3218

The shuts are oaused by

-

insufficient fluidity

of

the

molten metal,

-

extensive heat condictivity

of the

mould materials,

-unsatisfactory content

of

a moulding

sand,

It is

known that

fluidity

of a molten metal dependg

on its

temperature

and

chemical

composition* At the

same

temperature

of ovorhaating

pure

meto.ls

and eutectic

alloys

\

the

higher

fluidity

than alloys

solidifying

over

possess

rease

its flu idity, but the simplest way is the

increase

of its

pouring temperature* However^ this measure can

cease

the appearance of other undesirable defects, such

as hot

cracks,

shrinkage cavities and porosity,

bunning-

-on, etc*

Usually moulding materials used are not. wotted by

a molten metal.

Therefore, materials used for

the prepa­

coefficient of heat accumulating

ability decreases» Such

additions may be the following

saw-dust - for mixtures used

for cast iron and steel casting,

burnt asbestos and gypsum-

* for aluminium alloys*

Sometimes moulding mixtures with

low heat accumulating

lity

and so to

improve the fillin g moulds with metal,their

face

is oovorod

with a heat ia»-olaiing

paint,

composition

of which depends on the poured metal»

For the

decreasing

heat isolating powders

are used» They are chalfc,

- for aluminium alloy castings,

acetylene soot-for magnesium

and copper alloy castings»

As an example , the influence of

heat isolating

powders

on fillin g ability of a mould with molten cast

iron is

shown in Fig’ 2ClO

ohoise

of a running

system providing

smoother

flow by

a solid stream allow

to eliminate this

defect

in

surfa­

ces of

thin wall

castings

(3)* But

sometimes

a combin­

ation

of mentioned above

expedients

is

to be

used.

Laps-, seams„and cold

shots

Surface laps, seams

and cold shots are a variety

of a cold shuts.

Laps have the appearance of

a patch on

the casting surface alongated in the shape of a tongue.

Seams can be of

a different shape in

a surface.

But cold

shots have the appearence of small shotlilce spheres

metal which are

almost completely iistih et from the wast­

Surface roughness

Roughness and smoothness of casting surfaces is mainly

determined by the

size of the sand grains and temperature

of molten metal.

There is., a general agreement that, as the

permeability of the sand decreases, as result of small

voids between the

sand grains, surface finish improves.

Notes

1*

^he

content

of

the protective fluorio add­

ition

not less

than 6l.5 %

F,

22*5 % NH^» 4*5-5*5 B,

2*

The content of boric addition : not less than

99% boric

a cil,

3*

Paints

No.

1 and 2 are

used to paint moulds and

copes for Al-alloy castings,

4»

Paint

N6^3

i3 used to paint cores to be dried at£

temperature not more than 180°C,

5*

to

PaintBBo.4 and 5 are usedv"paint cores to be dried

at temperature 220-250°C.

in the

sand, metal may be

forced in between the

sand grains*

When the castings are large

so that the pressure

is high

and the temperature of the mould is maintained for a long

period,

metal may penetrate

to a considerable depth farming

a layer

of burning-on*