Uses of coatings in lost foam castings
The role of lost foam coatings is to support and protect the foam mold, prevent liquid metal from penetrating sand and sticking to sand, absorb decomposition products and allow decomposition gases to pass through the coating, maintain the integrity of the cavity formed after the foam mold evaporates, and keep the heat of the liquid metal from dissipating quickly. Therefore, the use of lost foam coatings 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 insulation, inability to pour, and carbon deposition.
Therefore, lost foam coatings should have sufficient refractoriness, certain mechanical strength, appropriate heat preservation and thermal conductivity, suitable air permeability and ability to absorb liquid decomposition products, smooth surface, sufficient brushing performance, and no chemical reaction with foam molds.
Functional analysis of expendable die casting coatings
Expendable die casting coatings have the functions of preventing mechanical and thermochemical sand sticking to castings, improving the rigidity of foam plastic molds, preventing deformation during embedding, and facilitating the rapid discharge of thermal decomposition gases of expendable molds. So why do expendable die casting coatings have so many functions?
Mainly analyzed from six aspects.
The coating for lost die casting has the following six characteristics:
(1) High strength and rigidity, which can prevent the shape from being deformed and damaged by dry sand during molding;
(2) High refractory performance, which can prevent sand from sticking to the surface of the casting during pouring;
(3) Excellent air permeability, which can quickly guide the product with gasified shape;
(4) Strong adhesion, which can prevent the coating from cracking or even falling off during the production process;
(5) Excellent coating and hanging properties, which can easily apply a coating of a certain thickness during use;
(6) Excellent sintering and peeling properties, that is, after pouring, a coating shell that is easy to peel off automatically is formed, thereby obtaining a casting with a bright surface.
Five refractory materials commonly used in lost die casting coatings With the development of lost die casting, people are paying more and more attention to lost die casting coatings. Refractory materials are the aggregate components of expendable die coatings.
There are many types of refractory materials, and the following five are generally selected:
A. Zircon powder The main components are ZrO2 and SiO2, i.e. zirconium orthosilicate. It has high refractoriness and is currently used as an anti-sand material for expendable die casting. This type of refractory material is often used when casting steel castings and large iron castings, which can reduce the cleaning workload of castings and obtain castings with bright surfaces.
b. Quartz powder The main component is SiO2. Because quartz powder has different crystal changes under different temperature conditions, it causes volume changes, thereby reducing its use value. It is generally used to cast small and medium-sized cast iron and non-ferrous metals such as cast aluminum and copper. Since expendable die casting is mainly used for small and medium-sized parts, quartz powder is widely used.
c. Alumina Alumina is corundum powder, and its main component is Al2O3. It is also a refractory material with good performance and can be used to cast steel castings and large iron castings.
d. Graphite powder Graphite powder is one of the most commonly used refractory materials in cast iron production. It has high refractoriness but is easily oxidized. Its thermal expansion coefficient is very low, generally within 50×10-7.
e. Kyanite Kyanite has a refractoriness of more than 1800 degrees and kyanite gradually transforms into mullite at high temperatures of 1100-1450 degrees. Mullite remains stable at 1800 degrees and decomposes into corundum at 1810 degrees. Kyanite is relatively expensive and is an ideal mineral to replace corundum.