Sodium silicate is a soluble silicate composed of alkali metal oxides and silicon dioxide. It typically appears as a viscous, transparent liquid.
In lost wax casting, water glass plays a key role as a binder. Its task is to firmly bind the dispersed refractory sand particles together, forming a hard ceramic shell around the wax pattern that can withstand the erosion of high-temperature molten metal. This ceramic shell is called the mold shell.
Features and Core Advantages
The sodium silicate process offers many irreplaceable advantages:
1. Extremely low cost and excellent economic efficiency
This is the core competitive advantage of the water glass process. Water glass itself is inexpensive, far lower than silica sol and ethyl silicate. This is a crucial factor for cost-sensitive applications.
2. Short shell production cycle and high production efficiency:
- The hardening process is chemical. After coating, carbon dioxide (CO₂) gas is introduced into the shell, completing the hardening process in just tens of seconds to minutes. The next layer can be applied immediately.
- This significantly improves production efficiency compared to silica sol, which requires a long natural drying time.
3. Simple process and easy operation:
No complex temperature and humidity control equipment (such as a silica sol drying room) is required. The curing process is quick and intuitive. The operator’s skill requirements are relatively low. It is easy to master and popularize.
4. Non-toxic and relatively environmentally friendly:
Water glass uses water as the solvent and CO₂ as the hardener. During the entire shell-making process, no toxic or harmful gases are emitted, resulting in a more environmentally friendly work environment.
Manufacturing Process of Sodium Silicate Shell Mold
The process of water glass craft follows the basic framework of lost wax casting. But, it has its own unique features in the shell making process:
1. Wax pattern making and tree assembly: The process is identical to the other steps. The wax pattern is created and welded to the casting system.
2. Paint preparation: Water glass is mixed with refractory powder (such as quartz powder) to create a thick slurry.
3. Painting and sanding: The wax pattern tree is dipped into the paint, evenly coating the surface. Coarse refractory sand is then applied.
4. CO₂ chemical curing: This is a critical step. The coated mold is placed in a curing chamber and aerated with CO₂ gas. The CO₂ reacts rapidly with water glass:
Na₂O·mSiO₂ + CO₂ → Na₂CO₃ + mSiO₂·nH₂O (silicone gel)
The resulting silicone gel instantly and firmly bonds the refractory sand particles together, forming a hard mold shell.
5. Repeated Coating: Repeat the “coating → sanding → hardening” process 5-7 times until the mold shell reaches the desired thickness and strength.
6. Dewaxing and Firing: The finished mold shell is placed in hot water or high-pressure steam to remove the wax. It is then fired in a high-temperature furnace to remove any residue and achieve a high-strength mold shell.
7. Pouring and Cleaning: Molten metal is poured in, and after cooling, the mold shell is broken into pieces. This produces the casting blank and is subsequently cleaned.
Limitations of the Sodium Silicate Process
The advantages of water glass technology are outstanding. But it also has some disadvantages:
- The casting surface quality is relatively rough: The silica gel formed by the reaction of water glass and CO₂ has a loose structure and contains sodium ions. This easily reacts with refractory materials to form low-melting-point substances, which can easily cause “powdering” on the mold shell surface. This results in a relatively rough casting surface, and the clarity of details is not as good as that of silica sol castings.
- Poor high-temperature performance of the mold shell: The mold shell has low high-temperature strength and insufficient refractoriness. It is prone to deformation or cracking when pouring hot molten steel, resulting in reduced dimensional accuracy of the casting.
- Poor collapsibility: After pouring, the mold shell is difficult to collapse automatically. Cleaning is laborious and increases the workload of post-processing.
- Prone to “silicon spot” defects: Sodium salts remaining on the surface of castings can easily cause chemical sand sticking, forming “silicon spot” defects that are difficult to clean.
Applicable Conditions of the Sodium Silicate Process
After understanding its advantages and disadvantages, we can clearly know the application boundaries of water glass technology:
| Application Field | Description |
| Carbon steel and low-alloy steel castings | Structural parts with no high requirement for surface finish, such as hardware tools, mechanical accessories, pipe valves, etc. |
| Cast iron castings | Such as artistic cast iron railings, manhole covers, etc. |
| Copper alloy and aluminum alloy castings | Medium and large castings that have certain surface requirements but prioritize cost more. |
| Large and thick-walled castings | For projects where surface quality and dimensional accuracy are not the primary considerations, and cost control is the key. |
| Single-piece and small-batch production | Low mold investment, flexible process, very suitable for trial production and small-batch orders. |
Summary
When your product budget is limited and you don’t demand extreme surface finish and precision, the water glass process is undoubtedly the most economical and practical choice. It offers a unique value proposition. QD is an expert in water glass processing. If you have any needs, please contact the QD team. We will provide you with the best solution.
FAQ
1. What are the greatest advantages of the water glass process?
Its core advantage is its exceptional cost-effectiveness. It can significantly reduce production costs. For many castings where surface finish requirements are not extremely demanding, it is the most cost-effective option. QD has extensive experience with the sodium silicate process and can help you maximize cost optimization while ensuring quality.
2. Is the water glass process fast for shell production?
Very fast. It uses CO₂ gas chemical curing. Each shell layer cures in just a few minutes, far exceeding the natural drying time required by the silica sol process. This allows us to shorten our production cycle and respond more quickly to your delivery needs.
3. Is the surface finish of castings made with water glass casting smooth?
Objectively speaking, its surface finish is inferior to that of the silica sol casting. If you’re looking for a mirror-like finish, you might want to consider the silica sol casting process. However, for many general-purpose and structural parts, careful process control can ensure that the surface quality of sodium silicate castings meets standard requirements.
4. How do I choose the most suitable process for my product? (Water Glass vs. Colloidal Silica)
It depends on your core requirements:
Prioritizing cost, cycle time, and large parts → Choose the Water Glass process.
Seeking extremely high surface finish, dimensional accuracy, and complex, thin-walled parts → Choose the Colloidal Silica process.
Or, you can contact the QD team directly. We will provide you with the most professional and cost-effective process recommendations.
5. I’m looking for trial production or mass production using a water glass process. How can I work with QD?
The process is simple: Simply provide your part drawings or samples. QD’s technical team will review the process and provide a quote. Once confirmed, production can begin.
We welcome you to visit our factory and see our water glass process production line and quality control system firsthand.