3/17/2011
Processing of Metals: Casting
Alessandro Anzalone, Ph.D.
Hillsborough Community College, Brandon Campus
Agenda
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Introduction
The Casting Process
Patterns
Sand Casting
Evaporative Casting Process
Shell Process
Permanent Mold Casting
Slush Casting
Centrifugal Casting
Investment Casting
Shaw Process
Die Casting
Furnaces and Metal Handling
Molten Metal Safety
Pouring Practice
Casting Cleanup
Casting Design and Problems
References
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Introduction
Casting is one of the oldest methods of manufacturing metals. Prehistoric
humans made tools by pouring molten metal into open molds made of
stone or baked clay. Cast objects over 4000 years old have been found
dating from ancient Assyrian, Egyptian, and Chinese cultures.
The process of casting metals is accomplished by pouring or forcing molten
metal into a mold cavity having a desired shape. When the metal has
solidified the casting is removed from the mold. Virtually any shape can be
produced by this method, in some cases with such precision that
subsequent machining is not required.
The Casting Process
When designing a metal part to be manufactured an engineer must choose a
method of production. The part can be made by one or more processes,
including machining from solid metal, welding fabrication, powder
metallurgy, pressing and cold forming, hot forging, or casting. The main
advantage of casting over other manufacturing processes is that parts
with intricate internal cavities/passages (e.g., faucets, exhaust
manifolds) can be made. This geometric feature would be difficult or even
impossible to produce with any other manufacturing process. Besides, the
casting process results in parts with smooth, flowing designs, either for
practical or decorative purposes. Also, the metal can be placed only where
it is required. Thus, the economy of using less metal for a part (especially
when a very expensive metal is used), the eye-pleasing appearance (such
as in machinery housings), and the possibility of producing intricate
shapes are the factors that set casting apart from other manufacturing
processes.
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The Casting Process
Sand casting is typically not a rapid method of production, whereas die
casting is a relatively rapid process. Other casting processes lend
themselves to production, but none should be considered to have high
production rates as compared with punch press work or powder
metallurgy.
Casting processes involve a large segment of the metals industry. These range
from the tiniest precision parts to huge castings for machinery sections
weighing many tons. Some metals that are too difficult to machine, such
as those used for aircraft turbine impeller blades, can be cast to a
precision shape not requiring any subsequent machining. Other softer
metals, such as aluminum, are used to form articles such as transmission
cases and valve covers for automobiles, It would take many hours of
machining to make a complicated carburetor part from solid metal in
machine shop, but it takes only seconds in a die casting machine.
The Casting Process
Several general conditions must be met for the production of good castings,
regardless of the method used:
1. A method of melting the metal to the correct temperature must be
available.
2. A mold cavity of the desired shape must he formed, with sufficient
strength to contain the metal without distorting or having too much
restraint on the molten metal as it solidifies. Also, the mold must be
designed to avoid porosity and cracking of the casting.
3. Molds must be arranged so that when molten metal is introduced into the
mold, air and gases can escape so the casting will be free from gas-related
defects.
4. As we will see, molds usually are split to facilitate the removal of the
pattern and/or the casting. Depending on the cross-sectional area of the
mold cavity, just the gravitational force of the liquid metal can he enough
to separate the mold halves. Some casting methods apply additional
pressure to the metal. In either case, a force adequate to hold the mold
halves together must be provided.
5. Any mold material (or cores) in internal cavities must have a provision for
its removal. Finishing operations are often required to remove any excess
material from the casting.
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Patterns
The first requirement in making a casting is to design and make a pattern.
Patterns are usually made of wood when only a few castings are needed.
For larger quantities and when wear (due to an abrasion by the molding
material) becomes a problem, patterns are made of metal, such as
aluminum or bronze. Hard, tough plastics are also used for patterns.
Patterns include several types — single-piece, split-piece, loose-piece,
match-plate, and cope and drag. The type of pattern used in the mold
depends on the required production. For example, small production is
economical with a single-piece pattern: however, large production
requires use of mechanized molding with application of a match-plate
pattern. A previously made casting can also be used as a pattern if
additional shrinkage is not a factor.
