Introduction to Casting Processes in Manufacturing

CASTING PROCESSES
Dr. Sachin Salunkhe,
Associate Professor, Mechanical Engineering
Vel Tech University, Chennai, India

CASTING PROCESSES
1. Sand Casting
2. Other Expendable Mold Casting Processes
3. Permanent Mold Casting Processes
4. Casting Quality
5. Metals for Casting
6. Product Design Considerations
7. Foundry Tools

CASTING PROCESSES
Casting is a manufacturing process in which a liquid material is
usually poured into a mold, which contains a hollow cavity of
the desired shape, and then allowed to solidify.

Two Categories of 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

Overview of Sand Casting
 Most widely used casting process, accounting
for a significant majority of total tonnage cast
 Nearly all alloys can be sand casted, including
metals with high melting temperatures, such as
steel, nickel, and titanium
 Castings range in size from small to very large
 Production quantities from one to millions

Figure 1 A large sand casting weighing over 680 kg (1500 lb) for
an air compressor frame (photo courtesy of Elkhart Foundry).

Steps in Sand Casting
1. Pour the molten metal into sand mold
2. Allow time for metal to solidify
3. Break up the mold to remove casting
4. Clean and inspect casting
 Separate gating and riser system
5. Heat treatment of casting is sometimes
required to improve metallurgical properties

Making the Sand Mold
 The cavity in the sand mold is formed by
packing sand around a pattern, then separating
the mold into two halves and removing the
pattern
 The mold must also contain gating and riser
system
 If casting is to have internal surfaces, a core
must be included in mold
 A new sand mold must be made for each part
produced

Sand Casting Production Sequence
Figure 1 Steps in the production sequence in sand casting.
The steps include not only the casting operation but also
pattern-making and mold-making.

The Pattern
A full-sized model of the part, slightly enlarged to
account for shrinkage and machining
allowances in the casting
 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

Types of Patterns
Figure 11.3 Types of patterns used in sand casting:
(a) solid pattern
(b) split pattern
(c) match-plate pattern
(d) cope and drag pattern

Core
Full-scale model of interior surfaces of part
 It is inserted into the mold cavity prior to
pouring
 The molten metal flows and solidifies between
the mold cavity and 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

Core in Mold
Figure 11.4 (a) Core held in place in the mold cavity by
chaplets, (b) possible chaplet design, (c) casting with
internal cavity.

Desirable Mold Properties
 Strength - to maintain shape and resist erosion
 Permeability - to allow hot air and gases to
pass through voids in sand
 Thermal stability - to resist cracking on contact
with molten metal
 Collapsibility - ability to give way and allow
casting to shrink without cracking the casting
 Reusability - can sand from broken mold be
reused to make other molds?

Foundry Sands
Silica (SiO2) or silica mixed with other minerals
 Good refractory properties - capacity to
endure high temperatures
 Small grain size yields better surface finish
on the cast part
 Large grain size is more permeable, allowing
gases to escape during pouring
 Irregular grain shapes strengthen molds due
to interlocking, compared to round grains
 Disadvantage: interlocking tends to
reduce permeability

Binders Used with Foundry Sands
 Sand is held together by a mixture of water and
bonding clay
 Typical mix: 90% sand, 3% water, and 7%
clay
 Other bonding agents also used in sand molds:
 Organic resins (e g , phenolic resins)
 Inorganic binders (e g , sodium silicate and
phosphate)
 Additives are sometimes combined with the
mixture to increase strength and/or
permeability

Types of Sand Mold
 Green-sand molds - mixture of sand, clay, and
water;
 “Green" means mold contains moisture at
time of pouring
 Dry-sand mold - organic binders rather than
clay
 And mold is baked to improve strength
 Skin-dried mold - drying mold cavity surface of
a green-sand mold to a depth of 10 to 25 mm,
using torches or heating lamps

Other Expendable Mold Processes
 Shell Molding
 Vacuum Molding
 Expanded Polystyrene Process
 Investment Casting
 Plaster Mold and Ceramic Mold Casting

Shell Molding
Casting process in which the mold is a thin shell of
sand held together by thermosetting resin binder
Figure 11.5 Steps in shell-molding: (1) a match-plate or
cope-and-drag metal pattern is heated and placed over a
box containing sand mixed with thermosetting resin.

