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castings are of considerable depth and the lower face of the mould cavity is subjected to the action of the high­ est static pressure. That is why in cases of swelling

is observed, it is usually found on parts or surfaces of

the castings which have been placed at the bottom of the mould in accordance with the casting position at pouring.

It is rather seldom but possible [4] to obtain swelling

due to the high static pressure in light castings set hor­

confined to light castings, since in heavy castings, the

hence, when it is fu ll, by creating the high hydrostatic pressure in the sprue which develops the high velocity or

This defect is mainly inherent in castings made of gray

or nodular iron. VThen a mould is fille d with molten iron

the metal contracts until solidification commences. On solidification of the eutectic* graphite precipitates from

molten iron and this phenomenon is accompanied by the inc­ reasing of the metal volume because it actually expands. The expansion depends upon chemical composition of gray

or nodular iron and mould variables. If enough carton is

free to precipitate, the over-all volume may increase dur­ ing solidification rather than decrease.

There are positive and negative sides of this phe-

nomenon.. As a matter of fact, the positive side is that graphite expansion in cast iron can, in a large extent, counterbalance solidification shrinkage . So it is pos­

On the other hand* if the mould cavity is not communicated with atmosphere, i .e . open top risers are absent, the eatrapped metal presses the mould during solidification and

This defect is called dilation. Dilation depends on how far the mould is .deformed by the pressure exerted cy the solidifying and expending casting.

in practice, the factors affecting casting dilation, and hence the amount of gross shrinkage that occurs, are

Fig, 2,4 illustrates the effects of dilation on sol­ idification of a gray iron cylinder being 8 in.high by 3^/4 in, in diameter under various moulding and risering conditions [6]

In dry dand, dilation does not occur; the graphite expansion counterbalances solidification contraction, and the cast iron ** shrinks ” very little ( Fig,2,4, a).

In green sand, however, when the casting is kept open to the atmosphere, dilation results in a cylinder which is bulged slightly outward; gross internal shrinkage is the result ( Fig. 2,4, b ) .

When atmospheric pressure cannot act through an in­ sulated ’’hot spot” , the outward dilation is balanced by inward contraction. ( Fig,2„4, c), and no grosp shrinkage results.

Pig,2Л . Schematic cross section of a gray iron cylinder solidified under various moulding and risering conditions [6] •

Fig,2.4,d illustrates that very large risers may

not be sufficient to feed shrinkage in a green sand cas«* ting and may| in fact, accentuate shrinkage by increasing the ferrostatic head and the ” hot spot

on the chemical composition of a grey cast iron but the essence of this described phenomena is the same.

Measures of the previous defect prevention are true for this case too, but when it is necessary to avoid the influence of a chemical composition of grey iron a great rate of cooling may be used.

Surface imperfections very widely as a result of the chemical, thermal, ferrostatic, dynamic or other in­ teractions between molten metal and mould.

Flash

Flash is a thin metal run around the parting line of the mould or core caused by the flow of liquid metal into the space between the halves of the mould or core.

It is readily removed in the cleaning operation but sonw

penetrates between sand grains due to the action of a high

This defect is known as veining when a network of fins oocurs over the whole of a fla t surface. Such the defect is usually inherent to castings made of copper base alloys. Probably the specific weight of alloys influences formation of fin and veining too.