Gating and Solidification: The Key to Eliminating Porosity
When turning to the investment casting process, it is important to understand all of the factors that go in to making a successful part. One of the first steps to avoid 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.
When designing an investment cast part, the gate location and size must be taken into consideration. The gate is a small opening that allows molten metal to flow freely to the cavity and can be thought of as the connection point between the feeding source, or sprue, and the part. The gate is ideally located on the thickest and heaviest section of the casting, allowing the metal to continuously flow and fill the part in its entirety before solidifying.
How does metal solidify during casting?
To fully understand gating, you must first understand how metal solidifies and what happens when the metal transitions from a liquid to a room temperature solid. Molten steel is measured at approximately 3000°F. From the initial pouring temperature, the metal will shrink volumetrically as it cools to its solidifying temperature of roughly 2500°F. As the metal continues to solidify, there is an additional shrinkage of volume, also referred to as solidification shrinkage, and once the metal finally becomes a solid, there is one final change in volume due to thermal expansion.
To give an example, if a 1" x 1" x 1" cube is filled up, the final room temperature part size would be 0.96" x 0.96" x 0.96". Because of the change in volume, part geometry and gate location and size must be designed so that as the metal starts to solidify, the casting is still being continually fed with liquid metal to replace the change in volume. Volumetric change is key in designing part geometry and gating.
What makes a successful cast part?
Gate location and size
After understanding the basics of metal solidification, we can start 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.
The gate is not the only determining factor in eliminating porosity. Part geometry can impact the directional solidification of the metal. The design must account for making 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 they cool from the outside in, a quality design will allow for 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 back through to the thickest section (the water source).
In designs that show initial porosity, adding features like feed ribs can eliminate any unwanted pockets. Part design can also be enhanced with structural changes, like tapering bottom floors, to reduce porosity. In some instances, however, part geometries cannot be altered. We can take advantage of radiant heat, designing the gate with inconsequential arms that keep thin walls hot enough to feed the part, including end plates, as it solidifies.
In working closely with customers, Signicast design engineers are able to better understand part function and optimize part geometry to meet the requirements of the application.
Casting Material Selection
In addition to design considerations, material choice directly impacts porosity within a part and the solidification of a casting. Each element within an alloy has different solidifying temperature, making an alloy like 17-4 stainless steel have a larger temperature range as it goes from a liquid to a solid due to the amount of elements within the alloy. The large temperature range inhibits flow, making it less forgiving than a low carbon steel. Knowing material requirements on the frontend of the project, design engineers have a better opportunity to predict part success and limit porosity before moving to the tooling stage.
Before heading 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 efficacy of gating 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.
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