Gating and solidification: the key to eliminating porosity

When using the investment casting process, it is important to understand all the factors involved in producing a successful part. One of the first steps in avoiding part failure is to ensure the required soundness of a part before it is cast. Investment casting is one of the few manufacturing processes that can preliminarily demonstrate component success. 

Gating systems

When designing an investment-cast part, the gate location and size must be considered. The gate is a small opening that allows molten metal to flow freely into the cavity and can be regarded as the connection point between the feeding source, or sprue, and the part. Ideally, the gate is located on the thickest and heaviest section of the casting, allowing the metal to flow continuously and fill the entire part before solidifying.

How metal solidifies during casting

Metal solidification is a critical factor in understanding gating. As molten steel cools from its initial pouring temperature of approximately 3000°F to its solidifying temperature of roughly 2500°F, it undergoes volumetric shrinkage. This shrinkage continues as the metal solidifies and again as it cools to room temperature, due to thermal expansion.

For example, a 1" x 1" x 1" cube, when filled with molten steel, will shrink to approximately 0.96" x 0.96" x 0.96" at room temperature. This volumetric change requires careful consideration of part geometry and gate location and size. Proper design ensures that the casting is continuously fed with liquid metal to compensate for shrinkage as solidification progresses.

What makes a successful cast part?

Gate location and size

After understanding the basics of metal solidification, we can begin to determine the size and location of the gate. If gate details are not taken into account, the gate could solidify before the metal has the opportunity to fill the casting and replace lost volume. This could create large pockets within the part that are not fully filled, also known as internal porosity. Porosity refers to the level of solidity achieved, that is, whether there are cavities or holes within the part. If a component is not carefully gated with part function in mind, this could lead to part failure.

Part design

The gate is not the only determining factor in eliminating porosity. Part geometry can affect the directional solidification of the metal. The design must provide thermal gradients steep enough to keep the feed paths clear from the sprue to the part. Since parts do not solidify all at once and cool from the outside in, a quality design will allow the metal to cool away from the gate first. An icicle shape is the prime example of a perfect casting, with the tip freezing first and the remainder of the icicle freezing from the smallest sections through to the thickest section (the water source).

In designs that show initial porosity, adding features such as feed ribs can eliminate unwanted pockets. Part design can also be improved with structural changes, such as tapering bottom floors, to reduce porosity. In some instances, however, part geometries cannot be altered. We can take advantage of radiant heat by designing the gate with inconsequential arms that keep thin walls hot enough to feed the part, including end plates, as it solidifies.

By working closely with customers, Signicast design engineers can better understand part function and optimise part geometry to meet application requirements.

Casting material selection

In addition to design considerations, material choice directly affects porosity within a part and the solidification of a casting. Each element within an alloy has a different solidifying temperature, giving an alloy such as 17-4 stainless steel a larger temperature range as it changes from liquid to solid due to the number of elements within the alloy. The large temperature range inhibits flow, making it less forgiving than a low-carbon steel. By understanding material requirements at the beginning of the project, design engineers have a better opportunity to predict part success and limit porosity before moving to the tooling stage.

Before moving into full production with a part design, it is important to identify potential porosity issues. At Signicast, we use solidification and flow software to determine the effectiveness of gate locations and predict casting porosity to validate the design. Our design engineers can help make beneficial design suggestions that will result in successful parts without delaying time to market.