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.
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