Patterns
Shrinkage Allowance When metal solidifies from the liquid state, it
shrinks. Thus, in order to reproduce casting of a desired dimension, a
shrinkage allowance must be added to the pattern. Each metal has a
different shrinkage. The following are shrinkages in in/ft for some metals:
Cast iron
Steel
Brass
Aluminum
Magnesium
1/8
1/4
3/16
5/32
5/32
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Patterns
Draft Patterns must have draft or taper to permit removal from the mold.
Draft is usually about 2° to 3°. Without draft on the pattern, parts of the
sand mold would be broken as the pattern is pulled out.
Other Allowances If there are areas on the casting that will have machined
surfaces, the pattern needs to provide machining allowance that will leave
additional metal for removal. The amount of machining allowance
depends on the roughness and accuracy of the finished casting. Enough
material must be left for subsequent machining in order to cut under sand
inclusions on the surface of the casting.
Sand Casting
In sand casting, a specially prepared sand that is mixed with different
binders and additives is used as a mold material. The appropriately
conditioned sand is then compacted around a pattern that has the shape
of a desired casting. Sand has the advantage of being highly refractory
(can resist high temperatures without melting), so metals like cast iron
and steel can be cast easily in sand molds. Although there are many
different molding materials, sand casting accounts for the greatest
tonnage of all castings produced.
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Sand Casting
Molding Sands
The basic characteristics of sand are that it is (1) easily molded and capable
of holding accurate detail, (2) reusable, and (3) inexpensive. Sand
molding processes can be classified into the following groups: green sand
molding, heat-cured resin binder processes, cold-box resin binder
processes, no- bake resin binder processes, and silicate and phosphate
bonds. The most common type of sand molding process is green sand
molding. Green refers to the fact that the molding sand contains moisture.
Other sand molding processes, because of their higher strength, are often
used for cores that form holes and hollow spaces in green sand molds.
The most common molding sand is a natural silica sand (SiO2); however,
other sand types such as zircon (ZrSiO4)and olivine are also used in
special applications. The sands can be classified into two groups: natural
and synthetic. Natural sand is the sand in its natural form. The synthetic
sands are natural sands that have been washed, screened, classified, and
blended to meet the requirements of a particular application. The
synthetic sands are most commonly used in foundries.
Sand Casting
General Characteristics of Molding Sands
Molding sand must have several characteristics:
1. Cohesiveness The ability to be packed (rammed) in a mold and retain
its shape. This property is achieved by adding various sand additives (e.g.,
clay, water, resins).
2. Refractoriness The ability to withstand high temperatures.
3. Permeability Porosity that allows gases to escape through the mold.
4. Collapsibility The ability to allow freedom for the solidifying, shrinking
metal to move without fracturing, and to allow the cast part to be
removed easily from the mold. This property is also achieved by adding
appropriate sand additives (e.g., corn flour, dextrin).
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Sand Casting
Preparation of Green Sand Mix
The green sand mixture (sand, clay, and water) is placed in a muller , where
the ingredients are thoroughly mixed in order to obtain the proper
consistency. A typical muller consists of a large tub in which an arm with
rollers swings around, forcing the rollers over the sand.
To achieve required molding properties, the sand grains must be of the right
size, and clay and water must be added in the right proportion. To obtain
consistently good castings, the mold must be of appropriate hardness,
strength, and permeability. Some standard tests are commonly applied in
the foundries to control process variables (e.g., grain size and grain
distribution, moisture content, green strength, hardness). These tests are
performed both during sand preparation as well as during mold making in
order to assure high-quality molds and castings.
Sand Casting
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Sand Casting
Molding
A series of steps in manually producing castings are necessary. Although
many kinds of patterns are used, such as split
split-piece,
piece, single
single-piece,
piece, and
loose-piece types, patterns for green sand molding are usually made in
two halves and are called match-plate patterns, or in the case of larger
castings cope-and-drag patterns. Cope refers to the top half and drag
refers to the bottom half.
