Mullite sand can effectively reduce sand sticking in precision casting through various mechanisms, as follows:

I. High fire resistance and low expansion coefficient

Mullite sand has a high fire resistance (usually over 1750℃), which can remain stable at high temperature without melting or deformation. Its low thermal expansion coefficient makes the relative position between sand particles change little when the temperature changes, which reduces the chance of metal liquid penetrating into the gap between sand particles, thus reducing the risk of sand sticking.

II. Good chemical stability

Mullite sand is mainly composed of alumina (Al₂O₃) and silica (SiO₂), which has high chemical stability and can effectively resist the chemical erosion of molten metal. This stability reduces the chemical reaction between molten metal and sand particles, thus reducing the possibility of chemical sand sticking.

III. Resistance to wetting of molten metal

The surface characteristics of mullite sand make it less wettable to molten metal, and it is difficult for molten metal to penetrate into the gap between sand particles. In addition, the spherical particle structure and low dust content of mullite sand further reduce the adhesion of molten metal.

IV. Optimized particle size distribution

Mullite sand has reasonable particle size distribution, usually fine-grained (such as -200 mesh, -325 mesh) or moderate particle size (such as 30-60 mesh, 60-80 mesh), which can provide good filling and air permeability. Fine-grained mullite sand can better fill the fine parts of the mold and reduce the occurrence of sand holes and pores, thus reducing the risk of sand sticking.

V. Reducing the penetration of molten metal

The high density and low porosity of mullite sand make it difficult to be penetrated by molten metal at high temperature. In addition, mullite sand can react in solid state at high temperature, and the interface wetting angle of the reaction product is small, which further reduces the penetration of molten metal.

VI. Use of additives

In practical application, mullite sand can also be used in combination with other additives (such as zircon powder and chromite powder) to further improve the sand adhesion resistance. These additives can improve the fire resistance and sintering resistance of sand molds and reduce the penetration and adhesion of molten metal.

VII. Coating and process optimization

Coating a layer of high-temperature resistant coating (such as silica sol) on the surface of mullite sand can further reduce the direct contact of molten metal. In addition, optimizing pouring temperature and speed, and reasonably designing exhaust passage can also reduce sand sticking.

VIII. Summary

Mullite sand effectively reduces sand sticking through its high refractoriness, low expansion coefficient, good chemical stability and wettability resistance of molten metal. Reasonable selection of particle size distribution and additives, as well as optimization of coating and process parameters, can further improve its anti-sand adhesion performance, thus improving the surface quality and yield of castings.