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1 Component Casting
1.1 Introduction 1
1.1.1 History of Casting 1
1.1.2 Industrial Component Casting Processes 1
1.2 Casting of Components 1
1.2.1 Production of Moulds 1
1.2.2 Metal Melt Pressure on Moulds and Cores 4
1.2.3 Casting in Nonrecurrent Moulds 5
1.2.4 Casting in Permanent Moulds 8
1.2.5 Thixomoulding 11
1.1 INTRODUCTION
1.1.1 History of Casting
As early as 4000 years BC the art of forming metals by
casting was known. The process of casting has not really
changed during the following millennia, for example during
the Bronze Age (from about 2000 BC to 400–500 BC), dur-
ing the Iron Age (from about 1100–400 BC to the Viking
Age 800–1050 AD), during the entire Middle Ages and
the Renaissance up until the middle of the Nineteenth cen-
tury. Complete castings were prepared and used directly
without any further plastic forming.
Figures 1.1, 1.2 and 1.3 show some very old castings.
In addition to improving the known methods of produc-
tion and refining of cast metals, new casting methods were
invented during the Nineteenth century. Not only were
components produced but also raw materials, such as bil-
lets, blooms and slabs. The material qualities were
improved by plastic forming, forging and rolling. An infe-
rior primary casting result cannot be compensated for or
repaired later in the production process.
Steel billets, blooms and slabs were initially produced by
the aid of ingot casting and, from the middle of the Twen-
tieth century onwards, also by the aid of continuous casting.
Development has now gone on for more than 150 years and
this trend is likely to continue. New methods are currently
being developed, which involve the production of cast com-
ponents that are in size as close to the final dimensions as
possible.
1.1.2 Industrial Component Casting Processes
As a preparation for a casting process the metal is initially
rendered molten in an oven. The melt is transferred to a so-
called ladle, which is a metal container lined on the inside
with fireproof brick. The melt will then solidify for further
refining in the production chain. This is performed by trans-
ferring the melt from the ladle into a mould of sand or a
water-chilled, so-called chill-mould of metal. The metal
melt is then allowed to solidify in the mould or chill-mould.
This chapter is a review of the most common and most
important industrial processes of component casting. The
problems associated with the various methods are discussed
briefly when the methods are described. These problems are
general and will be extensively analysed in later chapters.
In Chapter 2 the methods used in cast houses will be
described. The methods used in foundries to produce com-
ponents will be discussed below.
1.2 CASTING OF COMPONENTS
1.2.1 Production of Moulds
A cast-metal component or a casting is an object that has
been produced by solidification of a melt in a mould. The
mould contains a hollow space, the mould cavity, which
in every detail has a shape identical to that of the component.
In order to produce the planned component, a reproduc-
tion of it is made of wood, plastic, metal or other suitable
material. This reproduction is called a pattern. During the
production of the mould, the pattern is usually placed in a
mould frame, which is called a flask or moulding box. The
flask is then filled with a moulding mixture which is com-
pacted (by machine) or rammed (with a hand tool). The
moulding mixture normally consists of sand, a binder and
water.
When the compaction of the flask is finished the pattern
is stripped (removed) from it. The procedure is illustrated in
Figures 1.4 (a–d). The component to be produced is, in this
case, a tube.
Stage 1: Production of a Mould for the Manufactureof a Steel TubeThe cavity between the flask wall and the pattern is then
filled with the mould paste and rammed by hand orMaterials Processing during Casting H. Fredriksson and U. AkerlindCopyright # 2006 John Wiley & Sons, Ltd.
compacted in a machine. The excess mould paste is
removed from the upper surface, and the lower part of the
future mould is ready. The upper one is made in the same way.
Components due to be cast are seldom solid. They nor-
mally contain cavities, which must influence the design of
the mould. The cavities in the component correspond to
sand bodies, so-called cores, of the same shape as the cav-
ities. The sand bodies are prepared in a special core box, the
inside of which has the form of the core. The core box,
which is filled and rammed with fireproof so-called core
sand, is divided into two halves to facilitate the stripping.
