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University of Nevada Las Vegas |
MEG426/626 Manufacturing Processes |
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Department of Mechanical Engineering |
Fall Semester 2000 |
Casting Quality and Design Considerations
Casting Defects
:Misruns: is a casting that has solidified before completely filling the mold cavity.
Causes: 1). Less fluidity of molten metal,
2). Low pouring temperature,
3). Low pouring rate,
4). Mold cavity is too narrow.
Cold shut: occurs when two portions of the metal flow together, but there is a lack of fusion between them due to premature freezing.
Cold shots: are caused by solid globules of metal entrapped in the casting.
Cause: 1). Metal splattering,
Shrinkage cavity: is a depression in the surface or an internal void in the casting.
Cause: 1). Solidification shrinkage that restricts the amount of molten metal available in the last region to freeze.
Microporosity: refers to a network of small voids distributed through the casting.
Cause: 1). Localized solidification shrinkage of the final molten metal in the dendritic structure.
Hot tearing: occurs when the casting is restrained from contraction by an unyielding mold during the final stages of solidification or early stages of cooling after solidification.
Defects with Sand Mold Casting
:Sand blow: occurs at or below the casting surface near the top of the casting. It consists of a balloon-shaped gas cavity.
Cause: 1). Low permeability, 2). poor venting, and 3). high moisture content of the sand.
Pinholes: involves the formation of many small gas cavities at or slightly below the surface of the casting.
Cause: 1). Low permeability, 2). poor venting, and 3). high moisture content of the sand.
Sand wash: is an irregularity in the surface of the casting that results from erosion of the sand mold during pouring.
Scabs: is a rough area on the surface of the casting due to encristations of sand and metal.
Penetration: the metal fluid penetrates into the sand mold or sand core, forming a surface with mixture of sand and metal.
Mold shift: is the sidewise displacement between the cope and the drag.
Cause: by the buoyancy of the molten metal.
Core shift: is the displacement of the core relative to the molds.
Cause: by the buoyancy of the molten metal.
Mold crack: a crack may develop into which liquid metal can seap to form a fin on the final casting.
Cause: insufficient mold strength.
Design Considerations
Location of the parting line: may affect 1). The number of cores; 2). The use of effective and economical gating; 3). The weight of the final casting; 4). The method of supporting the cores; 5). The final dimensional accuracy, and 6). The ease of molding.
Geometric simplicity: will improve the castability and reduce the cost.
Corners: sharp corners and angles should be avoided.
Section thickness: should be uniform in order to avoid shrinkage cavities.
Draft: or taper is needed to facilitate removal of the pattern from the mold.
Sand casting: 10
Permanent casting: 20 to 30
Dimensional tolerances and surface finish
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Tolerances: ± 0.005 in. (± 0.12 mm) to ± 0.080 in. (± 2.0 mm)
Surface finish: 1000 m in (25 m m) to 30 m in (0.75 m m)
Machining allowances: must be left on the casting for the machining operation in sand casting. Usually about 1/16 to ¼ in.
Created by Dr. Wang