Casting Fundamentals
Transcript of Casting Fundamentals
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Casting is the process of forming metal objects by allowing molten metal to solidify in amould. The shape of the object is determined by the shape of the mould cavity. When
solidified, the desired metal object is taken out from the mould either by breaking the
mould or taking the mould apart. The solidified object is called the casting.
Advantages
Molten material can flow into very small sections so that intricate shapes can be
made by this process. As a result, many other operations, such as machining, forging,
and welding, can be minimized or eliminated.
It is possible to cast practically any material that is ferrous or non-ferrous.
Objects may be cast in a single piece which would otherwise require construction in
several pieces and subsequent assembly if made by other methods.
Metal can be placed in exact locations where it is needed for rigidity, wear,
corrosion, or maximum endurance under dynamic stress
The necessary tools required for casting moulds are very simple and inexpensive. As
a result, for production of a small lot, it is the ideal process.
There are certain parts made from metals and alloys that can only be processed this
way.
Size and weight of the product is not a limitation for the casting process.
Limitations
Dimensional accuracy and surface finish of the castings made by sand casting
processes are a limitation to this technique.
The metal casting process is a labour intensive process
Casting End Uses
Ferrous castings (Gray iron, malleable iron, ductile iron & steel castings)
Ingot moulds Farm equipment Engines
Refrigeration and heating Construction machinery Motor vehicles
Valves and fittings Railroad equipment Mining equipment
Metalworking machinery Pumps and compressors Hardware
Nonferrous castings
Aluminium
Auto and light truckAircraft and aerospace
Engines
Household appliances
Office machinery
Power tools
Refrigeration, heating & air conditioning
Copper-base
Valves and fittingsPlumbing brass goods
Electrical equipment
Pumps and compressors
Power transmission equipment
General machinery
Transportation equipment
Magnesium
Power tools
Sporting goodsAnodes
Automotive
Zinc
Automotive
Building hardwareElectrical components
Household appliances
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PATTERN is a form made of wood, metal, plastic, or composite materials around which amoulding material (usually prepared sand) is formed to shape the casting cavity of a mould.
Pattern MaterialCommonly used pattern materials are wood, metals and alloys, plastic, plaster of Paris,
plastic and rubbers, wax, and resins. The selection of pattern material depends on the sizeand shape of the casting, the dimensional accuracy, the quantity of castings required and
the casting process used. To be suitable for use, the pattern material should be:
Easily worked, shaped and joined
Light in weight
Strong, hard and durable
Resistant to wear and abrasion
Resistant to corrosion, and to chemical reactions
Dimensionally stable and unaffected by variations in temperature and humidity
Available at low cost
The most commonly used pattern material is wood, since it is readily available and of lowweight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood
is its absorption of moisture, which can cause distortion and dimensional changes. Hence,
proper seasoning and upkeep of wood is required.
Types of PatternsThe type of pattern used for a specific application depends primarily on the number of
castings required, the casting process to be used, the size of the pattern, and the casting
tolerances that are required. The stage of development of a casting design is also a factor. If
the casting is likely to be redesigned, an inexpensive prototype pattern is often used first.
Single Piece Pattern
The one piece or single pattern is the mostinexpensive of all types of patterns. This type of
pattern is used only in cases where the job is very
simple and does not create any withdrawal
problems. It is also used for application in very
small-scale production or in prototype
development. This type of pattern is expected to
be entirely in the drag and one of the surfaces is
expected to be flat which is used as the parting
plane. A gating system is made in the mould by
cutting sand with the help of sand tools.Split Pattern
Split pattern is most widely used type of pattern for
intricate castings. It is split along the parting
surface, the position of which is determined by the
shape of the casting. One half of the pattern is
moulded in drag and the other half in cope. The
two halves of the pattern must be aligned properly
by making use of the dowel pins, which are fitted,
to the cope half of the pattern. These dowel pins
match with the precisely made holes in the drag
half of the pattern. There are split patterns with
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more than two pieces, an example of which is shown below.
Match plate patterns are split patterns in which the cope and drag portions are mounted on
opposite sides of a plate, called the match plate, conforming to the parting line. The pattern,
as well as the associated gating and risering system, is usually made separately and then
mounted on the match plate, but can also be cast integrally with the plate. The size of thematch plate corresponds to the size of the flask used to make the final mould. Flask pin guides are
used to ensure accurate alignment of the match plate pattern in the flask. Multiple patterns
of small parts can be mounted on a single match plate. Common gates and risers on the
match plate can often be shared by the multiple patterns on the match plate. Match plate
patterns are used for moderate to high-volume production of small- and medium-size
castings. The moulding operation is simplified considerably by the use of match plate
patterns. The decrease in moulding costs and increased mould quality offset higher pattern
costs for higher-volume castings. Large patterns are usually not match plate patterns,
because of the limitations on flask sizes and the difficulties in moulding.
Loose piece pattern: It is a pattern with
loose pieces which are necessary to facilitatewithdrawal of the pattern from the mould. It
is used to produce undercuts. Loose pieces
are removed separately through the cavity
formed after the main pattern has been
removed. These loose pieces need to be
fastened loosely to the main pattern by
wooden dowel pins.
Sweep pattern: Need for large size symmetrical shaped patterns are eliminated with the
help of sweep pattern. Desired shape is swept into the sand mould by rotating the sweep
pattern about a central axis.
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Segmental patterns are sections of a pattern arranged to form a complete mould by placing
them in the mould suitably. After finishing one segment of the mould, the pattern is shifted
to next position and the mould is finished in a segment by segment manner. These are
generally used for circular work like rings, wheels, rims, gears etc.
Skeleton pattern: For very large simple castings, the pattern is made of wooden frame and
rib construction (skeleton) which form an outline of the casting. The openings in the ribbed
construction is filled and rammed with clay or sand.
Pattern AllowancesAlthough a pattern is used to produce a casting of desired dimensions, it is not
dimensionally identical to the casting. A number of allowances must be made on the
pattern- to ensure that the finished casting is dimensionally correct, to ensure that the
pattern can be effectively removed from the mould, and to allow for cores to be firmly
anchored.
Shrinkage allowance is the correction factor built into the pattern to compensate for the
contraction of the metal casting as it solidifies and cools to room temperature. The pattern
is intentionally made larger than the final desired casting dimensions to allow for
solidification and cooling contraction of the casting. The total contraction is volumetric, butis usually expressed linearly. Because different shrinkage allowances must be used for the
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individual types of metals cast, it is not possible to use the same pattern equipment for
different cast metals without expecting dimensional changes.
Material Dimension
Shrinkage
allowance
(inch/feet)
Material Dimension
Shrinkage
allowance
(inch/feet)
Grey cast
iron
Up to 2 feet 0.125
Aluminium
Up to 4 feet 0.155
2 feet to 4 feet 0.105 4 feet to 6 feet 0.143
Over 4 feet 0.083 Over 6 feet 0.125
Cast steel
Up to 2 feet 0.251
Magnesium
Up to 4 feet 0.173
2 feet to 4 feet 0.191 Over 4 feet 0.155
Over 4 feet 0.155
The patternmaker's shrink rule is a special scale that eliminates the need to compute
the amount of the shrinkage allowance that must be provided on a given dimension. For
example, on a 10.5 mm/m (1/8 in./ft) patternmaker's shrink rule, each meter (foot) is 10.5
mm (1/8 in.) longer, and each graduation on the shrink rule is proportionately longer than its
conventional length. Double shrinkage allowances must sometimes be made if a master
pattern is first made in wood and then used to make a metal match plate or cope and drag
production pattern. For example, an aluminium pattern made from a wood master pattern
would require a double shrinkage allowance on the wood pattern if a steel casting is to be
made. The total shrinkage allowance on the wood pattern would then provide for the
shrinkage of the aluminium pattern casting and of the steel casting made from the
aluminium production pattern.
It is even possible that several different
shrinkage allowances will be needed in one pattern,
depending on constraint conditions. For example, in
the figure, two different contraction situations exist.
Along dimension X, the casting has virtually no
constraint to contraction, and the pattern should be
made correspondingly larger along the X dimension
surfaces. Dimension Y, however, is restrained from
contraction by the core used to make the centre hole
and will require little or no shrinkage allowance on the
pattern dimensions.
