3218

- surface casting defects deteriorating their exte­ rnal appearance or making impossible to use them in prac­ tice because of their bad marketable state*

Causes of the arising of such defects are different

But knowing reasons of their arising it is easy to prevent and to eliminate the origin of defects in future.

The repairing or correction of the pattern and equ­ ipment find the increasing of the mould assembling accur­ acy allow one to reach required dimensional accuracy of a casting in a g i % alloy.

Cores may be the reason of casting sizes .imperfec­ tions too. Errors in the assembling of moulds and in # fixing of cores as well as a core distortion during bak­ ing and metal pouring are followed by a casting sizes change•

Dimentional inaccuracy due to,a core distortion is particularly dangerous because it cannot be, as a rule, detected by an inspection of the casting since the exte­ rnal dimensions are not affected. It can lead to a red­ uction in wall thicknesses and white iron can be formed in thin cross-sections of cast iron castings and the cas­ tings may be hard to machine or, what is worse, fa il in service. The core distortion takes place invariably due to Underbaking. In this case remedies are obvious*--.

Sometimes oversize castings result from rapping a pattern too hard when it is removed from the mould or from excessive allowance in the pattern for metal shrink.

At the process cf solidification from the liquid state and cooling to the room temperature in the solid state a casting metal contracts rod the casting changer.

its sizes. Different metals have different heat eapansion coefficient and it is obvious that alloys have vari­

Approximate values of free contraction of some metals and alloys

As a consequence of the mould resistance and varying rate cf cooling solid contraction of d ifferen t casting pa­ rts is usually non-uniform. But to take it into account at

Fig.2*l. Origin of stresses restricting

free contraction because of casting design and core and mould resistance

Practical linear contraction of cast alloys•

Note : Low limits are used at the presence of hindered srinkage of alloys poured into dry Sand moulds*

Warping and Cambering,

Warping is a distortion of a casting shape under the action of internal or residual stresses arising as a result

of non»uniform cooling and contraction due to thermal gra­ dients in various cro ssfo o t ions or parts of a casting*

A greater resistances to contraction in one part of the cas­ ting than in another is the reason of warping too*

- is -

Compression side

Tension side

(Я)

(b)

d)bracbut. Dotted linos show the right shape of castings. Arrows show directions of internal and rosidual stresses act­ ion after cooling*