What are investment casting tolerances?

Investment casting tolerances refer to the range of allowable deviations in the size and shape of metal castings during the production process. These tolerances are crucial to the performance, lifespan, and overall quality of the part. Especially in industries with extremely high precision requirements, such as aerospace, automotive, and medical devices, properly controlling investment casting tolerances often determines the success of the product.

Major Factors Affecting Investment Casting Tolerances

There are several factors that affect investment casting accuracy, including:

Mold Design and Complexity: The more complex the mold structure, the more challenging it is to control dimensional errors. Proper mold design can help improve dimensional consistency.

Metal Alloy Type: Different alloys shrink at different rates during cooling, which directly affects the final dimensional accuracy.

Cooling Shrinkage Behavior: All metals shrink during cooling. Accurately understanding the shrinkage behavior of the alloy is key to achieving target dimensions and controlling tolerances.

Wax Pattern Type: Medium-temperature and low-melting waxes, commonly used in investment casting, have different effects on tolerances. The wax-filled process can achieve tolerances of approximately 0.1–0.5 mm. Its advantages include high hardness, low shrinkage, good toughness, and strong dimensional stability.

Low-melt wax is often used for products with lower requirements for surface quality or dimensional accuracy. For example, when using low-melt wax for semi-silica sol composite casting of C15 alloy steel, tolerances of approximately ±0.1 mm can be achieved. Low-melt wax, typically composed of paraffin wax and stearic acid, is less expensive but offers slightly lower dimensional stability than wax-filled casting.

Overall, wax-filled casting offers advantages in dimensional accuracy, while low-melt wax is more cost-effective.

In addition, the volume and geometry of the casting also affect the achievable tolerance level. Large or complex parts are generally more prone to deviation, while small, simple parts are easier to maintain tight dimensional control.

Typical Tolerance Range

Investment casting is known for its high precision. Generally speaking, tolerances for small parts are approximately ±0.005 inches (±0.127 mm), while larger parts can be within ±0.010 inches (±0.254 mm). Specific values vary depending on metal type, part size, and process requirements. Compared to other processes like sand casting or die casting, investment casting offers higher dimensional accuracy and is therefore often used in applications requiring stringent precision.

Key Practices for Achieving Optimal Tolerances

To achieve ideal investment casting accuracy, manufacturers can take the following steps:

Optimize Mold Design: Work closely with the foundry to optimize mold structure for improved dimensional consistency.

Consider Secondary Operations: For extremely tight dimensional tolerances, machining or finishing can be used to further ensure accuracy.

These measures enable manufacturers to better collaborate with the foundry to ensure that products meet the expected precision standards.

At Chengshuo, our precision control focuses on two areas: wax pattern stability and mold shell quality.

Wax Pattern Stability: Wax shrinkage, deformation, and thermal resilience all affect final tolerances. We use modified, filled waxes to achieve CT5–CT6 accuracy grades, while low-melting waxes typically only achieve CT8–CT9.

Mold Shell Quality: Molding sand properties, including permeability and strength, are critical to casting stability. We rigorously screen our raw materials to ensure the sand possesses the best properties, ensuring consistent dimensional accuracy and excellent surface quality.