yvonne@nydcasting.com 
1. Common defects and preventive measures in the use of resin sand
1.1 Generation and preventive measures of pores
During the solidification process of metal, bubbles trapped in the molten metal form holes in the casting, which are called pores. By observing the shape of the holes, the characteristics of the hole wall, the surface morphology and the location of the holes, it can be distinguished from casting defects such as shrinkage, shrinkage, sand holes, and slag holes.
The main measures to prevent the generation of pores are as follows:
1.1.1 Choose the right type of resin 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.
1.1.2 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.
1.1.3 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.
1.1.4 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.
1.1.5 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.
1.1.6 Appropriately control the pouring temperature
Control the optimal pouring temperature range to ensure that there is no venting and no sand sticking
1.2 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:
1.2.1 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.
1.2.2 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.
1.2.3 Use more regenerated sand
The regenerated sand has good thermal stability after repeated use and the sand grain shape is also better.
1.2.4 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 turning point, and the lower part of the live block.
1.2.5 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.
1.3 The generation and prevention of mold sticking
The pattern is not easy to remove, or the sand sticks to the pattern when removing the mold, resulting in poor surface quality of the casting, or even waste. There are many reasons for mold sticking:
(1) Poor surface quality of mold or core box;
(2) Improper use of mold release agent;
(3) Too early demolding time, etc.
Therefore, the corresponding measures that can be taken to prevent mold sticking are as follows.
(1) Improve the surface quality of mold and core box.
(2) According to the different ambient temperatures, accurately control the optimal demolding time and reasonably select the mold release agent.
1.4 Reasonable control of curing time
In production, there are two extreme phenomena: curing too fast, sand mold and sand core become brittle and strength decreases; curing too slow, sand mold and sand core hardening is poor, resulting in low production efficiency and poor quality of sand mold and sand core.
1.4.1 Measures to prevent curing too fast
Curing too fast usually occurs in summer. The main measures to prevent curing too fast are:
Control the sand temperature, especially in summer, when the ambient temperature is high, measures should be taken to reduce the sand temperature to below 35℃ as much as possible.
Reduce the amount of curing agent added, or replace the curing agent with lower acidity.
Strictly prevent the wrong practice of increasing the amount of curing agent to speed up the curing speed and improve production efficiency.
1.4.2 Measures to prevent slow curing
Slow curing is more common in winter, or the water content of the raw sand is too high, or the mud content and micro powder content are too high. In addition, it often occurs in some enterprises that have just started to use the resin sand process and when the output is low. The main measures to prevent slow curing are:
① Increase the sand temperature to above 10℃.
② In winter, the ambient temperature in the workshop can be appropriately increased. At the beginning of each shift, the mold can be properly preheated.
③ 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.
④ Select high-quality raw sand. It is recommended to use scrubbing sand. The indicators should meet the requirements specified in 3.2.
2. Reasonable selection of resin and curing agent
The resin composition should be selected according to the type of casting, that is, the type of alloy, the size of the casting, the thickness and complexity of the wall. The type of curing agent should be reasonably matched with the resin in combination with the characteristics of the casting and the environmental conditions (mainly the ambient temperature).
2.1 Resins
Classified by nitrogen content of resins:
(1) W grade: N mass fraction is less than 0.3%, nitrogen-free resin, suitable for large steel castings;
(2) D grade: 0.3%-2.0% N, low nitrogen resin, suitable for alloy steel, small steel castings, ductile iron castings;
(3) Z grade: 2.0%-5.0% N, medium nitrogen resin, suitable for iron castings;
(4) G grade: high nitrogen resin, suitable for non-ferrous castings and general iron castings.
Classified by furfuryl alcohol mass:
High furfuryl alcohol resin mass fraction is above 85%, suitable for ductile iron castings and steel castings;
Medium furfuryl alcohol resin mass fraction is 70%-85%, suitable for iron castings;
Low furfuryl alcohol resin mass fraction is 50%-70%, suitable for non-ferrous castings.
2.2 Curing agent
Main considerations when selecting:
Total acidity is less than 20%, free sulfonic acid content is less than 4%, suitable for ambient temperature above 10℃;
Total acidity is greater than 20%, free sulfonic acid content is greater than 4%, suitable for ambient temperature below 10℃.
2.3 Matching resin and curing agent
In actual use, it is also necessary to pay attention to the matching of the type of resin with the type of curing agent. For example, inorganic acid curing agent is suitable for high nitrogen resin, and organic acid curing agent is suitable for medium and low nitrogen resin.
When selecting resin and curing agent, you can determine it according to your own conditions and after testing, and should also make timely adjustments according to changes in seasons, product categories and quality requirements.