Exercise 1
The casting shown is to be made in cast iron using a wooden pattern. Assuming onlyshrinkage allowance, calculate the dimension of the pattern. All Dimensions are in Inches.
Solut ion 1
The shrinkage allowance for cast iron for size up to 2 feet is 0.125 inch per feet
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For dimension 18 inch, allowance = 18 X 0.125 / 12 = 0.1875 inch 0.2 inchFor dimension 14 inch, allowance = 14 X 0.125 / 12 = 0.146 inch 0.15 inchFor dimension 8 inch, allowance = 8 X 0.125 / 12 = 0.0833 inch 0. 09 inchFor dimension 6 inch, allowance = 6 X 0.125 / 12 = 0.0625 inch 0. 07 inchThe pattern drawing with required dimension is shown below:
Draft or Taper Allowance is taper allowed on the
vertical faces of a pattern to permit its removal
from the sand or other moulding medium
without tearing of the mould walls. The amountof draft required depends on the shape and size
of the casting, the moulding process used, the
method of mould production, and the condition
of the pattern. A draft angle of approximately
1.5 is often added to design dimensions. The
draft angle may be higher when manual
moulding techniques are used. Interior surfaces
usually require somewhat more draft than
exterior surfaces, and deep pockets or cavities
may require considerably more draft.
The Machining or Finish allowance provides for sufficient excess metal on all cast surfaces
that require finish machining. The required machine finish allowance depends on many
factors, including the metal cast, the size and shape of the casting, casting surface
roughness and surface defects that can be expected, and the distortion and dimensional
tolerances of the casting that are expected. Accurate patterns combined with automated
moulding can often produce close-tolerance castings with a minimum machine finish
allowance that can reduce final machining costs considerably.
Exercise 2
The casting shown is to be made in cast iron using a wooden pattern. Assuming onlymachining allowance, calculate the dimension of the pattern. All Dimensions are in Inches.
The machining allowance for cast iron for size, up to 12 inch is o.12 inch and from 12 inch to
20 inch is 0.20 inch
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Distortion or Camber Allowances. Certain cast shapes, such as large flat plates and dome or
U-shaped castings, sometimes distort when
reproduced from straight or perfect
patterns. This distortion is caused by the
non-uniform contraction stresses during the
solidification of irregularly shaped designs.
Minor distortions are normally corrected by
mechanically pressing or straightening the
casting, but if distortions are consistent and
prominent, the pattern shape can be
intentionally changed to counteract the
casting distortions. The "distorted" pattern will then produce a distortion-free casting.
Rapping Allowance
Before the withdrawal from the sand mould, the pattern is rapped all around the vertical
faces to enlarge the mould cavity slightly, which facilitate its removal. Since it enlarges the
final casting made, it is desirable that the original pattern dimension should be reduced toaccount for this increase. There is no sure way of quantifying this allowance, since it is highly
dependent on the foundry personnel practice involved. It is a negative allowance and is to
be applied only to those dimensions that are parallel to the parting plane.
Core Prints: Castings are often required to have holes, recesses, etc. of various sizes and
shapes. These impressions can be obtained by using cores. So where coring is required,
provision should be made to support the core inside the mould cavity. Core prints are used
to serve this purpose. The core print is an added projection on the pattern and it forms a
seat in the mould on which the sand core rests during pouring of the mould. The core print
must be of adequate size and shape so that it can support the weight of the core during the
casting operation. Depending upon the requirement a core can be placed horizontal, verticaland can be hanged inside the mould cavity. A typical job, its pattern and the mould cavity
with core and core print is shown below.
Colour coding of patternsTypical colours used for some of the principal features are as follows:
As-cast surfacemain body of pattern: Red/orange
Machined surface: Yellow
Core-print: Black
Loose piece seating indication: Green
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Types of Moulding sand
Based on composition
Green sand Dry sand Synthetic sand Loam sand
Based on functionality
Core sand: Used for making cores. Facing sand: Specially prepared moulding sand which covers the pattern from
all around and forms the face of the mould cavity.
Parting sand: Consists of dried silica sand and sea sand, sprinkled on theparting surface to avoid sticking together of cope and drag. Also sprinkled
over the pattern for its easy removal.
Backing sand: It is the sand which backs up the facing sand.Moulding sand preparation
Remove all foreign and undesirable particles from the moulding sand. The sand is then screened. Using a mechanical mixer (MULLER), the sand ingredients are mixed in dry condition. Continue the mixing action until there is a uniform distribution of the ingredients
and optimum properties develop.
Aerating separates sand grains into individual particles.Core
Core is an obstruction which when positioned in the mould prevents the molten metal from
filling up the space occupied by it and thus produces a hollow casting.
Essential Characteristics of core Sufficient strength to support itself and to get handled without breaking. High permeability to let the mould gases escape through mould walls. Smooth surface to ensure a smooth casting. High refractoriness to withstand the hot molten metal. High collapsibility in order to assist the free contraction of solidifying metal. Ingredients should not generate mould gases.
Core making procedure
Core sand preparation Making the core
Hand rammed / machine madeCore venting
Reinforcing cores with wires, rods etc.
Baking the coreOvens, dielectric bakers
Finishing of coreCleaning, sizing, core-assembly
Setting the coreCore prints, chaplets
Chaplets
Chaplets are metal shapes which are positioned between mould and core surfaces tosupport the core.
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Chaplets firmly support the core to overcome vertical movement due to the buoyantforces exerted on core by the molten metal.
Chaplets should be of the same material being cast. Tin coated low carbon steels are used in ferrous foundries.
Types of cores
Horizontal core: Positioned horizontally in the mould Vertical core: Positioned vertically in the mould Hanging core: Supported from above & hangs in mould cavity Balanced core: Supported and balanced from one end only. Stop off core: To make a cavity in the casing which cannot be made with other
cores.
Ram up core: Placed in the sand along with pattern before rapping the mould. Kiss core: Does not require core seats and held in position by pressure exerted by
cope over drag.
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Types of moulds
Green sand moulds: Among the sand casting processes, moulding is often done with green
sand. Green sand can be defined as a plastic mixture of sand grains, clay, water and other
additives. The sand is called green because of the moisture content. Green sand moulding
has a great flexibility as a production process. The sand can be reused many times by
reconditioning it with water, clay and other materials. It is the least costly method of
moulding. Green sand moulds are suitable for producing small and medium sized castings.
Green sand moulds are not suitable for casting thin long projections. Thin long projections
of green sand in a mould cavity are washed away by the molten metal or may not even be
mouldable. Certain metals and some castings develop defects if poured into moulds
containing moisture. The dimensional accuracy and surface finish of green sand castings are
comparatively less. Large castings require greater mould strength and resistance to erosion
than that available with green sand moulds.
Dry sand moulds: Dry sand moulds are actually made with moulding sand in green condition
and then the entire mould is dried in ovens, before the molten metal is poured in them. In
sand used for making dry sand moulds, certain binders are added which harden when
heated. Dry sand moulds possess higher strength as compared to green sand moulds. They
are more expensive and consume more time in making compared to green sand moulds.
They generate less mould gases than green sand moulds. They possess higher permeability
than green sand moulds. They employ finer sands and hence produce smoother casting
surfaces.
Skin dried moulds: The mould is made with the moulding sand in the green condition and
then the skin of the mould cavity (1/4 to 1 inch) is dried with the help of gas torches or
radiant heating lamps. A skin dried mould possesses strength and other characteristics in
between green and dry sand moulds. If a skin dried mould is not poured immediately after
drying, moisture from green backing sand may penetrate the dried skin and make the same
ineffective.
Core sand moulds: A core sand mould is made by assembling a number of cores made
individually in separate core boxes and baked. The cores are made with recesses and
projections so that they can be fitted together to make the mould. A core sand mould ispoured without a moulding box surrounding the same. Core sand moulds possess high
collapsibility, baked strength and hardness. Core sand moulds are expensive as compared to
green and dry sand moulds.
Loam moulds: A loam mould is preferred for making large castings. Loam sand has clay
content of the order of 50% or so. Loam dries hard. Sweep or skeleton pattern may be used
for loam moulding. A loam mould is a time consuming one.
