Metal-Casting Processes and Equipment
The Casting Process
(a) pouring molten metal into a mold
(b) allowing it to solidify,
(c) removing the part from the mold.
Important considerations in casting operations are as follows:
• Flow of the molten metal into the mold cavity
• Solidification and cooling of the metal in the mold
• Influence of the type of mold material.
Casting Processes
• Melt
• Pour / force molten material (liquid) into
• hollow cavity (mold) of the desired shape.
• Cool / Solidify
• Remove
• Finish
• Bronze is one of
earliest metals know
man.
• Compositions vary,
most modern bron
88% copper and 12%
• may also co
manganese, alumi
nickel, phosph
silicon, arsenic, or zin
• a mixture of
sand, clay, and
other
materials,
• molds, for large
castings, often
without a
pattern.
• Loam is soil,
that is
composed of
sand, silt, and
clay
Capabilities
• A wide variety of sizes can be produced:
• As small as 1 mm
• As large as you like (with sand casting)!
• Tolerances
• 0.01 mm to 0.1 mm
• Surface finish
• 1-3 µm (die casting)
• 10-25 µm (sand casting)
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Typical Parts
• Machine tool beds
• Engine blocks
• Power trains
• Pump casings
• Pipes and plumbing fixtures
• Jewelry
• Fire Hydrants
• Railroad products
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Solidification of Metals
• a pure metal has a clearly defined
melting (or freezing) point,
• it solidifies at a constant
temperature,
• Pure aluminum, solidifies at 660°C,
iron at 1.537°C, and tungsten at
3.410°C.
Solidification of Metals
• Metals shrink while cooling
• generally also shrink when they solidify
• shrinkage can lead to microcracking and
associated porosity,
• which can in turn compromise the
mechanical properties of the casting.
Cast structures of
metals solidified
in a square mold:
Fluid Flow
• The molten metal is poured through a pouring
basin or cup;
• then flows through the gating system (consisting
of sprue,
• runners, and gates) into the mold cavity.
• the sprue is a tapered vertical channel through
which the molten metal flows downward in the Basic gravity-casting system
mold.
the tension of the surface film of a liquid caused by
the attraction of the particles in the surface layer by
the bulk of the liquid, which tends to minimize
surface area.
Viscosity is a measure of a fluid's resistance to flow.
It describes the internal friction of a moving fluid. A
fluidwith large viscosity resists motion because its
molecular makeup gives it a lot of internal friction.
Fluid Flow
Basic gravity-casting system
• Runners are the channels that carry the molten metal from the sprue into the
mold cavity or connect the sprue to the gate which the molten metal enters the
mold cavity).
• Risers (also called feeders) serve as reservoirs of molten metal to supply any
molten metal necessary to prevent porosity due to shrinkage during solidification.
Fluid Flow
• Two basic principles of fluid flow are relevant to gating design:
• Bernoulli’s theorem and the law of mass continuity.
• Bernoulli’s Theorem: based on the principle of the conservation of energy
• relates pressure, velocity, the elevation of the fluid at any location in the system,
• the frictional losses in a system that is full of liquid.
Fluid Flow
• Mass Continuity.
• for incompressible liquids and in a system with impermeable walls
• the rate of flow is constant.
• Q is the volume rate of flow
• Flow Characteristics. An important consideration of the fluid flow in gating systems is
the presence of turbulence,
• Lower Re values generally is regarded as harmless in gating systems.
• However, Re values in excess of 20,000 represent severe turbulence, resulting in
significant air entrainment
• minimizing turbulence generally involve avoidance of sudden changes in
• flow direction
• geometry of channel cross sections in gating
Fluidity of Molten Metal
The capability of molten metal to fill mold cavities is called fluidity,
• Viscosity. As viscosity and its sensitivity to temperature (viscosity index)
increase, fluidity decreases.
• Surface Tension. A high surface tension of the liquid metal reduces fluidity.
oxide films on the surface of the molten metal have a significant adverse
effect on fluidity.
• lnclusions. Because they are insoluble, inclusions can have a significant
adverse effect on fluidity.
• Solidification Pattern of the Alloy. The manner in which solidification takes
place an influence fluidity.
Fluidity of Molten Metal
The capability of molten metal to fill mold cavities is called fluidity,
• Mold Design. The design and dimensions of the sprue, runners, and risers
all influence fluidity.
