Reasons for preventing sand from molten sand casting castings
Slag inclusion defects refer to the defects formed by dry sand, paint and other inclusions as the molten iron enters the casting during the pouring process. On the surface of the machined castings, you can see spots of white or black-gray inclusions, distributed individually or in pieces. The white is quartz sand particles, and the black-gray is slag, paint, foam model residues and other inclusions after pyrolysis. . This kind of defect is commonly called "sand ingress" or "slag inclusion", and it is a very common defect in the production of lost foam casting. Almost all factories using lost foam casting are widespread, and it is difficult to completely eradicate them. Only by taking a variety of measures and careful operation in each process can the "slag inclusion" be reduced to a very low level and satisfactory results can be achieved.
After the lost foam castings are cooled and boxed before being cleaned, according to the surface conditions of the castings and the pouring system, it can be judged whether there are sand and slag inclusion defects. If the sprue cup, sprue, cross runner, sprue and gate surface or connection, and the surface of the casting are severely sticky or have cracked sticky sand, it is basically certain that the casting has slag inclusion and sand ingress defects. Smash the sprue rod or sprue bead, you can see white spots on the fracture, and in severe cases, the fracture will form a circle of white spots. Such castings, especially plate-shaped and round cake-shaped castings, will have white and black-gray spot defects on the processing surface after machining. If the operating procedures are not strictly controlled, the castings produced will seriously affect the quality of the castings and the progress of the order completion.
1. Causes of slag inclusion and sand ingress defects
Observations in the production practice of sanzhuji combined with the foundry proved that from the sprue cup, sprue, cross runner, inner runner to the casting, all parts may cause sand ingress, especially the joint part of the gating system and the casting. During the entire production process, the paint peeling and cracking on the surface of the white mold of the gating system, the paint peeling and cracking at the bonding part of the white mold, the paint peeling and cracking on the surface of the foam white mold, the sprue is not tightly closed and other factors are caused by slag inclusion and sand ingress. The most important cause of defects. Secondly, the selection of process parameters, such as the size of the net head of the pouring system, the pouring temperature, the degree of negative pressure, the particle size of dry sand, etc., as well as the transportation process of the model and the packing operation, all have an impact on the slag inclusion and sand ingress defects of the castings. Great influence. Only by adopting systematic measures and careful operation in these links can the slag inclusion defects of castings be reduced and basically eliminated, and high-quality castings can be obtained. Overcoming the defects of slag inclusion is a systematic project.
2. Methods and measures to reduce and overcome slag inclusion defects
Sand ingress and slag inclusion defects are a major problem in lost foam casting production. At present, there are mainly three types of products that have been successfully produced by lost foam casting, namely, wear-resistant parts, pipe fittings and box castings, all of which are rarely processed or not processed. For castings with many processing surfaces and high requirements, the slag inclusion defect is a key problem that needs to be solved. Combining the characteristics of lost foam sand casting, sanzhuji recommends the following measures to reduce and eliminate slag inclusion defects:
2.1 Coating
The role of the lost foam coating is:
2.11 Improve the surface finish of castings, reduce the roughness of castings by 2-3 grades, and improve the surface quality and performance of castings.
2.12 Reduce and prevent sand sticking and sand hole defects.
2.13 is conducive to sand removal and falling sand.
2.14 The liquid and gas melted in the lost foam during pouring are smoothly discharged into the casting sand through the coating layer, and the molten metal is prevented from penetrating into the sand mold to prevent the casting from producing pores, metal infiltration and carbon defects.
2.15 Improve the strength and rigidity of the pattern to prevent deformation and damage during transportation and sand filling vibration modeling, which is beneficial to improve the dimensional accuracy and yield of castings.
Coatings for lost foam casting require a series of strength, air permeability, refractoriness, thermal insulation, explosion resistance and crack resistance, rapid cooling and rapid heat resistance, moisture absorption, cleaning, coating, suspension, non-flowing, etc. Performance, to prevent slag inclusion defects, first requires the coating to have high strength and fire resistance. It is required that the coating layer applied on the surface of the white mold does not crack or crack during the drying and transportation process, that is, the coating should have sufficient room temperature strength; and during the pouring process, the coating layer will also If it does not fall off or cracks, it has high high temperature strength.