The cores normally obtain enough strength during the
baking process in an oven or hardening of a plastic binder.
Figures 1.4 (e) and 1.4 (f) illustrate the production process
of a core, corresponding to the cavity of a tube.
Stage 2: Production of the Core in what will become theSteel TubeWhen the mould is ready for casting the complete cores are
placed in their proper positions. Since the fireproof sand of
Figure 1.1 Stone mould for casting of axes, dating from 3000
BC.
Figure 1.2 A knife and two axes of pure copper, cast in stone
moulds of the type illustrated in Figure 1.1.
Figure 1.3 Picture of a cast bronze Buddha statue, which is more
than 20 m high. The statue was cast in the Eighth century AD. Its
weight is 780 � 103 kg. A very special foundry technique was used
in which the mould production and casting occurred simulta-
neously. The mould was built and the statue was cast in eight
stages, starting from the base. The mould was built around a frame-
work of wood and bamboo canes. Each furnace could melt 1 � 103
kg bronze per hour. Reproduced with permission from Giesserei-
Verlag GmbH.
Figure 1.4 (a) A pattern, normally made of wood, is prepared as
two halves. It is equipped with a so-called core print at the ends
as dowels. Reproduced with permission from Gjuterihistoriska
Sallskapet.
Figure 1.4 (b) Half the pattern and the patterns of inlet and cast-
ing runner are placed on a wooden plate in half a flask. A fine-
grained powder, for example lycopodium powder or talc, is distrib-
uted over the pattern to facilitate the future stripping of the pattern
[see Figure 1.4 (d)]. Reproduced with permission from Gjuterihis-
toriska Sallskapet.
2 Component Casting
the cores has a somewhat different composition than that of
the mould, one can usually distinguish between core sand
and mould sand.
A necessary condition for a successful mould is that it
must contain not only cavities, which exactly correspond
to the shape of the desired cast-metal component, but
also channels for supply of the metal melt. These are called
casting gates or gating system [Figure 1.4 (c)]. Other cav-
ities, so-called feeders, which serve as reservoirs for
the melt during the casting process, are also required
Figure 1.4 (c) The upper pattern half and the upper part of the
mould flask are placed on the corresponding lower parts. A thin
layer of fine-grained dry sand, so-called parting sand, covers the
contact surfaces. Special patterns of the future sprue and the fee-
ders are placed exactly over the inlets in the lower parts of the
tube flanges. Reproduced with permission from Gjuterihistoriska
Sallskapet.
Figure 1.4 (d) The mould parts are separated and the pattern
parts are stripped, i.e. lifted off the upper and lower parts of the
mould. The patterns of the inlet, sprue and feeders are also
removed. The figure shows the lower part of the mould after the
pattern stripping. Reproduced with permission from Gjuterihistor-
iska Sallskapet.
Figure 1.4 (e) The cavity in what will become the tube is formed
by a sand core, produced in a core box. The two halves of the core
box are kept together by screw clamps while the sand is rammed
into the mould. A cylindrical steel bar is placed as core grid in
the lengthwise direction of the core to strengthen the future core.
Reproduced with permission from Gjuterihistoriska Sallskapet.
Figure 1.4 (f) The core is lifted out of the parted core halves. The
core is often baked in an oven to achieve satisfactory strength.
Reproduced with permission from Gjuterihistoriska Sallskapet.
Figure 1.4 (g) The core is placed in the lower part of the mould. The parts of the mould are joined with the parting surfaces towards each
other. The dowels through the holes in the outer walls of the flask parts guarantee the exact fit of the corresponding cavities in the upper and
lower halves of the mould. A so-called casting box, which insulates the upper surface of the melt and prevents it from solidifying too early, is
placed exactly above the sprue and the parts of the mould are kept together by screw clamps. The mould is ready for use. Reproduced with
permission from Gjuterihistoriska Sallskapet.
