ROLE OF RISER IN SAND CASTING: Considerations for Desiging Riser, Freezing time, Feeding range, Feed Volume Capacity, Effect of Riser

 ROLE OF RISER IN SAND CASTING

Metals and their alloys shrink as they cool or
solidify and hence may create a partial
vacuum
within the casting which leads to casting defect known as shrinkage or
void. The primary function
of riser as attached
with the mould is to feed molten metal to accommodate shrinkage occurring during solidification
of the
casting. As shrinkage is very
common casting defect in casting and hence it should be avoided by allowing
molten metal to rise in riser after filling the mould cavity completely and
supplying the molten metal to further feed the void occurred during
solidification of the casting because of shrinkage. Riser also permits the
escape of evolved air and mold gases as the mold cavity is being filled with
the molten metal. It also indicates to the foundry man whether mold cavity has
been filled completely or not.
The suitable
design of riser also helps to promote the directional solidification and hence
helps in production of desired sound
casting.

 Considerations for Desiging Riser

While designing risers the following
considerations must always be taken into account.

(A) Freezing time


1. For producing sound casting, the
molten metal must be fed to the mold till it solidifies completely. This can be achieved when molten metal in riser
should freeze at slower
rate than the casting.

2. Freezing time of molten
metal should be more for risers than casting. The quantative
risering analysis developed by Caine and others can be followed while designing
risers.

(B) Feeding range

1. When large castings are produced in
complicated size, then more than one riser are
employed to feed molten metal depending upon the effective freezing
range of each riser.

2.  Casting should be divided into divided
into different zones so that each zone can be feed
by a separate
riser.

3. Risers should be attached to that
heavy section which generally solidifies last in the casting.

4. Riser should
maintain proper temperature gradients
for continuous feeding throughout freezing or solidifying.

(C)  Feed Volume Capacity

1. Riser should have sufficient volume to
feed the mold cavity till the solidification of the entire casting so as to
compensate the volume shrinkage or contraction of the solidifying metal.

2. The metal is always kept in molten
state at all the times in risers during freezing  of casting. This can be achieved by using
exothermic compounds and electric arc feeding arrangement. Thus it results
for small riser size and high casting
yield.

3. It is very important to note that
volume feed capacity riser should be based upon freezing time and freezing
demand.

Riser system is designed using
full considerations on the shape, size and the position or location of the
riser in the mold.

Effect of
Riser

Riser size affects on heat loss from top at
open risers. Top risers are expressed as a percentage of total heat lost from
the rises during solidification. Risers are generally kept cylindrical. Larger the riser,  greater
is the percentage of heat that flows out of top.
Shape of riser may   be
cylindrical or cubical or of cuboids kind. If shape is cylindrical i.e. 4
high
and 4
dia, insulated so that heat can pass only into
the circumferential sand walls, with a constant K value of 13.7 min./sq.ft.
Chvorinov’s rule may be used to calculate the freezing time for cylinder as 13.7 min. The freezing time
of a 4
”  steel cube of same sand is 6.1 minutes and   the
freezing time of a 2
, 8and 8
rectangular block is also 6.1
min. Since the solidification time as calculated of the cylinder is nearly
twice as long as that of either the block of the
cube. Hence cylindrical shape
is always better. Insulation and shielding of molten metal in riser also plays a good role for getting
sound casting

 

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