Investment Casting Tolerance Guide
Near-net-shape precision for complex geometries
Investment casting (lost wax) produces the most complex geometries of any casting process with excellent surface finish and near-net-shape tolerances. Understanding what's achievable as-cast helps you design parts that minimize secondary machining — often the most expensive part of the process.
Overview
Investment casting typically holds ±0.1–0.3 mm on linear dimensions depending on size and alloy. It bridges the gap between die casting (tighter, limited alloys) and sand casting (wider tolerances, any alloy). The process excels at complex stainless steel and superalloy parts.
Tolerance Specifications
| Feature | Standard | Precision | Notes |
|---|---|---|---|
| Linear Dimensions (< 25 mm) | ±0.10 mm | ±0.05 mm | Small features hold tight tolerances naturally. Wax injection is very repeatable at this scale. |
| Linear Dimensions (25–100 mm) | ±0.25 mm | ±0.13 mm | Standard tolerance per CT5-CT6 (ISO 8062). Precision requires premium shell and controlled pour. |
| Linear Dimensions (100–250 mm) | ±0.50 mm | ±0.25 mm | Larger parts have more thermal contraction variation. |
| Surface Finish (Ra) | Ra 3.2–6.3 µm | Ra 1.6 µm | As-cast finish. Finest in the casting world — often better than sand, comparable to die casting. |
| Flatness | 0.15 mm per 25 mm | 0.08 mm per 25 mm | Controlled by shell rigidity during cooling. Thicker shell = less distortion. |
| Wall Thickness (Minimum) | 1.5 mm (steel), 1.0 mm (aluminum) | 1.0 mm (steel), 0.75 mm (aluminum) | Depends on flow length. Longer fill paths need thicker walls. |
| Hole Diameter (As-Cast) | ±0.10 mm | ±0.05 mm | Ceramic cores for internal passages. Complex cores add cost and tolerance. |
| Draft Angle | 0.5°–1.0° | 0° (with side-pulls) | Investment casting needs minimal draft — a major advantage over die casting. |
Key Considerations
Near-Zero Draft
Unlike die casting, investment casting needs minimal draft (0.5° typical, 0° possible). This allows vertical walls and cleaner geometry.
Surface Finish Advantage
As-cast Ra 3.2 µm means many surfaces need no finishing. Specify "as-cast" for non-critical surfaces to save cost.
Consolidate Assemblies
Investment casting can produce geometry that would otherwise require welding or assembly of multiple machined parts — reducing both cost and tolerance stack-up.
Alloy Selection Impacts Tolerance
Low-carbon steels and aluminum hold tighter tolerances than high-alloy stainless or superalloys due to lower and more predictable shrinkage.
💰 Cost Impact of Tolerances
Precision investment casting requires premium wax injection, controlled shell layup, and 100% dimensional inspection — 25–40% higher than standard.
Standard CT5-CT6 tolerances are achievable with normal production methods and spot-check inspection.
Design critical features (bores, sealing surfaces, mating faces) with 0.5–1.0 mm machining stock. Achieve as-cast tolerances on everything else. This hybrid approach gives CNC precision where needed at casting economics.
⚠️ Common Tolerance Mistakes
- ⚠ Expecting die casting tolerances from investment casting — they're different processes with different capabilities.
- ⚠ Designing sections thinner than the alloy can reliably fill — consult your foundry on minimum wall for your geometry and alloy.
- ⚠ Not providing adequate machining stock on critical features — 0.5–1.0 mm is standard.
- ⚠ Over-specifying surface finish when as-cast (Ra 3.2 µm) is already better than most machined finishes.
- ⚠ Designing parts without considering gating and riser locations — these affect surface quality and may leave witness marks.
💡 Pro Tips
- ▸ Investment casting is ideal for stainless steel parts that would be very expensive to CNC machine — 316L, 17-4PH, CF8M.
- ▸ Rapid prototyping: 3D-printed wax or resin patterns can replace injection-molded wax for first articles.
- ▸ Our Vietnamese investment casting foundries produce CT5-CT6 quality with full material certs at 40–60% of Western pricing.
- ▸ Combine investment casting with CNC finishing for the best balance of geometry complexity and critical dimension accuracy.
- ▸ For structural parts, request radiographic (X-ray) inspection to verify internal soundness.
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