Basic Manufacturing Process - Ch.6.5

Q.12    Describe the construction and working of cupola.                           (AKTU - 2009-10, 10 - 11)
  A large variety of melting furnaces are available for foundry work. The selection of furnace will depend upon the quantity and type of material to be melted. In general any furnace should have following qualities.
1.    It should deliver molten metal at correct temperature.
2.    It should be less fuel consuming.
3.    It should need less maintenance.
Construction: -
    It consist of a cylindrical steel shell. The shell is lined with some refractory material to resist high temperature. At the bottom of shell the drop doors are provided. To prepare a better sand bed these door are utilized. The bottom sand bed prepared provides refractory bottom from molten metal. Just above the sand bed a metal tapping hole is provided. Above the slag hole there is a wind box. To this wind box air blowers are connected to supply the air at some pressure. This air through the wind box enters cupola through tuyeres. As shown in figure above the charging platform there is charging door from where pig iron coke and flukes are put into cupola.
Working: -
    Depending upon the combustion reaction in cupola it can be divided into four zones.
(i) Crucible zone: -
    It is between the top of sand bed and bottom of tuyeres. This is also referred by the names well or health.
(ii) Combustion zone: -
    The combustion or oxidizing zone is situated normally 150 to 300 mm above the top of tuyeres. Temperature in this zone may be of the order of 15500C to 18500C. Main reactions in this zone are 
(iii) Reducing zone: -
    This extends from top of combustion zone to top of coke bed. The reaction taking place in this zone is
(iv) Melting zone: -
    It starts from first layer of metal charge upto 900 mm. The highest temp. (15000C) is noted in this zone. 
Charging or Preheating Zone: -
    It starts from above the melting zone and holds out to the bottom of charging door.
    This zone contains the charge as alternate layers coke, flux and metal. The charge is preheated here out around 1100oC before coming to the melting zone.
Stack Zone: -
    This zone carries the hot gases developed in the furnace to the atmosphere.
Cupola Operation :
(a) Preparation of Cupola: -

    First of all, slag is removed out from the furnace. The damaged inner lining of furnace in repaired with fine clay and silica sand.
    A sand bed having slope is prepared upto tap hole.
(b) Firing: -
    Wood is set on fire on the bottom of sand and coke is added and air blast is turned on. After some time, extra coke is added up to the desired height.
(c) Charging Cupola: -
    After some burning, alternate layers of pig iron, coke and flux are charged though the charging door. Limestone is generally used as flux. Steel scrap is added to control the chemical composition.
(d) Soaking of the Iron: -
    When the furnace is fully charged, the charge is gradually heated to have the iron soaked.
(e) Switching the Airblast ON: -
    When the iron is soaked, air blast is switched on. Top hole is closed to accumulate the metal. The rate of charging must be equal to the melting rate so that furnace remains full.

Q.13    What is die casting. Explain with neat sketch.                                            (AKTU - 2010 - 11)
Ans.    Die-Casting: -

        Die casting is a precision casting technique that uses a permanent metal mould, or die, into which molten metal is poured directly. Metal is typically forced into the mould under pressure but gravity-feed systems are also used. Tooling costs and other capital costs are high due to the cost of designing dies. Operational costs, however, are relatively low, due to the high level of automation and the small number of production steps (i.e. direct pouring into a permanent mould rather than preparing destroyable patterns and/or moulds). The process, therefore, is best suited to mass production.
    Die casting is most suitable for non-ferrous metals with relatively low melting point (i.e. around 870oC) such as lead, zinc, aluminium, magnesium and some copper alloys. Casing metals with high melting points, including iron, steel and other ferrous metals, reduces die life.
Die-casting can be done using a cold chamber  or hot chamber process :
(i) Hot Chamber (Or Goose-Neck) Die Casting: -
    In this set up, the melting unit is in the machine itself that is the reason it is called “Hot chamber” die casting machine. Hot chamber die casting machine offers a fast cycling rate (up to around 15 cycles per minute). Molten metal is injected from the same chamber in which it is melted. This system involves no handling or transfer of molten metal.
    The goose neck container is always immersed in the molten metal pot. The molten metal from the container is forced into the die by means of a plunger, the molten metal due to pressure comes up and fills the cylinder. When the plunger moves down, the metal is forced into die.
    Hot chamber die casting is not applicable for higher melting point metals. There is a tendency for molten aluminium to pick up some iron as a result of extended time in contact with the casting equipment. Hot chamber is used for Tin, Lead and Zinc.
(ii) Cold Chamber Die Casting: -
    In this set up, metal is melted separately in a furnace and transported to die casting machine by means of small hand laddle. After the closure of die, the molten metal is forced into the die cavity and pressure is maintained during solidification. Since molten metal has to be transferred to die chamber for each cycle, the cold chamber casting process has a longer process cycle as compared to that of hot chamber machine. Aluminium, Manganese and Copper base alloys are preferred for cold chamber.
    A complete die casting process is shown in Fig. 2. Dies are usually made from two blocks of steel, each containing part of the cavity, which are locked together while the casting is being made. Retractable and removable cores are used to form internal surfaces. Molten metal is injected into the die and held under pressure until it cools and solidifies. The die halves are then opened and the casting is removed, usually by means of an automatic ejection system.

