21-12-2012, 06:42 PM
Fundamentals of Casting
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Casting, one of the oldest manufacturing processes, dates back to 4000 B.C. when copper arrowheads were made.
Casting processes basically involve the introduction of a molten metal into a mold cavity, where upon solidification, the metal takes on the shape of the mold cavity.
Simple and complicated shapes can be made from any metal that can be melted.
Casting Processes
Preparing a mold cavity of the desired shape with proper allowance for shrinkage.
Melting the metal with acceptable quality and temp.
Pouring the metal into the cavity and providing means for the escape of air or gases.
Solidification process, must be properly designed and controlled to avoid defects.
Mold removal.
Finishing, cleaning and inspection operations.
Casting Defects
Hot spots – thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created.
Precision Casting
Plaster Molding
Plaster of Paris (gypsum) is used as the mold material, other components are added to improve the strength and reduce the setting time. The pattern is dried at 400 oF and the two halves are assembled and molten metal is poured into the cavity.
The process produces fine detail with good dimensional accuracy and surface finish. Used to cast aluminum, zinc and copper-based alloys (low-melting temp.)
Ceramic Molding
Similar to plaster molding, except it uses ceramic as a mold which is suitable for high temp. applications. Mixture of zircon (ZrSiO4), aluminum oxide (Al2O3), silica (SiO2) and other bounding agents.
Design Considerations
Wall thickness should be kept uniform if possible. Non uniform wall thickness could lead to warpage and dimensional variations.
Minimum recommended; .025 in or .65 mm, up to .125 for large parts.
Round interior and exterior corners to .01-.015 in radius (min.), prevents an edge from chipping.