MOULDING IN DRY SAND 125
The liquid pressure will often cause the top part of a casting to " gather "
J in. or more in thickness. Risers relieve this strain by providing openings
into which the metal, otherwise confined, rises quietly. The risers are
closed with a ball of sand or clay during the pouring, and are floated off
by the filling of the mould.
The object of " feeding " or " pumping " is to supply additional hot
metal to compensate for the shrinkage of heavy masses. It is done through
a pouring basin, or a specially made opening and cup. Molten metal is
poured in, and a J-in. or f-in. rod inserted and pumped up and down until
the metal becomes too viscous to permit of further movement.
Moulding in Dry Sand
Moulding in dry sand is reserved for some massive castings that are
required perfectly sound, and free from minute specks and blow-holes.
Its principal applications are to steam and hydraulic cylinders. Only
strong mixtures of sand can be dried. This excludes all the green sands,
which, however, are frequently baked on the surface—" skin-dried ". The
porosity of the sand in a dry-sand mould when dried largely takes the place
of the venting with the wire done in a green-sand mould. The presence of
moisture even in small quantity in a mould imperfectly dried is therefore
a source of risk.
Practically all dried-sand moulds are enclosed wholly in boxes, and turned
over. They are put bodily into the stove to be dried. Since the sand is
very fragile after drying, all moulds are t: finned " in the joint faces previously
to or immediately following the delivery of the pattern, that is, they are
pressed down hard with the trowel for a distance of an inch or two back
from the mould boundaries, so that when dried and closed they will not
fracture. A slight fin is formed, but this is of no importance. Moulds
made in dry sand will bear harder ramming and more swabbing than those
of green sand. They are coated with wet blacking, while those of green
sand are dusted with plumbago powder.