Optimization Of Fluid-assisted Co-injection Molding Process For Complex Muti-layer Pipe

Fluid-assisted co-injection molding(FACIM),a new injection molding process which combines fluid-assisted injection molding(FAIM) and co-injection molding technology(COIM)technology,can be used to produce hollow products of complex shape with multi-layer structure.According to the fluid it used,it can be divided into two categories: water-assisted co-injection molding(WACIM) and gas-assisted co-injection molding(GACIM).The main influencing factors and their effects on the procuct of FACIM pipes were revealed via a combination of theoretical and experimental methods.And the optimal processing parameters set was obtained.The research results was helpful to understand the FACIM process well and provided some practical guidance for the manufacture of FACIM pipes.It would be helpful topromote the application and further development of FACIM process.The main contents of this research were summarized as follows:(1)The forming principle,characteristics,the research situation and practical application of FACIM process were stated briefly.The main subjects of this research thesis were derived from the reviews.(2)In order to find the best material combination,an experimental study of different materials which are HDPE,PP,ABS,PA66(including glass fiber volume of 20%),PS,TPE,TPU and other polymers in overflow FACIM(O-FACIM) was carried out based on a lab-developed FACIM system It was found that using HDPE material as the outer layer and PP as inner material resulted in a good product with a smooth inner wall and an uniform residual wall thickness(RWT).(3)The total RWT and the inner melt RWT of O-WACIM pipes were influenced by various processing parameters.In order to get the minimum values of the total RWT and the inner melt RWT,response surface methodology(RSM) was used to optimize the processing parameters.A pipe with a 20 mm diameter and a 200 mm length was chosen as a test case.HDPE and PP were used as the skin layer and the inner layer plastics in the experiments,respectively.The main influencing factors of the total RWT and the inner melt RWT of O-WACIM were determined via single factor experiments.The result of Plackett-Burman experiments showed that the key influence factors of the total RWT and the inner melt RWT of O-WACIM.Based on Box-Behnken experiments,a RSM analysis,and a optimization,the optimized processing parameters set to get the minimum values of the total RWT and the inner melt RWT was obtained: the water pressure 7.5 MPa,the water injection delay time 2 s,and the inner melt temperature 215?.Under the optimized conditions,thepredicted total RWT of Design-expert is in good agreement with the experimental results.The research result indicated that RSM can be used to optimize the processing parameters to get the minimal total RWT and the inner melt RWT of O-WACIM pipes.(4)The simulation of WACIM pipes with curved sections was performed on the CFD software of FLUENT.The simulation results showed that there is a linear relationship between the total and the inner residual wall thickness of WACIM and the diameter of the cavity in the straight portion.The total or inner wall thickness is controlled by the cavity diameter.The simulation results at the curve sections showed that the penetrations of the inner melt and water were always close to the inner concave side due to the higher local pressure gradient and temperature there,and leaded to total RWT and the inner melt RWT of the curved pipe in overflow WACIM at the outer convex side were thicker,while those at the inner concave side were thinner.The orthogonal experimental results showed that among the influcing factors including injection parameters(injection pressure,injection time delay) and cavity parameters(the bend angle,cavity diameter,bend radius),the cavity diameter has the most significant effect on the RWT.The total average RWT and the inner melt average RWT of the curved pipe in overflow gas assisted co-injection molding(O-GACIM) were thicker and less fluctuation than those of the O-WACIM pipe.The total average RWT and the inner melt average RWT of the short-shot water assisted co-injection molding(S-WACIM) were thinner than those of the O-WACIM pipe.And the total RWT and the inner melt RWT at the outer convex side at the curved section which was filled after the start injection of the inner melt and water were much thinner than those at the same section of the O-WACIM pipe.The total RWT and the inner melt RWT at the outer convex side increased,while those at the inner concave side decreased with increasing bending angle.More even RWTs of WACIM curved pipe can be achieved based on a proper process method,a reasonable curved geometry,and an improved cooling design of the mold cavity.(5)An experimental study of the penetration interfaces in overflow FACIM was carried out based on a lab-developed FACIM system.The influence of the cavity cross-section on the residual wall thickness of outer and inner layers and penetration rate of the inner melt and water were explored by using seven cross-section cavities for the investigation and comparison.It can be seen from the experimental results that both the penetration ratios of the inner melt and the fluid increased proportionally with increasing circle ratio for WACIM and GACIM parts.the penetration ratio of gas is slightly lower than that of water.For the cavities with the same circle ratio,the cavity with larger cross-section got a lower penetration ratios of the fluid.The penetration ratios of the fluid increased in the flow direction for both the WACIM and GACIM parts with different cross-sections.