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Fundamentals of Casting

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Preparing a mold cavity of the desired shape with proper allowance for shrinkage. ... produces fine detail with good dimensional accuracy and surface finish. ... – PowerPoint PPT presentation

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Title: Fundamentals of Casting


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).
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.

3
Sand Casting Terminology
4
Sand Casting Process
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.
6
Solidification Time
Sphere, cube and a cylinder with the same volume
7
Casting Defects
Hot tearing hot tearing, cracking, occurs if
casting is restrained from shrinking, during
solidification.
8
Casting Defects
These defects can be eliminated by proper mold
preparation, casting design and pouring process.
9
Casting Defects
Hot spots thick sections cool slower than other
sections causing abnormal shrinkage. Defects such
as voids, cracks and porosity are created.
10
Casting Defects and Design Consideration
11
Design Consideration
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.
13
Centrifugal Casting
True centrifugal casting
Centrifuging
14
Squeeze Casting
Cast parts have good mechanical properties, good
dimensional accuracy, fine microstructure, good
surface finish.
15
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.
16
Fabrication of Plastics
Injection Molding
17
Fabrication of Plastics
Hot-Compression Molding
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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