Research On Mechanism Of Water-assisted Injection Molding Short Fiber Reinforced Polymer Pipe Parts

Fiber reinforced polymer composites is not only can retain the properties of the original polymer matrix,but also can improve the mechanical properties of the molded parts.At this stage,they are widely used in actual production and life.Injection molding is one of the main processing methods of fiber reinforced polymer composites.Water-assisted injection molding(WAIM)technology is one of the most promising newly fluid-assisted injection molding technologies at this stage.Compared with gas-assisted injection molding technology and traditional injection molding technology,it has obvious advantages and wider application prospects.And water-assisted injection molding short fiber reinforced polymer products is not only can well retain the quality of WAIM pure resin,but also it has the advantages of excellent mechanical properties of composite products.However,the research on water-assisted injection molding technology at our country or abroad mainly focuses on the research of pure resin,while the research on water-assisted injection molding of fiber reinforced polymer is rare.Therefore,it is important to study the influence mechanism of relevant parameters on the short fiber reinforced polymer water-assisted injection molding products.In this paper,the combination of three-dimensional numerical simulation technology and experiment is used to study the mechanism of water-assisted injection molding of short fiber reinforced polymer round pipe parts,in order to improve the technical guidance for the actual production and processing in the future.The main work and related conclusions of this paper are summarized as follows:(1)The working principle and research progress of two molding methods for water-assisted injection molding technology were summarized.(2)Based on the rheology of polymer processing,the filling equation of multiphase fluid in the numerical simulation of short fiber reinforced polymer water-assisted injection molding was established,and the related evolution of fiber orientation model was summarized.(3)The numerical simulation experiment was carried out with the help of the existing commercial 3D injection molding software to study the effects of relevant injection molding process parameters and fiber weight percentage on the penetration length of high pressure water in short-shot water-assisted injection molding,and which was verified by experiments.It was found that in the numerical simulation of short-shot water-assisted injection molding,the penetration length of high-pressure water in 20% short glass fiber reinforce PP(SGFRPP)melts decreased with the increase of short shot size and water injection pressure;it increased with the increase of the water injection delay time;and the melt temperature increased only had slightly effected on water penetration length;in addition,it was found that the penetration length of the high pressure water decreased with the increase of the fiber content.The experimental results are consistent with the numerical results.(4)The effects of relevant process parameters and fiber content percentage on the residual wall thickness of overflow water-assisted injection molding round pipe were studied by numerical simulation and experiments.In the numerical simulation of overflow water-assisted injection molding,the results showed that the residual wall thickness of the product decreased with the increase of water injection pressure,and increased with the increase of water injection delay time,and the melt temperature had slightly effected on the residual wall thickness of the product;the experimental results are consistent with the numerical simulation results.In addition,it was also found that the residual wall thickness of the product became more and more uniform with the increase of the water injection pressure,and became more uneven with the increase of the water injection delay time;when the water injection pressure is low(6MPa),the stability of high-pressure water penetration process decreased with the increase of fiber content,and this phenomenon was more obvious in the end of the product.(5)Numerical simulation was carried out to study the change of fiber orientation across the thickness of the product in a molding cycle of OWAIM.It was found that when the high pressure water was not injected,the fiber orientation of the product along the different thickness showed a typical "skin-core" structure.With the injection of high pressure water,the fiber orientation at the core layer of the product was well improved.In addition,it has been found that the WAIM product can be divided into three regions(water channel layer,core layer,and shell layer)according to the fiber orientation across the thickness,and the fibers in the shell region of the product are only slightly oriented along the polymer flow direction,numerical simulation results consistent with the experimental conclusions.(6)The effects of relevant process parameters on fiber orientation in OWAIM products were studied by numerical simulation.It was found that with the increase of water injection pressure,the fiber orientation disordered region at the initial segment of the product became shorter;when the water injection delay time is 3s,the fiber orientation disordered region at the initial segment of the product becomes shorter,and in the middle and rear segment positions,the ordered orientation of the fiber orientation decreases as the water injection delay time increases.Compared to 210°C and 270°C,when the molding melt temperature is 250°C,the fiber ordered regions in the product are relatively thickest.Numerical simulations are consistent with experimental conclusions.(7)The effects of fiber content at lower pressure(6MPa)on the ordered orientation of fiber orientation in different regions of OWAIM products were compared by experiments.It was found that the ordered regions of fibers in the initial segment of the product decreased with the increase the fiber content of the material;while the ordered region of the fibers in the middle and rear segments of the part increased with the increases of fiber content in the material,and this phenomenon is most pronounced in the S5 region of the end of the part.When the water injection pressure is higher(10MPa),as the fiber content in the molding material increased,the ordered region of the fiber at the initial stage S1 of the product gradually decreased,but in the end regions S4 and S5 of the product,as the fiber content in the molding material increased,the fiber orientation in the water channel is more orderly.