Transcript Slayt 1 - Department of Mechanical Engineering

Casting Process
CASTING PATTERNMAKING :
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In pattern making, a physical model of
casting, i.e. a pattern is used to make
the mold. The mold is made by packing
some readily formed aggregated
materials, like molding sand, around
the pattern. After the pattern is
withdrawn, its imprint leaves the mold
cavity that is ultimately filled with
metal to become the casting.
n case, the castings is required to be
hollow, such as in the case of pipe
fittings, additional patterns, known as
cores, are used to develop these
cavities.
COREMAKING & MOLDING :
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In core making, cores are
formed, (usually of sand) that
are placed into a mold cavity to
form the interior surface of the
casting. Thus the annul space
between the mold-cavity surface
and the core is what finally
becomes the casting.
Molding is a process that
consists of different operations
essential to develop a mold for
receiving molten metal
ALLOY MELTING AND POURING :
Melting is a process of
preparing the molten material
for casting. It is generally done
in a specifically designated part
of foundry, and the molten metal
is transported to the pouring
area wherein the molds are filled
CASTING CLEANING :
•The casting is separated from the mold
and transported to the cleaning
department.
•Burned-on sand and scale are
removed.
•Excess metal is removed (Fins, wires,
parting line fins, and gates).
•Subsequently the casting can be
upgraded using welding or other such
as procedures.
•Final testing and inspection to check
for any defects
Advantages:
•Improves the surface appearance and finish of casting
•Improves overall quality and functionality by removing impurities, such
as sand, scale and excess metal
Finally the sand from the mold is separated and processed through a
reclamation system for further use.
INDUSTRIAL PROCESS DESCRIPTION
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The metal casting process has been divided
into the following five major operations:
Obtaining the Casting Geometry :
The process is referred as the study of the
geometry of parts and plans, so as to improve
the life and quality of casting.
Advantages of Good Casting Geometry
Reduces defects, post casting operations, and
rejected castings
Significantly reduce energy and
environmental impacts
Saves energy
Improves overall quality and life of casting
A key part of designing a mold involves the use of
cores. Cores are preformed masses of bonded
sand or some other material that are used to
make the internal passageways of a casting.
Castings may require a single core, a complex
assembly of cores or no cores at all. Like castings,
cores are made in a mold, called a coldbox.
 Typically cores are made of sand and may be
combined with other materials that bind the sand
together. Metal cores are used in permanent
mold and diecasting processes. The type of cores
used in each metalcasting process will also be
part of your decision making process.
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TYPES OF MOLDING PROCESS
1 Expendable Mold
1.1 Permanent
Pattern
1.1.1 Sand Casting
1.1.2 Plaster
Molding
1.2 Expendable
Pattern
1.2.1 Lost Foam
1.2.2 Lost Wax
(investment
casting)
1 Permanent Mold
1.1 Die
1.1.1 Hot Chamber
1.1.2 Cold Chamber
1.1.3Thixotropic
Mold prepertation =) metal heating =) pouring =) cooling =)
processing
EXPANDABLE MOLD
PERMANENT PATTERN
SAND CASTING
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SAND CASTING
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The set of channels through which a molten metal
flows to the mold cavity is called gating system.
Typical gating system consists of a pouring cup and
a sprue receiving the poured melt, runner – a
channel through which the melt is supplied to the
gates through which the molten metal enters the
mold cavity.
EXPANDABLE MOLD
EXPANDABLE PATTERN
INVESTMENT CASTING
•Use gravity to fill the mold.
•Mold is destroyed to remove casting
•Metal flow is slow
•Walls are much thicker than in die casting.
•Cycle time is longer than die casting because of inability of mold
material to remove heat.
The investment casting process uses expendable patterns made of
investment casting wax:
The wax patterns are commonly prepared by injection molding
technology which involves injection of wax into a prefabricated die
having the same geometry of the cavity as the desired cast part.
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INVESTMENT CASTING
EXPANDABLE MOLD
PERMANENT PATTERN
INVESTMENT CASTING
SHELL MOLDING
Shell moulding is a process for producing simple or complex near
net shape castings, maintaining tight tolerances and a high
degree of dimensional stability. Shell moulding is a method for
making high quality castings. These qualities of precision can be
obtained in a wider range of alloys and with greater flexibility in
design than die-casting and at a lower cost than investment
casting.
Shell molding
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Advantages
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Better surface finish
Better dimensional tolerances.
Reduced Machining.
Less foundry space required.
Semi skilled operators can handle the process.
The process can be mechanized.
Disadvantages
•The raw materials are relatively expensive.
•The process generates noxious fumes which must be removed.
•The size and weight range of castings is limited.
EXPANDABLE MOLD
PERMANENT PATTERN
INVESTMENT CASTING
LOST FOAM CASTING
Lost foam casting (LFC) is a type of investment casting
process that uses foam patterns as a mold. The method
takes advantage of the properties of foam to simply and
inexpensively create castings that would be difficult to
achieve using other casting techniques.
EXPANDABLE MOLD
PERMANENT PATTERN
LOST FOAM CASTING
Lost foam casting (LFC) is a type of investment casting
process that uses foam patterns as a mold. The method
takes advantage of the properties of foam to simply and
inexpensively create castings that would be difficult to
achieve using other casting techniques.
Lost foam casting
Lost foam, is similar to Investment or Lost wax,
in that the medium, or pattern device, is
Expendable, they melt or evaporate away,
leaving the cast part.
 They both have advantages, for the type of
function they were designed. One Process's
advantage, could be the other Process's weak
area.
 These points are brought up in the text portion of
Education Section.
