CHAPTER 4
PART 1
SAND MOLD CASTING
Ass.Pr.Dr. Nguyen Ngoc Ha
1. OVERVIEW OF SAND CASTING
Traditional way to cast metals
–How to make a sand casting
• Placing a pattern having the shape of the
desired casting in to the sand to make an
imprint
• Incorporating a gating system
• Filling the resulting cavity with molten
metal
• Allow the metal to cool
• Break away the sand mold
1. OVERVIEW OF SAND CASTING
Type of sand to use
–Most common – silica sand (SiO2)
– There are two types
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of sand:
• Naturally bonded (bank sand)
• Synthetic (lake sand)
–Preferred by foundries b/c its
composition can be controlled
1.1. Advantages
• Most widely used casting process, accounting
for a significant majority of total tonnage cast
• Nearly all alloys can be sand casted, including
metals with high melting temperatures, such
as steel, nickel, and titanium
• Parts ranging in size from small to very large
• Production quantities from one to millions
1.2. Disadvantages
• Porosity
• Poor dimensional control for some processes
• Poor surface finish for some processes
• Limitation on mechanical properties
• Safety hazard
• Environmental hazard
1.3. Sand-Cast Parts
A large sand casting weighing
680 kg for an air compressor
frame
1.4. Sand Casting Mold Characteristics
• Tolerances:
Non-Ferrous 1/32 to 6
Add .003 to 3, 3/64 from 3 to 6.
Across parting line add .020 to .090 depending on size.
(Assumes metal patterns)
• Surface Finish:
Non-Ferrous: 150-350 RMS
Ferrous: 300-700RMS
• Minimum Draft Requirements:
1° to 5°
Cores: 1° to 1 1/2°
• Normal Minimum Section Thickness:
Non-Ferrous: 1/8 - 1/4
Ferrous: 1/4 - 3/8
• Ordering Quantities: All quantities
1.5. Steps in Sand Casting
1.5. Steps in Sand Casting
2. TYPES OF SAND MOLDS
a) Green Sand Molds: The most common type
consisting of forming the mold from damp
molding sand (silica, clay and moisture)
b) Skin-dried Molds: It is done in two ways; (1)
The sand around the pattern to a depth of about
1/2 in(10 mm). is mixed with a binder so that
when it is dried it will leave a hard surface on the
mold. (2) Entire mold is made from green sand,
but a spray or wash, which hardens when heat is
applied, is used.
2. TYPES OF SAND MOLDS
c) Dry Sand Molds: These molds are made
entirely from fairly coarse molding sand mixed
with binders (linseed oil: bezir yağı or
gelatinised starch: nişasta). They baked before
being used. A dry sand mold holds its shape
when poured and is free from gas troubles
due to moisture.
Green Sand
• Contains bonding agents and water
• Typical “green sand” is:
– 88% silica
– 9% clay
– 3% water
3. SAND CASTING PATTERN
• Patterns may be made from a variety of materials:
– Wood
– Metal
– Hard Polymers
– EPS (Styrofoam)
3.1. Types of Sand Casting Patterns
• Variety of patters are used in casting and the choice
depends on the configuration of casting and number
of casting required
– Single-piece pattern
– Split pattern
– Follow board pattern
– Cope and drag pattern
– Match plate pattern
– Loose-piece pattern
– Sweep pattern
– Skeleton pattern
Characteristics of pattern materials
Characteristic Cast iron Plastic Steel Aluminum Wood
Machine-ability G G F G E
Wear resistance E F E G P
Strength G G E G F
Weight P G P G E
Repair-ability P G G P E
Resistance to:
Corrosion
Swelling
P
E
E
E
P
E
E
E
E
P
E, excellent; G, good; F, fair; P, poor
(a) Split pattern
(b) Follow-board
(c) Match Plate
(d) Loose-piece
(e) Sweep
(f) Skeleton
pattern
3.2. One-Piece Pattern
One-Piece (Using a Follow Board)
3.3. Split Pattern
• Split pattern, showing
the two sections
together and separeted.
(The light –colored
portions are core prints)
3.4. Match-Plate
3.5. Cope-and-Drag Split Patterns
Cope-and-drag pattern for producing two heavy
parts. (Left) Cope section; (right) drag section.
