Riser (casting)
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A riser or feeder is a reservoir built into a metal-casting mold to prevent cavities due to shrinkage. Because metals are less dense as liquids than as solids (with some exceptions), castings shrink as they cool. This can leave a void, generally at the last point to solidify. Risers prevent this by providing molten metal at the point of likely shrinkage, so that the cavity forms in the riser, not the casting.
This only works if the riser cools after the rest of the casting. Chvorinov's rule states that the solidification time t of molten metal is related to the constant C (which depends on the thermal properties of the mold and the material) and the local volume (V) and surface area (A) of the material, according to the relationship
Therefore, to ensure that the casting solidifies before the riser, the ratio of the volume to the surface area of the riser should be greater than that of the casting. The riser must satisfy two requirements: it must be large enough so that it solidifies after the casting (i.e. satisfies Chvorinovs rule) and it must contain a sufficient volume of metal to supply the shrinkage contraction which occurs on cooling from the casting temperature to the completion of solidification. This latter requirement will be more important for platelike shapes; the former will be more important for chunky shapes.
Hence the casting should be designed to produce directional solidification which sweeps from the extremities of the mold cavity toward the riser. In this way, the riser can feed molten metal continuously and will compensate for the solidification shrinkage of the entire mold cavity. If this type of solidification is not possible, multiple risers may be necessary with various sections of the casting solidifying toward their respective risers.
The activity of planning of how a casting will be gated and risered is called 'Foundry Methoding' or 'Foundry Engineering'. The efficiency of Methoding the risering plan can be expressed in terms of percentage, by dividing the weight of the casting shipped to the customer by the total pouring weight used to make the casting. Because risers exist only to ensure the integrity of the casting, they are removed after the part has cooled, and their metal is remelted to be used again. As a result, riser size, number, and placement should be carefully planned to reduce waste while filling all the shrinkage in the casting. The Methoding engineer normally aims for the highest efficiency in riser design while making a part quality with minimal or no rejections from shrink defects.
 Bronze casting showing sprue and risers (side view) |
 Bronze casting showing shrinkage in sprue (top view) |
[edit] References
- Kalpakjian, Serope, et al. (2001). Manufacturing Engineering and Technology. Published by Pearson Education.
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