Casting
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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).
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Transcript of Casting
Ken Youssefi Mechanical Engineering Dept., SJSU 1
Fundamentals of CastingCasting, 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).
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.
Ken Youssefi Mechanical Engineering Dept., SJSU 3
Sand Casting Terminology
Ken Youssefi Mechanical Engineering Dept., SJSU 4
Sand Casting Process
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.
Ken Youssefi Mechanical Engineering Dept., SJSU 6
Solidification TimeSphere, cube and a cylinder with the same volume
Ken Youssefi Mechanical Engineering Dept., SJSU 7
Casting Defects
Hot tearing – hot tearing, cracking, occurs if casting is restrained from shrinking, during solidification.
Ken Youssefi Mechanical Engineering Dept., SJSU 8
Casting Defects
These defects can be eliminated by proper mold preparation, casting design and pouring process.
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.
Ken Youssefi Mechanical Engineering Dept., SJSU 10
Casting Defects and Design Consideration
Ken Youssefi Mechanical Engineering Dept., SJSU 11
Design Consideration
Ken Youssefi Mechanical Engineering Dept., SJSU 12
Die CastingHot 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.
Ken Youssefi Mechanical Engineering Dept., SJSU 13
Centrifugal Casting
True centrifugal casting
Centrifuging
Ken Youssefi Mechanical Engineering Dept., SJSU 14
Squeeze Casting
Cast parts have good mechanical properties, good dimensional accuracy, fine microstructure, good surface finish.
Ken Youssefi Mechanical Engineering Dept., SJSU 15
Precision CastingPlaster 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.
Ken Youssefi Mechanical Engineering Dept., SJSU 16
Fabrication of Plastics
Injection Molding
Ken Youssefi Mechanical Engineering Dept., SJSU 17
Fabrication of PlasticsHot-Compression Molding
Vacuum Forming
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.