Casting of Components 3
[Figures 1.4 (c) and 1.4 (g)]. Their purpose is to compensate
for the solidification shrinkage in the metal. Without
feeders the complete cast-metal component would contain
undesired pores or cavities, so-called pipes. This phenom-
enon will be discussed in Chapter 10. When the casting gate
and feeders have been added to the mould, it is ready for
use.
Stage 3: Casting of a Steel TubeThe casting process is illustrated in Figures 1.4 (g), 1.4 (h),
and 1.4 (i).
1.2.2 Metal Melt Pressure on Moulds and Cores
During casting, moulds and cores are exposed to vigorous
strain due to the high temperature of the melt and the pres-
sure that the melt exerts on the surfaces of the mould and
cores.
To prevent a break-through, calculations of the expected
pressure on the mould walls, the lifting capacity of the
upper part of the mould and the buoyancy forces on
cores, which are completely or partly surrounded by melt,
must be performed. These calculations are the basis for dif-
ferent strengthening procedures such as varying compaction
weighting in different parts of the mould, locking of the
cores in the mould and compaction weighting on or cramp-
ing of the upper part of the mould.
The laws, which are the basis of the calculations, are
given below. The wording of the laws has been adapted
to the special casting applications.
The Law of Connected VesselsIf two or more cavities are connected to each other, the
height of the melt will be equal in all of them.
Pascal’s PrincipleA pressure that is exerted on a melt in a closed cavity, is
transferred unchanged to all parts of the mould wall.
Liquid Pressure and Strainp ¼ rgh F ¼ pA
The Hydrostatic ParadoxThe pressure on a surface element is universally perpen-
dicular to the element and equal to rgh where h is the
depth of the surface element under the free surface of
the melt, independent of the direction of the element.
The pressure on a lateral surface ¼ the weight of a
column with the surface as a basis and a height equal to
the depth of its centre of mass.
Archimedes’ PrincipleAn immersed body (core) seemingly loses an amount of
weight equal to the weight of the melt displaced by the
body.
The laws given on above are valid for static systems.
During casting the melt is moving and dynamic forces
have to be added. These forces are difficult to estimate.
The solution of the problem is usually practical. The calcu-
lations are made as if the system were static and the result-
ing values are increased by 25–50 %.
An example will illustrate the procedure. The pressure
forces are comparatively large and the moulds have to be
designed in such a way that they can resist these forces
without appreciable deformation.
Example 1.1A cavity consists of a horizontal cylindrical tube. Its length
is L and its outer and inner diameters are D and d, respec-
tively. The interior of the cylinder is filled with a sand core.
The density of the sand core is rs. The axis of the cylinder
is placed at the depth h below the free surface of the melt.
The density of the melt is rL.
Calculate
(a) the buoyancy force on the upper part of the mould
(b) the total buoyancy force on the sand core when the
cavity is filled with melt, and
Figure 1.4 (h) When the casting has solidified and cooled after
casting, the mould is knocked out. The casting is cleaned from
remaining sand. The feeders and inlet are removed through cutting
or oxygen shearing. The section surfaces are ground smoothed.
Reproduced with permission from Gjuterihistoriska Sallskapet.
Figure 1.4 (i) The complete steel tube.
4 Component Casting
(c) the mass one has to place on the upper part of the mould,
to compensate for the buoyancy forces, if d ¼ 50 mm,
D ¼ 100 mm, h ¼ 200 mm and L ¼ 300 mm.
The densities of the melt and the sand are 6.90 � 103 kg/m3
and 1.40 � 103 kg/m3 respectively.
