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Pig 2.5» Formation of fin*

94-

Fig* 2*6* Formation of a scab i

a) the preheating of the surface layer of the mould and arising of stresses!

b) the shearing the face layer of the mould| w

c) the crack formation' and scab arising*

The nature of scab formation is the following. When

molten metal is poured into a green sand mould the uncov­ ered surface of the mould is heated by radiation.

At the same time the air and gasses inside the mould cav­ ity are heated too and they give their heat to the facing layer of the »avity. The latter gets heated to a high temperature and losses its moisture because of the inte­

nsive evaporation, A part of the steam is removed from

the mould through risers and venting holes,the basic one passes through pores of the mould mixture. The moisture removed from the face condenses in some distance from the surface when the steam reaches a cool sand. So a zone of the higher moisture appears while further, behind it , we­ tness of the mixture remains constant. As the temperat­ ure of the sand behind the mould face rises, moisture ev­

aporates from the damp layer and condenses s t ill further

back. In other words the high moisture zone removes from

of the moulding sand have a different heat conductivity

and strength* The mould layers of the higher .moisture

%

have, relatively to other layers, the lowest strength and the highest heat conductivity. So varying the moisture distribution across the mould causes the corraspending

Quality of moulding materials may be the reason of scabbing too. Usually the most part of a moulding mixt­

ure is an used or, so called, "burnt" sand. So it is rep­ eatedly subjected to heatings from the molten metali The high temperature is the destructive agent continuously

deteriorating durability of the moulding sand. The hea­ vier the work, the more rapid silica grains crack and

The high hardness of ramming a sand and the devel­ opment of a high gas pressure in a region of too low per­ meability can also produce excessive stresses in the cope and cause scabbing.

Scabs due to sand expansion were discussed. Their

It is too rarely but sometimes scabs are obtained

on casting face when they have been made in dry sand mo­ ulds. Binding materials of dry sand mould usually contain some, too small, amount of the chemically combined water, the behaviour of whioh during the pouring of molten metal

may be adequate to that of green sand moulds. Indeed, such the amount of water cannot essentially decrease the

strength of the surface layer of the mould, but the defect

may occur if the mould has the large area of send which is being heated and cracks may form on its face unless the mould can deform.

Usually, elimination of scabbing is not a particul­ arly d ifficu lt task in foundries. Practically the use of

dry sand moulds allows foundrymen to solve this task convpletely but drying the moulds sharply increases their cost

change of the technological process of the casting manuf­