(Note: These are two separete pttern boards)
3.5. Cope-and-Drag Split Patterns
3.6. Loose Piece Pattern
4. MOLDING PROCEDURE
(a) A mechanical
drawing of the part is
used to generate a
design for the pattern.
Considerations such as
part shrinkage and draft
must be built into the
drawing. (b-c) Patterns
have been mounted on
plates equipped with
pins for alignment.
Note the presence of
core prints designed to
hold the core in place.
4. MOLDING PROCEDURE
(d-e) Core boxes
produce core halves,
which are pasted
together. The cores
will be used to
produce the hollow
area of the part
shown in (a).
4. MOLDING PROCEDURE
(f) The cope half of the
mold is assembled by
securing the cope
pattern plate to the flask
with aligning pins and
attaching inserts to form
the sprue and risers.
Continued on next slide.
(g) The flask is rammed
with sand and rthe plate
and inserts are removed.
4. MOLDING PROCEDURE
(h) The drag half is
produced in a similar
manner with the pattern
inserted. A bottom
board is placed below
the drag and aligned
with pins. (i) The
pattern , flask, and
bottom board are
inverted; and the
pattern is withdrawn,
leaving the appropriate
imprint.
4. MOLDING PROCEDURE
(j) The core is set in
place within the drag
cavity. (k) The mold is
closed by placing the
cope on top of the drag
and securing the
assembly with pins.
The flasks the are
subjected to pressure to
counteract buoyant
forces in the liquid,
which might lift the
cope
4. MOLDING PROCEDURE
(l) After the metal
solidifies, the casting is
removed from the
mold. (m) The sprue
and risers are cut off
and recycled, and the
casting is cleaned,
inspected, and heat
treated (when
necessary).
Sand-molding machines
• The oldest known method of molding, which still
used for simple castings, is to compact the sand
by hand hammering or ramming it around the
pattern. For most operations, however, the sand
mixture is compacted around the pattern by
molding machines, Fig.6. These machines have
the following advantages:
• Eliminate labor cost,
• Offer high quality casting by improving the
application and distribution of forces,
• Manipulate the mold in a carefully controlled
fashion,
• Increase the rate of production
Squeeze Heads
Various designs of squeeze
heads for mold making:
(a) conventional flat head;
(b) profile head; (c)
equalizing squeeze
pistons; and (d) flexible
diaphragm. Source: ©
Institute of British
Foundrymen. Used with
permission.
Automatic molding methods
Jolting the assembly
• Jolting the assembly can further assist
mechanization of the molding process. The flask,
molding sand, and pattern are placed on a
pattern plate mounted on an anvil, and jolted
upward by air pressure at rapid intervals, as
shown in Fig.7. The inertial forces compact the
sand around the pattern. Jolting produces the
highest compaction at the horizontal parting line,
whereas in squeezing, compaction is highest at
the squeeze head, Fig.6. Thus more uniform
compaction can be obtained by combining them,
as shown in F .
Jolting the assembly
(a) Schematic illustration of a jolt-type mold-making machine. (b)
Schematic illustration of a mold-making machine combines jolting
and squeeze
Vertical Flaskless Molding
• In this method, the halves of the pattern form
a vertical chamber wall against which sand is
blown and compacted, Fig.8. Then the mold
halves are packed horizontally, with the
parting line oriented vertically and moved
along a pouring conveyor.
Vertical Flaskless Molding
Vertical flaskless molding. (a) Sand is squeezed between two
halves of the pattern. (b) Assembled molds pass along an
assembly line for pouring.
5. CORES AND CORE MAKING
5.1. Introduction
• Casting processes are unique in their ability to
incorporate internal cavities -or reentrant
sections with relative ease.
• To produce these features, however, it is often
necessary to use cores as part of the mold.
• Figure shows an example of a product that
could not be made by any process other than
casting with cores.
• While these cores constitute an added cost,
they do much to expand the capabilities of the
process, and good design practice can often
facilitate and simplify their use.
5.1. Introduction
V-8 engine block
(bottom center) and
the five dry-sand cores
that are used in the
construction of its
mold. (Courtesy of
General Motors
Corporation, Detroit,
MI.)
