Sand Casting Alcohol Coating Layers: Plant-Validated 60 % Defect Cut

Optimizing Alcohol-Based Coating Layers in Sand Casting: A Plant-Validated Defect-Reduction Study

Abstract

This study quantifies the impact of alcohol-based coating layers on surface defect reduction in sand castings. Through SMEMACH-validated field trials, optimal layering protocols were established: 2-3 coats for small/medium castings (≤1 t), 3-4 coats for heavy sections (>5 t). Implementation reduced surface defects by 62%, scrap rates from 19% to 7%, and unit costs by 15 RMB (More than 2 US dollars). The process achieves 90 s touch-cure and 3 min full hardness via self-flaming, cutting mold turnover time by 75% compared to water-based systems.

1. Introduction

Surface defects (pores, sand inclusions, slag) cause 15-25% scrap in sand castings. While water-based coatings require 2-4 h oven drying, alcohol-based systems achieve touch-cure in 90 s and full hardness after 3 min self-flaming, cutting mold turnover time by 75%. However, foundry workers face operational dilemmas:

· Under-coating: <2 layers cause sand pick-up (50% rework rate in uncoated machine tool beds)

· Over-coating: >4 layers increase costs by 22% without proportional quality gains

This study, validated at SMEMACH partner foundries, establishes evidence-based layering protocols.

2. The Importance of Surface Coatings in Sand Casting

2.1 Core Functions

At 1400-1600°C, molten metal penetrates uncoated sand molds, causing:

· Surface roughness (Ra >25 μm without coating vs. Ra <6.3 μm with 3-layer coating)

· Micro-porosity (gas evolution rate 0.8 L/min/kg in uncoated vs. 0.12 L/min/kg in coated molds)

Coatings act as triple-function barriers:

· Thermal insulation: Zircon (ZrSiO₄) layers reduce heat transfer by 68%

· Impermeability: 0.3-0.6 mm coating (≈12-24 mil) blocks metal penetration

· Ventilation: Organic burnout creates 15-25 μm pores for gas escape

Case Study: A gearbox housing foundry reduced sand inclusion defects from 31% to 4% after adopting 3-layer alcohol coating.

2.2 Alcohol-Based Coating Advantages  

*Mandatory ATEX-rated exhaust during ignition (DNV GL Rules for Materials 2021)

3. Layering Protocol Development

3.1 Layer Determination Principles

Optimal layering balances:

· Coating thickness: 0.3-0.6 mm (30-60 μm per layer)

· Drying kinetics: 3-5 min inter-coat interval @22°C, <60% RH (extend to 8 min when RH>70%)

· Cost efficiency: Break-even at 3.2 layers for medium castings

Experimental Validation:

· Single layer: 0.18 mm thickness, 42% defect rate

· Double layer: 0.35 mm, 12% defects

· Triple layer: 0.52 mm, 3.8% defects

3.2 Geometry-Specific Protocols

Case Study: A 2.3 t machine column foundry achieved zero bottom sand inclusion using 4-layer protocol (45/35/25/15 °Bé) compared to 28% defects with 2 layers.

4. Process Optimization Guidelines

4.1 Viscosity Control

Baumé (°Bé) Adjustment Protocol:

·  Measure with hydrometer (Fig. 1)

·  Dilute: 1:1 with 95% ethanol if >50 °Bé (1.38 g cm⁻³)

· Thicken: Add 5-10% lithium bentonite if <20 °Bé (1.18 g cm⁻³)

Storage: Maintain coating at ≤30°C; ignition requires ambient ≥15°C for complete alcohol vaporization.

4.2 Ignition Timing

Three-Point Touch Test:

· Surface appears matte white

· No alcohol odor detectable

· Finger touch leaves no residue

Critical: Igniting too early (<2 min) causes brittleness; too late (>8 min) prevents adhesion.

4.3 Defect Troubleshooting

5. Implementation Results

5.1 Performance Metrics

At SMEMACH Trial Foundry:

· Defect reduction: 62% (from 19% to 7%)

· Cycle time: 4.2 h → 2.8 h per mold

· Cost saving: 15 RMB/piece (material + rework)

· CO₂ reduction: 1.8 t CO₂-eq per 1000 t castings

Statistical significance: χ² test, p<0.01 for yield improvement.

5.2 Continuous Improvement

Next Phase: Develop thermochromic coating (red→colorless at 200°C) for real-time thickness verification. Pilot deployment scheduled Q3 2025 at SMEMACH facilities.

6. Conclusions

6.1 Optimal Layering: 2-3 layers for medium castings, 3-4 for heavy sections

6.2 Critical Controls:

· Maintain 25-50 °Bé viscosity range

· Apply layers perpendicularly with 3-5 min intervals

· Ignite only after complete alcohol evaporation

6.3 Economic Impact: 14% higher throughput, 18% lower defect-related costs

References:

[1] DNV GL Rules for Materials and Welding. Ch. 3 Sec. 2. Oslo: DNV GL AS, 2021.
[2] Internal Report TR-2024-07: Alcohol Coating Field Trials. Shanghai: Research Institute, 2024.

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