The sticking of resin sand castings is mainly a comprehensive result of thermal, mechanical, and chemical interactions between them. The main types of sand sticking are mechanical sand sticking and chemical sand sticking. Both mechanical and chemical sand bonding are essentially formed by the permeation and mass transfer of molten metal.
1. Sand sticking mechanism
1.1 Mechanical sand bonding of resin sand castings
After the metal liquid infiltrates into the pores on the surface of the sand mold and solidifies, the sand particles are mechanically hooked onto the surface of the resin sand mold casting. The more liquid metal infiltrates and the deeper it gets, the more severe the mechanical sand sticking becomes. The comparison and variation of the two forces determine the tendency of mechanical sand sticking in resin sand castings. There must be one force that promotes the infiltration of liquid metal into the pores of the sand mold, and the other force that prevents infiltration. If the metal liquid does not wet the resin sand mold material, the required pressure for penetration, P, should be P=2 δ Cos θ/ In equation r, δ— Metal surface tension; θ— The wetting angle between liquid metal and casting material; R – radius of sand particle pores.
1.2 Permeability of Metal Liquids
The penetration force of molten metal is the force that penetrates into the pores of the sand mold, mainly the dynamic and static pressure of the molten metal on the mold. The greater these two forces, the easier it is for the molten metal to penetrate into the pores of the sand mold. Therefore, the lower part of tall resin sand mold castings is prone to mechanical sand sticking. However, the depth of molten metal infiltration is not entirely proportional to the pressure of the molten metal, because the molten metal loses heat after infiltration, gradually solidifies, and loses its flow ability. The higher the pouring temperature, the thicker the wall of the resin sand mold casting. The resin sand mold has good insulation effect, poor thermal conductivity, stronger metal flow ability, longer flow time, deeper metal liquid penetration, and more severe sand sticking. The core at the hot spot of resin sand castings and the sand core completely surrounded by molten metal are most prone to severe mechanical sand sticking. It is because these areas have poor heat dissipation conditions and can be heated to higher temperatures. The metal liquid remains in a flowing state for a long time, and the depth of invasion into the sand mold is large, which intensifies mechanical sand sticking.
1.3 Penetration Resistance of Metal Liquid
The penetration resistance of molten metal is the force that prevents it from penetrating the pores of the sand mold. There are mainly two types, one is the resistance of sand mold pores, and the other is the gas pressure in sand mold pores. The resistance of sand mold pores depends on the size of sand mold pores, and the smaller the size, the greater the resistance, making it difficult for metal liquid to penetrate. The size of pores depends on the particle composition, compactness of the sand mold, and the physical and chemical reaction processes at the interface during pouring and solidification.
The chemical bonding of resin sand castings is the result of the oxidation of the surface of the metal liquid, the generation of metal oxides such as iron oxide, and the chemical reaction with SiO2 in the molding sand to generate a low melting point iron silicate slag liquid. After solidification, it firmly adheres to the surface of the resin sand castings together with the sand particles. The surface of molten steel is prone to oxidation and the formation of an oxide layer, mainly FeO, with a melting point of 1370 ℃. It can wet quartz sand and is easy to penetrate into the sand mold, reacting with silicon dioxide to form a low melting point substance, ferrous silicate, with a melting point of 1205 ℃. If the sand mold contains alkaline substances, it will exacerbate the chemical sand sticking of resin sand castings, which is due to the formation of fusible silicates, According to the state diagram of the Na2O-FeO-SiO2 system, the one with the lowest melting point can be below 500 ℃. After the formation of low melting point substances, due to their good fluidity and ability to wet the mold wall, they are easy to penetrate into the pores of the sand mold, expanding the area of action of iron oxide and molding sand, intensifying chemical sand sticking. The result of chemical reactions also expands the pores between sand particles, which is more conducive to the infiltration of low melting point substances into the depths of the sand mold, thereby connecting the metal and sand mold into one. After the low melting point substance forms a sintered layer on the surface of the resin sand mold casting, if the sintered layer firmly adheres to the surface of the resin sand mold casting, it forms chemical bonded sand; If the sintered layer is easily peeled off from the surface of resin sand castings, not only will it not produce chemical sand sticking, but it will also form a measure to prevent chemical sand sticking.