Solution:
(a): Via the sprue the melt exerts outward pressure forces
acting on the surface elements of the upper part of
the mould. These are equal to the pressure forces that
act on each surface element in the figure but are
opposite in direction because the forces in this case
act from the melt towards the mould. The desired buoy-
ancy force is thus equal to the resultant in the latter case
but has an opposite direction. We will calculate the
resultant.
The pressure on the mould varies with its height. For this
reason it is difficult to calculate the resulting force directly.
We choose to calculate it as the difference between two
pressure forces, which are easy to find.
F ¼ LD hrLg � 1
2
pD2
4
� �LrLg ¼ LDrLg h � pD
8
� �ð10Þ
(b): The lifting force is equal to the weight of the melt,
displaced by the sand core, minus the weight of the
core. This force acts on the core prints [Figure 1.4 (a)
on page 2].
Flift ¼pd2
4
� �LrLg � pd2
4
� �Lrsg ¼ pd2
4
� �Lg rL � rsð Þ
ð20Þ
(c): The forces, directed upwards and acting on the upper
part of the mould, are equal to the sum of the forces
in equations (10) and (20) because the lifting force on
the core, via the core prints, also acts on the upper
part of the mould.
Ftotal ¼ LDrLg h � pD
8
� �þ pd2
4
� �Lg rL � rsÞð ð30Þ
This force, directed upwards, is equal to the weight of the
mass M and we get:
M ¼ Ftotal
g¼ 40:57 kg þ 3:24 kg ¼ 43:81 kg
Answer:
(a) The pressure force on the upper surface of the mould
is LDrLg h � pD8
� �:
(b) The lifting force is pd2
4
� �Lg rL � rsÞ:ð
(c) 44 kg.
1.2.3 Casting in Nonrecurrent Moulds
Sand Mould CastingSand moulding is the most common of all casting methods.
It can be used to make castings with masses of the
d
D
h
d
D
Ftotal ¼ 0:300 � 0:100 � 6:90 � 103� �
g 0:200 � p� 0:100
8
� �þ p� 0:0502
4
� �0:300 � g 6:90 � 1:40ð Þ � 103 N
¼ 40:57 g þ 3:24 g ¼ 43:81 g
Casting of Components 5
magnitude 0.1 kg up to 105 kg or more. It can be used for
single castings as well as for large-scale casting. In the lat-
ter case moulding machines are used. A good example is in
the manufacture of engine blocks.
In sand moulding an impression is made of a pattern of
the component to be cast. There are two alternative sand-
moulding methods, namely hand moulding and large-
scale machine moulding.
Hand moulding is the old proven method where the mould
is built up by hand with the aid of wooden patterns as
described earlier. This method has been transformed into a
large-scale machine method where the mould halves are sha-
ken and pressed together in machines. It needs to be possible
to divide the mould into two or several parts. Large-scale
moulds get a more homogeneous hardness, and thus also a
better dimensional accuracy, than do hand-made moulds.
The advantages and disadvantages of sand mould casting
are listed in Table 1.1.
The disadvantages of the sand moulding method have
been minimized lately by use of high-pressure forming,
i.e. the sand is compacted under the influence of a high
pressure. The method can be regarded as a development
of the machine moulding method of sand moulds. Normal
mould machines work at pressures up to 4 � 106 Pa (4 kp/
cm2) while the high-pressure machines work at pressures
up to (10–20) � 106 Pa (10–20 kp/cm2). The higher pres-
sure offers a better mould stability, which results in a better
measure of precision than that given by the low-pressure
machines. Development in sand foundries proceeds more
and more towards the use of high-pressure technologies.
Shell Mould CastingThe shell mould casting method implies that a dry mixture
of fine-grained sand and a resin binder is spread out over
a hot so-called brim plate, which covers half the mould.
The resin binder melts and sticks to the sand grains,
forming a shell of 6–10 mm thickness close to the pattern.
The shell is hardened in an oven before it is removed from
the plate with the pattern. The method is illustrated in
Figure 1.5.