Shell Molding
Figure 11.5 Steps in shell-molding: (2) box is inverted so
that sand and resin fall onto the hot pattern, causing a
layer of the mixture to partially cure on the surface to
form a hard shell; (3) box is repositioned so that loose
uncured particles drop away;

Shell Molding
Figure 11.5 Steps in shell-molding: (4) sand shell is heated
in oven for several minutes to complete curing; (5) shell
mold is stripped from the pattern;

Shell Molding
Figure 11.5 Steps in shell-molding: (6) two halves of the shell mold
are assembled, supported by sand or metal shot in a box, and
pouring is accomplished; (7) the finished casting with sprue
removed.

Advantages and Disadvantages
 Advantages of shell molding:
 Smoother cavity surface permits easier flow of
molten metal and better surface finish
 Good dimensional accuracy - machining often
not required
 Mold collapsibility minimizes cracks in casting
 Can be mechanized for mass production
 Disadvantages:
 More expensive metal pattern
 Difficult to justify for small quantities

Expanded Polystyrene Process
Uses a mold of sand packed around a
polystyrene foam pattern which vaporizes
when molten metal is poured into mold
 Other names: lost-foam process, lost pattern
process, evaporative-foam process, and
full-mold process
 Polystyrene foam pattern includes sprue,
risers, gating system, and internal cores (if
needed)
 Mold does not have to be opened into cope
and drag sections

Figure 11.7 Expanded polystyrene casting process: (1)
pattern of polystyrene is coated with refractory
compound;

foam pattern is placed in mold box, and sand is
compacted around the pattern;

molten metal is poured into the portion of the pattern that
forms the pouring cup and sprue. As the metal enters
the mold, the polystyrene foam is vaporized ahead of the
advancing liquid, thus the resulting mold cavity is filled.

 Advantages of expanded polystyrene process:
 Pattern need not be removed from the mold
 Simplifies and speeds mold-making,
because two mold halves are not required
as in a conventional green-sand mold
 Disadvantages:
 A new pattern is needed for every casting
 Economic justification of the process is
highly dependent on cost of producing
patterns

 Applications:
 Mass production of castings for automobile
engines
 Automated and integrated manufacturing
systems are used to
1. Mold the polystyrene foam patterns and
then
2. Feed them to the downstream casting
operation

Investment Casting (Lost Wax Process)
A pattern made of wax is coated with a refractory
material to make mold, after which wax is
melted away prior to pouring molten metal
 "Investment" comes from a less familiar
definition of "invest" - "to cover completely,"
which refers to coating of refractory material
around wax pattern
 It is a precision casting process - capable of
producing castings of high accuracy and
intricate detail

Investment Casting (Lost Wax Process)

Investment Casting
Figure 11.8 Steps in investment casting: (1) wax patterns are
produced, (2) several patterns are attached to a sprue to form
a pattern tree

Investment Casting
Figure 11.8 Steps in investment casting: (3) the pattern tree is coated
with a thin layer of refractory material, (4) the full mold is formed by
covering the coated tree with sufficient refractory material to make
it rigid

Investment Casting
Figure 11.8 Steps in investment casting: (5) the mold is held in an
inverted position and heated to melt the wax and permit it to drip out
of the cavity, (6) the mold is preheated to a high temperature, the
molten metal is poured, and it solidifies

Investment Casting
Figure 11.8 Steps in investment casting: (7) the mold is
broken away from the finished casting and the parts are
separated from the sprue

Investment Casting
Figure 11 9 A one-piece compressor stator with 108
separate airfoils made by investment casting (photo
courtesy of Howmet Corp.).

 Advantages of investment casting:
 Parts of great complexity and intricacy can
be cast
 Close dimensional control and good surface
finish
 Wax can usually be recovered for reuse
 Additional machining is not normally
required - this is a net shape process
 Disadvantages
 Many processing steps are required
 Relatively expensive process

Plaster Mold Casting
Similar to sand casting except mold is made of
plaster of Paris (gypsum - CaSO4-2H2O)
 In mold-making, plaster and water mixture is
poured over plastic or metal pattern and
allowed to set
 Wood patterns not generally used due to
extended contact with water
 Plaster mixture readily flows around pattern,
capturing its fine details and good surface
finish

