The moulding and casting process

Sadly very few working foundries still survive in the UK and the once familiar moulding and casting skills are rapidly disappearing. The manufacturing processes and terminology were well known, but are now alien to many people. The following section describes the processes that go into producing a casting from beginning to end.

An early type of cupola.

The molten iron is usually produced in a cupola furnace. This is a vertical cylindrical steel shell with a “well” at the bottom to collect the molten metal.

The inside can be made of fire bricks, but is normally constructed of steel, with a water jacket for cooling and lined with clay. The well at the bottom is lined with sand and the furnace is charged through a door at the top with pig iron or scrap iron, coke and limestone.

Air is necessary for combustion and this is blown into the furnace through several tuyeres (blast holes) near the base. There are cold blast cupolas where the air enters at atmospheric pressure and hot blast cupolas where the air is preheated by the heat from the exhaust gases. This type of cupola has an increased melting rate, produces hotter metal, and uses less coke.

The coke burns fiercely thanks to the air blast and the metal soon melts. The limestone acts as a flux, reacting with the sand and ash etc. to form slag, which floats on top of the molten metal. This is removed through a slag hole below the tuyeres.

The cupola extends above the charging door to produce a chimney where the exhaust gases are removed and cleaned before entering the atmosphere. The coke can be fired directly to start the combustion process, but electricity, oil or gas are often used with carbon being added to the charge.

A more modern cupola.

Cupolas at Hermit Industries in Upper Gornal.

Iron is usually cast in sand into which an impression of the item to be cast is made. The impression is made by a pattern, which is produced from the original drawing and is slightly larger than the final casting. Patterns were originally produced in wood, with several special types being used including jelutong from the Philippines, cherry wood, mahogany, maple, and white pine. Pattern making is an extremely skilful occupation; the patterns must be precisely shaped and accurately finished.

As the molten metal cools it shrinks, and to allow for this the pattern must be larger than the finished product. Pattern maker’s rules are used, which are scaled to allow for shrinkage in various metals. Although traditionally made of wood, many patterns today are made in resin that is shaped on a milling machine. The machines can be computer controlled and work directly from the original drawing data. In the past, every foundry had at least one pattern maker but today many buy their patterns from specialist companies.

If the casting is hollow, a core is inserted in the mould to shape the interior. Cores are made from silica sand (with a low clay content) and held together with a bonding agent such as sodium silicate, which can be bonded by passing carbon dioxide through the mixture. Such a mixture is very strong and has good dimensional stability. Cores were originally made in a core box but today they are usually formed by machine.

The mould is made in two halves, one forming the bottom and the other the top. Each half consists of a box into which sand is tightly packed around the pattern. When the pattern is removed its impression is left in the sand and this forms the mould.

A core box and filling machine.

Different types of fine sand are used with chemical additives that bind the sand together to give an accurate and detailed impression of the pattern. The sand is recycled and soon blackens from its contact with the molten iron.

The bottom half of a moulding box with 2 cores in place.

When the two halves of the mould have been prepared and any cores inserted they are fastened together in preparation for casting.

The mould will include a network of small flow channels that lead the metal into all of the corners of the impression in the sand.

Provision is also made for the escape of the hot gasses that are released when the molten metal enters the mould.

When the casting has solidified and cooled it is knocked out of the mould and taken to be dressed (cleaned up). Metal will have solidified in the flow channels and this will be attached to the casting and must be removed. Any extraneous pieces are cut off and ground flat using grinding wheels. Shot blasting can also be used to give a clean finish. Precision castings are often accurately machined and may be tested for unwanted cracks or flaws.

Crane Foundry products are made of grey cast iron, in which silicon causes the carbon to come out of the molten solution as graphite, to leave a matrix of pure soft iron. This type of iron produces sharp castings and has a high resistance to corrosion and wear. It also has a good thermal conductivity and so is ideal for saucepans and casseroles etc. and is easy to machine.

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