What are the compaction rates of green sand, coated sand and resin sand- Part 2
3. Resin sand compaction rate
The compaction rate of resin sand is usually controlled at 45% - 55%. The following are some methods to adjust the compaction rate of resin sand:
Raw material control
Resin and curing agent: The viscosity of the resin, the activity of the curing agent and the amount of both added will affect the compaction rate. If the resin viscosity is too high or the amount added is too much, the bonding force between the sand particles will be too large, and the compaction rate will be too high. The resin viscosity or the amount of resin can be appropriately reduced; if the curing agent activity is too high or the amount added is too much, the resin sand will solidify too quickly, the fluidity will be reduced, and the compaction rate may also be high. The type or amount of curing agent needs to be adjusted.
Raw sand: The particle size distribution, shape and surface characteristics of the raw sand have an impact on the compaction rate. If there is too much fine sand in the raw sand, the compaction rate will be too high, and the proportion of coarse sand can be increased; if the surface of the raw sand is too smooth, the friction between the sand particles is small, and the compaction rate may be low, so the raw sand with appropriate surface roughness can be selected.
Adjustment of sand mixing process
Sand mixing time: If the sand mixing time is too long, the resin is too evenly distributed on the surface of the sand particles, the bonding points between the sand particles increase, and the compaction rate will be high. The sand mixing time should be shortened; if the sand mixing time is too short, the resin and sand particles are not mixed evenly, and the compaction rate is unstable. The sand mixing time needs to be extended. Generally, the sand mixing time is about 3-10 minutes.
Sand mixing speed: If the sand mixing speed is too fast, the sand particles will move too violently, destroy the formed bonding bridge, and cause the compaction rate to decrease. The sand mixing speed can be reduced; if the sand mixing speed is too slow, the sand mixing efficiency is low, the resin and sand particles cannot be fully mixed, and the compaction rate will also be affected. The sand mixing speed can be appropriately increased.
Modeling operation optimization
Compacting method: Different compacting methods have different effects on the compaction rate. For example, vibration compaction can make the compaction rate of the lower part of the sand mold higher and the upper part lower; compaction compaction can make the compaction rate of the surface of the sand mold higher and the inner part lower. You can choose the appropriate compaction method according to the actual situation, or use a combination of multiple compaction methods to obtain a uniform compaction rate.
Compactness: During the molding process, the compaction degree should be controlled to avoid excessive or insufficient compaction. Excessive compaction will make the compaction rate too high, and the number of compaction times or the compaction pressure can be appropriately reduced; insufficient compaction will make the compaction rate too low, and the number of compaction times or the compaction pressure need to be increased.
What are the effects of the compaction rate of resin sand on its performance?
The compaction rate is too high
Poor air permeability: The compaction rate of resin sand is too high, the pores between sand particles become smaller, and the gas discharge is blocked. During pouring, the gas in the cavity is difficult to discharge, which can easily cause defects such as pores and shrinkage holes in the casting.
Abnormal strength: Too high compaction rate will make the resin sand over-compacted. Although the initial strength may be high, when the casting solidifies and shrinks, the sand mold has poor yield, which will hinder the shrinkage of the casting, causing stress inside the casting and even cracks. At the same time, it may also cause the mold to be damaged due to excessive brittleness during demolding or sand cleaning.
Reduced fluidity: Resin sand with too high compaction rate has poor fluidity, and it is not easy to fill the mold evenly during the molding process. For molds with complex shapes, it is difficult to ensure the consistency of compactness of various parts of the mold, affecting the dimensional accuracy and surface quality of the casting.
Too low compaction rate
Low mold strength: The compaction rate of resin sand is low, the sand particles are not tightly bonded, and the mold strength is insufficient. During the transportation, box closing and pouring process, the mold is easy to deform and collapse, resulting in dimensional deviation of the casting, and defects such as sand holes and sand loss may also occur.
Poor surface quality: Due to the low compaction rate and the loose surface of the casting, the molten metal easily penetrates into the gaps between the sand particles, resulting in sand adhesion defects, increasing the difficulty of cleaning, and increasing the surface roughness of the casting, affecting the appearance quality.
Poor dimensional stability: When loose resin sand is subjected to external force or high temperature, it is more likely to move and deform, making it difficult to maintain the precise size of the casting, reducing the dimensional accuracy of the casting.
