Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can create very large parts Some casting methods are suited to mass production

©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5/e
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 create 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, frying pans

Overview of Casting Technology
 Casting is usually performed in a foundry, which is a
factory equipped for
 Making molds
 Melting and handling molten metal
 Performing the casting process
 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 complex 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

Sand Casting Mold

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 the
cavity from the outside of mold
 Consists of a downsprue, through which metal
enters a runner leading to the main cavity
 At the top of the downsprue, a pouring cup is often
used to minimize splash and turbulence as the
metal flows into the 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 the chilling action of the 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 out of the
mold, as well as thermal properties of the metal

 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 Pure Metals

Most Alloys Solidify Over a
Temperature Range
 (a) Phase diagram for a copper nickel alloy
‑ system and
(b) cooling curve for a 50%Ni 50%Cu composition
‑

 Characteristic grain structure in an alloy casting, showing
segregation of alloying components in center of casting
Solidification of Alloys

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
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
 
  
 
n
TS m
V
T C
A

Mold Constant in Chvorinov's
Rule
 Mold constant Cm depends on:
 Mold material
 Thermal properties of casting metal
 Pouring temperature relative to melting point
 Value of Cm for a given casting operation can be
based on experimental data from previous operations
carried out using same mold material, metal, and
pouring temperature, even though the shape of the
part may be quite different

What Chvorinov's Rule Tells Us
 A casting with a higher volume to surface area ratio cools
‑ ‑
and solidifies more slowly than one with a lower ratio
 To feed molten metal to the main cavity, TTS for riser
must be greater than TTS for main casting
 Since mold constants of riser and casting will be equal,
design the riser to have a larger volume to area ratio so
‑ ‑
that the main casting solidifies first
 This minimizes the effects of shrinkage

Shrinkage during Solidification
and Cooling
 (0) starting level of molten metal after pouring; (1) reduction
in level caused by liquid contraction during cooling

Shrinkage during Solidification
and Cooling
 (2) reduction in height and formation of shrinkage cavity
caused by solidification; (3) further reduction in volume due to
thermal contraction during cooling of solid metal

Solidification Shrinkage
 Occurs in nearly all metals because the solid phase
has a higher density than the liquid phase
 Thus, solidification causes a reduction in volume per
unit weight of metal
 Exception: cast iron with high C content
 Graphitization during final stages of freezing
causes expansion that counteracts volumetric
decrease associated with phase change

Shrinkage Allowance
 Patternmakers correct for solidification shrinkage and
thermal contraction by making the mold cavity oversized
 Amount by which mold is made larger relative to final
casting size is called pattern shrinkage allowance
 Casting dimensions are expressed linearly, so
allowances are applied accordingly

Directional Solidification
 To minimize effects of shrinkage, it is desirable for
regions of the casting most distant from the liquid
metal supply to freeze first and for solidification to
progress from these regions toward the riser(s)
 Thus, molten metal is continually available from
risers to prevent shrinkage voids
 The term directional solidification describes this
aspect of freezing and methods by which it is
controlled

Achieving Directional
Solidification
 Directional solidification is achieved using
Chvorinov's Rule to design the casting, its orientation
in the mold, and the riser system that feeds it
 Locate sections of the casting with lower V/A
ratios away from riser, so freezing occurs first in
these regions, and the liquid metal supply for the
rest of the casting remains open
 Chills internal or external heat sinks that cause
‑
rapid freezing in certain regions of the casting

External Chills
 (a) External chill to encourage rapid freezing of the molten
metal in a thin section of the casting; and (b) the likely result
if the external chill were not used

Riser Design
 Riser is waste metal that is separated from the
casting and remelted to make more castings
 To minimize waste in the unit operation, it is desirable
for the volume of metal in the riser to be a minimum
 Since the shape of the riser is normally designed to
maximize the V/A ratio, this allows riser volume to be
reduced to the minimum possible value

Can create complex part geometries Can create both external and internal shapes Some casting processes are net shape; others are near net shape Can create very large parts Some casting methods are suited to mass production

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