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05-METAL CASTING

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MANUFACTURING PROCESS
(DTM 1042)
CHAPTER 6
METAL CASTING PROCESS
SCOPE OF LECTURE:
I. The Materials
II. The Process
III. The sand casting, die casting, continuous
casting and investment casting
IV. Common Problems in Cast Parts
LEARNING OUTCOMES: Students are able to
1. Draw a diagram and process flow for Casting sand
casting
2. Explain the process in castings.
3. Discuss the common defects in sand casting and
another castings.
CASTING in Action
Liquid
flow
casting
Casting is a method of making mold;
claylike molding compound
Casting Products
CASTING
A liquid or powder material shaped into parts
without the application of significant pressure
and does not require heat
Heat sometimes added to hasten hardening
No Significant Pressure
 Molds for casting can be made of wood, plaster,
plastic aluminium, rubber, etc.
Allows large parts to be made, no size limit
 Occasional voids in the part
 Poor dimensional control
Mold Casting
 The major categories are as follows:
1. Expendable molds, which typically are made of
sand, plaster, ceramics, and similar materials and
generally are mixed with various binders (bonding
agents) for improved properties.
2. Permanent molds, which are made of metals
that maintain their strength at high temperatures.
3. Composite molds, which are made of two or
more different materials (such as sand, graphite,
and metal) combining the advantages of each
material
FIGURE 11.1 (a) Typical gray-iron castings used in automobiles, including the transmission valve body (left)
and the hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General
Motors Corporation. (b) A cast transmission housing. (c) The Polaroid PDC-2000 digital camera with an
AZ191D die-cast, high-purity magnesium case. (d) A two-piece Polaroid camera case made by the hotchamber die-casting process. Source: (c) and (d) Courtesy of Polaroid Corporation and Chicago White Metal
Casting, Inc.
Sand casting
 A sand casting or a sand molded casting is a
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cast part produced by forming a mold from a
sand mixture and then pouring molten liquid
metal into the cavity in the mold.
The mold is then cooled until the metal has
solidified.
In the last stage, the casting is separated from
the mold
Sand casting typically has a low production rate.
The sand casting process involves the use of a
furnace, metal, pattern, and sand mold.
Process Flow Of Sand Casting
Product of sand casting
 Sand casting is used to produce a wide
variety of metal components with complex
geometries
 Some smaller sand cast parts: components
as gears, pulleys, crankshafts, connecting
rods, and propellers.
 Larger applications include housings for large
equipment and heavy machine bases.
 Sand casting is also common in producing
automobile components, such as engine
blocks, engine manifolds, cylinder heads, and
transmission cases.
Example…
Process in sand casting
 The process cycle for sand casting consists of six
main stages
1. Mold-making – The first step in the sand casting
process is to create the mold for the casting. A
sand mold is formed by packing sand into each
half of the mold.
2. Clamping - Once the mold has been made, it
must be prepared for the molten metal to be
poured. The surface of the mold cavity is first
lubricated to facilitate the removal of the
casting. Then, the cores are positioned and the
mold halves are closed and securely clamped
together.
3. Pouring - The molten metal is maintained at a
set temperature in a furnace. the molten metal
can be ladled from its holding container in the
furnace and poured into the mold. The pouring
can be performed manually or by an automated
machine. The filling time is very short in order
to prevent early solidification of any one part of
the metal.
4. Cooling -The molten metal that is poured into
the mold will begin to cool and solidify once it
enters the cavity. When the entire cavity is filled
and the molten metal solidifies, the final shape
of the casting is formed.
5. Removal - After the predetermined
solidification time has passed, the sand
mold can simply be broken, and the casting
removed. Shot blasting is sometimes used
to remove any remaining sand, especially
from internal surfaces, and reduce the
surface roughness.
6. Trimming - During cooling, the material from
the channels in the mold solidifies attached
to the part. This excess material must be
trimmed from the casting either manually
via cutting or sawing, or using a trimming
press.
Possible Defects in sand casting
Defect
Unfilled sections
Causes
Insufficient material
Low pouring temperature
Porosity
Melt temperature is too
high
Non-uniform cooling rate
Sand has low permeability
Hot tearing
Surface projections
Non-uniform cooling rate
Erosion of sand mold
interior
A crack in the sand mold
Mold halves shift
Die Casting
Die casting’s product
 Die casting is a manufacturing process that
can produce geometrically complex metal
parts through the use of reusable molds,
called dies.
 The die casting process involves the use of a
furnace, metal, die casting machine, and die.
 The metal, typically a non-ferrous alloy such
as aluminum or zinc, is melted in the furnace
and then injected into the dies in the die
casting machine.
 There are two main types of die casting
machines - hot chamber machines (used for
alloys with low melting temperatures, such as
zinc) and cold chamber machines (used for
alloys with high melting temperatures, such
as aluminum).
 However, in both machines, after the molten
metal is injected into the dies, it rapidly cools
and solidifies into the final part, called the
casting.
Die casting hot chamber
 Hot chamber die casting machine - Hot
chamber machines are used for alloys with
low melting temperatures, such as zinc, tin,
and lead. The temperatures required to melt
other alloys.
 The metal is contained in an open holding pot
which is placed into a furnace, where it is
melted to the necessary temperature.
 The molten metal then flows into a shot
chamber through an inlet and a plunger,
powered by hydraulic pressure, forces the
molten metal through a gooseneck channel
and into the die.
 Typical injection pressures for a hot chamber
die casting machine are between 1000 and
5000 psi.
