In this article we will discuss about:- 1. Meaning of Permanent Mould Casting or Gravity Die Casting 2. Characteristics of Permanent Mould Casting 3. Limitations.
Meaning of Permanent Mould Casting or Gravity Die Casting:
Like sand casting, this method also consists of pouring molten metal but in a metallic mould without external pressure. Moulds are generally made of cast iron or steel which are not destroyed or rebuilt after every casting. Since these moulds last for long periods, the process is called Permanent mould casting.
In this process, no external pressure applied but hydrostatic pressure created by the risers is mainly responsible for casting of metal in the mould. As no external pressure is applied, this process in sometimes called Gravity dies casting. The metal used for the mould should have such a composition as to withstand high temperature.
Generally they are made of grey cast iron (having high resistance to thermal shocks), alloy steels (for very high temperatures and withstanding surface erosion), or non-ferrous alloys. Inner surfaces of the mould are coated first with a refractory and then with lamp black or core oil. This is done in order to reduce the chilling effect on the cast metal and to facilitate the removal of casting, and prevent the adherence of the molten metal to the mould.
The moulds also have the facilities of setting cores for hollow castings and ejector pins for ejecting out casting from the mould after solidification. The moulds are generally made in two halves which are hinged at one end and have provision for clamping at the other end.
The heat of the molten metal is removed very rapidly because of fast conduction through metallic mould. The metal thus shrinks very fast, leaving an air space between the mould surface and casting. This air space protects mould material from the effect of heat.
The two halves of the mould may be mounted on a sliding carriage where one half is fixed on one side and other half is movable along a horizontal bed. The simplest type of permanent mould hinges at one end of the mould with provision for champing the halves together at the other (Refer Fig. 3.61).
For hollow casting either metal cores or dry sand cores can be used depending upon the shape of hollow portion required. For complicated shapes having varying diameter, the dry sand core is used. If the metallic cores are used, they have to be withdrawn before the metal starts solidifying.
The cores may be either fixed type (which are fastened to the mould body and can be withdrawn with the mould) or movable type (which are separate from the mould and these are drawn before parting the mould). This process can be successfully used for mass production (continuous casting) by placing several moulds on a circular table.
The table keeps on rotating and at one end molten metal is filled in the moulds and at other end the casting is removed after partial solidification. To avoid surface defects and chilling effect, the moulds are heated before the molten metal is poured into the moulds. The prevent cracks due to shrinkage strains, the castings are frequency removed before they are completely solidified.
After solidification, the casting tends to cling to one of the mould halves. Spring-loaded ejector pins (provided in one of the two halves in which the casting tends to cling) are used to eject the casting from the mould.
Thickness of mould walls usually varies from 20 to 25 mm. Ribs and flanges are provided on the outer surfaces for strengthening the mould. Moulds are usually coated with a refractory wash and then lamp black, which reduces the chilling effect on the metal and facilitates the removal of the casting.
The process is adaptable to casting both ferrous and non-ferrous metals, but is generally used for non-ferrous metals and their alloys (aluminium alloys, zinc alloys, magnesium alloys).
Characteristics of Permanent Mould Casting:
(1) Castings produced by this method have very close grain texture and hence porosity is decreased by half to that of sand casting. Surface appearance is better. The castings are free from sand with good surface finish.
(2) Castings produced by this method are superior in hardness and mechanical properties.
(3) These have heavier density than sand castings.
(4) Castings produced by this method have more favourable fine grained structure and, therefore, have improved surface finish.
(5) They have high dimensional accuracies. Castings are capable of maintaining clearance of the order of 0.06 to 0.3 mm.
(6) Permanent mould equipment’s are less costly to build than die casting dies but slightly more expensive than metal pattern equipment for sand casting.
(7) As porosity is less, the castings can be used for pressure tight vessels (upto 15 kgf/cm2).
(8) As solidification of metal is under equilibrium conditions, the casting is less susceptible to cooling cracks and thus stronger.
(9) Casting method requires less skill and at the same time number of rejections is less.
(10) Castings can be produced in large quantities and at faster rate thus making it an economical process.
(11) It requires less space.
Limitations of Permanent Mould Casting:
(i) Permanent mould castings are suitable for small and medium sized non-ferrous castings and are impracticable for large castings and alloys of high melting temperature.
(ii) As permanent moulds are costly to manufacture, their use is limited to large production of identical products. The initial cost of equipment is high and cost of mould maintenance is also high.
(iii) They have poor elongation.
(iv) Several defects like stress and surface hardness may be produced due to surface chilling-effects.
(5) Metal moulds are unyielding against the contraction of molten metal during solidification.
(vi) Anchoring points in the metal mould obstruct orderly contraction and cause internal stresses with the danger of shrink cracks.
(vii) Castings made in metal moulds are not so flexible as in sand mould since they have to be completely withdrawn without interference.