IIT BHU

MANUFACTURING TECHNOLOGY

CUPOLA FURNACE AND SAND CASTING ..

22/09/2015

NAME : Shubham Khatri

ROLL NO. : 14135077

CLASS : B. TECH. PART II

MECHANICAL ENGG.

SUBMITTED TO DR. MOHD. ZAHEER KHAN YUSUFZAI

What is cupola furnace? Cupola is a melting furnace for ferrous metals. It is basically used for the

melting and refining of pig iron into cast iron. Pig iron has a fairly high carbon

content combined with silica and dross constituents, making it very brittle. It

should not be used directly except for some limited application.

Advantages of cupola over others:- It melts iron with less fuel and more cheaply than any other furnace, can be run intermittently without any great damage from expansion and contraction in heating and cooling. Large or small quantities of iron may be melted in the same furnace with very little difference in the per cent, of fuel consumed, and the furnace can readily be put in and out of blast. Consequently in all cases where the strength of the metal is not of primary importance, the cupola is to be preferred for foundry work.

In the reverberatory furnace from 500kg to a ton of fuel is required to melt one ton of iron.

In the pot furnace one ton of coke is consumed in melting a ton of cast iron, and two and a half tons in melting a ton of steel.

In the blast furnace one ton to 1.3 tons of coke is consumed in the production of a ton of pig iron.

In the cupola furnace a ton of iron is melted with from 250kg to 400 kg of coke.

About Cupola

Structure Vertical cylindrical shell upto 6-12 mm

thick lined inside with acid refractory

bricks or fire bricks (SiO2 or Al2O3) etc.

Lines are thicker in the lower region

where temperature is high. The shell is

mounted on brickwork or steel columns.

There is a drop bottom for the removal

of debris of coke or slag. Constant

volume of air is required for combustion

is obtained from motorised blower. It

provides air to the wind pipe which takes

it to the wind box which is air jacket

around the shell. There are tuyeres which

supply the air to the cupola. Air is made

to first perform rotatory motion and then into the tuyeres. There is

also a valve to control the flow of air. There is a tapping door which is

the lowest point in the front of cupola through which molten iron is

obtained. Just above it on the

opposite side is the slag hole.

Metal, flux and coke are poured

into the furnace through the

charging door which is 3 to 6m

above tuyeres. Shell is continued

4 to 6m above charging door to form a chimney. At the top is the spark

arrester in the shape of cone opened on the base for smoke to escape.

Regions in Cupola 1. Crucible Zone

Top of sand bed to bottom of tuyere. Molten iron is

stored here.

2. Oxidizing zone Combustion of C to CO2, Mn to MnO2, Si to SiO2. Intense heat is

released which is liberated and heats the other zones. Temperature to

1550 to 1850 C

3. Reducing Zone Top of combustion zone to top of coke bed. Reduction of CO2 to CO

and temperature drops to 1200C.

4. Melting Zone First layer of metal charge above coke bed and extend upto 900mm.

Highest temperature is developed for the complete combustion of

coke and iron is melted here (1600C).

Considerable carbon pickup by the molten metal also occurs in this

zone according to reaction 3Fe +2CO=Fe3C +CO2

5. Preheating zone and charging zone Above melting and extends up to bottom of charging door. Contains

cupola charge as alternate layers of coke, metal and flux. Metal is

preheated before entering the melting zone.

6. Stack Zone Extends from above the preheating zone to the top of the cupola.

Carries gases that are released in the reactions.

The construction of a conventional cupola consists of a vertical steel

shell which is lined with a refractory brick. The charge is introduced into

the furnace body by means of an opening approximately half way up the

vertical shaft. The charge consists of alternate layers of the metal to be

melted, coke fuel and limestone flux. The purpose of adding flux is to

eliminate the impurities and to protect the metal from oxidation. The fuel

is burnt in air which is introduced through tuyeres positioned above the

hearth. The hot gases generated in the lower part of the shaft ascend

and preheat the descending charge. After that air blast is turned on with

tapping door and slag hole closed and the iron starts melting. The rate

of melting should be equal to the rate of charging. At the end when all

iron melts the tapping door and slag hole are opened and we get cast

iron.

If A is the internal area of the cupola then

A=Q/Q1

Q = designed cupola output, tonnes per hour

Q1= specific output per sq m of the cross-sectional area, tonnes per

hour. Q1=6 to 8 tonnes per hour.

