Metal Casting - Processes & Materials in Manufacturing

Metal Casting: Liquid metal is poured into a cavity or mold, upon solidification the metal has the opposite shape of the cavity.

Castings produce complex parts with complex internal cavities.

• Green sand molds made from sand which has been recycled. This green sand is combined with water and a tiny amount of clay and mix.
• The cores for the internal cavity must be made from clean sand.
• 2.5 millions tons of Michigan sand from the dunes are mined each year.
• The core sand can be recycled by heating processes.

There are several key factors to consider in casting:

• The flow of the liquid metal when pouring into the cavity or mold.
• Minimizing the internal porosity knowing the solidification and cooling of the metal material.
• Type of metal and material used
• Cores, risers, parting line, draft.

Order of processes in filling a casting:

• Pouring cup: cup where the liquid metal is poured
  
• Sprue: similar to a funnel, it constricts the flow of fluid to make it flow smoother.
• Runner: channel to the side riser.
  
• Side Riser: a small tank where the fluid fills up and allows a smooth flow into the casting.
  
• Gate: the entrance to the casting.
  
• Casting
• Top Riser

Heat Transfer

Chvorinov's Rule - thin intricate shapes will solidify faster than chunk shapes.
Solidification Time = C(casting volume/Casting Surface Area)^2

Volume Shrinkage: a liquid will occupy more volume than a solid, therefore, when a casting in cooling it will shrink. A riser will be attached to the casting to allow the liquid to fill the riser when the casting is filled, so that shrinkage cavities do not form. Since the casting will shrink it is necessary that the mold is larger than the desired overall part.

Process in Lost Foam

• Start with 5% pentane in raw polystyrene beads.
• Heat with hot air to expand these beads.
• Create pattern by putting these beads inside an aluminum mold. Inject steam to soften and expand the polystyrene beads.
• Use hot glue to join the foam pattern sections together.
• Coat the pattern with ceramic wash and then dry.
• Vibrate sand around foam pattern.
• Pour liquid metal into mold. The hot metal will vaporize the pattern.
• Remove casting. Cool the sand for recycle.

Defects in Castings

• Flash
• Shrinkage
• Internal Cavities
• Mis-run
• Runout
• Slag(metal oxides)

Info on Plastics Processing found here.