How to Design Thin-Wall Iron for Shell Molding (Ra & CT)

Table of contents

When shell molding is the right choice

Choose shell molding over green/resin sand when you need:

• Thin, repeatable walls with tight radii and crisp details.
• Higher as-cast accuracy (smaller ISO 8062 CT grades) to reduce machining stock.
• Cleaner as-cast surfaces (lower Ra) to help coating/paint or reduce finishing.
• Lower core gas and better venting control on small passages.

Trade-offs: higher tooling cost than green/resin sand; best ROI at small-to-mid volumes with thin features where yields improve.

Design rules for thin-wall iron (the short list)

Use mm/in together on drawings. Keep sections uniform; blend everything.

• Minimum wall (stable): 3.0–4.0 mm (0.12–0.16 in) on continuous spans.
• Local features can dip to 2.5–3.0 mm (0.10–0.12 in) if short and well-fed.
• Ribs & pads: rib thickness ≤ 0.6–0.7 × adjacent wall; pad thickness ≈ wall + 1–2 mm (0.04–0.08 in).
• Fillet radii: ≥ 0.5 × wall (absolute ≥ 1.0 mm / 0.04 in).
• Draft: 0.5–1.0° on external faces; 1.0–1.5° on internal pockets/cores.
• Bosses & holes: keep boss OD ≥ 2.5 × thread OD; avoid knife-edges—always add entrance fillets/chamfers.
• Uniformity: avoid thickness jumps > 1.5 × within 10–15 mm (0.4–0.6 in); taper transitions.

Read More:

• Wall Thickness & Uniformity Rules
• Fillets & Radii Guide

Typical capability: wall, CT grade & Ra

Ranges are indicative—actuals depend on size band, geometry, shell quality and melt control.

• ISO 8062
• Green Sand
• Resin vs Green

Cores, prints and spans (what breaks first)

• Shell cores are thin and accurate, but brittle over long free spans.
• Print length: target 1.5–2.0 × wall thickness (min 6–8 mm / 0.24–0.31 in).
• Core span: un-supported shell cores keep L/t ≤ 15–20; add bridges or chaplets if longer.
• Vent & gas: provide vent paths; avoid blind pockets that trap binder gas.
• Seat geometry: add generous fillets and flat landings on prints; avoid point contacts.

Gating, risering & process notes for thin sections

• Fast, calm fill: short runners, adequate choke; avoid dead-ends where metal cools before joining.
• Direction of fill: feed thin-to-thick, keep liquid metal fronts hot; minimize head height to reduce erosion.
• Hot spots: break them up with ribs/gradients; add small chills rather than giant risers on thin parts.
• Coating: use zircon/alumina on high heat-flux faces; verify DFT 0.15–0.30 mm (6–12 mil) and full dry to prevent burn-on/misruns.
• Metal temp: hold to a narrow superheat window—too hot increases penetration, too cold risks misruns.

What to put on the drawing (so suppliers quote apples-to-apples)

• Material: grade (e.g., EN-GJL-250 or EN-GJS-450-10); allow equivalent (ASTM A48/A536) if OK.
• As-cast CT: by ISO 8062 size band (e.g., CT7 small features, CT8 larger).
• Surface: as-cast Ra target by zone (e.g., shell faces Ra ≤ 6.3 µm; sealing faces after machining Ra 1.6–3.2 µm).
• Minimum walls: annotate regional t_min; call out ribs ≤ 0.7 × wall.
• Machining stock: faces/ODs +1.5–2.5 mm (0.06–0.10 in); bores by diameter band。
• Critical features (SC/CC): list for PPAP/FAI; request CMM/3D scan on datum scheme.

What YB Metal delivers

YB Metal Solution runs shell molding for gray/grey and ductile iron with:

• Tooling & process: heated tooling, shell cores, simulation-backed gating for thin sections.
• Metallurgy & QA: OES, hardness/microstructure, PPAP/FAI on request; CMM & 3D scan with datum lock.
• Finish: shot-blast, machining, and coatings (epoxy/powder) with DFT logs.