Ch3 specialcastproc Erdi Karaçal Mechanical Engineer University of Gaziantep

02/11/14 CHAPTER 3 SPECIAL CASTING
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CHAPTER 3
SPECIAL CASTING PROCESSES
3.1 INTRODUCTION
ME 333 PRODUCTION PROCESSES II
The process used for making a casting depends on;
-the quantity to be produced,
-the metal to be cast,
-the complexity of the part.
Sand molds are single-purpose molds, and are completely destroyed after the metal
has solidified.
Quite obvious the use of a permanent mold effect considerable saving in labor cost.
A summary of the various special casting methods, which will be discussed in this
chapter, is as follows:
A.Casting in metallic molds
B.Centrifugal casting
C.Precision or investment casting
D.Continuous casting
E.Shell molding

3.2 METAL MOLD CASTING PROCESSES
Permanent molds must be made of metals capable of withstanding high
temperatures.
Because of their high cost they are recommended only when many casting are to
be produced.
Although permanent molds are impractical for large castings and alloys of high
melting temperatures, they can be used advantageously for small and medium-sized
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castings that are manufactured in large quantities.
Die Casting: Die casting is a process in which molten metal is forced by
pressure into a metal mold known as a ‘die’.
The usual pressure is 100 to 125 atm.
It is the most widely used of any of the permanent mold processes.
There are two methods employed:
1. Cold-chamber Die Casting
2. Hot-chamber Die Casting

(1) Cold-chamber Die Casting:
Material to be cast is molten outside the machine.
Used for materials having high melting temperature Tm> 550°C, i.e. brass,
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aluminum, and magnesium.
(2) Hot-chamber Die Casting:
Materials to be cast is molten inside the machine.
Used for materials having low melting temperature Tm< 550°C, i.e. zinc, tin, and
lead.
(The principal distinction between the two is determined by the location of the melting
pot.
In the hot-chamber pot is included in the machine; and
In the cold-chamber pot is separate from the machine,
metal is introduced to injection cylinder by other means.)

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Fig. 3.1 Types of Die Casting

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Schematics of hot-chamber die-casting

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Schematics of cold-chamber die-casting

Die Casting Cavities
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Die Casting Products

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Advantages of die casting:
1. The process is rapid (since both dies and cores are permanent)
2. The surface quality is very good. (The smooth surface improves
appearance and reduces work required for other operations.)
3. Dimensional tolerances are very good comparing to sand casting. (The
size is so accurately controlled that little or no machining is necessary.)
4. The scrap loss is low since sprue, runners, and gates can be remelted.

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- The optimum production quantity ranges from 1,000 to 200,000 pieces.
- Max. Weight of brass die casting is about 2 kg, but aluminum die castings of
over 50 kg are common.
- Small to medium size castings can be made at a cycle rate of 100 to 800 die
fillings per hour.
- The life of dies depends on the metal cast and may range from 10,000 fillings
for brass to several million if zinc is used.
- If the part is big and complex, a single-cavity mold is used. If the parts are small
and quantity is large, multiple-cavity die can be used.
Optimum no. of parts: 1000-200,000
Weight of part: Brass about 2 kg
Aluminum about 50 kg
Die life: about 10,000 filling for Brass
Several millions for filling Zinc
Other methods of metal mold castings: low-pressure permanent mold casting,
gravity permanent mold casting, slush casting and pressed casting.

3.3 DIE-CASTING ALLOYS
A relatively wide range of nonferrous alloys can be die-cast. The principal base
metals used are ZINC, ALUMUNIUM, MAGNESIUM, COPPER, LEAD and TIN. They
are classified in two groups:
1.Low-temperature alloys (casting temp. below 5500C),
2.High-temperature alloys
Zinc-Base Alloys : Over 75% of die castings are produced from zinc-base alloys.
Melting point is around 4000C. So, they are cast by Hot-chamber die casting.
ASTM Number Al Cu Mg Zn
AG40A(XXIII) 4.1 0.1 max 0.04 Remainder
AC41A(XXV) 4.1 1.0 0.04 Remainder
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Aluminum improves mechanical properties.
Copper improves tensile strength and ductility.
Magnesium makes casting stable in microstructure.
Zinc alloys are widely used in: automotive industry, washing machines,
refrigerators, business machines, etc.