5.1. Introduction
• Consider the simple belt pulley shown
schematically in Figure 14-21. Various
methods of fabrication are suggested in the
four sketches, beginning with the casting of a
solid form and the subsequent machining of
the through-hole for the drive shaft.
• A large volume of metal would have to be
removed through a substantial amount of
costly machining.
• A more economical approach would be to
make the pulley with a cast-in hole of the
approximate final size.
5.1. Introduction
5.1. Introduction
• Figure 14-21b depicts an approach where
each half of the pattern includes a tapered
hole, Which receives the same green sand
that is used for the remainder of mold.
• While these protruding sections are an
integral part of the mold, they are also known
as green-sand cores.
• Unfortunately, green-sand cores have a
relatively low level of strength.
• If the protrusions are narrow or long, it might
be difficult to withdraw the pattern without
breaking them, or they may not have enough
strength to even support their own weight.
5.1. Introduction
• Dry-sand cores can be used to overcome some
of the cited difficulties.
• These cores are made independent from the
remainder of the mold and are then inserted
into core prints to hold them in position.
• The remaining sketches in Figure 14-21 show
dry-sand cores in the vertical and horizontal
positions
5.1. Introduction
• To function properly, cores must have the
following characteristics;
1. Sufficient hardness and strength (after baking or
hardening) to withstand handling and the forces
of the molten metal. Compressive strength
should be between 100 and 500 psi.
2. Sufficient strength before hardening to permit
handling in that condition.
3. Adequate permeability to permit the escape of
gases. Since cores are largely surrounded by
molten metal, they should possess exceptionally
good permeability.
5.1. Introduction
4. Collapsibility, After pouring, the cores must be
weak enough to permit shrinkage of the solidified
casting as it cools, thereby preventing cracking. In
addition, they must be easily removable from the
interior of the finished product via shakeout.
5. Adequate refractoriness, Since the cores are
largely surrounded by hot metal, they can
become quite a bit hotter than the adjacent mold
material.
6. A smooth surface.
7. Minimum generation of gases when heated
during the pour.
5.1. Introduction
• Various techniques have been developed to
enhance the natural properties of cores and core
materials.
• Internal wires or rods can be used to 'impart
additional strength. Collapsibility can be
enhanced by making the cores hollow or by
placing a material such as straw in the center.
• Enhanced collapsibility is particularly important in
steel castings, where a large amount of shrinkage
is observed.
• All but the smallest of cores must be vented to
permit the escape of trapped and evolved gases.
• Vent holes can be made by pushing small wires
into the core.
5.2. Types of Cores
Green sand core Dry sand core
Most commonly used binder is Linseed oil. The oil
forms a film around the sand grain and hardens when
baked at 180-2200C for 2 hours. Other binders are
wheat flour, dextrin, starch and several types of
thermosetting plastics.
Chaplets
• Cores serve to produce internal surfaces in castings
In some cases, they have to be supported by chaplets
for more stable positioning:
(a) Core held in place in the mold cavity by chaplets,
(b) chaplet design,
(c) casting with internal cavity
Chaplets
(Left) Typical chaplets. (Right) Method of supporting a
core by use of chaplets (relative size of the chaplets is
exaggerated).
Examples of sand cores showing core
prints and chaplets to support cores
6. MELTING AND POURING
6.1. Introduction
• The quality of casting depends on the method
of melting. The melting technique should
provide molten metal at required temperature,
but should also provide the material of good
quality and in the required quantity
6.1. Introduction
• Molten metal is prevented from oxidation by
covering the molten metal with fluxes or by
carrying out melting and pouring in vacuum
• Ladles which pour the molten metal from
beneath the surface are used
• The two main consideration during pouring are
the temperature and pouring rate
• Fluidity of molten metal is more at higher
temperature but it results into more amount of
dissolved gases and high temperature also
damage the mould walls and results into poor
surface quality of the casting
6.1. Introduction
• To control the amount of dissolved gases low,
the temperature should not be in superheated
range
• In ferrous metals, the dissolved hydrogen and
nitrogen are removed by passing CO. In non-
ferrous metals, Cl, He, or Ar gases are used.
• Therefore, fluidity and gas solubility are two
conflicting requirements. The optimum
pouring temp. is therefore decided on the
basis of fluidity requirements.The temp.
should be able to fill the whole cavity at the
same time it should enter inside the voids
between the sand particles.
