Performance parameters of lost foam coatings
The performance of coatings for lost foam casting is mainly thermal performance and air permeability, which determine its role in the pouring process. In actual operation, it is necessary to measure and/or control its viscosity, solid content, air permeability, pH value, etc. Only by ensuring that the performance of the coating is stable and consistent can the quality of lost foam casting products be guaranteed.
The performance of the coating has a great influence on the quality of lost foam castings. Performance of lost foam coatings: It should have high temperature resistance, suspension, anti-sand adhesion, flushing resistance, air permeability, leveling, and sintering properties. After brushing, a dense high-strength coating can be formed on the surface of the foam model. During high-temperature pouring, a small part should be melted so that the coating can be sintered. After cooling, the coating can be well peeled. At the same time, the coating should also have a small amount of volatile components during high-temperature pouring so that the coating can be well permeable.
(1) Viscosity
Viscosity describes the reaction of flow to shear force and is an important indicator for measuring the fluidity of the coating. Although viscosity mainly depends on the composition of the coating, it is also affected by the stirring process. Especially when the coating is used, the viscosity of the coating will be adjusted to the appropriate range as needed, so the measurement and monitoring of viscosity is particularly important. Changes in viscosity will affect other related properties of the coating, such as solid content, thickness and permeability, and thus affect the quality of the casting. The coating used for lost foam casting is generally shear-thinned. In the coating manufacturing process, extending the stirring time will cause the viscosity to decrease while the permeability is relatively stable. When used, the viscosity of the coating will further decrease with the extension of the stirring time. Excessive stirring may also cause the permeability to increase.
(2) Permeability
The permeability directly controls the speed at which the products of foam melting and decomposition escape, thereby determining the flow rate of the liquid metal, and then determining whether defects such as pores, cold shuts, insufficient pouring, and carbon deposition can be eliminated. Direct measurement of permeability is particularly important for coating quality control. Since the coating is used at high temperatures, it is difficult to directly measure its permeability in use. Existing methods are all performed at room temperature.
Generally, when discussing permeability, we refer to the permeability of the coating. As can be seen from the previous discussion, in the lost foam casting process, permeability should actually be considered in the entire dynamic system of metal/foam model/coating/molding sand. The permeability of the coating is only a part of the entire system. Changes in sand type, shape and temperature, whether vacuuming is performed, changes in foam model performance, etc. will cause changes in the permeability of the entire system and thus change the metal flow behavior. In actual operation, sometimes casting manufacturers hope to compensate for changes in the permeability of the entire system caused by changes in other process parameters by changing the permeability of the coating. However, simply changing the permeability of the coating is sometimes not enough to compensate for changes in the permeability of the entire system.
(3) Other properties
Other performance parameters include solid content, dry weight and wet weight, pH value, Baume degree, etc. The solid content of the coating mainly depends on the proportion of refractory fillers and affects the viscosity of the coating. The solid content of the coating is closely related to its anti-sand sticking property. Dry weight and wet weight are easy to operate and are a supplement to the measurement of viscosity and solid content of the coating. Monitoring pH changes can promptly detect bacterial and microbial growth in the coating. Baume measurement is not a very accurate quality control parameter, especially for coatings used in lost foam casting.
(4) Thermal conductivity
The thermal conductivity of the coating is crucial to obtaining high-quality lost foam castings. The measurement of thermal conductivity, specific heat, thermal expansion coefficient, etc. is mostly carried out during the research and development stage of the coating rather than during use.
