The silica sol process is an advanced technology that uses silica sol as a binder, combined with refractory materials, to create high-strength mold shells.
Silica sol is non-toxic, odorless, and environmentally friendly. Through gelation, it firmly bonds refractory sand particles together. Forming a precision casting mold shell that can withstand high-temperature molten metal.
Core Features and Advantages of the Silica Sol Process
Compared with the traditional water glass process, the improvement of the silica sol process is comprehensive:
| Characteristic Dimension | Silica Sol Process | Traditional Sodium Silicate Process |
| Surface Quality | Excellent. The inner surface of the mold shell has high finish, can perfectly replicate the details of the wax mold. The surface roughness of the casting can reach Ra 3.2 – 6.3μm. | General. The surface of the casting is relatively rough, often with defects such as “white frost”. |
| Dimensional Accuracy | Extremely high. The high – temperature mold shell has high strength and small deformation. The dimensional accuracy of the casting can reach CT4 – 6 levels. | General, with relatively low dimensional accuracy. |
| Applicable Materials | Extensive. Can be used for almost all alloys such as carbon steel, alloy steel, stainless steel, high – temperature alloy, aluminum alloy, and copper alloy. | Limited, especially not suitable for high – melting – point alloys. |
| Environmental Protection and Operation | Environmentally friendly. No ammonia smell, good operating environment, and little impact on workers’ health. | Has pungent ammonia smell, and the working environment is relatively poor. |
| Cost and Efficiency | The single – shell cost is relatively high, and the mold – making cycle is relatively long. The comprehensive yield is high, and the subsequent processing allowance is small. | The single – shell cost is low, and the mold – making cycle is short. The comprehensive yield is relatively low, and the waste product rate is high. |
Core Advantages:
- Ultra-high precision and finish: Capable of producing near-net-shape parts, significantly reducing the time and cost of subsequent machining.
- Excellent stability: The silica sol mold shell offers high high-temperature strength and excellent thermal shock resistance. It is less prone to cracking during casting, resulting in a high yield rate.
- Wide material compatibility: Particularly suitable for casting difficult-to-machine high-temperature alloys and stainless steels.
The Process Flow of Silica Sol Lost Wax Casting
The main steps of the silica sol process are as follows:
1. Wax Injection Molding: Liquid wax is injected into an aluminum alloy mold. A precision wax pattern is formed after cooling.
2. Wax Pattern Assembly: Multiple wax patterns are assembled onto a common sprue rod to form a “wax tree”.
3. Shell Forming (Core Step):
- Slurry Coating: Dip the wax tree into a slurry of silica sol and ultra-fine refractory powder, creating a uniform coating on the surface.
- Sanding: Sprinkle coarse refractory sand over the wet coating to increase the thickness and strength of the shell.
- Drying and Hardening: Allow to dry in a constant temperature and humidity environment (e.g., 23±2°C, 60±5% humidity) to allow the silica sol to gel and solidify.
- Repetitive Operation: Repeat the “coating-sanding-drying” process 6-9 times until the shell reaches the desired thickness and strength.
4. Wax Removal: The shelled wax tree is placed into a high-pressure steam dewaxing autoclave. High-temperature steam is used to completely melt and drain the internal wax patterns. The wax material is recycled, resulting in a hollow ceramic shell.
5. Shell Roasting: The shell is sent into a high-temperature furnace (approximately 1000°C) for roasting. This process removes residual wax and moisture, and enables the shell to attain its final high-temperature strength.
6. Smelting and Pouring: The molten metal is poured into the preheated shell at an appropriate temperature.
7. Shell Removal and Cleaning: After the metal has solidified and cooled, the outer shell is broken and removed by means of vibration, high – pressure water guns, etc., exposing the casting.
8. Cutting, Grinding and Polishing: The casting is cut off from the gating system, and necessary grinding, polishing, and inspection are carried out. Finally, the finished product is obtained.
Silica Sol: Applicable Conditions & Selection Tips
Applicable conditions
1. Product Structure and Technical Requirements:
- Complexity and Completeness: Ideal for producing parts with extremely complex shapes, intricate cavities, deep holes, and thin walls (down to 0.5mm). It can be cast in one piece.
