CONTENT
2.10 Cost analysis 85
Many of the technologies described herein are aimed at achieving a reduction in component weight. Indeed, the selection of material type is based, in part, on a careful balancing of the benefit of improved fuel economy from the use of lightweight materials against the increased costs that are often incurred (see Table 3.3). Many different cost models can be applied to the evaluation of material types in the applications discussed, but general trends are shown inFigure 2.49. The relative cost benefits/disadvantages of each material type only become fully apparent in the following chapters as process chain and other indirect effects become obvious.
However, since material selection and associated costs are initially determined by cost analysis at the design and engineering phase of the chain, an overall appreciation of the relative cost balance of the various materials is included here.
A luminum
Polymer Z inc-coated steel
V olume Total cost (£M):
Investment and cumulative piece cost
FIGURE 2.49
General cost basis for automotive skin materials
2.10
Cost analysis
85For a particular panel, there may be an increased cost for plastic compared to zinc-coated steel when manufacturing in excess of a certain annual volume. This is because although tooling costs for plastics are lower than for zinc-coated steel, raw material costs are higher. Thus, as total vehicle volume increases, the cost benefit derived from polymeric panels decreases until a certain break-even volume is reached when steel becomes the most economical solution. This break-even volume is the subject of on-going debate, although it is likely to be less than 200,000 cars. It should be noted that most medium- and high-volume models involve the production of over 250,000 cars/year, which explains why use of plastics/aluminum has been mainly limited to low-volume vehicles. Nonetheless, with improvements in tech- nology, the cost advantage for polymeric panels may potentially shift to higher volumes, making the alternatives to zinc-coated steel more attractive to the auto- motive industry.
The material costs quoted here must be considered as only an approximate guide.
Each material manufacturer will produce the common material grades at different cost levels depending on the exact specification of their production equipment. In addition, geographical differences can exist; for example, EZ coatings have been considered to offer a cost advantage over galvanneal coatings in Germany while the reverse has generally been true in the UK. This may go some way to explain slight differences in material policy between European carmakers.
For a true comparison of the economics of body materials the input detail should also extend to include different design and manufacturing strategies. A comprehensive cost analysis has been demonstrated by Dieffenbach.18 He compares five different systems that could be employed to design and manufacture a mid-range sedan: steel and aluminum unibodies, steel and aluminum space- frames, and a composite structure ea cost breakdown is shown below in Table 2.3.19At low volumes costs more strongly reflect investment levels, while at higher volumes material costs have a bigger influence, and these trends are mirrored in this study. Steel is characterized by high investment cost, lower material and faster production rate. Conversely, molded plastic has a lower investment cost, a higher material cost and slower production rate. The composite monocoque has the lowest cost for volumes up to about 30,000 vehicles per year, but from 30,000e60,000 vehicles per year the steel spaceframe shows the lowest cost. For higher volumes, the steel unibody shows the lowest cost. Neither the aluminum spaceframe nor unibody show a cost advantage, although the aluminum spaceframe competes fairly well (a 15% cost penalty), and compares with the steel unibody at high volumes. For outer panel assembly sets compression molded SMC has the lowest cost for volumes up to about 100,000 sets per year, above which steel has the lowest costs.
