Investment Casting Shell Making — Silica Sol

1. Executive summary — why silica sol matters

Silica sol is the binder that turns a packed refractory powder layer into a cohesive, high-fidelity facecoat and backer in modern precision casting shells.

Its colloidal behavior—notably particle size, SiO₂ content, stabilizer chemistry and ageing—governs slurry rheology, wet-film formation, green strength, fired density and thermochemical stability.

Small changes in sol specification, dilution or contamination can produce large, often non-linear effects on shell strength, permeability and final cast surface quality.

Therefore controlling silica-sol chemistry and its interaction with refractory powders is one of the highest-leverage activities in shell making.

2. The material: What is the silica sol used in investment casting?

Silica sol used in investment casting is a stable colloidal dispersion system, consisting of amorphous silicon dioxide (SiO₂) particles uniformly dispersed in an aqueous medium, stabilized by sodium oxide (Na₂O) as the alkaline stabilizer.

Unlike other binders (e.g., water glass, ethyl silicate), silica sol forms a dense, high-strength silicic acid gel network after drying and roasting,

which bonds refractory powders (zircon, alumina) tightly—laying the foundation for high-precision and high-strength investment casting shells.

The core characteristics of investment casting-grade silica sol are defined by its colloidal structure:

the SiO₂ particles (with a diameter ranging from 8 nm to 16 nm in typical applications) carry negative charges on their surfaces,

forming an electric double layer that maintains the balance between interparticle attractive and repulsive forces.

This balance is the key to silica sol’s stability; any external interference that disrupts this balance will trigger rapid gelation, rendering it unusable for coating preparation.

3. Stability of Silica Sol: Key Influencing Factors and Operational Implications

The stability of silica sol is the prerequisite for its application in investment casting shell making—any loss of stability will lead to premature gelation of coatings, resulting in defects such as shell cracking, peeling, and poor surface finish.

The stability of silica sol is mainly affected by two core factors: electrolyte interference and SiO₂ particle size, both of which have direct and significant impacts on on-site operation.

Impact of Electrolytes on Silica Sol Stability

Electrolytes have a decisive influence on the stability of silica sol, as they disrupt the balance between attractive (van der Waals forces) and repulsive (electrostatic forces) forces between SiO₂ particles.

Specifically, changing the pH value of silica sol or adding certain electrolytes will compress the electric double layer on the surface of SiO₂ particles, reduce the repulsive force between particles, and trigger agglomeration and gelation.

This principle directly dictates critical operational norms in shell making:

• Prohibition of Tap Water Use: Tap water contains a variety of electrolytes (e.g., calcium ions, magnesium ions, chloride ions) that can significantly accelerate silica sol gelation.
  Therefore, only deionized water or distilled water should be used for coating preparation and moisture supplementation to avoid electrolyte contamination.
• Restriction on Ionic Wetting Agents: Ionic wetting agents (anionic or cationic) act as electrolytes, disrupting the colloidal balance of silica sol.
  It is recommended to use non-ionic wetting agents (e.g., polyoxyethylene alkyl ethers) in minimal dosages to ensure coating wettability without compromising silica sol stability.

Impact of SiO₂ Particle Size on Stability and Shell Strength

The diameter of SiO₂ particles is a dual-factor that affects both silica sol stability and investment casting shell strength, presenting a trade-off that must be balanced in practical applications:

Effect on Silica Sol Stability

Generally, the larger the diameter of SiO₂ particles, the more stable the silica sol.
Larger particles have a lower specific surface area and weaker interparticle interactions, making them less prone to agglomeration and gelation.

Conversely, smaller SiO₂ particles have a larger specific surface area and stronger interparticle attractive forces, leading to higher sensitivity to external interference and easier gelation.

Additionally, under the same Na₂O (stabilizer) content, the smaller the SiO₂ particle diameter, the lower the pH value of the silica sol.

This is because smaller particles adsorb more Na⁺ ions on their surfaces, reducing the free Na⁺ concentration in the aqueous phase and thus lowering the alkalinity (pH value) of the system.

This relationship is critical for adjusting the pH of silica sol coatings to optimize stability and coating performance.

Effect on Investment Casting Shell Strength

The particle size of SiO₂ directly affects the mechanical strength of the investment casting shell, particularly the wet strength. Silica sol gelation is the result of SiO₂ particle agglomeration:

smaller particles have more contact points during agglomeration, forming a dense, interwoven gel network.

