Thermal Field Analysis And Experimental Research Of Electro-Permanent Magnet Cushion

Sheet metal drawing,as one of the important processes of stamping forming,is widely used in automobile,machinery,electronics and other industries in real life.Blank holder force is an important parameter in deep drawing.Reasonable blank holder force control technology can effectively improve the quality of formed parts.The electro-permanent magnet(EPM)blank holder technique uses magnetic force to provide blank holder force,which integrates power,transmission and executive components to form a loading system independent of the punch.The EPM blank holder technique can load the blank holder force independently,which simplifies the die structure to a certain extent.In the working process,the EPM blank holder technique changes the magnetic circuit direction of the cushion by injecting pulse current into the exciting coil,so as to realize the magnetization and demagnetization of the cushion,and then complete the loading and unloading of the blank holder force.Due to the frequent magnetization and demagnetization of EPM cushion during the stamping process,Joule heat generated by the exciting coil may affect the magnetic material.So it is of great significance to study the influencing factors of thermal field and the temperature distribution for the realization of EPM magnet blank holder.By using theoretical,finite element and experimental methods,the thermal field of the EPM cushions with double pole units and 36 pole units are quantified,and the thermal strain of the cushion caused by the temperature is analyzed.The cooling effect of forced convection conditions on the cushion is explored to improve the heating problem.Firstly,the heating source of the EPM cushion is calculated by theoretical method,coil parameters affecting the Joule heating are analyzed.Theoretical analysis shows that the product of coil cross-sectional area and coil turns is certain,when the magnetic flux density is constant,increasing coil turns or cross-sectional area will not reduce coil heat production.The phenomenon of demagnetization caused by temperature rise of magnetic material in the EPM cushion is analyzed.It can be seen that the permanent magnet Nd-Fe-B is more easily affected by temperature.Then,based on the finite element method,the thermal field distribution of the double pole units EPM cushion under natural and forced convection conditions is simulated.In order to study whether the maximum temperature will affect the normal operation of the cushion,the pulse current with a certain frequency is used to charge and demagnetize the magnetic for a period of time.The result shows that the maximum temperature is stable at338.800 K after 180 min,which is lower than the reversible demagnetization temperature of Nd-Fe-B permanent magnet and will not affect the normal use of the EPM cushion.Under the condition of forced convection,the maximum temperature is about 320 K.Compared with the temperature rise under natural convection condition,the result shows that the forced convection condition can greatly restrain the temperature rise,which can effectively restrain the temperature rise of suction cup.Then,taking the cushion which includes 36 magnetic pole units as the research object,the thermal field of the cushion under the natural convection condition of is analyzed by finite element method,the thermal deformation of the cushion due to temperature rise is also analyzed.The result shows that under natural convection condition,the maximum temperature of 36 pole units EPM cushion reaches 320.199 K K after 480 min,which is lower than the temperature of reversible demagnetization of magnetic material,so the temperature rise of the cushion is in a safe range.Furthermore,the thermal deformation of the 36 pole units EPM cushion is very small,which is only 0.14 mm,which will not affect the geometry of the cushion.The simulation results of thermal field of the EPM cushion under forced convection show that the temperature is remarkable lower compared with the temperature distribution of the cushion under natural convection.Finally,the temperature rise of the EPM cushion with double magnetic pole units and double magnetic pole units is studied by experiment.The temperature of the cushion is measured under natural and forced convection conditions respectively.Under the two convection conditions,the maximum temperature of the cushion will be stable below340 K,which is lower than the maximum temperature allowed by the normal operation of the permanent magnet.The experimental results are close to the results of the finite element analysis,which verifies the accuracy of the finite element simulation of the temperature rise phenomenon of the double magnetic poles unit EPM cushion.Under the condition of forced convection,the temperature rise of the double pole units EPM cushion decreases significantly.The feasibility of blank holder with 36 pole units EPM cushion was verified by experiments.08 Al blank with thickness of 1 mm and diameter of 100 mm was selected for the EPM drawing mold integrated with 36 magnetic pole units EPM cushion.The experimental results show that the EPM cushion of 36 pole units can provide enough blank holder force for the blank,which can meet the production demand of deep drawing.