A practical guide for engineers and buyers: what green sand casting can and cannot do—minimum wall thickness, ISO 8062 tolerances, surface finish (Ra), typical machining stock, and a cost curve by part size/complexity. Includes a quick comparison to resin sand and shell molding, and an RFQ checklist.
What is green sand casting best for? (capability snapshot)
- Part sizes: from palm-size to >100 kg (220 lb). Automated lines excel ≤20 kg (≤44 lb).
- Materials: excellent for gray/grey iron (EN-GJL / ASTM A48) and ductile iron (EN-GJS / ASTM A536).
- Complexity: single- to moderate-core parts; heavy core packages are possible but cost/yield suffer.
- Throughput: highest among sand processes; best piece-price at medium–high volumes.
Minimum feasible wall thickness (mm / in, typical):
- Gray iron: 4–6 mm (0.16–0.24 in) on auto lines with sound design.
- Ductile iron: 5–7 mm (0.20–0.28 in).
- Very thin features are safer with shell molding; large/heavy sections often prefer resin sand for stability.
Tolerances & dimensional control (ISO 8062)
Casting tolerances (as-cast), indicated by size
| Envelope size (max dim) | Typical CT grade (ISO 8062-3) | Notes |
|---|---|---|
| ≤ 200 mm (≤ 7.9 in) | CT7–CT8 | Auto line, well-designed gating |
| 200–500 mm (7.9–19.7 in) | CT8–CT9 | Most common industrial parts |
| ≥ 500 mm (≥ 19.7 in) | CT9–CT10 | Heavier/long parts; consider resin sand |
GD&T tip: control machined features with position/profile/flatness, not with tight as-cast tolerances. Reference ISO 8062-3 for CT and ISO 1302 for surface texture.
Typical room-temperature properties (indicative)
Draft, shrinkage & machining stock
- Draft angles: external 1.0–1.5°, internal 1.5–2.0° (green sand).
- Shrinkage allowance (pattern): gray ~ 0.8–1.0%, ductile ~ 1.0–1.2%.
- Typical machining allowance (both units):
| Rough weight | Green sand allowance |
|---|---|
| ≤ 5 kg (≤ 11 lb) | 1.0–1.5 mm (0.04–0.06 in) |
| 5–20 kg (11–44 lb) | 1.5–2.5 mm (0.06–0.10 in) |
| ≥ 20 kg (≥ 44 lb) | 2.5–3.5 mm (0.10–0.14 in) |
Surface finish (Ra) and cleanliness
- As-cast Ra: 6.3–12.5 μm (250–500 μin) with typical green sand and shot-blast.
- Improving Ra: finer sand (AFS GFN), better mold wash, controlled blast media, and coverage.
- Coating prep: follow paint system (e.g., ISO 12944); mask sealing faces/threads.
Design rules that make green sand successful
Uniform sections & transitions
- Keep walls uniform; use fillets (≥ 2–3× wall) to avoid hot spots.
- Add ribs instead of thickening plates; target a balanced modulus.
Cores & venting
- Minimize core count; specify core prints and seating tolerances.
- Coat cores (zircon/graphite) to prevent burn-on and gas porosity; ensure proper venting.
Parting line & handling
- Choose a simple, stable parting line; design lifting points early.
- Provide datum features that survive molding and blasting, easing later machining and CMM checks.
Cost curve by part size & complexity (index, gray iron = 1.00)
Use this as a sourcing guide; your quote will reflect pattern strategy, cores, yield and volume.
| Weight & complexity | Cost index (green sand) | Cost drivers & notes |
|---|---|---|
| ≤ 5 kg (≤ 11 lb), low cores | 0.85–1.00 | High line efficiency; low cleaning time |
| 5–20 kg (11–44 lb), simple cores | 1.00–1.20 | Sweet spot for auto lines |
| 20–50 kg (44–110 lb), multiple cores | 1.20–1.45 | More risering/hand work; yield ↓ |
| 50–120 kg (110–265 lb), heavy cores | 1.40–1.70 | Consider resin sand for stability |
| Thin-wall (near minima) any size | +0.10–0.20 | Higher scrap risk, tighter control |
| Pressure-tight (leak test) | +0.05–0.15 | NDT/leak testing, possible impregnation |
Volume effect: piece-price drops as pattern amortization spreads; break-even often around 500–3,000 pcs/year depending on tool cost and yield.
Green sand vs resin sand vs shell (when to switch)
| Criterion | Green sand | Resin sand | Shell molding |
|---|---|---|---|
| Best part size | Small→medium | Medium→large/heavy | Small→medium, thin-wall |
| Typical CT | CT7–CT9 | CT8–CT10 | CT6–CT8 |
| As-cast Ra | 6.3–12.5 μm | 3.2–6.3 μm | 1.6–3.2 μm |
| Min wall (gray/ductile) | 4–6 / 5–7 mm | 6–10 / 7–12 mm | 3–4 / 4–5 mm |
| Cores | Few–moderate | Moderate–many | Low–moderate |
| Piece price | Lowest at volume | Medium | Medium-high |
| When to choose | Volume, moderate accuracy | Large/heavy, stability | Thin-wall, high accuracy |
Quality plan YBmetal runs on green sand lines
- Melt & sand control: spectrometer chemistry per heat; sand KPIs (AFS GFN, moisture, LOI).
- Process records: pouring temp, inoculation, yield, and rework reasons.
- Mechanical tests: Brinell hardness for each batch; tensile per heat/lot for specified grades (EN-GJL/EN-GJS / ASTM A48/A536).
- NDT as required: MT/UT on criticals; pressure/leak test for pressure bodies; impregnation only when spec’d.
- Dimensional: FAI + CMM, 3D scan for thin-wall or complex housings; machining datums validated on fixtures.
- Traceability: heat/batch/serial; photo records of metallography (nodule count/shape for ductile).
RFQ checklist (send with drawings)
- Annual volume & batch size; target price (optional).
- Material & grade (e.g., EN-GJL-250 / EN-GJS-500-7; ASTM equivalents).
- Preferred process (green sand) or performance targets (CT, Ra, min wall).
- Critical features & GD&T; machining drawing/datums.
- Leak/NDT requirements; sampling plan.
- Coating/packaging; destination.
- PPAP level/documents needed (FAI/CMM, material certs).
FAQs
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