Get a handle on investment casting

April 20, 1998

6 Min Read
Get a handle on investment casting

As the Internet and global commerce expand, high expectations of "round-the-clock" communications availability increase. Rising sophistication, coupled with more complex switching boxes and electronics, increase the burden of downtime on providers and users. To protect electrical enclosers from tampering, break-ins, and the elements, more companies specify locking mechanisms that stave off potential security breeches.

That's why engineers at A.L. Hansen Manufacturing (Waukegan, IL) designed a stainless steel security handle with double-locking capability. The 70A handle's single-component design, tight tolerances, and complex geometry made it difficult to forge, die cast, or machine. Moreover, the part required good density, uniformity and superior strength. Because virtually any volume or alloy can be applied with investment casting, it was the preferred method for production. But, as A.L. Hansen learned, its benefits can only be realized if design engineers work closely with process engineers, early in development.

To cut costs, the firm contracted outside to cast the new handle. Delivery was slow because an overseas supplier couldn't produce a satisfactory finish, or hold tolerances. A second vendor failed to supply enough components without porosities in the part, or bumps in the surface finish. The two vendors delivered parts in a piecemeal fashion, without regard for A.L. Hansen's scheduling. Parts were weak, compromised product integrity, and had poor fitup and aesthetics. After almost losing the account, A.L. Hansen's Purchasing Manager Lynn Kretsch finally contacted Lake Geneva, WI-based Northern Precision Casting Co. (NPC) for expertise in casting highly intricate parts.

To get a handle on casting density and surface finish problems, A.L. Hansen partnered closely with NPC to reduce porosity, achieve dimensional tolerances, and improve surface finish. NPC demonstrated proficiency in problem solving as well as creative engineering, according to A.L. Hansen's Engineering Manager Chuck Bullock. To determine the source of problems, they first examined A.L. Hansen's prints, patterns, and tooling design submitted for the handle. To encourage proper mold filling and improve casting densities and surface finish, NPC recommended the following production changes:

- Relocate the gate to improve casting density, maximize flow into the mold, and ensure complete filling.

- Enlarge the gate to encourage proper mold filling.

- Use A.O.D. (argon oxygen decarburization), a refined virgin material that when remelted has low slag and inclusions, to further enhance the surface finish.

- Change ceramic-shell design and formulation to reduce metal-mold reaction and entrapped gas.

"Together, these solutions allowed complete filling of the mold to ensure that no porosities occurred in the part or in the finish," says Bullock.

Effective handle performance depends on a good fit between the escutcheon and a one-piece handle-shank component. Because dimensional tolerances are critical, NPC modified the process to allow for slight variability within parameters. Wax patterns reflect precise part geometry but are larger to compensate for volumetric shrinkage. Process controls and critical-dimension gauging ensure parts meet tolerances after gate removal.

Throughout production, NPC also applied special process handling procedures to eliminate dings and handling defects. "NPC's solutions eliminated the problems with porosities and surface finishes," asserts Bullock. "The part integrity was good, the finishes were smooth, and the tolerances met specs. These improvements allowed the handle to perform effectively, and enhanced the visual appeal as well."

High-security handle. The one-piece handle and shank design withstands higher torque than two-piece designs, with a 5/16-inch square shank that rotates 90(degrees). Stainless steel construction and an electropolish finish provide corrosion resistance and extend life. Its double-locking feature consists of a padlock and a second tool-activated release mechanism inside the escutcheon. Only authorized personnel using a special tool can secure or release the cabinet door. Blind-mount handle fasteners, only accessible from inside the cabinet, further enhance security.

In addition to solving the problems, NPC acted promptly. "NPC knew we needed to get up to production quickly and make this project a priority," Kretsch says. "They produced 1,000 pieces in the first few weeks, far exceeding what we had received from the previous vendor over several months." As a result, A.L. Hansen has expanded in the telecommunications market.

"The A70 handle satisfies the need created by large telecommunications companies that want to increase security on their outdoor electrical cabinets," A.L. Hansen Marketing Manager Dennis Belmont explains. "NPC's problem-solving capability and responsiveness certainly helped the product meet its full potential."

Seven steps to quality metal parts

Although investment casting--or the "lost wax" process as it is known, has roots in China's Shang Dynasty (1766 to 1122 B.C.)--it has evolved into a cost-effective metal-forming process for producing high-quality metal parts. When performed by experienced and knowledgeable casting specialists, investment casting:

- Increases design freedom

- Provides superior repeatability

- Can use a wide variety of alloys

- Yields light, strong metal parts with superior finishes

- Reduces labor, tooling, and machining costs

- Allows rapid prototype development

1. DIE CONSTRUCTION. Create a wax duplicate pattern that matches the part's configuration and finish. A simple hand-operated, single-cavity die can be easily modified for prototypes, or multiple-cavitytooling is fully automated for volume production runs.

2. WAX-PATTERN INJECTION AND ASSEMBLY. Inject dies with wax to produce heat-expandable patterns for each part. Disposable patterns are slightly larger to accommodate shrinkage. Multiple wax patterns, connected by "gates" to wax sprues, form a cluster.

3. CERAMIC INVESTMENT. Clusters are dipped into a ceramic slurry, drained, and coated with fused silica sand. After drying, repeated dipping with progressively coarser ceramic material eventually forms a self-supporting shell ranging from 3/16- to 5/8-inch thick.

4. DE-WAXING AND CURING. Once dried, shells are heated in a steam autoclave to melt and evacuate wax from the mold. Then the ceramic mold is fire cured at temperatures between 1,600F to 2,000F.

5. ALLOYING AND CASTING. At pouring temperature levels, add elements to achieve proper chemistry and improve castability. Depending on the application, a vacuum- or centrifugal-force pour can be used.

6. SHELL RELEASE AND CASTING CLEANING. Once sufficiently cooled, mechanical vibration and chemicals remove the ceramic shell. Technicians separate parts from the cluster and prepare the casting for secondary operations such as heat treating, machining, or applied finishes.

7. QUALIFICATION. After visual and dimensional inspection, parts are x-rayed to ensure component integrity.

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