Permanent moulds or metal moulds: A metal mould is generally made up of gray cast iron
or steel. They are manufactured by casing and consequent machining of the mould cavity. A
metal mould is made in two parts to facilitate the removal of the cast object. Metal moulds
are preferred for casting non-ferrous metals and alloys. Metal moulds produce surfaces withfine grain structure, high dimensional accuracy and very good surface finish.
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Other types of moulds include
Cement bonded sand moulds Plaster moulds
Graphite moulds Shell moulds
Investment moulds Ceramic moulds
Sodium silicate CO2 moulds
Moulding methods
Bench moulding: Moulding carried out on a bench of convenient height.
Used for small and light castings
Both cope and drag are rammed on the bench
Floor moulding: Moulding carried out on foundry floor
Used for medium & large sized castings
Normally drag portion is in the floor and cope portion rammed
in a flask inverted on the drag.
Pit moulding: Used for very large castings
Pits are normally constructed of concrete walls and
sometimes floors to withstand great pressures during
pouring. Because the drag part in the pit cannot be rolled
over, the sand under the pattern must be rammed in. A bed
of coke, cinders, or other means of venting the pit bottom
must be provided. The cope is rammed over the pit with
pattern in position.
Machine moulding: Used for mass production of castings
Produce identical and consistent castings.
Various moulding operations like sand ramming, rolling themould over, withdrawing the pattern etc. are done by
machines
Moulding machines
Jolt machine
Jolt-squeezer machine
Sand slinger
Jolt moulding machine
Jolt-type moulding machines operate with the pattern mounted on a pattern plate which inturn is fastened to the machine table. The table is fastened to the top of an operating air
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piston. A flask is placed on the pattern and is positively located by pins relative to the
pattern. The flask is filled with sand, and the machine starts the jolt operation. This is usually
accomplished by alternately applying and releasing air pressure to the jolt piston, which
causes the flask, sand, and pattern to lift a few inches and then fall to a stop, producing a
sharp jolt. This process is repeated a predetermined number of times, depending on sand
conditions and pattern configuration. Because the sand is compacted by its own weight,
mould density will be substantially less at the top of a tall pattern. The packing that results
from the jolting action will normally be augmented by some type of supplemental
compaction, usually hand or pneumatic ramming. When ramming is complete, push-off
pins, bearing against the bottom edges of the flask, lift the flask and completed mold half off
the pattern.
Jolt-squeezer machine
Jolt squeeze moulding machinesoperate in much the same manner as jolt-type moulding
machines. The main difference is that the supplemental compaction takes place as the
result of a squeeze head being forced into the moulding flask, thus compacting the loose
sand at the top. The required pressure can be applied pneumatically or hydraulically. In
many cases, the squeeze head will be one piece and may even have built-up areas to
provide more compaction in deep areas that are hard to ram. In other cases, the squeeze
head may be of the compensating type, which consists of a number of individual cylinders,each exerting a specified force on the rear mould face. Some machines exert the same force
on all areas of the mould, while other machines allow the operator to adjust squeezing
pressure in zones. Jolt squeeze machines are available in many sizes and are suitable for
many different purposes and production levels. They can be operated manually or
automatically. The operator has the option of independently adjusting the number of jolts
from zero to any number and adjusting the squeeze pressure from zero up to pressure that
is considered excessive. Hand or pneumatic ramming is often combined with this process;
supplemental ramming normally takes place after jolting but before squeezing.
Sand slinger
Sand slinger moulding machines deliver the sand into the mould at high velocity from arotating impeller. Moulds made by this method can have very high strengths because a very
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dense mould can be made. Density is a function of sand velocity and the thickness through
which the high-velocity sand must compact previously placed sand. Sand slingers may or
may not be portable. Some ride on rails to the mould, while others have the moulds brought
to the slinger. Generally speaking, larger moulds have the slinger brought to the mould,
while smaller moulds are brought to the moulding station.
Although slingers are useful in producing larger moulds, it should be noted that the
sand entry location and angle are critical to the production of good moulds. Entry location is
controlled by the operator, while entry angle and, to some extent, location are controlled by
internal adjustment. Error can and does lead to soft spots in the mould or to excessive
pattern wear. A considerable amount of operator skill is required to achieve consistent
results.
Gating system
The gating system refers to all passage ways through which molten metal passes to enterinto the mould cavity. The gating system is composed of
Pouring cups and basins
Sprue
Runner
Gates
Risers
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The goals for the gating system are
To minimize turbulence to avoid trapping gases into the mould To get enough metal into the mould cavity before the metal starts to solidify To avoid shrinkage
Establish the best possible temperature gradient in the solidifying casting so that theshrinkage if occurs must be in the gating system not in the required cast part.
Incorporates a system for trapping the non-metallic inclusionsPouring cups: A pouring cup is a funnel shaped cup which forms the top portion of the
sprue. A pouring cup makes it easier for the ladle or crucible operator to direct the flow of
metal from crucible to sprue. The pouring cup may be cut out of the sand in the upper
surface of the cope above the sprue.
Pouring basins: It can be made out of metal or be cut in the cope of sand mould. A pouring
basin makes it easier for the ladle or crucible operator to direct the flow of metal from
crucible to sprue. It helps in maintaining the required rate of liquid metal flow. It reduces
turbulence and vortexing at the sprue entrance. It is helpful in separating dross, slag etc.
from molten metal before it reaches the sprue.
Sprues: A sprue feeds metal to the runner which in turn reaches the mould cavity through
gates. A sprue is tapered with its bigger end at the top. The larger section being at the top
will freeze after the smaller section at the bottom and compensates for the shrinkage till the
lower end solidifies. Sprues up to 20 mm diameter are round in section whereas largersprues are often rectangular. A round sprue has a minimum surface exposed to cooling and
offers the lowest resistance to flow of metal. There is less turbulence in a rectangular sprue.
Gates: A gate is a channel which connects runner with the mould cavity. Gate feeds liquid
metal to the casting at a rate consistent with the rate of solidification. A small gate is used
for casting which solidifies slowly and vice versa. More than one gate may be used to feed a
fast freezing casting. A gate should not have sharp edges as they may break during pouring.
Moreover sharp edges may cause localized delay in freezing leading to voids and inclusions
in the casting. A gate may be built as a part of the pattern or it may be cut in the mould with
the help of a gate cutter. The major types of gates are top gate, bottom gate and parting
line side gate.
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Top gate Bottom gate Parting line side gate
Molten metal just drops on
the sand in the bottom of
the mould cavity until a pool
is formed and this is kept in a
state of agitation until the
mould is filled.
Liquid metal fills rapidly in
the bottom portion of the
mould cavity and rises
steadily and gently up themould walls.
Liquid metal enters the
mould cavity from the side of
the mould at the parting line.
Moulding is simpleGreater complexity in
mouldingSimple to construct
Favourable temperature
gradients enable directional
solidification from casting
towards gate which serves as
riser too.
It is difficult to achieve
directional solidification
especially when the bottom
gate has a riser at the top of
the casting.
Hottest metal reaches the
riser thereby promoting
directional solidification.
Dropping liquid stream
erodes the mould surface.
Erosion of mould surface is
very less. Less erosion
There is lot of turbulence
and pick up of air and other
gases
Little turbulence only.
In case the parting line is not
near the bottom of mould
cavity, turbulence will occur.
Splashing of molten metal
increases chances of
oxidation.
There is no splashing. Splashing is less.
Runner: It is generally located in the parting plane which connects the sprue to its gates,
thus letting the metal enter the mould cavity. The runners are normally made trapezoidal in
cross section. It is a general practice for ferrous metals to cut the runners in the cope andthe gates in the drag. This is to trap the slag and dross which are lighter and thus trapped in
the upper portion of the runners. For effective trapping of the slag, runners should flow full
as shown below.
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The runner is extended a little further after it encounters the gate. This extension is
provided to trap the slag in the molten metal. The molten initially comes along with the slag
floating at the top of the ladle and this flow straight, going beyond the gate and then
trapped in the runner extension.
Riser: Risers serve as reservoirs to supply the molten metal necessary to prevent shrinkageduring solidification.
Functions of Risers
Provide extra metal to compensate for the volumetric shrinkage
Allow mould gases to escape
A casting solidifying under the liquid metal pressure of riser is comparatively sound.
A riser full of molten metal indicates that the mould cavity is filled up.
Open riser Blind riser
The top of the open riser is open and isexposed to the atmosphere.
