A Numerical Simulation Method For The Thermoforming-Injection Integrated Forming Process Of Continuous Fiber Reinforced Thermoplastic Composites

The continuous fiber reinforced thermoplastic composites(CFRTP)have the advantages such as high production efficiency,high impact resistance,recyclable,and can be assembled by surface welding,which is a new material with great potential.The conventional CFRTP thermoforming process suffers from the shortages such as low design freedom,and material damages caused by secondary manufacturing.Therefore,the thermoforming-injection integrated forming process has been developed which can significantly improve the manufacturing efficiency and part quality.However,due to the high processing complexities,inappropriate processing conditions,part design and mold structures can lead to defects.The simulation technique can identify the potential problem at early stage,and optimize the molding conditions in the scientific approach by simulating the actual molding process,however there is no research regarding the thermoforming-injection integrated forming process currently.Hence for the first time,this thesis established the simulation method to the thermoforming-injection integrated forming process,and the major contributions are listed as below.There exists coupling behavior between the melt plastic and CFRTP in the thermoforming-injection integrated forming process,which cannot be solved by the conventional one-way coupling method.By analyzing the molding process,this thesis made reasonable assumptions and simplifications to the problem,established the velocity boundary conditions,pressure boundary conditions and thermal boundary conditions,and proposed the integrated simulation strategy to the thermoforming-injection process.This thesis proposed a transient mold temperature simulation model based on the implicit heat flux conservation constraint,which can solve the large temperature variation within the molding cycle of the thermoforming-injection integrated forming process.This model adopts the three-dimensional mold meshes as input to solve the disconnected complex mold by applying the implicit heat flux conservation constraint at the contact interfaces.Apart from that,this model applies the simplified melt plastic temperature model as boundary condition,and cycle averaged mold temperature model as initial condition,hence to simulate the transient mold temperature under the stable production stage efficiently and accurately.The actual molding experiment was performed to validate the mold temperature model proposed in this work,it is found that the maximum temperature error was 3.8%.This thesis proposed a stable velocity-pressure coupling simulation method under the moving mesh,which is capable of simulating the significant filling domain variation caused by CFRTP thermoforming.As the moving boundary causes mesh deformation,hence the arbitrary lagrangian eulerian based governing equations were applied to describe the motion of the melt plastic.To fix the mesh mistakes and improve the numerical stability during mesh deformation,a penalty method which optimizes the mesh by penalizing the eigen values of the deformation gradient tensor was adopted.To improve the simulation performance and stability,the stablized velocity-pressure coupling scheme by enriching the linear tetrahedral element with an extra central velocity and streak line upwinding scheme were given for simulating the melt plastic filling.Designed the stability experiments for the mesh deformation optimization and filling methods,and the effectiveness were proven.An actual injection compression molding experiment was performed to validate the filling method under the compressed mesh.By validating the flow front simulation with the short shot experiment,it is found the simulation error was3.7%.This thesis proposed a dual-mesh based interface coupling method for solving the two-way coupling problem between melt plastic and CFRTP,in which the fiber rotation based objective stress rate model was adopted to simulate the CFRTP thermoforming,the partitioned coupling scheme was applied to implement the melt plastic and CFRTP deformation calculation,and the monolithic coupling scheme based on the implicit heat flux conservation constraint was applied to implement the temperature calculation.Besides,a bonding strength prediction model for the CFRTP and melt plastic interface was proposed based on the theory of polymer chain diffusion and entanglement.The bias extension and hemi-sphere tests were conducted for validating the CFRTP deformation simulation.During the hemi-sphere test,the simulation and the experimental results matched well,it was found that the error of the maximum shear angle between simulation and experiment was 6.9%.The validation for the bonding strength was performed,and it is shown that the bonding strength prediction error was 4.8%.Finally,an actual thermoforming-injection molding experiment was carried out to validate the simulation method.The simulation was carried out based on the actual molding parameters,and the key results such as mold temperatures,melt plastic filling behavior and CFRTP deformation were validated.The maximum error of the transient mold temperatures was within 2.8%.The polymer flow fronts were measured at different molding stages and the error between simulation and experiment was within 3.5%.The final composites intra-plane deformation error was less than 2.5%.The error of the CFRTP shear angle was less than 1.2%.Based on the facts above,the simulation method proposed in this thesis can accurately simulate the thermoforming-injection integrated forming process.Hence it is valuable for the scientific molding trials to improve the part quality and successful rate.