Q. What kind of defects are likely to result in castings due to the following?
(i) Core prints on the core are undersigned.
(ii) Core prints on the core are oversized
(iii) Molten metal is poured at inadequate temperature.
Ans. A properly designed casting, a properly prepared mould and correctly melted metal should result in a defect free casting. But due to lack of proper exercise and surrounding there are some defects in casting. The defects may be of various types. For the given results in casting the following defects occurs :
(i) Core prints on the core are undersize. Due to this result the defects are :
(b) Uneven wall thickness
Mismatch : Mismatch is a shift of the individual parts of a casting with respect to each other. This may occur due to mould shift or core shift. The causes can be an inexpert assembling of the two halves of the mould, from wear of pin bushes and pins and dimensional discrepancy between the core prints of the pattern and the core prints of the core.
(ii) Core prints on the core are oversized. Due to this result the defects are
(a) Crushes (b) Uneven wall thickness
Crushes : A crush is a deformation of mould surface due to pressing or scrapping of the sand during setting of core or assembly of the mould boxes. Setting of over sized cores in undersized core prints and lack of care in assembly often lead to the occurrence of crushes.
(iii) Molten metal is poured at inadequate temperature :
Due to this result, the defects are :
(a) Hot tears (b) Shunts
(c) Misrun or Short run
Hot tears : Hot tears are hot cracks which appear in the form of irregular crevices with a dark oxidized fracture surface. They arise when the solidifying metal does not have sufficient strength to resist tensile forces produced during solidification. They occur chiefly from an excessively high temperature of casting metal. This defect can be avoided by improving the design of the casting and by having a mould of low hot strength and large hot deformation.