Numerical Simulation Study On Hot Melt Welding Of High Density Polyethylene Pipe

High density polyethylene pipe(HDPE pipe)has the advantages of moderate stiffness,good flexibility,reliable connection and excellent transport performance,so it is more suitable for gas pipe than other plastic pipes.With the development of the west east gas pipeline project and the construction of urban pipeline network,the use of polyethylene gas pipe in China is increasing rapidly.The high density polyethylene pipe for gas transmission must be absolutely safe and reliable,and the quality control must be very strict.Therefore this paper carried out the numerical simulation of hot melt welding of high density polyethylene pipe,the welding performance,improve welding process,improving pipe application security are very useful high density polyethylene pipes.The research contents of this paper are as follows:(1)According to the characteristics of welding process of hot melt,and CJJ63-2008"polyethylene gas pipeline engineering technical regulations",through the method of ANSYS indirect coupling to establish high density polyethylene pipe hot melt welding three-dimensional finite element model.The model considers the problem of senior high school entrance examination convective latent heat pipe and air,solid-liquid phase change heat transfer and temperature dependence of viscoelastic material,and the welding process is divided into heating stage,switching stage and cooling stage,the heating phase analytical solution,cooling phase contrast test,verify the accuracy of the results of finite element simulation of the temperature field.The distribution of stress field in the welding process,the model can accurately simulate the temperature field in the welding process and the distribution of stress field.(2)To establish the analytical model of temperature field in heating process.Its essence is to move the heat transfer boundary(Nuemann solution)research,mathematical model is established,the relationship between distribution and liquid-solid temperature distribution,the temperature along the axial direction on the location and time of the heating time and the thickness of the molten layer.(3)The influence of welding heating time,switching time and heating plate temperature on the temperature distribution and the thickness of molten layer was analyzed.The thickness of the molten layer increases with the increase of the heating plate temperature and the heating time.To obtain the same melting layer thickness,the higher the temperature of the heating plate,the shorter the time required.In the switching stage,the weldment loses the heat source and convection heat with the air.The heat loss is serious,so the shorter the switching time,the better.(4)The analysis of temperature field shows that during the heating process,because of the poor thermal conductivity of materials,the temperature is transferred from the welded end face to the non welded end surface,and the thickness of the molten layer is about 3mm-4mm.In the process of cooling,the thickness of the molten layer becomes thinner,and the cooling rate of the inner and outer surfaces is different,which makes it easy to produce a large number of micro shrinkage holes,the bonding strength is not enough,and the quality of the welded joint decreases.It is suggested that measures should be taken in the cooling stage of hot melt welding to make the cooling rate of the inner and outer surfaces of the pipeline consistent,which has an important influence on the quality of welded joints.(5)The analysis of the stress field shows that during the heating process,the welding end face expands with heat,and the expansion tendency is limited by the relatively cold zone nearby,forming thermal compression and producing compressive stress.The maximum value of the instantaneous stress appears in the inner surface of the pipe,5mm-10mm the distance from the heating end.Because of the viscoelastic property of the material,the cooling edge of the pipe is relaxed during the cooling process,and the final internal stress relaxation is finished,and there is no residual stress.The analysis results have important guiding significance to the actual welding process,and provide reference for welding technicians in formulating welding process parameters.