What Is A Riser In Casting?

A riser in the casting process is a passage in the casting mold which prevents casting shrinkage defects and is designed into the casting mold. Once the liquid metal has solidified, the riser is removed from the mold.

A riser reduces the number of defects in the casting materials, hence improves the number of defect-free products of the process. Improving this reduces additional costs for the manufacturer as post-casting work is not required to rectify the defects.

A riser results in a greater requirement of molten metal, making the design of the riser essential for efficiency.

How Does A Casting Riser Work?

When the molten metal is poured through the casting gating system to enter the mold, it enters the riser once the mold is full. The ‘rising’ of the molten metal is what gave the riser its name.

As metals are less dense in a liquid state compared to a solid state, voids form in the casting due to cooling/shrinkage. A void/cavity tends to form at the last point of solidification.

A riser prevents the formation of the void by supplying additional molten metal to the mold as the metal solidifies. As a result, the void then forms within the riser and not the casting.

For a riser to work, there are three requirements. 1) The riser cools after the casting 2) The riser has sufficient material to compensate for the shrinkage seen in the casting 3) The casting solidifies in the direction of the casting.

1 and 2 are both related to the position of the riser in the casting mold. 3. is centred around the volume of molten metal used to fill the mold. With careful design and planning, all 3 factors can be met.

Chvorinov’s rule

Chvorinovs rule is a quick and simple way of determining the minimum size of a riser. It is achieved by setting the cooling time of the riser longer than the casting. 25% is the standard time difference.

T_riser = 1.25T_casting

Using the above formula, calculations can be completed to determine a suitable riser size.

A shape with a large volume and surface area is generally used as a riser to ensure a longer cooling time. For this reason, a cylindrical shape is a popular choice.

How Is a Riser removed From a Casting?

A riser is removed from a casting mold by tumbling or air blasting, however, this is dependent on the size and thickness. When these processes do not work, an oxy-acetylene torch, chipping hammers and sledges may be used to remove them.

Types of Risers

Open Riser

An open riser is a riser that has an opening to the atmosphere from the mold. Open risers tend to be larger than other types because they experience greater heat loss. An open riser is generally a cylindrical shape.

Closed/Blind Riser

A closer or blind riser is a riser that is contained within the mold. In some instances, a vent is used for a blind riser to expose it to the atmosphere.

Top and Side Riser

Riser names are also determined by their location on the casting mold. For example, a riser located on the side of the mold is known as a side riser.

Summary

Risers are crucial to preventing casting defects within the casting material. It is achieved by allowing the voids to form in the riser. However, the positioning of the riser is essential to determining its effectiveness.

There are a variety of different risers, however the most common is a cylindrical shape due to its longer cooling time. The nomenclature follows a simple pattern with them identified by positions (e.g. top, side etc.) and if the riser is open to the atmosphere (e.g. open or closed).

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