 Advantages of plaster mold casting:
 Good accuracy and surface finish
 Capability to make thin cross-sections
 Disadvantages:
 Mold must be baked to remove moisture,
which can cause problems in casting
 Mold strength is lost if over-baked
 Plaster molds cannot stand high
temperatures, so limited to lower melting
point alloys

Ceramic Mold Casting
Similar to plaster mold casting except that mold is
made of refractory ceramic material that can
withstand higher temperatures than plaster
 Can be used to cast steels, cast irons, and
other high-temperature alloys
 Applications similar to those of plaster mold
casting except for the metals cast
 Advantages (good accuracy and finish) also
similar

Permanent Mold Casting Processes
 Economic disadvantage of expendable mold
casting: a new mold is required for every
casting
 In permanent mold casting, the mold is reused
many times
 The processes include:
 Basic permanent mold casting
 Die casting
 Centrifugal casting

Permanent Mold Casting Processes

The Basic Permanent Mold Process
Uses a metal mold constructed of two sections
designed for easy, precise opening and closing
 Molds used for casting lower melting point
alloys are commonly made of steel or cast iron
 Molds used for casting steel must be made of
refractory material, due to the very high pouring
temperatures

Permanent Mold Casting
Figure 11.10 Steps in permanent mold casting: (1) mold is
preheated and coated

Permanent Mold Casting
Figure 11.10 Steps in permanent mold casting: (2) cores (if used)
are inserted and mold is closed, (3) molten metal is poured into
the mold, where it solidifies.

Advantages and Limitations
 Advantages of permanent mold casting:
 Good dimensional control and surface finish
 More rapid solidification caused by the cold
metal mold results in a finer grain structure,
so castings are stronger
 Limitations:
 Generally limited to metals of lower melting
point
 Simpler part geometries compared to sand
casting because of need to open the mold
 High cost of mold

Applications of Permanent Mold Casting
 Due to high mold cost, process is best suited to
high volume production and can be automated
accordingly
 Typical parts: automotive pistons, pump
bodies, and certain castings for aircraft and
missiles
 Metals commonly cast: aluminum, magnesium,
copper-base alloys, and cast iron

Die Casting
A permanent mold casting process in which
molten metal is injected into mold cavity under
high pressure
 Pressure is maintained during solidification,
then mold is opened and part is removed
 Molds in this casting operation are called dies;
hence the name die casting
 Use of high pressure to force metal into die
cavity is what distinguishes this from other
permanent mold processes

Die Casting Machines
 Designed to hold and accurately close two
mold halves and keep them closed while liquid
metal is forced into cavity
 Two main types:
1. Hot-chamber machine
2. Cold-chamber machine

Hot-Chamber Die Casting
Metal is melted in a container, and a piston injects
liquid metal under high pressure into the die
 High production rates - 500 parts per hour not
uncommon
 Applications limited to low melting-point metals
that do not chemically attack plunger and other
mechanical components
 Casting metals: zinc, tin, lead, and magnesium

Figure 11.13 Cycle in hot-chamber casting: (1) with die closed
and plunger withdrawn, molten metal flows into the chamber

Figure 11.13 Cycle in hot-chamber casting: (2) plunger
forces metal in chamber to flow into die, maintaining
pressure during cooling and solidification.

Cold-Chamber Die Casting Machine
Molten metal is poured into unheated chamber
from external melting container, and a piston
injects metal under high pressure into die cavity
 High production but not usually as fast as
hot-chamber machines because of pouring step
 Casting metals: aluminum, brass, and
magnesium alloys
 Advantages of hot-chamber process favor its use
on low melting-point alloys (zinc, tin, lead)

Cold-Chamber Die Casting
Figure 11.14 Cycle in cold-chamber casting: (1) with die
closed and ram withdrawn, molten metal is poured into
the chamber

Cold-Chamber Die Casting
Figure 11.14 Cycle in cold-chamber casting: (2) ram forces
metal to flow into die, maintaining pressure during
cooling and solidification.

