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sheet metal quality was also proven to be in vain. Extreme care taken with the lubrication conditions can ease the split problem: however. this is not cost-cffective for the process of mass production. Also the large quantity of lubrication oil used in the stamping process may pollute the shop floor. Hcnce. the most efficient way left to solve this problem is to provide more metal for the cup area before the punch starts to form the cup. To achieve this end, modification of the blank-holder surface to form a better binder-wrap shape which provides more metal around the cup area was considered. The binder-wrap is the deformed shape
of the sheet-blank at the closure of the blank-holders.However, due to the same geometrical reason, i.e. The considerable distance between the cup and one side of the blank holder, the optimum blank-holder surface is not casy to obtain. Finally. a separate die face designed for the first-draw operation aided by a special wedge mechanism mounted in the lower die frame provided more metal for the cup area and enabled the production of sound products without split defects.
Fig . 1 . A two-piece rear floor panel
Fig.2. A split occurring at the cup wall
Fig.3. The location of the drawn cup in the sheet-blank
TRUE STRAIN
Fig .4. The stress-strain curve for the sheet-blank.
3. Analysis of the original design
The problem of splitting is usually related to the strain distribution in the critical area. The strain distribution in any cross section of the formed part is determined by two factors: one is the amount of metal-flow resulting from drawin over the blank-holder; the other is the amount of stretch created by the contacts between the punch and the die [6]. To quantify the effect of the tooling geometry on the metal flow, the original design was analyzed by circle-grid analysis (CGA) and the finite-element method(FEM).
3.1. Circle-grid analysis
Circle grid analysis has been used widely in the press shop to measure the strain distributions and thus to enable an analysis of the formability of the sheet metal by plotting the measured strains on the forming-limit diagram. The circle grids have a major advantage over the other types of grid, such as square grids, since they do not have any preferred orientations, This advantage lies in that the deformation of the circles will result in ellipses,the two principal directions being displayed clearly by the major and minor axes. The magnitudes of the principal strains can then be calculated from the measurements of the lengths of the major and minor axes, By plotting these measured major and minor strains on the forming limit diagram, the severity of areas of a formed part can be evaluated.
In the present study, a production floor-panel made using the original die design was first analyzed with circle-grid analysis, The steel sheet used for production is 0.7 mm thick and of DDQ quality, with material properties as shown in Fig. 4. The corresponding forming-limit diagram for this material provided by the steel supplier is shown by the solid curve in Fig. 5. Any area of a formed part which is deformed with the strains located on or close to this curve will tend to split. In practice, the forming-limit curve shifted down by 10%, as shown by the dashed Line in Fig. 5, is used as the design curve. The area above the forming limit curve is called the failure zone; the area between the forming-limit curve and the design curve is termed the marginal zone; and the area below the design curve is named the safe zone. In general,the strain distributions in any area of a formed part should fall in the safe zone to make the stamping process stable, a stamping process being said to be stable if it is less sensitive to process variations.
Before being stamped, the critical area of the sheet-blank was imprinted with 5 mm diameter circles with 6-mm spacing between their centres. To imprint the circles the critical area of the sheet-blank was first cleaned using a special cleaner, then a stencil with the correct grid pattern was placed in position on the part.Using the electrolyte as conductor, the area covered by the stencil was marked with the grid pattern by an etching process. To prevent the marked area from rusting, a piece of cloth saturated with cleaner was used to wipe off excess electrolyte and residual 0xides in the marks.