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Full text of "Modern Mechanical Engineering Vol-I"

MOULDING  IN  GREEN  SAND                    113
CHAPTER   I
Moulding in Green Sand                                        11
ll
The term " green sand " does not denote any one of the specific mixtures                                ?J
used, but it signifies that the sand is moistened and rendered coherent with                                ||
water, so that it becomes sufficiently self-sustaining to retain the shape
imparted to it by the pattern, and to resist the pressure of molten metal.
It differs therefore from moulds made in dried sand and in loam, and from
cores, which are desiccated. By far the largest proportion of moulds, large
and small, is made in green sand. As there is no drying process, fuel and
time are saved. Green-sand work embraces three systems of working:
open sand, bedding-in, and turning over, or rolling over.
Moulding in Open Sand.—This is but a crude and very elementary
method, and one which is of extremely limited application, being almost
exclusively employed for making foundry appliances, loam and core plates,
back plates, moulding boxes, and sometimes balance weights for cranes.
It signifies that the mould is not covered with a cope, and the consequence
is that the upper surface of the casting so poured is left rough and uneven
as the metal solidifies. The necessary details may be stated briefly.
A Levelled Bed essential.—If the bottom of an open mould is not level,
the thickness of the casting will not be equal all over. The bed is levelled
by bedding two parallel straight-edges—"winding strips"—in the sand of the
floor, levelling them lengthways with a spirit-level, and, in relation to each
other, with a parallel straight-edge set across them, and a spirit-level. The
sand is flat-rammed a little higher than the top edges of the bedded-in strips,
and then strickled off level by them. On this bed the mould is made,
seldom from a pattern, but usually from a skeleton frame, or as often from
sectional pieces. No venting is required as in closed moulds, and no specific
sand mixtures, the moulds being made in the floor.
The Formation of Mould Outlines.—If these are produced from entire
patterns, as core grids generally are, the pattern is laid on the levelled bed,
and the sand rammed around and within it, and strickled off. In most cases
some portions have to be stopped-off to suit various core outlines. In
others, a grid larger than the pattern is required. Here the pattern is rammed
in one position, then removed to another adjacent position, and rammed
again. Generally, open moulds are constructed with sectional pattern parts.
The outlines are marked on the levelled bed by the moulder or pattern-
maker. External portions are rammed against short sweeped pieces, moved
around and rammed in successive lengths. Large central holes are rammed
against concave sweeps. Small holes are formed with cores, measured in,
and held down with weights. Straight sides are rammed against straight
strips. In all this work the depth of the mould exceeds that of the casting
thickness by from J- in. to f in., and flow-off gullies are cut at the height
corresponding with the thickness required. This is necessary, because it
VOL. I.                                                                                                                                8