Role of lost foam casting coating in lost foam casting
The function of lost foam casting coating is to support and protect the foam model, prevent liquid metal from infiltrating into sand and sand, absorb the decomposition products and allow the decomposition gas to pass through the coating, maintain the integrity of the cavity formed by the evaporation of the foam model, and maintain the liquid metal heat will not be released quickly. Therefore, the use of lost foam casting coating can reduce the surface roughness of lost foam castings, ensure the accuracy of castings, and reduce or prevent defects such as sand sticking, sand holes, pores, metal penetration, cold shut, insufficient pouring, carbon deposition and so on. Therefore, lost foam casting coating should have adequate refractoriness, certain mechanical strength, proper thermal insulation and thermal conductivity, proper permeability and absorptive capacity of liquid decomposition products, smooth surfaces, adequate brushing properties, and no chemical reaction with foam models.
Functional analysis of lost foam casting coating
Lost foam casting coatings have the effect of preventing mechanical and thermal sand sticking, and improving the rigidity of foam plastic models, preventing deformation during burying, and rapidly discharging thermal decomposition gas in the lost foam mold. So why does lost foam casting coating have such a large number of functions? What we will do is to analyze it from six aspects.
Lost foam casting coating has these six characteristics: (1) high strength and stiffness to prevent pattern deformation and damage by dry sand during molding; (2) High fire resistance to prevent sand sticking on the casting surface during pouring; (3) Excellent air permeability, which can quickly export the products of pattern gasification; (4) Strong adhesion to prevent the coating from cracking or even falling off in the production process; (5) Good coating and hanging performance, which can easily apply a certain thickness of coating when in use; (6) Good sintering and peeling property, that is to form a coating shell easy to peel off automatically after pouring, so as to obtain castings with smooth surface.
Five kinds of refractories commonly used in lost foam casting coatings
With the increasing development of lost foam casting, people pay more and more attention to lost foam casting coatings. It is reported that refractories are the aggregate components of lost foam casting coatings. There are many kinds of refractories, and the following five kinds are usually selected:
A. Zircon powder is mainly composed of ZrO2 and SiO2, namely zirconium orthosilicate. It has high fire resistance and is the best anti sticking sand material used in lost foam casting at present. When pouring steel castings and large iron castings, this kind of refractory material is often used, which can reduce the cleaning workload of castings and obtain castings with smooth surface.
b. The main component of quartz powder is SiO2. Because quartz powder has different crystal transformation under different temperature conditions, its volume changes, which reduces its use value. It is generally used to cast small and medium-sized cast iron and non-ferrous metals such as cast aluminum and cast copper parts. Because lost foam casting is mainly used for small and medium-sized parts, quartz powder is widely used.
c. Alumina, namely corundum powder, is mainly composed of Al2O3. It is also a kind of refractory with excellent performance, which can be used to cast steel castings and large iron castings.
d. Graphite powder graphite powder is one of the refractory materials widely used in the production of cast iron. It has high fire resistance, but it is easy to oxidize. Its coefficient of thermal expansion is very low, generally at 50 × Within 10-7.
e. The fire resistance of Kyanite is above 1800 degrees, and kyanite is gradually transformed into mullite at 1100-1450 degrees. Mullite is still stable at 1800 degrees, and it decomposes into corundum at 1810 degrees. Kyanite is relatively cheap and is an ideal mineral to replace corundum materials.