3. Reduce the consumption of binder
The binder in furan resin self-hardening sand includes resin and curing agent. If the amount of binder added is too high, it is easy to cause more defects in castings, lower quality and higher casting costs. Among the ordering technical indicators formulated by the resin sand technology promotion group of the former Ministry of Machinery and Electronics, an important indicator is that the amount of resin added (mass fraction) should be controlled at about 1%. In actual production, due to the influence of various factors, the amount of resin added is often higher than 1%. The factors affecting the amount of binder added should be deeply analyzed, and targeted measures should be taken to minimize the amount of binder added. The measures that can be taken in production are as follows:
3.1 Reasonable selection of the strength of resin sand
The strength of resin sand increases with the increase of resin addition, but after the addition exceeds a certain limit, the strength is not further improved significantly. In addition, the strength of resin sand should not be too high. It should be reduced as much as possible while meeting the process requirements.
The strength value of resin sand should vary depending on the type of alloy, the size of the casting and the process conditions. At present, the recommended tensile strength of resin sand is 0.8-1.0MPa, the tensile strength of sand core is 1.0-1.2MPa, and the complex sand core is 1.6-2.0MPa. The amount of resin added (mass fraction) in most factories is controlled at 0.8%-1.2%, and the curing agent added (mass fraction) is generally 30%-40% of the resin.
3.2 Use high-quality raw sand
It is recommended to use bagged four-sieve scrubbed and dried sand. The quality of raw sand should meet the following requirements: concentration greater than 85%, micro powder content (mass fraction) not exceeding 0.5% (micro powder content refers to the part of silica sand with a particle size of 0.075mm to the bottom plate, that is, the part below the 0.106mm sieve); mud content (mass fraction) not exceeding 0.2%, acid consumption value not exceeding 5mL/50g, and moisture not exceeding 0.2%; the shape of raw sand particles is measured by bulk density, and the bulk density of raw sand used for resin sand should not be less than 1.55g·cm-3.
The choice of particle size is: 40-70 mesh sand is used for medium and large pieces, 50-100 mesh sand is used for thin and small pieces without paint, and non-standard 70-100 mesh sand can also be used, with a pH of 7. SiO2 content (mass fraction) should be greater than 97% for steel castings and greater than 90% for iron castings.
3.3 Use more regenerated sand
Compared with raw sand, the regenerated sand has the following characteristics:
(1) Good thermal stability. The main component of raw sand is SiO2. At about 870℃, quartz turns into quartzite, and the volume expands. After cooling and then heating, the volume change is small, that is, the volume stability of quartzite after phase change is good.
(2) Low acid consumption. The acid consumption of regenerated sand is lower than that of new sand, generally less than 3mL/50g. This is mainly due to repeated use. During the sand mixing process, the curing agent has neutralized part of the alkaline substances. On the other hand, repeated rubbing removes the alkaline oxides on the surface of the original sand.
(3) The grain shape of regenerated sand is more reasonable. After repeated rubbing of sand, the sharp corners become purer and the roundness is improved, that is, the grain shape is better.
Well-controlled regenerated sand outperforms raw sand; poorly controlled, it performs worse. Micro powder must be under 0.8%. Ignition loss: cast steel ≤2.0%, ductile iron ≤3.0%, general cast iron ≤3.5%.
3.4 Improve the quality of sand mixing
The equipment used in the resin sand mixing process and the quantitative accuracy affect the amount of binder added to a certain extent.
At present, the double-arm continuous sand mixer is widely used in production. The mixing process of resin sand is single sand secondary mixing, that is, first add the curing agent to premix, and then add the resin to mix. Experiments and practice have proved that the continuous sand mixer is better than the intermittent sand mixer. To achieve the same molding sand strength, the former adds 10%-20% less resin (mass fraction) than the latter. Regardless of the type of sand mixer used, the curing agent should be added first, fully premixed, and then the resin should be added to mix.
3.5 Control the sand temperature
The sand temperature affects the strength of the sand core, the degree and uniformity of hardening, and the curing time of the sand mold. The most ideal sand temperature for resin sand production is 20-30℃. In production, the sand temperature should be reasonably controlled according to the output size and equipment conditions, especially in winter and summer, and different measures should be taken.
4. Reduce the sand-iron ratio and reduce the amount of sand used.
Actual production shows that lowering the sand-iron ratio reduces resin and curing agent usage. It improves casting quality and lowers costs. A 1-point reduction cuts costs by 200-300 yuan.
The main ways to reduce the amount of sand used are:
(1) Use fillers. Fillers can be added to parts with a large amount of sand. Fillers include old sand blocks, wood blocks, bricks, stones, etc.;
(2) Use a molding sand box. Resin sand has high strength. Under normal circumstances, the sand consumption of the sand tire is 50mm. When there is a sand box for molding, it can be appropriately reduced. Using a molding sand box can make the sand consumption around the casting almost consistent, which can greatly reduce the amount of sand used;
(3) Use hollow sand cores. Self-hardening sand creates hollow cores like coated sand. It lowers the sand-iron ratio and improves permeability. For cylindrical cores, insert and remove a stick or pipe after solidification.