Sand Casting
The cope-and-drag pattern is placed in a flask that is made in two halves (cope
and drag) . First, the drag half of the flask is filled with sand. The sand is then
rammed into place and afterward struck off (leveled off) even with the top of
g
g The drag
g half of the flask and p
pattern is also
the flask with a straightedge.
rammed in the same manner. The pattern also has an extension called a core
print if a core is used. This space in the sand mold provides a support for the
ends of the sand core. One or more vertical holes are provided for pouring the
metal. This hole, called a sprue, has a pouring cup on the top. The role of the
pouring cup is to make pouring easy and to keep the sprue constantly filled
with metal. At its bottom the sprue is connected to the runner, which is
connected to the mold cavity through the gate. When the molten metal is
poured it fills the mold cavity and other holes in the top of the mold, called
risers. The risers are used as reservoirs to feed liquid metal to the mold cavity
as it shrinks while solidifying. They also provide for escaping gases and allow
impurities to float to the top of the riser and out of the casting. The cope-anddrag pattern is made in segments that can be separated, such as the riser and
downsprue, which are removed from the pouring side of the cope half of the
flask, and the pattern is removed from the parting line side after it is turned
over. Finally, the patterns are removed from the mold sections. Cores are
installed, the two halves of the flask are assembled, and the mold is ready for
casting.
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Sand Casting
http://www.youtube.com/watch?v=oRIbaGRB6tI&feature=related
Sand Casting
The Advantages and Disadvantages of Sand Casting
The greatest advantages of sand casting are that almost any metal can be poured
p , or weight
g off the
in the sand mold,, and there is almost no limit on size,, shape,
part. Sand casting pro- vides the most direct route from pattern to casting.
Tooling costs are low and the gravity-casting process is economical. Among the
limitations involved in sand casting is the need for machining in order to finish
the castings, especially large ones having rough surfaces. Other disadvantages
are that it is not practical in the green sand process to cast parts with long and
thin sections, and a new mold is required for every pour.
Sand castings are extensively used for machine tool housings, bases, slideways,
and other parts, and they are also extensively used in the automotive industry
for making engine blocks.
Castings must be defect free, and their quality must be consistent from casting to
casting. Statistical process control (SPC) techniques are used often to improve
quality and reduce rejection of parts.
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Evaporative Casting Process
In the evaporative casting process, the pattern, sprue, and riser are made of
foamed polystyrene. They can be made as a single piece, or they can be
made separately and then glued together. The completed polystyrene
pattern is then coated with a thin layer of refractory material. The coated
pattern is placed in the mold and dry sand is compacted/vibrated around
the pattern. When the metal is poured, the heat vaporizes the polystyrene
pattern almost instantaneously, leaving the mold shape intact as it is
being filled with metal. This process can be used for casting parts of any
size and shape, since the patterns do not need to be removed prior to
casting, thus eliminating the need for draft, on the pattern. Also,
problems related to mold shift are eliminated, since the entire pattern is
assembled before being covered with sand, allowing for corrections prior
to pouring. Another advantage of the process is the low cost of sand
preparation, since no additives or binders are used for sand conditioning.
The process is economical only for prototyping or large production, that
is, only if the polystyrene patterns themselves can be mass produced in a
die.
Evaporative Casting Process
http://www.productionnavigator.com/ventura/productnav/productie/Verlorenschuimgieten1.gif
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Shell Process
The shell molding process is a type of sand casting process that provides a
finer detail and smoother finish because the sand is finer and it is
combined with plastic resin to make a smooth mold surface. The process
can be automated for mass production.
http://www.custompartnet.com/wu/images/shell-mold-casting/shell-mold-casting-small.png
Permanent Mold Casting
The greatest disadvantage of sand casting is that a new mold must be made
for each casting. In addition, some inherent dimensional inaccuracies are
present in the sand casting. These disadvantages gave rise to the
development of a permanent mold. Despite the name, permanent molds
can be reused at most for several thousand pours, after which they lose
their true shape and must be scrapped. Most permanent molds are made
of gray cast iron or steel. Graphite molds are often used for casting highertemperature metals. The molds are made by machining processes and are
hand finished or polished. A refractory wash is applied to the mold prior
to casting in order to prolong its life. When cores are needed, they can be
made from metal, in which case they are reused, or they can be made from
sand, in which case they cannot be reused. The mold halves may be hinged
or mounted on a casting machine so they can be opened and closed
quickly and accurately.
