Dongguan Casting tells you: Why do sand castings have sand defects?
The firmly bonded sand sticking defects on the surface of sand castings can be divided into mechanical sand sticking and chemical sand sticking. Mechanical sand sticking is caused by the metal liquid drilling into the pores between the sand grains. Chemical sand sticking is caused by the oxidation of metals such as SiO2 and FEO in the sand mold. A chemical reaction occurs between the materials, producing ferrous silicate (fayalite), which is bonded to the casting. The chemical composition of ductile iron castings contains high carbon content, so ferrous silicate is not produced, and the sand sticking produced is only mechanical sand sticking.
1. Large pieces of ductile iron are stuck with sand.
Large parts are produced using clay dry type, resin sand type, water glass sand type, zircon powder, chromite powder, brown corundum powder, magnesite powder, graphite powder and other refractory aggregates. However, in order to save production costs, coating manufacturers may Too many expensive materials, such as quartz powder, bauxite powder, coke powder, etc., will be added to the powder to reduce its anti-sand sticking effect;
2. In the wet type, small ductile iron parts are stuck with sand.
The sand of iron castings mostly contains coal powder, which produces a large amount of reducing gas during casting, which will not cause chemical sand sticking. The following are the effects of various factors on mechanical sand sticking.
(1) The compactness of the sand mold.
The tightness of manual molding and vibration molding is generally low, the sand grains on the sand surface are relatively loose, the metal liquid is more likely to penetrate into the pores between the sand grains, and the pits and corners of the sand cavity may also be loose. Workers can tighten it with their fingers. Use a hammer to tighten locally. Whether the high-density molding with high productivity is locally loose depends on the fluidity of the sand, so many factories try to reduce the tightness of the sand to improve the fluidity of the sand. Use micro micros during the sand adding and compaction process. The vibration is very effective. It also depends on the tight pressure of the molding machine.
(2)Pouring temperature.
Metal liquid has high temperature and good fluidity, and easily penetrates into the pores between sand grains, resulting in mechanical sand sticking and surface roughness. However, from the perspective of avoiding casting holes, cold insulation and other defects, the pouring temperature cannot be lowered arbitrarily. The production of complex thin films Wall castings require higher pouring temperatures.
(3) Particle size and air permeability of molding sand
On the one hand, the thickness of the green sand must ensure smooth exhaust after pouring. On the other hand, the air permeability of the green sand should not be too high to prevent molten metal from easily seeping into the sand holes.
