Investigate Of Class "A" Surface Defects Through Experiments And Numerical Simulation

0.PrefaceFierce market competition has driven automotive manufacturers to improve the quality of their cars and trucks in every aspect The Class "A" surface,i.e.the exposed surface of a vehicle's outer panel,determines the appearance of the vehicle and needs to be clean. However,many defects such as die mark,draw line,skid line,loose metal,surface low, distortion,etc are frequently introduced into the Class "A" surface during the manufacturing process.In plants,time-consuming and costly die rework and manual panel finishing are required to clean up these defects.Among them,the surface distortion and skid line are the two most challenging problems,and difficult to resolve in physical tryouts.Up to date the origins of these defects have not been well understood.These two categories of surface defects can be generated in draw die operations or trim/flange die operations.Surface low and distortions are surface irregularities that deviate from the nominal design.The deviation may be caused by incomplete forming,insufficient stretching,buckling in the panel,or spring-back.The resolution of surface distortion can be achieved by revising product design or die development(binder,addendum and draw bead),or through adjusting binder travel and binder pressure etc.However,without a clear understanding of the root cause,all these remedies can only be performed on a trial-and-error basis.In this paper our study was based on the project of National Natural Science Funds "the basic theory,calculation method and key technique of stamping and die design" and General Motor Grant "investigate and correct class "A" surface defects through experiments(draw die related surface low and distortion)".We designed a serious of experiments dies which represented the surface distortion and low of auto panel.Moreover,we also designed dynamic and stable measuring system.We analyzed the root-cause and mechanism of surface distortion based on stamping tests and numerical simulation.1.Technical details of the experiment dieWe designed experimental dies containing representative automobile outer panel product features that usually cause surface low and distortion.Prior to designing the dies,we collected and reviewed the typical product features in automotive outer panels.The geometry of a typical depression feature can be described by five design variables: Depression Depth(H),Upper Sectional Radius(R_U),Lower Sectional Radius(R_L),Plan View Radius(R_P) and Plan View Angle(α_p).The values of the five design variables vary from part to part.In practice,however,only the Plan View Radius Rp,Plan View Angleα_P and Depth H vary in a wide range.For example,the plan view angle can be anywhere from 60°to 240°.The upper sectional radius R_U and lower sectional radius R_L do not vary as much. In our design of the experiment,for simplicity,we considered variation of Rp,α_P and H while keeping R_U and R_L constant.To reduce material and construction costs through utilizing common die components,we made die inserts for these product features.The plan view angle of each insert takes one of four values(α_P=60°,90°,120°,240°) and the plan view radius has four variations(R_P=10mm,20mm,30mm,40mm) in each insert(exact values to be determined).There different depression depths have been experimented.2.Stamping test and numerical SimulationWe did stamping test of the four different redrawing characters.The surface defects can be seen around the corner of finish panels after fiction of oilstone.The preliminary tryout on these experimental dies was carried using CR4 steel,a typical material for exterior panels. Various static measurements and dynamic measurements were performed.The measurement results have shown that the designed experimental dies successfully replicated the surface distortion phenomena,which are very common in automotive surface panels.These results suggested the experimental dies,which we designed can replicate the surface defects phenomena,which are very common in automobile's outer panel.3.Correct test correct simulation of surface defectsWe designed three types of test and simulations calculations about correct solution for the surface distortion.The first method was no gap between die and panel after stamping. The results of simulation and examination showed that the surface low has been improved but there was still micro-distortion on the die.The surface distortion can be removed by the second correct solution according the results of experiment and simulation.We only did the simulation of the third solution,and the simulation results showed the low and distortion reduced obviously.