Study On Mechanism Of Water-assisted Injection Molding Of Short Fiber Reinforced Polymer Curved Pipe

The water-assisted injection molding(WAIM)technology of short fiber reinforced polymer elbows is very promising,so it is of great significance to carry out research on the important indicators such as residual wall thickness and fiber orientation.Based on the simplified derivation of the basic governing equation,the constitutive equation and the viscosity model,determined the boundary conditions,proposed the White-Metzner model,and systematically study the mechanism of the effects of process parameters on the residual wall thickness(RWT)and fiber orientation with the self-made WAIM device.The main research contents of this paper are as follows:1.Based on the numerical simulation,the effect of injection volume,fiber percentage,melt temperature,water injection delay time and water injection pressure on the RWT of WAIM curved pipe was analyzed by experiments.The results show that:(1)In the short shot water-assisted injection molding(SWAIM),with the increase of injection volume,fiber percentage,melt temperature,water injection delay time and water injection pressure,the RWT of WAIM curved pipe is not uniform along the melt flow direction,which increases slightly from P1 to P2(or P3),decreases obviously from P3 to P6,and increases slightly from P6 to P8;(2)In the overflow water-assisted injection molding(OWAIM),with the increase of fiber percentage,melt temperature,water injection delay time and water injection pressure,the RWT of OWAIM curved pipe increases along the melt flow direction,and the increase is slow.2.Based on the SWAIM and the OWAIM,the numerical simulation and residual wall thickness detection of different sections of the curved pipe were carried out,and the effect of process parameters on the RWT in the two forming methods was analyzed by using the single factor method.The results show that:(1)In the long straight section,with the increase of fiber percentage,melt temperature,water injection delay time,and water injection pressure,the RWT of the SWAIM curved pipe is smaller than that of the OWAIM curved pipe.For example,under the conditions of 40%fiber percentage,280℃ melt temperature,6 s water injection delay time and 10 MPa water injection pressure,the RWT of the SWAIM curved pipe is not only smaller than that of the OWAIM curved pipe,but also the difference is obvious;(2)In the transition section of straight bend angle,with the increase of process parameters,the change trend of RWT of the SWAIM curved pipe is consistent with that of OWAIM,but the experimental measurement value is smaller than that of OWAIM,and the difference of RWT between SWAIM and OWAIM is the smallest when the melt temperature is 240℃ and the water injection pressure is 8 MPa,and the difference of RWT between SWAIM and OWAIM is the largest when the water injection delay time is 2 s and the fiber mass content is 20%;(3)In the curved section,the RWT of the SWAIM curved pipe is smaller than that of OWAIM,and with t the increase of water injection pressure,the rise of melt temperature and the decrease of water injection delay time,the RWT of the two molding methods in the curved section decreases.3.Based on the OWAIM,the four factors and four levels orthogonal experimental analysis was carried out to reflect the RWT deviation rate in the curved section.The results show that,the order of the factors affecting the RWT deviation rate is fiber percentage,water injection pressure,water injection delay time and melt temperature,and the optimal combination is 40%fiber percentage,10 MPa water injection pressure,0 s water injection delay time and 280℃ melt temperature.4.Based on the SEM character and the numerical simulation analysis were carried out on P1,P3 and P7 of curved pipe with the the SWAIM and the OWAIM,the effects of injection volume,fiber percentagew,melt temperature,water injection delay time and ater injection pressure on the fiber orientation along the melt flow direction were studied.The results show that:(1)The fiber orientation distribution along the melt flow direction in the OWAIM is more continuous than that in the SWAIM with the change of parameters and positions;(2)The change of shell layer and water channel layer plays a decisive role in the fiber orientation along the melt penetration of high-pressure water injection;(3)With the increase of shear rate along the melt flow direction increases,the fiber rotates and the degree of orientation along the melt flow direction increases;(4)The larger the bending angle is,the more stable the effect of process parameters on the fiber orientation is.5.By comparison,the overflow method has more injection molding advantages in terms of fiber orientation.The four factors and four levels orthogonal experimental analysis was carried out to reflect the fiber orientation along the melt flow direction.The results show that,the order of the factors affecting the fiber orientation degree is water delay time,fiber percentage,water pressure and melt temperature,and the optimal combination is 0 s water delay time,20%fiber percentage,9 MPa water pressure and 260℃ melt temperature.