2. Main measures to prevent sand sticking
2.1 Reasonable selection of raw sand
Selecting a particle size configuration with multiple (four) sieves of sand reduces the gap between sand particles, thereby reducing the permeability of metal liquid to the sand mold (core). Using chrome iron ore sand with high fire resistance as surface sand, with a particle size distribution between 40 and 70. By adjusting the amount of resin curing agent added, the usable time of resin sand was increased, ensuring the strength of the molding sand. Try to use recycled sand as much as possible. After repeated use, recycled sand has good thermal stability and sand particle shape. The greater the compactness of the core, the better the surface quality of the model, and the easier it is to prevent sand sticking. Furthermore, based on the addition amount of different furan resin hardeners and the peak tensile strength curve of self hardening sand, it can be seen that when the amount of hardener is 0.24%, due to the low acidity value in the sand, the hardening process is extremely slow, seriously affecting the formation of demolding strength of the sand mold, and the final strength is also low; When the amount of hardener added is 0.72%, the acidity is too strong, the hardening reaction rate is too fast, the resin cross-linking structure is incomplete, the resin film and binder bridge become brittle, and the final strength is significantly reduced; When the amount of hardener added is 0.48%, the acidity is relatively moderate, and the hardening reaction proceeds according to objective laws. Under the condition of not increasing the resin amount, an ideal hardening effect is obtained. From the curve, it can be seen that when the resin addition amounts are 1.0%, 1.1%, and 1.2%, the peak tensile strength occurs within the range of 0.35% to 0.40% of the hardener addition amount. That is to say, to improve the strength of the sand mold, only increase the resin addition amount, and the hardener addition amount does not need to increase accordingly. This indicates that there is an optimal hardener addition amount for furan resin sand that is independent of the resin addition amount. In the past, the amount of hardener added was generally measured as 40% to 60% of the resin added, but it should be changed to a percentage of the weight of sand.
2.2 Choosing high-quality coatings is the main way to prevent sand sticking
Coatings are used to reduce the surface roughness of sand molds to prevent them from reacting with metals and isolate sand particles from heating. Coatings are generally divided into water-based coatings and alcohol based coatings.
The refractory powder in high-quality coatings should have a high melting point, forming a thermal protection barrier to ensure the separation of sand and metal, good adhesion, and the thermal expansion of the coating should be coordinated with the sand. Good coverage and low surface tension. Has high resistance to metal penetration and excellent resistance to erosion. The volatile material content should be low at the pouring temperature, and the particle size of refractory materials should ensure that they remain suspended for a reasonable period of time. Require excellent thixotropy, in order to quickly resuspend after sinking and stirring.
We adopted a combination of bottom layer coating and surface layer coating. The bottom layer was brushed with a relatively thin alcohol based zircon powder coating with certain permeability, allowing it to penetrate into the sand mold by about 1.5 mm. Then, two to three coats of surface layer coating were applied to make the coating layer reach 2-3 mm. The operation method is to brush horizontally and vertically, and brush the corners three to four times. This greatly reduces the probability of sand sticking.
3. Pouring temperature
The pouring temperature should not be too high. The appropriate pouring temperature should be selected based on the shape, size, wall thickness, weight and other characteristics of resin sand castings. In principle, the pouring temperature should be reduced as much as possible without producing defects such as porosity and insufficient pouring.
As a molding sand using furan resin as a binder, when the steel liquid enters the mold cavity, the resin is sintered due to the action of high-temperature metal liquid, resulting in a decrease in the strength of the molding sand. Liquid metal penetrates into the pores between the sand particles, forming sticky sand. As the pouring temperature increases, the tendency of resin sand castings to stick to sand also increases. Most of the resin sand castings made of clay sand in our factory have a pouring temperature of 1560-1580 ℃, while other special steels have a slightly higher pouring temperature, while others have a lower pouring temperature. By conducting experiments on various pouring temperatures, the sand sticking phenomenon was significantly reduced when the pouring temperature was controlled between 1550-1570 ℃, while ensuring the absence of cold shuts and wrinkles, resulting in a very significant effect.
4. Conclusion
The mechanism of sand sticking in resin sand castings is quite complex, and it is mostly a comprehensive result of mechanical and chemical sand sticking. Sand sticking defects can be effectively controlled through the above methods. Each example still needs further discussion.