What level of dimensional accuracy can engineers expect from sand casting?

Typical Tolerance Standards for Sand Casting

Engineers can generally expect a dimensional accuracy of ISO CT10 to CT13 (or equivalent ASTM Grade D) for sand castings, depending on the part size, geometry, and specific sand process. This translates to a typical linear tolerance of approximately ±0.5 mm to ±2.0 mm per 300 mm of dimension. For example, a 100-mm feature might be held to ±0.8 mm, while a 500-mm feature could have a tolerance of ±2.5 mm. It is crucial to understand that these are general benchmarks, and tighter tolerances on critical features often require post-machining.

Key Factors Influencing Achievable Accuracy

Several variables directly impact the final dimensional accuracy of a sand-cast part:

• Mold Material: Chemically-bonded (no-bake) sands typically provide better accuracy and surface finish than traditional green sand due to greater mold rigidity and stability.

• Pattern Equipment: Patterns created via CNC Machining offer higher precision than manual methods, directly translating to a more accurate mold cavity.

• Part Geometry: Simple, uniform shapes are easier to hold to tolerance. Complex geometries with thin walls, deep pockets, or extensive cores are more prone to distortion and variation.

• Alloy Selection: Different alloys have varying shrinkage rates during solidification. For instance, Aluminum A356 has a predictable shrinkage which allows for more accurate pattern compensation compared to some steels.

Designing for Sand Casting and Managing Expectations

Successful sand casting projects involve designing with the process's inherent variability in mind. This includes incorporating generous draft angles, appropriate fillet radii, and avoiding sharp corners to minimize stress and distortion. Critical features like sealing surfaces, bolt holes, and mating interfaces should be designed with post-machining in mind, as the as-cast surface and tolerance are insufficient for precise assembly. Early collaboration with our die castings Engineering team is essential to set realistic tolerances, optimize the design for manufacturability, and plan for any necessary secondary operations to achieve the final required dimensions.