The casting process of gray iron
The casting process of gray iron includes the three elements known as the “three musts” in the casting industry: good iron, good sand, and good process. The casting process is one of the three major factors, alongside iron quality and sand quality, that determine the quality of castings. The process involves creating a mold from a model in the sand, and then pouring molten iron into the mold to create a casting.
The casting process includes the following components:
1. Pouring basin: This is where the molten iron enters the mold. To ensure the consistency of the pour and remove any impurities from the molten iron, there is usually a slag collection basin at the end of the pouring basin. Directly below the pouring basin is the sprue.
2. Runner: This is the horizontal part of the casting system where molten iron flows from the sprue to the mold cavity.
3. Gate: This is the point where the molten iron enters the mold cavity from the runner. It is commonly referred to as the “gate” in casting. 4. Vent: These are holes in the mold that allow air to escape as molten iron fills the mold. If the sand mold has good permeability, vents are usually unnecessary.
5. Riser: This is a channel used to feed the casting as it cools and shrinks. Risers are used to ensure that the casting has no voids or shrinkage cavities.
The key points to consider when casting include:
1. The orientation of the mold: The machined surface of the casting should be located at the bottom of the mold to reduce the number of shrinkage cavities in the final product.
2. Pouring method: There are two main methods of pouring – top pouring, where the molten iron is poured from the top of the mold, and bottom pouring, where the mold is filled from the bottom or middle.
3. Positioning of the gate: Since molten iron solidifies quickly, it’s important to position the gate in a location that will ensure proper flow into all areas of the mold. This is particularly important in thick-walled sections of the casting. The number and shape of the gates should also be considered.
4. Type of gate: There are two main types of gates – triangular and trapezoidal. Triangular gates are easy to make, while trapezoidal gates prevent slag from entering the mold.
5. Relative cross-sectional area of the sprue, runner, and gate: According to Dr. R. Lehmann, the cross-sectional area of sprue, runner, and gate should be in the ratio A:B:C=1:2:4. This ratio is designed to allow the molten iron to flow smoothly through the system without trapping slag or other impurities in the casting.
The design of the casting system is also an important consideration. The bottom of the sprue and the end of the runner should both be rounded to reduce turbulence when molten iron is poured into the mold. The time taken for the pour is also important.
