Economic disadvantage of expendable mould casting is that a new mould is required for every casting. The basic difference between sand casting and permanent mould casting is the use of metal (or permanent) mould, that is, the mould is not destroyed as in case of sand casting process and the same mould can be reused many times after cleaning for producing another casting.

The processes include:
1. Basic permanent mould casting (Gravity Die Casting)
2. Pressure Die casting
3. Centrifugal casting.

In permanent mould casting process, permanent moulds are used and the castings are made by pouring molten metal into the mould under the force of gravity. The process is normally used for cast iron and, occasionally, for a nonferrous casting. The permanent moulds are made from highly refractory materials (due to the very high pouring temperatures) or metal (steel or cast iron) and given a refractory coating. The mould is provided with fins on its outer surface for efficient air-cooling. The inner surface of the mould cavity is sprayed with an oil-carbon-silica mixture before each pouring. Typically, permanent moulds constructed of two parts, similar to cope and drag as in case of sand casting and designed for easy, precise opening and closing. Permanent mould castings usually have better mechanical properties than sand castings because solidification is more rapid. Permanent mould casting process ensures product of good quality, uniform mechanical properties and better surface finish. Metal moulds usually are made of high-alloy iron or steel and have a production life of up to 1,20,000 or more castings. Typical products made by this process include kitchenware, automobile pistons, pump bodies, and certain castings for aircraft and missiles. A typical permanent mould is shown in Figure 1 (a-d).

(a) permanent two piece mould is preheated and coated with silicon spray


(b) cores (if used) are inserted and mould is closed, (c) molten metal is poured into the mould, where it solidifies


(d) Movable mould half section is moved back and the casting is obtained

Figure 1: Steps in typical Permanent Mould Casting

Die-casting is the process of making castings by using permanent moulds called die. (You should not confuse the term die used here with the die used in forging, extrusion and drawing processes.) This process is used for casting a low melting temperature material, e.g., aluminium and zinc alloys. The die used for diecastings is very much similar to mould. Designed to hold and accurately close two mould halves and keep them closed while liquid metal is forced into cavity. Die-casting is similar to permanent mould casting except that the metal is injected into the mould under high pressure of 10-210 MPa instead being fed by gravity. Pressure is maintained during solidification, then mould is opened and part is removed. Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mould processes. This result in a more uniform structure, good surface finish and good dimensional accuracy. For many parts, post-machining can be totally eliminated, or very light machining may be required to bring dimensions to size. The process, when applied to a plastic casting, is called injection moulding.

Die casting process can be classified into two types viz.
1 Hot chamber die-casting process, and
2 Cold chamber die-casting process

8.3.1 Hot Chamber Die-Casting Process
The hot chamber die-casting process is shown in Figure 2 . When the piston is in the extreme left position, molten material in the hot chamber around the cylinder fills the cylinder (Figure 2 (a)).

(a) Cycle in hot-chamber casting - with die closed and plunger withdrawn,
molten metal flows into the cylinder


(b) Cycle in hot-chamber pressure die casting - piston forces metal in cylinder to flow into die (mould),
maintaining pressure during cooling and solidification


(c) Cycle in hot chamber pressure die casting - piston is withdrawn back, movable die slides forward,
casted product is ejected out by ejector pins

Figure 2: Hot chamber Pressure Die-casting process

(a) Cycle in cold-chamber casting - with die (mould) closed and piston withdrawn,
molten metal is poured into the chamber/ cylinder


(b) Cycle in cold-chamber casting - piston forces metal to flow into die (mould),
maintaining pressure during cooling and solidification



Figure 3: Cold chamber die-casting process

A family of casting processes in which the mould is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity. When a body rotates about an axis a centrifugal force is created, this centrifugal force is used in getting the shape of the final casting. Typical centrifugal casting equipment is shown in Figure 8. In centrifugal casting, a permanent metal mould revolves at very high speeds in a horizontal, vertical or inclined position and the molten metal is poured. The poured molten metal is thrown on the walls of the mould cavity by centrifugal force. Centrifugal force is utilized to distribute liquid metal over the inner surfaces (walls) of the mould. The metal solidifies in the form of a hollow cylinder. Centrifugal castings can be made in almost any required length, thickness and diameter. The cylinder wall thickness is controlled by the amount of liquid metal poured. The centrifugal force of this process keeps the casting hollow, eliminating the need for cores. Using centrifugal casting process it is possible to produce castings of good quality, dimensional accuracy and external surface detail. Typical examples of the products manufactured by this process include pipes, gun barrels, bushings, flywheel, pressure vessels.

Note:
1. For producing a hollow part, the axis of rotation is placed at the centre of the desired casting.
2. The speed of rotation is maintained high so as to produce a centripetal acceleration of the order of 60g to 15g.
3. The centrifuge action segregates the less dense nonmetallic inclusions near the centre of rotation. It should be noted that the casting of hollow parts needs no core in this process.
4. Solid parts can also be cast by this process by placing the entire mould cavity on one side of the axis of rotation.

2.4.1 Advantages of Centrifugal Casting Process
1. The castings, produced by this method, are very dense.
2. By having several mould cavities, more than one casting can be made simultaneously.
3. Hollow sections can be obtained without using Cores.

(a) Setup for Horizontal or True Centrifugal casting process


(b) Casted Pipe obtained from true centrifugal casting process (axi-symmetric and fully hollow)




(c) Inclined Centrifugal Casting Process (i) Metal is being poured into the rotating mould and metal is
getting slashed along the walls (ii) Metal solidified along the walls providing the hollow casting


(d) Different shapes that can be obtained from various centrifugal casting process,
(i) semi centrifugal casting process and (ii) Centrifuge Casting Process

Figure 4: Centrifugal casting process

Slush Casting is a special type of permanent mould casting, where the molten metal is not allowed to completely solidify. A slush casting is produced by pouring the liquid material into an open-top permanent mould, after the desired wall thickness is obtained, the mould is inverted therefore, not yet solidified molten metal is drained out (Figure-5). This is useful for making shell-like ornamental objects from low melting point metals such as zinc, tin or lead alloys, for example, candlesticks, lamps, lamp base, toys, statues, masks. Low-meltingpoint metals such as lead, zinc, and tin are used. The exterior appearance is important, but the strength and interior geometry of the casting are minor considerations.

                                     
Figure 5: Images depicting steps in Slush Casting Process

Continuous casting process can be used for producing castings of large length with a uniform cross-section. In this process continuous pouring of molten metal into a horizontal or vertical mould (pouring die) opened at both ends, cooling it there rapidly till metal solidifies and removing the solidified product in a continuous form from the other end of the mould (Figure 6). Cooling jacket are made, sometimes, integral with the furnace. It also incorporates an integrated valve to stop the metal flow into the mould when desired. The molten metal flows into the mould from below the metal surface so that impurities are not included in it. As it flows down it is cooled rapidly by quick dissipation of heat by the circulating water in the jacket around the metal mould. Withdrawing rolls below the mould help in pulling down the solidified casting at a controlled speed. A saw is fitted to cut the casting into desired lengths. The shapes that can be produced by this process include square, rectangular, hexagonal etc. The process is used now-a-days to produce blooms, billets, slabs and tubings of very long lengths directly from molten metal in steel plants replacing the traditional casting of ingots in separate moulds and later convert them into blooms or billets. The process of continuous casting is largely used for casting steel, brass, copper, bronze, aluminium, grey cast iron and alloy cast iron.

Figure 6: Continuous Casting Process