FUNDAMENTALS OF METAL
CASTING
1. Overview of Casting Technology
2. Heating and Pouring
3. Solidification and Cooling
FUNDAMENTALS OF METAL CASTING
1. Overview of Casting Technology
2. Heating and Pouring
3. Solidification and Cooling
Solidification Processes
Starting work material is either a liquid or is in a highly
plastic condition, and a part is created through
solidification of the material
Solidification processes can be classified according to
engineering material processed:
Metals
Ceramics, specifically glasses
Polymers and polymer matrix composites (PMCs)
Classification of solidification processes
Casting of Metals
Process in which molten metal flows by gravity or other
force into a mold where it solidifies in the shape of
the mold cavity
The term casting also applies to the part made in the
process
Steps in casting seem simple:
1. Melt the metal
2. Pour it into a mold
3. Let it freeze
Capabilities and Advantages of Casting
Can create complex part geometries
Can create both external and internal shapes
Some casting processes are net shape; others are
near net shape
Can produce very large parts
Some casting methods are suited to mass production
Disadvantages of Casting
Different disadvantages for different casting processes:
Limitations on mechanical properties
Poor dimensional accuracy and surface finish for some
processes; e.g., sand casting
Safety hazards to workers due to hot molten metals
Environmental problems
Parts Made by Casting
Big parts
Engine blocks and heads for automotive vehicles, wood burning
stoves, machine frames, railway wheels, pipes, church bells,
big statues, pump housings
Small parts
Dental crowns, jewelry, small statues
Overview of Casting Technology
Casting is usually performed in a foundry
Foundry = factory equipped for making molds, melting
and handling molten metal, performing the casting
process, and cleaning the finished casting
Workers who perform casting are called foundrymen
The Mold in Casting
Contains cavity whose geometry determines part shape
Actual size and shape of cavity must be slightly
enlarged to allow for shrinkage of metal during
solidification and cooling
Molds are made of a variety of materials, including
sand, plaster, ceramic, and metal
Open Molds and Closed Molds
Two forms of mold: (a) open mold and (b) closed
mold for more complex mold geometry with gating
system leading into the cavity
Two Categories of Casting Processes
1. Expendable mold processes – use an expendable mold which must
be destroyed to remove casting
Mold materials: sand, plaster, and similar materials, plus
binders
2. Permanent mold processes – use a permanent mold which can be
used to produce many castings
Made of metal (or, less commonly, a ceramic refractory
material
Advantages and Disadvantages
More intricate geometries are possible with expendable mold
processes
Part shapes in permanent mold processes are limited by the need
to open the mold
Permanent mold processes are more economic in high production
operations
Steps in Sand Casting
• There are six basic steps in making sand castings:
• Patternmaking
• Core making
• Molding
• Melting and pouring
• Cleaning
• Pattern making
• The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as
molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become
the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.
• Core making
• Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between the
core and mold-cavity surface is what eventually becomes the casting.
• Molding
• Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a
pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the
mold.
• Melting and Pouring
• The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and
the molten metal is transferred to the pouring area where the molds are filled.
• Cleaning
• Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to
improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting
for defects and general quality is performed.
Sand Casting Mold
Casting Terminologies
• Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of
the flask in the molding structure, it is referred to by various names such as drag – lower molding flask, cope – upper
molding flask, cheek – intermediate molding flask used in three piece molding.
• Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.
• Parting line: This is the dividing line between the two molding flasks that makes up the mold.
• Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay,
and moisture in appropriate proportions.
• Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better
surface finish to the castings.
• Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped
cavities in the castings.
• Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured.
• Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it
controls the flow of metal into the mold.
• Runner: The channel through which the molten metal is carried from the sprue to the gate.
• Gate: A channel through which the molten metal enters the mold cavity.
• Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as “feed
head”.
• Vent: Small opening in the mold to facilitate escape of air and gases.
Terminology for Sand Casting Mold
Mold consists of two halves:
Cope = upper half of mold
Drag = bottom half
Mold halves are contained in a box, called a flask
The two halves separate at the parting line
Forming the Mold Cavity in Sand Casting
Mold cavity is formed by packing sand around a pattern, which has
the shape of the part
When the pattern is removed, the remaining cavity of the packed
sand has desired shape of cast part
The pattern is usually oversized to allow for shrinkage of metal
during solidification and cooling
Sand for the mold is moist and contains a binder to maintain its
shape
Use of a Core in the Mold Cavity
The mold cavity provides the external surfaces of the cast part
In addition, a casting may have internal surfaces, determined by a
core, placed inside the mold cavity to define the interior geometry of
part
In sand casting, cores are generally made of sand
Gating System
Channel through which molten metal flows into cavity from outside of
mold
Consists of a downsprue, through which metal enters a runner
leading to the main cavity
At the top of downsprue, a pouring cup is often used to minimize
splash and turbulence as the metal flows into downsprue
Riser
Reservoir in the mold which is a source of liquid metal to
compensate for shrinkage of the part during solidification
The riser must be designed to freeze after the main casting in
order to satisfy its function
Heating the Metal
Heating furnaces are used to heat the metal to
molten temperature sufficient for casting
The heat required is the sum of:
1. Heat to raise temperature to melting point
2. Heat of fusion to convert from solid to liquid
3. Heat to raise molten metal to desired
temperature for pouring
Pouring the Molten Metal
For this step to be successful, metal must flow into all
regions of the mold, most importantly the main cavity,
before solidifying
Factors that determine success
Pouring temperature
Pouring rate
Turbulence
Solidification of Metals
Transformation of molten metal back into solid state
Solidification differs depending on whether the metal is
A pure element or
An alloy
Cooling Curve for a Pure Metal
A pure metal solidifies
at a constant
temperature equal to
its freezing point
(same as melting
point)
Solidification of Pure Metals
Due to chilling action of mold wall, a thin skin of solid
metal is formed at the interface immediately after
pouring
Skin thickness increases to form a shell around the
molten metal as solidification progresses
Rate of freezing depends on heat transfer into mold,
as well as thermal properties of the metal
Solidification of Pure Metals
Characteristic grain
structure in a casting of
a pure metal, showing
randomly oriented
grains of small size
near the mold wall, and
large columnar grains
oriented toward the
center of the casting
Solidification of Alloys
Most alloys freeze over a temperature range
Phase diagram for a copper-nickel alloy system and
cooling curve for a 50%Ni-50%Cu composition
Solidification of Alloys
• Characteristic grain
structure in an alloy
casting, showing
segregation of
alloying components
in center of casting
Solidification Time
Total solidification time TTS = time required for casting
to solidify after pouring
TTS depends on size and shape of casting by
relationship known as Chvorinov's Rule
n
V
TTS Cm
A
where TTS = total solidification time; V = volume of the
casting; A = surface area of casting; n = exponent
with typical value = 2; and Cm is mold constant.
Draft Allowances
Pattern having no draft on vertical edges Pattern having draft on vertical edges
Core and Core Prints
Where core is required, provision should be made to support the core inside the mold cavity. Core prints
are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat
in the mold on which the sand core rests during pouring of the mold. The core print must be of adequate
size and shape so that it can support the weight of the core during the casting operation. Depending
upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold
cavity.
Core print