GIVEN INTERNAL DIAMETER = 0.46m

A = 16.6cm2

Suppose we have iron is to coke ratio of n: 1 and Q1=q tonnes/

(cm2*hour)

IRON MELTED Q=A*q=16.6q tonnes/hour

Coke required = 16.6q/n tonnes/hour

Air Supply:- Air supply in cupola is that about 8.4 cu m of air required per kg of coke at normal atmospheric pressure and

temperature.

The internal diameter of cupola determines the amount of coke consumed

and the amount of metal melted per unit time. 14cm2 of cupola plan

burns about 1kg of coke per hour. The height of cupola effects the

melting rate. The height upto the charging door is about 3 to 5 times

the diameter.

Sand Casting Sand casting consists of

1. Placing pattern having shape of

the desired product.

2. Incorporating a gating system.

3. Filling of the resulting cavity with

molten metal.

4. Allowing the molten metal to

solidify.

5. Breaking the sand mold.

6. Removing the castings.

Moulding Sand

GREEN SAND: It is a mixture of silica sand with 18 to 30% clay, having total water from 6 to 8 %. The clay and water furnish the bond for green sand.

It is fine, soft, light, and porous. It is fine, soft light, and porous. Being

damp, when squeezed in the hand, it retains the shape, the impression given to

it under pressure.

DRY SAND: The sand obtained after green sand has been dried or baked is called dry sand.

PARTING SAND: Parting sand is used to keep green sand from sticking to the pattern and also allow the sand from cope and drag from clinging. This

is clean clay free silica sand.

CORE SAND: It is used for making cores. It is dried properly to increase the strength of the sand. It is also made porous so as to allow the passing of

gases through it as the gases have to pass through it.

Different Types Of Patterns

SINGLE PIECE:

A pattern that is made without partings or

joints or any loose pieces is called single piece

pattern. Gating system has to be made

separately for each mould.

SPLIT PATTERN:

When the pattern cannot be made in a single piece because

of difficulties encountered in moulding them. So they are

split into two halves one in the drag and one in cope. For

proper synchronization one half has dowel pins and other

has the hole so they can be matched.

MATCH PLATE PATTERN:

When the split patterns are mounted on rectangular

plates with the drag part containing the gating system.

GATED PATTERN:

When the pattern itself contains a gating system embedded

in it, it is called a gated pattern. In mass production a

number of castings are produced in a single multi cavity

structure the patterns are joined together by runner and

ingates.

LOOSE PIECE PATTERN:

Sometimes patterns are composed of loose pieces. Such

patterns are employed when it is difficult to remove the

pattern completely at once. Thus the part which can be

removed is removed and the rest which need turning or

moving to remove them are removed.

SWEEP PATTERN:

Symmetrical moulds and cores can be

produced with the help of sweep

patterns. It consists of a board having

the desired shape of the mould and it is

attached to a central axis about which it

is rotated.

SKELETON PATTERN:

These patterns are employed for the making of very large

patterns. It provides the contour and shape of the

pattern. It consists of a ribbed structure with rectangular

and circular openings between the ribs.

Types Of Defects

SHRINKAGE CAVITY:

It is the void or depression due to uncontrolled

solidification of the metal. This happens when the extra

metal is not supplied by the riser. This can happen due to

solidification of ingates or riser prior to our casting. We

can prevent this by relocation of riser or building a proper

gating system.

BLOW HOLES:

Blow holes are smooth, round holes appearing in the form of cluster of a

large number of small holes below the surface of a casting. These are

entrapped bubbles of gases with smooth walls. They are caused by the

presence of excessive moisture in the sand or when the permeability of the

sand is low, sand grains are too fine, sand is rammed too hard or when venting

is sufficient.

COLD SHUT: A cold shut is an external defect caused due to improper fusion of the two meeting streams of the molten metal. This is caused due to

improper or insufficient temperature of the molten metal, thin sections in

cavities, improper gating system, slow pouring, and poor fluidity of the metal.

SHIFTS: This is due to core misplacement or mismatching of top and bottom part of the casting.

FIN:

A thin projection of metal, not intended which occurs at the

parting surface of the mould or core section. Insufficient

weight on the mould or improper clamping of the flasks

may produce a fin.

