Research On In-mold Overmolding Of Continuous Fiber Reinforced Composites

The principle of the traditional continuous fiber reinforced thermoplastic composite(CFRT)injection molding process is to place the fiber reinforced preform in the mold cavity,and mold the CFRT sample by injecting the matrix plastic on the preform.This process has the advantages of high efficiency,short cycle,continuous production and complex structure of molded products,but it has disadvantages such as low product performance,poor interface bonding effect,and easy detachment of the injection molding layer.In view of the above shortcomings,this paper develops a CFRT in-mold over-injection molding process by taking the grid-like rigid prepreg as the reinforcement preform and the high-fluidity matrix as the research object.The continuous fiber reinforced polypropylene(CFRT-PP)product with "reinforced concrete" structure with excellent mechanical properties,high interfacial bonding strength and good coating quality was successfully prepared by this process,and the following main conclusions were drawn:(1)The in-mold over-injection molding process and the apparent quality of the sample were analyzed,and the influence of the matrix filling process and various factors on the coating quality and warpage deformation of the sample under different injection directions was explored.It is concluded that the melt flow is relatively uniform during the mold filling process,and the melt flows through the preform holes;higher matrix fluidity and preform rigidity can significantly improve the coating quality and reduce the degree of warpage deformation.According to the maximum warpage amount of the sample,the influence of process parameters on the warpage degree was studied and the warpage deformation mechanism was analyzed,which proved the feasibility of preparing CFRT by in-mold over-injection molding.(2)The CFRT-PP samples were prepared with pure PP as the matrix.Combined with mechanical property test,density test,SEM test,SZM observation method,interface performance test,etc.,the effects of injection speed,mold temperature,etc.The influence of the apparent density of the sample and the bonding strength of the interface,and the influence of the structure and shape of the preform and the content of the compatibilizer on the performance of the sample.It is concluded that an appropriate increase of the mold filling melt temperature and holding pressure can improve the mechanical properties and interface bonding effect of the sample,but an excessively high melt temperature will lead to the melting of PP fibers and thus reduce the performance.The interfacial bonding mechanism was analyzed,and it was revealed that the change of the properties of the samples was caused by the different structural forms of the preforms.(3)Using long glass fiber reinforced polypropylene(LFRT-PP)as the matrix,the effects of preform layers,LFRT-PP glass fiber content,and injection speed on the coating quality of CFRT-PP samples and the effect of preform structure on the coating quality were studied.The effect of matrix fiber fracture;and orthogonal experiments were used to explore the optimal mechanical properties of CFRT-PP samples under different preform layers,LFRT-PP glass fiber content and injection speed.It is concluded that appropriately reducing the number of preform layers,reducing the glass fiber content of LFRT-PP and increasing the injection speed can effectively improve the coating quality of CFRT-PP samples and reduce coating defects;The length of the matrix fiber was reduced;while selecting fewer preform layers,lower matrix fiber content and higher injection speed,the comprehensive mechanical properties of the CFRT-PP samples were better.(4)By comparing the mechanical properties of CFRT-PP samples under different molding methods.It is concluded that the mechanical properties of the CFRT-PP molding sample pressurized with 5 layers of preforms are the best,with a tensile strength of 206.14 MPa and a bending strength of 201.38 MPa.Compared with the LFRT-PP specimen,the maximum tensile strength and flexural strength are increased by 89.21% and 32.83%,respectively.Therefore,the CFRT-PP specimens prepared by the in-mold overmolding process have excellent mechanical properties and feasibility.