The green sand casting process requires the addition of coal dust, bentonite and water in the sand mixing process, which is mixed and stirred by GS high efficiency rotor sand mixer, and then the sand is made by Jolt-squeezing moulding machines or Hydraulic multi-piston moulding machines.
The green sand casting process requires the addition of coal dust, bentonite and water in the sand mixing process, which is mixed and stirred by GS high efficiency rotor sand mixer, and then the sand is made by Jolt-squeezing moulding machines or Hydraulic multi-piston moulding machines. Bestech Machinery summarizes the role of coal dust in the green sand casting process in conjunction with Sand casting equipment.
Coal dust for casting is a powder made by grinding bituminous coal, which is widely used to produce wet sand for sticking on cast iron parts. It has a good anti-sticky sand effect and can significantly improve the surface quality of castings. It should be said that the most important advantage of coal dust is its low price and abundant supply.
1,Anti-sticky sand effect of coal dust
Although coal dust has long been widely used auxiliary materials, but the mechanism of its anti-sticky sand effect are so far still not completely clear, the existing several statements for reference, this aspect still needs to continue to study.
1.1 gas film said role: sand with coal dust, after pouring, due to volatile volatilization in coal dust and coal dust combustion incomplete CO, in the metal, casting interface to form a protective gas film, which will reduce FeO. Moreover, because the gas film has a certain pressure, it can prevent silicate and metal from penetrating into the sand mold.
1.2 Coke film effect: After casting, the metal, casting interface is in a state of oxygen deprivation. Under such conditions, the coal powder starts to melt at about 450℃ and melts completely at about 650℃. Liquid coal, it is easy to evenly coated on the surface of silica sand. After the volatile fraction of coal evaporates, it becomes coke. This coke is firmly attached to the surface of the sand, it will not be melted by FeO, and can even make FeO reduction. Moreover, the coal powder melting and coking, the volume to expand (casting coal can generally expand 6 to 8 times the volume), the expansion occurs, in addition to coating the surface of the sand grains, may also fill the gaps between the sand grains.
1.3 bright carbon role: "bright carbon" so far there is still no precise definition, can be said to be a kind of carbon film generated by the pyrolysis of gas hydrocarbons. In terms of its structure, it is different from carbon black, coke and graphite. The structure and nature of carbon film generated by hydrocarbon pyrolysis depends on the formation conditions, the main factors are the nature of hydrocarbon and its concentration, the pyrolysis temperature, the time of pyrolysis, the surface (size and state) of carbon film deposition, etc.
Therefore, the volatile fraction produced by the heat of the pulverized coal in the type sand and the carbon film deposited on the surface of the cast after pyrolysis is bright carbon. The bright carbon film protects the surface of the silica sand from interacting with FeO and producing fusible silicates, thus preventing metal penetration.
2,Side effects of coal dust
Coal dust, the traditional anti-sticky sand auxiliary material, is still widely used, but its side effects are also increasingly important to people
After the addition of coal dust in the sand, the amount of gas generation increases, and the possibility of pore generation increases
The coking of pulverized coal is a heat-absorbing reaction, which will cause the iron to cool faster. In addition, due to the expansion of coal powder when melting, blocking the gap between the sand grains, it will make the type sand permeability decline. All of the above factors are not conducive to the filling of the castings with hemlock. Therefore, when producing thin-walled castings, sometimes the coal is not added to the sand.
After adding coal powder in the mould sand, the fume increases during casting and produces many carbonaceous fine dust. It is recommended to equip casting dust removal hood, which can quickly extract the fume and dust produced during casting.
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