yvonne@nydcasting.com 
Clay wet sand molds have poor rigidity, causing cavity expansion. Factors include sand quality, mold compactness, pouring temperature, and molten iron pressure. The actual volume expansion can be between 2% and 8%.
Since the volume expansion of the mold cavity varies greatly, the principles for setting risers will of course vary depending on the specific situation.
1. Thin-walled castings
Castings with a wall thickness of less than 8mm generally do not have obvious mold wall movement, and the liquid shrinkage of the molten iron after filling the mold is not too large, so a riser-free casting process can be used.
2. Castings with a wall thickness of 8 to 12mm
For this type of casting, if the wall thickness is uniform and there is no large hot spot, as long as the low-temperature pouring is strictly controlled, a riser-free casting process can also be used.
If there is a hot spot, and shrinkage cavities and shrinkage are not allowed inside, a thin-necked riser should be set according to the size of the hot spot.
3. Castings with wall thickness of more than 12mm
When using clay wet sand molds to produce such castings, the mold wall movement is quite large, and it is difficult to produce castings without internal defects. When formulating a process plan, you can first consider using a thin-necked riser and strictly control low-temperature pouring. If this solution cannot solve the problem, you have to design a special riser.
When using clay wet sand molds to produce ductile iron castings, if you want to set a riser, it is best to do the following:
Use a thin inner gate to solidify after the mold is filled. After the inner gate solidifies, the casting and the riser form a whole and are not connected to the pouring system;
When the casting shrinks in liquid state, the riser replenishes the casting with molten iron;
When the casting expands due to graphitization, the molten iron flows to the riser to release the pressure in the mold cavity. Reduce its effect on the mold wall;
When the casting body shrinks secondary after graphitization and expansion, the riser can provide the casting with molten iron to compensate for shrinkage.
It may not sound complicated, but in fact, many factors must be considered when designing a riser. Moreover, no effective specific solution has been found so far, and there is no set of data that is easy to use. In production, exploration and experimentation are often necessary to balance the quality of castings and the process yield.
Regarding the design of risers, the following two methods can be referred to.
1. Top riser
R.W. Heine of the United States has conducted a systematic study on the riser setting problem of producing ductile iron castings with clay wet sand molds, and proposed a variety of process solutions to suit different conditions, including riserless casting process, edge pressure riser process and top riser process.
A top riser is set between the two hot nodes of the casting. With the help of the heat effect of the riser, the temperature of the thinner part below the riser is increased, and the riser can replenish the molten iron to the two hot nodes through this place.
When the casting undergoes graphitization expansion, the molten iron can flow to the riser through the hole of the spacer, releasing the pressure in the cavity, and the cavity expands very little.
When the casting shrinks secondary after graphitization expansion, the pressure head in the riser can make the molten iron flow to the casting to compensate for shrinkage.
A spacer is added between the riser and the casting to facilitate knocking down the riser after sand falling.
2. Controlled pressure riser
The controlled pressure riser was proposed by S.I.Karsay. Its mechanism of action is the same as Heine’s top riser. In terms of structure, a dark riser with a larger riser neck section is set on the side of the top of the casting.
After the mold cavity is filled, the thin inner gate solidifies, the casting and the dark riser become a connected whole, and a thin shell that is not very strong condenses at the mold-metal interface, forming a system isolated from the outside world.
The liquid shrinkage of the casting before graphitization expansion is supplemented by transporting molten iron to the casting by the riser.
When the casting undergoes graphitization expansion, the molten iron flows to the riser to release the pressure. In the later stage of graphitization expansion, the pressure in the system is slightly higher than the atmospheric pressure, but it is not enough to exceed the bearing capacity of the thin shell and wet sand mold, so as not to cause mold wall movement.
When the casting undergoes secondary contraction after graphitization expansion, the pressure in the system is still slightly higher than the atmospheric pressure, so that defects such as shrinkage cavities and shrinkage porosity can be avoided inside the casting.
This riser prevents high pressure during graphitization expansion, avoiding mold wall movement. It also maintains pressure to prevent shrinkage defects. Therefore, it is called a controlled pressure riser.