f,	MOULDING SANDS	137
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^1	CHAPTER V
Moulding Sands
The vast majority of moulds is made in sand mixtures.   The methods
.	that lie outside of these are of a special character, as chill casting, casting in
permanent moulds, and die-casting, which, though of growing importance,
bear but a small proportion to the large volume of work made in sand. This
material is of pre-eminent utility because it is easily rammed or moulded
into any outline, it is so highly refractory that it is not fused by the
temperature of molten metal, it is adhesive enough to retain the shapes
imparted to it, is porous enough to permit of the escape of gases generated
in the mould by the molten metal, and, being quarried in many districts,
its cost is low.
Sand is never used in the crude raw state in which it arrives from the
quarries. It is wet, lumpy, non-homogeneous, and has to be subjected to
preliminary treatment in machines. And few sands are employed alone
)	without admixture, though some are used thus because of their self-venting
properties.   The judicious mixing of sands to secure the best results for
if	different classes of moulds is one of the tasks of the foreman, who has
generally to work with those kinds that are obtained locally.
i	Facings.—The essential mixture is the facing sand.   This is prepared
to line the mould for a thickness of 2 in. to 3 in. next the pattern.   Elsewhere
:	the flask is occupied with the " black " or ^<c floor " sand, which occupies the
foundry floor to a depth of about a couple of feet, and which consists of the
•	accumulations of years from former moulds.    It has lost its original properties
I	by repeated bakings, but when riddled and moistened with water it is used
for box-filling, serving as a backing to the facing sands. Broadly, these are
grouped as being " weak " or " strong ". The difference is that the first
contains a smaller proportion of heavy clayey material than the second,
also less coal dust. The function of the latter material is to prevent the
occurrence of " sand-burning ". While the infusible silica is the basis of
sands, a proportion of alumina is essential to provide the bond of coherence.
Oxide of iron is also present, and both these substances are fusible at pouring
temperatures. The coal dust lessens risk of resulting roughening of the
" skin " of the casting, by forming a film of one of the oxides of carbon
between the sand and the casting, a result which is assisted by the plumbago
facings dusted or brushed on the moulds. It follows that the larger the
•	proportion of clay present in strong sands, the larger must be the quantity
J	of coal dust.    The amount will range trom one of coal to six or eight of sand
in the strong sands to one in fifteen in the weaker mixtures.   Large moulds
in which the metal remains hot for a long time require more coal dust than
|	small moulds that cool quickly.    The determination of the strength of a
,*	mixture for a given mould is one of much importance.   Different grades are
•	desirable for different parts of the same mould.   Areas subject to great
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