The primary function of a riser in casting is to prevent defects caused by material shrinkage during solidification.
In the casting process, molten metal solidifies and cools within the mold cavity. As materials cool and transition from liquid to solid, they typically undergo a reduction in volume, a phenomenon known as shrinkage. Without compensation, this shrinkage can lead to voids, cracks, or porosity within the final casting, compromising its structural integrity and appearance.
## How the Riser Compensates for Shrinkage
A riser, also known as a feeder, is essentially an extra reservoir attached to the casting mold. Its critical role is **to act as a reservoir of molten metal to compensate for shrinkage of casting**.
Here's how it works:
1. **Molten Metal Supply:** The riser is filled with molten metal alongside the main mold cavity.
2. **Delayed Solidification:** Risers are designed to solidify *after* the main casting. This is often achieved by making the riser larger or using insulating/exothermic sleeves around it.
3. **Feeding the Casting:** As the molten metal in the main casting starts to cool and shrink, it draws liquid metal from the still-molten riser. This flow of metal from the riser fills the space created by the shrinking material in the casting.
4. **Preventing Voids:** By continuously feeding molten metal, the riser ensures that shrinkage cavities are formed within the riser itself (which is later removed) rather than within the finished part.
This continuous feeding process is crucial for producing solid, defect-free castings, especially in thicker sections where shrinkage is more pronounced.
## Types of Shrinkage Risers Help Mitigate
Risers primarily address solidification shrinkage, which is the reduction in volume that occurs as the metal changes from a liquid to a solid state. While metals also experience liquid shrinkage (volume reduction as the liquid cools before solidifying) and solid shrinkage (volume reduction after solidification as the solid cools to room temperature), solidification shrinkage is the most significant cause of internal voids that risers are designed to prevent.
By providing this essential reservoir of molten metal, risers play a vital role in ensuring the quality, density, and dimensional accuracy of the final cast product.
