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The Production of Castings | Moulding and Casting Processes A Brief History of Metal Forming Initially metals were used for adornment – combs, necklaces and bracelets for example, which were made from gold and silver, the easiest of metals to extract from their ores. Then he discovered the more common and useful copper, which he learned to harden by alloying it with other metals to form bronze. The Greeks and the Romans in the centuries immediately before Christ became the first true masters of the art of casting metal; their beautiful bronze sculptures, coins, ornaments and utensils are the legacies we continue to admire. Last of all came iron, the most common metal but the most difficult to work, Unlike the other metals whose relatively low melting points allowed them to be cast in moulds, iron had to be forged, that is heated to make it soft and then beaten into the necessary shapes. It was to be several hundred years before the discovery of a method of attaining temperatures sufficiently high to keep iron molten long enough to cast into shapes, Nevertheless the iron age was truly the beginning of history. When man learned to shape this metal he began to transform life. For the first time he had made himself materials that could perform miracles in providing tools, utensils and weapons and, in the distant future, machines. In 1709 Abraham Darby developed coke at Coalbrookdale, very often regarded as the birthplace of the industrial revolution. In fact if it were not for coal and the skill of iron casting it may not have been a revolution – just a prolonged evolution. It was the coke used to smelt iron that gave the great inventors the material from which they could make their machines, the most revolutionary being the steam engine developed by James Watt in the 1760’s. In 1780 the world saw its first iron bridge made from cast material; then over the turn of the century the great iron works really came into their own as steam was harnessed to power iron locomotives and ships. Thousands of miles of cast iron railways were laid and scores of iron bridges were built across rivers and straits. Steel, which had been made in relatively small amounts since 1733 when Benjamin Huntsman developed his crucible process, suddenly became freely available when Sir Henry Bessemer invented his famous converter in 1855 which made large scale production possible. Electric motors, motor cars, diesel engines, underground railways, photography, the wireless, the computer; all appeared in this most inventive of centuries, On the domestic front vacuum cleaners, water closets, bicycles and scores of other products were invented. And the men of metal contributed their skills to them all. The twentieth century opened up with the first successful powered flight and the introduction of the mass production of motor cars. Techniques of metal casting changed to cater for the enormous demands of these new industries and more so when, within a few years, both modes of transport were being used in the first all-metal war between 1914 and 1918. Aluminium, introduced to the metal industry in 1909, provided the strong lightweight material that was to satisfy man’s long time universal yearning to fly. Ultimately its derivatives opened up the very heavens for him. Space exploration commenced in the 1950’s, man landed on the moon in 1969 and countless satellites and space probes were projected into the universe. Once again the art of metal moulding came into its own as an essential part of the aerospace industry. Today the casting industry contributes to information technology, food production, telecommunications, nuclear power and world finance. It can be confidently predicted that as the human race continues its progress, whatever new materials are utilised, the casting industry will be there to serve it. Whoever enters the metal casting business today will doubtless help to take it into its seventh millennium as possibly man’s oldest and most essential skill. DESIGN Designers need a wealth of information before they reach for their set squares, or more likely these days, turn to their computers. The specified shape and size of the final product is obviously needed but with metal casting the designer needs to know what stresses and conditions the products will have to withstand so that the correct metal can be chosen. He will need to know how many castings are needed, too. All these factors dictate which moulding techniques are chosen. PATTERNMAKING With one production technique, wax is used to form the pattern, (see Investment Casting). Patterns must be precise in their shape and finish, for any mistakes are reproduced in the moulds which are made from them and from which the final castings are formed. They must be made to allow for the shrinkage of the metal when it cools and they can include channels to allow metal to flow into the casting shape. From the initial pattern a prototype or production sample is usually made with which the customer can experiment to ensure that the final casting will be exactly as required. MOULDMAKING Moulds are usually made in at least two parts and for very large castings they may even start out as large holes dug into the sand floor of the foundry. Different types of sand are used for moulding with additives like water and clay and various chemicals, depending on the size of the mould and the types of metal that are being cast. One important feature of the mould is the running system which is a network of small channels that leads the molten metal down into the casting shape. The shapes and sizes of these channels have to be carefully calculated to ensure that the molten metal does not solidify before it gets to the casting shape and to make sure that it does not flow too fast when it could wear away the mould. Many castings are designed to have cavities in them – engine blocks, for example. These voids, which have to be as accurate as the outer moulds, are made by forming their shape in moulding material. The shapes, or cores as they are known, are placed in the mould and after the molten metal has solidified, the core material is removed leaving a precisely shaped cavity behind. CASTING When the casting has solidified and cooled, it is knocked out of the mould. Superfluous metal such as that which has solidified in the flow channels is removed – this clean up operation is known as fettling. Grinding and often shot blasting is then used to produce a clean finish. Some castings may also go through a series of tests, such as x-raying or pressure testing to ensure that they do not contain any unwanted cracks or flaws. The metal may also be tested to check, amongst other things, its strength, its resistance to sudden knocks, chemicals or high temperatures. This is really the last step in the casting process but many castings require some further shaping or finishing before becoming the final engineering component. This can involve any or all of the engineering machining processes including drilling, and turning to produce the exact dimensions and features required. The casting can then be assembled with other components, often other castings, this becomes another engineering product on which modern society depends for its economic wealth and life style. Moulding and Casting Processes Moulding is the process of making the hollow shape into which molten metal is poured to produce a casting. There are many different moulding and casting methods but basically they are all variations of the processes outlined below. The V Process employs a different technique which is explained below. Guides to...| Sand Casting | Shell Moulding | Die Casting | Investment Casting Sand moulding is probably the most basic of all moulding methods. Essentially it is the process described previously i.e. loose sand is packed around a pattern to form a hollow shape which, when filled with molten metal, becomes a casting. Additional process information.
