Key Technologies Of Thermoforming-Injection Integrated Forming Process For Woven Carbon Fiber Reinforced Thermoplastic Composites

Woven carbon fiber reinforced thermoplastic composite(CFRTP)has significant application prospects in the fields of aerospace,automotive and medical fields,with the advantages of excellent comprehensive performance,designability,impact resistance,creep resistance,mass production and recycling.Currently,the conventional forming processes for continuous CFRTP composites include winding forming,automated fiber placement,pultrusion forming,compression molding and thermoforming.Among,thermoforming is the main method for manufacturing woven CFRTP composite parts quickly and economically,but there are problems such as the inability to form complex functional structures and the lack of complex assembly features.In this paper,a new method named thermoforming-injection integrated forming was proposed to manufacture woven CFRTP complex components,which manufacture the bearing skeleton by thermoforming and complex functional accessories by injection over-molding.Based on high-performance polyether-ether-ketone(PEEK)composites,the several key technologies have been investigated,such as the preparation process and properties of high-performance woven CF/PEEK composite sheets,the formability of woven CF/PEEK composite sheets during thermoforming process,the bonding properties of CF/PEEK composite parts formed by injection over-molding,and a typical parts are designed for application verification.The main research results are as follows.Thermal-force conditions have a great influence on the structure and properties of woven CF/PEEK composite sheets.In this paper,the mechanical properties,thermal properties and microstructure of woven CF/PEEK composite sheets fabricated under different temperatures and pressures were studied.Then,the influence mechanism of molding process on the structure and properties of composite sheets was analyzed,and high performance woven CF/PEEK composite sheet were fabricated.The results show that the mechanical properties of the woven CF/PEEK composite sheets tend to initially improve as the temperatures and pressure increase,and subsequently decline when the temperature and pressure exceed 400 °C and 5 MPa,respectively.It is primarily determined by several complex mechanisms,such as those associated with the matrix properties,void defects,and fiber/resin interaction.When the molding process parameters are 400 °C-5 MPa-10 min,the woven CF/PEEK composite sheets with excellent mechanical properties can be prepared.The tensile strength,bending strength and interlaminar shear strength(ILSS)of the sheet are 774 MPa,934 MPa and 74 MPa,and the tensile modulus and flexural modulus are 69 GPa and 68 GPa,respectively.The understanding of deformation mechanism and formability of woven CF/PEEK composite sheets at evlated temperature can effectively guide the forming of CF/PEEK skeleton.In this paper,the deformation behavior and formability of woven CF/PEEK composites at different forming conditions were studied,and the deformation and failure mechanism were systematically discussed.The results show that the deformability of PEEK at rubbery state is excellent.PEEK exhibits a relatively obvious strain rate strengthening phenomenon in small deformation,while the strengthening phenomenon gradually decreases as the temperature is increased.In the thermoforming process,the deformation of the woven CF/PEEK composite sheet is caused by plastic deformation,shear deformation,and squeeze deformation,and there is no plastic thinning and fiber slippage.Increasing the temperature will greatly improve the formability of the sheet,while the increase of the rate will slightly reduce the formability of the sheet.Therefore,the formability of woven CF/PEEK composite sheets at 325 °C and 2 mm/min is excellent,and the flexural failure angle,limit drawing ratio and limit Ericksen ratio are 55°,0.51 and 0.55,respectively.The CF/PEEK composite part manufactured by injection over-molding are a multi-layer composite structure composed of a substrate and the accessories,and the interface structure and properties of the part are great effected by the injection process.In this paper,the shear strength and bonding mechanism of the bonding interface of CF/PEEK composite parts under different injection conditions were studied,and the process range of firmly combined in the interface is obtained.The results show that the mold temperature and melt temperature have a great influence on the interfacial bonding strength of the composite parts,while the holding pressure has little effect on it.The effect of matrix diffusion-reaction on ILSS is much greater than mechanical interlocking and wetting adsorption,and a strong matrix diffusion-reaction layer can be formed at the interface when the mold temperature and melt temperature are higher than 250 °C and 400 °C.However,when the melt temperature is higher than 420 °C,a long-time high temperature environment causes partial thermal decomposition of the matrix,thereby greatly reducing ILSS.Moreover,the “rivet-type bonding” structures were formed at the interface,there is a matrix diffusion-reaction exists in the matrix enrichment area,mechanical interlocks and diffusion-reaction existed in the pits,and weak diffusion-reaction and cracks existed in the fiber surface.Based on the above research,a CF/PEEK typical part was designed to verifie the thermoforming-injection integrated forming process.The results show that forms a good resin inter-melting layer is formed in the bonding interface between the skeleton and the accessory of the CF/PEEK composite parts,which means a firmly bonded interface.Meanwhile,a small four-sided warping deformation of the part appears appears due to the inconsistent two-phase shrinkage,and the deformation value is about 1.43 ~ 2.09 mm.In addition,the compression resistance of the part is strong,and the compression failure load of the assembly columns is 12.8 kN,while the reinforcement and skeleton remain intact and firmly bonded at 80 kN.The above shows that the method named thermoforming-injection integrated forming proposed in this paper provides a feasible way for the integrated forming of complex parts,which is of great significance for expanding the engineering application of thermoplastic composite and developing high-performance and lightweight manufacturing technology.