INTERNAL AIR POCKET: This appears as small holes inside the casting and is caused by pouring boiling metal or by rapid pouring of boiling

metal in the mould. Faulty and poor quality of metal and excessive moist sand

also causes air pockets.

SAND HOLES: Sand holes are found on external surface or inside of the casting. They are caused by loose sand which enters into the melt during

pouring.

Preparation Of Mould For Split Pattern First half of the pattern was kept in the middle of the drag box. Parting sand

was then put on it. Then green sand was properly put in the box and it was

properly rammed with peen rammer and round rammer. The surface was then

made smooth with the help of strike off bar. The drag box was then inverted

and again parting sand was put over it. Then the other half of the pattern was

fitted into it and then down spruce and riser were adjusted and cope box was

put over it. Then sand was filled in the cope box and rammed properly. Funnel

type shape was made for pouring basin. Guide pins were put at opposite

corners. Vent holes were made and then cope box was taken from above.

Watering was done around patterns and they were pulled from drag and cope

boxes. U shaped cavities were made in the drag box in places where riser and

down spruce were put.

CO2 Casting The process is basically a hardening process for moulds and cores. If CO2 is passed through a sand mix containing sodium silicate, the sand becomes extremely strongly bonded.

Principle

The principle working of this process is based on the fact that, instead of using an oil or resin that requires heat for bonding, CO2 gas is passed through as and mix containing sodium silicate as a result of which, the sand immediately becomes extremely hard and strongly bonded as sodium silicate becomes a stiff gel.

Chemical Reaction

Na2O.(x)SiO2 + (x)H2O + CO2 Na2CO3 + SiO2(x)H2O (sodium silicate) (sodium carbonate) (silica gel)

When x=3,4 or 5,most often x=2

Sand Used The sand which should be used for this casting should ideally have following properties

1. The sand used must be dry. 2. Minimum moisture content of 0.25%. 3. It should be free of clay. 4. Maximum strength when sand grain size is at 80 mesh size.

Additives The following additives are suitable in this process with moulding sand

Coal powder Wood flour Kaolin clay Aluminum oxide Invert sugar Dextrine Sea coal

Why Carbon Dioxide?

It is used as a catalyst in the making of sodium silicate molds and cores in the CO2 moulding.

During the process CO2 forms a weak acid which hydrolyzes the sodium silicate thus forming an amorphous silica gel.

The reaction proceeds rapidly in the early stages of gasification.

Compressive strength reaches maximum when critical amount of gas is passed.

The mixed sand must be covered otherwise it will react with the CO2 in the air.

There is often a crust that forms from this contact and it is removed and thrown away.

Construction of mould

1. Suitable proportion of silica sand and sodium silicate binder (3-5% based on sand weight) are mixed together to prepare the sand mixture. 2. Additives like aluminum oxide, molasses etc. are added to impart favorable properties and to improve collapsibility of the sand`. 3. Then the proper mould pattern is placed and the mould is packed. 4. When packaging is complete, CO2 is forced into the mould at a pressure of about 1.45 kgf/cm2 for a certain time.

Estimates The time taken to harden a small or medium size sand-body ranges from 1-2 minutes.

Volume of CO2 required can be calculated if quantity of sodium silicate present is known.

For every 1 kg of sodium silicate, 0.5 0.75 kg of gas is required.

Advantages

Instantaneous strength development.

The process is safe to human operators.

Very little gas evolution during pouring of molten metal.

Reduces large requirement for number of mold boxes and core dryers.

Provides great dimensional tolerance and accuracy in production.

Moisture is completely eliminated from the molding sand.

This process can be fully automated.

Compared to other casting methods cores and moulds are strong.

Reduces fuel cost since gas is used instead of other costly heating generating elements

Disadvantages

Poor collapsibility of mould is a major disadvantage of this process.

The sand mixture has the tendency to stick to the pattern and has relatively poor flow ability.

There is a significant loss in the strength and hardness of moulds which have been stored for extended periods of time.

Over gassing and under gassing adversely affects the properties of cured sand.

Applications Co2 casting process is ideal where speed and flexibility is the prime

requirement.

Molds and cores of a varied sizes and shapes can be molded by this process.

In commercial operations, one can assure customers of affordable castings which require less machining.

The molding process which can be fully automated is generally used for casting process that require speed, high production runs and flexibility.