When the liquid metal enters the mold, the straight gate is tightly closed, and the coating layer on the surface of the casting and the pouring system does not fall off, cracks and cracks are the first conditions to prevent slag inclusion defects. If the runner is not tightly sealed, the coating layer will fall off. Cracks and cracks, a large amount of sand, paint and inclusions will enter the metal to form slag inclusion defects. Strength and air permeability are two important properties of coatings. Sometimes the coatings used in the pouring system are required to have higher fire resistance than the casting coatings to resist the long-term erosion of high-temperature metals without falling off and cracking. The operator must ensure the uniformity of the paint during the painting process.
2.2 Packing operation
During packing, the coating layer on the surface of the pattern group (model + gating system) is not allowed to have any shedding, cracks or cracks, especially at the junction of the sprue and the runner, the junction of the runner and the inner runner, and the inner As long as there is looseness, cracks, or weak connection between the gate and the mold, sand may enter. This requires the joint strength to be high, the paint to be relatively thick, the gating system to have sufficient rigidity, and when necessary, a brace or reinforcement sleeve should be set. The pattern group should be stable when placed on the bottom sand of the sand box. It is not allowed to start sanding and vibration modeling when it is placed in the air to avoid shock cracking of the coating layer. Don''''''''''''''''t add sand directly to the appearance, use a hose to add sand first, and then use rain shower equipment to sprinkle sand before shaking. When starting to vibrate the shape, the vibration should be slight and the amplitude should be small, and then vibrate sharply after the dry sand is buried in the shape. During vibration molding, the gating system, especially the sprue, is not allowed to break or bend to avoid the paint layer from cracking. The sprue must be tightly sealed to prevent sand from entering. The entire packing, sanding, vibration, and modeling operation process must be very careful, and it must be ensured that the coating layer of the pattern group does not fall off, cracks or cracks before pouring. Before pouring, the sprue cup should be cleaned again to ensure that there is no floating sand, dust and debris.
2.3 Pouring head, temperature and time
The higher the pressure head during pouring, the greater the erosion of the pouring system and the mold, and the greater the possibility of sand ingress caused by the scouring of the coating. The pressure head is different for castings of different sizes. To choose a ladle with suitable capacity, the ladle should be as low as possible the pouring height, and the ladle mouth should be as close as possible to the pouring cup, avoid using a large ladle to pour small jobs. The higher the pouring temperature, the higher the performance requirements of the coating, the easier it is to produce defects such as sticky sand and slag, and the appropriate pouring temperature should be selected. For gray iron castings, the tapping temperature can be around 1480°C, and the pouring temperature should be 1380-1420°C; the tapping temperature of ductile iron castings should be above 1500°C, and the pouring temperature should be 1420-1450°C; the pouring temperature of steel castings should be 1480-1560°C . The pouring time of a box of iron castings requiring 300-500 kg of molten iron can be controlled within 10-20 seconds.
2.4 Negative pressure
The pouring process of lost foam casting is generally carried out under vacuum conditions. The role of negative pressure is to compact dry sand, accelerate exhaust, and improve filling capacity. Pouring under vacuum sealing conditions improves the working environment. The degree of negative pressure has a great influence on the quality of castings. Excessive negative pressure increases the possibility of inhaling dry sand and inclusions when the molten metal flows through the cracks and cracks, and also increases the sticky sand defects of the castings. Too fast filling speed increases the scouring ability of the metal to the runner and the mold, and it is easy to cause the paint to fall off and enter the metal, and it is also easy to wash out the paint layer and cause sand to enter. For iron castings, the appropriate negative pressure is generally 0.025-0.04MPa.
2.5 Set up slag stop, slag skimming and slag collecting riser
Setting slag retaining and slag skimming in the gating system and setting slag collecting risers on the castings and taking slag retaining and slag skimming measures can help to improve the defects of sand and slag inclusion.
2.6 Molding sand
Excessively coarse or fine molding sand will affect the occurrence of slag inclusion and sand sticking defects, and too coarse particle size will increase sticky sand and slag inclusion defects. Iron castings generally use dry quartz sand (washed sand) with a grain size of 30/50.
2.7 Using hot metal purification technology
The entire molding process of lost foam castings must consider the problem of molten iron purification, which is one of the key technologies of lost foam casting. The whole process including molten iron smelting, overheating, and pouring into the mold must consider the purification problem, and filtration technology is one of them.
Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. specializes in the production of Green Sand Molding Line,Self-hardening sand molding line,foundry machines,Sand molding machine,Sand casting equipment,sand mixer,GS high efficiency rotary mixer,Resin sand mixer,no-bake resin sand mixer,Jolt squeeze molding machine/Jolt-squeezing moulding machines,Multi- Piston Moulding Machine/Hydraulic multi-piston moulding machine,sand casting molding machine,sand reclamation equipment，foundry molding machine,flaskless moulding machine,shot blasting machine,dust collector,according to the amount of old sand recovered Carry out plan customization, and provide sand reclamation equipment installation, commissioning, and training. Welcome guests to visit the factory.
After the lost foam castings are cooled and boxed before being cleaned, according to the surface conditions of the castings and the pouring system, it can be judged whether there are sand and slag inclusion defects. If the sprue cup, sprue, cross runner, sprue and gate surface or connection, and the surface of the casting are severely sticky or have cracked sticky sand, it is basically certain that the casting has slag inclusion and sand ingress defects. Smash the sprue rod or sprue bead, you can see white spots on the fracture, and in severe cases, the fracture will form a circle of white spots. Such castings, especially plate-shaped and round cake-shaped castings, will have white and black-gray spot defects on the processing surface after machining. If the operating procedures are not strictly controlled, the castings produced will seriously affect the quality of the castings and the progress of the order completion.
1. Causes of slag inclusion and sand ingress defects
Observations in the production practice of sanzhuji combined with the foundry proved that from the sprue cup, sprue, cross runner, inner runner to the casting, all parts may cause sand ingress, especially the joint part of the gating system and the casting. During the entire production process, the paint peeling and cracking on the surface of the white mold of the gating system, the paint peeling and cracking at the bonding part of the white mold, the paint peeling and cracking on the surface of the foam white mold, the sprue is not tightly closed and other factors are caused by slag inclusion and sand ingress. The most important cause of defects. Secondly, the selection of process parameters, such as the size of the net head of the pouring system, the pouring temperature, the degree of negative pressure, the particle size of dry sand, etc., as well as the transportation process of the model and the packing operation, all have an impact on the slag inclusion and sand ingress defects of the castings. Great influence. Only by adopting systematic measures and careful operation in these links can the slag inclusion defects of castings be reduced and basically eliminated, and high-quality castings can be obtained. Overcoming the defects of slag inclusion is a systematic project.
2. Methods and measures to reduce and overcome slag inclusion defects
Sand ingress and slag inclusion defects are a major problem in lost foam casting production. At present, there are mainly three types of products that have been successfully produced by lost foam casting, namely, wear-resistant parts, pipe fittings and box castings, all of which are rarely processed or not processed. For castings with many processing surfaces and high requirements, the slag inclusion defect is a key problem that needs to be solved. Combining the characteristics of lost foam sand casting, sanzhuji recommends the following measures to reduce and eliminate slag inclusion defects:
2.1 Coating
The role of the lost foam coating is:
2.11 Improve the surface finish of castings, reduce the roughness of castings by 2-3 grades, and improve the surface quality and performance of castings.
2.12 Reduce and prevent sand sticking and sand hole defects.
2.13 is conducive to sand removal and falling sand.
2.14 The liquid and gas melted in the lost foam during pouring are smoothly discharged into the casting sand through the coating layer, and the molten metal is prevented from penetrating into the sand mold to prevent the casting from producing pores, metal infiltration and carbon defects.
2.15 Improve the strength and rigidity of the pattern to prevent deformation and damage during transportation and sand filling vibration modeling, which is beneficial to improve the dimensional accuracy and yield of castings.
Coatings for lost foam casting require a series of strength, air permeability, refractoriness, thermal insulation, explosion resistance and crack resistance, rapid cooling and rapid heat resistance, moisture absorption, cleaning, coating, suspension, non-flowing, etc. Performance, to prevent slag inclusion defects, first requires the coating to have high strength and fire resistance. It is required that the coating layer applied on the surface of the white mold does not crack or crack during the drying and transportation process, that is, the coating should have sufficient room temperature strength; and during the pouring process, the coating layer will also If it does not fall off or cracks, it has high high temperature strength.
When the liquid metal enters the mold, the straight gate is tightly closed, and the coating layer on the surface of the casting and the pouring system does not fall off, cracks and cracks are the first conditions to prevent slag inclusion defects. If the runner is not tightly sealed, the coating layer will fall off. Cracks and cracks, a large amount of sand, paint and inclusions will enter the metal to form slag inclusion defects. Strength and air permeability are two important properties of coatings. Sometimes the coatings used in the pouring system are required to have higher fire resistance than the casting coatings to resist the long-term erosion of high-temperature metals without falling off and cracking. The operator must ensure the uniformity of the paint during the painting process.
