Investment casting—also known as lost-wax casting—remains one of the most versatile metal-forming techniques.
Within this realm, water glass (sodium silicate) investment casting stands out for its cost efficiency and capacity to produce complex ferrous components.
In this guide, we delve deeply into every aspect of the process, providing data-driven insights and referencing industry standards to support engineering decisions.
1. Introduction: Understanding Water Glass Investment Casting
Water glass investment casting uses sodium silicate (Na₂SiO₃) as the ceramic binder to form a multi-layer shell around wax patterns.
Whereas silica-sol processes rely on colloidal silica, water-glass draws on an abundant, low-cost binder that has served foundries since the mid-20th century.
Historically, artisans in Asia and Europe applied primitive alkaline silicates to shell molds; over time, chemists refined the SiO₂:Na₂O ratio (often 2.5:1 by weight) to optimize strength and setting speed.
Today, water-glass casting fills a critical niche: it delivers ISO 8062 CT7-CT9 tolerances and surface finishes of Ra 6–12 μm while keeping per-part shell material costs under $0.50/kg—a fraction of silica-sol systems.
Consequently, manufacturers leverage it for medium-precision, budget-sensitive applications such as agricultural machinery, pump housings, and heavy-duty valves.
2. What Is Water Glass Casting?
Water glass casting, also known as sodium silicate investment casting, is a specific type of investment casting that utilizes water glass (sodium silicate solution) as the binder material for building ceramic shells around wax patterns.
It is an efficient and economical process that produces net-shape or near-net-shape metal components with moderate precision and surface quality.
This method is especially well-suited for casting medium- to large-sized ferrous components with relatively simple to moderately complex geometries.
Definition and Core Principle
In water glass casting, the central principle remains consistent with all lost-wax casting processes: a disposable wax model is coated with multiple ceramic layers to form a shell.
Once the shell is cured and hardened, the wax is removed (dewaxed), and molten metal is poured into the cavity.
After cooling and solidification, the shell is broken away to reveal the cast metal component.
The distinguishing feature of this process is the use of water glass (Na₂SiO₃ solution) as the binder in the ceramic slurry.
Compared to colloidal silica (used in higher-precision silica sol investment casting), water glass provides:
- Lower material cost
- Faster drying time
- Higher production throughput
3. Why Use Water Glass?
Water glass investment casting, though not the most refined process available,
continues to be widely adopted across multiple industries due to its outstanding balance between cost-efficiency, mechanical reliability, and production scalability.
By using sodium silicate (Na₂SiO₃) as the binder, this method offers significant advantages,
particularly for medium-complexity components that do not require ultra-tight tolerances but must meet functional and structural demands.
Cost-Effectiveness Without Sacrificing Strength
One of the primary reasons manufacturers choose water glass casting is its economic efficiency.
Sodium silicate is abundant, non-toxic, and much less expensive than colloidal silica used in high-end precision casting. On average:
- Binder cost per liter of water glass is 30–50% lower than that of silica sol.
- Shell materials, such as quartz sand, are cheaper than fused silica or zircon.
- Shorter drying cycles (4–8 hours/layer) enable higher daily output, reducing overall lead time.
Result: Lower per-part production cost—particularly effective for medium-volume orders (>1,000 pcs).
Adequate Dimensional Accuracy for Industrial Use
Although water glass casting cannot rival silica sol in tight tolerance achievement, it still provides acceptable dimensional accuracy for most structural and functional parts:
- Achievable tolerance: ISO 8062 CT7–CT9
- Linear tolerance deviation: ±0.5% to ±1.5% of nominal dimension
- Surface finish: Ra 6–25 μm, depending on slurry quality and mold processing
This level of precision is sufficient for gear blanks, valve housings, brackets, agricultural fittings, and many other functional components.
Superior Mechanical Strength of Shells
Water glass-based shells offer robust green and fired strength, enabling the process to accommodate larger and heavier components (typically 1–80 kg per piece). This is possible due to:
- Higher solids content (~40–50 wt%) in water glass binder
- Strong bonding with quartz or silica-based refractories
- Rapid setting time, which reduces defects due to shell deformation
Applications requiring structural integrity over fine appearance benefit the most from this.
Process Simplicity and Operational Flexibility
Water glass investment casting is also easier to implement and scale in small to medium-sized foundries:
- Binder preparation requires no pH adjustments or surfactant additives.
- Ambient curing is faster and less sensitive to humidity than colloidal silica systems.
- Less stringent temperature control is required during shell drying and firing.
- The Reusability of the wax and the simplicity of slurry handling reduce material waste.