Shell moulding is a variation of sand moulding in which the mould is formed of a thin layer or shell of a special sand. The shell is formed by coating a hot metal pattern with resin impregnated sand. The heat melts the resin which then holds the grains of sand together forming a shell. Shell mouldings are invariably machine made and the process is used when a large number of one type of casting is needed. It is also one of the methods used to make cores when the same type of sand is blown into a metal die the same shape as the core. Shell moulds have to be supported when molten metal is poured into them since they quickly collapse with the high temperature. But this disintegration is useful, particularly in mass production, as it minimises the costs of removing the castings from the moulds. The high precision metal patterns used in shell moulding are expensive but they are durable and reduce the unit costs of the moulds when produced in large numbers. They also allow better control of shape and size than conventional sand casting and produce castings with a smoother surface which reduces the cost of finishing.
This type of casting differs from other methods in that the mould is made of a durable material, usually heat resistant metal, and can be used many times. The cost of producing a complex die is very high, but similar to the shell moulding method, large numbers of accurate castings can be made so that individual casting costs are low. Costs of finishing are also reduced because die casting produces castings that are accurate and very smooth. Because a metal die is used it is confined to relatively small castings and to low melting point metals such as aluminium and zinc alloys. These find very many applications in everyday life particularly in motor cars and domestic appliances. In the simplest systems of die casting, molten metal is poured into the mould and allowed to solidify. The die is then opened, the casting removed and the die re-closed to receive more molten metal to start the cycle over again. Often the die is set into a machine and the whole process is controlled automatically, including the addition of molten metal.
was invented during 1971 in Japan. A thin heated plastic film is placed over the pattern half. A vacuum draws the film tightly around the pattern profile which is then surrounded by a flask/moulding box. The flask is then filled with dry, un-bonded fine sand. The sand is tightly packed around the pattern by vibrating the flask A second sheet of film is placed on the back of the flask, a vacuum draws out any remaining air and the completed mould half is then stripped from the pattern. The mould halves are made the same way. Still under a vacuum to retain the mould shape, the mould halves are closed together. The molten alloy is then poured directly into the mould cavity. Once the casting has solidified the vacuum is released and the sand and completed casting fall free. Note: Products that can be produced by the V Process are limited by the film's physical limitation. The film can only stretch as far vertically as it is spaced horizontally. For example, the vertical distance between features can be no greater than then height difference between the features. i.e. one-to-one ratio. Process Capabilities
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1. The pattern, in this example made of
wood, is similar in shape to the finished casting
2. The pattern is packed in sand in a
moulding box and removed to leave a cavity the shape of the casting.
Channels are cut into the sand (running system) to allow the molten
metal to flow into this cavity
3. The complete mould is assembled and
molten metal poured in
4. After cooling, the sand and the casting
are knocked out of the box. The casting maintains the shapes of
the channels that carried the molten metal attached to it
5. Extraneous pieces are cut off and
the casting ground smooth using abrasive wheels. Often more finishing,
such as machining and testing takes place
6. The finished product, a cast iron
grate, in position in the road
1. A pattern is made
in metal, usually in at least two pieces
2. The separate pieces
are mounted on a flat metal backing plate and extra pieces added
which form the channels through which the molten metal flows
3. The heated pattern
plate is placed on top of a box containing special resin sand
4. The
box and pattern are inverted so that the resin sand covers the
hot plate
5. When the box and
pattern are returned to their original positions a thin ’shell’
of sand stays on the pattern. Normally this takes place automatically
on a shell moulding machine
6. The shell is removed
and the two matching shells are glued or clamped together to form
a mould
7. Molten metal is
poured into the completed mould
8. The finished casting,
including the shape of the channels that carried the molten metal,
is removed from the mould
9. The extra pieces of metal
are removed and the casting machined and ground to the finished
size
10. The finished
casting in place, in this case a crankshaft in a 2 cylinder engine
1. A ’die’ is made
by machining an accurately shaped cavity in a block of heat resisting
metal. The die is in two halves so that the finished casting can
be removed
2. The die
also has a hole and channels through which the metal flows. It
may have various rods and pegs to form internal holes in the casting
3. The parts of the die are clamped
together and molten metal poured in
4. The die
is opened and the casting removed. The casting is the finished
size and shape except for the extra pieces where the molten metal
was poured in
5. The extra pieces are
removed and the casting ground smooth
6. Some finishing
machining may be necessary, in this example screw threads are
being cut at the ends
7. The finished product, a brass ’T’
junction in domestic pipework