• The refractory compound will provide a smoother surface on the pattern and to improve its high temperature resistance.
• Moulding sands usually include bonding agents.
• This process is also called as Lost-Foam process (or) Lost Pattern Process (or) Evaporative-Foam Process.
• The foam pattern includes risers, sprue, gating system, internal core.
• Parting lines and draft considerations are reduced.
In this process, a sand mold is held together by vacuum pressure and not by a chemical binder.
The term vacuum in this process refers to the making of the mould, rather than the casting operation. Casting operation is same as any other process.
Advantages :
(1) No binders are used and hence sand is readily recovered in vacuum molding.
(2) Mechanical ramming is not required.
(3) Since no water is mixed with the sand, moisture related defects are absent from the product.
Disadvantages :
(1) Relatively slow and not readily adaptable to mechanization.
For casting, metal has to be heated above its melting point. The heating is done in a furnace. Depending upon the fuel used, the furnace may be classified as Electric, Oil fired (or) Coal fired etc., Where metal free from any impurities is required, electric furnaces are used. In oil fired and coal fired furnaces, the flame plays upon the hot metal and the molten metal picks up impurities by coming in contact with flames. Electric furnaces are costly and equally costly to operate. Usually, for non-ferrous metals and alloys, oil fired crucible furnaces are used. The metal is placed in large graphite crucibles and heated on the outside surface of crucibles, so that flames do not come in actual contact with metal.
CONSTRUCTION:
For melting cast iron, a cupola furnace is used. It is one of the most economical and convenient ways of providing a supply of molten cast iron. Cupola uses coke as fuel. Coke is produced by heating ordinary steam coal in an inert atmosphere. It gives more intense heat than coal.
Cupola consists of a long cylindrical steel shell with its interior lined with refractory fire-bricks. It is erected vertically up and rests on short pillars about 0.85 meters above the ground level. The bottom of the cupola is provided with steel doors which are also lined with fire resistant material and covered with a layer of good quality sand.
At a suitable height, near the top of this steel shell, an opening is cut, which is used for charging fuel and raw materials into the furnace. A wind box, connected to a motorized blower is provided at a height of one meter or so above the bottom closing doors. From this wind box, small air passages are provided into the cupola shell for supply of air to aid combustion of fuel. These air passages are called Tuyeres. At the bottom, above the door, a tapping spout is provided to tap molten metal and about 350 mm above the tapping hole another hole called the Slag Hole is provided at the back of the cupola, through which liquid slag can be forced out under air pressure.
The cylindrical space provided between the tap hole and slag hole is called the molten metal well, in which the molten metal accumulates between two-taps. All these features are depicted in below Fig.,
OPERATION OF CUPOLA
In the cupola the first operation is to repair the lining of the door and area around it and then closing the hinged doors. Doors are jammed shut, so that there is no possibility of their opening while the cupola is in operation. Then fire is lighted at the bottom with the help of some wood and kerosene oil rags. When the fire is burning strongly, coke is added from the top charging door until the height of coke bed reaches about half a meter above Tuyeres. After those tuyeres are opened partially, air blower is switched on and alternate layers of metal, flux (in the form of limestone pieces) and coke are charged from the top. These alternate layers settle down on coke bed. When the cupola is filled up to the level of charging door, the tuyeres are opened fully and the heating of charge begins. The coke near the tuyeres start burning and the coke bed becomes intensely hot. The metal in lower layers near the coke bed starts melting. Lime stone breaks down to CaO and CO2. The calcium oxide reacts with impurities like silica and other oxides forming slag (CaSiO3). Slag is lighter and floats upon the molten metal layer. Ultimately. when enough metal has melted, the slag is blown out by opening the slag hole. The metal is then tapped by puncturing the tap hole with a long steel rod with one end shaped like a cone. The molten metal will start flowing into the metal chute and is collected into ladles (refractory lined steel buckets to which long handles are welded) and taken away for pouring into mould's. The tap hole id then closed by plugging it with a lump of fire clay.