Moreover, standard equipment and conventional casting skills are sufficient to run a water glass foundry efficiently, making this process attractive for both emerging markets and experienced manufacturers.
Environmental and Health Considerations
Water glass binders are inorganic, non-toxic, and water-soluble, reducing risks associated with VOCs (volatile organic compounds) and hazardous fumes during shell preparation.
Compared to resin-based binders:
- No organic solvents are required
- Less stringent exhaust and fume handling systems needed
- Dewaxing emissions are lower due to cleaner shell combustion
This supports ISO 14001 environmental compliance and workplace safety improvements.
4. Process Overview: From Wax to Metal
Below is a step-by-step breakdown, highlighting key parameters and differences versus silica-sol casting.
Wax Pattern Creation
- Tolerance: ±0.05 mm
- Materials: Paraffin-microcrystalline blends (ash <0.05 wt%)
- Volume: 10–50 parts per tree
Tree Assembly
- Sprue Design: 5–10% of part volume
- Heat Stakes or Wax Adhesive: Ensures robust joints
Shell Building with Water Glass Binder
- Slurry Composition: 30–35 wt% Na₂SiO₃, pH 11.5–12.5, viscosity ~10 mPa·s
- Stucco Grades: #100 mesh (150 µm) prime coat; #50–#30 (300–600 µm) backup coats
- Coats & Drying: 4–7 dips; 1–2 h ambient or 60 °C oven per coat
- Total Shell Thickness: 5–15 mm
Dewaxing (Steam or Hot Water)
- Temperature: 160–180 °C
- Pressure: 5–7 bar steam autoclave
- Duration: 20–30 min
- Wax Recovery: >85% reclamation
Firing of Ceramic Mold
- Ramp Rate: 5 °C/min to 800 °C; hold 2 h
- Final Temperature: 900–1000 °C for 2–4 h
- Purpose: Remove residual organics; vitrify silicate binder
Metal Pouring and Cooling
- Alloy Types: Carbon steel (1 450–1 550 °C), low-alloy steel (1 500–1 600 °C), ductile iron (1 350–1 450 °C)
- Superheat: +20–50 °C above liquidus
- Pour Rate: 10–20 kg/s for typical industrial crucibles
Shell Removal and Finishing
- Knockout Methods: Shot-blast at 0.4–0.6 MPa, mechanical vibration
- Cleanup: Grit blasting and light grinding
- Final Surface: Ra ~6–8 µm before machining
Key Difference vs. Silica Sol: Water glass sets by drying, not acid or heat-induced gelation.
Dewax uses wet removal, avoiding high-temperature burnout but requiring effluent management.
Consequently, cycle times may be shorter (2–3 days) than silica-sol’s 3–5 days, but shell refractoriness peaks at ~900 °C rather than 1200–1300 °C.
5. Binder System: The Chemistry Behind Water Glass
The binder system is the cornerstone of the water glass investment casting process.
It determines the mechanical strength, dimensional stability, and thermal behavior of the ceramic shell. In water glass casting, sodium silicate—commonly referred to as “water glass”—is used as the primary binder.
Understanding its chemical composition, behavior, and limitations is essential for optimizing casting quality, minimizing defects, and controlling production costs.
What Is Sodium Silicate?
Sodium silicate (Na₂O·nSiO₂) is an alkaline aqueous solution of silica and soda ash, forming a viscous, glassy substance that hardens on drying.
The ratio of silicon dioxide (SiO₂) to sodium oxide (Na₂O) is known as the silicate modulus—a key indicator of binder properties.
- Typical modulus range: 2.4 to 3.0
- Viscosity (25 °C): 0.5–1.5 Pa·s
- pH: 11–13 (strongly alkaline)
- Solid content: 35–45%
- Appearance: Transparent to light amber liquid
A higher modulus indicates a higher SiO₂ content, which improves shell strength but may increase viscosity and reduce workability.
Mechanism of Action: How It Binds
Sodium silicate binds ceramic particles through evaporative hardening and polymerization:
- Water evaporation causes the silicate gel to concentrate and harden.
- In the presence of CO₂ or acidic environments, it undergoes irreversible polymerization, forming a strong, glassy matrix.
This quick-setting nature supports faster drying cycles compared to silica sol, particularly in environments with good airflow and low humidity.
