Optimising Part Geometry for Investment Casting

Investment casting is known for its ability to produce complex geometries with remarkable precision. However, to fully capitalise on this manufacturing method, part geometry must be carefully optimised to ensure a smooth casting process and achieve the desired performance. In this article, we’ll explore the critical aspects of investment casting part geometry and provide guidance on how to optimise your designs.

Understanding Part Geometry in Investment Casting

Investment casting allows for near-net-shape parts, meaning that components are cast to their final shape with minimal need for secondary machining. This flexibility is particularly beneficial for intricate designs, but there are some geometric considerations that can influence the success of the casting. By understanding these key factors, designers can create parts that are both manufacturable and functional.

Maintain Uniform Wall Thickness

One of the most important aspects of optimising part geometry is ensuring uniform wall thickness. Variations in thickness can lead to issues such as shrinkage, warping, or cracking during the cooling phase. To avoid these problems, aim for a consistent wall thickness throughout the part, typically between 2mm and 6mm, depending on the material and size. If variation in thickness is unavoidable, use gradual transitions to minimise stress concentrations.

Incorporate Draft Angles

Incorporating draft angles into your design is essential for easy removal of the wax pattern from the mould. Without sufficient draft, the pattern may stick to the mould, leading to defects or damage. A draft angle of at least 1 to 2 degrees is recommended for most vertical surfaces. For larger or more complex parts, a greater draft may be necessary to ensure smooth removal and reduce the risk of defects.

Avoid Sharp Corners

Sharp corners and edges can be problematic in investment casting as they create areas of high stress and are more prone to cracking. Wherever possible, replace sharp corners with fillets or rounded edges to distribute stress more evenly. A minimum radius of 2 to 3 mm is generally recommended to improve the strength of the part and reduce the likelihood of casting defects.

Minimise Undercuts and Overhangs

While investment casting can produce complex shapes, undercuts and overhangs should be minimised. These features can make tool design and removal of the ceramic mould more difficult. If your design requires undercuts, consider using cores or breaking the part into multiple sections that can be assembled later. Reducing these challenging geometries can improve casting quality and reduce production time.

Design for Material Flow

Proper material flow is crucial to achieving high-quality cast parts. When designing your part geometry, it’s important to consider how the molten metal will flow through the mould. Thick sections of the part can create turbulence or cause cooling inconsistencies, leading to defects such as porosity or incomplete fills. By designing parts with smooth transitions and consistent cross-sections, you can promote even material flow and improve the final cast quality.

Plan for Shrinkage

All metals shrink as they cool, and this must be accounted for when optimising part geometry. The amount of shrinkage varies depending on the material, with most metals shrinking between 1.5% and 2%. Collaborate with your foundry to understand the shrinkage rates for your chosen material and adjust your part dimensions accordingly to ensure the final part meets your specifications.

Avoid Thin Protrusions

Thin protrusions such as fins, tabs, or narrow ribs are prone to breakage during the casting process, especially if they are long and unsupported. If your design includes thin features, try to keep them short or reinforce them with fillets or additional support structures. Thicker or more robust geometries will be more durable and result in fewer defects.

Use Generous Radii for Internal Features

Internal features, such as holes or slots, should incorporate generous radii rather than sharp edges. Sharp internal corners can cause stress concentrations and lead to cracking or other defects during cooling. Incorporating larger radii in these areas will help reduce stress and improve the overall durability of the part.

Simplify Complex Geometries

While investment casting is well-suited for complex shapes, overly intricate designs can increase the risk of casting defects and raise production costs. Whenever possible, aim to simplify complex geometries. If a part is particularly complicated, it may be beneficial to split the component into multiple castings that can be assembled later, rather than attempting to cast everything in a single piece.

Collaborate With Your Foundry

Finally, collaboration with your investment casting foundry is critical. Foundry engineers can offer valuable insights into optimising part geometry for manufacturability.

Engaging with your foundry early in the design process can help avoid potential issues and ensure that your part geometry aligns with the capabilities of the investment casting process.

The Importance of Optimising Part Geometry

Optimising part geometry for investment casting is essential for achieving high-quality components. By following these guidelines — such as maintaining uniform wall thickness, incorporating draft angles, and minimising complex features — designers can create parts that are not only functional but also easier to manufacture.

The result is a more efficient, cost-effective process with fewer defects and higher overall quality.