Metal Casting

1. Fundamentals: Melting - Pouring - Fluidity

and Fluid Flow - Gating system -
Solidification.
2. Processes (Expendable Mould Casting
and Permanent Mold Casting ): Examples
3. Cleaning, Finishing and Tolerances
4. Continuous Casting
5. Defects
6. Inspection and Tests
7. Heat Treatment
8. Design of Casting
9. Cost Evaluation
 Part 4

Metal – Casting Processes

References:
[1] Serope Kalpakjian & Steven R. Schmid, “Manufacturing Engineering and Technology”,
Any edition (2001 to 2008), Prentice Hall.

[2] Prof. Karl B. Rundman: Dept. of Materials Science and Engineering, Michigan Tech.
University, “Metal Casting”

[3] U.K.Singh. Manish Dwivedi, “Manufacturing Processes”, New Age International

Publishers, 2009.

[4] Jaromir Roučka et al., “Casting Alloy Filtration”, Brono, 2000.

[5] John Campbell and Richard A. Harding, “The Fluidity of Molten Metals”, IRC in
Materials, The University of Birmingham,
[6] B. Borowiecki*, O. Borowiecka, E. Szkodzińka, “Casting defects analysis by the Pareto
method”, A R C H I V ES o f F O U N D R Y ENG IN EER I NG V o l u me 1 1 , S p e c i a l I s s u
e3/2011,33-36
[7] Rajesh Rajkolhe, J. G. Khan, “Defects, Causes and Their Remedies in Casting Process:
A Review”, International Journal of Research in Advent Technology, Vol.2, No.3, March
2014 E-ISSN: 2321-9637, pp.375-383

[8] Castings – System of dimensional tolerances and machining allowance, International

Standard – ISO 8062
Metal – Casting Processes
Casting Processes
The casting process involves pouring of liquid metal into a mold cavity and allowing it to
solidify to obtain the final casting. The flow of molten metal into the mold cavity depends
on several factors like minimum section thickness of the part, presence of corners, non-
uniform cross-section of the cast, and so on. The casting processes can be broadly
classified into expendable mold casting and permanent mold casting processes.

Expendable Mold Casting

Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster,
and investment (lost-wax technique) molds. All these methods use temporary, non-
reusable molds. After the molten metal in the mold cavity solidifies, the mold is broken to
take out the solidified cast. Expendable mold casting processes are suitable for very
complex shaped parts and materials with high melting point temperature. However, the
rate of production is often limited by the time to make mold rather than the casting itself.
Metals often cast by this way are iron, steel, aluminum, brass, bronze, magnesium, some
zinc alloys, nickel-based alloys

Following are a few examples of expendable mold casting processes.

Sand Casting
Sand casting is widely used for centuries because of the simplicity of the process. The
sand-casting process involves the following basic steps: (a) place a wooden or metallic
pattern in sand to create a mold, (b) fit in the pattern and sand in a gating system, (c)
remove the pattern, (d) fill the mold cavity with molten metal, (e) allow the metal to cool,
and (f) break the sand mold and remove the casting.
Sand Casting
The sand-casting process is usually
economical for small batch size production.
The quality of the sand casting depends on
the quality and uniformity of green sand
material that is used for making the mold.
Figure schematically shows a two-part sand
mold, also referred to as a cope-and-drag
sand mold. The molten metal is poured
through the pouring cup and it fills the
mold cavity after passing through
downsprue, runner and gate. The core
refers to loose pieces which are placed
inside the mold cavity to create internal
holes or open section. The riser serves as a
reservoir of excess molten metal that
facilitates additional filling of mold cavity to
compensate for volumetric shrinkage
during solidification. Sand castings process
provides several advantages. It can be
employed for all types of metal. The tooling
cost is low and can be used to cast very
complex shapes. However, sand castings
offer poor dimensional accuracy and
Figure Schematic set-up of sand
surface finish. molding / casting process
Shrinkage allowance
-Shrinkage allowance (pattern to
be larger than part at room
temp)
- Done by using shrink rules
which take into account the
shrinkage allowance (1’ will be
1’ 3/16’’ in a shrink rule for
brass)

