Green sand casting sand performance analysis

Green sand casting is the most widely used casting method because of its high productivity and low production cost of castings, but the scrap rate remains high due to the low quality of sand in many foundry factories and the poor surface quality of castings. sanzhuji combined with hydrostatic molding line to produce engine block, cylinder head blanks, large size, high technical requirements, high complexity of shape, minimum wall thickness of 3.5mm, comprehensive The scrap rate is 3% to 5%. Among them, the scrap rate caused by wet sand accounts for about half. The reason is that the strength of wet sand is low, water content is high, the amount of gas, poor liquidity, the casting is easy to produce defects.
　　
1. Control of sand composition
1.11 Mud content, type sand and old sand mud is composed of effective bentonite, coal dust and ash. The so-called "ash" includes failed bentonite and coal dust, dust brought in by new sand, coal dust, bentonite and other raw materials, as well as the fine powder made of broken silica sand particles. The content of mud in the sand or old sand is called mud content.
1.12 The excessive ash of the sand will make the water content increase, permeability decreases, the casting is easy to produce porosity class defects. and in the case of wet compression strength remains unchanged, sand toughness, hot and wet tensile strength significantly reduced, the casting is easy to produce sand hole class and sand trapping defects. In the process of casting engine block and cylinder head, a large amount of core sand is mixed with old sand, which makes the sand mud score too little. The mud is too little, which will lead to low water content and water absorption capacity of the sand, and a small deviation in the amount of water will cause dramatic fluctuations in the properties of the sand, and the sensitivity of the sand to moisture is greatly enhanced.
1.13 The main influencing factors of low mud content are high temperature and low moisture of old sand; large amount of cold core sand mixed in; loss of ash caused by dust removal system, etc.
1.14 If the sand processing system is designed without old sand cooling equipment, and the calculation margin of the whole sand system capacity is small before technical transformation, the daily molding sand circulation is too many times, and the hot sand problem is more serious, and the old sand temperature is up to 70℃ or more in summer. and the sand temperature is too high on the sand performance, molding operations, casting quality and environmental health have adverse effects.
1.15 The use of resin as a sand core binder has improved the performance of the falling sand, so that the number of collapsed and broken core sand into the old sand is greatly increased. The particle size of wet sand is 70/140 mesh, while the particle size of resin sand core is used 50/100 mesh or 70/40 mesh, and too many loose particles of sand core are mixed into the old sand to make the particle size coarse, and the mud content of the sand is low, resulting in rough surface of the casting. In addition, part of the core sand is not sintered into the old sand, because the core sand surface with a layer of resin mold, will lower the strength of the sand, affecting the performance of the sand. So the casting factory in the choice of core sand, generally require the core sand particle size than the own sand particle size of one grade.
1.16 If the mud content of the sand is too low, the test data is often less than 9% (process requirements: 9.5% ~ 11.5%), is caused by the old sand mixed with a large number of collapsed resin sand core, so that the sand suitable water content is too low, the air permeability is too high, the performance is difficult to control. The ideal mud content of cast iron sand for high density molding should be 10%~13%, and the ideal mud content of old sand is 8~11%.

1.2 sand moisture, on the one hand, wet sand can not be too dry, otherwise the bentonite is not fully wetted, resulting in mold difficulties, sand fragile, the surface of the wear strength is low, the castings are easy to generate sand holes and erosion defects. On the other hand, the sand should not be too wet, too wet sand easy to make the casting pinhole, porosity, choking fire, water explosion, sand trapping and sand defects. In addition, it will also cause the sand viscosity, liquidity is reduced, and then the sand cavity surface compactness is uneven, complex local filling is not solid. Therefore, from the perspective of reducing the casting porosity defects, require the most suitable dry and wet state of the sand moisture content as low as possible.

1.3 sand effective composition, the effective composition of the sand mainly refers to the effective bentonite and effective coal dust, for a higher degree of automation of the static pressure molding line, the general requirements of effective bentonite control at 7% to 9%, effective coal dust control at 4% to 6%, too high or too low will affect the quality of the sand.

2. Performance of sand
2.11 sand strength, wet sand if the wet state strength is not enough, in the process of starting the mold, handling sand, under the core and together, the sand may break and collapse, when pouring may break the sand and cause sand hole defects or even "run fire"; due to the lack of sufficient strength of the sand, after pouring ferrofluid will cause displacement of the wall and lead to casting loosening and expansion sand defects. For tall castings, due to the high metal pressure head, pouring scouring force, high temperature effect for a long time, the sand strength requirements are higher. However, the strength should not be too high, because the high-strength sand needs to add more bentonite, not only affect the moisture and permeability of the sand, but also to mix the sand, compact and drop sand and other processes bring difficulties. For the castings prone to thermal cracking defects also require a lower strength.
2.12 wet compression strength, static pressure molding line, high production efficiency, casting size accuracy, more stringent requirements for the performance of the sand. For the static pressure caused by the wet pressure strength should not be too high, sand strength is too high, you must add a lot of bentonite, reduce the mobility of the sand, although KW static molding with airflow pre-firming function, but in the use of sand with airflow to the edge of the sand box flow, resulting in uneven sand, uneven distribution of cavity hardness. and the vibration sand machine crushing effect is low, and it is difficult to crush large pieces of sand with high wet compression strength. General hydrostatic molding sand wet compression strength should be controlled at 150 ~ 170kPa more appropriate.
2.12 hot wet tensile strength, with wet sand casting thicker and larger flat class castings, the most likely to produce sand trapping defects, so the sand should have a certain hot wet tensile strength. Adding coal dust, residual oil, asphalt, wood flour and dextrin and other additions to the sand can make the sand thermal compressive stress reduction, so as to play a role in anti-sand trapping.
2.13 Sand surface strength, wet sand type should be high enough surface abrasion resistance, to withstand the mold, blowing, core and pouring and other processes of abrasive action. If the sand surface abrasion resistance is not high, the surface sand particles by the action of external forces easily off, not only directly affect the surface roughness of the casting, but also cause sand holes, sticky sand and other casting defects. From modeling, mold to close the box, between pouring, sand open placed for a period of time, during which the casting surface moisture evaporates constantly, the so-called "air dry phenomenon", may lead to a sharp decline in surface wear resistance and surface strength. The "air-drying phenomenon" is especially serious when the interval before closing the box is long, the environmental humidity is low, and the sand temperature is high. According to the relevant information, adding a-starch to the sand can increase the surface strength of the sand.

2.2 Sand fluidity, with good fluidity of the sand can ensure that the sand hardness distribution is uniform, angles, tabs clear, the casting surface is smooth. For KW static pressure molding, sand with good fluidity is extremely critical. The fluidity of wet sand is not only affected by the compact rate, but also the particle size and shape of the original sand. In general, improve the amount of bentonite sand or add dextrin can improve the toughness of the sand, but will make the liquidity decline.

2.3 sand toughness, sand should have a certain toughness, otherwise in the mold, core, shape and transport when the corners of the sand and hanging sand parts by impact and vibration easy to touch broken or fall off. But the toughness of the sand should not be too high, so as to avoid its liquidity decline and affect the tightness and uniformity of the sand.

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