Key Advantages of Sodium Silicate Binder
Water glass binders offer multiple benefits, especially for cost-driven applications:
| Feature | Performance |
|---|---|
| Cost | 30–50% lower than colloidal silica |
| Shell drying time | Fast: 4–8 hours per layer |
| Availability | Globally abundant, easy to store |
| Bonding strength | Moderate to high (~1–3 MPa dry strength) |
| Environmental impact | Low VOCs, water-based, non-flammable |
These characteristics make sodium silicate ideal for medium-precision ferrous casting and large-volume runs where economics take precedence over surface finish.
Limitations of Water Glass Binders
Despite its practicality, sodium silicate is not without drawbacks:
| Limitation | Technical Impact |
|---|---|
| Hygroscopic nature | Shells absorb moisture over time, weakening structure |
| Lower refractoriness | Degrades above ~1250 °C, limiting high-temp alloy use |
| Poor moisture resistance | Risk of shell softening in high-humidity storage |
| Alkalinity | Can corrode handling equipment and irritate skin |
| Shrinkage mismatch | Higher risk of shell cracking during cooling |
Compared to silica sol binders, which offer superior high-temperature resistance and dimensional stability, water glass has reduced reliability for tight-tolerance, high-performance alloys like titanium or superalloys.
Modifier Additives and Enhancements
To improve performance and reduce defects, water glass binders are often modified using:
- pH stabilizers: Boric acid, citric acid (to control gelation rate)
- Hardening agents: CO₂ gas injection or ammonium chloride
- Organic binders: Small additions to improve flexibility
- Surfactants: Reduce slurry viscosity and improve wetting
Recent advances have introduced hybrid binders—mixing sodium silicate with colloidal silica—to balance cost and shell performance.
These hybrids improve shell thermal shock resistance and casting surface quality by up to 25%.
Standards and Quality Metrics
Water glass binders must be monitored for key performance metrics:
| Property | Test Method | Acceptable Range |
|---|---|---|
| Modulus | Titrimetric or ICP-OES | 2.4–3.0 |
| pH | pH meter (25 °C) | 11.5–13.0 |
| Viscosity | Brookfield viscometer | 0.5–1.5 Pa·s |
| Gel time (CO₂ test) | Lab gassing rig | <30 seconds |
| Dry bonding strength | ASTM C1161 | ≥1.0 MPa (at 25 °C) |
6. Shell Materials and Construction Techniques
Water-glass shells rely on silica-based refractories:
- Prime Coats: #100–#140 mesh fine quartz (75–150 µm) for detail capture
- Intermediate Coats: #60–#80 mesh (200–300 µm) for strength
- Backup Coats: #30–#50 mesh (300–600 µm) for rigidity
Foundries typically apply 4–7 layers, balancing strength (3–5 MPa at 500 °C) against permeability (10–30 Darcy).
They maintain drying rooms at 22–28 °C, <50% RH to prevent shell cracking. In contrast, silica-sol shells often incorporate zircon or alumina fillers to achieve 6–8 MPa strength at 800–1200 °C.
7. Casting Metals and Compatibility
Water glass excels with ferrous alloys:
- Carbon Steel (e.g. AISI 1080): Poured at 1500 °C; tensile strength ~450 MPa
- Low-Alloy Steel (e.g. 4140): Poured at 1550 °C; tensile ~650 MPa
- Ductile Iron: Poured at 1 350 °C; elongation ~10–15%
- Manganese Steel: Poured at 1450 °C; hardness ~250 HB
However, it poorly supports reactive or light alloys (Al, Mg, Ti) due to binder alkalinity and residual moisture. These require vacuum or inert-binder systems (silica-sol or aluminous shells).
8. Dimensional Accuracy and Surface Finish
- Tolerances: ISO CT7–CT9 (±0.1–0.2% of nominal length)—suitable for features down to 2 mm thickness
- Surface Roughness: Ra 6–12 µm; with additional prime coats, parts can reach Ra ~4–6 µm before machining
- Comparison: Sand casting yields Ra 25–50 µm and CT11-CT14 tolerances; silica-sol delivers Ra 1.6–3.2 µm and CT4-CT6 tolerances
A 100 mm steel bracket cast via water-glass typically requires 0.5–1.0 mm of machining stock to achieve Ra < 1.6 µm, versus 0.2 mm for silica-sol castings.
9. Quality Control and Inspection Protocols
Foundries implement rigorous QA:
- Shell Inspection: Ultrasonic thickness gauges, visual crack checks
- Dewax Verification: Residual wax <0.5 wt%; shell hardness >3 MPa
- Casting Inspection:
-
- Radiography (ASTM E446) to detect ≥1 mm porosity
- Dye Penetrant (ASTM E165) for surface cracks ≥50 µm
- CMM Measurement: Critical dims to ±0.05 mm
Process documentation adheres to ISO 9001 and, where applicable, AS9100 for aerospace parts, ensuring full traceability from slurry batch to final heat treatment.