Aluminum-Base Alloys : They are used due to their lightness in mass and
resistance to corrosion. Compared to zinc alloys they are more difficult to cast
(melting point around 5500C). Since molten aluminum will attack steel if kept in
continuous contact with it, the cold-chamber process generally is used.
ASTM
Number Cu Si Mg Al Uses
S12A&B -- 12 -- Remainder Large Intricate castings
S5C -- 5 -- Rem. Gen. Purpose
G8A 3 -- 8 Rem. High strength, res. corro.
SG100A&B -- 9.5 0.5 Rem. Gen. Purpose, Good Property,
excellent casting charac.
SC84B 3.5 9 -- Rem. Good machinability and
castability
Principal elements used as alloys with aluminum are SILICON, COPPER, and
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MAGNESIUM.
Silicon increases hardness and corrosion resisting properties, copper increases
mechanical properties, magnesium increases lightness and resistance to
impact. They are generally used in aerospace industry and production of
pistons.

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Copper-Base Alloys : Die casting of brass and bronze have presented a
greater problem due to their high casting temperature. Temperatures are
around 870 to 10400C, which need heat resisting die material. Copper alloys are
cold chamber die-cast.
ASTM
Number Cu Si Sn Pb Zn Uses
Z30A 57 min -- 1.5 1.5 30 min Yellow brass, good
machinability
Z5331A 65 1 -- -- Rem. Gen. Purpose casting, Corr.
res., castability
Z5144A 81 4 -- -- Rem.
High strength, hardness,
wear resistance-but most
difficult to mold
Copper-base alloys have extensive use in miscellaneous hardware, electric-machinery
parts, small gears, marine, automotive and aircraft fittings, chemical
apparatus, and numerous other small parts.

Magnesium-Base Alloys : It is alloyed principally with ALUMINUM, but may
contain small amounts of SILICON, MANGANESE, ZINC, COPPER, and
NICKEL. They have the lowest density. Their casting temperatures are around
670-7000C, so, cold chamber die casting is suitable.
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ASTM Number Al Zn Mn Si Cu Ni Mg
B94 9 0.5 0.13 0.5 0.3 0.03 Rem.
Zinc parts Aluminium parts
Copper parts
Magnesium parts

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• Die Materials:
- Hot-worked tools steels
- Mold steels
- Maraging Steels
- Refractory Metals
Properties:
1.High hot strength
2.High temperature wear resistance
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3. 4. CENTRIFUGAL CASTING (Savurma Döküm)
Centrifugal casting is the process of rotating a mold while the metal solidifies so
as to utilize centrifugal force to position the metal in the mold.
Castings of symmetrical shape lend themselves particularly to this method,
although many other types of casting can be produced.
Fig. 3.2 Centrifugal casting machine for casting steel or cast iron pipe

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Setup for true horizontal centrifugal casting

Centrifugal casting is often more economical than other methods.
Cores in cylindrical shapes and risers or feed-heads are both eliminated.
The castings have a dense metal structure with all impurities forced back to the
centre where frequently they can be machined out.
Piston rings weighing 50-100 grams to paper mill rolls weighing over 42 tons have
been cast in this manner.
Aluminum engine block uses centrifugally cast iron liners.
In some alloys, the heavier elements tend to be separated from the base metal,
known as “gravity segregation”.
The metal is forced against the walls on the mold with a centrifugal force of
approximately 70 g, which is a force 70 times that of the force of gravity alone on
the casting. Forces as high as 150 g have been used but are seldom found
necessary unless very thick-walled cylinders are being cast.
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Semi-centrifugal casting
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- In this method, centrifugal force is used to produce solid castings rather than
tubular parts.
- Density of the metal in the final casting is greater in the outer sections than at the
center of rotation.
- The process is used on parts in which the center of the casting is machined away,
such as wheels and pulleys.

3.5. PRECISION OR INVESTMENT CASTING (Hassas Döküm)
Precision or investment casting employed techniques that enable very smooth
highly accurate castings to be made from both ferrous and non-ferrous alloys.
The process is useful in casting unmachinable alloys and radioactive metals.
There are a number of processes employed, but all incorporate a sand, ceramic,
plaster (alçı), or plastic shell made from an accurate pattern into which metal is
poured.
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Advantages of investment techniques are;
(1) intricate forms with undercuts can be cast;
(2) a very smooth surface is obtained with no parting line;
(3) dimensional accuracy is good;
(4) unmachinable parts can be cast;
(5) may replace die casting for short runs.
Main disadvantage is that it is expensive and not suitable for big parts.

