What is the reason for delamination in the shell made of water glass sand during the investment molding process?

In investment casting (lost wax casting), when using the water glass sand shell making process, sand shell delamination (shell peeling) is one of the common defects, which can lead to rough surface, burrs, and even structural fracture of the casting. The following are possible reasons and solutions:

1. Analysis of the main reasons

(1) The problem of water glass adhesive is that the water glass modulus (SiO ₂/Na ₂ O ratio) is not suitable. The modulus is too high (>3.2): it hardens too quickly, the shell is brittle, and it is prone to cracking and delamination. Modulus too low (<2.6): slow hardening, insufficient strength, poor interlayer bonding. Improper density of water glass (>1.40g/cm ³) leads to poor permeability and insufficient infiltration of the lower layer; If the density is too low (<1.28g/cm ³), the bonding strength is insufficient.

(2) Improper hardening process, insufficient concentration or time of hardening agent (NH ₄ Cl or CO ₂), incomplete hardening, residual unreacted water glass inside the shell, shrinkage and delamination after drying. The CO ₂ ventilation time is too long or the pressure is too high. The surface is excessively hardened, and the inner layer is not hardened, forming a "hard shell soft core" structure.

(3) When the sand is unevenly sprinkled or the sand particle size distribution is unreasonable in the shell making operation, and the proportion of coarse sand (such as 20-40 mesh) is too high, the mechanical interlocking between layers is weak. When there is too much fine sand (such as 70-100 mesh), the permeability is poor and the hardening gas cannot penetrate. Insufficient interlayer drying causes the next layer to be coated before the previous layer is completely dried, resulting in a decrease in adhesion due to residual moisture.

(4) Environmental factors such as high humidity (>70% RH), water glass absorbing moisture, delayed hardening reaction, and low shell strength. When the temperature is too low (<15 ℃), the hardening reaction rate decreases and the interlayer bonding is not firm.

(5) Material contamination of wax molds with residual release agents (such as silicone oil) or dust can affect the bonding between water glass and sand particles.

2. Solution

(1) Optimize the water glass parameters and adjust the modulus to 2.8-3.0 (by adding NaOH or silica sol). Control the density between 1.30-1.36g/cm ³ (dilute or concentrate with water). (2) Standardize the hardening process with NH ₄ Cl hardening solution concentration of 20-25% and soaking time of 5-10 minutes. CO ₂ hardening: pressure 0.1-0.2MPa, ventilation time 30-60 seconds (adjusted according to shell thickness).

(3) Improve the sand grading for shell making operation: use coarse sand (20-40 mesh) for the bottom layer, medium sand (40-70 mesh) for the transition layer, and fine sand (70-100 mesh) for the surface layer. Interlayer drying: After each layer is coated, it should be naturally dried for 2-4 hours (or forced ventilation drying).

(4) Environmental control environment temperature: maintain 20-25 ℃, humidity ≤ 60%. Wax mold cleaning: Use alcohol or specialized cleaning agents to thoroughly remove the release agent.

(5) Other measures include adding reinforcing agents: adding 1-2% silane coupling agent (such as KH550) to water glass to improve interlayer adhesion. Secondary hardening: After the shell is made, it is fully immersed in hardening solution for reinforcement.

3. Defect investigation process

1. Check the layering position: separation of surface layer and transition layer → wax mold cleaning or surface layer sand particle problem. Separation of transition layer and back layer → hardening process or sanding grading issues.

2. Testing shell strength: Use a hardness tester to measure the hardness of each layer, and adjust the hardening parameters when the difference is greater than 15%.

3. Observing the cross-sectional morphology: more sand particles falling off → insufficient adhesive; Smooth cross-section → insufficient hardening.

4. Alternative process suggestion: If delamination occurs repeatedly, the following alternative solution can be considered: silica sol water glass composite shell: silica sol for the surface layer (high precision), and water glass for the back layer (low cost).

All silica sol process: Low delamination risk, but high cost. By systematically adjusting material, process, and environmental parameters, the problem of delamination in water glass sand shells can be effectively solved. Suggest conducting small-scale experiments to validate the optimization plan first.