Riser design is a crucial aspect of the casting process, ensuring that castings are free from defects such as shrinkage cavities and porosity. Risers, also known as feeders, act as reservoirs of molten metal that feed the casting as it solidifies and contracts.

Riser Design for Steel Castings

In steel casting, the primary objective of riser design is to ensure adequate feeding of molten steel to compensate for volumetric shrinkage during solidification. Steel tends to have high shrinkage rates, making effective riser design critical.

For steel castings, two main types of risers are used: open risers and blind risers. Open risers are exposed to the atmosphere, whereas blind risers are enclosed within the mold. Blind risers are more efficient in minimizing heat loss.

Risers should be located at the heaviest sections of the casting where shrinkage is most likely to occur. The size and shape of the riser should be designed to ensure a sufficient volume of molten metal. Cylindrical risers are commonly used for their ease of removal and effectiveness in feeding. Insulating sleeves and exothermic materials are often used to maintain the temperature of the riser, ensuring it remains molten longer than the casting.

Riser Design for Iron Castings

For iron castings, particularly gray and ductile iron, the goal is to manage the expansion characteristics during solidification. Unlike steel, some types of iron expand during the final stages of solidification, reducing the need for large risers.

Blind risers are preferred for iron castings due to their efficiency. For ductile iron, smaller risers can often suffice because of the graphite expansion during solidification.

Smaller, more compact risers can be used for iron castings. The shape can vary but should minimize the surface area exposed to heat loss while maximizing feeding efficiency. Chills (materials that absorb heat) are sometimes used in conjunction with risers to control the cooling rate and promote directional solidification towards the riser.