Selecting casting technology: permanent mold's perspective.

The wheel.

Along with fire and fermentation, it is set in the crown of accomplishments of mankind. So simple, yet so useful. The automobile simply would not be the same without wheels.

Each year, more than 16 million cars and light trucks are produced in North America with an average of five wheels (don't forget the spare) on each one. This totals more than 80 million wheels each year (not including the after-market) manufactured predominantly from steel (60%) or aluminum (40%).

If wheels are to be made in steel, then they typically are forged. If wheels are to be made in aluminum, then they typically are permanent mold cast. But what permanent mold casting process is best to provide the necessary economics and aesthetics for the high-production casting of aluminum wheels?

This article explores an economic analysis of permanent mold casting processes using the aluminum wheel as the benchmark casting. The permanent mold processes compared are low-pressure (LP), gravity, counter pressure (CP) and squeeze. While cast wheels because of their maturity in design and low profit margins (with high volumes) make a perfect baseline for a comparison of these technologies, all permanent mold casters (and those considering permanent mold) can see how these technologies stack up as well as learn the proper steps required for an analysis for other components.

Analysis Approach

The cost modeling approach used to perform the economic analysis of the casting technologies is based in three philosophies.

1. A manufacturing basis is critical for comparative cost estimation used to set technology strategies. Performing a cost analysis from a manufacturing basis is preferred because it allows engineers--and engineering managers--those most often tasked with making technology selection decisions--to work with and update the analysis. Costs are then broken down into discrete elements that include material, labor and equipment. The inputs that make up these elements--such as material price and part mass, number of laborers, wage and production speed, equipment investment and utilization-then can be estimated by a manufacturing-oriented user even if no hard data exists for these inputs.

For these costs, a level playing field for analyzing the four casting technologies is ensured by minimizing the possible variables between material systems and manufacturing systems. For example, geographical differences are eliminated by using a common set of cost factors for labor rates, utilities and building costs. Design differences for each process are minimized by assuming a constant macro geometry for a given wheel regardless of material system. Therefore, the wheel will look the same regardless of how it is made. The only differences for each technology are wheel weight and wall thickness.

2. Cost should be simulated from the bottom up, rather than from the top down. The implication of simulating cost from the bottom up is that it focuses the analysis on technology and ignores artificial differences that arise from differing degrees of business efficiency. By breaking the process used to make wheels into discrete manufacturing operations and by breaking each operation into specific elements of cost, cost estimating becomes analytical.

3. Reported costs should include the allocation of all capital investments on a per piece basis, including tooling. Without this assumption, the cost of machines and tooling (which come at a real cost to the manufacturer) become a separate analysis and a separate decision-making point. When evaluating new technologies and setting future technology strategies, a "fully allocated" cost basis is most appropriate.

Economic Assumptions

For the analysis, the baseline wheel casting design is a medium complexity 16-in, wheel. Each of the casting technologies starts with the same raw material (A356 aluminum alloy) at the same price ($0.85/lb). The annual production volume for this analysis was assumed to be 1.5 million wheels/year. This value is typical of high volume OEM sales but it does not typify lower volume specialty sales or after market sales.

For each of the technologies, some basic assumptions must be made to perform the analysis. Table 1 shows these assumptions. In addition, Table 2 shows the process related variables that are held constant across each of the machines because these values differ only by environment, not by technology.

The Winner?

Figure 1 shows the results from the economic analysis. As shown gravity cast wheels are the lowest cost ($32.29 lb) and squeeze cast are the highest ($40.06).

As with many high-volume automotive processes, materials are the largest single element of manufacturing cost. Gravity casting has a small disadvantage with material compared to the other processes because it has higher scrap rates. Squeeze casting has a slight advantage because it can produce a lower weight wheel with the necessary mechanical properties.

The other significant differences in cost per wheel per casting technology arise in the "value-added' segments of manufacturing costs that include labor, equipment, tooling, finishing, etc. Following is a look at some of those differences for each process.

Low pressure permanent mold casting is the dominant technology used to produce high-volume wheels. It combines a fast cycle time with moderately priced equipment and a high degree of automation. As a result, at high annual production volumes, it is the low-cost wheelmaking technology. Low pressure also is consistent and reproducible, resulting in high quality with low rejects.