 After the molten metal has been injected into
the die cavity, the plunger remains down,
holding the pressure while the casting
solidifies.
 After solidification, the hydraulic system
retracts the plunger and the part can be
ejected by the clamping unit.
FIGURE below Schematic illustration of the hot-chamber die-casting
process.
Hot chamber die casting
machine - Opened
Hot chamber die casting
machine - Closed
Die casting cold chamber
 Cold chamber die casting machine - Cold
chamber machines are used for alloys with
high melting temperatures that can not be
cast in hot chamber machines because they
would damage the pumping system.
 Such alloys include aluminum, brass, and
magnesium.
 The molten metal is still contained in an open
holding pot which is placed into a furnace,
where it is melted to the necessary
temperature.
 The metal is poured from the ladle into the
shot chamber through a pouring hole.
 The injection system in a cold chamber
machine functions similarly to that of a hot
chamber machine, however it is usually
oriented horizontally and does not include a
gooseneck channel.
 A plunger, powered by hydraulic pressure,
forces the molten metal through the shot
chamber and into the injection sleeve in the
die.
 The typical injection pressures for a cold
chamber die casting machine are between
2000 and 20000 psi.
 After the molten metal has been injected into
the die cavity, the plunger remains forward,
holding the pressure while the casting
solidifies
 After solidification, the hydraulic system
retracts the plunger and the part can be
ejected by the clamping unit
FIGURE below Schematic illustration of the cold-chamber die-casting
process. These machines are large compared to the size of the casting,
because high forces are required to keep the two halves of the dies closed
under pressure.
Cold chamber die casting
machine - Opened
Cold chamber die casting
machine - Closed
Investment casting
 Investment casting is one of the oldest
manufacturing processes, dating back
thousands of years, in which molten metal is
poured into an expendable ceramic mold.
 The mold is formed by using a wax pattern - a
disposable piece in the shape of the desired
part. The pattern is surrounded, or "invested",
into ceramic slurry that hardens into the
mold.
 Investment casting is often referred to as
"lost-wax casting" because the wax pattern is
melted out of the mold after it has been
formed.
 Investment casting can make use of most
metals, most commonly using aluminum
alloys, bronze alloys, magnesium alloys, cast
iron, stainless steel, and tool steel.
 Parts that are typically made by investment
casting include those with complex geometry
such as turbine blades or firearm
components.
 High temperature applications are also
common, which includes parts for the
automotive, aircraft, and military industries.
The process steps in investment casting
The process steps in investment casting
1. Pattern creation -The wax patterns are typically
injection molded into a metal die and are
formed as one piece. Cores may be used to
form any internal features on the pattern.
Several of these patterns are attached to a
central wax gating system (sprue, runners, and
risers), to form a tree-like assembly. The gating
system forms the channels through which the
molten metal will flow to the mold cavity.
2. Mold creation -This "pattern tree" is dipped
into a slurry of fine ceramic particles, coated
with more coarse particles, and then dried to
form a ceramic shell around the patterns
and gating system. This process is repeated
until the shell is thick enough to withstand
the molten metal it will encounter. The shell
is then placed into an oven and the wax is
melted out leaving a hollow ceramic shell
that acts as a one-piece mold, hence the
name "lost wax" casting.
3. Pouring -The mold is preheated in a furnace
to approximately 1000°C (1832°F) and the
molten metal is poured from a ladle into the
gating system of the mold, filling the mold
cavity. Pouring is typically achieved
manually under the force of gravity, but
other methods such as vacuum or pressure
are sometimes used.
4. Cooling - After the mold has been filled, the
molten metal is allowed to cool and solidify
into the shape of the final casting. Cooling
time depends on the thickness of the part,
thickness of the mold, and the material
used.
5. Casting removal - After the molten metal has
cooled, the mold can be broken and the
casting removed. The ceramic mold is
typically broken using water jets, but several
other methods exist. Once removed, the
parts are separated from the gating system
by either sawing or cold breaking (using
liquid nitrogen).
6. Finishing - Often times, finishing operations
such as grinding or sandblasting are used to
smooth the part at the gates. Heat
treatment is also sometimes used to harden
the final part.
FIGURE below Manufacture of total knee replacements. (a) The Zimmer NexGen
mobile-bearing knee (MBK); the femoral portion of the total knee replacement is the
subject of the case study. (b) Assembly of patterns onto a central tree. (c) Dipping of
the tree into slurry to develop a mold from investment. (d) Pouring of metal into a
mold. Source: Courtesy of M. Hawkins, Zimmer, Inc.
Continuous Casting
Schematic diagram of continuous casting
Continuous Casting
► Method for making clear plastic sheets
► Parts are called cast sheets
► Material used is a liquid resin i.e. acrylic syrup
► The casting is done horizontally, the effect of
gravity is uniform over the entire surface. Therefore
minimum molecular orientation. Uniform shrinkage
in all directions.
► Thickness of the sheets varies
Advantages over extruded sheet
 Cast sheet can be crosslinked
 Cast sheet can be made with higher molecular
weight resins
 Cast sheet can be made distortion-free, no die
 Cast sheet has superior optical quality
Problems in Casting
1. Cracking of parts – due to exotherm so high
2. Bubbles in parts – air entrapped
3. Non-uniform density – viscosity not right
4. Surface imperfections – improper filling
TUTORIAL
1. State definition of casting and sketch
diagram for sand casting
2. Sketch the process flow of sand
casting
3. Explain two (2) defects in sand
casting
4. List two (2) types of die casting