6.2. Furnaces for Casting Processes
• Furnaces most commonly used in foundries:
– Cupolas
– Direct fuel-fired furnaces
– Crucible furnaces
– Electric-arc furnaces
– Induction furnaces
Cupolas
• Vertical cylindrical furnace equipped with
tapping spout near base
• Used only for cast irons, and although other
furnaces are also used, largest tonnage of cast
iron is melted in cupolas
• The "charge," consisting of iron, coke, flux,
and possible alloying elements, is loaded
through a charging door located less than
halfway up height of cupola
Cupola
Direct Fuel-Fired Furnaces
Small open-hearth in which charge is heated by
natural gas fuel burners located on side of
furnace
• Furnace roof assists heating action by
reflecting flame down against charge
• At bottom of hearth is a tap hole to release
molten metal
• Generally used for nonferrous metals such as
copper-base alloys and aluminum
Crucible Furnaces
Metal is melted without direct contact with
burning fuel mixture
• Sometimes called indirect fuel-fired furnaces
• Container (crucible) is made of refractory
material or high-temperature steel alloy
• Used for nonferrous metals such as bronze,
brass, and alloys of zinc and aluminum
• Three types used in foundries: (a) lift-out
type, (b) stationary, (c) tilting
Crucible Furnaces
Three types of crucible furnaces:
(a) lift-out crucible,
(b) stationary pot, from which molten metal must be ladled, and
(c) tilting-pot furnace
Electric-Arc Furnaces
• Charge is melted by heat generated from an
electric arc
• High power consumption, but electric-arc
furnaces can be designed for high melting
capacity
• Used primarily for melting steel
Electric arc furnace for steelmaking
Induction Furnaces
Uses alternating current passing through a coil to
develop magnetic field in metal
• Induced current causes rapid heating and melting
• Electromagnetic force field also causes mixing
action in liquid metal
• Since metal does not contact heating elements,
the environment can be closely controlled, which
results in molten metals of high quality and purity
• Melting steel, cast iron, and aluminum alloys are
common applications in foundry work
Induction furnace
6.3. Ladles
• Moving molten metal from melting furnace to
mold is sometimes done using crucibles
• More often, transfer is accomplished by ladles
Two common types of ladles: (a) crane ladle,
and (b) two-man ladle
7. CLEANING AND FINISHING
1. Casting is taken out of the mould by shaking and the
Moulding sand is recycled often with suitable additions.
2. The remaining sand, some of which may be embedded
in the casting, is removed by means of Shot blasting.
3. The excess material in the form of sprue, runners,
gates etc., along with the flashes formed due to flow of
molten metal into the gaps is broken manuaaly in case
of brittle casting or removed by sawing and grinding in
case of ductile grinding.
7. CLEANING AND FINISHING
6. The entire casting is then cleaned by either
shot blasting or chemical pickling.
7. Sometimes castings are heat treated to
achieve better mechanical properties.
7.1. Trimming
• Removal of sprues, runners, risers, parting-line
flash, fins, chaplets, and any other excess
metal from the cast part
• For brittle casting alloys and when
cross-sections are relatively small, appendages
can be broken off
• Otherwise, hammering, shearing,
hack-sawing, band-sawing, abrasive wheel
cutting, or various torch cutting methods are
used
7.2. Removing the Core
• If cores have been used, they must be
removed
• Most cores are bonded, and they often fall out
of casting as the binder deteriorates
• In some cases, they are removed by shaking
casting, either manually or mechanically
• In rare cases, cores are removed by chemically
dissolving bonding agent
• Solid cores must be hammered or pressed out
7.3. Surface Cleaning
• Removal of sand from casting surface and
otherwise enhancing appearance of surface
• Cleaning methods: tumbling, air-blasting with
coarse sand grit or metal shot, wire brushing,
buffing, and chemical pickling
• Surface cleaning is most important for sand
casting, whereas in many permanent mold
processes, this step can be avoided
• Defects are possible in casting, and inspection
is needed to detect their presence
7.4. Heat Treatment
• Castings are often heat treated to enhance
properties
• Reasons for heat treating a casting:
– For subsequent processing operations such as
machining
– To bring out the desired properties for the
application of the part in service
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