. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DESIGN DETAILS
Manufacturability Key in Complex Casting With Low Scrap Rate
Jiten Shah, Product Development & Analysis, Naperville, Illinois CASTING PROFILE
Cast Component: Control knuckle. Casting Process: Permanent mold. Material: A356 aluminum. Weight: 7 lbs. (3.2 kg). Casting Application: Automotive.
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Generous fi llets and radii provide sound and durable castings with consistent quality. • Sharp corners in permanent mold casting lead to lower tool life and, often, castings with porosity. Generous fi llets and radii help introduce liquid metal into the mold cavity smoothly with less turbulence. Turbulent fl ow re-oxidizes the liquid metal, leading to oxide- type defects. Generous draft allows smoother ejection of the casting from the mold cavity and a better surface fi nish.
rear control knuckle was developed as a permanent mold casting for an automotive application. Designing for ease of casting, which included incorporating generous radii, fi llets and draft, helped keep the scrap rate low.
The cost of tooling maintenance, dimensional stability and reproducibility dictate drilling holes vs. casting them. • Core pin life depends on the thermal mass it encounters during solidifi cation and cooling, and the dimensional accuracy of the as-cast cored hole can be affected if not designed properly. Often, it becomes a tooling maintenance problem. In the case of this rear knuckle, the small diameter of the holes would have led to a short core pin life, so it was decided to cast these features solid and drill the holes later. Sometimes, the orientation of the cored hole also makes it impossible to design a core pin for as-cast cored holes.
Heavy sections do not necessarily mean a stronger casting. • A356 aluminum alloy’s as-cast me- chanical properties are sensitive to sec- tion thickness, which infl uence cool- ing rate and subsequent microstructure and grain size. Often, better overall performance and weight savings is achieved with a carefully designed thinner section, such as using a cored pocket in a low stress area. Heavy sections also may lead to shrinkage and porosity if they are isolated.
Jiten Shah is president of Product Development & Analysis (PDA) LLC, Naperville, Ill., and a 25-year casting design and manufacturing veteran.
20 METAL CASTING DESIGN & PURCHASING MAY/JUNE 2011
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