Molding is the key process point of casting, molding sand quality directly affects the quality of casting, Sanzhuji combined with more than ten years of experience in producing sand treatment equipment and casting customer feedback, summarized and concluded mainly from the following four points to determine the quality of molding sand, the specific content is as follows:

1. Quality requirements of raw materials

1.1 the original sand

Scrubbing sand, silt content 1.0%, SiO2 85%, grit degree 70/140 mesh. Due to the vertical parting injection molding is a high-density modeling, in order to reduce sand mold thermal expansion, to avoid the sand grains fall off from the sand mold surface due to extrusion caused by casting defects, particle size requirements should not be too concentrated, the original sand is best to use 4 sieve sand,4 sieve concentration rate of more than 85%, the main peak sieve (100 mesh) amount controlled within 40%. The amount of new sand added is below 5%.

1.2 pulverized coal

Pulverized coal ash should 10%, the ash content of coal powder is too high, making the sand silt content increase, affecting the sand performance. Sulfur content 1.5% or less; The coal particles with particle size ≥95% pass through the 140-mesh screen, and large particles are not allowed to exist in the coal particles, because the combustion time of molten metal during pouring is long, preventing molten iron from approaching the mold wall. After solidification of molten iron, the surface pits of the casting will be caused, affecting the surface roughness of the casting. The volatilization content is one of the main indexes to measure the quality of coal powder. The volatilization content of fine coal powder is high. During pouring, reducing gas is easy to be formed in the mold cavity, which precipitates a large amount of bright carbon and improves the appearance quality of the casting. However, when the volatiles exceed 40%, the air volume of molding sand increases, and the casting is prone to produce defects such as porosity and insufficient casting. Therefore, the volatile component is generally in 30% ~ 38%.

1.3 bentonite

On the surface of liquid metal, wrinkle skin and other defects are formed. The second option is to choose only the dark riser. Because most of sanzhuji clients is the production of large castings, the resin sand mold process modelling, cannot use negative pressure make EPS shape of pyrolysis products quickly discharge cavity, but as a result of iron casting pouring temperature is high, the EPS pyrolysis fully, with molecular gaseous pyrolysis product is given priority to, make the air pressure inside the cavity with the increase of dozens of times, hundreds of times, so that the product cracking under the intense pressure within the cavity through coating layer have been squeezed into the sand. The use of dark riser can ensure the pressure in the cavity can improve the filling efficiency and slag discharge effect. It has been proved that the surface wrinkling and porosity defects of casting with only dark riser are much less. The casting temperature is 1380℃, because the casting is very long and the shape is complex, and the lost shape is continuously absorbed by the pyrolysis and gasification of iron temperature, so that the temperature of the leading edge of the metal liquid is constantly reduced. Therefore, in order to ensure the filling effect and prevent the occurrence of cold separation, the casting must be done at high temperature. In the process of casting, the way of first slow and then fast and then slow should be adopted to avoid the sudden rise of gas pressure in the mold cavity at the beginning of casting, and when the cracking product has no time to discharge, there will be reverse spraying, thus causing injury. In the pouring process, the pouring cup is always in a state of full, and can not be cut off flow, to ensure that the straight runner is in a closed state, can not be filled with air. Because EPS in the mold cavity is cracked under high temperature and pressure, rather than burned under high temperature and oxygen, otherwise it will appear wrinkling defects.

Since the products after the cracking of the lost mold are mainly small molecule gases, the pressure in the mold cavity is very high, so the lifting force of the box is very large, and the box is easy to expand, which not only makes the casting easy to be deformed, but also makes it easy to appear the capping seam. Since the tooling (sand box) we use is made of steel, it is necessary to press the box even if the card is punched in the sand box. According to the actual production experience, the weight of the pressure box is about 4 times the casting weight.

The bed made by the above process is up to 8m in length and 19T in weight, which not only has good appearance quality, but also has high dimensional precision and small processing amount, which improves the quality and reduces the cost.

