SURAM MOHAN KUMAR, (Ph.D) 2
Assistant Professor, Dept. of ME, AVNIET, Hyderabad, Telangana, India
in larger lots. However, castings are less strong as compared to wrought components produced
by processes such as forging etc. However castings offer the possibility of having slightly
improved properties in certain part of the casting by techniques such as use of chill etc. In casting
process, very little metal is wasted.
Patterns are replicas of the casting required. It is similar in shape and size to the final product,
but not exactly. Usually, the mould is prepared in wet sand, to which some binder is added to
hold sand particles together. The pattern is then withdrawn from inside the sand mould in such a
manner that the impression/ cavity made in the mould is not damaged or broken in anyway.
Finally molten metal is poured into this cavity and allowed to solidify and cool down to room
Since most metals shrink in volume, when solidifying from liquid state and again on cooling, it is
obvious, that the pattern should be made slightly larger than the size of finished casting. This
difference in size of the pattern is called shrinkage allowance. For cast iron, this allowance is 1%
and for aluminium, it is about 1.6%.
On many occasions, castings produced in the foundry shop are machined subsequently. The
object of machining is to get exact sizes and better surface finish on the component. If such is the
case, a layer of 1.5–2.5 mm thick material has to be provided all round the casting. This is done
by making the pattern suitably bigger than the casting. This increase in size of pattern is called
Another important allowance provided on patterns is called draft allowance. It facilitates
withdrawal of pattern from the mould. It is provided on vertical surfaces. The idea is to give an
inclination of 2–3 degrees to vertical surfaces, so that while lifting the pattern, the upper surface
is wider and withdrawing the pattern with draft provided will not damage the sand mould. On
inner vertical surfaces, draft is provided in such a way that top surface is narrower and bottom
portion of pattern is wider. Apart from the above allowances, some other allowances are
sometimes given to compensate for inherent distortion or bending of castings. Sharp corners and
bends are also radius sized while making a pattern.