Cores and Furnaces

CORES

A core can be defined as a body of sand which is used to form a cavity of desired shape and size in a casting. Core are prepared separately in core boxes.

Types of cores:

Horizontal core: 

It is most common and simple type of core. It is assembled in the mold with its axis horizontal. It is supported in the mold at its both ends.

Vertical core: 

It is quite similar to a horizontal core except that it is fitted in the mold with its axis vertical.

Balanced core: 

It is used to produce a blind hole along a horizontal axis in a casting. As a matter of fact it is nothing but a horizontal core, with the exception that it is supported only on one end, the other end remaining free in the mold cavity.

Hanging or cover core: 

A core which hangs vertically in the mold and has no support at its bottom is known as hanging core. In such case it is obvious that the entire mold cavity will be contained in drag only.

MELTING FURNACES

A melting furnace is a very necessary equipment in foundry shop. It is used to melt the metal to be casted.

TYPES OF FURNACES

The main types of furnaces used in foundries for melting of various varieties of ferrous and non-ferrous metals and alloys are described as:

Crucible furnaces: 

These are the simplest of all the furnaces used in foundries. They are used in most of the small foundries where melting is not continuous and a large variety of metal is to be melted in small quantities. In these furnaces the entire melting of metal takes place inside a melting pot called crucible, which is made of clay and graphite. These furnaces can be classified as:

a. Coke fired furnaces: 

These furnaces are generally installed in a formed pit and are used for melting small quantities of ferrous metals for producing iron casting and also non-ferrous metals and alloys. They are provided with refractory lining inside and a chimney at the top. Coke is used as fuel. Broken pieces of metal are placed in the crucible. Bed coke is fired in the furnace and the crucible placed into it. Afterwards more coke is placed all around the crucible.

b. Oil and gas fired furnaces: 

These furnaces utilize oil or gas as a fuel. In fact a mixture of gas and air or oil and air is fed into the furnace which burns inside to produce the desired temperature. The furnace essentially consists of cylindrical steel shell, provided with refractory lining inside and proper passage for entry of the fuel mixture. The crucible is seated on a pad formed at the bottom.

Cupola furnace: 

For melting of cast iron in foundry the cupola furnace is used. It has a construction in the form of a hollow vertical cylinder made of strong mild steel plates. The kindling material, generally soft and dry piece of wood, is first placed over the sand bed followed by a small amount of coke charge known as bed charge. 

The coke for this charge is put in the furnace through the charging door. The kindling material is ignited through the hole. This fire spreads slowly into the coke around the kindling material. 
Additional coke is fired until the bed charge acquires the required height. Cover plated opposite the tuyeres are opened to allow the free entry of air to aid combustion and they are left open till the entire bed charge is fully ignited. A carefully weighted proportionate amount of metal, pig iron, scrap and flux is then fired over the bed charge followed by a weighed quantity of coke. 
They are repeated in alternate layers, of course a predetermined quantity of each, until the cupola is full to the charging door. If the cupola, on account of its fixed capacity, is unable to take up the entire material to be melted at a time, the remainder is fed into it after the initial charge has been melted.

ADVANTAGES OF USING CUPOLA

1. The initial cost is comparatively lower than other types of furnaces of same capacity.
2. Operation and maintenance of the furnace does not involve too many complications.
3. Cost of operation and maintenance are comparatively lower.
4. The floor area required is hardly a fraction of that required for other furnaces of similar capacity.
5. It can be operated for a number of hours at a stretch.
6. It does not involve very complicated problems in its design which is comparatively simpler.

PRECAUTIONS IN OPERATING THE CUPOLA

1. A superior refractory lining should be used to withstand high temperature produce inside the furnace.
2. The man who fires the coke and charge should place the metal charge in the center.
3. The molten metal should be tapped out well before its level rises too high in the well.
4. The tap hole should be properly closed by means of well prepared clay bott or plug.
5. In closing the tap hole care should be taken to press the plug downwards in the hole so that the splash of the molten metal does not fall on the hands.
6. The amount of air supply should be properly controlled. An excess amount of air will result in lowering of temperature inside.