Study On Weld Line Control Technology Of Thin-walled Transparent Plastic Parts

Weld line is a defect often encountered in injection molding products.It will affect the beauty of products and reduce the mechanical properties of products,and it is more obvious in thin-walled transparent plastic parts.Thin-walled transparent plastic parts are used in microfluidic devices as microfluidic chips.The existence of weld line in microfluidic chip cover affects the detection of light transmission and reduces the accuracy of the detection results of microfluidic systems.To solve this problem,the mould design method for local heating is proposed in this paper.The object of this study is the weld line formed on the microfluidic chip cover,and the material used in the experiment is SMMA.First,a local heating mould is designed according to the assumption of local heating.The cooling pipes are used to control the overall temperature of the mould,and the heating rods are used for local heating in the formation positions of the weld lines.The local heating temperature control device is designed to adjust the temperature distribution on the mold cavity.The temperature distribution on the mold cavity is simulated and analyzed by the finite element software.Then the effectiveness of local heating to improve weld line is explored.The change law of the convergence angle of the weld line is studied by the short shot experiment.The effect of local heating on the length,width and depth of the surface V groove of the weld line are studied by means of tool microscope and laser microscope.The effects of local heating on the V groove of weld line under different melt temperature and injection pressure are compared and analyzed.The results show that with the increase of mould temperature or mould local temperature,the length,width and depth of V groove will decrease.At different melt temperatures,the average width of the V groove under local heating is reduced by 63.7%,and the average depth is reduced by 54.9%;under different injection pressure,the average width of the V groove under local heating is reduced by 69.6%,and the average depth is reduced by 70%.Local heating makes the weld line more and more thin.Thirdly,the improvement of mechanical properties of weld line by local heating is explored.The molecular orientation of weld line is simulated and analyzed by Moldflow,and the section morphology of weld line is observed and analyzed by SEM.The influence of local heating on the tensile strength of weld line is studied,and the effect of local heating on the tensile strength of weld line is compared and analyzed under different melt temperature and injection pressure.The results show that increasing the local temperature of the mould can improve the tensile strength of the weld line.At different melt temperatures,the average tensile strength of the weld line increased by 6.5% under local heating.The average tensile strength of the weld line under local heating increased by 6.2% under the different injection pressure.Finally,the effect of local heating on the warpage,birefringence and transmittance of microfluidic chip cover are investigated.Due to the local heating and the temperature distribution are the same in the moving mold and stationary part of mold,the local heating does not cause warpage.The stress birefringence distribution of products is complex,and local heating reduces residual stress to a certain extent.Local heating has a positive effect on the transmittance of thin-walled transparent products,and increasing the mold temperature or increasing the local temperature of the mold can improve the transmittance of the products.Through the study of this paper,local heating effectively improves the effect of weld line on the microfluidic chip cover,and the uneven distribution of temperature on the mold cavity surface does not bring about the quality of the molding.