Green sand determines the casting quality of vertical molding lines(1)

In the green sand casting production process, the sand is the key, its performance control is good or bad, stable quality or not, will have a huge impact on the quality of castings. Take sanzhuji''s vertical fractional injection molding line for example, the iron is melted by medium frequency electric furnace, the castings are mainly thin-walled cast iron (gray cast iron and ductile iron) parts, the products include auto parts, pipe parts, valve parts, etc. sanzhuji combined with sand casting equipment for many years of production practice, through the improvement of GS high-efficiency rotor sand mixer, in the sand mixing quality control has done a lot of work, and achieved good results.

1 raw material quality requirements
1.1 Raw sand
Scrubbed sand with mud content: 1.0%, SiO2: 85%, 70/140 mesh. Due to the vertical fractal injection molding is a high-density molding, in order to reduce the thermal expansion of the sand, to avoid the casting defects caused by the extrusion of sand particles from the surface of the sand, the particle size requirements should not be too concentrated, the best sand 4 sieve concentration rate of 85% or more, the main peak sieve (100 mesh) volume control within 40%. The amount of new sand replenishment is below 5%.
1.2 Coal powder
Ash content of pulverized coal should be lt;10% ash content of pulverized coal is too high, which makes the mud content of sand increase and affects the use performance of sand. Sulfur content ≤ 1.5%; pulverized coal particle size ≥ 95% of the particles through a 140 mesh sieve, and pulverized coal does not allow the presence of large particles, because of its long burning time in the pouring process met metal liquid, preventing the iron liquid near the wall, to be solidified, it will cause the casting surface pits, affecting the surface roughness of the casting. The volatile fraction is one of the main indicators of the quality of coal powder, good coal powder volatile fraction content is high, when pouring, the cavity is easy to form reducing gas, precipitation of a large number of bright carbon, improve the appearance of the casting quality. But volatile fraction exceeds 40%, the sand gassing volume increases, the casting is easy to produce porosity, pouring and other defects. Therefore, the volatile fraction is generally 30%-38%.
1.3 bentonite
Selected sodium bentonite. Wet compressive strength ≥ 120kPa, blue absorption (g/100g) ≥ 38, particle size over 200 mesh ≥ 90%.

