Cost Variables

Fig. 2

Depending on the end-use and the specific properties needed in the component, the manifold could be produced as a Class 40 gray iron casting. Here, the yield (65%) and molding rate (135 molds/hr.) are higher, helping to reduce cost to $47.50/casting.

The breakdown of costs for the three components is shown in Table 1. Please note that costs differ from facility to facility and part to part.

Table 1. Comparison of Costs for Iron Manifold

Figure 3 shows a 356 aluminum one-way flow valve. A cost-estimating model was used to show the differences between producing the casting via green sand molding on a 20 x 24 in. (508 x 610 mm) automatic molding machine and a 16 x 20 in.  (406 x 508 mm) automatic molding machine as well as producing the casting via the permanent mold process.

Fig. 3

The casting is 4.18 lbs. (1.8 kg) and requires one 9-in. (228-mm) long shell core (Fig. 4) that weighs 3.8 lbs. (1.7 kg). With an annual volume of 30,000, if the component were sand cast on a 20 x 24 in. automatic molding machine, it would cost $11.35/casting. This type of molding machine would allow for the production of 70 molds/hr. in a four-on configuration with a 55% yield. The estimate also accounts for belt grinding 180 castings/hr. and heat treatment.

Fig. 4

Also, both automatic molding machines were running well within their capabilities. The example assumes a 5% scrap rate, but that could grow as the limits of the molding machine are tested more.

“If you are pushing the limits of the process controls, the costs go up exponentially,” Creese said. “Once you start reaching the upper limits of the process, there is going to be more scrap because it is harder to control—and the costs start taking off from there.”

To show the difference in tooling cost, the same aluminum component was modeled as a semi-permanent mold casting with the same shell core. Using the same production rates, scrap rate and post-casting operations, the price to produce the semi-permanent mold component in a two-on configuration is $10.73/casting.

The permanent mold process is the least expensive in this case because the volume is high enough to justify the investment in the metal tooling. The yield is greater than in sand casting (in this case 8% higher), which reduces the melt cost per casting due to the fact that melt labor is predicated on a per pound basis as opposed to an hourly basis. That drops the labor rate significantly. Also, because the molding process is automatic, there is no need for operators, and the costs are less.

The cost breakdown for the three aluminum components is shown in Table 2.

Table 2. Comparison of Costs for Aluminum One-Way Flow Valve

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