2.2 Packing operation
During packing, the coating layer on the surface of the pattern group (model + gating system) is not allowed to have any shedding, cracks or cracks, especially at the junction of the sprue and the runner, the junction of the runner and the inner runner, and the inner As long as there is looseness, cracks, or weak connection between the gate and the mold, sand may enter. This requires the joint strength to be high, the paint to be relatively thick, the gating system to have sufficient rigidity, and when necessary, a brace or reinforcement sleeve should be set. The pattern group should be stable when placed on the bottom sand of the sand box. It is not allowed to start sanding and vibration modeling when it is placed in the air to avoid shock cracking of the coating layer. Don''''''''''''''''t add sand directly to the appearance, use a hose to add sand first, and then use rain shower equipment to sprinkle sand before shaking. When starting to vibrate the shape, the vibration should be slight and the amplitude should be small, and then vibrate sharply after the dry sand is buried in the shape. During vibration molding, the gating system, especially the sprue, is not allowed to break or bend to avoid the paint layer from cracking. The sprue must be tightly sealed to prevent sand from entering. The entire packing, sanding, vibration, and modeling operation process must be very careful, and it must be ensured that the coating layer of the pattern group does not fall off, cracks or cracks before pouring. Before pouring, the sprue cup should be cleaned again to ensure that there is no floating sand, dust and debris.
2.3 Pouring head, temperature and time
The higher the pressure head during pouring, the greater the erosion of the pouring system and the mold, and the greater the possibility of sand ingress caused by the scouring of the coating. The pressure head is different for castings of different sizes. To choose a ladle with suitable capacity, the ladle should be as low as possible the pouring height, and the ladle mouth should be as close as possible to the pouring cup, avoid using a large ladle to pour small jobs. The higher the pouring temperature, the higher the performance requirements of the coating, the easier it is to produce defects such as sticky sand and slag, and the appropriate pouring temperature should be selected. For gray iron castings, the tapping temperature can be around 1480°C, and the pouring temperature should be 1380-1420°C; the tapping temperature of ductile iron castings should be above 1500°C, and the pouring temperature should be 1420-1450°C; the pouring temperature of steel castings should be 1480-1560°C . The pouring time of a box of iron castings requiring 300-500 kg of molten iron can be controlled within 10-20 seconds.
2.4 Negative pressure
The pouring process of lost foam casting is generally carried out under vacuum conditions. The role of negative pressure is to compact dry sand, accelerate exhaust, and improve filling capacity. Pouring under vacuum sealing conditions improves the working environment. The degree of negative pressure has a great influence on the quality of castings. Excessive negative pressure increases the possibility of inhaling dry sand and inclusions when the molten metal flows through the cracks and cracks, and also increases the sticky sand defects of the castings. Too fast filling speed increases the scouring ability of the metal to the runner and the mold, and it is easy to cause the paint to fall off and enter the metal, and it is also easy to wash out the paint layer and cause sand to enter. For iron castings, the appropriate negative pressure is generally 0.025-0.04MPa.
2.5 Set up slag stop, slag skimming and slag collecting riser
Setting slag retaining and slag skimming in the gating system and setting slag collecting risers on the castings and taking slag retaining and slag skimming measures can help to improve the defects of sand and slag inclusion.
2.6 Molding sand
Excessively coarse or fine molding sand will affect the occurrence of slag inclusion and sand sticking defects, and too coarse particle size will increase sticky sand and slag inclusion defects. Iron castings generally use dry quartz sand (washed sand) with a grain size of 30/50.
2.7 Using hot metal purification technology
The entire molding process of lost foam castings must consider the problem of molten iron purification, which is one of the key technologies of lost foam casting. The whole process including molten iron smelting, overheating, and pouring into the mold must consider the purification problem, and filtration technology is one of them.
Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. specializes in the production of Green Sand Molding Line,Self-hardening sand molding line,foundry machines,Sand molding machine,Sand casting equipment,sand mixer,GS high efficiency rotary mixer,Resin sand mixer,no-bake resin sand mixer,Jolt squeeze molding machine/Jolt-squeezing moulding machines,Multi- Piston Moulding Machine/Hydraulic multi-piston moulding machine,sand casting molding machine,sand reclamation equipment，foundry molding machine,flaskless moulding machine,shot blasting machine,dust collector,according to the amount of old sand recovered Carry out plan customization, and provide sand reclamation equipment installation, commissioning, and training. Welcome guests to visit the factory.