Table : Typical Draft (Taper) Allowances

Figure - Schematic illustration of a full split pattern and core box to produce a
wheel type casting. Note that draft is required on the vertical surfaces to allow
the pattern to be drawn away from the mold. The core that will be made in the
core box will form a cylindrical cavity to reduce machining.
 Shakeout, Cleaning and Finishing

-Final operation in casting is to separate casting from mould.

-Shakeout is designed to:

-Separate the moulds and remove casting from mould
-Remove sand from flask and cores from cast
-Punch out or vibratory machines are available for this task

-Blast cleaning is done to remove adhering sand from casting or remove oxide scale
and parting line burs.

-Final finishing operations include Grinding, Turning or any forms of machining

 Sand Properties and Defects
Table: Desirable Properties of a Sand-based Molding Material

•Round grains - better permeability, minimize clay required.

•Angular grains - better strength due to interlocking.

Sand Properties and Defects

•Large grains - better permeability, better high temps.
•Smaller grains - better surface finish.
•Uniform particle size - better permeability.
•Sand expands on heating; (also phase transformation in silica). Expands next to cavity but
not elsewhere, sometimes get sand expansion defects especially on long flat surfaces
 Making Sand Moulds
Ramming , Jolting and Squeezing
- Hand ramming (For small number of castings), Pneumatic hand rammer as well.

- Moulding Machines

- Jolting - sand on pattern then drop flask several times to pack sand.

- Squeezing - platen/piston squeezes sand against pattern. Non-uniform sand

density.

- Combined Jolt & Squeezing - Uniform density.

 Sand Testing

- Grain Size – tested by shaking in 11 sieves of decreasing mesh size and the weight in
each of them gives an idea of grain size.

- Moisture – electrical conductivity, or weigh 50g sample of sand after heating for
sometime at 110°C for water to evaporate

- Clay – wash 50g of sample sand with alkaline water (NaOH) to remove clay, dry the
sand and weighed to find the clay content

- Permeability – measure of escape gases – done by standard ramming of sand and

passing air through it at known pressure. Pressure loss between orifice and sand
gives permeability

- Compressive Strength – also mould strength. Use the prepared sample and
compressively load it find when it fails

- Hardness – Resistance of sand to spring loaded steel ball

- Compactability – 45% is good. Amount of height change from loose sand to

application of standard load
 Sand Moulds
Table: Green – Sand Casting

-Dry sand moulds are durable

-Long time required for drying and increased cost of operation
- Compromise is skin dried mould
- Commonly used for large castings
- Binders (linseed oil etc) added in the sand face to improve the strength of
the skin
Shell molding
Shell molding is similar to sand casting. Normally a machined pattern of grey iron or
aluminum is used in this process. The pattern is heated to 2500C to 2600C, and the sand
resin mixture is poured over its surface. The heated pattern melts the resin creating
bonds between the sand grains. After a dwell period the pattern and sand inverted, and
extra sand is cleaned off. The mold cavity is now formed by a hardened shell of sand.
The mold is then heated in an oven for further curing. The shell thus formed constitutes
one half of the mold. Two such halves are placed over one another to make the
complete mold. The sands used in shell molding process are usually finer than the same
used in sand casting. This process is ideal for complex shaped medium sized parts.
Figure represents the steps of shell mold casting. This method can be employed for
making an integrate shapes, thin and sharp corners small projection which are not
possible in green sand mold. Subsequent machining operations are also reduced due to
more dimensional accuracy.
Fig. Schematic set-up of shell mold casting process
Table: Shell-Mold Casting
 Investment Casting

Investment casting is also referred to as lost-wax casting since the pattern is made
of wax.
- By using this method, complex castings with good dimensional accuracy and fine
surfaces can be obtained.
- The wax can be reused.
- Compared to sand and die casting processes, the investment casting is expensive.

