Fundamentals of Casting

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Fundamentals of Casting : 

Ken Youssefi Mechanical Engineering Dept., SJSU 1 Fundamentals of Casting 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. Example of cast parts: frames, structural parts, machine components, engine blocks, valves, pipes, statues, ornamental artifacts….. Casting sizes range form few mm (teeth of a zipper) to 10 m (propellers of ocean liners).

Casting Processes : 

Ken Youssefi Mechanical Engineering Dept., SJSU 2 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.

Sand Casting Terminology : 

Ken Youssefi Mechanical Engineering Dept., SJSU 3 Sand Casting Terminology

Sand Casting Process : 

Ken Youssefi Mechanical Engineering Dept., SJSU 4 Sand Casting Process

Solidification Time : 

Ken Youssefi Mechanical Engineering Dept., SJSU 5 Solidification Time Solidification time = C(volume/surface area)2 Where C is a constant that depends on mold material and thickness, metal characteristics and temperature.

Solidification Time : 

Ken Youssefi Mechanical Engineering Dept., SJSU 6 Solidification Time Sphere, cube and a cylinder with the same volume

Casting Defects : 

Ken Youssefi Mechanical Engineering Dept., SJSU 7 Casting Defects Hot tearing – hot tearing, cracking, occurs if casting is restrained from shrinking, during solidification.

Casting Defects : 

Ken Youssefi Mechanical Engineering Dept., SJSU 8 Casting Defects These defects can be eliminated by proper mold preparation, casting design and pouring process.

Casting Defects : 

Ken Youssefi Mechanical Engineering Dept., SJSU 9 Casting Defects Hot spots – thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created.

Casting Defects and Design Consideration : 

Ken Youssefi Mechanical Engineering Dept., SJSU 10 Casting Defects and Design Consideration

Design Consideration : 

Ken Youssefi Mechanical Engineering Dept., SJSU 11 Design Consideration

Die Casting : 

Ken Youssefi Mechanical Engineering Dept., SJSU 12 Die Casting Hot chamber, low-melting alloys; tin, zinc, lead. Cold chamber, high-melting alloys; aluminum, copper. Higher pressure needed High production rates, good strength, good dimensional accuracy and surface finish, and good quality parts.

Centrifugal Casting : 

Ken Youssefi Mechanical Engineering Dept., SJSU 13 Centrifugal Casting True centrifugal casting Centrifuging

Squeeze Casting : 

Ken Youssefi Mechanical Engineering Dept., SJSU 14 Squeeze Casting Cast parts have good mechanical properties, good dimensional accuracy, fine microstructure, good surface finish.

Precision Casting : 

Ken Youssefi Mechanical Engineering Dept., SJSU 15 Precision Casting Plaster Molding Ceramic 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.) 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.

Fabrication of Plastics : 

Ken Youssefi Mechanical Engineering Dept., SJSU 16 Fabrication of Plastics Injection Molding

Fabrication of Plastics : 

Ken Youssefi Mechanical Engineering Dept., SJSU 17 Fabrication of Plastics Hot-Compression Molding

Design Considerations : 

Ken Youssefi Mechanical Engineering Dept., SJSU 18 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.

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