Quick Answer
DFM for investment casting — investment casting permits tighter tolerances, thinner walls, and more complex geometry than sand casting, but has specific design rules. Key rules: wall thickness 1.5–4 mm, draft 0.5–1.5°, generous fillet radii (≥1.5 mm), no parting line on critical surfaces, and proper gate location. Review these rules before finalizing your design to avoid expensive tooling changes.
Why Investment Casting Is Different from Sand Casting
Investment casting uses a ceramic shell mold that is dimensionally stable and smooth — unlike sand, which is coarse and impression-prone. This allows:
| Capability | Investment Casting | Sand Casting |
|---|---|---|
| Tolerance | ±0.1–0.3 mm | ±0.5–1.0 mm |
| Surface finish | Ra 3.2–6.3 µm | Ra 12.5–25 µm |
| Minimum wall thickness | 1.5–2 mm | 3–5 mm |
| Sharp edges | Yes (0.5 mm radius achievable) | No (needs 3–5 mm radius) |
| Undercuts | Excellent (ceramic cores) | Moderate (pattern split) |
| Internal cavities | Excellent (ceramic cores) | Good (sand cores) |
But the process constraints are different from sand casting, requiring specific design attention.
Wall Thickness Design
Investment casting wall thickness guidelines:
| Part Size | Minimum Wall (mm) | Recommended (mm) | Uniformity Rule |
|---|---|---|---|
| Small (< 50 mm) | 1.0–1.5 mm | 2–3 mm | ±25% max transition |
| Medium (50–150 mm) | 1.5–2.0 mm | 2.5–4 mm | ±30% max transition |
| Large (150–500 mm) | 2.0–3.0 mm | 3–5 mm | ±30% max transition |
Why uniformity matters: Even though investment casting allows thinner walls, uniformity still matters. Gradual transitions reduce shrinkage defects and improve dimensional stability.
Draft Angle Requirements
Ceramic shell molds are more rigid than sand, so less draft is needed:
| Surface Type | Minimum Draft | Recommended Draft | Why |
|---|---|---|---|
| External surfaces | 0.5° | 1–1.5° | Ceramic shell is rigid, less drag |
| Internal surfaces (cores) | 1° | 1.5–2° | Cores still need draft for removal |
| Deep features | 1.5–2° | 2–3° | Deep features need more draft |
Fillet Radii
Sharp internal corners cause stress concentrations and casting defects:
| Corner Type | Minimum Radius | Recommended Radius | Why |
|---|---|---|---|
| Internal corners | 1–2 mm | ≥ wall thickness | Reduces stress, eliminates hot spots |
| Junction of 3+ walls | 3–5 mm | 5–10 mm | Hot spots concentrate at junctions |
| External corners | 0.5–1 mm | 1–2 mm | Sharp edges are possible but fragile |
Rule: Internal corner fillet radius should be at least equal to wall thickness.
Minimum Feature Sizes
| Feature | Minimum Size | Recommended | Notes |
|---|---|---|---|
| Letters and numbers (embossed) | 1.5 mm high, 0.3 mm deep | 2–3 mm high, 0.5 mm deep | Embossed easier than engraved |
| Holes (through) | 2 mm diameter | ≥3 mm diameter | Thin walls limit hole size |
| Blind holes | 3 mm diameter | ≥4 mm diameter | Depth limited to 2× diameter |
| Threads (as-cast external) | M3 | M5 or larger | Machined threads preferred |
| Threads (as-cast internal) | M4 | M6 or larger | Internal threads difficult |
Parting Line Design
Investment casting parting lines are typically simpler than sand casting:
- Flat parting plane: Most investment castings use a single flat parting plane
- Complex 3D parting: Possible but expensive; avoid if possible
- Critical surfaces: Place parting line away from sealing surfaces, bearing surfaces, and appearance surfaces
- Flash removal: Flash at the parting line must be removed; design for this operation
Gate Location
The gate is where molten metal enters the casting. Gate location affects:
- Filling pattern: Gate should allow smooth filling without turbulence
- Surface finish at gate: Gate area will have rougher surface (Ra 6.3–12.5 µm); place gate away from appearance surfaces
- Gate removal: Gate must be cut off; place gate where cutting is easy and finish is acceptable
- Feeding: Gate connects to riser for feeding heavy sections
Before finalizing the sourcing decision, many OEM buyers also compare Investment Casting Process, DFM Review, Casting Tolerances, and Surface Finish to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
Buyer Checklist
- Wall thickness within recommended range (2–4 mm)?
- Wall thickness uniform (no transitions > 30%)?
- Fillet radii specified on all internal corners (≥1.5 mm)?
- Draft angles specified on all surfaces (≥0.5° external, ≥1° internal)?
- Minimum feature sizes achievable (holes ≥2 mm, letters ≥1.5 mm)?
- Parting line location acceptable (away from critical surfaces)?
- Gate location acceptable (away from appearance surfaces)?
- Surface finish requirements achievable as-cast (Ra 3.2–6.3 µm)?
- Has foundry confirmed DFM before tooling?
FAQ
Can investment casting produce undercuts?
Yes — using ceramic cores. Undercuts that cannot be removed from the ceramic mold require cores. Discuss core feasibility with your foundry; complex cores add cost.
What is the thinnest wall investment casting can achieve?
About 1.0–1.5 mm for small parts. This is at the limit and risks misrun defects. Practical minimum is 1.5–2.0 mm.
Can you have sharp edges in investment casting?
Yes — external edges can be as sharp as 0.5 mm radius. However, sharp edges are fragile during handling. Internal corners must have fillets (≥1.5 mm radius) to avoid defects.