In the future, the challenges for each category include: lowering tooling costs and scrap production (down to 25%) for steel unibodies; lowering raw material costs, e.g. by continuous casting, for aluminum unibodies; full exploitation of the potential 40% mass reduction available from the steel spaceframe; meanwhile, the aluminum spaceframe would benefit by the adoption of SMC (or similar) cladding (24%
Table 2.3 Body-in-White Cost Analysis
Key Design Inputs for Selected Case Study Alternatives Presented by Dieffenbach19 Steel
Unibody
Aluminum Unibody
Steel Spaceframe
Aluminum
Spaceframe Composite Monocoque Geometry
Overall vehicle mass (kg) 315 188 302 188 235
Mass as % of
Steel unibody 100% 60% 96% 60% 75%
Spot joints (#) 3250 3400 1000 n/a
Seam joints (cm) n/a n/a 4000 6000
Piece Count
Total piece count (#) 204 224 137 137 41
Count as % of steel unibody
100% 110% 67% 67% 20%
Number of stampings 187 207 40 40 n/a
Number of castings n/a n/a 30 30 n/a
(Continued)
2.10Costanalysis87
Table 2.3 Body-in-White Cost AnalysisdCont’d
Key Design Inputs for Selected Case Study Alternatives Presented by Dieffenbach19 Steel
Unibody
Aluminum Unibody
Steel Spaceframe
Aluminum
Spaceframe Composite Monocoque Number of roll/
hydroformings
n/a n/a 50 n/a n/a
Number of extrusions n/a n/a n/a 50 n/a
Number of moldings n/a n/a n/a n/a 7
Number of foam cores n/a n/a n/a n/a 34
Panels (inners/outers) 17 17 17 17 17
Materials
Material prices ($/kg) $0.77–0.92 $3.00–3.50 $0.77–2.20 $2.00–3.00 $3.13 Material density
(g/cm3)
7.85 2.70 7.85 2.70 1.59
Body-in-White Cost Analysis: Key Fabrication Input for Selected Case Study Alternatives Stamping Casting Hydroforming Extrusion Molding
Cycle times (s) 8–12 50–60 30–40 3–10 600–1200
No. laborers/fab’n line 4–6 2 2 2 2
Machine costs ($M) $1.3–7.5 $0.8–1.5 $1.0–2.0 $1.0–2.0 $0.5–1.0
Tool set costs ($M) $0.2–6.0 $0.1–0.2 $0.1–0.5 $3k–7k $0.1–1.2
CHAPTER2Designandmaterialutilization
cheaper than aluminum), which would make it cost competitive up to 80,000 units per year. The composite monocoque is characterized by relatively expensive materials and clearly the challenge here is to reduce raw material costs, especially for carbon fiber composites.
A second approach proposed by Dieffenbach18 is to use a stainless steel spaceframe clad with self-colored composite panels, where potential savings are made by deletion of various levels of the painting operation. This idea highlights another method of utilizing materials development to reduce costs: the concept of prepainted strip (see Chapter 9). However, it does pinpoint one target area that could produce massive savings and that is the paint shop. Costs presented for steel versus stainless steel are shown inTable 2.4.
Thus, comparing costs can be an extremely complex process requiring an inti- mate knowledge of the expected design and production scenario before accurate forecasts can be attempted. It is important to appreciate that the application of new material technologies as a means of vehicle weight reduction will usually be decided by the vehicle program development manager who may be willing to pay a cost penalty to reduce weight. This penalty may be influenced by the need for the vehicle to remain in a certain weight class or to move the vehicle into a lower weight class.
For example, in the USA higher profit luxury vehicles have a negative effect on a company’s CAFE rating. Production of a large number of heavy vehicles in this class may incur a cost penalty and the program manager may decide that the cost penalty of introducing a new materials technology will be compensated by the ultimate weight positioning of the final vehicle.
In conclusion, the main evolutionary phases of the automotive body structure have been reviewed and the role of materials introduced with respect to properties, costs and performance expected in service. The following chapter moves on to the production processes for each of these materials to provide a fuller understanding of their strengths and weaknesses, and so enable the exact specifications to meet design, process chain and environmental requirements at minimum cost, both direct and indirect.
Table 2.4 Relative Costs of Steel Unibody vs Stainless Steel Spaceframe18 Steel Unibody
Stainless Steel Spaceframe Structure
Panels Assembly Paint
$748
$191
$261
$415
$522
$191
$115
$314
Total $1,615 $1,142
The stainless steel spaceframe is found to have a cost advantage of about $375 (23%) if paint is not included, and $475 (30%) if paint is included