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PERMANENT MOLD
DIE CASTING
Liquid metal injected into reusable steel mold, or die,
very quickly with high pressures .
Die casting is a process in which the molten metal is
injected into the mold cavity at an increased pressure
The mold used in the die casting process is called a
die.
PERMANENT MOLD
DIE CASTING
COLD CHAMBER
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In a cold chamber process, the molten metal is ladled
into the cold chamber for each shot. There is less time
exposure of the melt to the plunger walls or the plunger.
This is particularly useful for metals such as Aluminum,
and Copper (and its alloys) that alloy easily with Iron at
the higher temperatures.
PERMANENT MOLD
DIE CASTING
HOT CHAMBER
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In a hot chamber process the pressure chamber is connected to
the die cavity is immersed permanently in the molten metal. The
inlet port of the pressurizing cylinder is uncovered as the plunger
moves to the open (unpressurized) position. This allows a new
charge of molten metal to fill the cavity and thus can fill the
cavity faster than the cold chamber process. The hot chamber
process is used for metals of low melting point and high fluidity
such as tin, zinc, and lead that tend not to alloy easily with steel
at their melt temperatures.
Manifacturing techniques
TERMS OF CASTING
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Riser: A column of metal placed in the mold to
feed the casting as it shrinks and solidifies. Also
known as a "feed head."
Riser: A column of metal placed in the mold to feed the casting as it shrinks and
solidifies. Also known as a "feed head."
Runner: The channel through which the molten metal is carried from the sprue to
the gate.
Cores: A separated part of the mold, made of sand and generally baked, which is
used to create openings and various shaped cavities in the casting.
Gate: A channel through which the molten metal enters the casting cavity.
Sand: A sand which binds strongly without losing its permeability to air or gases.
TERMS OF CASTING
Parting Line: Joint where mold separates to permit removal of the pattern. The
axe which shows how and where to open the mold
Draft: Slight taper given to a pattern to allow drawing from the sand.
Chaplet: A metal support used to hold a core in place in a mold. Not used
when a core print will serve.
Binders: Materials used to hold molding sand together.
Pouring: Filling the mold with molten metal.
Centrifugal Casting: Process of filling molds by pouring metal into a mold which
is spinning or revolving about an axis. Cast iron pipe can be created using this
method.
Shrinkage: The decrease in volume when molten metal solidifies.
SELECTING THE RIGHT METAL CASTING
PROCESS
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For any Metal Casting Process, selection of right alloy, size,
shape, thickness, tolerance, texture, and weight, is very vital.
Special requirements such as, magnetism, corrosion, stress
distribution also influence the choice of the Metal Casting
Process.
Views of the Tooling Designer; Foundry / Machine House
needs, customer's exact product requirements, and secondary
operations like painting, must be taken care of before selecting
the appropriate Metal Casting Process.
Tool cost.
Economics of machining versus process costs.
Adequate protection / packaging, shipping constraints,
regulations of the final components, weights and shelf life of
protective coatings also play their part in the Metal Casting
process.
SAND CASTING
Advantages
Least Expensive in
small quantities (less
than 100)
Ferrous and non ferrous metals may
be cast
Possible to cast very
large parts.
• Least
expensive tooling
Disadvantages
Dimensional
accuracy inferior to
other processes,
requires larger
tolerances
Castings usually
exceed calculated
weight
Surface finish of
ferrous castings
usually exceeds 125
RMS
Recommended
Application
Use when
strength/weight
ratio permits
Tolerances,
surface finish and
low machining
cost does not
warrant a more
expensive process
PERMANENT AND SEMI-PERMANENT
MOLD CASTING
Advantages
Disadvantages
Less expensive
than Investment
or Die Castings
Only non-ferrous
metals may be cast
by this process
Dimensional
Tolerances closer
than Sand
Castings
Less competitive
with Sand Cast
process when three
or more sand cores
are required
Castings are dense
and pressure tight
Higher tooling cost
than Sand Cast
Recommended
Application
Use when
process
recommended
for parts
subjected to
hydrostatic
pressure
Ideal for parts
having low
profile, no cores
and quantities in
excess of 300
PLASTER CAST
Advantages
Smooth "As Cast"
finish (25 RMS)
Closer dimensional
tolerance than Sand
Cast
• Intricate
shapes and fine
details including
thinner "As Cast"
walls are possible
• Large parts
cost less to cast than
by Investment
process
Disadvantages
More costly than
Sand or
Permanent MoldCasting
Limited number of
sources
Requires
minimum of 1 deg.
draft
Recommended
Application
Use when parts
require smooth "As
Cast" surface
finish and closer
tolerances than
possible with Sand
or Permanent
Mold Processes
INVESTMENT CAST
Advantages
Disadvantages
Close dimensional
tolerance
Costs are higher
than Sand,
Permanent Mold or
Plaster process
Castings
Complex shape, fine
detail, intricate core
sections and thin
walls are possible
Ferrous and nonferrous metals may
be cast
As-Cast" finish (64 125 RMS)
Recommended
Application
Use when
Complexity
precludes use of
Sand or
Permanent Mold
Castings
The process cost is
justified through
savings in
machining or
brazing
Weight savings
justifies increased
cost
DIE CASTING
Advantages
Good dimensional
tolerances are
possible
Excellent part-part
dimensional
consistency
Parts require a
minimal post
machining
Disadvantages
Recommended
Application
Economical only in very Use when quantity
large quantities due to
of parts justifies the
high tool cost
high tooling cost
Not recommended for
hydrostatic pressure
applications
For Castings where
penetrant (die) or
radiographic inspection
are not required.
Difficult to guarantee
minimum mechanical
properties
Parts are not
structural and are
subjected to
hydrostatic pressure