Permanent Mold or Gravity Die Casting
Permanent Mold or Gravity Die Casting: This process is commonly known as permanent mold casting in U.S.A and gravity die casting in England. A permanent mold casting makes use of a mold or metallic die which is permanent. A typical permanent mold is shown in Fig. 1. Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity. However, the liquid metal solidifies under pressure of metal in the risers, etc. The metallic mold can be reused many times before it is discarded or rebuilt. These molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness. The mold is made in two halves in order to facilitate the removal of casting from the mold. It may be designed with a vertical parting line or with a horizontal parting line as in conventional sand molds. The mold walls of a permanent mold have thickness from 15 mm to 50 mm. The thicker mold walls can remove greater amount of heat from the casting. For faster cooling, fins or projections may be provided on the outside of the permanent mold. This provides the desirable chilling effect. There are some advantages, disadvantages and application of this process which are given as under.
- Fine and dense grained structure is achieved in the casting.
- No blow holes exist in castings produced by this method.
- The process is economical for mass production.
- Because of rapid rate of cooling, the castings possess fine grain structure.
- Close dimensional tolerance or job accuracy is possible to achieve on the cast product.
- Good surface finish and surface details are obtained.
- Casting defects observed in sand castings are eliminated.
- The cost of metallic mold is higher than the sand mold. The process is impractical for large castings.
- The surface of casting becomes hard due to chilling effect.
- Refractoriness of the high melting point alloys.
- This method is suitable for small and medium sized casting such as carburetor bodies, oil pump bodies, connecting rods, pistons etc.
- It is widely suitable for non-ferrous casting.
Fig. 1 A typical permanent mold
SLUSH CASTING: Slush casting is an extension of permanent mold casting or metallic mold casting. It is used widely for production of hollow casting without the use of core. The process is similar to metallic mold casting only with the difference that mold is allowed to open at an early stage (only when a predetermined amount of molten metal has solidified up to some thickness) and some un-solidified molten metal fall down leaving hollowness in the cast object. The process finds wide applications in production of articles namely toys, novelties, statutes, ornaments, lighting fixtures and other articles having hollowness inside the cast product.
PRESSURE DIE CASTING: Unlike permanent mold or gravity die casting, molten metal is forced into metallic mold or die under pressure in pressure die casting. The pressure is generally created by compressed air or hydraulically means. The pressure varies from 70 to 5000 kg/cm2 and is maintained
while the casting solidifies. The application of high pressure is associated with the high velocity with which the liquid metal is injected into the die to provide a unique capacity for the production of intricate components at a relatively low cost. This process is called simply die casting in USA. The die casting machine should be properly designed to hold and operate a die under pressure smoothly. There are two general types of molten metal ejection mechanisms adopted in die casting set ups which are:
(i) Hot chamber type
(a) Gooseneck or air injection management
(b) Submerged plunger management
(ii) Cold chamber type
Die casting is widely used for mass production and is most suitable for non-ferrous metals and alloys of low fusion temperature. The casting process is economic and rapid. The surface achieved in casting is so smooth that it does not require any finishing operation. The material is dense and homogeneous and has no possibility of sand inclusions or other cast impurities. Uniform thickness on castings can also be maintained.
The principal base metals most commonly employed in the casting are zinc, aluminum, and copper, magnesium, lead and tin. Depending upon the melting point temperature of alloys and their suitability for the die casting, they are classified as high melting point (above 540°C) and low melting point (below 500°C) alloys. Under low category involves zinc, tin and lead base alloys. Under high temperature category aluminum and copper base alloys are involved.
There are four main types of die-casting machine which are given as under.
- Hot chamber die casting machine
- Cold chamber die casting machine.
- Air blown or goose neck type machine
- Vacuum die-casting machine