DESIGNING CAST-IN AND MACHINED FEATURES
As-cast mounting holes in this aluminum diecast headlight part were positioned in the main die cavity without core pins. Movable slides and core pins wear out cycle after cycle and add vari- ability.
As-cast holes in iron castings reduce mass and provide better properties around the cored hole. Some cores can be oriented perpendicular to the parting plane with careful core design by casting engineers. The minimum cored hole diameter is driven by sec- tion thickness and molding process. Typically, for green sand castings, the minimum diameter is 0.5 in., with the section thickness not in excess of 1 in.
It was decided to machine rather than cast the small mounting holes on this construction frame connector due to their size (below). The slot (left) was cast-in using a core to replace expensive machining. Casting process modeling can be used to ensure cored pockets are not going to pose metal penetration and sand burn problems, which will make decoring diffi cult. When the core is surrounded by metal from both sides, its thickness is driven by the section thickness of the metal and core cross-section thickness.
Locating cored pockets and holes co-planar to the parting plane makes die fabrication easier. Locating mounting holes as part of the main die cavity provides high precision tolerance.
30 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2014
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