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Casting

• Classification of solidification processes
  • Metals
  • Glassworking
  • Polymers and PMCs
• Casting
  • Steps
    1. Melt the metal
    2. Pour it into a mold
    3. Let it freeze
• : Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity

Capabilities and Advantages of Casting

1. Can create complex part geometries
2. Can create both external and internal shapes
3. Some casting processes are net shape; others are near net shape
4. Can produce very large parts
5. Some casting methods are suited to mass production

Disadvantages of Casting

1. Limitations on mechanical properties
2. Poor dimensional accuracy and surface finish for some processes
3. Safety hazards to workers due to hot molten metals
4. Environmental problems

The Mold in Casting

• Contains cavity whose geometry determines part shape
• Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling

Two Categories of Casting Process

• Expendable mold processes: 석고, 회반죽
• Permanent mold processes: 내화물

Advantages and Disadvantages

• More intricate geometries are possible with expendable mold processes
• Part shapes in permanent mold processes are limited by the need to open mold
• Permanent mold processes are more economic in high production operations

Forming the Mold Cavity

• Mold cavity is formed by packing sand around a pattern, which gas the shape of the part
• When the pattern is removed, the ramaining cavity has desired shape of cast part
• The pattern is usually oversized to allow for shrinkage of metal as it solidifies and cools
• Sand for the mold is moist and contains a binder to maintain shape

Cores in the Mold Cavity

• The mold cavity provides the external surfaces of the cast part
• In addition a casting may have internal surfaces, determined by a core, places inside the mold cavity to define the interior geometry of part

Gating System

• Channel through which molten metal flows into cavity from outside of mold
• Consists of a downsprue, through which metal enters a runner leading to the main cavity
• At top of down sprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue

Riser

• Reservoir in the mold which is a source of liquid metal to compensate for shrinkage during solidification
• The riser must be designed to freeze after the main casting in order to satisfy its function
• 의도적으로 main cavity보다 느리게 굳도록 design
• liquid metal을 보충

Heating the Metal

• The heat required is the sum of
  • Heat to raise temperature to melting point
  • Heat of fusion to convert from solid to liquid
  • Heat to raise molten metal to desired tempreaure for pouring

Pouring the Molten Metal

• Factors that determine success
  • Pouring temperature
  • Pouring rate
  • Turbulence

Chvorinov’s Rule

• TST: total solidification time
• TST =

  

• V = Volume of the casting
• A = Surface area of casting
• n = exponent usually taken to have a value = 2
• C = mold constant