3218
.pdfsure is |
arised when a fast moving stream of |
metal strikes |
a mould wall as a sudden obstacle* |
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It |
is obvious that static pressure is |
highest when |
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
izontally |
for |
pouring. |
In this case the main reason of |
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the |
defect |
is |
too weak ramming of |
a moulding mixture in |
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the |
flask. |
The pressure on the mould face is sufficient |
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to |
compact |
the |
rammed sand sti^ |
further and therefore |
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swelling increases as mould hardness decreases. |
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Swelling due to |
dynamic pressure is almost entirely |
confined to light castings, since in heavy castings, the
velocity of |
the |
metal |
fa lls when the ingate |
is passed. |
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On the other |
hand, |
in |
order to obtain sound light castings |
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the weight rate |
of |
pouring the .molten metal must be hig |
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her when thin walls |
castings are produced. |
It is |
attained |
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by using a sprue |
of |
the sufficiently great |
height |
and, |
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
sible |
to feed castings made |
of this alloy with mueh sma |
|
lle r |
risers or under proper |
conditions, without |
them at |
a ll whil# other metals are needed considerable |
feeders. |
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
the swelling |
or increasing of a casting dimensions to the |
size greater |
than that of the original pattern occur 15] • |
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
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the size and shape of the casting ; |
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the action of atmospheric or ferrostatic pressure} |
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the |
characteristics of the moulding material ; |
- the |
size, shape, location, and thermal constitu |
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tion |
of gates and risers ; |
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metal chemistry, |
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] •
Foundry Department |
„Casting |
defects and |
Dr. Vo Bastrakov. |
measures of |
their prevention** |
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 other |
hand, very |
small risers completely feed |
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a dry |
sand casting |
( Fig.2,4, |
e ), |
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It is obvious |
that the value of dilation and the |
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volume |
and shape of |
shrinkage |
flavity may changedepending |
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
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Fins and voining |
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When facing surfaces of moulds come into contact |
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With molten metal they may crack and fins |
may be formed |
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as |
a result of |
the entry |
of metal |
the |
cracks of the |
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facing |
layer. |
Sometimes |
a fin |
may be |
obtained when mou |
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lds |
are |
rummed unusually soft. |
In this case molten metal |
penetrates between sand grains due to the action of a high
ferrostatic |
pressure. |
This |
can happen only if the mould |
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is so |
soft |
that there |
is some risk of |
a run-out. |
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The fin usually has a narrow base |
and tapers to |
a |
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sharp edge ( Fig.2.5 )• |
When its reason is the soft |
mould |
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it may be of |
a great length. |
On rectangular cores it |
usu |
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ally |
occurs at an edge, but can also occur on round or |
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fla t |
surfaces. |
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The defect origin |
is supposed to |
be explained by |
the expansion of the |
sand grains at the contact with mol |
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ten metal. The incidence of |
the defect is |
influenced by |
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both the sand grains |
and the |
binder, but the exact mecha |
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nism by which it occurs is s t ill not clear |
|43 • |
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.