A blind riser is closed at its top. However a
vent or permeable core at the top of theriser may be provided to have some
exposure to the atmosphere.
Open riser is not placed in the drag. Blind risers can be placed at any position in
the mould.
Open riser is generally larger than a
comparable blind riser.
A blind riser is smaller than a comparable
open riser.
An open riser is more difficult to remove
from the casting.
A blind riser can be removed more easily
from a casting.
Being exposed to atmosphere, the liquid
metal in the top portion of the riser startssolidifying immediately after the mould
filling is completed, because there is a major
heat loss to atmosphere by radiation.
Being surrounded by moulding sand from all
sides, the metal in blind riser cools slowly.
An open riser is easy to mould than a blind
riser.
It is difficult to mould a blind riser.
An open riser will not draw liquid metal from
solidifying casting.
A blind riser may draw liquid metal from
solidifying casting as a result of partial
vacuum in riser.
Slag trap systemProper design of gating system prevents the slag from entering the mould cavity.
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Strainer core, perforated metal sheets and ceramic filters are also used for removing slag.
Gating ratioIt is the ratio of cross-sectional areas of the sprue, runner and gates. This ratio, numerically
expressed in the order c.s.a of sprue: c.s.a of runner: c.s.a of gate, defines whether a gating
system is increasing in area (unpressurized) or constricting (pressurized).
Pressurized Versus Unpressurized gating systems
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The difference between these two systems is in the choice of the location of the flow-
controlling constriction, or choke, which will determine the ultimate flow rate for the gating
system. This decision involves the determination of a desired gating ratio. Common
unpressurized gating ratios are 1:2:2, 1:2:4, and 1:4:4. A typical pressurized gating ratio is
4:8:3.
Pressurized gating systems Unpressurized gating systems
The total cross sectional area decreases
towards the mold cavity.
The total cross sectional area increases
towards the mold cavity.
Back pressure is maintained by the
restrictions in the metal flow.
Restriction only at the bottom of sprue.
Flow of liquid (volume) is almost equal from
all gates.
Flow of liquid (volume) is different from all
gates
Back pressure helps in reducing the
aspiration as the sprue always runs full.
Aspiration in the gating system as the system
never runs full.
More turbulence and chances of mold
erosion.
Less turbulence
Chills
Chills are metal shapes inserted in moulds to speed up the solidification of a particular
portion of the casting. Chills equalise the cooling rate of thin and thick sections and thus
prevent hot tears. Chills promote progressive and directional solidification. The use of chills
becomes necessary when it is not possible to locate a riser on the casting.
External chills are rammed up in the mould walls. Direct external chill comes in contact with
the liquid metal. An indirect external chill is rammed and embedded behind the mould
cavity wall. Internal chills fuse into and become a part of the casting and therefore shouldbe made of same metal as that of casting.
Fluid flow in metal casting
Reynolds Number: Nature of flow in the gating system can be established by calculating
Reynold's number.
RN is Reynold's number
v is mean velocity of flow
D is diameter of tubular flow
is viscosity
is fluid density
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When the Reynold's number is less than 2000 stream line flow results and when the number
is more than 2000 turbulent flow prevails. As far as possible the turbulent flow must be
avoided in the sand mould as because of the turbulence sand particles gets dislodged from
the mould or the gating system and may enter into the mould cavity leading to the
production of defective casting. Excess turbulence causes
Inclusion of dross or slag
Air aspiration into the mould
Erosion of the mould walls
Bernoulli's Equation: According to Bernoullis theorem,
z is the elevation above certain vertical plane, p is the pressure at that elevation, is density
of fluid, v is velocity of liquid at that elevation and g is acceleration due to gravity.
Conservation of energy in the system requires that
Where the subscripts 1 and 2 represent two different elevations and f represents the
frictional loss as the liquid travels downward through the system. The frictional loss includes
energy loss at liquid-mould wall interface and turbulence in liquid.
Continuity equation: For incompressible fluids in a system with impermeable walls, the rate
of flow is constant.
Where Q is flow rate, A is cross-sectional area and v is average velocity of liquid. Subscript 1
and 2 refers to two different locations in the system.
Assuming that the pressure at the top of the sprue is equal to the pressure at the bottom
and that there are no frictional losses, the relationship between height and cross-sectional
area at any point of sprue is given by
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Heat transfer in metal casting
A typical temperature distribution at the mould-liquid metal interface is shown below. Heat
from liquid metal is given off through the mould wall and the surrounding air. The shape of
the temperature distribution curve depends on the thermal properties of the molten metal
and the mould.
The solidification time is a function of the volume of a casting and its surface area.
According to Chvorinovs rule,
Where C is a constant that reflects mold material, metal properties and temperature.
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Design of gating system
Pouring cup & basin: Pouring basin is designed in such a way that when liquid metal enters
the sprue, it should be a proper uniform flow system as under full flow conditions. This can
be achieved by
Streamlining the pouring basin
Use of strainer core/DAM/sprue plug
It should be easy and convenient to fill the pouring basin. The diameter of the cup should be
large enough to avoid metal splashing.
Sprue: As the liquid metal passes down the sprue, it loses its pressure head and gains
velocity. In a uniform cross-sectioned or parallel sprue, the metal contracts and is pulled
away from sprue walls. As a result, turbulence occurs. Moreover, a vortex tends to form in
the sprue. Turbulence and vortex formation results in mould erosion. A tapered sprue is
provided to overcome these problems. In a properly tapered sprue, the liquid metal lies
firmly against the walls which reduce turbulence and elilminates application. Sprue taper
almost follows the equation
The smallest area in the feeding channels controls the flow rate into the mould cavity and
consequently controls the pouring time. This area is called choke area. Usually the choke
area occurs at the base of the sprue. A proper choke area can be calculated using the
Bernoullis theorem.
A is choke area, W is casting weight, is density, t is pouring time, C is efficiency factor of
gating system, H is effective head.Runner and gates: In a good runner and gate design,
Abrupt changes in section and sharp corners which create turbulence and gas
entrapment should be avoided.
A suitable relationship provided by the gating ratio must exist between the cross-
sectional area of sprue, runner and gates. Selection of gating ratio depends on
whether the gating system is to be a pressurised one or an unpressurised one.
The use of pressurized or unpressurized system of gating depends on the metal
being cast. Ideally, in a system, pressure should be just enough to avoid aspiration
and to keep all feeding channels full of liquid metal.
The maximum liquid metal tends to flow through the farthest gate. A more uniformdistribution of liquid metal in the feeding system can be maintained by changing the
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gating ratio. Total gating area is reduced by making gates farthest from sprue of
smaller cross-section. Then less volume of metals flow through the farther gates and
makes a uniform distribution of liquid metal at all gates.
Further good distribution is obtained if runner beyond each gate is reduced in cross-
section to balance the flow in all parts of the system and to equalise further velocity
and pressure.
Uniform distribution of liquid metal can be achieved through a parallel runner if the
gates are placed at certain angles with the runner.
Streamlining the gating system also reduces turbulence and air aspiration.
Streamlining includes removing sharp corners, tapering sprue, providing radius at
sprue entrance and exit, providing a basin instead of pouring cup etc.
Riser: A riser should perform its functions in the most economical manner, ie, the yield for
the casting should be high.
Wc is the weight of casting, Wrs is weight of the riser, sprue etc.
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Yield can be increased by reducing the weight of riser, sprue etc. Weight of the riser can be
reduced by making its size small. Riser size can be reduced by making solidification more
directional ie, by extracting heat more quickly from the casting than from the riser. Use of
chills significantly helps in reducing the riser size. Proper riser location also is very
important.
The efficiency of a riser is defined as
Where I is the initial volume of metal in the riser, F is the final volume of metal in the riser
and therefore I F is the amount of metal supplied to the casting by the riser.
Efficiency of riser may be increased by
delaying the solidification of metal in the riser or by making the solidification of
the casting rapid.
assisting the movement of riser metal into the casting
A number of methods are employed for increasing the efficiency of riser.
Locating risers in suitable locationsUsing insulating materials and exothermic materials
Use of chills and padding
Using mould materials of different heat conductivities
Topping up
Electric arc feeding
All the above methods help in maintaining the freezing time of riser more than that of
casting.
For large castings, more risers are provided to cover the total feeding range. The total
number of risers should be optimum to achieve maximum casting yield.
