key issues such as the high-pressure-water penetration,RWT and fiber orientation distribution of the process were studied using the methods of numerical simulation and experimental research.The main research contents and results are as follows:The RWT is an important index affecting the mechanical properties of SFRP water-assisted injection molded parts,and has a nonlinear relationship with process parameters.The sample data of RWT were obtained by the simulation of overflow WAIM,as well as the methods of central composite experimental design.The response surface multivariate quadratic model between the process parameters and RWT was constructed using the linear regression method.The sensitivities of RWT to the process parameters(melt temperature,mold temperature,delay time,water injection pressure and water injection temperature)were investigated using variance analysis,and the interaction effects of the significant process parameters on RWT were analyzed as well.The results show that the water injection pressure,delay time and water injection temperature are the predominant factors for RWT;the value of RWT decrease as the water injection pressure rises,and increase with the increasing delay time and water injection temperature.The values of RWT of SFRP water-assisted injection molded parts were obtained at 10 locations distributed along axial direction.The standard deviation was used to characterize the uniformity of RWT distribution.The nonlinear mathematical agent model between process parameters and RWT uniformity was established using the hybrid of the artificial neural network and the genetic algorithm.The influence of process parameters on the uniformity of RWT was investigated using the method of variance analysis.The results showed that the melt temperature,water injection delay time and water injection temperature have an important influence on the uniformity of RWT.To improve the uniformity of RWT,The hybrid model was applied as the fitness function of the genetic algorithm in the optimization process,obtaining the minimum RWT standard deviation 0.0972.The numerical simulation was carried out using the optimized process parameter,and the standard deviation of RWT was 0.09,indicating the RWT uniformity was improved.The short fiber orientation(SFO)distribution in Water-assisted injection molding(WAIM)is more complex than that in traditional injection molding(CIM).Taking the hollow straight tube as an example,the 3D numerical simulation of the SFO in overflow WAIM was carried out,and the validity of the prediction model of SFO was verified by comparing the simulation results with the SEM photographs.The values of the orientation tensor components at different thicknesses of the section were extracted,and the mechanism of SFO in the overflow WAIM was analyzed.The simulation results showed that the distribution of SFO in the RWT had the characteristics of outer-shellinner layer structure,and the short fibers in the outer layer and the shell layer are mainly oriented along the axial direction,and that in the circumferential direction was second,and that in the radial direction was the weakest.The inner wall of the RWT near the water channel was affected by the high-pressure water column,causing the short fibers tend to be freely oriented.The SFO tensor components in different sections along the axis of the part were analyzed.Due to the different stress fields and velocity fields,the SFO tensor component of the three directions changed in the axial direction,and the overflow port also had an influence on the SFO distribution.Due to the layer structure of the SFO in the RWT,the average orientation method was employed to characterize the SFO of the entire parts based on the laminate theory.Through the methods of the orthogonal experimental design,range analysis and variance analysis,the mathematical model between the process parameters and the SFO was established,and the influences of the process parameters(melt filling time,melt temperature,mold temperature,delay time and water pressure and water temperature)on the SFO were studied.The results showed that the water pressure,melt temperature and water temperature had a significant effect on the SFO in RWT.As the water pressure and the melt temperature increase,the degree of SFO decreases,and as the water injection temperature increases,the degree of SFO increases.The improvement of the mechanical properties of the plastic parts is mainly reflected in the direction of SFO.In order to improve the performance of resistant the fluid pressure of the SFRP water-assisted injection molded parts,it was desirable that more short fibers orient along the circumferential direction.In this paper,the influence of main process parameters on the SFO tensor circumferential component was investigated.It was found that the circumferential component increased with the increase of melt temperature and water pressure,and decreased with the increase of the delay time.By optimizing the process parameters,the maximum SFO tensor circumferential component was 0.23.