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
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, frying
pans
 All varieties of metals can be cast, ferrous and
nonferrous
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 oversized 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, simply a container in the

shape of the desired part; and (b) closed mold, in which the
mold geometry is more complex and requires a gating system
(passageway) leading into the cavity.
Two Categories of Casting Processes
1. Expendable mold processes – uses an
expendable mold which must be destroyed to
remove casting
 Mold materials: sand, plaster, and similar
materials, plus binders
2. Permanent mold processes – uses a
permanent mold which can be used over and
over 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
Sand Casting Mold

(b) Sand casting mold.

 Sand Casting Mold Terms
 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
 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
 Pouring (Sprue Design)
Pouring Basin

Near Top
Sprue Diameter, d1

Diameter Velocity, V1
Height, h

Bottom
Diameter, d2
Velocity, V2

Let Q = Pour Rate (volume/unit time) or (in3/sec or liter/s)

Q = Flow Area * Velocity
Q = A*V
Near Top, Q = A1V1 Bottom, Q = A2V2
 Sprue Diameters

Design Concept: To avoid gas entrapment or Aspiration

FLOW Continuity  constant flow rate

A1V1  A2V2
d 2
d 2
d V d V
2 2
1
V1  2
V2 1 1 2 2
4 4
V1
d2   d1 NOTE: V2 > V1 so d2 < d1
V2
Thus diameter decreases from top to bottom
Sprue Height

At 1, Total Energy, E1
1
E1  PE  KE
1 2
E1  mgh  mv1
Height 2
v1  0 (pouring) – In the basin
(h)

2
Thu E1  mgh
s
1 2
At bottom E2  mgh  mv 2 h0
2
1 2
E2  mv 2
2
 Conservation of Energy

E1  E2
1 2
mgh  mv 2
2
v2  2 gh h can be calculated if ν2
is known
Desired Area at Base of Sprue

Q Q
Ab  OR Ab 
V2 2 gh
 Desired Sprue Height for a given
Flow Rate

2
1 Q
h  
2 g  Ab 

Mold Fill Time (MFT)

Volume Poured
MFT 
Q
Example – Flow Analysis
Example – Sprue Design
Example – Riser Design
In the casting of steel under certain mold conditions, the mold constant in
Chvorinov's Rule is known to be Cm = 4.0 min/cm2, based on previous
experience. The casting is a flat plate whose length = 30 cm, width = 10 cm,
and thickness = 20 mm. Determine how long it will take for the casting to
solidify.

A steel casting has a cylindrical geometry with 4.0 in diameter and weighs 20
lb. This casting takes 6.0 min to completely solidify. Another cylindrical-
shaped casting with the same diameter-to-length ratio weighs 12 lb. This
casting is made of the same steel and the same conditions of mold and
pouring were used. Determine: (a) the mold constant in Chvorinov's Rule; and
(b) the dimensions, and (c) the total solidification time of the lighter casting.
Note: The density of steel = 490 lb/ft3.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
Riser Design - Try

A riser in the shape of a sphere is to be designed for a sand casting

mold. The casting is a rectangular plate, with length = 200 mm, width
= 100 mm, and thickness = 18 mm. If the total solidification time of
the casting itself is known to be 3.5 min, determine the diameter of
the riser so that it will take 25% longer for the riser to solidify.

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e
 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
 Technical Issues in Casting

A. Heating / Pouring / Solidification

Solid : Heat / Energy Required

Liquid : Pouring Temperature (Degree of Superheat)

Molding : Sprue Design

Pouring : Pouring Rate, Mold Fill Time

Solidification : Solidification Time, Riser Design

B. Process Selection
(i) Performance Index / Rating
(ii) Cost per Part Analysis
 Heat Required

a. Solid (Room Temperature) Solid (Melting Point)

Heat, H s  Mass  Specific Heat  Temperatur e Change

H s  mc s T
Temp H s  mc s Tm  T0 
Tp
L

Tm S+L

S
To

Time
 Heat Required Continued

b. Solid (Melting Point) Liquid (Melting Point)

L+
Tm Latent Heat, H f  mh f
S
hf Latent heat of Fusion per unit mass.

c. Liquid (Melting Point) Liquid (Pouring Temp.)