Molds for Die Casting
 Usually made of tool steel, mold steel, or
maraging steel
 Tungsten and molybdenum (good refractory
qualities) used to die cast steel and cast iron
 Ejector pins required to remove part from die
when it opens
 Lubricants must be sprayed into cavities to
prevent sticking

Advantages and Limitations
 Advantages of die casting:
 Economical for large production quantities
 Good accuracy and surface finish
 Thin sections are possible
 Rapid cooling provides small grain size and
good strength to casting
 Disadvantages:
 Generally limited to metals with low metal
points
 Part geometry must allow removal from die

Centrifugal Casting
A family of casting processes in which the mold is
rotated at high speed so centrifugal force
distributes molten metal to outer regions of die
cavity
 The group includes:
 True centrifugal casting
 Semicentrifugal casting
 Centrifuge casting

True Centrifugal Casting
Molten metal is poured into rotating mold to
produce a tubular part
 In some operations, mold rotation commences
after pouring rather than before
 Parts: pipes, tubes, bushings, and rings
 Outside shape of casting can be round,
octagonal, hexagonal, etc , but inside shape is
(theoretically) perfectly round, due to radially
symmetric forces

True Centrifugal Casting
Figure 11.15 Setup for true centrifugal casting.

Semicentrifugal Casting
Centrifugal force is used to produce solid castings
rather than tubular parts
 Molds are designed with risers at center to
supply feed metal
 Density of metal in final casting is greater in
outer sections than at center of rotation
 Often used on parts in which center of casting
is machined away, thus eliminating the portion
where quality is lowest
 Examples: wheels and pulleys

Centrifuge Casting
Mold is designed with part cavities located away
from axis of rotation, so that molten metal
poured into mold is distributed to these cavities
by centrifugal force
 Used for smaller parts
 Radial symmetry of part is not required as in
other centrifugal casting methods

Additional Steps After Solidification
 Trimming
 Removing the core
 Surface cleaning
 Inspection
 Repair, if required
 Heat treatment

Casting Quality
 There are numerous opportunities for things to
go wrong in a casting operation, resulting in
quality defects in the product
 The defects can be classified as follows:
 General defects common to all casting
processes
 Defects related to sand casting process

A casting that has solidified before completely
filling mold cavity
Figure 11.22 Some common defects in castings: (a) misrun
General Defects: Misrun

Two portions of metal flow together but there is
a lack of fusion due to premature freezing
Figure 11.22 Some common defects in castings: (b) cold shut
General Defects: Cold Shut

Metal splatters during pouring and solid globules
form and become entrapped in casting
Figure 11.22 Some common defects in castings: (c) cold shot
General Defects: Cold Shot

Depression in surface or internal void caused by
solidification shrinkage that restricts amount of
molten metal available in last region to freeze
Figure 11.22 Some common defects in castings: (d) shrinkage cavity
General Defects: Shrinkage Cavity

Metals for Casting
 Most commercial castings are made of alloys
rather than pure metals
 Alloys are generally easier to cast, and
properties of product are better
 Casting alloys can be classified as:
 Ferrous
 Nonferrous

Product Design Considerations
 Geometric simplicity:
 Although casting can be used to produce
complex part geometries, simplifying the
part design usually improves castability
 Avoiding unnecessary complexities:
 Simplifies mold-making
 Reduces the need for cores
 Improves the strength of the casting

 Corners on the casting:
 Sharp corners and angles should be
avoided, since they are sources of stress
concentrations and may cause hot tearing
and cracks
 Generous fillets should be designed on
inside corners and sharp edges should be
blended

 Draft Guidelines:
 In expendable mold casting, draft facilitates
removal of pattern from mold
 Draft = 1 for sand casting
 In permanent mold casting, purpose is to aid
in removal of the part from the mold
 Draft = 2 to 3 for permanent mold
processes
 Similar tapers should be allowed if solid
cores are used

Draft
 Minor changes in part design can reduce need
for coring
Figure 11.25 Design change to eliminate the need for using
a core: (a) original design, and (b) redesign.

 Dimensional Tolerances and Surface Finish:
 Significant differences in dimensional
accuracies and finishes can be achieved in
castings, depending on process:
 Poor dimensional accuracies and finish for
sand casting
 Good dimensional accuracies and finish for
die casting and investment casting

 Machining Allowances:
 Almost all sand castings must be machined
to achieve the required dimensions and part
features
 Additional material, called the machining
allowance, is left on the casting in those
surfaces where machining is necessary
 Typical machining allowances for sand
castings are around 1.5 and 3 mm (1/16 and
1/4 in)

Introduction to Casting Processes in Manufacturing

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