Two shell halves are made. After hardening they are glued
together. Before casting, the mould is placed into a container
filled with sand, gravel or other material, which gives
increased stability to the mould during the casting process.
A shell with a smooth surface and a good transmission
ability for gases is obtained with this method, which can be
used for most casting metals. The advantages and disadvan-
tages are given in Table 1.2.
TABLE 1.1 The sand mould casting method.
Advantages Disadvantages
Most metals can be cast Relatively poor
dimensional accuracy
Relatively complicated Poor surface smoothness
components can be cast
Single components can be
produced without too
high initial costs
Figure 1.5 Shell mould casting stages 1 to 6.
TABLE 1.2 The shell mould casting method.
Advantages Disadvantages
High dimensional accuracy High initial cost for the model equipment, which must be made of cast iron
Good reproduction of the shape of the component
Good surface smoothness Profitable series size for masses between 0.1–1 kg must be at least
50 000–100 000 components
Easy finishing of the surfaces of the component
No burnt sand sticking to the surface Small maximum mass of components, due to fragility of the mould
No sand inclusions Maximum mass is 60–70 kg.
Components with thin walls can be cast
Complicated core systems are possible
6 Component Casting
Precision Casting or Shaw ProcessIn the Shaw process, a parted mould is made of fireproof
material with silicic acid as a binding agent. The mould
is heated in a furnace to about 1000 �C. The method gives
roughly the same measure of precision as the shell mould
casting method but is profitable to use for small series
and single castings because the pattern of the mould can
be made of wood or gypsum. The Shaw process is espe-
cially convenient for steel.
Investment CastingInvestment casting is also a precision method for compo-
nent casting. In this method, a mould of refractory material
is built on a wax copy of the component to be cast. An
older name of the method is the ‘lost wax melting
casting’ process. The method is illustrated in Figures
1.6 (a–f).
In investment casting a wax pattern of the component
has to be made. The wax pattern is then dipped in a mixture
of a ceramic material and silicic acid, which serves as a
binding agent. When the mould shell is thick enough it is
dried and the wax is melted or burnt away. Then the
mould is burnt and the casting can be performed.
Investment casting can be used for all casting metals.
The mass of the casting is generally 1–300 g with maximum
masses up to 100 kg or more. The advantages and disadvan-
tages are listed in Table 1.3.
Investment casting offers very good dimensional
accuracy. With the proper heat treatment after casting the
component acquires the same strength values for stretch
and fracture limits as do forged or rolled materials.
The investment casting method and the Shaw method are
complementary to each other in a way. The Shaw method is
used when the casting is too big for investment casting or
Figure 1.6 (a) Wax patterns are cast in a special tool made for
the purpose. Reproduced with permission from TPC Components
AB.
Figure 1.6 (b) A so-called cluster is built of the wax patterns.
The trunk and the branches are inlets. Reproduced with permission
from TPC Components AB.
Figure 1.6 (c) The cluster is dipped into ceramic slurry, pow-
dered with ceramic powder and dipped again. The procedure is
repeated until the desired thickness has been achieved. Reproduced
with permission from TPC Components AB.
Figure 1.6 (d) The wax is melted away and the mould is burnt in
an oven. The wax can only be used once. Reproduced with permis-
sion from TPC Components AB.
Figure 1.6 (e) Casting is performed directly into the hot mould.
Reproduced with permission from TPC Components AB.
Casting of Components 7
when the series is too small to be profitable with the invest-
ment casting method.
1.2.4 Casting in Permanent Moulds
Gravity Die CastingIn gravity die casting permanent moulds are used. Such a
mould is made of cast iron or some special steel alloy
with a good resistance to high temperatures (the opposite
property is called thermal fatigue).
The gravity die casting method is often used for casting
zinc and aluminium alloys. It is difficult to cast metals with
high melting points due to the wear and tear on the mould,
which is caused by thermal fatigue.
