3218

had a high expansion and deformation in the temperature range from 400° to 600°C. Castings made in these moulds were spoiled by_scabs completely* Although the two mat­ erials had similar expansion by the time the temperature reached 1100°C no scabs formed on surfaces of castings made in olivine sard moulds* It my be explained by uni­ form expansion of the olivine sand without any heat shocks and when the temperature, of the sand reached 1100°C the clay softened and the rammed sand became plastic.

penetration defects in ferrous castings. Large castings

made in chamotte have exceptional external appearance and

show

do not ^ scabs and. sand spots as do ordinary sand cas­ tings, Chamotte is outstanding in its resistance to ero­ sion by molten steel and is useful for runner blocks and cores as well as moulds.

Some of the refractory materials listed in Table 2.3, including carbon, thoria, beryllia, and magnesia, are used in rammed refractory moulds for casting the very reactive metals t titanium, zirconium and uranium.

toward which the facing sand can expand freely without

a deformation and crack formation during the pouring of

The good result of scabbing prevention was also ob­

tained by screening a moulding face with the help of an

aluminium f o il [22] (Pig.2.9, c ) . The screen reflects

the greatest amount of heat radiated from the surface of molten metal protecting the facing layer from overheating and preventing its deformation. This method is especia­ lly effective when the time of pouring is rather long ari developed cope surface is subjected to the heat radiation for a long time»

But when this time is not so long it is possible tc strengthen a cope facing layer by simple sprigging the surface with moulding tacks or nails ( Fig«2.9, d ).

Buckles

Buckles are defects which usually also apnear in cope surfaces of castings. Buckles and scabs are alike in appearance, the only difference being that buckles are relatively extensive overlapping of metalc Usually this defect is caused by the extensive expansion of cope surf-

aces due to too rapid preheating by radiation from the rising metal.

In total reasons of origin and measures of elimin­ ation of the defect are alike to those of scabs.

Rat tails

Rat ta ils are narrow depressions, usually aligned

along the edges of the incoming metal streams. As a rule,

and at the strictly horizontal disposition of the mould,

streams incoming into the mould cavity move on the fla t

face along the direction of ingates not coming into con­

tact one with another. While the main amount of the flo « ' is

other hand, some parts of the mould face between the str<# earns are le ft uncovered with liquid metal that, causes the nonuniform heating of different areas of the bottom face.

Under the proper conditions the nonuniform heat expansion Of the areas may cause cracking the sand face and buclclnng

edges gets more permeable than the other parts of the face.

Such the situation is kept during the pouring and some

period of time after it until the temperature gradient between different zones of the face disappears.

them, apart from mentioned measures, by change in the method of running and gating or by change of moulding mix­

tures especially facing one* The most effective additions

to the sand for cast iron are fibrous materials and wood

vention of the defect but they do not guarantee freedom from rat ta ils .

It is better and more reliable to prevent the def­ ect by change in the method of running. The use of an uniform distribution of molten metal through ingates to