In contrast, larger particles have fewer contact points, resulting in a loose internal structure of the gel.

Practically, shells made with small-particle-size silica sol (8–10 nm) exhibit significantly higher wet strength and dry strength than those made with large-particle-size silica sol (14–16 nm).

This is crucial for preventing shell damage during handling, dewaxing, and transfer.

However, the trade-off is that small-particle-size silica sol is less stable and requires stricter control of operating conditions (e.g., temperature, humidity, electrolyte contamination).

4. Viscosity of Silica Sol: Key Parameter for Coating Formulation and Performance

Viscosity is one of the most critical performance parameters of silica sol, directly determining the fluidity of the coating, the powder-liquid ratio (P/L ratio) of the formulation, and the uniformity of the coating layer.

A deep understanding of silica sol viscosity and its influencing factors is essential for optimizing coating performance.

Viscosity Requirements for Investment Casting

Silica sol used in investment casting requires low viscosity to ensure good fluidity of the coating and enable the preparation of high P/L ratio coatings (critical for shell strength and surface quality).

According to industry data and academic research:

• Silica sol with a kinematic viscosity of less than 8×10⁻⁶ m²/s is suitable for general investment casting applications.
• For high-precision castings requiring superior surface finish and detail replication, silica sol with a kinematic viscosity of less than 4×10⁻⁶ m²/s is preferred,
  as it can be formulated into coatings with excellent fluidity and uniform coverage.

Factors Influencing Silica Sol Viscosity

Silica sol is a colloidal dispersion system, and its viscosity is affected by multiple factors—contrary to the simple assumption that viscosity depends only on volume concentration (per Einstein’s theory):

Volume Concentration of SiO₂ Particles

Einstein’s theory states that the viscosity of a colloidal dispersion depends on the volume concentration of the dispersed phase (SiO₂ particles) and is independent of particle diameter.

However, this applies only to ideal, dilute colloidal systems. In practical industrial silica sol,
even with the same volume concentration of SiO₂, viscosity can vary significantly due to other factors.

Thickness of the Adsorbed Layer on Particle Surfaces

Each SiO₂ particle in silica sol is surrounded by an adsorbed water layer, whose thickness varies with particle size, surface properties, and stabilizer content.

A thicker adsorbed layer increases the effective volume of the particles, leading to higher viscosity—even at the same SiO₂ volume concentration.

This explains why two silica sols with the same SiO₂ content may have different viscosities.

Compactness of SiO₂ Particles

The compactness of SiO₂ particles, determined by the production process, also affects viscosity.

If the silica sol production process is improper (e.g., incomplete hydrolysis, uneven particle growth), the SiO₂ particles will be loose and porous.

Loose particles occupy a larger volume than dense particles of the same mass, resulting in higher viscosity of the silica sol.

Other Influencing Factors

Additional factors affecting silica sol viscosity include temperature (viscosity decreases with increasing temperature),
pH value (viscosity is lowest at the optimal pH range for stability), and storage time (prolonged storage may cause slight agglomeration, increasing viscosity).

5. Relationship Between Silica Sol Density and SiO₂ Content

The density of silica sol is directly related to its SiO₂ content, as SiO₂ has a higher density than water.

This relationship is critical for on-site coating formulation, as it allows operators to quickly estimate the SiO₂ content by measuring density—ensuring consistent coating performance.

The following is the typical correlation between silica sol density and SiO₂ content (verified by industrial practice):

In investment casting, silica sol with a SiO₂ content of 30% (density ≈1.21 g/cm³) is the most commonly used, as it balances stability, viscosity, and coating performance.

When the SiO₂ content exceeds 35% (density ≥1.27 g/cm³), the silica sol exhibits a significant tendency to gel, requiring stricter control of storage and operating conditions.

6. Water States in Silica Sol and Their Implications for Shell Making

Water in silica sol exists in three distinct states, each with different thermal stability and impacts on coating and shell performance.

Understanding these water states is critical for optimizing coating formulation, drying processes, and avoiding shell defects.