Cores of steel or sand can be used. It is also possible to
introduce details of materials other than the cast metal, for
example, bearing bushings and magnets. The advantages
and disadvantages of the method are listed in Table 1.4.
Due to the high mould cost, series of less than 1000
components are not profitable. In these cases, another cast-
ing method must be chosen. There is also an upper limit,
which is set by the thermal fatigue of the mould. In alumi-
nium casting the maximum number of components is
around 40 000.
High-Pressure Die CastingThe molten metal is forced into the mould at high pressure
as indicated by the name of the process. The method is
described in Figure 1.7.
The permanent mould is made of steel and the mould
halves are kept together by a strong hydraulic press. The
method can only be used for metals with low melting
points, for example zinc, aluminium and magnesium
alloys.
The mechanical properties of the components are good
with this method, better than with the gravity die casting
method. However, weak zones may occur in the material
due to turbulence in the melt during the mould filling.
Due to high machine and mould costs, the high-pressure
die casting method will be profitable only if the number of
cast components exceeds 5000 to 10 000. The method is
useful for production of large series of components, for
example in the car industry.
The ‘life time’ of the high-pressure die casting machine
varies from about 8000 castings for brass to 800 000 cast-
ings for zinc alloys.
The advantages and disadvantages of the method are
listed in Table 1.5.
TABLE 1.3 The investment casting method.
Advantages Disadvantages
Good accuracy High mould cost
Good mechanical properties Size limitations
Good surface finish
Thin components can be cast
No shape limitations
Can be used for all casting metals
TABLE 1.4 The gravity die casting method.
Advantages Disadvantages
Good mechanical properties High mould cost
High dimensional accuracy Materials with
low melting point only
High surface smoothness
Figure 1.6 (f) The ceramic mould is knocked out after casting
and solidification and the complete component is revealed. Repro-
duced with permission from TPC Components AB.
Figure 1.7 High-pressure die casting machine. During casting,
the molten metal is transferred into the shot cylinder. The piston
is then pushed inwards and forces the melt into the mould.
8 Component Casting
Low-Pressure Die CastingThe principle of this method is illustrated in Figure 1.8.
Contrary to the high-pressure die casting machine, the
low-pressure casting machine contains no pushing device
and no piston. Nor is it necessary to apply the high pressure,
required in high-pressure die casting, at the end of the
casting process.
Air, or another gas, is introduced into the space above
the melt. The gas exerts pressure on the melt and causes
it to rise comparatively slowly in the central channel and
move into the mould. The mould is kept heated to prevent
solidification too early in the process. This is a great advan-
tage when small components, with tiny protruding parts, are
to be cast. In this way it is possible to prevent them from
solidifying earlier than other parts of the mould. This is
one of the most important advantages of this casting
method.
The walls of the component to be cast can be made
rather thin. The low melt flow gives little turbulence in
the melt during the mould filling and very little entrapment
of air and oxides. When the casting has solidified the pres-
sure is lowered and the remaining melt in the central chan-
nel sinks back into the oven.
A list of the advantages and disadvantages of the method
is given in Table 1.6.
Squeeze CastingSqueeze casting is a casting method that is a combination of
casting and forging. It is described in Figures 1.9 (a–c).
When the mould has been filled the melt is exposed to a
high pressure and starts to solidify. The pressure is present
during the whole solidification process so that pore forma-
tion, which causes plastic deformation, is prevented and the
mechanical properties of the castings are strongly improved
as compared to conventional casting.
TABLE 1.5 The high-pressure die casting method.
Advantages Disadvantages
The process is rapid Very high workshop costs due to high pressure and high thermal fatigue
Very thin and complicated components can be cast
Very high precision compared with conventional casting The rapid filling process is very turbulent and the melt absorbs large
Little work remains after casting amounts of gas
Inset parts, for example bearings and bolts, Components containing cores are normally impossible to cast
can be introduced from the beginning Only metals with low melting points can be cast
Figure 1.8 Low-pressure die casting machine. The melt is
included in an air-tight chamber connected to a compressor.
