What are the compaction rates of wet sand, coated sand, and resin sand?
1. Compaction rate of wet sand
The compaction rate of clay sand is generally controlled at 32% - 45%. Generally, the compaction rate of clay sand is controlled at 40%-50% during manual molding, and the compaction rate is controlled at 35%-45% during automatic molding line molding.
The methods for controlling the compaction rate of clay sand are as follows:
Control moisture content: Moisture is the key factor affecting the compaction rate of clay sand, which is generally controlled by adjusting the amount of water added. Usually, the moisture content of molding sand is regularly tested with equipment such as a rapid moisture meter, and the amount of water added to the molding sand is adjusted according to the results to meet the compaction rate requirements.
Adjust the composition of molding sand: Increasing the clay content can increase the bonding force and compaction rate of molding sand, while increasing the proportion of new sand will reduce the compaction rate. Therefore, the proportion of clay, new sand, and old sand can be adjusted according to actual conditions. At the same time, an appropriate amount of additives such as coal powder can be added to improve the performance of molding sand and indirectly affect the compaction rate.
Optimize the sand mixing process: the sand mixing time and sand mixing strength have an impact on the compaction rate. If the sand mixing time is too short, the clay and other binders cannot evenly cover the sand particles, and the compaction rate of the molding sand is unstable; if the sand mixing time is too long, the clay film on the surface of the sand particles will be too thick, resulting in a decrease in the compaction rate. The optimal sand mixing time and sand mixing strength should be determined through experiments to ensure the stability of the compaction rate of the molding sand.
Strengthen the compaction operation control: During the molding process, the compaction method and compaction degree should be uniform and consistent. For example, when compacting with a vibrating table, the vibration time and frequency should be controlled; when compacting with sand shooting, the sand shooting pressure and time should be adjusted to ensure that the compaction rate of the molding sand meets the requirements.
What are the effects of compaction rate on the quality of castings?
Too high compaction rate
Poor air permeability: If the compaction of the molding sand is too high, the porosity will decrease, and the gas will be difficult to discharge, which will easily produce defects such as pores and shrinkage holes in the castings.
Poor surface quality of castings: Too high compaction rate will increase the hardness of the molding sand and reduce the yield. When the casting solidifies and shrinks, it will hinder the shrinkage of the metal, causing cracks on the surface of the casting and affecting the surface quality.
Difficult demoulding: The molding sand is too compact, and the friction with the mold increases. It is easy to damage the mold during demoulding, affecting the dimensional accuracy of the casting, and may also cause sand adhesion defects in the casting.
The compaction rate is too low
Insufficient mold strength: The compactness of the molding sand is not enough. The casting is prone to deformation and collapse during transportation, box closing and pouring, resulting in dimensional deviation of the casting or even scrapping.
The filling capacity is affected: Loose molding sand cannot effectively prevent the erosion of molten metal, and sand erosion may occur. After the molten metal enters the mold cavity, part of the molding sand will be involved, forming defects such as sand holes and sand inclusions, reducing the intrinsic quality of the casting.
The surface roughness of the casting increases: Because the molding sand is not compact enough, the molding sand particles are easy to fall off under the pressure of the molten metal, which increases the surface roughness of the casting.
2. Compactness of coated sand
The compactness of coated sand is generally controlled at 35% - 45%. In actual production, it can be fine-tuned according to specific casting requirements, process conditions, etc. The following are some methods to adjust the compactness of coated sand:
Raw material adjustment
Sand particle size distribution: If the compactness is high, the proportion of coarse sand can be appropriately increased, and the content of fine sand can be reduced to make the sand particles more loosely stacked and reduce the compactness; if the compactness is low, the proportion of fine sand can be increased to make the filling between sand particles tighter and improve the compactness.
Resin dosage: Excessive resin dosage will make the coated sand sticky and the compactness is high, so the resin dosage can be appropriately reduced; insufficient resin dosage will lead to a low compactness, and the resin dosage needs to be increased to increase the bonding force between sand particles, thereby improving the compactness.
Optimization of sand mixing process
Sand mixing time: Prolonging the sand mixing time can make the resin more evenly coated on the surface of the sand particles, but too long will cause the resin film on the surface of the sand particles to be too thick and the compaction rate to be high, so the sand mixing time needs to be shortened; if the sand mixing time is too short, the resin is unevenly distributed and the compaction rate is unstable. The sand mixing time should be appropriately extended to ensure the stability of the compaction rate.
Sand mixing speed: If the sand mixing speed is too fast, the friction between the sand particles will increase, which may cause damage to the resin film on the surface of the sand particles and affect the compaction rate. The sand mixing speed can be reduced; if the sand mixing speed is too slow, the sand mixing efficiency is low, and the resin and sand particles are mixed unevenly. The sand mixing speed can be increased to allow the resin to better combine with the sand particles and stabilize the compaction rate.
Molding process control
Sand shooting pressure and time: If the sand shooting pressure is high and the time is long, the coated sand will be compacted in the mold with a high compaction rate. The sand shooting pressure can be appropriately reduced or the sand shooting time can be shortened; if the sand shooting pressure is low and the time is short, the compaction of the coated sand is insufficient and the compaction rate is low. The sand shooting pressure needs to be increased or the sand shooting time needs to be extended.
Mold temperature: If the mold temperature is too high, the resin in the coated sand will solidify prematurely, resulting in a decrease in the compaction rate. The mold temperature should be lowered; if the mold temperature is too low, the coated sand has poor fluidity and the compaction rate may be high. The mold temperature can be appropriately increased to improve the molding performance of the coated sand and make the compaction rate reach a reasonable range.
What are the effects of the compaction rate of coated sand on its performance?
Too high compaction rate
Reduced air permeability: Too high compaction rate will reduce the pores between the coated sand particles and narrow the gas discharge channel, resulting in a decrease in air permeability. During the pouring process, the gas in the cavity is difficult to discharge smoothly, and it is easy to form defects such as pores and shrinkage holes in the casting.
Poorer disintegration: Too high a compaction rate will make the coated sand too compact, and the bonding force between the sand particles will be enhanced. After the casting solidifies, the disintegration of the mold will be worse. This will make it difficult to clean the sand of the casting, increase production costs and labor intensity.
Fluidity is affected: Too high a compaction rate will reduce the fluidity of the coated sand, making it difficult to evenly fill various parts of the mold during the molding process. Especially for some molds with complex shapes, it may cause insufficient local compactness or looseness of the mold, affecting the dimensional accuracy and surface quality of the casting.
Too low a compaction rate
Insufficient strength: Too low a compaction rate of the coated sand will reduce the contact points between the sand particles, resulting in weaker bonding and lower strength of the mold. During the transportation, box assembly and pouring process, the mold is prone to deformation and damage, and cannot withstand the scouring and pressure of the molten metal, resulting in defects such as sand holes and sand falling in the casting.
Surface quality decreases: Due to the low compaction rate, the surface of the casting is not dense enough. Under the action of the molten metal, the molding sand particles are easy to fall off, which increases the surface roughness of the casting and affects the appearance quality of the casting. At the same time, the loose molding sand is also easy to be infiltrated by the molten metal, resulting in sand adhesion defects, which increases the difficulty of cleaning the surface of the casting.