• Mold Material and its Surface Characteristics. The higher the thermal
conductivity of the mold and the rougher its surfaces, the lower the fluidity
of the molten metal.
• Degree of Superheat. Superheat (defined as the increment of temperature
of an alloy above its melting point) improves fluidity by delaying
solidification.
• Rate of Pouring. The slower the rate of pouring molten metal into the
mold, the lower the fluidity because of the higher rate of cooling when
poured slowly.
• Heat Transfer. This factor directly affects the viscosity of the liquid metal
Fluidity of Molten Metal
Heat Transfer
Heat Transfer
• The effects of mold geometry and elapsed time on skin thickness and shape:
• the unsolidified molten metal has been poured from the mold at different time
intervals, Ranging from 5 seconds to 6 minutes.
• The skin thickness increases with elapsed time,
• the skin is thinner at internal angles (location A in the figure) than at external angles
(location B).
• The latter condition is caused by slower cooling at internal angles than at external
angles
Allowances
Modifications that must be incorporated to a pattern is called
allowances. Pattern should be larger than the desired shape to
• Shrinkage allowance: compensate for shrinkage during solidification.
• Finish allowance: if a better surface is to be obtained via machining.
• Distortion allowance
• Rap (shake) allowance: To facilitate removal, pattern is shaked, which in turn
enlarges the mold. Thus, pattern should be made smaller.
• Draft (taper) allowance: Taper are necessary to facilitate pattern’s
withdrawal. Otherwise, sand particles may break away from the mold due to
the interface friction. Hence, pattern should be made larger.
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Shrinkage
• Because of their thermal expansion characteristics, metals usually shrink (contract)
• Shrink during solidification and while cooling to room temperature.
• Which causes dimensional changes and sometimes warping and cracking,
• the result of the following three sequential events:
1. Contraction of the molten metal as it cools prior to its solidification.
2. Contraction of the metal during phase change from liquid to solid (latent heat of
fusion).
3. Contraction of the solidified metal (the casting) as its temperature drops to
ambient temperature.
• The largest potential amount of shrinkage occurs during the cooling of the casting
to ambient temperature.
Note that some metals (such as gray cast iron) expand. (graphite has a relatively high specific volume, and when it precipitates
as graphite flakes during solidification of the gray cast iron, it causes a net expansion of the metal.)
Defects
A-Metallic projections, consisting of fins, flash, or projections such as swells and rough surfaces.
B-Cavities, rounded or rough internal or exposed cavities including blowholes, pinholes, and
shrinkage cavities
C-Discontinuities, cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained
from shrinking freely, cracking and tearing may occur. coarse grain size and the presence of low-
melting-point segregates along the grain boundaries (intergranular) increase the tendency for hot
tearing. Cold shut is an interface in a casting that lacks complete fusion because of the meeting of
two streams of liquid metal from different gates.
D-Defective surface, surface folds, laps, scars, adhering sand layers, and oxide scale.
E-Incomplete casting, misruns (due to premature solidification), insufficient volume of the metal
poured, and runout (due to loss of metal from the mold after pouring). Incomplete castings also can
result from the molten metal being at too low a temperature or from pouring the metal too slowly.
F-Incorrect dimensions or shape, due to factors such as improper shrinkage allowance, pattern-
mounting error, irregular contraction, deformed pattern, or Warped casting.
G-Inclusions, Which form during melting, solidification, and molding; these are generally
nonmetallic. They are regarded as harmful because they act as stress raisers and thus reduce the
strength of the casting.
Common Defects In Castings.
Solidification Time
• Solidification takes time
• Total solidification time TST
• time required for casting to solidify after pouring
• TST depends on size and shape of casting
• by relationship known as Chvorinov's Rule
Porosity
• Porosity in a casting may be caused by
shrinkage, entrained or dissolved gases,
or both.
• Porous regions can develop in castings
because of shrinkage of the solidified
metal.
• Thin sections in a casting solidify sooner
than thicker regions; as a result,
• Adequate liquid metal should be
provided to avoid cavities caused by
shrinkage.
• Internal or external chills, as those used
in sand casting
Two Categories of Metal Casting
Processes
1. 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
2. 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
3. Composite molds, which are made of two or more different materials (such as sand, graphite,
and metal) combining the advantages of each material.
• have a permanent and an expendable portion and are used in various casting processes
• to improve mold strength, control the cooling rates, and optimize the overall economics of the
casting process.