The properties of cast iron improves with addition of small amounts of Ferro-Manganese and Ferro-Silicon. Since most of the manganese and silicon already present in scrap cast iron, pig iron and a little bit of thin steel scrap, which forms the metallic charge dropped into cupola, is oxidized and lost, addition of Ferro Manganese and Ferro Silicon has to be done to the molten metal in the ladles before pouring.
After the day's work is over, extra coke is charged into the cupola along with last charge. After all the metal has melted, the air blower is switched off and the bottom door of cupola opened. What-ever unburnt coke etc., is left, is allowed to fall to the ground beneath the cupola door. This is necessary otherwise the left over coke, slag and metal etc., may join up in one mass, then its removal will become extremely difficult. The size of a cupola is denoted by its internal diameter.
Moulding processes may be classified as Hand moulding (or) Machine moulding moulding according to whether the mould is prepared by hand tools (or) with the aid of some moulding machines. When large number of castings to be produced, hand moulding consumes more time, labour and also accuracy and uniformity in moulding varies. To overcome this difficulty, machines are used for Moulding.
Based on Ramming methods Moulding machines are classified as follows:
(1) Jolt Machine,
(2) Squeeze machine,
(3) Jolt-Squeeze machine, and
(4) Sand slinger.
(1) Jolt machine:
A Jolt machine consists of a flat table mounted on a piston cylinder arrangement and can be raised (or) lowered by means of compressed air.
In operation, the mould box with the pattern and sand is placed on the table. The table is raised to a short distance and then dropped down under the influence of gravity against a solid bed plate. The action of raising and dropping is called Jolting .
The disadvantage of this type is that, the density and hardness of the rammed sand at the top the mould box is less when compared to its bottom portion.
(2) Squeeze machine:
A Squeeze machine is very useful for Shallow patterns. In squeeze machine, the mould box with pattern and sand in it is placed on a fixed table as shown in fig., A flat plate (or) rubber diapharm is brought in contact with the upper surface of the loose sand and pressure is applied by a Pneumatically operated piston. The squeezing action of the plate causes the sand particles to get packed tightly above and around the pattern.
Squeezing is continued until the mould attains the desired density. In some machines, the squeeze plate may be stationary with the mould box moving upwards.
Moulding force (Mf) = P (π. d2/4)-W
Where P - Pressure in squeezing cylinder,
d - Piston diameter
w - Weight of flask pattern and sand.
(3) Jolt squeeze machine:
Jolt-Squeeze machine combines the operating principles of 'Jolt' and 'Squeeze' machines resulting in uniform ramming of the sand in all portions of the moulds.
The machine makes use of a match plate pattern placed between the cope and drag box. The whole assembly is placed on the table with the drag box on it. The table is actuated by two pistons in air cylinders, one inside and the other. One piston called as Jolt piston rises and drops the table repeatedly for a predetermined number of times, while the other piston called Squeeze piston pushes the table upwards to squeeze the sand in the flask against the squeeze plate. In operation, sand is filled in the drag box and jolted repeatedly by operating the Jolt piston.
(4) Sand slinger:
A Sand slinger is an automatic machine equipped with a unit that throws sand rapidly and with great force into the mould box. The figure shows a sand slinger. The sand slinger consists of a rigid base, a sand bin, a bucket elevator, a swinging (or) movable arm, a belt conveyor and the sand impellor (ramming head). Prepared sand lying in the sand bin is packed up by the elevator buckets and is dropped on to the belt conveyor which takes the same to the impellor head.
The sand is discharged into the moulding box at a rate of 300 to 2000 kg/min. This force is great enough to ram the mould satisfactorily. The density of the ramming sand can be controlled by varying the speed of the impellor. Rest of the operations, viz., removal of pattern, cutting gates etc., are done manually.
In moulding boxes, sand is filled and rammed at the same time. The density of sand which is the result of sand's inertia is uniform throughout the mould. In the initial stages of ramming, the blades are rotated at slow speeds, around 1000-2000 rpm to avoid damage to the pattern due to the abrasive action of the high velocity sand particles.
