Experimental Research On Gas-Water-Assisted Injection Molding Process

Gas-Water-Assisted Injection Molding(GWAIM)is a new type of injection molding process that uses high-pressure gas to push the molten polymer in the mold cavity forward to fill the mold cavity to obtain a hollow pipe.Then high-pressure water is injected into the cavity obtained after the gas penetrates,and the gas is discharged while the pressure is maintained and cooled,and finally the water is drained and the mold is opened to obtain the molded pipe.Based on the research of Gas-Assisted Injection Molding(GAIM)and Water-Assisted Injection Molding(WAIM),this paper combines the two molding processes into a new GWAIM process,the pipe fittings have the excellent characteristics of smoother inner wall quality and more uniform wall thickness,and the related research is lacking.The quality of pipe fittings made by different forming processes was compared by experimental method.The influence of process parameters and different forming materials on GWAIM pipe fittings quality is studied,so that the technology can be widely used in practical life.The main contents of this research are summarized as follows:(1)Explain the process and characteristics of GWAIM process forming,briefly introduce the research status of GAIM and WAIM technology at home and abroad,and determine the research content and ideas of this article.(2)Based on the existing injection molding machine,gas-assisted molding equipment,water injection system,mold temperature controller and injection mold,according to the molding principle of GWAIM process,on the basis of the original WAIM experimental equipment,the GWAIM experimental platform was built through transformation:The device of gas injection-water injection was designed.The gas injection and water injection channels are concentrated in the same valve body to realize the switch of gas injection and water injection;The exhaust-drainage device was designed.The air cylinder controls the advance and retreat of the valve core.The valve core is used to break the surface melt condensation coating layer of the end,so that the gas and water in the pipe can be discharged from the outside;the vent hole was designed on the side of the movable mold core to ensure that the gas and water are discharged from the side of the mold,the installation hole of the exhaust-drain device was designed between the cavity and the overflow cavity.(3)Polypropylene(PP)was used as raw material,the short shot method was used to conduct experiments on three molding processes of GAIM,WAIM and GWAIM to prepare pipe fittings.The residual wall thickness,uniformity,and inner wall quality of the pipe fittings obtained by different molding processes were compared.It is found that the relationship of the residual wall thickness of the three kinds of forming pipe fittings is GAIM> GWAIM> WAIM,and the residual wall thickness of GAIM and GWAIM pipe fittings is not much different,the standard deviation of GWAIM pipe fittings wall thickness is the smallest,and its fluctuation is the smallest.By calculating the concentricity,hollowness and standard deviation,the wall thickness uniformity of three kinds of formed pipe fittings along the flow direction is analyzed.For the pipe fittings of GAIM and GWAIM molding process,the concentricity of the P1 position is low.After the P2 position,the concentricity gradually increases and there is a tendency to gradually maintain a balance.The WAIM molding process pipe fittings have been in a relatively stable state.The hollow rate of the three kinds of process pipe fittings is WAIM> GWAIM> GAIM,the quality of the inner wall of the pipe fittings GWAIM is brighter than the inner wall of GAIM pipe fittings,the wall thickness is thinner,and the wall thickness is more uniform than WAIM;Three kinds of process pipe fitting samples were prepared for scanning electron microscope(SEM)and polarized light microscope(PLM)observation.The sample can be subdivided into the near mold wall layer,the middle layer and the near inner wall layer along the wall thickness layer.The experiment found : The microscopic morphology of the GAIM process sample is generally uneven and rough.The microscopic morphology of the WAIM process sample is generally a long strip shape.The microscopic morphology of the GWAIM process sample is thin and layered with smooth texture.From the mold wall layer to the inner wall layer,the microscopic unevenness of the section gradually decreases;the three process samples have similar crystal structures.There are dense and fine spherulites in the inner wall layer,the middle layer,and the mold wall layer,but the shape and size are irregular.Under the same process,the spherulites of the near mold wall layer and the near inner wall layer are more densely distributed,the middle layer spherulites are relatively sparse,and the near mold wall layer is denser than the near inner wall layer.The distribution of spherulites in the near-inner wall layer and the near-mold wall layer of the GWAIM process samples is denser than that of the GAIM samples,and the crystal distribution of the middle layer of the samples formed by the three processes is similar.(4)The influence of main process parameters on the residual wall thickness of GWAIM pipe fittings was investigated through single factor experiment and orthogonal experiment.Polypropylene(PP)was selected as the raw material to carry out the experimental study of short shot GWAIM.The influence of process parameters on the residual wall thickness of pipe fittings was investigated by single factor method,and it was found that gas injection pressure,gas injection delay time,and mold temperature have obvious effects on the residual wall thickness of pipe fittings.Melt temperature,water injection pressure,and water injection delay time are The residual wall thickness of the pipe has little effect.An orthogonal experiment with six factors and five levels was carried out,and the influence of the main process parameters on the quality of the formed pipe fittings was studied using the range analysis method and the variance analysis method.It was found that the process parameters along the flow direction have different effects on the residual wall thickness of the pipe fittings P1,P2,P3,and P4.The best combination of process parameters to obtain the thinnest wall thickness is: melt temperature 220 ?,gas injection pressure 6MPa,injection gas delay time 7s,water injection pressure 6MPa,water injection delay time 9s,mold temperature 70?;gas injection pressure has the highest contribution rate to the wall thickness of GWAIM pipe fittings P1,P2,P3,and it is significant to the wall thickness of pipe fittings P1 and P2 influence;the wall thickness of the pipe fittings along the flow direction shows a decreasing trend;in addition,the influence of the process parameters on the wall thickness difference of the pipe fittings is also studied.The size relationship of the influence of the process parameters on the wall thickness standard deviation of the pipe fittings is: mold temperature> gas injection pressure> Melt temperature> water injection pressure>water injection delay time> gas injection delay time.(5)Four materials of HDPE,PS,PA6,and PBT were selected for WAIM and GWAIM experiments.It was found that HDPE,PS,PA6 are suitable for GWAIM process and show excellent wall thickness.GWAIM process has a wider materials applicability than WAIM,but it still has material applicability problem.The order of the overall residual wall thicknesses of diferent materials GWAIM pipe from thick to thin is PA6,HDPE,PS,PBT.The residual wall thickness of PA6 pipe is the most uniform.Observation and comparison of the cross-sectional morphology of WAIM and GWAIM specimens of different materials by scanning electron microscope(SEM)shows that the microscopic morphology of GWAIM pipes formed by different material properties will be quite different.