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Full text of "Modern Mechanical Engineering Vol-I"

FOUNDRY  WORK
The Die Moulds.—These are made in mild steel for the white metal
mixtures, but in one of the alloy steels for those having an aluminium base.
No portion of the mould is made in sand. Cores are of steel, and they have
to be drawn endwise with a lever from the casting. Sliding undercut parts
are similarly treated. Jointing is done when necessary for the removal of
the castings. Vent channels are cut. Ingates are severed while the casting
is in the mould. Means are provided for the mechanical ejectment of the
castings. In the more complicated moulds, where several operating levers
are involved, fool-proof methods are included to prevent cores and other
sliding pieces from being moved out of their proper sequence. Dies are
cleaned after casting with compressed air directed through a hose.
Furnaces.—These are essentially troughs of cast iron in which the alloy
is kept molten with a gas flame. The mould is usually carried above the
furnace, often on a tilting table. The pressure is put on with a piston in
a cylinder immersed in the metal. But many patents have been taken for
other methods, with the object of avoiding the blow-holes which are a frequent
cause of wasters. Some employ air pressure, others, centrifugal force, with
a vacuum, the idea being that blow-holes are due to the entanglement of air,
which is doubtful. The case is not analogous to that of green-sand moulding.
The cause would appear to be the chilling of the metal against the walls of
the mould, forming an unyielding shell before the interior has solidified.
The remedy is, to have an ingate large enough to fill the mould rapidly, to
bring the metal in where the sections are heaviest, and to inject under adequate
pressure.
The Castings and their Alloys.—In the selection of metals to form alloys
for die-casting, shrinkage is the predominant factor. For, although casting
is done under pressure which is not released until solidification has set in,
some shrinkage must occur. Allowance must be made for this in making
the dies, or means provided to counteract it. Further, the strength of an
alloy to resist elongation by reason of the shrinkage stresses set up during
cooling has to be known with some approximation to correctness, because
otherwise, by using an unsuitable alloy, fracture may occur in the mould.
The case is different from that of ordinary moulds. The cores, being of
steel instead of sand, will not yield, sc the metal must have strength to
elongate, or it will rupture. And this varies with the1 proportions of the
elements, and with the temperature. This therefore is a matter for ex-
periment. Antimony is used to lessen the amount of shrinkage of alloys.
Only a small quantity, from i per cent to z per cent, is required in the zinc-
base alloys, but in the lead-base group it may be alloyed up to 25 per cent.
Classification of Alloys.-—Die-casting alloys are grouped as those having
low melting-points, below about 800° F., and those that fuse above that
temperature. The first are by far the most extensively used, comprising
the numerous white metals; the second are the aluminium and copper alloys.
Alloys are classified according to their bases, signifying by this the metal
which occurs in the largest proportion, and so determines the leading charac-
teristics of the alloy. These are zinc, tin, lead, and aluminium. A very large