Iron foundry basics
Figure 9: Casting and runner system
10
Practical Action
Moulding
Moulding, in simplest terms, consists of
burying or pressing a pattern into
specially prepared sand, which will
retain the shape or imprint of the
pattern when the latter is removed. Into
the impression that is left, molten metal
is poured and allowed to cool, after
which it is removed in a solid state. The
making of the mould, however, is not
quite as simple as it would appear from
this description. Sometimes, in fact,
castings are required with hollow
portions and this is done by inserting
cores, which adds to the intricacy of the
mould. Other problems can arise, which
make moulding a highly skilled and
exact art. For example, gases are
generated, both in the mould and the
cores, when molten metal comes into
contact with the damp or green sand, as
it is known in the foundry. For certain
castings where sand is not sufficiently
permeable, these gases have to be led
away through fine vents so that they do
not find their way into the metal and
cause bubbles or blow holes. Again, the
rush of molten metal may cause the two
halves of the mould to separate, or the
cores to float, with the result that
castings are obtained which are thicker
than they should be, or not true to
pattern. To avoid these problems, the
art of moulding will be explained in
more detail, but will only cover small
moulds, that is those which are easily
handled by one man and made in a
moulding box.
Figure 10: Top half box (cope)
A moulding box consists of two
rectangular frames of equal size, each
of which is open at the top and bottom.
The bottom box, or drag, has a fixed
locating pin at each end, which fits into
two holes in the top box or cope. Boxes
are usually of metal construction and
vary in size.to suit the size of pattern to
be used (Figs. 10, 11 and 12).
When choosing a moulding box, care
must be taken to allow sufficient room
around the pattern to obtain a good
solid mass of sand, between the edges
of the pattern and the sides of the box
and also in depth. A flat board is
required on which to place the moulding
box. The half pattern (without the dowel
pins) is placed on the board and the
bottom half of the moulding box (the