Eliminate Burn-On & Sand Inclusions: Practical Foundry Guide

Table of contents

Definitions & identification

Sand Inclusion

Loose sand grains or lumps are entrained into the melt; after solidification you see subsurface inclusions or honeycomb-like pits. After shot-blast the surface shows cratered pits or specks; sectioning reveals silica/sand particles or dark inclusion bands.

Burn-On (sintering/metal penetration)

High-temperature metal chemically/physically penetrates the mould/core sand, forming a metal–sand composite layer that looks glossy but rough. It is hard to remove by blasting and accelerates tool wear in machining. Typical at hot spots, sharp corners, and faces opposite ingates.

Quick cues

• Inclusion: closer to direct impingement and recirculation zones; clear inclusion boundary after pickling; hardness lower than base metal.
• Burn-On: “sand-welded” layer; residual porosity even after grinding; metallography shows a penetration layer.

Root-cause mechanisms (Why)

Sand system (green/resin sand)

• Particle size skewed fine → poor permeability, friable skin → spalling (inclusion).
• Bentonite/water imbalance → low strength or over-bonding → scabbing/peel.
• High LOI → excess residual carbon and gases → burn-on/blows.
• Elevated sand temperature → binder degradation and surface powdering.
• Poor return-sand management → excessive fines; distorted grading curve.

Pouring thermo-mechanics

Practical “closest match” pairs used in sourcing. Always verify exact requirements for your part.

• Excess superheat (iron/steel) → better wetting of sand → burn-on.
• Turbulent filling (tall sprue, abrupt runner transitions) → sand erosion/entrainment → inclusion.
• Hot spots (direct impingement, thick sections) → high heat flux → coating failure.
• Long contact time of liquid front with sand face → metal penetration.

Coating & application

• Insufficient refractoriness (e.g., using low-refractory fillers in high-heat areas).
• Dry film thickness (DFT) too thin/uneven at corners/ingates.
• Incomplete drying (residual alcohol/water → foaming, scabbing).
• Solids/viscosity drift due to settling; inadequate agitation.

Diagnostic checklist (fast, actionable)

Patterned locations

• Inclusion: upper mould scour zones, leeward faces around sharp inner corners, behind ingates (recirculation).
• Burn-On: hot spots, small R corners, near runners/risers.

Shop tests & lab items

• Sand: AFS clay (active/total), moisture, GFN, wet compression strength, permeability, LOI, return-sand fines %.
• Coating: density/°Bé, viscosity (e.g., Ford #4 s @25 °C), solids %, fineness, DFT mapping.
• Process: pouring temperature & superheat, fill time, slow-motion video of the gating (look for sand roll-in/eddies).
• Metallography/hardness: penetration-layer thickness; chill tendency (gray/ductile iron).
• Surface roughness: Ra (µm) after blast vs drawing; 3D scan overlay when needed.

Refractory coating solution window (the core)

Objective: with the right refractory system + controlled DFT + correct drying, build a non-wetting barrier in high-heat/scour regions to suppress both inclusion and burn-on.

System selection (from high to medium heat duty)

• Zircon-based (alcohol or water-borne): highest refractoriness and non-wetting; first choice for steel and high-heat iron areas (hot spots, ingate impact).
• Alumina/Mullite systems: robust in most high-temp zones; water-borne is more eco-friendly.
• Graphite/Wollastonite hybrids: aid surface finish and release for gray/ductile iron in non-extreme heat; avoid as the sole barrier at direct-hit hot spots.

Critical process parameters (mm/in shown in parallel)

• Solids (wt %): keep within supplier’s band, typically 50–70 %.
• Viscosity: lock to an internal window (e.g., 20–35 s Ford #4 @25 °C; model-specific).
• °Bé / Density: trend chart per batch; check every shift.
• Agitation: 10–15 min low-speed before use; re-agitate every 30–60 min to avoid settling.
• Dry film thickness (DFT):
• General faces: 0.15–0.30 mm (6–12 mil)
• Hot spots/direct impact: 0.25–0.40 mm (10–16 mil)
• Sharp corners with R < 3 mm (0.12 in): add +20–30 % vs adjacent faces
• Application: dipping > brushing/spraying for uniformity. For multi-coats, ensure full dry between passes.
• Drying/ignition: alcohol systems must fully evaporate (weight-loss check or “hand-back cool” test). Water-borne needs hot-air/low-temp bake; no free water/alcohol left.
• Recoat/repair: control DFT per pass; log cumulative DFT on router; spot-reinforce leeward faces and opposite ingates.

In-process QC & records

• Per shift log: density, viscosity, temperature, agitation time, ≥3 DFT points per casting.
• Build a “coating ↔ defect” Pareto by location/lot/equipment to tune DFT & formula.
• Any change (lot, recipe, drying method) → trigger FAI/PPAP-level re-validation.

Green sand control window

Goal: enough skin strength and scour resistance while keeping permeability and stable grading; reduce spalling and sand roll-in.

Tooling & process co-optimization

• Gating system: reduce direct impingement; control sprue height; smooth runner transitions; add inclusion traps.
• Hot-spot management: local chills; thickness gradients; replace knife-edges with blended radii.
• Filling/pouring cards: target fill-time window; record pouring temperature & superheat curves.
• Cleaning: specify shot-blast saturation/coverage so you don’t “polish over” real defects.
• Drawing & acceptance: mark functional faces with Ra (µm) and a surface class/defect class (ISO/ASTM references); set AQL for acceptable non-functional burn-on area/depth.

Acceptance, recurrence prevention & cost impact

• PPAP/FAI content: sand reports, coating batch records, DFT map, pouring temp/time curves, metallography (penetration thickness), surface Ra.
• TCO lens:
• DFT too thin → rework/scrap ↑
• DFT too thick → drying energy/time ↑, throughput ↓, coating usage ↑
• The optimum window is where unit cost and first-pass yield intersect.
• Prevention: integrate a location–frequency–severity defect atlas into FMEA and the Control Plan; add SPC checkpoints on critical faces.

Case snapshot (pump housing, gray/ductile iron)

• Symptom: broad burn-on opposite ingate; Ra > 12.5 µm after blast; machining tool wear high.
• Fixes: redirected ingate + chill at the hot spot; switched to zircon alcohol-based coating on the face; raised DFT from 0.15 → 0.30 mm (6 → 12 mil); tuned green sand LOI from 4.2 % → 3.1 % and compactability from 39 % → 45 %.
• Result: burn-on defect rate ↓ 78 %, machining tool life +35 %, FPY +9.6 % over 6 lots.

How YB Metal delivers

YB Metal Solution integrates metallurgy, moulding, coating, and inspection under one roof to stabilize surface quality on gray/grey iron, ductile iron, and steel castings:

• Process capability: green sand, resin sand, shell moulding; gating simulation support; prototype to mass production.
• In-house testing: OES spectrometry, tensile/hardness, microstructural analysis, CMM and 3D scanning for surface/DFT/shape verification.
• Quality framework: APQP/PPAP (up to Level 3), FAI, SPC on critical faces, leak/pressure tests upon request.
• Delivery: typical prototype lead time 2–4 weeks; PPAP as agreed per customer program.
• Need help tuning your coating window or building a defect atlas?