Riser shape is decided considering the following factors:The junction area between riser and casting should be optimum minimum to reduce
fettling costs.
According to Chvorinovs rule, for greater efficiency, the riser should be cylindrical
rather than square or rectangular of equal mass.
Cylindrical risers are tapered in order to avoid turbulence, aspiration etc.
Riser size is determined by meeting two different requirements freezing time and feed
volume to obtain directional solidification and thus a sound casting.
Riser location depends upon
The design and complexity of casting
Type of cast metal
Number of risers
Ease of moulding
Ease of riser removal after the casting has solidified
Cupola furnace
Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry
operations. It is economical for the production of gray cast iron, modular cast iron and some
malleable iron castings.
Cupola construction
A Cupola is a cylindrical steel shell constructed (welded or riveted) from boiler plate (6 to10mm thick), open at both its top and bottom and is lined with firebrick and clay
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At bottom, the cupola is supported on cast iron legs. The bottom opening of Cupola is closed
by cast iron door. This door when closed is supported by an iron prop.
Air from the blower comes through the blast pipe and enters Wind box which surrounds the
cupola and supplies air evenly to all the tuyers.
Tuyers extend through the steel shell and refractory wall to the combustion zone and supply
air necessary for combustion. Tuyers may be fitted in one or more rows and have
dimensions 50mmX150mm or 100mmX300mm.
There is a tap hole in the Cupola from where the molten metal is taken out. The fire in the
Cupola is also lit through the tap hole.
There is a slag hole a little higher than tap hole through which slag is removed.
Cupola remains either open or has a spark arrester at its top.
A Cupola is provided with a charging platform and a charging door at suitable heights to
feed the charge in the Cupola.
Cupola capacities vary from 1 to 15 tons of melted iron per heat.
The height of the Cupola is about 6 metres and the inside diameter ranges from 75 cm to
2.5 metres.
Cupola operation: The different steps involved in Cupola operation are
Preparation of Cupola: The bottom door is opened and the contents left from previousmeltings are dumped & removed. Slag, coke and iron sticking to the side walls of the
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furnace are chipped off. Damaged fire bricks are replaced by new ones. Eroded refractory
lining is patched with the help of a pneumatic gun which blows the patching mixture at
sufficient velocity. The original refractory lining has the composition Silica (52 to 62%),
Alumina (31 to 43%), Titania (1.5 to 2.5%) and fluxing oxides (3 to 6%). However the
patching mixture consists of silica and fireclay. Once the furnace lining is reconditioned,
bottom door is closed and supported by a prop. A layer of tempered sand sloping towards
the tap hole is rammed over the bottom to provide a slope for better metal flow.
Lighting the fire: Coke is placed over soft and dry wooden pieces and the wooden pieces are
ignited through the tap hole. Air necessary for the combustion of coke enters from the
tuyers. When the initial coke is burning well, an additional amount of the same is added
through the charging door to the desired height (normally 75 cm). For initiating fire in
Cupola, electric spark ignitor and gas torches are also used.
Charging of Cupola: Charging of Cupola means adding alternate layers of limestone (flux),
iron (metal) and coke (fuel) upto the level of charging door. Flux aids forming slag to remove
impurities and retards oxidation of iron. The fuel used in Cupola can be good grade sulphurcoke, anthracite coal or carbon briquettes. Metal charge consists of pig iron, cast iron scrap
and steel scrap. The ratio of metal to fuel by weight ranges from 4:1 to 12:1.
Melting: After the Cupola is fully charged, a soaking period of about 30 minutes to 1 hr is
given to permit the charge to preheat. Blowers are not started during the soaking period. At
the end of the soaking period, the blast is turned on. The coke becomes fairly hot to melt
the metal charge. After the air blast has been on for about ten minutes, molten iron starts
accumulating in the hearth and appears at tap hole. The tap hole is closed with a plug and
molten metal is allowed to collect for about five minutes.
Slagging and metal tapping: After enough molten iron has collected, the slag hole isopened; slag comes out, is collected in a container and disposed off. The plug inserted in the
tap hole is knocked out and molten iron is poured into the moulds. Additional charge is
dropped through the charging door at a rate at which the charge is consumed so that
Cupola remains always full. Intermittent tapping is usually accompanied by intermittent
slagging. The length of one heat may be sixteen hours or less.
Dropping down the bottom: Near the end of Cupola heat, charging of Cupola is stopped. All
the contents in the Cupola are allowed to melt till one or two charges are left above the
coke bed. At this stage, the air blast is shut off, the prop under the bottom door is knocked
down and the remains in the Cupola are dropped down. Dropped Cupola remains are
quenched with water immediately and the metal and coke are recovered from the same foruse in next heats.
Zones of Cupola
Well: Molten iron collects in this zone before being tapped.
Superheating, combustion or oxidizing zone: All the oxygen in the air blast is consumed here
owing to the combustion taking place in this zone. The chemical reaction occurring is
C + O2CO2 + Heat
The temperature of the combustion zone varies from 1550C to 1850C.
Reducing zone or protective zone: It extends from top of the combustion zone to the top of
the coke bed. It has reducing atmosphere and protects the metal charge from oxidation. An
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endothermic reaction takes place in this zone, some of the hot CO 2 moving upward through
coke gets reduced.
CO2 + C2 CO Heat
This reduces the heat in the reducing zone and temperature is in the order of 1200C.
Melting zone: Melting zone starts from the first layer of metal charge above the coke bedand extends up to a height of 90cm or less. Iron melts in this zone and trickles down through
the coke bed to the well zone. The temperature in the melting zone is above 1600C.
Preheating zone: Preheating zone starts from above the melting zone and extends up to the
bottom of the charging door. Gases like CO2, CO and N2 rising upwards from combustion
and reducing zones preheat the Cupola charge in this region to about 1100C.
Stack zone: Stack zone extends from above the preheating zone to where Cupola shell ends.
Hot gases from Cupola pass through the stack zone and escape to atmosphere. Stack gases
normally contain 12% CO2, 12% CO and 76% N2.
Advantages of CupolaSimple design & easier construction
Low initial cost as compared to other furnaces of same capacity.
Simple to operate and maintain in good condition.
Economy in operation and maintenance
Less flow space requirements as compared to other furnaces of same capacity.
Can be continuously operated for hours.
Limitations of Cupola
Since molten iron and coke come in contact with each other, certain elements like Si,
Mn are lost while others like sulphur are picked up. This changes the final
composition of molten iron.Close temperature control is difficult to maintain.
Centrifugal Casting: In this process, the mould is rotated rapidly about its central axis as the
metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting
on the metal as it solidifies. The slag, oxides and other inclusions being lighter get separated
from the metal and segregate towards the centre. This process is normally used for the
making of hollow pipes, tubes, hollow bushes, etc., which are axisymmetric with a
concentric hole. Since the metal is always pushed outward because of the centrifugal force,
no core needs to be used for making the concentric hole. The mould can be rotated about a
vertical, horizontal or an inclined axis or about its horizontal and vertical axessimultaneously. The length and outside diameter are fixed by the mould cavity dimensions
while the inside diameter is determined by the amount of molten metal poured into the
mould. Since centrifugal force feeds the molten metal under pressure many times higher
than that in static casting, this process improves casting yield significantly (85 to 95%),
completely fills mould cavities, and results in a high-quality casting free of voids and
porosity. Thinner casting sections can be produced with this method than with static
casting. There are three types of centrifugal casting
True centrifugal casting Semi-centrifugal casting Centrifuge centrifuge casting
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True centrifugal casting is used to produce cylindrical or tubular castings by spinning the
mould about its own axis. The process can be either vertical or horizontal, and the need for
a centre core is completely eliminated. Castings produced by this method will always have a
true cylindrical bore or inside diameter regardless of shape or configuration. The bore of the
casting will be straight or tapered, depending on the horizontal or the vertical spinning axis
used. Castings produced in metal moulds by this method have true directional cooling or
solidification from the outside of the casting toward the axis of rotation. This directional
solidification results in the production of high-quality defect-free castings without shrinkage.
Semi-centrifugal casting is used to produce castings with configurations determined entirely
by the shape of the mould on all sides, inside and out, by spinning the casting and mould
about its own axis. A vertical spinning axis is normally used for this method. Cores may be
necessary if the casting is to have hollow sections. Directional solidification is obtained by
proper gating. Typical castings of this type include gear blanks, pulley sheaves, wheels,
impellers, and electric motor rotors.