Gravity casting is popular for producing lower volumes of wheels. This process is characterized by high initial equipment investments ($750,000) that are defrayed by having multiple die shuttle systems. For example, with six molds and a 300-sec cycle time, sequential pours can be made during the cooling cycle. As a consequence, the perpiece cost of capital equipment is low. Furthermore, the tooling cost for gravity casting is the lowest of the technologies. However, gravity casting has long cycle times and requires higher levels of labor.

Squeeze casting produces wheels with low porosity and comparatively higher strength. As a result, it is possible to reduce the weight of a wheel by converting it from one of the other processes to squeeze. However, squeeze casting cannot be justified economically on just conversion opportunities. The weight savings are not adequate to make up for the vastly greater capital equipment cost ($4.5 million) and tooling investment ($450,000).

Squeeze casting is a new technology and it boasts the shortest cycle time of all the alternatives, More importantly, it produces the highest strength component and would not normally be considered for casting a medium complexity wheel. The role of squeeze casting is to replace forged steel wheels with aluminum wheels at a weight savings.

Since the annual production figure for wheels is beyond the realm for medium and small size foundries, a sensitivity analysis (Fig. 2.) was also performed to show the effect of annual production volume on the economics of the casting technologies. In a sensitivity analysis, one variable is changed while the others are held constant. This allows the effect of one variable to be isolated and properly analyzed.

Wheel Futures

The future for wheels is up to individual consumers. If the popularity of alloy wheels continues then casting will continue to grow as the means to manufacture wheels. In addition, as weight continues to be a driving force for automotive engineers, more forged steel wheels will be converted to aluminum, making squeeze casting and other untested processes more viable.

This article was adapted from a paper, "Wheelmaking Technologies: A Comparison of Their Economies and Aesthetics," presented at the AFS 5" International conference on Permanent Mold Casting of Aluminum in 2000.

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Table 1

Critical Assumptions for Each Casting Technology

Process                Low Pressure     Gravity     Counter   Squeeze
                                                    Pressue
Component                 Medium Complexity Wheel

# of Dies                   1              6           1         3
Gross Material (%)          30            40          30        30
Dross (%)                   3              5           3         5
Gross Scrap (%)             8              9           8        15
Net Scrap (%)               0              0           0         0
Cycle Time (sec)           300            300         450       180
Laborers                   0.5            1.5         0.5        2
Equipment Cost ($000)      200            750         200      4500
Tooling Cost ($000)         40            210         40       1350
Tool Life (cycles)        50,000        50,000      50,000    150,000
Work Space (sq m)          250            400         250       250
Power (kW)                 250            250         250       250

Table 2

Constant Cost Factors Across Technology


Annual Production Volume              1.5 million/year
Length of Production Run                  3 years

Working Days Per Year                  250 days/year
Working Shifts Per Day                  3 shifts/day
Working Hours Per Shift                8 hours/shift

Direct Labor Wage                          $12/hr
Direct Labor Benefits                  35% of salary
Indirect Labor Salary                   $40,000/year
Indirect Labor Benefits                35% of salary
Indirect : Direct Labor Ratio         25% of laborers
Indirect : Direct Machine Ratio       25% of machines

Fabrication/Finishing Equipment Life      12 years
Assembly Equipment Life                   7 years
Building Life                             25 years
Working Capital Period                    2 months
Capital Interest Rate                     10%/year
Cost of Building Space                   $400/ sq m

For More Information

Visit www.moderncasting.com to view the entire paper, "Wheelmaking Technologies: A Comparison of Their Economies and Aesthetics," AFS 5th International Conference on Permanent Mold Casting of Aluminum, AFS, Des Plaines, IL (2000).

Proceedings from the AFS 5th International Conference on Permanent Mold Casting of Aluminum, AFS, Des Plaines, IL (2000).

RELATED ARTICLE: Inside This Story:

* Four permanent mold casting technologies are examined in a cost analysis of wheel manufacturing.

* Low-pressure permanent mold casting proves to be the most economical process for high production with gravity casting the next to consider.

* A sensitivity analysis relates the economic comparison of technologies to lower production volume components.

About the Author

John V. Busch is the former president of IBIS Associates, Inc., Waltham, Massachusetts, a consultant organization that specializes in economic analyses for manufacturing.