Patterns are produced from wax (mum) or plastics which are subsequently melted
from the mold, leaving a cavity having all the details of the original pattern.
Present practice requires that a replica of the part to be cast made from steel or
brass.
From this replica a bismuth or lead-alloy split mold is made.
After wax is poured into the mold and solidification takes place, the mold is opened
and the wax pattern removed.
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Replica Split pattern Wax pattern
Wax Tree Plaster Coated
Fig. 3.3
Processes of
Precision or
Investment
Casting

Several patterns are usually assembled (Wax Tree) with necessary gates and risers
by heating the contact surfaces (wax welding) with a hot wire.
This cluster is molded by silica sand, plaster or ceramic slurries.
After the mold material gets strength, the mold is placed upside down and heated in
an oven for several hours to melt out the wax and to dry the mold.
The casting can be produced by gravity, vacuum, pressure, or centrifugal casting.
When the solidification finished, mold is broken away and gates and risers are cut-off.
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Replica Split pattern Wax pattern
Wax Tree Plaster Coated
Fig. 3.3
Processes of
Precision or
Investment
Casting

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Figure
Schematic
illustration of
investment
casting, (lostwax
process).
Castings by this
method can be
made with very
fine detail and
from a variety of
metals.
23

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Investment casting of an integrally cast
rotor for a gas turbine.
(a)Wax pattern assembly.
(b)Ceramic shell around wax pattern.
(c)Wax is melted out and the mold is filled,
under a vacuum, with molten superalloy.
(d)The cast rotor, produced to net or near-net
shape.
Crosssection and microstructure
of two rotors:
(top) investment-cast;
(bottom) conventionally cast.
Examples

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3. 6. CONTINUOUS CASTING )
Continuous casting
consists of pouring
molten metal into one
end of a metal mold
open at both ends,
cooling rapidly, and
extracting the solid
product in a continuous
length from the other
end.
This is done with copper,
brass, bronze,
aluminum, and to a
growing extent, cast iron
and steel.
Fig. 3.4 A continuous casting rolling process

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Continuous casting is done for a number of purposes.
It is suitable for any shapes of uniform cross-section: round,
square, rectangular, hexagonal, fluted, gear toothed, and
many other forms; solid or hollow.
A growing use is to produce blooms, billets, and slabs for
rolling structural shapes. This is cheaper than rolling from
ingots.
A bloom has a square cross section with a minimum size of
15 by 15 cm.
A billet is smaller than a bloom and may have any square
section from 4 cm up to the size of a bloom.
Slabs may be rolled from either an ingot or a bloom. They
have a rectangular cross-sectional area with a minimum width
of 25 cm and a minimum thickness of 4 cm. The width is
always three or more times the thicknesses, which may be as
much as 40 cm. Plates, skelp and thin strips are rolled from
slab.

Continuous casting offers several advantages:
1. It yields 10% or more over rolling from ingots. Ingots have an appreciable
amount of porous end, which returns back to furnace. This waste is eliminated in
continuous casting.
2. A hollow center occurs from shrinkage in continuous casting but it is welded shut
after four rolling passes. Continuous cast structure is more uniform and dense.
3. Physical properties and surface finishes are comparable to those obtained in
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other metal mold processes.
Continuous casting is essentially automatic, and unit labor cost is low. Dies or
molds are made of copper or graphite.

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3. 7. SHELL MOLDING
Shell moulding is a casting process in which the mold is a thin shell (typically 9mm)
made of sand held together by a thermosetting resin binder.
Steps in shell moulding: (1) a match-plate or cope-and-drag metal pattern is heated and placed over a
box containing sand mixed with thermosetting resin;
(2) Box is inverted so that sand and resin fall onto the hot pattern, causinga layer of the mixture to
partially cure on the surface to form a hard shell;
(3) Box is repositioned so that loose, uncured particles drop away;
(4) Sand and shell is heated in oven for several minutes to complete curing
(5) Shell mold is stripped from the pattern
(6) Two halves of the shell mold are assembled, supported by sand or metal shot in a box,
(7) The finished casting is removed

Advantages:
1. Good surface finish (up to 2.5
mm)
2. Good dimensional accuracy
(±0.25 mm)
3. Suitable for mass production
Disadvantages:
Expensive metal pattern
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Area of application:
Mass production of steel casting of less than 10 kg
Two halves of a shell mold pattern

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3. 8. Expandable-pattern casting (Lost foam Process)
This process uses a polystyrene pattern, which evaporates upon contact with
molten metal to form a cavity for the casting; this process is also known as lost-foam
casting.
It has become one important casting process for ferrous and nonferrous
metals, particularly for the automative industry.

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3. 9. Casting Techniques for single-crystal components
Methods of casting turbine
blades: (a) directional
solidification; (b) method to
produce a single-crystal blade;
and (c) a single-crystal blade with
the constriction portion still
attached.

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THE END

Ch3 specialcastproc Erdi Karaçal Mechanical Engineer University of Gaziantep

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Ch3 specialcastproc Erdi Karaçal Mechanical Engineer University of Gaziantep