2 Type sand performance quality control
2.1 Wet compression strength
If the wet strength of the sand is not sufficient, the sand may break and collapse in the process of mold starting, sand transport, core removal and mold closing; the sand cannot withstand the scouring and impact of the metal liquid during casting, resulting in sand hole defects and even iron leakage. Generally, the wet compression strength is used to express the wet strength of sand, which is generally controlled at 150-200kPa.
2.2 Air permeability
The air permeability of the sand should not be too low to avoid choking and casting porosity defects during casting, but it must not be understood that the higher the air permeability of the sand, the better. Because the air permeability is too high indicates that the pores between the sand grains are large, the metal liquid is easy to penetrate and cause surface roughness, and may also produce mechanical sticky sand. The air permeability of wet sticky sand is determined according to the type and temperature of the casting metal, the size and wall thickness of the casting, the molding method, and the size of the gassing of the sand, etc. It is generally controlled at 90-160.
2.3 compactness and water content
Firmness and moisture content are important parameters to measure the comprehensive performance of the sand. Wet sand moisture must be moderate, otherwise the bentonite is not fully wetted to affect sand mixing, which leads to molding difficulties and sand mold brittle and fragile, low surface abrasion resistance, castings are prone to produce sand eye and erosion defects. General tightness control at 32%-40%, the best 34%-38%. The moisture content of sand for high density molding is 2.8%-3.8%. The compactness/water content ratio of sand is generally 10-12.
2.4 Mud content
The ideal mud content of cast iron sand for high-density molding (including coal dust) is 10%-13%, should not be ≥ 14%; the ideal mud content of old sand is 8%-11%, should not be ≥ 12%.
2.5 Effective clay
Effective bentonite of modeling sand is all still have the bonding ability of bentonite. General high-density molding of the effective bentonite control at 6%-8%.
2.6 Effective amount of pulverized coal (combustion loss)
Coal dust decomposition of the gas phase at high temperatures, deposited on the surface of the sand to form "bright carbon", to prevent the casting surface sticky sand, improve the surface quality of the casting. If the effective pulverized coal content in the sand is too high, while the sand permeability and poor, due to the increase in the amount of gas in the cavity, the casting is prone to porosity, cold compartment, under-pouring and other defects. For this reason, most foundries calculate the effective pulverized coal content by measuring the amount of gas generation in the sand and additional materials, generally controlled at 3.0%-4.5%.
According to sanzhuji''s experience and with reference to other foundries'' practice, we changed the method of determining the effective pulverized coal content to determining the volatile fraction of the sand. The volatile fraction is measured by placing the sample sand into a crucible and placing it in a muffle furnace, calcining it at 649°C for 1h and then weighing it. On-site control of the effective coal dust content through the change in the amount of volatile fraction of sand is more practical, the measurement of gassing is not very accurate, because the organic matter in the sand (sand core) is also gassing, it is not easy to distinguish. General sand volatile content control at 2.0% - 3.0% (mass fraction). 
2.7 Particle size
Sand particle size directly affects the air permeability and casting surface roughness, but the original sand particle size does not represent the actual sand particle size in the sand, because in the casting process part of the sand may be broken into fine powder, another part may be sintered into coarse particles, and different coarse and fine sand core collapse will also be mixed into the old sand, so the determination of the mud content of the sand with sieving method to determine the particle size, general sand particle size AFS value control at 55 -65.
2.8 Quality control of reused sand (old sand)
2.81 Cooling of reused sand: Since the vertical parting molding line is a continuous operation of molding and pouring, the reused sand is repeatedly recycled, and its temperature will directly affect the temperature of the sand, thus affecting the overall performance of the sand: for one thing, the hot sand has the tendency to stick to the colder bin wall and form a hopper shrinkage, so that only a small part of the system sand is used frequently and the turnover rate of the sand increases, so that only a small part of the system sand is used frequently and the turnover rate of the sand increases. Second, the hot sand and mold contact, due to moisture condensation and bonding together, resulting in rough cavity surface and sand loose when demolded; Third, the higher the sand temperature, the greater the sand water vapor pressure, the faster the evaporation rate of water on the sand surface, the more serious the air-drying phenomenon, the lower the strength of the sand after air-drying, the air-drying of the cavity when the iron is flushed during pouring The dried sand has a lower strength, and the molten iron scours the air-dried sand in the cavity during casting, resulting in trachoma defects. Therefore, how to reduce the temperature of the reuse sand will become one of the key factors of the molding line can produce high-quality castings. The purpose of the sand temperature control is that the final sand temperature delivered to the sand blender should be less than 45°C. The best way to do this is to have the sand blender after the sand drop. The best method is to install cooling devices (cooling drum, boiling cooling bed, double-disk cooler, etc.) after the sand falling machine. The cooling effect of boiling cooling bed is better, because the reused sand is tumbling in the boiling cooling bed and spraying water mist at the same time, thus increasing the contact area and time of water and sand, and achieving the purpose of reducing the sand temperature. and the traditional practice of spraying water to the sand on the conveyor belt does not effectively cool the reuse sand and is not obtainable. The reason for this is that in the traditional method.Water does not have enough time to fully contact with all sand grains, and the mixture of locally hot sand and water has the potential to saturate the surrounding air with humidity, in which case it is not possible to change water from liquid to gas and remove the water vapor formed effectively.
2.82 reuse sand (old sand) moisture control: In order to better use the bentonite in the reuse sand, the reuse sand must maintain sufficient moisture before entering the sand mixer, so that the bentonite absorbs more water, greater play in the reuse sand bentonite bonding effect and reduce the amount of bentonite added. Therefore, water should be added as early as possible in the sand processing system, and the moisture content of the reused sand should be kept at 70%-80% of the moisture content of the blended sand before mixing, so that the bentonite has sufficient expansion time, provided that it does not block any sand processing equipment. In this way, not only can shorten the sand mixing time, but also can reduce the fluctuation range of the wet compressive strength of the sand after mixing, and the sand mixing effect is good. Therefore, it is required that the reused sand enters the old sand hopper with sufficient retention time, and the effective clay in the old sand will be partially rehydrated.
2.83 Influence of core sand in reused sand: The addition of sand core has influence on the performance of wet type sand. After casting only part of the sand core of the sand surface value is burned, and not burned off still linger in the sand, because the core hit part of the sand core (resin as binder), the core sand surface resin is not hydrophilic, the replenishment of the wet bentonite is difficult to bond with it, resulting in brittle sand. Brittle sand can not guarantee the quality of the sand at the corners, and with the scouring of the iron liquid is very easy to produce sand eyes. In addition, brittle sand will also bring adverse effects to the starting mold. In order to prevent the impact of the core sand addition on the wet sand, the amount of new sand added should be kept at 2% - 5% of the total amount of mixed sand. and when arranging the production of castings, attention should be paid to the reasonable combination of core and coreless products to reduce the amount of core sand in the sand as much as possible.

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.