Tp
H l  mc l T
Tm
H l  mc l T p  Tm 

d. Total Heat Required (H)

H  H s  H f  Hl
note : mass, m    V
Example 1- Heating
 Note: 1 cal = 4.2 J
Solution

Disc, D = 40 cm Thickness, t = 5 cm
Pure Aluminum; Latent heat, hf = 389.3 J/kg
t Specific Heat, Cs = 0.21 Cal/g-oC
D Cl = 0.21 Cal/g-oC
Density ρ = 2.70 g/cm3
Temperatures: To = 25 C Tm = 660 C Tp = 800 C

Volume of Casting, V = (πD2/4)t = (π*402/2)*5 = 6283 cm3

Heated Volume, VH = 1.05V = 6597 cm3

Heated Mass, m = ρV = 2.70 x 6597 = 17,812 g

Heat Required = Ht(To to Tm) + Ht(Fusion) + Ht(Tm to Tp)
= mCs(Tm – T0) + mhf + mCl(Tp – Tm)
= 9.95 MJ + 6.93 MJ + 2.19 MJ
= 19.07 MJ
 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)

Cooling curve for a pure metal during casting.

 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
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
rather than at a single temperature

(a) Phase diagram for a copper-nickel alloy system and (b)

associated cooling curve for a 50%Ni-50%Cu composition
during casting.
Characteristic grain structure in an alloy casting, showing
segregation of alloying components in center of casting.
 Solidification Time
 Solidification takes 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 
TST  Cm  
 A
where TST = total solidification time; V =
volume of the casting; A = surface area of
casting; n = exponent with typical value = 2;
and Cm is mold constant.
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 main cavity, TST for
riser must greater than TST 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
 Total Solidification Time (TST) - Examples

Chvorinov’s Rule n
V 
Total Solidification Time, TST, TST  Cm  
Where V = Volume of Casting
 A
A = Surface Area of Casting exposed to Mold and Cores

n = exponent ( n =2 for expendable mold castings)

n = 1 for permanent mold castings

V/A = Casting Modulus

Cm = Mold Constant. This is a function of Mold Material and Cast Material

Mold Material: Specific Heat, Thermal Conductivity, Density

Cast Material: Heat of Fusion, Density, Melting Temperature, Thermal

Conductivity
 Examples of Casting Modulus (V/A)

1. Cube
V a 3 A  6a 2

a
V a
a
Modulus, 
a
A 6

2.
Sphere a
4 3
V  a A  4a 2
3
V a
Modulus, 
A 3
3. Solid Cylinder / Disk

V  r h 2

A  2rh  2r 
h
2

4. Hollow Cylinder / Disk

 
r1 r2

V  r  r h
2
2
1
2

h
A = Exposed Area


A  2r2 h  2r1h  2 r22  r12 
5. Modulus of Composite Shapes

(a)
r

r1 r2
h

t h

w (b)
t
b

b
 Riser Design

Concept: Riser should be last section to

solidify
Steps
1. Calculate TST for the Casting
2. Decide on TST for Riser such that TST riser  TST casting
3. Determine Riser Modulus
2
V 
TST riser  Cm  
 A  riser
1

V   TST  2
Solve     
 A  riser  Cm 
 Cylindrical Riser

Modul
 
 D 2

4
h  
  TST
 


1
2 D
 
 Dh  D 
2
us of  Cm 
 4  riser
Solid h
Cylind
er: Determine D/h ratio.
 Put D in terms of h, solve for h
OR
 Using D/h ratio and knowing h,
solve for D
 Spherical Riser

1
a  TST  2
 
3  C
a

 m 
 Shrinkage in Solidification and Cooling

Shrinkage of a cylindrical casting during solidification and cooling:

(0) starting level of molten metal immediately after pouring; (1)
reduction in level caused by liquid contraction during cooling
(dimensional reductions are exaggerated for clarity).
Shrinkage in Solidification and Cooling

Reduction in height and formation of shrinkage cavity caused by

solidification shrinkage; (3) further reduction in height and
diameter due to thermal contraction during cooling of solid metal
(dimensional reductions are exaggerated for clarity).
 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 account for solidification
shrinkage and thermal contraction by making
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
 Pattern Allowances

Pattern: A replica of the part to be cast and is used to prepare the mold.
It is made either of wood or metal. Metals: Al, Mg, commonly used.

It is made somewhat larger than the final part for various reasons.

This excess in dimensions is referred to as “pattern allowance”.

Shrink Rule: A special ruler with the expansion added to the dimensions

Regular Rule – 1 ft

Shrink Rule
 Major Pattern Allowances

1. Shrinkage Allowance:
A linear allowance added to the dimensions to
compensate for contractions of the casting.
Liquid Solid (Room Temp)
Solid (Freezing
Temp)

Desired Part
Major Pattern Allowances
2. Machining Allowance:
An allowance added to the dimensions so that we
may later machine it off to produce better surface
finish and tolerance.
 Directional Solidification
 To minimize damaging 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 remote
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
 Desired directional solidification is achieved
using Chvorinov's Rule to design the casting
itself, 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 geometry of the riser is normally
selected to maximize the V/A ratio, this allows
riser volume to be reduced to the minimum
possible value