Ecxperimental Research On Residual Wall Thickness And Glass Fiber Orientation Of Gas Projectile-assisted Injection Molding Process

Gas Projectile-Assisted Injection Molding(G-PAIM)is a new type of injection molding process developed by introducing projectiles on the basis of Gas Assisted Injection Molding(GAIM).It uses high-pressure gas to push the projectile penetrate the high-temperature melt in the mold cavity,and makes hollow pipes through pressure-holding cooling.Compared with the Fluid Assisted Injection Molding(FAIM)process,this process can realize the controllability of the wall thickness of the pipes,and the produced pipes have a more uniform wall thickness,for the transportation industry,G-PAIM products have broad application prospects.The G-PAIM experimental platform was built by transforming the existing Water Assisted Injection Molding(WAIM)experimental platform.The G-PAIM process was experimentally studied,and the influence of process method and projectile size on the quality of formed pipes was investigated;the influence of processing parameters on the residual wall thickness(RWT)of G-PAIM pipes was explored and optimized.For G-PAIM products of short glass fiber reinforced polypropylene,the characteristics of glass fiber distribution and orientation and the effects of processing parameters and glass fiber content on glass fiber distribution and orientation were studied,the differences in the distribution and orientation of glass fibers at different observation positions were compared.The main contents and conclusions of this paper are as follows:(1)The process flow and principle of GAIM,WAIM,and G-PAIM were described,and the research status at home and abroad was introduced,and the research content and ideas of this paper were determined.(2)The polypropylene pipes with a diameter of 16 mm were prepared by GAIM-O,GAIMS,G-PAIM-O,and G-PAIM-S,and the influence of process methods on the quality of pipes was explored.The study found that in the straight section of the pipes,the order of the RWT of the pipes formed by the four processes is GAIM-O>GAIM-S >G-PAIM-O>G-PAIM-S,the penetration length of the GAIM-S is longer than that of the G-PAIM-S,the wall thickness uniformity of pipes formed by G-PAIM-S along the flow direction is the best.In the bending section of the pipes,the outer RWT is larger than the inner RWT,and the greater the deflection angle is,the greater the difference between the inner and outer RWT is.In terms of the quality of the inner wall of the pipes,there is foaming on the inner wall of GAIM pipes,while G-PAIM pipes have "drag marks" formed after projectile penetration at the bend of pipes.(3)The effect of projectile size on penetration in G-PAIM process was investigated.The study found that when the projectile has a certain length and a small diameter,the projectile is prone to rollover under the driving of gas.The gas will overtake the projectile from the side and the projectile cannot penetrate the melt smoothly.The RWT of the pipes is mainly determined by gas penetration.When the diameter of the projectile is constant,the projectile with a smaller length is easier to penetrate the melt under the driving of gas.Therefore,when processing projectiles,the diameter of the projectile should not be too small,for projectiles with smaller diameters,the projectiles should not be too long.(4)The effect of processing parameters on the RWT of G-PAIM-O pipes was investigated by control variable method.The results showed that with the increase of melt temperature,the RWT of the pipes decreases first and then increases;With the increase of the gas injection delay time,the RWT of the pipes shows a significant increase trend;With the increase of the gas injection pressure,the RWT of the pipes decreases,which is obvious;With the increase of melt injection pressure,the RWT of the pipes decreases first and then increases;With the increase of the mold temperature,the RWT of the pipes decreases,which is not obvious.(5)Taking the RWT of pipes as the response value,the G-PAIM processing parameters were optimized by response surface method.Through the Plackett-Burman experiment,it was found that the gas injection delay time and the gas injection pressure have a very significant influence on the RWT of the pipes.and the melt temperature has a significant effect on the RWT of the pipes,while the mold temperature and melt injection pressure have no significant effect on the RWT of the pipes.Therefore,the factors that have a significant influence on the RWT of G-PAIM pipes were screened out: gas injection delay time,gas injection pressure,and melt temperature.According to the significant factors selected by Plackett-Burman experiment,the steepest ascent experiment was designed,and the center of Box-Behnken experiment was determined.Through Box-Behnken experiment,it was found that the interaction between melt temperature and gas injection delay time is not obvious,and the interaction between melt temperature and gas injection pressure,gas injection delay time and gas injection pressure is obvious.At the same time,the optimal processing parameter combination is obtained by response surface method.(6)Using short glass fiber reinforced polypropylene as the pipe forming material,the influence of processing parameters and glass fiber content on the distribution orientation of glass fiber was explored through experiments,and the difference in glass fiber distribution orientation at different positions of pipes made by G-PAIM-S was compared.The study found that when the glass fiber content is 20%,the distribution of glass fiber in the wall thickness layer is relatively uniform by changing the melt temperature and melt injection pressure in turn;When the gas injection delay time is changed to 8s,the glass fiber in the wall thickness layer is not uniform,and the distribution density of the glass fiber near the mold wall layer is the largest;When the gas injection pressure is changed to 8MPa,the glass fiber is distributed in the middle layer and the near mold wall layer,and the glass fiber is basically not observed in the near airway layer.When the processing parameters were changed,the processing parameters have different effects on the glass fiber orientation of the wall thickness layer,but the orientation law of glass fibers in the wall thickness layer is the same: the glass fiber orientation is the best in the near airway layer,and the worst in the near mold wall layer.When the glass fiber content is10% and 20%,the distribution of the glass fiber in the wall thickness layer is relatively uniform;when the glass fiber content is 30%,the distribution of the glass fiber in the wall thickness layer is not uniform,the distribution density of the glass fiber in the mold wall layer is the largest and the smallest in the near airway layer,and when the glass fiber content is 20%,the orientation of the glass fiber in the wall thickness layer is the best.For the pipes made by the G-PAIM-S,the distribution of the glass fiber in the wall thickness layer at the primary filling stage(S1)and the secondary filling stage(2)is relatively uniform,and the orientation rule is the same;But in the near airway layer,the orientation of the glass fiber at S1 is better than that at S2,in the near mold wall layer,the orientation of glass fiber at S2 is better than at S1.