Sodium bentonite was selected. Wet compressive strength ≥120kPa, blue absorbance (g/l00g)≥38, particle size ≥90% over 200 mesh.

2. Molding sand performance quality control

2.1 Wet compressive strength

If the green sand strength is not enough, the sand mold may be damaged and collapse in the process of molding, handling sand mold, core, and molding. Sand mold can not withstand the flushing and impact of metal liquid during casting, resulting in sand hole defects, or even iron liquid leakage. Generally, the wet compressive strength is used to express the wet strength of molding sand, which is generally controlled at 150 ~ 200kPa.

2.2 permeability

The air permeability of molding sand should not be too low in order to avoid choking fire and casting blowhole defects during pouring, but it can never be understood that the higher the air permeability of molding sand is, the better. The high permeability indicates that the pores between sand grains are large, and the metal liquid is easy to permeate, resulting in rough surface and the possibility of mechanical sand adhesion. The permeability of the green sand is generally controlled between 90 and 160 according to the type and temperature of the pouring metal, the size and wall thickness of the casting, the molding method, and the amount of air generated by the green sand.

2.3 Compactness and water content

Compactness and water content are important parameters to evaluate the comprehensive properties of molding sand. The moisture of the wet molding sand must be moderate, otherwise the bentonite is not fully wetted to affect the sand mixing, which will lead to the difficulty of molding and the sand mold is brittle and fragile, the surface wear resistance is low, and the casting is easy to produce sand holes and erosion defects. If the molding sand is too wet, the casting is liable to have porosity, sand inclusion, sand adhesion and other defects.
Generally, the compacting rate is controlled at 32% ~ 40%, and the best is 34% ~ 38%. The water content of high density molding sand is 2.8% ~ 3.8%. The ratio of compactness to water content of molding sand is usually 10 ~ 12.

2.4 hydraulic

The silt content (pulverized coal) of cast iron molding sand with high density modeling is 10% ~ 13%, not ≥14%; The ideal silt content of old sand is 8% ~ 11%, not ≥12%.

2.5 Effective clay

The effective bentonite of molding sand refers to all bentonite which still has the adhesive ability. The effective bentonite of general high-density modeling is controlled at 6% ~ 8%.

2.6 Effective pulverized coal amount (fire loss)

The "bright carbon" is formed on the surface of sand by gas phase decomposition under high temperature to prevent the surface of casting from sticking to sand and improve the surface quality of casting. If the effective pulverized coal content in the molding sand is too high, and the permeability of the molding sand is poor, due to the increase in the amount of gas in the molding cavity, the casting is prone to produce such defects as porosity, cold isolation, and insufficient pouring. For this reason, most foundry enterprises calculate the effective pulverized coal content by measuring the air output of molding sand and additives, which is generally controlled within 3.0% ~ 4.5%.

On the basis of Sanzhuji''''''''s experience and with reference to other foundry enterprises, we changed the determination of effective pulverized coal content to the determination of sand volatilization. The determination of volatile matter is obtained by placing the sample sand into a crucible with a cap and placing it in a muffle furnace, calcining it at 649℃ for 1h and then weighing it. It is more practical to control the effective pulverized coal content through the variation of the volatile component of molding sand on site, and the measurement of gas generation is not very accurate, because organic matter (sand core) in molding sand is also gas generation, which is not easy to distinguish. The volatile content of general molding sand is controlled in 2.0% ~ 3.0%(mass fraction)

2.7 the particle

Sand grain size directly affect the permeability and casting surface roughness, but the original grain size does not represent the actual sand grain size of the sand, because in the process of casting parts sand may break into fine powder, the other part may burn into coarse grain, and of different thicknesses sand core, and be broken after will be mixed with sand, so it should be determined by sieving method determination of silt sand granularity, general sand granularity AFS value control in 55 ~ 65.

