Permanent Mould Casting:
Permanent mould casting, also sometimes called gravity die casting employs moulds which can be used more than once and hence are permanent. These moulds are usually made in more than one piece to facilitate removal of the finished casting. The mould is assembled; and held together by clamps, screws or toggles during pouring. In the assembled position the parts of the mould make a complete mould with sprues, runners, gates, vents and blind risers. Vent channels may be provided for escape of entrapped air if it is found that the air within the mould cavity cannot escape properly during pouring through the space between parting surfaces.
The moulds are preheated at the start of the run to avoid thermal shock to the moulds. A refractory parting coat is given to the mould once in each cycle by spraying or brushing. French chalk or calcium carbonate suspended in sodium silicate binder is the commonly used refractory coat for aluminium and magnesium castings. It protects the mould and promotes casting ejection.
During operation the mould temperature should be controlled to remain within a close range depending on the metal poured to produce good castings.
Permanent moulds are usually made of close grained alloy cast iron which is resistant to heat and repeated changes in temperature. Bronze moulds may be used for casting lead, tin and zinc and wrought alloy steel moulds for some bronzes.
Cores are usually made of alloy steel. Sometimes sand or plaster cores may be used in which case the process is called semi permanent mould casting. Sand and plaster cores are cheaper but the structure, surface finish and dimensional accuracy of cored openings are only as good as that of sand or plaster casting. Metals commonly cast by the permanent mould casting process include lead, tin, zinc, aluminium, magnesium alloys, certain bronzes and cast iron.
Some typical products include refrigerator compressor cylinder block heads, connecting rods, gear blanks, automobile pistons, and kitchen ware and type writer parts.
Weights of castings produced may vary from a few grams to 150 kg but generally range below 25 kg. The life of the moulds varies from 3000 to 10000 castings for cast iron to as many as 100000 castings with softer materials.
Advantages of permanent mould casting process:
· The advantages of permanent mould casting process over sand casting include production of a fine grained structure, smoother surfaces, closer dimensional tolerances, lower floor space requirement and an economical production for large quantities. The fine grain structure produced results from the chilling action of the metal moulds and imparts better mechanical properties to the casting.
· The surface finish obtained in permanent mould castings ranges from 2.5 to 3 microns RMS and dimensional accuracy produced is of the order of ± 0.25 to 1.25 mm / mm across a parting line. Small cored holes up to 6 mm diameter can be produced with metal cores.
Production rates of the order of 15 to 30 castings per hour per mould can be attained. The limitations of the process are higher mould cast, restriction of size and shape of the castings and the lack of flexibility in making any changes in the gating and risering systems.