Q. Discuss briefly the causes and remedies of Shrinkage cavities casting defects.
OR
Why shrinkage cavities are caused in the casting ? Discuss methods used for eliminating them.
Ans. Shrinkage cavities caused by improper directional solidification are most with small sections abruptly.
What happen in these sections is that there being a greater mass at the junction point compared to the legs, the junction point area becomes a hot spot with directional freezing progressing towards the hot spot, which in turn feeds the legs and develops the shrinkage cavity.
The hot spot can be eliminated by either making the section more uniform or using chills near the cross-section of larger mass. Although every effort must be made to prevent isolation of heavy sections which can become 'hot spots', sometimes is becomes difficult.
Under such circumstances, it is left to the foundry man to control the freezing by means of :
(i) Special manipulation of the position of risers in mould,
(ii) Controlling the speed of pouring,
(iii) Utilizing hot metal in the risers,
(iv) Using mould materials of different thermal characteristics.