it exceeds such a ratio, the centrifugal force inside the rotating mold will not produce a sound casting.
A Test Case for Conversion JBT Aerotech, Salt Lake City,
which produces jetways, offers a great example of centrifugal casting’s potential benefits for both supplier and customer. For two decades, the OEM placed orders with green sand casting facilities for two large brass rotundas (Fig. 4) that attached to a hydraulic column linking the jetway to the terminal (Fig. 5). When JBT sent out a request
for quote, State Brass Foundry and Machine, also of Salt Lake City, submitted a bid that decreased costs and lead times in comparison to the green sand facility. Operating a vertical centrifugal casting machine since 1995 (Fig. 6), State Brass pro- duced the two 34 x 27.5 x 7-in. rings with the following advantages over
its green sand predecessor. • Scrap Rates: The centrifugal casting process limits shrink- age and inclusion defects, which greatly reduces the scrap rate for State Brass. When the part was
Fig. 3. In a properly designed centrifugal casting, the progressive solidification front in a centrifugal casting (left) travels from the outer dieameter inward, as opposed to the two solidification fronts competing with each other in an incorrectly designed traditional casting (right).
• Improved Yield: Considering the vertical centrifugal casting process
produced in green sand, roughly one casting in 10 required weld repair or needed to be scrapped. Following the conversion of cen- trifugal casting, the scrap rate was effectively zero.
• Lead Times: By converting to a vertical centrifugal casting process, State Brass was able to yield more parts per hour. The facility was capable of produc- ing two parts in roughly three hours in green sand. It could pour both parts in a half-hour, blast and send them directly to the machine shop without grind- ing. According to State Brass, the new process saved two hours per casting, allowing 16 parts to be produced in one eight-hour shift versus three or four when produced via green sand.
does not require gating and riser- ing, State Brass greatly improved its casting yield in production of the components, saving 50-75 lbs. of metal per part, which provided a signicant savings in a casting that starts out as a 385-lb. piece and is machined into 73-lb. and 57-lb. components.
Fig. 4. State Brass cast the 34-in. rings in its vertical centrifugal machine.
• Quality & Consistency: Because of directional solidification in the centrifugal casting process, the brass components exhibited improved mechanical properties. While not a primary concern for the customer, the increased qual- ity meant more consistent, higher performing parts.
Nov/Dec 2013 | METAL CASTING DESIGN & PURCHASING | 27
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