Die Casting of Metals: Meaning and Advantages | Industries | Metallurgy

In this article we will discuss about:- 1. Meaning of Die Casting 2. Die Casting Machines 3. Advantages 4. Limitations.

Meaning of Die Casting:

Die casting utilises two blocks of heat resistant metal machined to meet along the plane of the parting line and having cavities machined accurately and smoothly into each to form opposite halves of the shape to be cast around the edges of the mould. Fine vents are cut to allow air to escape as the metal enters. If air is not allowed to escape it would be trapped and produce blow holes in the casting.

In die casting, molten or semi-molten metal is either poured under gravity (gravity die casting) or is forced under high pressure (20 to 2000 kgf/cm²) into a die (mould) and pressure is maintained till solidification stage (pressure die casting). As a result, a very smooth and accurate part is obtained.

Die casting is used for mass production and is most suitable for non-ferrous metals and alloys of low fusion temperature. By this method castings of nearly all sizes can be produced economically. The process is rapid and the surface is smooth, consequently little or no finishing operation is required.

Size can be controlled, so that no machining is required. The material is dense and homogeneous and has no possibility of sand inclusions or other impurities. Thin wall can be produced and uniform thickness can be maintained.

In gravity die casting, no external force is applied other than the head of the liquid. A feeder is incorporated so that when the level of the metal in the feeder is above that at the highest point of the casting, head of liquid in feeder forces the metal into all parts of the mould cavity. The feeder is cut off after solidification. It is usually used for shorter production runs.

As metal flows under high pressure in pressure die casting, within a fraction of second the fluid alloy fills the entire die including all minute cavities, therefore, the intricate casting can be produced successfully. Also as the die is metallic, the cooling rate is high and thus mass production is possible.

Further close dimensional tolerances with good surface finish can be obtained. Due to high initial cost of die casting machine, this process will be advantageous in cases where 10000 or more pieces are to be produced, where complex shapes are desired and where tolerances matter and sections are thin. Surface quality is excellent and features such as screw threads, locating flanges etc. can be readily incorporated, as well inserts of other metals.

The high initial cost of the casting process is easily made up by the reduction or total elimination of machining and finishing costs for a component required in large numbers. The principal base metals most commonly employed in die casting are zinc, aluminium, copper magnesium, lead and tin.

Depending upon the melting temperature of alloys and their suitability for die casting, they are classified as high melting point (above 500°C) and low melting point (below 500°C) alloys. Under the low temperature category come zinc, tin and lead base alloys and these have the advantages of lower cost of production and low die maintenance cost.

Under high temperature category come aluminium and copper base alloys. These alloys create lot of difficulties in die casting due to destructive effects of high temperature. The erosion of dies increases with temperature and at high temperature the molten alloy has solvent action on the machine part and dies (aluminium has more pronounced effect).

Zinc has emerged as incorporating the best overall properties for die-casting. It is possible to obtain castings weighing 20 kg and having wall thickness of as less as 1.25 mm. A die-casting machine is a heavy, rigid machine which supports the die halves, moving one into alignment with other.

It must have sufficient strength to lock and hold the dies together under considerable pressure, so that no leakage occurs at the parting line. The machine must resist the injection pressure. Very little deflection or stretching can be allowed in any part of the machine.

Die Casting Machines:

In die casting machines, the following provisions are necessary:

(i) Die-set mechanism for opening and closing the dies,

(ii) Means of forcing the metal in the die,

(iii) Device to keep the movable die half pressed till the solidification of molten meal takes place,

(iv) Arrangement for automatic insertion and removal of cores from die casting, and

(v) Ejector pins to eject casting from the die.

The dies may be closed and locked by hydraulic means, mechanical means or mechanical toggle clamping or combinations of one or more.

Types of Die Casting Machines:

Depending upon the means for molten metal supply to the die, i.e. the location of the melting chambers, there can be two types of die casting machines.

(1) Hot chamber die casting machine (heating cham­ber is an integral part of the machine unit). It is used for casting zinc, tin, lead, and the low melting-point alloys.

(2) Cold-chamber die casting machine (heating chamber is outside the machine unit). It is used for casting aluminium, magnesium, copper-base alloys and other high- melting point non-ferrous alloys.

(3) A slight modification of the above two types gives third type of die casting machine, i.e. vacuum die casting machine.

(1) Hot Chamber Die Casting Machine:

It has further two types of arrangements:

(a) Submerged plunger type casting machine.

(b) Direct air pressure die-casting machine.

In hot-chamber die-casting machines, the metal melting unit forms an integral part of the machine. It mainly consists of a hot-chamber and a goose-neck type metal container made of cast iron. This machine is mainly used for low melting point alloys and metals like zinc, lead, etc. Metals and alloys having high melting point and those of which have affinity for iron can’t be cast by this machine, which would otherwise attack the dies and damage the machine.

In submerged plunger type machine (Fig. 3.62), the goose-neck type container always remains immersed in the metal pot. The die is made in two parts (stationary die platen and movable die platen). The molten metal from the metal container is forced inside the die with the help of a plunger submerged in the molten metal and operated hydraulically. The plunger moves inside the cylinder having a hole in its bottom.

