Coating is an important part of lost foam casting process and one of the key factors affecting the filling of liquid metal. The main functions of lost foam casting coating are as follows:
(1) improve the strength and rigidity of the foam model to prevent deformation or damage during transportation and sand filling vibration.
(2) prevent molten metal from infiltrating into dry sand during casting process to avoid sand sticking defects. At the same time, prevent dry sand from falling into the gap between metal liquid and foam pattern to avoid collapse.
(3) coating layer facilitates rapid discharge of foam antipyretic products and prevents porosity and carbon defects.
In order to make the coating achieve the above functions, the performance indexes that need to be controlled are: fire resistance, strength, air permeability, rapid cooling and heat resistance, coating property, dripping property, etc.
The fire resistance mainly depends on the type of refractory aggregate. Due to the high pouring temperature of cast steel, bauxite, corundum and zircon powder are generally used as refractory aggregate for cast steel coating, whose fire resistance exceeds 1800 ℃, which can effectively prevent sand sticking defects of steel castings.
The strength of coating mainly depends on the type and amount of binder, followed by the type and particle size distribution of refractory aggregate. For the bridge bearing steel castings studied in this project, the bending strength of the coating is generally required to be no less than 0.8 MPa.
It is related to many factors, such as coating thickness, coating thickness and coating thickness. The air permeability of coating has the greatest influence on the quality of steel castings, so the higher the air permeability is, the better the coating strength is.
The coating selected this time is domestic commercial coating. For the large volume of bridge bearing steel castings, the method of dip coating and brush coating is often used. Three times of coating are required before and after each coating. The drying time is 12h-16h, and the coating thickness is about 3mm-4mm after final drying.