    The die is cleaned between each casting cycle, preheated and lubricated to reduce wear on the die, to improve surface quality and to aid ejection. Mould coating material can also be used to protect the molten metal from the relatively cool and conductive surface of the mould. Cooling systems are often used to maintain the desired operating temperature.
Advantages -
·     Once capital is in place, operating costs are low relative to most other casting processes. This is due to the reduced number of process steps, the elimination of temporary moulds and patterns from the process, and the lower volume of materials that need to be handled.
·     Dies can sustain very high production rates (i.e. over 400 shots per hour). Total cost of castings can be relatively low at high levels of production.
·     High design flexibility and complexity allows products to be manufactured from a single casting instead of from an assembly of cast components.
·     Good accuracy, consistency and surface finish are possible, with high metal yields.
·     Cleaning, machining, finishing and fabrication costs are low.
·     There are low levels of waste due to elimination of refractory material, leading to a cleaner work environment.
Limitations -
·     Capital costs for equipment and dies are high.
·     Pressure dies are very expensive to design and produce.
·     Die casting is not applicable to steel and high melting point alloys.
·     Casting size is limited to a maximum of about 35 kg.
·     Cabinets for the electronics industry, hand and power tools for industrial and home use, general hardware appliances, pump parts, plumbing parts, parts for the automotive industry, sports and leisure, home appliances, a lot of communication equipments, etc.
·     All general hardware type parts, hardware for doors, drawers, and furniture, plumbing fixtures, automotive products, parts for the lighting industry, all hand tools, power tools, sporting goods, etc.

Q.14.    Write short notes on centrifugal casting.                                                    (AKTU - 2010 - 11)
Ans.    Centrifugal Casting: -

        In this method molten metal is poured in the centre of a rotating mou1d (at the axis of rotation). The metal flows under pressure into the mould cavities by the centrifugal force. The impurities being lighter in weight are left behind at the centre; so a dense and strong casting is, produced.
There are three methods of centrifugal castings:
(a) True Centrifugal Casting: -
    In this method, the axis of rotation of the mould and that of the casting are the same. The molten metal is poured into a revolving mould. The centrifugal force throws the metal towards the outer walls. The outside of the mould is watercooled, therefore, the metal immediately solidifies forming a hollow casting. The thickness of the casting wall can be controlled by controlling the amount of molten metal poured in the mould. This method is used to cast hollow cylindrical objects such as C.L pipes, liners, gun barrels, etc.
(ii) Semi-Centrifugal Casting: -
    In this method the mould rotates about its vertical axis and the molten metal is poured through a central sprue. The speed of rotation is low, so centrifugal force and pouring pressure produced are low. The impurities are not collected at the centre. This method is used to produce large sized symmetrical castings such as discs, pulleys, spoked wheel, gears, etc.
(iii) Centrifuging or Pressure Casting: -
    This method is used to obtain large number of small sized castings of irregular shapes. The axis of rotation of the casting does not coincide with the axis of the mould. The casting cavities are located near the outer periphery of the mould. The molten metal is admitted at the centre. The centrifugal force produces sufficient pressure to force the metal into the cavities through radial gates.

[I] Choose Correct Answers
1.     Mold can be made of
    (a) Metal    (b) Sand        (c) Plaster     (d) All of these
2.     Chills in a casting are used to
    (a) Increase melting point        (b) Increase freezing point
    (c) Obtain directional solidification    (d) None of these
3.     Higher shrinkage allowance is required for
    (a) Aluminum            (b) Bronze    (c) Mild steel    (d) Grey cast iron
4.     Which is the best material for pattern making
    (a) Bronze     (b) Brass    (c) Cast iron    (d) Aluminum
5.     Which of the following is not an element of a gating system
    (a) Pouring basin/cup    (b) Riser     (c) Sprue    (d) Runner      (e) Drag
6.     In a cupola, Tuyeres are used to
    (a) Supply air in the cupola            (b) Feed the charge inside the cupola
    (c) Discharge the waste products from cupola    (d) None of these
7.     Hot chamber die casting is suitable for
    (a) Zinc, Tin and Lead base alloys   
    (b) Aluminum, Magnesium and Copper base
    (c) Both (a) and (b)            (d) None of these
8.    Moulding sand is called green sand because it contains.                                           (AKTU - 2009-10)
    (a) bakelite    (b) moisture    (c) no moisture        (d) dry sand
9.    Blow holes are the common defects found in                                                        (AKKTU - 2008-09)
    (a) Welding    (b) machining    (c) Forming        (d) Casting.

[II] Fill in the blanks
10.     A pattern is defined as .......................... of the desired casting, which, when packed in a molding material produces a cavity called ..........................
11.     Shrinkage allowance takes care of .......................... of metals during solidification of casting.
12.     Foundry sand containing moisture is called ..........................
13.     Centrifugal casting makes use of a .......................... mould.
14.     Hole is produced in casting with the help of .......................                                (AKTU - 2009 - 10)
15.     Core is used to produce .............................. in casting.                                        (AKTU - 2008-09)
16.  Allowance provided on surfaces parallel to direction of withdrawn in casting is known ..........................                                                                                                          (AKTU - 2008-09)
17.  Shrinkage allowance on master pattern is ...............                                                (AKTU - 2010-11)
[III] True or False Statements
18.     Metallic patterns are cast from wooden pattern.
19.     Sand molds are used in permanent mold casting.
20.     Die castings have less defects than sand casting.
21.     Bottom gates are used for small sized castings.
22.     The castability of Al is very poor.                                                                         (AKTU - 2008-09)

Answer - 

1. (d)    2. (c)    3. (d)    4. (d)    5. (e)    6. (a)    7. (a)    8. (b)        9. (d)
10. replica, mold    11. Contraction or shrinkage    12. green sand
13. rotating    14. core        15.  A cavity of desired shape & size
16. Shrinkage allowance        17. double   
18. (T)    19. (F)   
20. (T)    21. (F)    22. (F)