How to Design Core Prints & Vents & Coating for Iron Castings

Table of contents

What goes wrong (and why cores cause scrap)

Typical core-driven defects in iron castings:

• Core shift / rotation → off-position bores, thin walls, leak paths.
• Gas blows / blisters / veining → trapped binder gas, poor venting or wet coating.
• Burn-on / penetration → hot faces under-coated or over-heated.
• Erosion / scabbing → high metal velocity at core noses or weak surfaces.
• Breakage / float → prints too small for buoyancy & metallostatic head.

Core prints: geometry, strength & shift control

Geometry rules (design stage)

• Print length (per side): 1.5–2.0 × core wall thickness, min 6–8 mm (0.24–0.31 in).
• Print draft: 1.0–1.5° to protect edges; flat landings (no knife edges).
• Fit/clearance at seating: allow 0.2–0.5 mm (0.008–0.020 in) per side to avoid crush; keep seating faces coplanar.
• Anti-rotation features: three-point seating, small keys, or asymmetric prints.
• Datum logic: seat cores against cast datums, not parting flash; avoid relying on chaplets.

Strength vs buoyancy (safe sizing)

Buoyancy on a core ≈ displaced metal weight minus core weight:

F_buoy ≈ (ρ_metal − ρ_core) · V_core · g

For iron: ρ_metal ~ 6.9–7.2 g/cm³, ρ_core ~ 1.5–1.8 g/cm³ → Δρ ≈ 5.2 g/cm³ (≈ 52 kN/m³).

To resist float and shear at prints:

A_required ≥ (SF · F_buoy) / τ_allow

• SF (safety factor): 2–3 for pour turbulence & head.
• τ_allow (allowable shear) at the glue/print interface: design with 0.3–0.5 MPa conservative.
• Split area across all prints; add mechanical stops where possible.

Practical tip: if prints get too large, add a core pocket/step to increase seating area without growing overall part size.

Core shift control

• Locate close to datums; short overhang beyond prints.
• Use hard stops or nesting features where machining allows.
• Chaplets only as last resort—if used, specify alloy, location, and NDT/etch inspection on pilots.

Venting: diameters, density & flow paths

Goal: give binder gas the shortest, hottest way out—to parting, risers or dedicated bleeds.

• Vent diameters (wire or drilled): Ø0.5–1.5 mm (0.020–0.060 in); start small and add density as needed.
• Spacing (deep pockets / long cores): grid 25–50 mm (1–2 in) toward high points; staggered if thin.
• Pathing: aim vents uphill to the highest elevations; avoid dead-end pockets.
• Print vents: design grooved prints or vent slots leading to parting vents.
• Permeability: use sands with adequate AFS GFN (see §4) and keep binder LOI controlled to limit gas load.
• Do not vent into steel chills; vent around them to parting or risers.

Coating (wash): selection, DFT & drying

When to use which:

• Zircon or alumina on high heat-flux faces (thin walls, gate impingement) to cut burn-on/penetration.
• Graphite/silicate on general faces to stabilize texture and reduce erosion.

Targets

• Dry-film thickness (DFT): 0.15–0.30 mm (6–12 mil), uniform; avoid runs/“orange peel”.
• Application: dip, flow-coat or brush; control viscosity per supplier; keep agitation consistent.
• Drying: full dry—no solvent smell, knife-scratch pass, stable weight (two-weigh check).
• Re-bake if doubtful. Wet coating is the #1 cause of gas blows and roughness.

Span, stiffness & L/t guidelines

• Free span ratio (unsupported shell cores): L/t ≤ 15–20. Add bridges or chaplets (last resort) if longer.
• Core noses at gates: radius & coat; avoid knife tips.
• Print land width: ≥ core wall thickness for brittle sands; shorter if shell.

Gating interactions: gas, erosion & burn-on

• Impingement control: don’t point high-velocity streams at raw cores; use kickers or splash pads.
• Fill direction: feed thin → thick so cores see hot metal quickly (less misrun).
• Temperature window: superheat too high → penetration; too low → misrun over cold cores.
• Coating choice + metal temp + mold hardness decide whether you see burn-on vs misrun—tune as a system.

What to put on the drawing (copy–paste)

Core prints & seating

• Core prints: length ≥ 1.5–2.0 × core wall; draft 1–1.5°; flat landings.
• Print-to-seat clearance: 0.2–0.5 mm per side.
• Anti-rotation: 3-point seating / keying; chaplets only if approved.

Vents

• Vent wires Ø0.5–1.5 mm; density 25–50 mm grid toward high points.
• Print vents to parting or risers; no blind pockets.

Coating

• High heat-flux faces: zircon/alumina; DFT 0.15–0.30 mm; full dry (scratch test).
• General faces: graphite/silicate as needed; uniform DFT.

Process KPIs (for PPAP/FAI)

• Core sand AFS GFN window recorded; LOI & gas evolution logged.
• Cold-box tensile / shell strength per supplier spec.
• Coating DFT logs; drying records; vent map.

Inspection

• Critical core features (SC/CC) listed; 3D scan/CMM vs CAD on datum scheme.
• Leak test plan if pressure-tight; microstructure & hardness if required.

Helpful internal links:

• Sand Inclusion & Burn-On
• Shell Molding Thin-Wall Guide
• Resin vs Green

Quick checklists (DFM & launch)

DFM (before tooling)

• L/t ≤ 15–20 on free spans; add bridges or pockets.
• Prints sized for SF 2–3 against buoyancy; anti-rotation designed.
• Vent plan to parting/risers; no blind pockets.
• Coating plan by zone with DFT targets.

Pilot / PPAP

• Vent map & coating DFT records attached.
• Core strength & LOI/gas evolution data logged.
• 3D scan of cored features; leak/NDT if applicable.
• Corrective actions for any blows, veining, or burn-on.

What YB Metal delivers

YB Metal Solution de-risks cored castings with:

• Core feasibility pack: print sizing calc, vent layout, coating specification.
• Process proof: pilot coupons, DFT logs, vent maps, and CMM/3D scans of cored features.
• Production control: binder/LOI & gas evolution windows, bake/dry records, and NCR/8D if deviations occur.

Need a core plan for your part? Upload your drawing —YB Metal will return prints/venting/coating recommendations and a quote.