Risers in Casting
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 of Riser in Casting process
- 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.
A major concern in the design of castings is the size and placement of risers. Risers are extremely useful in affecting the solidification-front progression across a casting and are an essential feature in the mold layout described previously.
Types of Risers in Casting
In Casting process there are mainly two types of raisers are used
- Open riser (Top Riser)
- Blind riser (Side Riser)
1. Open Riser (Top 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.
2. Blind Riser (Side Riser)
Blind Riser 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.
Blind risers are good design features and maintain heat longer than open risers do.
Risers are designed according to six basic rules:
- The riser must not solidify before the casting. This rule usually is satisfied by avoiding the use of small risers and by using cylindrical risers with small aspect ratios (small ratios of height to cross section). Spherical risers are the most efficient shape, but are difficult to work with.
- The riser volume must be large enough to provide a sufficient amount of liquid metal to compensate for shrinkage in the casting.
- junctions between the casting and the riser should not develop a hot spot where shrinkage porosity can occur.
- Risers must be placed so that the liquid metal can be delivered to locations where it is most needed.
- There must be sufficient pressure to drive the liquid metal into locations in the mold where it is needed. Risers therefore are not as useful for metals with low density (such as aluminum alloys) as they are for those with a higher density (such as steel and cast iron).
- The pressure head from the riser should suppress cavity formation and encourage complete cavity filling.
Question and Answers
- The purpose of riser is to
Answer : Feed the molten metal to the casting in order to compensate for the shrinkage.