33 2001 August • JOM Economics Automotive Materials This paper examines the economics of sub- stituting tube-hydroformed parts for stamped assemblies. Tube hydroforming has been her- alded for its ability to decrease weight and increase stiffness compared with stamped solutions. The economics of this substitution question is examined in three case studies where tube-hydroformed components have replaced stampings using technical cost modeling, a technique developed at the Mas- sachusetts Institute of Technology. The cases illustrate different factors that influence the relative cost of tube hydroforming compared to stamping. INTRODUCTION As the automobile industry continues to call for reduced cost and weight in components and assemblies, designers and manufacturers have responded with novel manufacturing technologies that either use steel in a more economical way or use alternative materials to fulfill Substituting Tube-Hydroformed Parts for Automotive Stampings: An Economic Model Bruce Constantine, Richard Roth, and Joel P. Clark Table I. Assumptions for Analyses Material Data Material Price $0.75/kg Scrap Price $0.05/kg Material Density 7.85 g/cm 2 Material Specific Heat 690 J/kgK Tensile Strength 400 Mpa Manufacturing Data Product Life 4 y Lot Size 5,000 parts/lot Exogenous Data Days/Year 240 d/y Hours/Day 24 h/d Wage (Including Benefits) $40/h Interest Rate 25% Equipment Life 20 y Building Life 25 y Building Cost $1,500/m 2 Fixed Overhead Rate 35% Electricity Unit Cost $0.10/kWh increasingly demanding design require- ments. The mere existence of a new man- ufacturing technology, with its requisite competitive advantages and limitations, is not itself a sufficient impetus to change the manufacturing habits of an industry, however. Rather, the manufacturing technology must demonstrate a clear dis- ruption to the existing best-of-practice results before it is adopted. Most often, design teams decide which manufacturing method to use. Although the decision-making process in these teams is very complex, the advantages of the new technology must be demon- strable to motivate technology change. The design for manufacturing and as- sembly movement suggests that auto- mobile design teams proceed through a series of design decisions 1 , beginning with splitting the larger vehicle into sub- systems and modules, then choosing materials and the manufacturing method. Once these de- cisions have been made, component designs are created that optimize the capabilities of the chosen material system and manufacturing technol- ogy. Although this pro- cess is ideal, whether it functions this simply in practice is questionable. For automotive design teams, tube hydroform- ing appears to be a com- petitive manufacturing technology. First used more than 30 years ago in simple applications such as modifying pipe geometries, 2 tube hydro- forming has become a real challenger to the in- cumbent technology: stamping. Compared to traditional stamping, it promises greatly simpli- fied modules through parts consolidation, weight reduction via im- proved part design, and improved stiffness and structural strength of the Figure 1. Radiator support parts: (a) stamped; (b) hydroformed. b a components. Although performance im- provements have been widely heralded in literature, the question remains: What is the economic reality of hydroforming compared to stamping? Technical cost modeling, a technique developed at Massachusetts Institute of Technology’s (MIT) Material Systems Laboratory and described in the sidebar, is used to apply this question to three design scenarios: a radiator support assembly, an instru- ment panel beam assembly, and an ex- haust manifold. All three scenarios use a common set of materials properties, manufacturing data, and economic con- ditions (Table I). RADIATOR SUPPORT ASSEMBLY Modeling Assumptions The radiator support assembly in the current generation Dodge Dakota/ Durango represents a real case in which a tube hydroformed assembly replaced a stamped assembly. The manufacturer, Variform Corporation, 4 provided MIT with some limited information about the two part assemblies. Diagrams of the disassembled assembly, Figure 1, were assumed based on diagrams of similar disassembled radiator support assem- blies manufactured by Variform; the dis- assembly assumptions are consistent with their claim that the part count was