The Application and Research Progress of Silica Sol in Precision Casting

       Precision casting (also known as investment casting) is an advanced process capable of producing high-precision and complex-structured metal parts, and is widely used in aerospace, medical devices, automotive manufacturing and other fields. In the precision casting process, the preparation of the mold shell is a crucial step, directly affecting the surface quality, dimensional accuracy and yield of the casting. Silica sol, as an inorganic binder, has become one of the core materials for the preparation of precision casting molding shells due to its excellent high-temperature resistance, stability and environmental friendliness.

   一、 The characteristics of Silica Sol and its role in precision casting Silica sol is a colloidal solution formed by dispersing nano-scale silica particles in water, and it has the following characteristics: 

    1. High adhesiveness: The SiO₂ particles in silica sol form a three-dimensional network structure through dehydration and condensation, endowing the mold shell with good room temperature and high-temperature strength. 

    2. Low coefficient of thermal expansion: The silica sol mold shell has excellent dimensional stability at high temperatures, reducing the risk of casting deformation. 

    3. Environmental friendliness: With water as the dispersion medium, there are no toxic volatile substances, which conforms to the trend of green manufacturing. 

    4. Surface wettability: It combines closely with refractory materials (such as zircon sand and corundum powder), enhancing the density of the shell. 

    In precision casting, silica sol is mainly used for: 

• Binder: replacing the traditional ethyl silicate hydrolysate for the preparation of multi-layer shells; 

• Coating carrier: Mixed with refractory powder to form a surface coating, it enhances the surface finish of castings. 

• Reinforcing agent: Enhances the thermal shock resistance and creep resistance of the shell through modification.

二、Preparation process and Optimization of silica sol type shell 

1. Typical shell-making process Silica sol type shell usually adopts the "impregnation - sanding - drying" cycle process:

• Surface coating: silica sol + fine-grained zircon powder (particle size <200 mesh), forming a high-precision surface; 

• Reinforcing layer: silica sol + bauxite or mullite sand (particle size 60-100 mesh), enhancing the overall strength of the shell. 

• Drying conditions: Temperature 25-30℃, humidity 40-60%, drying time for each layer 4-8 hours. 

    2. Process Optimization Research 

• Drying Process improvement: By adopting microwave drying or vacuum drying technology, the drying time is shortened by more than 50%. 

• Layered structure design: The surface layer uses nano-silica sol (particle size 10-20nm) to reduce surface defects of castings; 

• Development of composite adhesives: The combination of silica sol with ethyl silicate or aluminum sol improves the high-temperature strength of the shell (bending strength at >1500℃ reaches 3-5 mpa).