Centrifuge centrifugal casting has the widest field of application. In this method, the casting
cavities are arranged about the centre axis of rotation like the spokes of a wheel, thuspermitting the production of multiple castings. Centrifugal force provides the necessary
pressure on the molten metal in the same manner as in semi-centrifugal casting. This
casting method is typically used to produce valve bodies and bonnets, plugs, yokes,
brackets, and a wide variety of various industrial castings.
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Advantages of centrifugal casting
Relatively lighter impurities such as sand, slag, oxides and gas float more quickly
towards the centre of rotation from where they can be easily machined out.
Dense and fine grained metal castings can be produced.
There is proper directional solidification from outside towards inwards of the
casting.
Gating system is not required & hence more casting yield.
There is no need of a central core to make a pipe or tube.
Process can be adopted for mass production.
Limitations of centrifugal casting
True centrifugal casting is limited to certain shapes.
Equipment costs are high
Skilled labour required
Applications of centrifugal casting
Bearings for electric motors and industrial machinery.
Cast iron pipes, alloy steel pipes and tubings
Liners for IC engines
Rings, pots and other annular components
Investment Mould Casting Process (Lost wax process):
The investment casting process begins with the production of wax patterns of the desired
shape of the castings. The patterns are prepared by injecting wax or polystyrene in a metal
dies. Dies may be made either by machining cavities in steel blocks or by casting a low
melting point alloy around a metal master pattern. Waxes employed are beeswax, paraffin
etc. A number of wax patterns are attached to a central wax sprue to form an assembly.
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The wax pattern assembly is dipped into a slurry of a refractory coating material,
sprinkled with silica sand and is permitted to dry. This pre-coating provides a good surface
finish to the castings. Typical slurry consists of silica flour suspended in ethyl silicate
solution. The pre-coated wax pattern assembly is then invested for the production of mould.
Investment moulds may be formed by either solid moulding or shell moulding.
Solid moulding: The wax pattern assembly is placed in a metal flask. Ceramic slurry
(investment) is then poured into the flask and is allowed to harden around the wax pattern
assembly. The investment hardens after about 8 hours of air drying. A typical investment
moulding mixture consists of sand, water, calcium phosphate and MgO.
Shell moulding: Pre-coated wax pattern assembly is dipped in ceramic slurry and
immediately dusted with powder ceramic. A number of dips and subsequent dusting build a
shell thickness of the order of 6 to 12mm. The slurry is made of fused silica and alumina
along with liquid binders.
Solid moulds are placed upside down in furnaces to remove the wax pattern. Wax
patterns from shell moulds are removed either by exposing it to a furnace or by using a
suitable solvent.Molten metal is brought in small ladles to the pre-heated moulds for pouring.
Preheating vaporizes any remaining wax in the moulds. Also metal may flow more easily and
fill every detail of the preheated mould. After solidification, castings are removed from
the mould for cleaning, finishing and inspection.
Advantages of investment casting
Castings possess excellent details, smoother surfaces and close tolerances.
Castings do not contain any disfiguring parting line.
Sections as thin as 0.75 mm may be cast.
Since molten metal is poured in preheated moulds, the resultant cooling rate is slow
and the process produces large grain size as well as sounder and denser castings.Limitations of investment casting
Production of wax patterns and then investment moulds etc. make the process
relatively expensive.
There is a size limitation for the castings. Majority of the castings produced weigh
less than 0.5 kg.
Since the pattern is expendable, one wax pattern is required for each casting.
Relatively slow process.
The use of cores makes the process more difficult.
Applications of investment casting
To fabricate difficult-to-work alloys into highly complex shapes such as turbine
blades.
Impellers and other pump and valve components.
In dentistry and surgical implants.
For making jewellery and art castings.
Milling cutters and other tools
Corrosion resistant and wear resistant alloy parts.
Shell Moulding
Shell moulding replaces, conventional sand moulds by shell moulds made up of relativelythin and rigid shells of uniform wall thickness.
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A metal pattern having the profile of required casting is heated to 180 - 250C in an oven.
Pattern after being heated is taken out of the oven and sprayed with a lubricating agent. It is
necessary to prevent the shell from sticking to the metal pattern. Metal pattern is then
turned face down and clamped over the open end of the dump box. The dump box containsa mixture of sand and formaldehyde resin. The dump box is inverted so that dry sand - resin
mixture falls on the face of hot metal pattern. The resin sand mixture in contact with the
pattern gets heated up, the resin softens and fuses to form a soft and uniform shell of about
6mm thickness on the surface of the pattern. As the dump box is turned to its original
position, excess sand resin mixture falls back into the dump box leaving a shell on the
pattern. The pattern along with the shell is passed into an oven where the resin-sand
mixture cures and the shell acquires rigidity. The shell is then stripped from the pattern
plate with the help of ejector pins which are an integral part of the metal pattern. After the
shells so obtained have cooled, two mating shells are securely fastened together to form a
complete mould. The heat of the molten metal starts burning resin binder of the mould. Bythe time the casting is solidified, the binder completely burn out and on tapping, the shell
mould disintegrates easily.
Advantages of shell mould casting
Castings as thin as 1.5 mm and of high definition can be cast satisfactorily.
Castings possess excellent surface finish.
Reproduces details with sharp clean edges eliminating the need of subsequent
machining.
Less foundry space requirement.
Semi-skilled operators can handle the process.
Shells can be stored for a long time before use.Shell moulding can be mechanised.
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Limitations of shell mould casting
Uneconomical on small scale production
Shapes in which proper parting and gating cannot be obtained are not suitable for
shell moulding.
The maximum size of the casting is limited by the maximum size of the shell which
can be feasibly produced and poured.
Break down sand from shell moulds are not recoverable.
Applications of shell mould casting
Ideal for mass production of small intricate castings
For casting automotive rocker arms and valves.
Camshafts, bushings, brackets, shafts and gears
Hydraulic castings in SS and copper alloys.
Continuous casting
Round ingots, slabs, square billets and sheets can be cast by a continuous process
directly from molten metal. Continuous casting is accomplished by pouring molten metal
into a mould open at both ends and by keeping it filled at all times. The metal at the lower
end of the mould is cooled so that it solidifies and the solid product thus formed is extracted
in a continuous length from the lower end.
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Molten metal is transferred from the furnace into a special ladle called tundish. From
the tundish, molten metal is poured into the top of a bottomless graphite mould of the
desired shape. The molten metal should be slag free and should be poured with minimum
turbulence.
Graphite moulds are self lubricating and are not wetted by molten metal. Massive
graphite moulds eliminate the need of water cooling. Brass or copper moulds are used in
some cases. Water cooling and lubrication is mandatory for such moulds.
The process is started by placing a dummy bar in the mould up on which the
first liquid metal falls. The liquid metal gets cooled and is pulled by the pinch rolls along with
the dummy bar. Heat from the molten metal dissipates fast through the mould walls and a
skin of solid metal forms quickly at the mould-metal interface and shrinks from the mould
walls. The shrinking effect provides a very small gap between the metal and mould thereby
reducing friction between the two and permitting cast shape to move continuously through
the mould.
Pinch and guide rolls regulate the rate of settling of cast shape and keepproper alignment. As the casting passes out of the pinch rolls, it is cut to desired length by a
saw or oxyacetylene torch. The cut lengths are straightened, rolled and inspected. Argon
provides an inert atmosphere to avoid atmospheric contamination of molten metal. X-ray
unit controls the pouring rate of molten metal from the ladle.
Heat extraction should be in such a way that directional solidification is promoted.
The rate at which heat is removed from the molten metal must be synchronised with the
molten metal input and the rate of removal of casting.
Advantages of continuous casting
100% casting yield
Process is cheaper than rolling from ingotsGrain size can be regulated by controlling cooling rates.
Process is essentially automatic and labour cost is low.
Applications of continuous casting
Continuous casting can produce any shape of uniform cross-section such as
rectangular, square, hexagonal, gear toothed, solid or hollow.
Production of blooms, billets, slabs and sheets.
Bushings and pump gears.
Copper wire/bar.
