5. Solidification of Casting
• During solidification metal experience shrinkage which
results in void formation.
• This can be avoided by feeding hot spot during
solidification.
• Riser are used to feed casting during solidification.
6. What Are Risers?
• Risers are added reservoirs designed to feed liquid metal to the
solidifying casting as a means for compensating for solidification
shrinkage.
• Riser must solidify after casting.
• Riser should be located so that directional solidification occurs
from the extremities of mold cavity back toward the riser.
• Thickest part of casting – last to freeze, Riser should feed
directly to these regions.
7. Why Risers?
• The shrinkage occurs in three stages,
1. When temperature of liquid metal drops from Pouring to Freezing
temperature
2. Whenthe metal changes from liquid to solid
state, and
3. Whenthe temperature of solid phase drops
from freezing to room temperature
• The shrinkage for stage 3 is compensated by providing shrinkage
allowance on pattern, while the shrinkage during stages 1 and 2 are
compensated by providing risers.
8. Risering
• A riser or feeder head is a vertical passage made in the cope to
store the liquid metal and supply the same to the casting as it
solidifies.
Functions:
• Store sufficient liquid metal and supply the same to the casting
it solidifies there by avoiding volumetric shrinkage of the
casting.
• The riser must be kept open to the atmosphere and placed in
such a location that it maintains a positive pressure of liquid
metal on all portions of the casting it is intended to feed.
• The cylindrical shaped riser are generally recommended
compared to spherical shaped risers which although consider
as the best.
9. Types of riser
1. Open riser
• The top surface of the riser will be open to the atmosphere.
• The open riser is usually placed on the top of the casting.
• Gravity and atmospheric pressure causes the liquid metal in
the riser to flow into the solidifying casting.
10. 2. Blinder riser
• It is completely enclosed in the mould and not exposed to the
atmosphere .
• The metals cools slower and stay longer promoting directional
solidification.
• The liquid metal is fed to solidifying casting under the force of
gravity alone.
12. Function of Gating System
A good gating system should help easy and complete filling
of the mould cavity.
It should fill the mould cavity with molten metal with least
amount of turbulance.
It should prevent mould erosion.
It should establish proper temperature gradient in the
casting.
It should promote directional solidification.
It should regulate the rate of flow of metal into the mould
cavity.
13. Design Considerations of Casting
1. Always keep the stressed areas of the part in
compression.
2. Round all external corners.
3. Wherever possible section thickness throughout should
be held as uniform as compatible with overall design
considerations.
4. Avoid the concentration of metal at the junction.
5. Avoid very thin sections.
14. Solidification Time For Casting
• Solidification of casting occurs by loosing heat from the
surfaces and amount of heat is given by volume of
casting .
• Cooling characteristics of a casting is the ratio of
surface area to volume.
• Higher the value of cooling characteristics faster is the
cooling of casting.
Chvorinov rule state that solidification time is inversely
proportional to cooling characteristics.
Solidification time
Where
Ts = Solidification time V = Volume of casting
K = mould constant
SA = Surface area