When the pressure is increased in the chamber, melt is pressed
upwards through the refractory tube into the mould. Reproduced
with permission from Addison-Wesley Publishing Co. Inc., Pearson.
TABLE 1.6 The low-pressure die casting method.
Advantages Disadvantages
High metal yields are obtained Lower productivity than in high-pressure die casting.
Little work remains after casting. Use of cores is possible More expensive moulds than in conventional sand casting
Easy to automate
Dense structure of the component, compared to Only metals with low melting points can be cast
chill-mould casting and high-pressure die casting
Lower workshop costs than in high-pressure die casting
Better mechanical properties than in conventional sand casting
Casting of Components 9
Centrifugal CastingIn centrifugal casting centrifugal force is used in addition to
gravitational force. The former is used partly to transport
the melt to the mould cavity and exert a condensing
pressure on it and partly, in certain cases, to increase
the pressure, thus allowing thinner details to be cast and
making surface details of the metal-cast components more
prominent.
There are three types of centrifugal casting, distin-
guished by the appearance of the mould and its construction
and the purpose of the casting method:
true centrifugal casting;
semicentrifugal casting or centrifugal mould casting;
centrifugal die casting.
The principal differences between the three methods are
described in Table 1.7.
True Centrifugal Casting
This method is characterized by a simple mould with no
cores. The inner shape of the casting is thus formed entirely
by the mould and centrifugal force. Typical products pro-
duced in this way are tubes and ring-shaped components.
Figure 1.9 Squeeze casting. (a) The melt is poured into the lower
mould section; (b) the melt is exposed to a high pressure from the
upper mould section; (c) after solidification the upper mould
section is removed and the casting is ejected by the aid of the
ejector. Reproduced with permission from The Metals Society.
TABLE 1.7 Schematic description of various centrifugal casting methods.
Characteristics True centrifugal casting Semicentrifugal casting Centrifugal die casting
Horizontal axis of rotation
Vertical axis of rotation
Inclined axis of rotation
10 Component Casting
The dominating product, with respect to mass, is cast iron
tubes.
The permanent mould, i.e. the mould, is normally
cylindrical and rotates around its central axis, which can
be horizontal, vertical or inclined. Figure 1.10 shows a
sketch of the most common tube casting machine.
Semicentrifugal Casting
During semicentrifugal casting (Figure 1.11) the mould is
rotated around its symmetry axis. The mould is complicated
in most cases and may contain cores. The detailed shape of
the casting is given by the shape of the rotating mould. The
centrifugal force is utilized for slag separation, refilling of
melt and increase of the filling power in order to cast com-
ponents with thin sections. Cogwheels are an example of
components that can be cast using this method.
Centrifugal Die Casting
The principle of centrifugal die casting is illustrated in
Figure 1.12. The mould cavities are symmetrically grouped
in a ring around a central inlet. From this the metal melt is
forced outwards under pressure into the mould cavity and
efficiently fills all its contours. The centrifugal force supplies
the necessary pressure to transport the melt to all parts of the
mould.
The method is extensively used for casting in moulds
prepared by the investment casting method. It is often
used in the dental industry to cast gold crowns for teeth.
1.2.5 Thixomoulding
Thixomoulding should logically have been treated under
the heading ‘Casting in Permanent Moulds’ in Section 1.2.4.
The reason why it has been extracted from its proper posi-
tion is that it differs radically from all other mould casting
methods and deserves special attention.
Thixomoulding is a very promising method for casting
components of various sizes. Fleming originally developed
it in 1976 at MIT in the US and introduced it under the
name semi-solid metal processing (SSM). During the late
1990s the method was primarily used for the casting of
magnesium on an industrial scale.
