30-01-2013, 01:56 PM
METAL CASTING PROCESSES
METAL CASTING.ppt (Size: 3.48 MB / Downloads: 65)
Two Categories of Casting Processes
Expendable mold processes - mold is sacrificed to remove part
Advantage: more complex shapes possible
Disadvantage: production rates often limited by time to make mold rather than casting itself
Permanent mold processes - mold is made of metal and can be used to make many castings
Advantage: higher production rates
Disadvantage: geometries limited by need to open mold
Overview of Sand Casting
Most widely used casting process, accounting for a significant majority of total tonnage cast
Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium
Castings range in size from small to very large
Production quantities from one to millions
Making the Sand Mold
The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern
The mold must also contain gating and riser system
If casting is to have internal surfaces, a core must be included in mold
A new sand mold must be made for each part produced
Desirable Mold Properties
Strength ‑ to maintain shape and resist erosion
Permeability ‑ to allow hot air and gases to pass through voids in sand
Thermal stability ‑ to resist cracking on contact with molten metal
Collapsibility ‑ ability to give way and allow casting to shrink without cracking the casting
Reusability ‑ can sand from broken mold be reused to make other molds?
Advantages and Disadvantages
Advantages of shell molding:
Smoother cavity surface permits easier flow of molten metal and better surface finish
Good dimensional accuracy - machining often not required
Mold collapsibility minimizes cracks in casting
Can be mechanized for mass production
Disadvantages:
More expensive metal pattern
Difficult to justify for small quantities