Study On The Mechanism Of Flow-induced Fiber Orientation And Property Control In Water-assisted Injection Molded Short-glass-Fiber-reinforced Polypropylene

Short-glass-fiber-reinforced thermoplastic composites(SGFRTCs)have been widely used to fabricate structural parts due to their extremely high strength-to-weight ratio,excellent resistance to higher temperature and chemical corrosion,good creep resistance,and low price,etc.Injection molding is one of the most important processing methods for molding parts of SGFRTCs,and the advantages of SGFRTCs in the practical applications are closely related to the injection molding process.Waterassisted injection molding(WAIM)is a promising molding process developed on the basis of conventional injection molding(CIM).Compared with CIM or gas-assisted injection molding(GAIM)processes,the involvement of assisted injection medium water resulting in WAIM process has many unique advantages,and the properties of WAIM parts are also more excellent.In addition,WAIM parts of SGFRTCs are not only possess the quality of WAIM parts of pure resin,but also have the advantages of excellent properties of composite parts.However,there are few studies on WAIM parts of SGFRTCs presently,especially on the formation mechanism of flow-induced fiber orientation,which stunts the development and application range of WAIM process.As we all know,it is a critical requirement to have an insight into the mechanism of flowinduced fiber orientation in WAIM of fiber-reinforced polymer for improving the structural rigidity and service life of molded parts.In this work,therefore,the mechanism of flow-induced fiber orientation in WAIM parts of short-glass-fiberreinforced polypropylene(SGF/PP)was clarified through numerical research and experimental verification,so as to provide a technical guidance for obtaining WAIM parts of SGFRTCs with lightweight and excellent properties.The main conclusions are as follows:(1)The fibers orientation distribution across the part thickness of WAIM parts and CIM parts,as well as the flow fields during filling process of WAIM and CIM processes are investigated and compared.The results showed that the hierarchical structure of fiber orientation within WAIM SGF/PP part is mainly ascribed to the strong shearing effect generated by high-pressure water penetration.(2)Fiber orientation in the WAIM part depends not only on its position both in the axial position and along the radial position,but also on the processing methods and processing parameters.(3)In the SSWAIM parts,fiber orientation distribution varies greatly at different positions of the molded part.At the front section of the part,the fiber orientation presented a typical ‘shell layer-core layer-water channel layer’ hierarchical structures across the part thickness.Most fibers in the shell layer,especially those in the region near the mold wall(i.e.,subsurface layer),were quite orderly oriented along the flow direction,while the fibers adjacent to the water channel were randomly oriented.At the rear of the molded part,most of the fibers were quite orderly oriented along the melt flow direction,especially those in the core layer;and only in the thin region adjacent to the water channel and the mold wall,the fibers tended to be randomly oriented.Furthermore,within the range of investigated processing parameters,a smaller shortshot size,shorter water injection delay time,higher water pressure,and lower melt temperature could significantly facilitate more fiber orientation across the part thickness.(4)In the OWAIM parts,the residual wall thickness can be divided into three layers according to the fiber orientation states,that is shell layer,core layer,water channel layer.Most fibers in the shell layer,especially those in the region near the mold wall(i.e.,subsurface layer),were quite orderly oriented,while the fibers adjacent to the water channel were randomly oriented.The core region was a transitional area where the fibers close to shell layer tended to be orderly orientated,while those fibers near the water channel layer tended to be disorderly orientated.Additionally,melt temperature is the main processing parameter affecting the fiber orientation.A higher water injection pressure and melt temperature,lower water injection delayer time could significantly facilitate more fiber orientation across the part thickness.(5)Compared with CIM process,since the additional water-related parameters are involved in WAIM process,resulting in its process moldability is significantly reduced,especially for molding fiber-reinforced thermoplastics.To address the aforementioned problem,an improved molding method,i.e.,OSSWAIM,is proposed on the basis of advantages of SSWAIM and OWAIM processes.And the results of our investigations show that OSSWAIM process could not only improve the defects in the SSWAIM or OWAIM parts,such as secondary penetration,the nonuniformity of residual wall thickness,discontinuous penetration etc.,but also improve the fiber orientation and processibility.These results put forward a new idea for molding parts with better quality.In addition,the effect of main processing parameters on the fiber orientation across the residual wall thickness in OSSWAM part are also investigated,and the process optimization research is also carried out on the uniformity of residual wall thickness and the maximum tensile load of OSSWAIM parts by using the orthogonal experiment method.It is found that the tensile property of WAIM parts is mainly determined by the fiber orientation across the residual wall thickness,and the properties of WAIM parts can be regulated by simply adjusting the processing parameters.