Permeability is an important index of molding sand
Air permeability of molding sand refers to the ability of molding sand to allow gas to penetrate, and it is also an important indicator of molding sand performance. When molten metal is poured into the sand mold, a large amount of gas will be generated. The molding sand has low air permeability and poor mold exhaust capacity, which will lead to choking fire or formation of pores.
Determinants of Permeability
The air permeability is determined by two factors: one is the total volume of voids between sand particles, that is, porosity. The other is the size of the gap. It is generally believed that the higher the porosity in the compacted mold, the better the air permeability. Therefore, when using fine sand, people often worry about the defects of Blowholes in castings due to poor air permeability and poor exhaust during pouring. In fact, the difference of voids in molding sand with different particle sizes is not big, and the influence of voids on air permeability is also small, so the main factor affecting air permeability is the size of voids between sand particles.
Under the condition that the original sand particle size is basically uniform, the porosity in the sand mold has nothing to do with the size of sand particles, but only depends on the compaction conditions of molding sand. In fact, although the porosity of sand mold varies according to different compaction conditions, the difference will not be too large. After tamping under the same conditions, the air permeability of molding sand prepared with uniform sand particles with different particle sizes varies greatly, although the porosity is basically the same, showing a sharp decline trend with the refinement of sand particles
Permeability of Fused Ceramic sand
Fused Ceramic sand has excellent performance in air permeability. Fused Ceramic sand particles are spherical, with good fluidity, easy compaction and good air permeability. The surface of Fused Ceramic sand is smooth and the structure is dense, so that the binder can be evenly covered. Fused Ceramic sand is a neutral material, which can be used as acid or alkali binder.
The size distribution of raw sand particles significantly affects the quality of castings. When using coarse sand particles, the metal liquid is easy to penetrate into the core sand particles, forming a poor surface of the casting. The use of finer sand particles can produce a better and smoother casting surface, but it requires a higher amount of binder, which will reduce the permeability of the core and may cause porosity defects.
Fused Ceramic sand, due to its nearly round particle shape, excellent fluidity, high air permeability, and wide particle size distribution in the production process, can be combined or mixed with a single mesh, has its own unique grading characteristics in addition to the common characteristics in the casting practice, so that it is not segregated and stratified in the transportation and transportation process. It has good wet strength in the application of wet mold sand and self -hardening sand. For the sand casting process using binder, it is an effective way to use multi sieve distribution to make smaller particles fill the gap between larger particles and inlay each other, increase the binder "connecting bridge", and then improve the core bonding strength.