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Full text of "Modern Mechanical Engineering Vol-I"

MOULDING  IN GREEN  SAND                     123
the surfaces of the patterns. The air and gas escape directly through the
top of the cope. That from the drag is brought out through large horizontal
channels driven between the bottom of the box and the sand floor on which
it rests. Vents that come into the joint faces are led into shallow gutters
cut in those faces, and surrounding the mould, to come out through the box
joints. The vents from large bedded-in moulds in the bottom are taken
down to a cinder bed, to be discharged through pipes outside. A similar
practice is adopted when very large masses of sand occur in closed moulds.
Bodies of clinker or coke are introduced. Into these the vents are led. The
gases are discharged quietly through these without risk of partial explosion
and shock to the mould. Vents from large moulds are generally ignited with
a hot skimmer. The amount of venting in moulds varies. Close, loamy
sand, and portions that are rammed hard require the maximum amount.
Insufficient venting is productive of blow-holes, and of scabbing.
Delivery and Mending-up.—Some rapping is necessary to loosen patterns
for delivery. Its severity has to be greater with increase in depth, in the
proportion of vertical faces, and in area. The point of the bar is inserted
in the pattern, or in a hole in a rapping plate, and the bar is struck with a
hammer laterally from all sides. During the early stages of delivery, the
top of the pattern is rapped slightly with a wooden mallet to assist its detach-
ment from the sand. The edges of the mould all round adjacent to the
pattern are swabbed with water to lessen risk of the sand being torn up.
But some fractures, except in the case of semicircular and allied outlines
that are most favourable to delivery, almost invariably occur. These have
to be repaired by a process of mending-up.
When a mould is very badly damaged, it is better to put the pattern back,
and re-ram the parts, but this is not practicable when it is of unwieldly
dimensions. Portions of the pattern are sometimes detached for making
broken sections good; often supplementary pieces are prepared to avoid
such removal. In most instances the moulder mends up with any odds and
ends suitable—straight strips, sweeps—or he bends sheet lead to outlines.
Nails may be thrust into the broken sections, and swabbed to assist the
coherence of the sand, and a stronger sand, skin-dried, will often be useful.
In mending-up, it is not easy to preserve correct dimensions and outlines.
Often this is of little or no consequence, but it is so when work has to be set
in fixtures for machining. This is one of the reasons why machine-moulded
castings should then have preference.
Pouring Arrangements.—Many moulds free from faults in the making
have produced damaged or waster castings because of improper methods
of supplying them with metal. Molten iron, steel, and brass are heavy,
and sand is fragile. The ideal method is to bring the metal in, in a position
which varies in different classes of moulds, and to let it distribute itself, and
rise quietly, instead of rushing and beating against weak sections of sand,
against cores, or parts that have to be machined. Moulds of moderate
depth are generally poured from the top (figs. 16 and 36), the metal being
brought into the thickest portion of the casting, such as a central boss, the