When the plunger moves up, the molten metal comes up and fills the cylinder which is then forced into the die cavity through the nozzle in the goose-neck of the container in the down stroke of the plunger, the movable die platen also moves hydraulically and the movements of plunger and the movable die platen are synchronized such that when metal is being forced, the two die halves are in match and on solidification when plunger is moving up, the movable die platen moves away and the casting is removed. The pressure used is of the order of 100 to 150 kg/cm².

In direct air pressure type die casting machine, direct pressure (about 40 kg/cm²) is used for forcing the metal inside the die. The goose-neck container is operated by a lifting mechanism. Initially it is submerged in the molten metal and is filled by gravity. Then it is raised so as to bring the nozzle in contact with the die opening and is locked in that position.

Compressed air then forces the metal into the die and pressure is maintained till solidification. When solidification is complete, the goose-neck is lowered down and casting is removed by ejector pins after opening the dies and withdrawing the cores, if any.

Metal injection speed and pressures are controllable to suit different metals and castings for fast operation. To obtain uniformity, a predetermined and automatically controlled cycle for various operations should be used.

Hot Chamber:

Hot chamber machine is primarily used for zinc with an injection pressure of 1500 kg/cm². Alloys of zinc, tin and lead are particularly recommended for these machines, because they do not have affinity of iron. Hot chamber machines are used with low melting alloys because of the increased corrosion of the machine parts.

(2) Cold Chamber Die Casting Machine:

This machine is used for casting alloys which require high pressures and have high melting temperatures such as brass, aluminium and magnesium. In this machine the metal melting unit is not an integral part of the machine and metals are melted in a self-contained pot in an auxiliary furnace.

The molten metal is ladled in the plunger cavity next to the dies and forced into the die cavity by a hydraulically operated plunger and pressure is maintained till solidification. These machines can either have vertical plunger or horizontal plunger for forcing molten metal into die and are built very strong and rigid to withstand the heavy pressures. The process of casting mainly consists of four steps. (Refer Fig. 3.64).

(i) Pouring the molten metal below the plunger and placing the moveable platen and cores in position;

(ii) Forcing the molten metal into the dies by means of plunger;

(iii) Withdrawing of cores and opening dies;

(iv) Ejecting the casting from moveable dies platen.

In actual die casting machines, the problem of rapid oxidation of the steel dies at high temperatures is overcome by using better die materials, circulating water through plates adjacent to the dies and using metal in semi-liquid or plastic state so that lower temperatures are encountered.

An advantage of cold die casting machine is that even semi- molten metal can be easily cast because the pressure applied on metal is very high and also the casting produced will be very dense and defects likes blow holes and porosity are eliminated.

(3) Vacuum Die Casting Machine:

In ordinary hot and cold chamber die casting machines, some air is entrapped inside the die during the producers of casting which produces major defects in castings like blow holes etc. To overcome this difficulty vacuum die casting machine is used in which a vacuum pump creates vacuum in the die cavity.

A seal is used which cuts off the vacuum pipe connection to die after proper vacuum is attained in die so that the metal does not flow from die to the vacuum pipe. Also due to vacuum, the metal flows quickly and automatically in this machine.

Finishes:

All die castings are somewhat susceptible to corrosion, especial in moist or salty atmospheres. The surface pitting which appears may not be serious but it may be undesirable, so that a chemical treatment is necessary to inhibit corrosion.

The protect the surface, it may be given any one of the large number of finishing operations described in Chapter 8. These also improve the appearance of casting for long periods. Electroplating nickel followed by chrome is most widely used and considered to be most satisfactory.

Advantages of Die Casting:

(i) It requires less floor space than is required by other casting processes.

(ii) Die casting provides for precision manufacture with a subsequent reduction in machining cost. Close dimen­sional control (tolerance of ± 0.07 mm for zinc upto 25 mm size and ± 0.07 mm for each additional 25 mm) is possible.

(iii) Thin sections of complex shape are possible by die casting. Under favourable conditions, minimum thickness that can be cast is 0.5 mm for zinc and 0.9 mm for aluminium.

(iv) Die casting provides for greatly improved surface finish when compared to other casting methods. Finishing operations are thus minimum.

(v) True shape as that of original shape of die cavity can be obtained. Details can be reproduced successfully with a high degree of precision.

(vi) Castings produced by die casting are usually less defective, owing to increased casting soundness. These are strong with dense metal structure.

(vii) The increased soundness and reduction of defects provides increased yield.

(viii) The labour cost involved in die casting is less with the minimum of job training.

(ix) The rate of production of castings is very high, which can be as much as 800 castings per hour.

(x) The true shape of die can be retained for a longer period e.g., it is possible to produce upto 100,000 castings in case of zinc base alloys, 75,000 castings in case of copper base alloys and 50,000 castings in case of copper base alloys with a single die.

Limitations of Die Casting:

(a) The cost of die and equipment used is high. The life of die decreases rapidly if metal temperature is high.

(b) There is a limited scope of non-ferrous alloys that can be used for die casting.

(c) There are many complex features which limit the application of die casting.

(d) The size of castings produced by this method is limited.

(e) Special skill is required for maintenance and su­pervision of die.

(f) The minimum economic quantity for die casting is around 20,000.

(g) Die castings usually contain some porosity due to the entrapping of air.