What Is Die Casting - Its Introduction, Production Process, Applications, Advantages, Types of Machines 

Introduction :

The die-casting process, developed in the early 1900s, is a further example of permanent-mold casting. The European term for this process is pressure die casting. Die casting is a permanent-mold casting process in which the molten metal is injected into the mold cavity under high pressure. Used for low melting point (non ferrous) metals. Die casting is a manufacturing process that can produce geometrically complex metal parts through the use of reusable molds, called dies, hence the name die casting. Molds used in die-casting operations are usually made of tool steel, mold steel, or maraging steel. Tungsten and molybdenum with good refractory qualities are also being used, especially in attempts to die cast steel and cast iron.

This process is a further development of Permanent mold casting.

1. A permanent mold casting process in which molten metal is injected into mold cavity under high pressure, typical pressures are (7 to 350 MPa).

2. Pressure is maintained during solidification, then mold is opened and part is removed.

3. Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes.

4. It differs from sand casting. Sand casting uses a mold made of sand which is a poor conductor of heat, so the cooling process is very slow, molten metal is simply poured into the mold and the mold is expendable. In permanent mold casting, by contrast, the mold is made from steel or other metal which is a good conductor, so the cooling is fast, it can be reused and the molten metal is being injected.

Applications :-

Typical parts made by die casting are housings, business-machine and appliance components, hand-tool components, and toys. The weight of most castings ranges from less than 90 g to about 25 kg. Equipment costs, particularly the cost of dies, are somewhat high, but labor costs are generally low, because the process is semi- or fully automated. Die casting is economical for large production runs. In the die-casting process, molten metal is forced into the die cavity at pressures ranging from 0.7 to 700 MPa.

Advantages of die casting :-

(1) high production rates possible;
(2) economical for large production quantities;
(3) close tolerances possible, on the order of 0.076 mm for small parts;
(4) good surface finish;
(5) thin sections are possible, down to about 0.5 mm and
(6) rapid cooling provides small grain size and good strength to the casting.

Die Casting Machines :

Die casting operations are carried out in special die casting machines which is designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity

There are two basic types of die-casting machines: hot-chamber and cold-chamber machines.

In hot-chamber machines, the metal is melted in a container attached to the machine, and a piston is used to inject the liquid metal under high pressure into the die. Typical injection pressures are 7 to 35 MPa. Production rates up to 500 parts per hour are not uncommon. Hot-chamber die casting imposes a special hardship on the injection system because much of it is submerged in the molten metal. The process is therefore limited in its applications to low-melting-point metals that do not chemically attack the plunger and other mechanical components. The metals include zinc, tin, lead, and sometimes magnesium.

In cold-chamber die-casting machines, molten metal is poured into an unheated chamber from an external melting container, and a piston is used to inject the metal under high pressure into the die cavity. Injection pressures used in these machines are typically 14 to 140 MPa. Compared to hot-chamber machines, cycle rates are not usually as fast because of the need to ladle the liquid metal into the chamber from an external source. Nevertheless, this casting process is a high production operation. Cold-chamber machines are typically used for casting aluminum, brass, and magnesium alloys. Low melting-point alloys (zinc, tin, lead) can also be cast on cold-chamber machines, but the advantages of the hot-chamber process usually favor its use on these metals.

THE FOUR STEPS OF DIE CASTING :

There are four key steps in the process of die casting, the die casting machine should be at the required temperature, to ensure the molten metal not to solidify too quickly. According to the size of the casting, heating can take from several hours to several minutes. The four steps are:

1) Spray the mold with lubricant and close it, allowing for an easier removal of the cast object later on.

2) Inject the molten metal into the die. The metal is inserted at an extremely high pressure, which allows the metal to conform to the precise shape of the die. Air escapes into overflow wells, and out vents, and metal fills the molds

3) Cool the mold, and wait for the metal to solidify. In some cases, the mold may be immersed or sprayed with cold water to help the casting become solid faster. A high pressure is maintained inside the mold, which ensures the metal doesn't change properties while inside the die.

4) Open the two molds halves and remove the solid cast by knockout pins eject the part. The parts are cut off the runners and sprues.