Thixomoulding can be used for casting many alloys. It
is a very promising new method, which will probably
develop rapidly and is expected to obtain a wide and
successful application in industry. Zinc and aluminium
alloys followed magnesium as suitable for thixomoulding
on an industrial scale. Thixomoulding of Zn and Al alloys
has been commercialized and other types of alloys will
follow.
Principle of ThixomouldingIn ordinary mould casting it is necessary to melt the alloy
and superheat the liquid to above its melting point. Suffi-
cient heat must be provided to retard the crystallization pro-
cess, i.e. reduce the formation of so-called dendrites, and
maintain sufficient fluidity of the melt until the mould has
Figure 1.10 Sketch of a tube casting machine which works
according to the principle of true centrifugal casting.
Figure 1.11 Machine designed for semicentrifugal casting of
double cogwheels. Reproduced with permission from Karlebo.
Figure 1.12 Mould for centrifugal die casting. Reproduced with
permission from Karlebo.
Casting of Components 11
been completely filled. Dendrites and dendrite growth will
be discussed in Chapter 6.
In contrast to the superheating required for conventional
mould casting, thixomoulding is carried out at a tempera-
ture between the liquidus and solidus temperatures of the
alloy (Figure 1.13). At this temperature the alloy consists
of a viscous mixture of a solid phase with growing dendrites
and a liquid phase. This can be seen from the phase
diagram, which shows the composition of the alloy as a
function of temperature (Figure 1.13).
The solidifying metal is exposed to shearing forces,
which break the dendrites into pieces and a fairly homoge-
neous mixture is formed. The solidifying metal consists
of spherical solid particles in a matrix of melt. The
appearances of the structures of the partly molten alloy
before and after the mechanical treatment are seen in
Figure 1.14.
The homogenous viscous solidifying metal is the mate-
rial used for casting.
Thixomoulding EquipmentFigure 1.15 shows a machine designed for thixomoulding
of Mg alloys.
Room-temperature pellets of the alloy are fed into the
rear end of the machine. An atmosphere of argon is used
to prevent oxidation at high temperature. The pellets are
forwarded into the barrel section where they are heated to
the optimal temperature below the melting point of the
alloy. The solidifying metal is carried forward and simulta-
neously exposed to strong shear forces when a powerful
screw is rotated around its axis.
The solidifying metal is forced through a nonreturn valve
into the accumulation zone. When the required amount of
the solidifying metal is in front of the nonreturn valve,
the screw forces it into the preheated metal mould and a
product of desired shape is formed. The injection of the
Figure 1.14 The structures of a partly molten MgAl alloy before
and after mechanical treatment. Courtesy of Thixomat Inc., Ann
Arbor, MI, USA.
Figure 1.15 Thixomoulding machine designed for moulding Mg alloys. Courtesy of Thixomat Inc., Ann Arbor, MI, USA.
Figure 1.13 Phase diagram of the Mg–Al system. An alloy
with 90–89 % Mg and 9–10 % Al at a temperature of 560–
580 �C represents a suitable mixture for thixomoulding die casting.
Reproduced with permission from the American Society for
Metals.
12 Component Casting
solidifying metal into the mould occurs under pressure. The
process is reminiscent of the squeeze casting process,
described on pages 9–10.
The advantages and disadvantages of thixomoulding are
listed in Table 1.8.
TABLE 1.8 Method of thixomoulding compared with conventional mould casting.
Advantages Disadvantages
No transfer operations required High equipment cost
Good dimensional stability, i.e. good dimensional accuracy of products Two-step process
Low temperature, which reduces the melt costs, gives low corrosion and Risk of formation of oxides and other inclusions
low porosity of products
Good mechanical properties of products in most cases In some cases coarser structure and less favourable
mechanical properties
The Ar atmosphere results in little oxidation, which contributes to low corrosion
No secondary machining and heat treatment of the cast components are required
Environment-friendly process
Casting of Components 13