http://www.youtube.com/watch?v=owgcK0TswBM&feature=related
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Permanent Mold Casting
http://www.custompartnet.com/wu/images/permanent-mold-casting/permanent-mold-casting.png
Slush Casting
Slush casting is used with permanent molds to make a shell of metal in the
mold. The molten metal is poured into the mold and allowed to solidify to
a certain wall thickness against the mold, and the remainder of the molten
metal is dumped out, thus producing a shell. Toys, lamp bases, and
ornamental objects are made with this process.
http://www.carhobby.com/55Mscc.JPG
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Centrifugal Casting
Centrifugal casting is a process in which molten metal is poured into a
rapidly revolving mold. The liquid metal is forced to conform to the shape
of the mold by centrifugal forces many times the force of gravity. In this
process the mold rotates about either a horizontal or a vertical axis. No
core is needed to make an inner surface, since this process naturally
produces a hollow shape such as pipe. The thickness of the mold controls
the cooling rate and therefore the grain structure of the cast part. Also, if a
hard outer wear surface is needed with an inner machinable soft metal,
two dissimilar metals can be used—the outer one to harden when
solidified and the inner one to remain relatively soft.
http://www.efunda.com/processes/metal_processing/images/casting_centrifugal.gif
Centrifugal Casting
http://www.centrifugal.net/images/index_bottomright.jpg
http://www.hycast.com.au/images/CentrifugalCasting.jpg
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Investment Casting
One of the oldest methods of casting metals is the investment casting (i.e., lost
wax) process. The pattern is made of wax coated (invested) with a thick layer
of refractory material. After the refractory material has hardened, the wax is
poured out of the mold and reused for another p
pattern. The mold
melted and p
is then preheated close to pouring temperature, and molten metal is then
poured. This method is used for dentistry, arts and crafts, and for any type of
very accurate near-net-shape casting. When it is used for industrial purposes,
significant production rates are possible.
http://www.youtube.com/watch?v=WXFRRg8YMT0
Shaw Process
In this process, a rubbery jelling agent and a slurry of refractory aggregate is
poured over the pattern. This rubbery mold hardens sufficiently to be easily
stripped off the pattern and will return to the exact shape of the pattern. The
g
to burn off the volatile elements,, and it is then placed
p
in a
mold is ignited
furnace and brought to a high temperature. The mold is then ready for
pouring. The advantage of this process is good permeability and good
collapsibility, which allow for production of delicate and intricate shapes with
fine detail and higher- quality castings.
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Die Casting
Die casting is similar to permanent molding in that a metal mold made in two
halves is used. The difference is that the metal is not gravity poured into the
mold (die), but instead the metal is injected under high pressures ranging from
,
PSI. This requires
q
massive machines that are generally
g
y
1000 to 100,000
operated hydraulically to exert the hundreds of tons of force necessary to hold
the two halves of the die together when the molten metal is being injected at
such high pressures.
http://www.azom.com/work/gmQ9Dmtd0mw9jnoTHN6z_files/image008.gif
Die Casting
http://www.welsonmold.com/images/02/Die-CastingParts/Die-Casting_4.jpg
http://www.lucidhardware.com/images/Die_Casting.jpg
http://www.youtube.com/watch?v=yQlrc4pBGkw&feature=related
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Furnaces and Metal Handling
The cupola furnace has been of primary importance in the melting of cast iron for
foundry work. Coke (produced from coal) is used for producing heat in the
cupola furnace. A typical cupola furnace is a circular steel shell lined with a
q pp with a blower,, air duct,,
refractoryy material such as fire- brick. It is equipped
and wind box with tuyeres for admitting air into the cupola. A sand bottom
keeps the molten metal from burning through and is sloped so that the iron
may flow out when the tap hole is opened.
http://www.industrialmetalcasting.com/gifs/cupola-furnace.gif
http://www.theworkshop.ca/casting/course/MTB70/2/cupola_furn.jpg
Furnaces and Metal Handling
http://www.indiamart.com/foundmaticengineers/ladles.html
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Molten Metal Safety
The hazards inherent in the handling of molten metal require thoughtful planning
and engineering to ensure they are avoided. Molten metal is a threat to the
safety of people and equipment if it is mishandled; therefore, the handling
process,, for example,
p
p , cranes,, ladles,, fork trucks,, and lifting
g devices,, must be
reliable and operate with a minimum of difficulty. The floor space or aisleways
in which such equipment is used must be designed so they are free of
obstructions and (if possible) people. Auxiliary equipment such as troughs,
molds, and stirring and skimming equipment must be engineered to avoid
spilling or splashing of metal.
