A riser is a specially designed cavity in the mold that stores molten metal. Its purpose is to compensate for the shrinkage of the casting during solidification, thereby preventing shrinkage cavities and porosity. Risers also serve the functions of venting and slag collection.
Types of Risers
There are many types of risers, which are generally classified as follows:
• Top riser: Positioned at the highest part of the casting. It not only facilitates venting and slag flotation but also benefits from gravity feeding. Large and heavy steel castings often use this type of riser for feeding.
• Side riser: Set on the side of the mold adjacent to the section being fed, generally in the form of a blind riser. This type is advantageous for mechanized molding.
Both top and side risers can be further divided into open risers and blind risers.
• Open risers are easy to mold and allow observation of the molten metal level in the cavity. They also make it convenient to apply exothermic or insulating agents on the riser surface, and for large castings, operations such as stirring the riser metal or topping up can be carried out to enhance feeding efficiency. Open risers are not restricted by flask height, and riser sleeves can be used to achieve the required height. However, their feeding efficiency is lower than that of blind risers.
• Blind risers require the flask to be taller than the riser, which increases flask volume, but they are more flexible and can be placed closer to the hot spots of the casting. Large castings, especially steel castings, commonly use open risers, whereas medium and small malleable iron and ductile iron castings primarily adopt blind risers. Blind risers are particularly suitable for machine molding.
Riser Shapes
The shape of the riser should provide sufficient volume while minimizing the relative cooling surface area (i.e., having a sufficiently large modulus). It must also maintain adequate metal head pressure to prolong solidification time and improve feeding effectiveness.
Although spherical risers have the largest modulus, they are rarely used due to their low pressure and manufacturing difficulties. In practice, the most commonly used shapes include cylindrical risers, spherical-top cylindrical risers, oval cylindrical risers, and annular risers.
• Cylindrical risers are the most widely used.
• For gear rims and wheel castings, where the hot spot is elongated, oval cylindrical risers are often employed.
• For cylinders, sleeves, and hub areas of wheels, annular risers are more suitable.
Feeding Principles of Common Risers
Basic Requirements
Risers must satisfy the following conditions:
1. The solidification time of the riser must be greater than or equal to that of the casting section it feeds.
2. The riser must contain enough molten metal to compensate for the shrinkage of the casting during cooling.
3. During feeding, there must be an uninterrupted feeding channel between the riser and the casting hot spot, with the expansion angle opening toward the riser.
Riser Placement
The positioning of the riser directly affects casting quality and feeding efficiency. Improper placement not only fails to eliminate shrinkage cavities and porosity but may also cause other defects (such as cracks). The following principles should be followed:
1. Risers should be placed as close as possible to the hot spots of the casting, either above or to the side.
2. Risers should preferably be located at the highest and thickest parts of the casting. If hot spots occur at different heights, separate risers can be used, but the feeding areas should be separated with chills to prevent the upper riser from having to feed both the casting and the lower riser, which could otherwise cause shrinkage cavities and porosity in the upper part of the casting.
3. Risers should avoid stress concentration areas and should not be placed at locations prone to cracking or stress accumulation, otherwise the tendency for cracking will increase.
4. One riser should ideally feed multiple hot spots to improve feeding efficiency.
5. Risers should be placed on machining surfaces whenever possible to reduce finishing work.
6. Risers should not be located in critical or heavily loaded areas of the casting, as the coarse grain structure in the riser area may reduce mechanical properties.
Post time: Sep-12-2025