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Full text of "Modern Mechanical Engineering Vol-I"

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commercial success. Both the temperature of pouring and the coefficient
of expansion are high. With the increased shrinkage the dies are strained
badly, and castings crack. The cost in any case is prohibitive. To produce
at a profit, the life of a die should be equal to that of 10,000 casts. It has
not been found possible to exceed 1000 in the brasses. To pour iron would
also destroy these moulds. Yet under different conditions this is done, as
described in the next section.
Permanent Moulds.—These are made of cast iron, and iron castings
are produced in them. The advantages gained are: (a) the saving of time
otherwise spent in making a sand mould for every cast; (b) the more rapid
removal of the castings from the moulds when set. The development,
almost wholly confined at present to the United States, is a remarkable one.
Its applications are chiefly to pipes required in large numbers, the cores
for which are also made in iron. The same grades of iron may be used for
castings made in permanent moulds as for those in sand, but iron that would
not give satisfactory results in the latter will do so in the first named. Also,
harder or softer castings can be obtained from the same metal, depending
on the time during which they are permitted to remain in the moulds. A
surface chill can be imparted, or the casting may be soft throughout. In
any case, cored castings must be removed before they shrink tightly on their
cores, otherwise they must be broken up.
The mass of metal in a permanent mould must be large, because a thin-
walled mould would become heated so quickly that a rapid succession of
castings could not be produced. For castings of 15 Ib. weight and upwards,
the mould should be of about seventy times the weight of the casting.
Castings are removed soon after the outside has set. This will usually occur
in from six to ten seconds, but the time will depend a great deal on the
weight, the degree of hardness, &c., required. That this can be varied,
though using metal of the same chemical composition, is one of the valuable
features of these moulds. If a casting is allowed to remain long in contact
with its mould, it becomes chilled, a large proportion of the carbon remain-
ing in solution, in the combined form; but if the casting is removed at a
bright yellow colour while the interior is still viscous, the exterior will
become annealed, and the casting will be soft, the carbon passing mostly
into the graphitic state.
It has been proved that iron which is unsuitable for sand casting is excellent
for permanent mould work. An iron with a percentage of phosphorus as
high as 1*5 per cent, and of sulphur o-i per cent, and silicon 2-5 per cent, is
as strong as one with smaller proportions. The explanation is, that these
remain in normal solution, not having time to separate out.
Permanent mould work has its limitations, due to the fact that iron cores,
which must be drawn out endwise, are used. This limits the forms of pipes
to those with straight or regularly curved cores. The same hindrance
occurs in die-casting. As the castings have to be removed quickly and the
moulds are massive, a good deal of mechanism is necessary for rapid and easy
handling. In general, from one to two castings are poured and removed