Personal protective equipment varies with the metal involved but usually includes
safety glasses, face shields, hard hat, molder’s boots (or spats to cover
shoelaces and shoe openings), heat-resistant
heat resistant leggings and aprons, and heat
heatresistant gloves; some plants require employees to wear flame- retardant
cotton clothing and strictly forbid synthetic clothing. Some of these
precautions are related to the heat involved in working around furnaces and
with molten metals, but they all will also help protect the individual from
coming in contact with molten metal in case there is an explosion.
Molten Metal Safety
A dangerous aspect of molten metal is the possible consequences of its coming in
contact with water. The temperatures at which many metals are molten are
many times the boiling point of water; if molten metal manages to trap or
p y transform to steam and explode
p
cover moisture or water,, the water will rapidly
as it expands. This explosion can hurl molten metal many feet, endangering
people and equipment.
Molten metal can also explode with devastating force and consequences if it
comes in contact with an oxide of another metal under just the right (or
wrong) circumstances. This type of explosion very violent and depends on the
fact that energy is required to free metals from their oxides, therefore if the
reverse happens, that is, if a molten metal suddenly converts back to its oxide,
there is a tremendous release of energy. This can happen with all metals, but
aluminum is possibly the worst; it is estimated that the conversion of 1 lb of
aluminum to aluminum oxide releases three times the energy of a pound of
nitroglycerine! Some of these types of explosions have been known to level
whole plants.
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Pouring Practice
Ladles are usually constructed of steel and lined with firebrick or other refractory
material. The interior of the ladle is heated or kept hot while in use so as not to
cool the molten metal. In small foundries, a handheld, shank-type ladle is
pour small q
quantities. This type
yp requires
q
two p
persons to use it.
often used to p
The teapot ladle can contain more metal and is supported on an overhead
monorail or crane. The handwheel is turned to tilt the ladle and make the
pour. This and the bottom-pour types keep the slag and oxidized metal from
going into the mold. In some operations the molds are placed on a pouring
floor, whereas in other, automatic or semiautomatic, operations the molds are
carried along a conveyor to the ladles where a measured amount of molten
metal is poured into the mold.
Casting Cleanup
After the solidified castings are removed from the mold, they are cleaned in a
shakeout, and small parts are often put in a tumbler. The risers and gates must
be knocked off (often this can be done on cast iron because of its brittleness) or
g or use of an abrasive cutoff wheel.
cut off byy means of sawing
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Casting Design and Problems
Sufficient allowance of metal should always be provided when machining
operations are to be performed. Size of the casting, and surface roughness,
are involved in this decision. The location of the parting plane is also very
important, since the pattern must be extracted at this plane without
disrupting the sand mold. The following are other factors to be
considered:
1.
2.
3.
4.
5.
Weight of the casting and mold strength
Effective gating and sufficient riser
Number of cores and their placement
Required dimensional accuracy
Radii, thickness of sections, and amount of shrinkage
Cracking can occur at sharp corners and where thick sections join with thin
sections. Proper radii can reduce this problem. Also, the cooling rates are
greater for the thin section than for the thick one; this also causes
cracking at the juncture. Cooling rates may be increased in a thick section
by using “chills,” metal sections embedded in the mold to absorb heat.
References
1.
2.
3.
3
4.
5.
6.
7.
R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, ,
Modern Materials and Manufacturing Processes, Prentice Hall, 3rd
edition, 2003, ISBN: 9780130946980
http://www.metmuseum.org/toah/ho/02/wae/ho_48.180.htm
http://www.metmuseum.org/toah/ho/02/wai/ho_47.100.80.htm
p //
g/
/ / /
/
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http://www.simpsongroup.com/images/mixmuller.jpg
http://www.eriebronze.com/gallery/Pour014.jpg
http://www.eriebronze.com/gallery/Autoline.jpg
http://thelibraryofmanufacturing.com/metalcasting_sand.html
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