Pressure Die CastingIn pressure die casting, molten metal is forced into die cavity under pressure. The
pressure is generally obtained by compressed air or hydraulically. The pressure varies from
70 to 5000 kg/cm2
and is maintained while the casting solidifies. A die casting machine
performs the following functions
Holding the two die halves firmly together Closing the die Injecting molten metal into die Opening the die Ejecting the casting out of die
Cast and wrought dies are used for the purpose. Die material selected should be able towithstand thermal erosion, mechanical erosion and chemical attack. Single cavity dies,
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multiple cavity dies and combination dies are used as per requirement. One die half is
stationary and is known as cover die whereas the other die half called ejector die moves in
order to open or close the complete die. The two die halves are kept in perfect alignment
with the help of dowel pins. Both stationary and movable cores may be employed in die
casting. Ejector pins employed to push the casting out may be actuated manually or
mechanically. Vents are provided for the escape of air present in the die cavity as the
molten metal enters the same. Based on the molten metal injection mechanism, pressure
die casting is of two types.
Hot chamber die casting
In hot chamber die casting machine, the melting unit constitutes an integral part of
the process. The molten metal possess normal amount of superheat and therefore less
pressure is needed to force the liquid metal into the die. Hot chamber process is of two
types Gooseneck or air injection type and submerged plunger type.
In gooseneck type, the cast iron gooseneck is so pivoted that it can be dipped beneath thesurface of the molten metal to receive the same when needed. The molten metal fills the
cylindrical portion and the curved passageways of the gooseneck. Gooseneck is then raised
and connected to an air line which supplies air at a pressure of 30 to 45 kg/cm2. The air
pressure forces the molten metal into the closed die. After the casting has solidified, the
gooseneck is again dipped beneath the molten metal to receive molten metal again for next
cycle. In the mean time, die halves open out, casting is ejected and die closes in order to
receive molten metal for producing the next casting.
Advantages
Simple in construction and operation.
No moving parts as compared to plunger type machineLimitation
Production rate is lower when compared to plunger type machine
Submerged plunger type machine has an injection cylinder which is partially submerged in
the pot containing molten metal. The molten metal enters the cylinder through the port and
plunger forces it through the nozzle into the die. Pressure exerted on the molten metal is of
the order of 140 to 200 kg/cm2. When the metal has solidified, die is opened and the casting
is ejected. The die is then once again closed, plunger is drawn to up position and the process
repeats.
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Advantages
Exerts pressure more effectively when compared with air injection
type machine
Limitation
Cannot be used with such alloys which affect the fit of plunger and
cylinder.
Air entrainment is more.
Cold chamber die casting
Melting unit is not an integral part of the cold chamber die casting machine. Molten
metal is brought and poured into the die casting machine with the help of ladles. Molten
metal poured is at a lower temperature as compared to that poured in hot chamber die
casting machine. Hence higher pressures (200 to 2000 kg/cm2) are employed in this process
to inject molten metal into die cavity.
The cold chamber die casting machine consists of a pressure chamber or cold chamber of
cylindrical shape fitted with a ram or piston operated by hydraulic pressure. Dies are made
of strong heat resistant materials to withstand high pressures and temperatures. A
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measured quantity of molten metal is brought in a ladle from the furnace and poured into
the cold chamber after the die is closed. The ram forces the metal into the die. Once the
casting is solidified, the movable half of die slides away and die opens. Ram then moves in
backward direction and the ejector is advanced to force the casting out of die.
Advantages
Castings produced are of greater density and more sustained
dimensional accuracy.
Separation of furnace from the working parts of die casting machine
increases its life and efficiency.
Limitations
Dies have to be extra strong to withstand high pressures
Advantages of pressure die casting
Dies are capable of retaining their accuracy and usefulness for long periods of
production.
High production rates.
Very thin sections can be cast without any difficulty.
Close dimensional tolerances.
Intricate shapes can be die cast.
Good surface finish obtained.
Can be mechanised and used for mass production.
Semi-skilled workers may be employed
Less defective than sand castings.
Less floor space required
Economical for large scale production
Limitations of pressure die castingFerrous alloys are not cast
Size restriction is there for castings.
Proper evacuation of air from die cavity is required to avoid porosity in castings.
Longer period of time for going into production.
Dies may produce an undesirable chilling effect on the die castings.
Applications of pressure die casting
Zinc based alloys for automobile parts, refrigerators, washing machines etc.
Aluminium based alloys for automobile and air craft industry.
Copper based alloys for electrical machine components and chemical apparatus.
Magnesium based alloys for binocular and camera bodies.Lead based alloys for radiation shielding and battery parts
Tin based alloys for bearings and containers.
Gravity die or permanent mould casting
Gravity die or permanent mould cavity makes use of a mould which is permanent.
The mould or die can be used several times before it is discarded or rebuilt. Molten metal is
poured into the mould under gravity only. No external pressure is applied to force the liquid
metal into the mould cavity. However the liquid metal solidifies under pressure of metal in
the risers etc.
Permanent moulds are made of dense, fine grained, heat resistant cast iron, steel,bronze, graphite etc. A permanent mould is made in two halves in order to facilitate the
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removal of casting from the mould. The parting line may be horizontal or vertical. The
mould walls have thickness from 15 mm to 50 mm. Mould walls are made thicker opposite
thicker sections of the casting to provide desired chilling effect. Fins or projections are
provided on the mould wall for faster cooling. Pouring cup, sprue, gates and riser are built in
the mould halves itself. Simple mechanical clamps are adequate for clamping the die halves
for small moulds. Large permanent moulds need pneumatic or other power clamping
methods. Cores if any are placed before closing the die halves. Lubricating coatings if
sprayed helps removal of castings and core from the mould. The mould is pre-heated before
pouring the molten metal. Molten metal is poured into the mould under gravity. Castings
are ejected from the mould after they are solidified.
Advantages of permanent mould casting when compared to sand casting
Closer dimensional tolerance and accuracy.
Very good surface finish.
Chilling effect of the metal mould helps in producing fine grained structure.
Mass production of castings is more economical.
Less floor space is needed.
Faster rate of production
A number of casting defects can be completely eliminated.
Less skilled labour required
Limitations of permanent mould casting when compared to sand casting
Higher cost
Shape and size restriction for castings
Gating system once machined cannot be changed and hence no chance for lateradjustments
Uneconomical for small production runs
More chances for chilling problems
Applications of permanent mould casting
Carburettor bodies
Hydraulic brake cylinders
Refrigeration castings
Connecting rods and automotive pistons
Aircraft and missile castings
Washing machine gearsOil pump bodies
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Squeeze casting
Squeeze casting, also known as liquid-metal forging, is a process by which molten
metal solidifies under pressure within closed dies positioned between the plates of a
hydraulic press.
Squeeze casting consists of metering liquid metal into a preheated, lubricated die
and forging the metal while it solidifies. The load is applied shortly after the metal begins to
freeze and is maintained until the entire casting has solidified. After solidification, casting is
ejected out with the help of ejector pins.
Advantages of squeeze casting
Near Net Shape components reduce machining costs
Localised reinforcement can give increased properties
Solidification under load eliminates shrinkage and gas porosity
Fine grain microstructure
Excellent casting yield as no running and feeding systems are required.
Applications of squeeze casting
Aluminium domes
Ductile iron mortar shells
Steel bevel gears
Stainless steel blades
Super alloy disks
Aluminium automotive wheels and pistons
Gear blanks made of brass and bronze.
Slush casting
This process is used to produce hollow castings when the external features of the
casting are important and the castings are not destined for engineering use. The uniformity
of wall thickness may not be an important consideration for such castings.
The mould is filled with molten metal and held stationary until a thin skin of solid
metal freezes against the mould walls. The mould is then inverted and the liquid metal is
drained out.
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The time required for this casting operation is sufficient to freeze a metal shell in the mould,
corresponding to the shape of the cavity wall. The thickness of the wall of the casting
depends on the time interval between the filling and the inverting of the mould, as well as
on the chemical and physical properties of the alloy and the temperature and composition
of the mould. Usually lead and zinc alloy castings are produced by slush casting. After giving
enough time for solidification, casting is taken out.
Vacuum casting
A vacuum is created within the mould cavity and the metal is pulled rather than pushed into
the mould. Excellent mechanical properties and high production rates are often realized in
vacuum casting because of the low mould temperatures associated with the method.
