Openings, called risers, are then provided from each
of the high points of the wax pattern to the top of the mould, these risers
ending at the top in a shallow basin. The molten metal comes up into these
risers and cares for contraction of the casting, as well as avoiding
defects in the collar of the weld. After the moulding material is well
packed, these gate patterns are tapped lightly and withdrawn, except in the
case of the metal pipes which are placed at points at which it would be
impossible to withdraw a pattern.
_Preheating._--The ends to be welded are brought to a bright red heat
by introducing the flame from a torch through the preheating gate. The
torch must use either gasoline or kerosene, and not crude oil, as the crude
oil deposits too much carbon on the parts. Preheating of other adjacent
parts to care for contraction is done at this time by an additional torch
burner.
The heating flame is started gently at first and gradually increased. The
wax will melt and may be allowed to run out of the preheating gate by
removing the flame at intervals for a few seconds. The heat is continued
until the mould is thoroughly dried and the parts to be joined are brought
to the red heat required. This leaves a mould just the shape of the wax
pattern.
The heating gate should then be plugged with a sand core, iron plug or
piece of fitted fire brick, and backed up with several shovels full of the
moulding mixture, well packed.
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