Riser (casting)

A riser, in the context of metal casting, is a reservoir of molten metal designed to compensate for shrinkage that occurs during the solidification process. Risers are an integral part of the mold and are strategically positioned to feed liquid metal to sections of the casting that are prone to shrinkage defects, ensuring a sound and homogenous casting.

Function:

The primary function of a riser is to provide a source of molten metal to counteract the volume reduction that happens as the metal cools and solidifies within the mold cavity. This shrinkage can lead to voids, porosity, and dimensional inaccuracies within the casting. Risers are designed to solidify last, effectively drawing molten metal from the riser into the casting as needed, filling any developing voids.

Types:

Risers can be classified in several ways, based on their location and how they are connected to the casting:

• Open Risers: These risers are exposed to the atmosphere. While easy to implement, they tend to lose heat more quickly, potentially leading to premature solidification.
• Blind Risers: These risers are completely enclosed within the mold and are not exposed to the atmosphere. This helps to maintain the molten metal temperature for a longer period.
• Top Risers: Located on the top surface of the casting.
• Side Risers: Located on the side of the casting.

Risers can also be categorized based on their shape and dimensions:

• Cylindrical Risers: A common and simple design.
• Sleeved Risers: Risers with a ceramic or insulating sleeve to reduce heat loss.

Design Considerations:

Effective riser design requires careful consideration of several factors, including:

• Riser Size: The riser must be large enough to supply sufficient molten metal to compensate for shrinkage.
• Riser Location: The riser must be strategically placed to feed critical sections of the casting that are prone to shrinkage.
• Riser Solidification Time: The riser should be designed to solidify later than the casting it is feeding. This is often achieved through the use of insulating materials or heating methods.
• Riser Neck: The connection between the riser and the casting should be sized appropriately to allow for efficient metal flow without causing premature solidification.

Riser Removal:

After the casting has solidified and cooled, the risers are typically removed from the casting by cutting, machining, or other methods. The resulting surface is then finished to match the rest of the casting. The metal reclaimed from risers can often be remelted and reused.