Simulation And Experimental Research On Fiber Breakage In Injection Molded For Long Fiber Reinforced Thin-Walled Products

Compared with short fibers,long fiber reinforced thermoplastic composites(LFRT)still have the ability to form complex structural products with high efficiency and near net shape at one time through injection molding process while improving the strength,stiffness,impact resistance and dimensional stability of parts.However,the injection molding process is a typical melt flow behavior dominated by shear,and the strong shear will inevitably lead to fiber damage.Especially for thin-walled injectionmolded products,on the one hand,the flow channel in the thickness direction of the mold cavity is very narrow,on the other hand,the formation of the frozen layer near the mold wall makes the flow channel narrower,and the long fiber in the narrow space will be difficult to move freely with melt,presenting the phenomenon of stress,breakage/damage under constrained state.In this case,the current unconstrained fiber damage model based on free flow field is no longer suitable for fiber breakage prediction of thin-walled products.In this context,the mechanism of macroscopic load transfer from matrix to fiber of composites was studied,a fiber breakage model during the filling process of longfiber reinforced injection-molded thin-walled products was constructed,the quantitative relationship between shear rate and fiber damage was established,and the experimental verification was carried out.The main research contents are as follow:(1)The mechanical model of macroscopic load transfer from matrix to fiber was derived,the calculation models of tensile stress and interfacial shear stress under macroscopic strain were constructed,and the fiber reinforcement criterion was established.The mechanism that product properties and fiber reinforcement become stronger with fiber length increasing was clarified theoretically and the reason why the matrix can transmit the maximum stress to the fiber when the fiber length exceeds the critical value was revealed.In addition,this model provides a theoretical basis for quantitative control of fiber length distribution and its effect on mechanical properties during injection molding of LFRT.(2)Based on OSEEN perturbation equation,the force equation of fiber on uniform flow field was derived,the force model of constrained fiber in the injection shear flow field was constructed,the calculation method for the critical length of fiber breakage was put forward and the fiber damage criterion was established.The prediction software "Fiber Break Calculation and Analysis System for Composite Injection Molding" was developed by using Python language,which realizes the quantitative prediction of fiber breakage and its critical length in injection flow field.(3)The effects of shear rate,melt temperature and orientation on the maximum tensile stress and fiber breakage critical length of long fiber reinforced polypropylene composites were studied numerically.It is found that the maximum tensile stress of fiber becomes stronger with the increase of shear rate and fiber aspect ratio,and decreases with melt temperature increasing.In addition,the critical length gradually decreases with the increase of shear rate,the decreasing speed is fast at first and then slow,and finally tends to a basically stable state,showing that the fiber length is reduced to a certain extent and it is no longer easy to break,which theoretically explains the reason why the minimum remaining length of fibers in injection molded products is almost always between 0.2 mm and 0.6 mm in previous experimental studies.(4)Based on the developed software,the fiber length distribution of typical thinwalled injection products reinforced by long fiber was simulated and predicted.A batch of long fiber reinforced composite products were molded by injection molding under the same conditions as the simulation process,and the fiber length and the proportion of fiber volume and quantity in different positions of the products were calculated by using the fiber length distribution analysis system.The statistical results show that samples farther from the gate have more short fibers.(5)The comparison of experimental and simulation results shows that the prediction accuracy of fiber length distribution in the three samples is 87.8%,91.4%and 83.6% respectively,the corresponding relative errors of fiber number average length are 3.2%,2.5% and 1.0% respectively,and the relative errors of weight average length are 1.5%,2.4% and 10.5% respectively,which are in good agreement with the experimental results.It shows that the fiber breakage model and the prediction system developed in this paper can accurately predict the fiber break and length distribution in injection molding process.The work of this paper enriches and develops the numerical prediction theory for fiber-reinforced injection molding,which makes the fiber length regulation in the injection molding process rise from the qualitative level to the scientific quantitative calculation,provides the necessary conditions for further accurate calculation of the mechanical properties of injection molded products,and has important application value for the performance prediction and regulation of long fiber-reinforced thermoplastic composite products.