Three States of Water in Silica Sol

1. Free Water: This is unbound water that exists in the aqueous phase of silica sol, not adsorbed or chemically bonded to SiO₂ particles.
   It is completely lost when heated to below 110℃. Free water is the key to maintaining the fluidity of the coating,
   as it lubricates SiO₂ particles and refractory powder, ensuring uniform mixing and coating application.
2. Adsorbed Water: This water is physically adsorbed on the surface of SiO₂ particles through hydrogen bonding. It is lost when heated to 140–220℃.
   Adsorbed water is tightly bound to the particles and does not contribute to coating fluidity but affects the gelation rate of silica sol.
3. Crystalline Water: This water is chemically bonded to SiO₂ particles (forming hydrated silica), lost when heated to 400–700℃.
   Adsorbed water and crystalline water are collectively referred to as “bound water,” which affects the drying rate and final strength of the shell.

Key Implications for Shell Making

Effect of Water States on Coating Fluidity

Free water is critical for coating fluidity: insufficient free water leads to high coating viscosity, poor spreadability, and uneven coating thickness;
excessive free water reduces the P/L ratio, weakening shell strength and increasing the risk of coating sagging.

The balance of free water and bound water is therefore a key consideration in coating formulation.

Relationship Between Water States, Particle Size, and SiO₂ Content

• At the same SiO₂ particle size, the higher the SiO₂ content, the higher the proportion of bound water (adsorbed + crystalline water).
  This is because more SiO₂ particles provide a larger surface area for water adsorption and chemical bonding.
• At the same SiO₂ content, the smaller the particle size, the higher the proportion of bound water.
  Smaller SiO₂ particles have a larger specific surface area, enabling more water adsorption.

Effect on Powder-Liquid Ratio (P/L Ratio)

The particle size of SiO₂ directly affects the P/L ratio of the coating when using the same refractory powder (e.g., zircon powder).

According to academic research (cited from Professor Xu’s paper), for silica sol with 30% SiO₂:

• When the average diameter of SiO₂ particles is 14–16 nm, the optimal P/L ratio is 3.4–3.6.
• When the average diameter of SiO₂ particles is 8–10 nm, the optimal P/L ratio is 2.9–3.1.

To verify this difference, comparative tests can be conducted using 830 silica sol (particle size 8–10 nm) and 1430 silica sol (particle size 14–16 nm), with three critical test controls:

using the same zircon powder, ensuring the same cup viscosity, and simultaneously measuring coating density and thickness.

Moisture Supplementation in On-Site Operation

Water in silica sol evaporates continuously during storage and use, increasing the SiO₂ content and viscosity, and increasing the risk of gelation.

For a 1-meter-diameter slurry bucket, the daily water evaporation is approximately 1–2 liters—thus, daily moisture supplementation with deionized water is mandatory.

Notably, this evaporation rate is only a general reference; the actual water loss is affected by environmental conditions such as drying room temperature, air conditioning operation, humidity, and wind speed.

In unstable operating environments, water loss may fluctuate significantly, requiring on-site measurement to determine the exact supplementation amount.

While some methods for determining water supplementation are described in “Practical Technology of Investment Casting”,
their operability is limited. Industrial operators are encouraged to explore and share more practical methods.

7. Gelation Process and Roasting Temperature of Silica Sol

The gelation process of silica sol is a critical step in investment casting shell making, as it determines the formation and strength of the shell.

Understanding the gelation mechanism and optimal roasting temperature is essential for avoiding shell defects such as cracking and insufficient strength.

Gelation Process of Silica Sol

The gelation of silica sol is a process of SiO₂ particle agglomeration and network formation, which occurs in two stages:

1. Hydrated Gel Formation: Initially, silica sol forms a water-containing hydrated gel with poor strength, which can be partially redissolved in water.
   This phenomenon is clearly observable during the pre-wetting process of wax patterns—hydrated gel on the shell surface can redissolve when in contact with pre-wetting silica sol.
2. Dry Gel Formation: Only when all free water is lost (through drying), the hydrated gel transforms into a dry gel with high strength, resistance to high temperatures, and no redissolution.
   Insufficient drying of the backcoat shell results in incomplete conversion to dry gel, leading to insufficient strength and increased risk of shell cracking during dewaxing.