10. Economic Considerations and Cost Analysis
| Factor | Water Glass | Silica Sol | Sand Casting |
|---|---|---|---|
| Binder Cost | $0.20–0.40/L | $4–6/L | $0.10–0.20/L |
| Sand Cost | $30–50/ton | $200–300/ton (zircon) | $20–30/ton |
| Shell Build Time | 2–3 days | 3–5 days | 1–2 days |
| Typical Part Cost (steel) | $50–$200 | $150–$500 | $30–$120 |
| Net-Shape Machining Savings | 30–50% | 60–80% | 0–20% |
11. Industrial Applications
Water glass casting suits medium- to large-scale ferrous components, including:
- Pump and Valve Bodies: Complex internal geometries, Ra < 12 µm
- Agricultural Equipment: Tractor housings, plow assemblies
- Heavy Machinery: Mining shovels, gearbox housings
- Off-road Vehicle Components: Chassis brackets, brake housings
12. Comparative Analysis: Water Glass vs. Other Methods
When selecting a casting process, engineers must weigh accuracy, surface finish, material compatibility, tooling investment, and production scale against unit cost.
Water glass investment casting occupies a middle ground—it offers better precision and finish than sand casting, yet at a fraction of the cost of silica‐sol investment casting.
Likewise, it accommodates ferrous alloys that die casting cannot. The table below distills these trade-offs into key metrics across five common methods.
| Casting Method | Dimensional Accuracy (CT Grade) | Surface Finish (Ra, µm) | Alloy Suitability | Tooling Cost | Production Volume | Relative Cost | Notable Advantages |
|---|---|---|---|---|---|---|---|
| Water Glass Casting | CT7–CT9 | 6–25 | Ferrous (steel, ductile iron) | Medium | Medium to High | Low | Cost-effective, robust shells, good for large parts |
| Silica Sol Casting | CT5–CT7 | 3–12 | Ferrous & non-ferrous | High | Medium | High | Finest detail, superior finish, high-temp stability |
| Sand Casting | CT10–CT13 | 25–50 | Broad (steel, iron, aluminum) | Low | Low to Very High | Very Low | Extremely low tooling cost, flexible part size |
| Die Casting | CT4–CT6 | 1–5 | Non-ferrous (Al, Zn, Mg) | Very High | Very High | Medium–High | Rapid cycle times, excellent repeatability |
| Lost Foam Casting | CT8–CT10 | 12–50 | Aluminum, iron | Low–Medium | Medium | Medium | Single-piece molds, complex geometries without cores |
Key Takeaways:
- Water Glass vs. Silica Sol: Water glass reduces binder and refractory costs by up to 70%, while delivering CT7-CT9 tolerances and Ra 6–25 µm finishes.
In contrast, silica sol reaches CT5-CT7 and Ra 3–12 µm but demands higher-cost colloidal silica and zircon flour. - Water Glass vs. Sand Casting: Water glass narrows accuracy to CT7-CT9 (versus CT10-CT13) and improves surface finish by 2–4×,
making it ideal when sand casting’s roughness and loose tolerances cannot meet functional requirements. - Water Glass vs. Die Casting: Although die casting achieves the tightest tolerances (CT4-CT6) and smoothest finishes (Ra 1–5 µm), it restricts alloy choice to non-ferrous metals and incurs very high tooling costs, limiting its viability for ferrous components and lower volumes.
- Water Glass vs. Lost Foam Casting: Both methods handle complex shapes, but water glass yields better surface quality (Ra 6–25 µm vs. 12–50 µm) and stronger ceramic shells, while lost foam offers simpler mold setup without shell building.
13. Conclusion
Water glass investment casting delivers an optimal balance of cost, complexity, and precision for ferrous components.
With binder costs under $0.50/kg, tolerances to CT7, and surface finishes down to Ra 6 µm, it enables manufacturers to produce intricate, heavy-duty parts at a fraction of specialized investment-casting costs.
Furthermore, robust QA protocols aligned with ISO 9001 and ASTM standards ensure consistent quality for critical applications.
Looking ahead, advances in automated shell building, optimized silicate formulations, and hybrid binder systems may further enhance the method’s accuracy and environmental footprint.
Nevertheless, when engineers need a cost-effective, reliable solution for medium-precision steel and iron castings, water glass investment casting remains a time-tested, industry-proven choice.
LangHe is the perfect choice for your manufacturing needs if you need high-quality water glass investment casting services.
