Original Date: 08/14/2000
Revision Date: 01/18/2007
Best Practice : Investment Casting Process
The Investment Casting Process operates as a transfer method to manufacture investment cast parts. All of the process steps must be strictly controlled. An investment casting team studied and streamlined all of the transfer steps (e.g., developing the design, using wax injection to create the pattern, pouring the liquid metal into the ceramic mold), enabling Howmedica Osteonics to significantly reduce its lead times.
In 1992, Howmedica Osteonics developed a cellular environment throughout its facility so that team-focus approaches could be directed at specific process improvements. Also that year, the company began concerted efforts to improve the quality and cost of its Investment Casting Process, a core part of its business since 1980. An investment casting team was set up as a support cell to streamline the transfer method used to manufacture parts. Previously, the company incurred high processing costs and a large inventory due to its 11% rejection rate of parts and excessive lead times.
The Investment Casting Process operates as a transfer method to manufacture investment cast parts. Among the steps are developing the design, producing the wax injection mold, using wax injection to create the pattern, assembling the pattern into a tree arrangement, adding the ceramic slurry to create the mold shell, removing the wax from the mold, preheating the mold, and pouring the liquid metal into the ceramic mold. All of the process steps must be strictly controlled. The investment casting team studied and improved all of the transfer steps. As a result, Howmedica Osteonics now produces ceramic shells from a rarely used alumina silicate ceramic material. The shelling technique involves dipping the entire pattern assembly into the ceramic slurry, draining it, and coating it with fine ceramic sand. After drying, this process is continuously repeated using progressively coarser grades of ceramic material until a self-supporting shell is formed. The various transfer steps allow the dimensional and metallurgical variations to improve through shrinkage control, mold preparation, ceramic composition of the mold, and metal casting temperatures.
Since streamlining the Investment Casting Process, Howmedica Osteonics has significantly reduced its lead times (Figure 2-1). The rejection rate of parts has also decreased from 11.4% in 1992 to 2.72% in 1999. These rates were reduced by implementing a new shell formula, revising cast techniques, establishing new setups, and refining process controls.
Figure 2-1. Investment Casting Lead Time Analysis
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