yvonne@nydcasting.com 
If furan resin self-hardening sand is used improperly, casting defects such as pores, mechanical sand sticking, and mold sticking often occur.
1. Generation and prevention measures of pores
During metal solidification, trapped bubbles form pores in the casting. The shape, wall characteristics, surface texture, and location of pores help distinguish them from shrinkage, sand holes, and slag defects.
The main measures to prevent the generation of pores are as follows:
1.1. Choose the right type of resin sand and appropriately reduce the amount of resin added
Too much resin or curing agent added, and too high a nitrogen content in the resin, will cause an increase in gas generation. Therefore, to prevent the generation of pores, low-nitrogen resins should be selected and the amount of resin and curing agent added should be reduced as much as possible.
1.2. Improve the quality of regenerated sand and reduce its burning loss and micro powder content
If the old sand is not regenerated well, the burning loss of the old sand will be out of control, and the micro dust content will be too high, which will cause the gas emission of the mold (core) sand to increase and the air permeability to decrease. When this phenomenon occurs, the proportion of new sand should be appropriately increased, and the old sand regeneration equipment should be improved at the same time to make the regenerated sand meet the required indicators.
1.3. Strictly control the coating time and drying process of the paint
Poor hardening of sand molds and sand cores, improper coating or drying, poor coating quality or insufficient drying will cause excessive residual moisture in the sand mold (core) and increase the gas emission.
1.4. Improve the casting process design and reasonably set the exhaust channel in the sand mold (core)
Improve the casting process design, appropriately increase the pressure head, so that the sand core can be vented smoothly; the sand mold should reasonably set the exhaust hole, and the sand core outlet should be connected and led out with the sand mold air hole when closing the mold to prevent the outlet from being blocked by sand or the iron liquid from entering the core head outlet during pouring; control the pouring speed to avoid pouring too slowly and fill the pouring system with metal liquid.
1.5. Adjust the original sand particle size
Appropriately adjust the original sand particle size. On the premise of ensuring the surface quality of the casting, the original sand can be appropriately coarser to improve the permeability of the sand mold.
1.6. Appropriately control the pouring temperature
Control the optimal pouring temperature range to ensure that neither the air hole nor the sand sticking occurs
2. The generation and prevention measures of mechanical sand sticking
Mechanical sand sticking is the phenomenon that the metal liquid penetrates into the pores of the sand wall of the casting mold under pressure, resulting in the mixing and bonding of metal and sand particles. Sand sticking is the result of the interface reaction between the molten metal and the mold. After the molten metal is injected into the mold, a series of physical and chemical reactions will occur between the molten metal, the mold material and the gas in the mold at the interface. Sand sticking is the comprehensive result of these many reactions.
The main measures that can be taken to prevent sand sticking are as follows:
2.1. Reasonable selection of raw sand
Select the particle size distribution of multi-(four) screened sand to reduce the gap between sand particles and increase the penetration resistance of the molten metal.
2.2. Reasonable selection of coatings and brushing processes
The function of the coating is to isolate the mold (core) and the molten metal, so the coating layer must have a certain thickness and a certain penetration depth. In areas that are severely heated, double-layer coatings can be used. The bottom coating mainly ensures that the coating has a certain penetration depth, while the surface coating must ensure a certain thickness; improve the coating formula and improve the coating performance.
The coating should be selected according to the alloy type, casting size and casting process.
2.3. Use more recycled sand
The resin sand that has been recycled after repeated use has good thermal stability and better sand grain shape.
2.4. Improve the surface quality of the pattern and the compactness of the mold (core) sand
The surface quality of the mold (core) depends on the surface quality of the pattern and the compactness of the mold (core) sand. Therefore, the better the surface of the pattern, the higher the compactness of the mold (core) sand, and the easier it is to prevent sand sticking. When molding, try to improve the compactness of the mold (core), especially the compactness of the lower part of the pattern boss, the concave corner turn, and the lower part of the live block.
2.5. Control the pouring temperature
The pouring temperature should not be too high. The appropriate pouring temperature should be selected according to the shape, size, wall thickness, weight and other characteristics of the casting. In principle, the pouring temperature should be reduced as much as possible without producing defects such as pores, insufficient pouring, cold shut, etc.
3. The generation and prevention measures of mold sticking
The pattern is not easy to remove, or the sand sticks to the pattern during demolding, resulting in poor surface quality of the casting, or even waste. There are many reasons for mold sticking:
Poor surface quality of the pattern or core box;
Improper use of mold release agent;
Too early demolding time, etc.
Therefore, the corresponding measures that can be taken to prevent mold sticking are as follows.
3.1. Improve the surface quality of the mold and core box.
3.2. According to the different ambient temperatures, accurately control the optimal demoulding time and reasonably select the demoulding agent.
4. Reasonably control the curing time
In production, there will be two extreme phenomena: curing too fast, the sand mold and sand core become brittle, and the strength is reduced; curing too slow, the sand mold and sand core are poorly hardened, resulting in low production efficiency and poor quality of sand molds and sand cores.
4.1. Measures to prevent curing too fast
Curing too fast usually occurs in summer. The main measures to prevent curing too fast are:
Control the resin sand temperature, especially in summer, when the ambient temperature is high, measures should be taken to try to reduce the sand temperature to below 35℃.
Reduce the amount of curing agent added, or replace the curing agent with less acidity.
Strictly prevent the wrong practice of increasing the amount of curing agent to speed up the curing speed and improve production efficiency.
4.2. Measures to prevent slow curing
Slow curing is more common in winter, or the raw sand has too high water content, or the mud and micro powder content is too high. It also often occurs in some companies that have just started to use resin sand technology and when the output is low. The main measures to prevent slow curing are:
- Increase the sand temperature to above 10℃.
- In winter, the ambient temperature in the workshop can be appropriately increased, and the mold can be appropriately preheated at the beginning of each shift.
- Adjust the amount of curing agent added. In winter, the amount of curing agent can be appropriately increased and a curing agent with higher acidity can be selected.
- Select high-quality raw sand. It is recommended to use scrubbing sand.