- Process steps:
- The pattern is prepared using a metallic die.
- A tree of patterns can be made to increase the productivity.
- The tree is coated with a refractory material. After drying, the process is repeated
to obtain a sufficient coating layer.
- To remove the wax, the mold is held in an inverted position and heated.
- To obtain a clean mold and to remove any contaminants, the mold is reheated to
a high temperature.
- The molten metal is poured into the mold cavity.
- After the complete solidification of metal, the mold is broken away from the
casting.
- Finally, the separated cast pieces are obtained.
Investment (top) and Conventional Castings
Table: Investment Casting
Lost Foam Casting
Permanent Mold Casting processes
Permanent mold casting processes involve the use of metallic dies that are permanent
in nature and can be used repeatedly. The metal molds are also called dies and provide
superior surface finish and close tolerance than typical sand molds. The permanent
mold casting processes broadly include processes such as pressure die casting, squeeze
casting and centrifugal casting.

- Die Casting
- It is a permanent –mold casting . The molten metal is forced into the die cavity under
pressure .
-Product weight: < 90 g – 25 kg [1].
- The die casting is especially suited for small and medium sized parts with good details,
fine surfaces and high dimensional accuracy.
- Most die castings are made from nonferrous metals (such as zinc, copper, and
aluminum based alloys).
-Very high production rates can be achieved in pressure die casting process with close
dimensional control of the casting. However, the process is not suitable for casting of
high melting temperature materials as the die material has to withstand the melting (or
superheated) temperature of the casting. Pressure die castings also contain porosity due
to the entrapped air. Furthermore, the dies in the pressure die casting process are
usually very costly.

- There are two basic types: Hot – chamber and cold – chamber processes.
- Hot – chamber Process
- The molten metal is injected into
the mold cavity using the injection
unit. This unit is immersed in the
molten metal.

- Pressure: up to 35 MPa, ( The

average is about 15 MPa) [1].

-The alloys which can be cast: Low

– melting – point alloys (such as tin,
zinc, and lead).

-More details are described in

ref.[1].
- Cold – chamber Process
- In this process, the molten metal is
poured into the shot chamber (injection
cylinder). Through this chamber, the
molten metal is forced into the die cavity.

- Pressure: 20 MPa – 70 MPa [1]

- The alloys which can be cast: High –

melting – point alloys (such as aluminum,
some magnesium alloys, copper – based)
[1]
Squeeze casting
Molten metal is poured into a metallic mold or die cavity with one-half of the die
squeezing the molten metal to fill in the intended cavity under pressure as shown in
Figure. Fiber reinforced casting with SiC or Al2O3 fibers mixed in metal matrix have
been successfully squeeze cast and commercially used to produce automobile pistons.
However, squeeze casting is limited only to shallow part or part with smaller
dimensions.

Fig. Schematic set-up of squeeze casting process

Centrifugal casting

- In centrifugal casting process, the molten metal poured at the center of a rotating
mold or die. Because of the centrifugal force, the lighter impurities are crowded
towards the center of the case.

- The centrifuge action segregates the less dense nonmetallic inclusions near to the
center of rotation that can be removed by machining a thin layer.

- No cores are therefore required in casting of hollow parts although solid parts can
also be cast by this process.

- The centrifugal casting is very suitable for axisymmetric parts.

- Very high strength of the casting can be obtained.

- Since the molten metal is fed by the centrifugal action, the need for complex metal
feeding system is eliminated.

- Both horizontal and vertical centrifugal castings are widely used in the industry.
Figure schematically shows a set-up for horizontal centrifugal casting process. Figure
typically shows large pipes that are made using centrifugal casting process.
 Centrifugal Casting

Horizontal Vertical
 Figure Schematic set-up of horizontal centrifugal casting process

Figure Metallic pipes

made using centrifugal
casting process
Casting cost