Investment Casting Design Guide
Design investment castings that fill completely, cool evenly, and finish cleanly
Investment casting (lost wax) produces complex, near-net-shape parts in virtually any alloy — stainless steel, carbon steel, aluminum, bronze, superalloys. It's ideal for parts that are too complex for die casting but need better surface finish than sand casting. However, the process has its own rules around wall thickness, gating, and solidification that must be respected for consistent quality.
Core Design Principles
Directional Solidification
Metal must solidify from thin sections toward the gate/riser. Isolated thick sections create shrink porosity.
Wax Pattern Constraints
The part starts as a wax pattern. If the wax can't be injected or extracted from the die, the casting can't be made.
Shell Strength
The ceramic shell must withstand the force of molten metal. Thin shells on large flat areas can crack, causing run-outs.
Near-Net-Shape Thinking
Investment casting gets you 80–90% there. Design to minimize machining — but plan for it on critical surfaces.
DFM Rules & Guidelines
Wall Thickness
Too thin and the metal freezes before filling. Too thick and shrink porosity forms inside.
Minimum: 1.5 mm for steel, 1.0 mm for aluminum, 2.0 mm for superalloys. Maximum: keep under 25 mm to avoid centerline shrinkage. Transition gradually (3:1 taper).
Walls thinner than 1.0 mm in ferrous alloys. Abrupt thickness changes. Isolated thick sections without feed paths.
Draft Angles
Investment casting needs less draft than die casting — but some draft helps wax pattern ejection from the metal die.
Minimum 0.5° on external surfaces. 1° on internal surfaces. Zero draft is possible on short surfaces (< 10 mm) but increases wax die cost.
Negative draft (undercuts in the wax die) without planning for soluble wax or ceramic cores.
Fillets & Transitions
Sharp corners create stress concentrations in both the casting and the ceramic shell. They also impede metal flow.
Internal fillets: R ≥ 1.0 mm (R2.0 mm preferred). Blend wall thickness changes over at least 3× the thickness difference. Round all edges minimum R0.5 mm.
Sharp internal corners (causes hot spots and cracks). Knife edges on external features.
Holes & Internal Features
Small holes and complex internal passages may need ceramic cores, adding cost and complexity.
Cast-in holes: minimum Ø3 mm for blind, Ø2 mm for through. Depth: max 2× diameter without cores. For complex internal passages, discuss ceramic core feasibility early.
Holes smaller than Ø2 mm (use drilling as secondary op). Long, narrow internal channels without core support.
Flatness & Straightness
Investment castings warp during shell removal and heat treatment. Large flat surfaces are hardest to keep flat.
Add ribs or curvature to large flat surfaces. Allow 0.3 mm per 25 mm for flatness. Straightness: ±0.5 mm per 100 mm. Machine datum surfaces if needed.
Large, unsupported flat areas > 100 mm. Tight flatness specs on as-cast surfaces without machining allowance.
Tolerances
Investment casting is more precise than sand casting but less precise than machining. Standard tolerances vary by size.
Linear: ±0.1 mm per 25 mm (±0.4% of dimension). Angular: ±0.5°. Surface finish: Ra 3.2–6.3 µm as-cast. Machine critical features to tighter specs.
Expecting CNC precision on as-cast features. Flatness tighter than 0.1 mm on surfaces > 50 mm without machining.
⚠️ Common Design Mistakes
- ⚠ Designing isolated thick sections (bosses, hubs) without feed paths — they solidify last and develop shrink porosity.
- ⚠ Requesting zero draft on all surfaces — possible on short features but dramatically increases wax die cost and pattern rejection rate.
- ⚠ Not providing machining stock on datum surfaces — as-cast surfaces aren't flat enough for reliable fixturing.
- ⚠ Specifying alloys that aren't investment-casting-friendly — some alloys pour and feed well; others are prone to hot tearing.
- ⚠ Over-specifying surface finish — investment castings are already Ra 3.2–6.3 µm. Polishing to Ra 0.8 µm is expensive and often unnecessary.
💡 Pro Tips
- ▸ Combine multiple brazed/welded components into a single casting — investment casting excels at complexity consolidation.
- ▸ Add "cast-in" text and part numbers — they're free in the wax die and provide permanent traceability.
- ▸ Discuss gating strategy with your foundry — gate location affects surface finish, porosity, and dimensional accuracy.
- ▸ For structural parts, specify HIP (Hot Isostatic Pressing) to close internal porosity — critical for aerospace and medical applications.
- ▸ Investment casting minimum order quantities can be as low as 25–50 pieces — it's more accessible than most engineers think.
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