Casting Fundamentals

7/30/2019 Casting Fundamentals

1/44

JO/VJCET Page 1

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


2/44

JO/VJCET Page 2

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


3/44

JO/VJCET Page 3

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.


4/44

JO/VJCET Page 4

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


5/44

JO/VJCET Page 5

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


6/44

JO/VJCET Page 6

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


7/44

JO/VJCET Page 7

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


8/44

JO/VJCET Page 8

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.


9/44

JO/VJCET Page 9

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.


10/44

JO/VJCET Page 10

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.


11/44

JO/VJCET Page 11

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


12/44

JO/VJCET Page 12

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


13/44

JO/VJCET Page 13

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


14/44

JO/VJCET Page 14

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.


15/44

JO/VJCET Page 15

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.


16/44

JO/VJCET Page 16

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.


17/44

JO/VJCET Page 17

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


18/44

JO/VJCET Page 18

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


19/44

JO/VJCET Page 19

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


20/44

JO/VJCET Page 20

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.


21/44

JO/VJCET Page 21

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


22/44

JO/VJCET Page 22

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.


23/44

JO/VJCET Page 23

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


24/44

JO/VJCET Page 24

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


25/44

JO/VJCET Page 25

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


26/44

JO/VJCET Page 26

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


27/44

JO/VJCET Page 27

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.


28/44

JO/VJCET Page 28

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.


29/44

JO/VJCET Page 29

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.


30/44

JO/VJCET Page 30

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.


31/44

JO/VJCET Page 31

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.


32/44

JO/VJCET Page 32

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,


33/44

JO/VJCET Page 33

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.


34/44

JO/VJCET Page 34

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


35/44

JO/VJCET Page 35

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


36/44

JO/VJCET Page 36

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


37/44

JO/VJCET Page 37

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.


38/44

JO/VJCET Page 38

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.


39/44

JO/VJCET Page 39

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


40/44

JO/VJCET Page 40

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


41/44

JO/VJCET Page 41

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


42/44

JO/VJCET Page 42

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.


43/44

JO/VJCET Page 43

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

Documents

Casting Fundamentals