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Abstract: Surface low/distortion is one of the most challenging surface deflections that have a great effect on the exterior appearance of automobiles. Most studies on surface distortion/deflection have focused on evaluation and visualization techniques, the research work on correction or prevention of surface low/distortion is limited, and there is no perfect surface low/distortion corrective method that can satisfy the needs of the engineering. A B-spline based geometry morphing algorithm is proposed and then a new program based on UG-NX platform is developed to modify the die face in the surface low/distortion areas. To verify this developed system, the experimental dies that can replicate the surface low/distortion phenomenon successfully is put to use. Five geometric variables are introduced to describe the basic geometry of typical depression features of automotive outer panels. 57948
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The experimental dies are then designed to reflect various combinations of these geometric parameters. The stamping experiments are conducted on cold rolled grade 5(CR5) sheet steel and various static measurements, such as oil-stoning, laser scanner, etc, are performed to measure and record the surface low/distortions. Three approaches including good bearing, holds in blank and die face morphing that aim to correct low/distortions are tried out and surface low/distortions are observed in the specimen with reverse draw depth of 10 mm. The measurement results show that die morphing is a practical and effective method to correct the surface low/distortion. The correction method proposed can be used to minimize the occurrence of surface low/distortion in die manufacturing, which has certain reference significance to the correction of surface low/distortion. Key words: sheet metal forming, surface low/distortion, surface deflection, corrective method, morphing algorithm 1 Introduction The increasing social demand for the function and aesthetics has driven automotive manufacturers to improve the quality of their vehicle’s outer panels. However, many defects such as die mark, draw line, skid line, loose metal, surface low, low/distortion, etc, are frequently introduced into the exposed surface of a vehicle’s outer panels during stamping process, affect surface quality of panels and the external appearance of the vehicle body seriously. These defects mostly appear in area where sudden changes occur on the large flat exterior panels, such as corners of a sunroof, the handle portion of the exterior door panels, and the fuel filler opening, etc. Fig. 1 illustrates the typical surface defects of an automotive outer panel. Surface low/distortion, also called surface deflection, is one of the main surface defects. It is a deviation that the die surface deviates from the ideal design. The deviation is influenced by materials properties, sheet thickness, process condition, etc.
* Corresponding author. E-mail: pinghu@dlut.edu.cn This project is supported by Key Project of National Natural Science Foundation of China (Grant No. 10932003), National Hi-tech Research and Development Program of China (863 Program, Grant No. 2009AA04Z101), National Basic Research Program of China (973 Program, Grant No. 2010CB832700) © Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2012 Fig. 1. Typical surface defects of an automotive outer panel In earlier studies, SHIMOMURA, et al[1], SATOH, et al[2] and ISHIGAKI, et al[3], visually observed the growth and disappearance of surface deflection in automotive outer panels. For the past few years, more and more researchers have paid attention to the evaluation and visualization techniques. PARK, et al[4], and ANDERSON[5] tried both numerical and experimental approaches to evaluate surface defects of sheet metal parts. SUN, et al[6], developed a 2D and 3D evaluation criterion to measure and to analyze the surface deflection mathematically. YOSHIDA, et al[7–8], designed the test for studying formation of surface distortion. However, the test does not represent a typical forming process used in practical applications. FU, et al[9], designed and manufactured different experimental dies containing typical automobile outer panel product features that commonly cause surface low/distortion. By the stamping tests, he replicated the surface low/distortion phenomenon successfully through experimental dies. Furthermore, FU, et al[10], analyzed the effect of forming condition, material and curvature radius on dent resistance of panels. PARK, et al[11], clarified the effects which elastic recovery exerts on the mechanism and on the extent of the occurrence of surface deflection in actual panels. However, the origins of the surface low/distortion have not been well understood. On the other hand, the research work on correction or prevention of surface low/distortion is limited. In stamping plants, the correction of these defects, such as revising product design, die development (binder, addendum and draw bead), adjusting the binder travel and binder pressure, still depends on trial-and-error, which requires a lot of time and expense. It is necessary to find a method that aims to correct surface low/distortion with lower time consumption and cost, hence, which has application value in engineering practice. Morphing technology has been commonly used in die modification for different purposes[12–15]. In this paper, a geometry morphing method is proposed to correct the surface low/distortion, which based on FU’s experiment[9]. We first briefly explain the replication of the surface low/distortion, including relevant geometric variables and the design of experimental dies.