CASTING PROCESS

Centrifugal casting is done by pouring molten metal into a rotating mould. The centrifugal force acting on the mould helps in feeding and positioning the metal in the mould. Mould rotation is continued till after the metal is solidified.

Centrifugal casting results in denser and cleaner metal as heavier metal is thrown to parts of the mould away from the centre of rotation and the lighter impurities like slag, oxides and inclusion are squeezed out to the centre.

The castings produced have a close grain structure, good detail, high density and superior mechanical properties. Elaborate gating and risering systems are not required as very simple systems will do the job. There is also a considerable saving of material. 

 

Types of centrifugal casting:

Centrifugal casting can be divided into three categories namely true centrifugal casting, semi centrifugal casting and centrifuging.

True centrifugal casting:

The true centrifugal method of casting is used to produce hollow castings with a round hole. The characteristic feature of this process is that the hole is produced by the centrifugal force alone and no cores are used.

The mould is rotated about the axis of the hole with the axis held horizontal, inclined or vertical. The outside surface of the job may be round, square, hexagonal etc. and should be symmetrical with the whole axis. The central hole should be round to be formed without cores.

Long castings like cast iron soil pipes are cast with the moulds rotated about a horizontal axis. Castings with relatively short lengths are poured with moulds rotated about an inclined or vertical axis. Rotation about the vertical or inclined axis is convenient but the central hole produced will be slightly parabolic with smaller diameter at the bottom because the metal has a tendency to settle down due to gravity. The speed of rotation for true centrifugal casting should be high enough to hold the metal on to the mould wall till it solidifies. A low speed of rotation would result in raining or slipping of the metal inside the mould. Too large a speed of rotation on the other hand may result in internal stresses and possible hot tears. A speed which would provide a centrifugal force of 60 to 75 times the force of gravity on horizontal moulds and 100 times force of gravity for vertical moulds is found to be suitable. The moulds used for the process may be metal moulds or refractory or sand lined moulds. Common products produced by true centrifugal casting include pipes, oil engine cylinders, piston ring stock, gear blank stock, bearing bushes and the like.

Semi-centrifugal casting:

In semi-centrifugal casting process no attempt is made to produce a hole without a core. The centrifugal force resulting from rotation of the mould is used to properly feed the casting to produce a close grained clean casting.

The process is suitable for large axis-symmetrical castings like gear blanks, fly wheels and track wheels. Any hole round or otherwise is made with the use of a core. The mould is clamped to a turn table with casting axis along the axis of rotation.

The metal is poured along or near the axis to feed the points farthest from the axis of rotation under pressure. If made solid the central portion tends to be porous and with inclusion which are removed in subsequent machining.

Centrifuging:

Centrifuging or centrifuge casting is employed to force metal under pressure into moulds of small castings or castings not symmetrical about any axis of rotation. The moulds are made around a central axis of rotation, to balance each other.

The metal is poured along this axis of rotation through a central sprue and made to flow into mould cavities through radial ingates cut on the mould interface. Centrifuging helps in proper feeding of castings resulting in clean, close grained castings.

Die Casting:

The die casting process uses steel dies into which metal is forced under pressure through a runner and gate to fill the dies. The pressure (70 to 5000 kg/cm²) is maintained while the casting solidifies after which the dies are separated, cores are withdrawn and the casting is ejected.

Metals and alloys that are die cast include zinc, aluminium, and magnesium, copper, lead and tin.

Typical applications of die casting process include automobile components, household appliances, railway and aircraft fittings, bath room hardware, business machines, locks, pullers and many other similar parts.

Die Casting Dies:

The dies used for die casting resemble a permanent mould. They are generally made in two parts arranged to open and close with a vertical parting. When mounted on a die casting machine one of the die halves remains stationary during operation and is called a cover die.

The other half moves for opening and closing and is called the ejector die. Die casting dies are made of special die steels which are resistant to heat checking, hammering and mechanical wear and are also dimensionally stable. Die cavities are machined to very close accuracies. Vents and overflow wells are provided in the dies for escape of air. The dies may be water cooled to speed up cooling of the casting.

Die casting machines:

A die casting cycle consists of the following steps:

(i) Closing the die halves

(ii) Clamping the die halves securely together

(iii) Forcing the liquid metal into the dies

(iv) Opening the die halves and

(v) Ejecting the casting.

Die casting machines are designed to perform all these functions. To be effective these machines should be strong and rigidly built to take up die weights and provide holding pressures against the pressure of the molten metal. The machine frame should hold the die halves rigidly in correct alignment. The die holding forces should be in excess of the maximum force developed by the molten metal to ensure leak proof joint in the dies. The die closing and locking arrangements generally used in the die casting machines include hydraulic, hydraulic mechanical or mechanical devices depending on the capacity of the machine.

Modern die casting machines are of two basic types namely

Hot Chamber Die Casting Machine:

It is also called a gooseneck machine because of the shape of the metal passage way. In this machine the melting pot, usually made of cast iron, is a part of the machine. The gooseneck containing a cylinder and metal passage way is kept immersed in the metal pot. The plunger in the gooseneck cylinder is actuated either hydraulically or pneumatically. In operation the plunger is withdrawn letting the liquid metal into the gooseneck cylinder through the port provided.

When the die halves are closed and ready for casting the plunger forces the liquid metal entrapped in the cylinder into the die through the gooseneck passage and a nozzle. After a predetermined time interval the plunger is retracted allowing the liquid metal in the gooseneck channel and nozzle to fall back into cylinder.

The die halves are opened and the solidified casting is ejected from the die. Hot chamber machines are designed to operate almost automatically and fast. A press button operation will make the machine go through a complete cycle of activities including closing the die halves, forcing the metal into the die, holding the pressure for a predetermined time, withdrawing the plunger, opening the die, ejecting the casting and stop ready for the next cycle. The operator then removes the casting, inspects the dies, gives spray lubrication to the dies and starts the next cycle. Metal injection speeds and pressures can be controlled to suit different metals and castings.

Since the melting pot plunger and cylinder of a hot chamber die casting machine are made of cast iron and cast iron reacts with metals like aluminium at elevated temperatures, only low melting-point metals can be cast by this method. There is also a limit on the maximum pressure which can be applied. Hot chamber machines are mostly operated below 14 kPa. Alloys of lead, tin and zinc are the most common metals cast by this process.

Cold chamber die casting machine:

The metal in this case is melted in a separate furnace and the required quantity of metal is ladled to the machine. A plunger operated hydraulically forces the metal into the die. Injection pressures of 28 kPa to 250 kPa are possible in cold chamber machines. The machine is semiautomatic in that after the metal is ladled into the cold chamber the rest of the operation is automatic. Hot chamber machines are made in capacities varying from 0.25 to 7.5 MN and cold chamber ones from 1 to 10 MN.