Parison Thickness Optimazation And Study On Shrinkage Of Extrusion Blowmolding Filler Pipe

With the gradual popularization of the concept of automobile lightweight,people have higher requirement for the weight and quality of automobile parts.With the development of material science,some mechanical properties of engineering plastics have approached or exceeded some metal materials,so replacing metal with plastics has become a trend.Blow molding as a low cost,high efficiency and stable production process,has become the third most commonly used plastic processing method.It is also the fastest growing plastic processing method.Blow molding products have been widely used in aerospace,automotive industry,medical health,food packaging and many other fields.When parts with complex structure,numerous features and strict quality requirements,affected by materials,production equipment and blow molding process,blow molding products often have some problems such as nonuniform wall thickness and over-tolerance size caused by shrinkage,which prolong production cycle,improve production cost,reduce production efficiency,and affect the performance of parts.Therefore,it is urgent to adopt new tools and means to control the extrusion blow molding process and solve the forming defects.The subject of this paper is extrusion blow molding for filler pipe which produced by Shanghai Longda Plastic Technology Co.,Ltd.The material of the filler pipe is HDPE5502.The filler pipe,with complex structure,has high requirements on the wall thickness of the parts,the dimension tolerance of the fitting surface and the flatness of the cutting surface.It is necessary to control the thickness strictly during parison extrusion.When the parison fits to the surface of mold cavity,the holding time and cooling time will affect the dimension shrink of the filler pipe.So it is important to set a reasonable time for opening mold.Based on the POLYFLOW,the key factors in the extrusion process of parison were analyzed.The effects of parison temperature,inlet flow rate and friction coefficient on the extrusion swell and parison quality were obtained.The analysis results show that,when the parison temperature is 170?,180?,190?,the influence of temperature on extrusion swell is not obvious.With the increase of inlet flow rate,the extrusion swell ratio decreases,but the shear rate increases significantly.When the friction coefficient of die is small,it has little effect on the extrusion swell ratio and shear rate.When the friction coefficient exceeds 1×10~5,the extrusion swell ratio and shear rate increase rapidly,and the end of the parison is rolled up,which affects the quality of the parison.Based on the idea of reverse optimization,the extruded parison is used as the initial parison of blow molding,and the parison wall thickness is optimized iteratively with the optimization objective of wall thickness.The wall thickness of filler pipe approaches the target value and filler pipe has a uniform thickness distribution after four iterations.The final parison wall thickness curve is input into the parison thickness control system for trial production.Based on CFD fluid-solid heat transfer coupling method,the cooling process of filler pipe is simulated considering the effect of internal and external cooling on the shrinkage of filler pipe,and the curve of shrinkage with time is obtained within 50s.The results show that the longer the cooling time,the smaller the shrinkage of filler pipe size,but the longer the production cycle and the lower the production efficiency.Through the actual mold testing,the filler pipe with uniform thickness distribution can be obtained by using the optimized parison thickness curve.According to the shrinkage curve,setting the cooling time as 20s,the size of filler pipe is within the required tolerance range.