Sand Casting
• Sand is used as mold material. (Silica and SiO2 common materials)
• Cheap
• High melting temperature
• Sand grains are mixed with small amounts of other materials:
• To improve moldability
• To increase cohesive strength
• Patterns are used to prepare molds.
• To remove pattern, mold should be made, at least in two pieces.
• A new mold is prepared for each casting.
Steps in Sand Casting
1. Pour molten metal into sand mold
2. Allow metal to solidify
3. Break up the mold to remove casting
4. Clean and inspect casting
5. Heat treatment of casting is sometimes required to
improve metallurgical properties
Steps in the production sequence in sand casting
MAJOR FEATURES OF MOLDS IN SAND CASTING
I. The flask, which supports the mold itself. Two-piece molds consist of a cope on
top and a drag on the bottom; the seam between them is the parting line. When
more than two pieces are used in a sand mold, the additional parts are called
cheeks.
2. A pouring basin or pouring cup, into which the molten metal is poured.
3. A sprue, through which the molten metal flows downward.
4. The runner system, which has channels that carry the molten metal from the
sprue to the mold cavity. Gates are the inlets into the mold cavity.
5. Risers, which supply additional molten metal to the casting as it shrinks during
solidification. Two types of risers-a blind riser and an open riser-are
6. Cores, which are inserts made from sand. They are placed in the mold to form
hollow regions or otherwise define the interior surface of the casting. Cores also
are used on the outside of the casting to form features such as lettering on the
surface or deep external pockets.
Letters
Symbols
Surfaces
cavities
cope and drag: mold halves
Patterns.
• Patterns are used to mold the sand
mixture into the shape of the casting
and may be made of wood, plastic, or
metal.
• The selection of a pattern material
depends on the size and shape of the
casting, the dimensional accuracy and
the quantity of castings required, and
the molding process.
• Because patterns are used repeatedly
to make molds, the strength and
durability of the material selected for a
pattern must reflect the number of
castings that the mold will produce.
Warp: make or become bent or twisted out of
The Pattern shape, typically as a result of the effects of heat
or damp.
• A full-sized model of the part,
• slightly enlarged to account for shrinkage and machining allowances
Pattern materials:
• Wood - common material because it is easy to work, but it warps
• Metal - more expensive to make, but lasts much longer
• Plastic - compromise between wood and metal
Pattern Types
• Single pattern:
• Split pattern:
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Patterns.
• One-piece patterns, also called loose or solid patterns, generally are
used for simpler shapes and low quantity production; they generally
are made of wood and are inexpensive.
• Split patterns are two-piece patterns, made such that each part
forms a portion of the cavity for the casting; in this way, castings with
complicated shapes can be produced.
• Match-plate patterns are a common type of mounted pattern in
which two-piece patterns are constructed by securing each half of
one or more split patterns to the opposite sides of a single plate
a) solid pattern b) split pattern c) match-plate pattern
d) cope and drag pattern
Core
• Full-scale model of interior surfaces of part
• inserted into the mold cavity prior to pouring
• The molten metal flows and solidifies between the mold cavity
• the core to form the casting's external and internal surfaces
• May require supports to hold it in position in the mold cavity during
pouring, called chaplets
• Like molds, cores must possess: strength, permeability, the ability to
withstand heat, and collapsibility;
chaplets, support the core when
the core has to be suspended
inside the cavity
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Sand Casting Video
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• Investment casting
• wax pattern is coated with a refractory ceramic material.
• hardened ceramic material takes the shape of the casting.
• The wax is melted out molten metal is poured into the cavity.
Shell Molding: Casting process in which the mold is a thin shell of sand held
together by thermosetting resin binder
refractory is a
heat-resistant
material: that is,
a mineral that is
resistant to
decomposition
by heat,
pressure, or
chemical
attack.
magnesia,
dolomite and
limestone..
Ceramic Mold Casting
• mold is made of refractory ceramic materials
• that can withstand higher temperatures than plaster
• cast steels, irons, and other high-temperature alloys
• Advantages (good accuracy and finish)
• The processes include:
• Basic permanent mold casting
• Die casting
• Centrifugal casting
Coke: a solid fuel made by
heating coal in the absence of
air so that the volatile
components are driven off.
cupola furnace
Slag: stony waste
matter separated
from metals during
the smelting or
refining of ore.
Tuyère: a nozzle
through which air is
forced into a smelter,
furnace, or forge.
Crucible:a container in which metals or other substances can
be heated to very high temperatures
