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Industrial summary The stamping process of manufacturing a one-piece rear floor panel of a passenger car has been investigated in the present study. An analysis of the original die design, in which a split defect occurred at the drawn-cup wall, was performed using circle-grid analysis as well as the 3-D finite-element method. The split defect is due to the large area of sheet metal under the blank-holder that limits the flow of the metal towards the cup area. An optimum die design, which consists of a separate die face and a wedge mechanism mounted in the lower die frame, was proposed to provide additional metal for the cup area and to eliminate the split defect without adding an extra opcration. This optimum die design was validated by the results of circle-grid analysis performed for the first and second draw operations and also by the good-quality panels produced.
Keywords: Stamping dies; Rear fioor panel; Splits; Circle-grid analysis.
1. Introduction
Splits are amongst the major defects occurring commonly in the stamping process. A lot of research effort has been made to investigate the causes of and solutions to the split problem during the last few decades [1-4],the methods used including forming-limit analysis and the finite-element method. Since Keeler and Backofen [5] first introduced the concept of forming-limit diagrams (FLDs) in 1963, they have been used widely in the analysis of sheet metal forming in press shops. The FLDs indicate the strains which lead to failure and thus provide a useful tool to determine if the forming process is likely to be prone to splitting, whilst the finite-element method can calculate the strain distributions in the stamped parts accurately and thus predict if the split defect is likely to occur.
In general, the solution to problems of splitting is to provide more metal to the critical area before the major drawing process starts, This can be achieved either by decreasing the blank-holder pressure or improving the lubrication conditions, but the most straight-forward method is to add an extra operation solely for the purpose of feeding more metal into the critical area.
However, the extra operation increases the productioncost by adding one more set of dies and additional man-power; and hence in reality, it shouid be avoided.
In the present study, an optimum die design in whicha separate die face and a wedge mechanism mounted in the lower die frame is proposed to eliminate the occurrence of a split-defect in the stamping process for a onepiece floor panel of a passenger car. The special die face and wedge mechanism were designed to provide additional metal for the critical area where the split defect occurred, without adding an extra operation. Both circlegrid analysis and 3-D finite-element simulations were performed to analyzc this split defect.
2. Problem description
The common design for a rear floor panel of a passenger car is usually a two-piece type, namely, welding two stamped pieces together, as shown in Fig.l. The twopiece type design is chosen mainly due to thedifficulty encountered in stamping a one-piece rear floor panel, in which a split tends to take place at the wall of the deeply drawn cup used for storing the spare tire, as shown in Fig.2. The occurrence of the split is attributed to the substantial distance between the cup wall and one side of the blank-holder, as shown by line A-B in Fig. 3, that restricts the metal under the blank-holder from flowing into the cup area. Whilst in the two-piece design this distance is much shorter and sufficient metal can flow easily into the cup to prevent the edge of the cup from splitting, due to cost-effective considerations, a onepiece rear floor panel is always desired, so that the split problem must therefore be overcome.
The original procedure in the press shop for the production of the one-piece rear floor panel consists of four operations: first draw, second draw. trimming, and flanging. The first-draw operation was designed only to produce a cup shape, as shown in Fig. 3. As for the ribs around the cup, these were formed in the second draw operation. Like most stamping processes, the main defor-mation of the rear floor panel is completed in the first draw operation. The conventional draw process allows the punch to pull more metal into the die cavity from the blank-holder. To facilitate metal-flow, no draw beads were employed on the blank-holder surface. However,due to the large draw depth and the geometric difficulty mentioned above, a split was still found at the cup wall near the bottom after the first-draw operation.as shown in Fig. 2. The location of the split defect indicates that the considerable distance betwcen one side of the cup wall and the blank-holder does prevent the metal from flowing towards the cup. Some efforts have been made to help the metal to flow toward the cup area. The attempt of decreasing the blank-holder pressure led to morewrinkles at the root of the cup area but without eliminating the split. Improvement of the