Any good aluminum casting foundry knows to allow for shrinkage as the metal goes from its liquid to solid state. But how much do you know about exactly what happens during that shrinkage process? As a leading aluminum foundry in the U.S. for over 70 years, we work closely with our customers to communicate specific tolerances and requirements. This allows our foundry to design patterns and casting processes with shrinkage and other factors in mind, ensuring the best possible results for our customer’s needs. Let’s talk about aluminum casting shrinkage.
Why Does an Aluminum Casting Shrink?
Aluminum casting naturally shrinks as the metal goes from its liquid to its solid state. This shrinkage usually occurs in three distinct phases:
- Liquid Shrinkage: This is the reduction in volume as the molten aluminum cools while still in the liquid form. This phase is usually not a major concern as the metal can still fill the mold. This is also why as a foundry; we’ve devised a riser system for the molds that act as reservoirs of molten aluminum that feed the castings as it shrinks and solidifies.
- Solidification Shrinkage: This is the most significant phase where the metal undergoes a phase change from liquid to solid. This is also where the riser feeders on top of the mold can be most helpful to keep feeding the casting as it shrinks.
- Solid Shrinkage: After solidification, the aluminum continues to shrink as it cools down to room temperature. Usually, even the gating system will be solid at this point, unable to continually feed the casting itself. That is why a certain amount of shrinkage is built into the pattern itself to compensate for this last stage of shrinkage. The actual pattern is built a percentage larger than the final casting measurements.
Other Shrinkage Factors to Keep In Mind That May Impact the Final Product
Material Properties and Shrinkage Rates
Different aluminum alloys have different shrinkage rates. Our team will work with you to ensure that the choice of alloy you pick is right for the final dimensions of the casting you want to end up with. Discussing this information helps aluminum foundries like ours to manage customer expectations regarding the precision of the castings.
Impact on Dimensional Accuracy
Likewise, because shrinkage affects the overall dimensional accuracy of the casting, it’s important to know that patterns are typically designed larger to compensate for shrinkage. However, despite any aluminum foundry’s best efforts, minor deviations may occur, especially in complex shapes.
Potential Defects from Poor Shrinkage Control
If shrinkage is not properly accounted for during the casting process, it can lead to internal defects like shrinkage cavities or porosity. These defects can weaken the casting or result in failure during use. This is why choosing an aluminum foundry with a full understanding of proper shrinkage control is so important to ensure a strong and reliable final product.
Machining Allowances
After shrinkage, castings may require machining to achieve the desired final dimensions. Our foundry accounts for this by designing patterns with extra material that can be removed during machining, ensuring precise tolerances in critical areas. We currently machine 95% of the castings poured in our foundries. Accuracy and repeatability are built into our CNC machining centers, the oldest of which is less than three years of age. From multiple vertical machining centers equipped with the 5th axis to our large horizontal machining centers, Precision Enterprises can process nearly any size casting. With decades of experience machining aluminum castings, you can expect parts that are precisely to print and on time, every time.
Expertise and Quality You Can Trust
With over 70 years of experience in casting and machining technology, we’re the experts you need in an aluminum foundry. Let us review your project and have our in-house engineering staff provide cost savings ideas; tooling, materials, alloys, tolerances, finishes…and more. Request a quote today.
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