Roasting Temperature of Silica Sol Shells

Before pouring, silica sol shells must be roasted to remove residual moisture, organic matter, and to enhance shell strength through crystalline transformation:

• Dehydration Stage (Below 700℃): During roasting, bound water (adsorbed and crystalline) is gradually lost, and the amorphous SiO₂ network is further densified.
• Crystalline Transformation Stage (900℃): At approximately 900℃, amorphous SiO₂ undergoes a crystalline transformation (converting to cristobalite),
  which significantly increases the mechanical strength and high-temperature stability of the shell.
• Optimal Roasting Temperature: The typical roasting temperature for silica sol shells is 950–1050℃,
  which ensures complete dehydration, organic matter removal, and sufficient crystalline transformation—balancing shell strength and thermal shock resistance.

8. Practical Considerations for Silica Sol Application in Shell Making

To maximize the performance of silica sol and avoid common defects, the following practical considerations must be observed in on-site operation:

1. Strict Control of Electrolyte Contamination: Use only deionized water for coating preparation and moisture supplementation;
   avoid using ionic wetting agents and ensure all equipment (slurry buckets, mixers, viscosity cups) is clean and free of electrolyte residues.
2. Optimal Selection of SiO₂ Particle Size: Choose silica sol particle size based on casting requirements: small-particle-size silica sol (8–10 nm) for high-strength, high-precision shells; large-particle-size silica sol (14–16 nm) for general castings requiring better stability.
3. Viscosity and P/L Ratio Optimization: Monitor silica sol viscosity regularly; adjust the P/L ratio based on particle size and SiO₂ content to ensure coating fluidity and shell strength.
4. Scientific Drying and Moisture Control: Implement a strict shell drying schedule to ensure complete removal of free water;
   adjust drying parameters (temperature, humidity, wind speed) based on the water states in silica sol.
5. Roasting Process Optimization: Ensure the roasting temperature reaches 950–1050℃ to achieve complete crystalline transformation and maximize shell strength;
   avoid insufficient roasting (leading to incomplete dehydration) or over-roasting (causing shell brittleness).

9. Troubleshooting — common failure modes & fixes

10. Thinking Question: Key Notes for Silica Sol Pre-Wetting

Pre-wetting is a critical step in investment casting shell making, where wax patterns are pre-wetted with silica sol to improve coating adhesion and uniformity.

Based on the characteristics and performance of silica sol discussed above, the key notes for silica sol pre-wetting are summarized as follows:

1. Viscosity Control: Pre-wetting silica sol should have a lower viscosity (kinematic viscosity <6×10⁻⁶ m²/s) than coating silica sol to ensure uniform coverage on the wax pattern surface without forming a thick film.
2. Stability Assurance: Pre-wetting silica sol must be free of electrolyte contamination and maintained at a stable pH (8–10) to avoid premature gelation, which would affect adhesion.
3. Moisture Content: The moisture content of pre-wetting silica sol should be consistent with the coating silica sol to prevent uneven drying and coating peeling.
4. Avoid Redissolution: Ensure the pre-wetting silica sol does not cause excessive redissolution of the existing shell layer (if applying multiple coats). This can be achieved by controlling the pre-wetting time and silica sol pH.
5. Cleanliness: The pre-wetting silica sol should be kept clean, free of refractory powder and debris, to avoid surface defects on the shell.

11. Conclusion

Silica sol is the core binder in investment casting shell making, and its performance is fundamentally determined by colloidal properties such as stability, particle size, viscosity, density, and water state.

Electrolyte sensitivity and SiO₂ particle size directly influence stability and gelation behavior, requiring a careful balance between slurry stability and shell strength.

Viscosity and density serve as key control parameters for slurry formulation and powder-to-liquid ratio optimization.

The gelation, drying, and high-temperature transformation of silica sol are critical to shell integrity.

Proper control of free and fixed water ensures adequate dry-gel formation, preventing shell cracking during dewaxing, while high-temperature firing strengthens the amorphous SiO₂ network to withstand molten metal and thermal shock.

In practice, high-quality shells depend on strict control of contamination, particle size selection, moisture balance, and firing conditions.

As investment casting moves toward higher precision and more demanding applications, continued optimization of silica sol systems will remain essential to improving shell reliability, casting quality, and production efficiency.

FAQ

Can I use tap water to top up silica sol?

No—tap water contains ions that destabilize the colloid and can induce premature gelation.

Why does a finer sol improve wet strength but reduce shelf life?

Finer particles pack more densely (better strength) but have a larger adsorbed water/facilitated polymerisation tendency that lowers colloidal stability.

How often should I rheologically test slurries?

At least weekly for production stability; after any lot change of sol or refractory powder; daily if production is sensitive.