Metal Casting
Metal Casting
Metal Casting
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Lesson Outcomes
By the end of this lessons, students should be able to
explain:
1. The generic metal casting process
2. The sand casting process and example products
3. The investment casting process and example
products
4. The die casting process and example products
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The metal casting process
The casting process generally involves:
1. Pouring molten metal into a mold
patterned after the part to be manufactured
2. Allowing the metal to cool and solidify
3. Removing the solid part from the mold
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Types of molds
Casting processes can be classified by the type of mold used:
• Expendable molds
– After the casting has solidified, mold is broken to remove the
casting
– Typically made of sand, plaster, ceramics
• Permanent molds
– Used repeatedly (not broken after solidification)
– Made of metals
• Composite molds
– Consists of permanent and expendable portions
– Made of two or more different materials (eg: sand, graphite, metal)
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Overview
Shell mold Good dimensional accuracy Part size limited; expensive
and surface finish; high patterns and equipment required.
production rate.
Expendable pattern Most metals cast with no limit Patterns have low strength and
to size; complex shapes can be costly for low quantities
Permanent mold Good surface finish and High mold cost; limited shape
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dimensional accuracy; low and intricacy; not suitable for
porosity; high production rate. high-melting-point metals.
Overview
Investment Intricate shapes; excellent Part size limited; expensive
surface finish and accuracy; patterns, molds, and labor.
almost any metal cast.
Permanent mold Good surface finish and High mold cost; limited shape
dimensional accuracy; low and intricacy; not suitable for
porosity; high production rate. high-melting-point metals.
Die Excellent dimensional accuracy Die cost is high; part size limited;
and surface finish; high usually limited to nonferrous
production rate. metals; long lead time.
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Examples
(a) (b)
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Schematic illustration of a sand mold, showing various features.
Sand Casting: Sand mold features
1. Flask
– Supports the mold
2. Pouring basin
– A hole into which the molten metal is poured
3. Sprue
– A vertical channel that allow the metal to flow downward
4. Runner System
– Channels that allow metal to flow from the sprue to mold cavity
5. Risers
– Store and supply additional molten metal to the casting as it shrinks during
solidification
6. Cores
– Inserts made from sand. Placed in mold to form hollow regions.
7. Vents
– Allow gasses and exhaust air to exit
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Sand casting: Mold Patterns
•Used to shape the sand mixture into the shape of the casting
•Used repeatedly to make sand molds
•Can be made of wood, plastic or metal
•Coated with parting agent to ease removal of mold from pattern
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Investment casting
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Investment Casting: Steps
Schematic
illustration of
investment
casting, (lost-wax
process).
Castings by this
method can be
made with very
fine detail and
from a variety of
metals. Source:
Steel Founders'
Society of
America.
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Investment Casting: Steps
1. Mold is normally made of metal
2. Pattern is then made by injecting wax or plastic into mold
3. A series of patterns can be assembled into a pattern tree
4. The pattern tree is then dipped into a slurry (eg: fine silica
and binders)
5. Stucco coating is applied repeatedly to increase thickness,
forming a one-piece mold
6. The one-piece mold is dried and heated to melt out the
wax (original pattern).
7. The one-piece mold is heated further to remove water and
burn off any remaining wax.
8. Molten metal is poured into the one-piece mold. After
solidification, the one-piece mold is broken up and the
casting is removed.
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Die Casting
• In permanent-mold casting, two halves of a mold are made
from materials that resist erosion and thermal fatigue such
as cast iron, steel and graphite.
• The die casting process:
– A piston traps a volume of molten metal
– Piston then forces molten metal into the die cavity through a
nozzle
– Metal is held under pressure (up to 35 MPa) until it solidifies.
– The part is then ejected by an ejector mechanism
– The die is normally cooled using circulating water or oil
• Example products:
– Engine blocks, hand tools, motor housings
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Hot-chamber die-casting
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Heated chamber
Cold-chamber die casting
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Die Casting Cavities
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Hot Chamber Die
Casting Machine
(b)
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