Three Dimensional Viscoelastic Numerical Simulation And Mechanism Analysis For Jetting Phenomenon During Injection Molding

Jetting is a filling pattern which melt squirts into the cavity without wetting mold walls.Compared with the conventional meltfront advancing this type of filling can easily result in some part defects such as snake streaks,weld lines and air traps.Jetting is a bad flow which should be avoided in real polymer manufacturing.The diversity and irregularity of the jetting induced factors increase the difficulty of theoretical analysis and numerical prediction for jetting,which leads no commerical softwares are available to predict jetting phenomenon successfully.Therefore,numerical simulation and mechanism analysis for jetting can not only develop the theoretical system of injection molding,but also provides significant tools for the engineers to design mold structures and process conditions.A three dimensional flow theory was established in terms of the intrinsic viscoelastic characteristics of polymer melt,and the corresponding numerical approach based on finite volume method was proposed to fit the therotical characteristics.Aiming at the nonlinear characteristic of the govern equations and constitutive equation,an iterative algorithm was proposed to determine the flow solutions and a computational code was developed.The program successfully predict jetting phenomeanon and the succeed reptile buckling flow.Based on the numerical results the influence of viscoelastic characteristics,processing considtions and mold structures on jetting were thoroughly studied.In addition,some effective mold structures and process conditions which can avoid jetting were proposed in this thesis.The main achievements of this thesis are listed as follows:(1)Continuity equation,energy equation and momentum equation which accounts for the gravity and inertia force were established in terms of fluid theory.The capability and advangtage of Giesekus model to characterize the viscoelastic behavior was analysed.Then the three dimensional melt flow models in terms of viscous(Cross-WLF)and viscoelastic(Giesekus)models were constructed respectively.(2)An algorithm for melt flow is proposed using finite volume method.Based on the coupling character of velocity and pressure,a modified PISO method is proposed to calculate the velcity and pressure.As velocity,temperature and stress are highly interdependent,an iterative approach between Navier-Stokes and heat conduction is proposed.These twofold iterations can highly improve the stablity and efficiency.(3)Three stages of jetting evolution,i.e.columar jetting,buckling flowing,meltfront filling,were successfully simulated.The reptile filling in thin cavity and corkscrew filling in thick cavity were also successfully predicted.The jetting mechanism that inertia force exceeds viscous force and dominates the flow during columnar stage was found by comparing the effects of inertia force and viscous force on melt flow.(4)Melt viscoelaticity has influence on jetting.The amplitude and the frequency of the buckling flow decrease as Wi increases,and the time switching to meltfront filling from reptile flow reduces a little.Anisotropy also has an impact on jetting.Jetting distance decreases as the nonlinear parameter ? of Giesekus equation inceseases.This parameter also induces the bending of the columar in initial jetting stage and a larger reptile swing front.(5)Injection speed has a significant effect on jetting and buckling flow.High injection speed leads to a longer jetting length,a lower swinging frequency and a larger reptile diamater.Melt temperature has little effect on the jetting length,but has heavy influence on reptile flow.High melt temperature decreases the swinging frequency and increases the reptile diameter;and reduces the switch time required from reptile flow to meltfront flow.(6)The gate location plays a significant role on inducing jetting.The further the gate is away from the mold wall,the greater probability for jetting occurrence,and induces more complete jetting and reptile flow.On the other hand,the closer gate to the mold wall induces less probability for jetting occurence and a shorter jetting distance.The melt flow in this type of mold may switches to the meltfront flow as soon as entering the cavity,and prevents jetting hapening.