Mechanical Sand burning-on and Chemical sand burning-on
The sand burning-on defects on the casting surface can be divided into mechanical sand burning-on and chemical sand burning-on.
The reason for chemical sand burning-on is the metal oxide in the high-temperature molten metal, which is mainly the chemical reaction between FeO, MnO and silica of molding sand to produce ferrous silicate (iron olivine, manganese silicate, manganese olivine, etc.) low melting point compounds, which can penetrate into the molding sand layer and have strong adhesion to castings and molding sand, It can firmly adhere the mold sand layer to the casting surface to form chemical sand burning-on, which mainly occurs in steel castings (especially heavy wall and high manganese steel).
Mechanical sand bonding is that the metal liquid is directly drilled into the pores between the sand particles, and is connected with the molding sand machinery by the surrounding and hooking action of the metal, without chemical reaction.
Influence of various factors on mechanical sand burning-on
1) Compactness of sand mold
The compactness of manual molding is generally low, the sand particles on the surface of the sand mold are relatively loose, and the pits and corners of the sand mold cavity are prone to loose. In addition, it also depends on the setting of the hydraulic pressure or air pressure of the compaction device.
2) Casting temperature
With high temperature and good fluidity of molten iron, it is easy to penetrate into the pores between sand particles and produce mechanical sand burning-on. However, in order to avoid defects such as porosity and cold shut, the pouring temperature should not be arbitrarily reduced. Higher pouring temperature is especially needed in the production of complex thin-wall castings.
3) Grain size and air permeability of molding sand
On the one hand, the size of green molding sand should ensure smooth exhaust after pouring, and on the other hand, the permeability of green molding sand should not be too high to prevent molten iron from penetrating into the pores between sand particles. Otherwise, the casting surface will become rough and even have local mechanical sand burning-on.
How to solve sand burning-on
The measures to solve the sand burning-on of castings are generally to use effective coatings and improve the properties of raw sand/mold. The common anti-burning-on sand special sand is chromite sand, which has a long history of use and good effect. However, it is rarely used as a single sand due to its large proportion, poor grain shape, high price, strong alkalinity and other reasons.
Since the end of the last century, people have developed a variety of special sands, which generally have the advantages of nearly round grain shape, high fire resistance, high particle strength and wide particle size distribution. The best one is fused ceramic sand, which has good anti-burning-on effect.
Properties of Fused Ceramic Sand
Fused ceramic sand has high fire resistance.
Fused ceramic sand has good crushing resistance.
Fused ceramic sand is approximately spherical, with smooth surface and good air permeability.
The coefficient of thermal expansion of fused ceramic sand is relatively low.
Compared with other molding sands, fused ceramic sand has better sand burning-on prevention performance due to its higher fire resistance and smoother surface.
It should be noted that relatively speaking, the finer the original sand particle size, the wider the particle size distribution, and the more conducive to reducing sand burning-on. The tighter the mold (sand core) and the higher the strength, the more conducive to reducing sand burning-on, especially mechanical sand burning-on.