The metal in the fill tube acts as a riser, and excellent metal yields are obtainable.
The process lends itself to permanent mould casting automation, and the result is the ability
to produce large quantities of high-quality castings at a competitive price. The process is
usually associated with smaller castings and requires specialized, complex mould designs toinduce the vacuum properly.
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Shakeout/Cleaning/Finishing
Shakeout: It is the removal of casting from the moulding box. In manual shakeout, the
mould assembly is dumped upside down on the ground. It will disintegrate the sand and the
casting can be pulled out from the sand with a hook bar. In mechanised foundries, sand
around the casting is broken by striking with a metal rod. Mechanical shakeout is anotheroption. Moulding boxes containing the castings are placed on the vibrating platform of the
shakeout unit. Moulding boxes are shaken to loosen and break up sand portions of the
mould. Moulding boxes and castings remain on the shakeout platform whereas loose sand
falls into a sand hopper situated below the shaking platform.
Fettling or cleaning: Fettling includes
Removal of cores from the casting Removal of adhering sand and oxide scale from the casting surface Removal of gates, risers, runners etc. from the casting Removal of fins and other unwanted projections from the castings
Removal of cores: Hammering or vibrations imparted to cores loosen and break them up.Sand portions sticking inside the castings are removed by poking action using a metal rod.
Cores from larger castings may be removed efficiently by pneumatic rapping and hydro-
blasting.
Cleaning of casting surfaces: Adhering sand on casting surfaces can be removed using hand
methods or mechanical equipment. Hand methods involve the use of wire brush, file, pick,
crowbar etc. Hand methods are slow and tedious. Mechanical methods include tumbling, air
blasting and hydro-blasting.
Tumbling: The tumbling barrel is filled with castings, star shaped hard iron pieces,
granite chips, pieces of graphite electrodes etc. the barrel ends are closed and the
barrel is rotated at about 30 rpm. Castings tumble over each other removing theadhering sand from casting surfaces. Tumbling operations are slow and dusty.
Air blasting: Compressed air propels abrasive particles against a casting to clean its
surface. The air pressure is about 7kg/cm2. Air blasting can be of two types sand
blasting and shot blasting. Metallic shots or grits are used with shot blasting whereas
special grade sand is used as abrasive in sand blasting. Blast cleaning has an
additional advantage of improved surface properties of the casting. Air blasting has
dust problems.
Hydro-blasting: Water stream carrying abrasive particles clean the casting surfaces.
Water pressure applied is about 140kg/cm2. The process is dust free. The process is
more rapid and effective. Large initial cost is a limitation. It is generally applied forlarge castings.
Chemical cleaning methods utilise chemicals like caustic soda to react with and break the
surface oxide layer. In electrolytic method, casting is made cathode and the oxide layer is
reduced. Pickling involves immersing the casting in acid for some time and later neutralising
by dipping in lime water. Pickling removes sand from the surfaces and inaccessible pockets
of the castings.
Removal of gates and risers can be done by
Chipping hammers Flogging or knocking off Shearing Sawing
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Abrasive wheel slitting Machining Flame cutting Plasma cutting
Castings are trimmed to remove fins, chaplets, parting line flash etc. The methods employed
are chipping, sawing, flame cutting, grinding, abrasive belt machines, shearing etc.
Finishing of castings: Finishing is performed to
Smooth the areas of castings from where gates and risers have been removed.
Remove any excess metal if left on the casting.
Improve surface finish and appearance.
Different finishing operations carried out on castings are grinding, rotary filing, machining,
chemical treatment, polishing, buffing, blast cleaning, painting etc.
Heat treatment of castings: Heat treatment of castings has two main purposes
Relief of stresses developed in cooling, repair welding or machining.
Development of structure sensitive properties by metallurgical changes.The various heat treatments for ferrous castings involve annealing, normalising, quench
hardening, tempering, stress relieving etc. Non ferrous castings may undergo solution
hardening and precipitation hardening heat treatments as required.
Quality control in foundries
Quality control in mould making
Patterns should be checked for dimensions and allowances before moulding. Patterns and core boxes should be kept in good condition. Cores should be tested for suitability as per desired requirements. Moulding sand should be tested for its different properties before use and should
be rammed to correct density.
Cores should be positioned correctly. Flask equipment should be checked as regards its shape, surface, pins, alignment
etc.
Quality control in melting
Incoming metal charge should be analysed regarding its chemical composition. Furnace is to be selected based on the nature of the material. Metal charge should be clean, dry and correctly weighed. Molten metal temperature should be properly controlled. Samples of molten metal should be chemically analysed. Molten metal should be degassed before pouring into the mould.
Quality control in Fettling, Cleaning & Heat treatment
Gates, risers etc. should be removed with care so that cracks are not initiated in thecastings.
Chisel marks should be smoothed away. Finishing operations should not produce cracks or burning off marks. Heat treatment variables should be properly controlled.
Inspection & Testing of castings
Destructive Testing
Tensile testing
Hardness testing
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Impact testing
Fatigue testing
Creep testing
Non-destructive testing
Visual inspection
Die penetrant test
Leak test
Radiography
Magnetic particle inspection
Ultrasonic inspection
Casting Defects
Mismatch or Mould sift: There is mismatching at the top and bottom parts of the casting at
the parting line.
Causes
Faulty placing of the top and bottom halves of the pattern.
Worn out, loose, bent or ill-fitting moulding box clamping pins.
Blowholes: Blowholes are entrapped bubbles of gas with smooth walls. Blowholes may
occur in clusters or may be isolated. Blowholes visible on the surface of a casting are calledopen blows.
Causes
Excess moisture in the moulding sand.
Low permeability of moulding sand
Rusted and damp chills, chaplets and inserts.
Inadequate venting of cores and moulds
Misrun:A misrun is caused when the metal is unable to fill the mould cavity completely and
thus leaves unfilled cavities.
Causes
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Too cold molten metal
Too thin casting section
Too small gates
Too many restrictions in the gating system
Lack of fluidity of molten metal
Interrupted flow of metal from ladle to mould.
Cold shut: A cold shut is caused when two liquid metal streams while meeting in the mould cavity,
do not fuse together properly thus forming a discontinuity in the casting.
Causes
Too cold molten metal
Too thin casting section
Too small gates
Too many restrictions in the gating system
Lack of fluidity of molten metal
Hot tears: They are cracks which appear in castings during solidification due to high tensile
or shear stresses.
Causes
Very hard ramming and therefore excessive mould hardness.
Higher dry and hot strength of the sand mould
Insufficient collapsibility of the core
Too much solidification shrinkage
Faulty design causing some portions of the casting to be restrained while cooling.
High sulphur content
Too low pouring temperature
Cut and washes: These appear as rough spots and areas of excess metal, and are caused by
erosion of moulding sand by the flowing metal. The former can be taken care of by the
proper choice of moulding sand and the latter can be overcome by the proper design of the
gating system.
Other casting defects include
Fins and flash: They usually occur at the parting line and result in excess metal which has to
be ground off.
Crush: It is the displacement of sand while closing a mould, thereby deforming mould
surfaces. A crush shows itself as an irregular sandy depression in the casting.
Drop: A drop occurs when mould surface cracks and breaks, thus pieces of sand fall into themolten metal.
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Scab: It occurs when a portion of the face of a mould lifts and the metal flows underneath in
a thin layer. Liquid metal penetrates behind the surface layer of the sand.
Pinholes: They are numerous very small holes revealed on the surface of a casting after the
surface has been cleaned by shot blasting. This occurs when sand has high moisture content
or gas generating ingredients.Shrinkage defects: If the solidification shrinkage is not compensated by providing risers etc.,
voids will occur on the surface or inside the casting.
Inclusions: Any separate undesirable foreign particle present in the metal of a casting is
known as inclusion. An inclusion may be oxides, slag, dirt or moulding sand broken from
mould surface.
Metal penetration: Molten metal enters into the space between the sand grains and result
in metal penetration and rough casting surface.
Fusion: Sand may fuse and stick to the casting surface with a resultant rough glossy
appearance.
Swells: A swell is an enlargement of the mould cavity due to molten metal pressure on
mould walls.
Semisolid Metal Forming
The metal or alloy has a nondendritic, roughly spherical, fine-grained structure when
it