- Dimensional Accuracy and Surface Quality: When product specifications require stringent dimensional tolerances (typically reaching CT4-6) and surface roughness (Ra 3.2-6.3μm), colloidal silica is the preferred process. It minimizes machining and achieves near-net-shape results.
2. Industries and Applications:
- Aerospace: Critical hot-end components such as engine turbine blades, guide vanes, and casings. These parts have complex shapes and are made of difficult-to-machine high-temperature alloys. They place extremely high demands on internal metallurgical quality and surface integrity.
- Medical: Surgical instruments, orthopedic implants, and other applications require extremely high surface finish and biocompatibility. The stability and cleanliness of the silica sol process can meet medical-grade standards.
- Automotive: Turbocharger impellers, engine manifolds, and lightweight structural parts. These components require high performance, heat resistance, and lightweight construction. They must withstand high temperatures and high speeds.
- Energy and Industrial Pumps and Valves: Pump casings, impellers, and valve components. Corrosion- and wear-resistant materials such as stainless steel and duplex steel are commonly used.
3. Material Characteristics:
This process is suitable for nearly all alloys. But, its value is particularly evident when casting expensive, hard, and difficult-to-machine materials such as high-alloy steels, stainless steels, tool steels, cobalt- and nickel-based superalloys, and titanium alloys.
4. Volume and Cost-Effectiveness:
It’s ideal for small and medium-volume production. While the mold and shell cost per piece is higher, for R&D, customized products, or products with low overall lifecycle demand, mold-making costs are significantly lower than those of hard die casting (such as die casting).
High-volume production can also be achieved economically through efficient tree assembly solutions. QD offers automated tree assembly lines to improve the efficiency of large-scale production.
Selection Tips
When developing a new casting, consider the following factors to determine whether to choose the silica sol process:
- Prioritize precision and finish: If your product has extremely high dimensional and surface requirements and extensive post-processing is undesirable or impossible, silica sol is the preferred choice.
- Examine material properties: If you are casting expensive or difficult-to-machine materials such as stainless steel or high-temperature alloys, the high yield and material savings of the silica sol process can offset its higher initial cost.
- Evaluate total cost: Don’t just look at the cost of a single mold shell; calculate the comprehensive costs (including scrap rate, machining costs, and time costs). For high-value parts, silica sol is often more economical.
- Choosing a partner: A reputable foundry (such as QD Casting) not only has stable processes but also offers professional advice on mold design, gating and riser solutions, and cost optimization. This helps you avoid potential flaws from the design stage, which is crucial to project success.
FAQ
1. What’s the biggest difference between silica sol and water glass processes?
The core difference lies in the binder. Silica sol uses nano-silica colloidal silica, while water glass uses a sodium silicate aqueous solution.
2. What are the disadvantages of the silica sol process?
The main disadvantages are the long production cycle and high cost per mold. Therefore, it is more suitable for products with high quality requirements and high added value, rather than for low-cost, high-volume, simple parts.
3. Are all metals suitable for casting using the colloidal silica process?
Almost. The mold shell has high refractoriness (reaching temperatures exceeding 1700°C) and excellent thermal stability. It can be used to cast a wide range of metals, from aluminum and copper alloys to various steels, stainless steels, and superalloys such as nickel and cobalt. Its adaptability is very broad.
4. How can you tell if a casting is produced using the colloidal silica process?
The most intuitive way is to look at the surface finish and detail clarity. Castings produced using the colloidal silica process have very smooth surfaces and sharp corners. In contrast, conventional sand castings or water glass investment castings have relatively rough surfaces and blurred details.
5. What are the advantages of a professional manufacturer like QD Casting?
Their strengths lie not only in their advanced silica sol shell-making lines and testing equipment like spectral analysis, but also in their extensive technical expertise and quality management system. They offer a one-stop solution from mold design, rapid prototyping, casting, heat treatment, to finishing, ensuring they provide customers with high-quality castings with high consistency and low defect rates.