2.8 Quality control of recycled sand (used sand)
Recycling sand cooling:
 
(a) for vertical parting line modelling modelling, pouring continuous operation, recycling sand recycling use, its temperature will directly affect the temperature of the molding sand, sand which affects the overall performance: first, the hot sand has a tendency to paste in the cold silo wall and form a hopper shrink tube, the system only a few are frequently used in the sand, sand the turnover increase quickly, further raise the sand temperature; Secondly, hot molding sand and mold contact, due to water condensation and bond together, resulting in mold cavity surface rough and loose sand; Thirdly, the higher the sand temperature is, the greater the sand type steam pressure is, the faster the water evaporation on the sand type surface is, the more serious the air drying phenomenon is, and the sand strength is lower after air drying. During casting, iron liquid scour the air drying sand in the mold cavity, leading to sand hole defects in the casting. Therefore, how to reduce the temperature of reclaimed sand will be one of the key factors for molding line to produce high quality castings. The goal of temperature control of recycled sand is that the temperature of the old sand delivered to the blender should be lower than 45℃. The best method is to install a cooling device (cooling drum, boiling cooling bed, double disc cooler, etc.) after the sand falling machine. Among them, the boiling cooling bed has a better cooling effect, because the recycled sand billows in the boiling cooling bed at the same time with the spray spray, thus increasing the contact area and time between water and sand, and achieving the purpose of reducing the sand temperature. However, the traditional method of spraying water on the sand on the conveyor belt cannot effectively cool the reused sand, so it is not available. The reason is that the water in the traditional method does not have enough time to come into full contact with all the sand particles, and the local hot sand and water mixture may saturate the humidity of the surrounding air, in which case it is impossible to convert the water from liquid to gas and effectively remove the formed steam.

(b) Water control of recycled sand (old sand) : In order to make better use of the bentonite in recycled sand, the recycled sand must be kept sufficiently hydrated before entering the blender, so that the bentonite can absorb more water, exert the bonding effect of the bentonite in recycled sand and reduce the amount of bentonite added. Therefore, water should be added in the sand treatment system as early as possible, and the water content of recycled sand should be kept at 70% ~ 80% of the water content of mixed sand before sand mixing, so that the bentonite has sufficient swelling time, provided that no sand treatment equipment is blocked. In this way, not only the mixing time can be shortened, but also the fluctuation range of the wet compressive strength of molding sand after mixing can be reduced, and the mixing effect is good. Therefore, it is required to keep the water of recycled sand at 1.25% ~ 2.0% before it enters the old sand hopper. If the recycled sand has enough time in the hopper, the available clay in the old sand will partially rehydrate.

(c) Influence of recycled sand core: The addition of sand core has an impact on the performance of green sand. After casting mold casting, only part of the sand core surface resin is burned off, and the unburned is still stuck in the molding sand, because the sand core is mostly coated sand core (resin as binder), the resin on the surface of the core sand is not hydrophilic, the added moist bentonite is difficult to bond with it, resulting in the production of brittle sand. Brittle sand can not guarantee the quality of sand edges, and it is easy to produce sand holes with the scouring of molten iron. In addition, brittle sand can also bring bad effects to molding. In order to prevent the influence of core sand on green sand, the amount of new sand added should be kept at 2% ~ 5% of the total amount of mixed sand. In addition, the reasonable combination of core-free and core-free products should be paid attention to when arranging casting production to reduce the amount of core sand in molding sand as much as possible.

3. Production and application

3.1 Sand mixing process

In the sand mixing process, the sand, bentonite, water and additives should be mixed uniformly (GS high-efficiency rotor sand mixer); Knead the various materials to evenly coat the clay film around the sand. Feeding sequence is generally the first dry mix, and then add water wet mix. It has been observed, however, that after the moulding sand has passed through the moulding sand, there are always some hard clumps, the size of small grains of rice, wrapped in the moulding sand. The composition of these clumps is actually the sinter and clay ball of bentonite and sand. Therefore, it is suggested that the sand mixing process should be changed into adding water and wet mixing to dissolve the sand mass after adding sand, and then adding other powder.

Old sand + new sand + water (preferably atomized water); After 15s of wet mixing, add bentonite + pulverized coal + additive, mix 120 ~ 150s. The mixing time is not easy to be too long, otherwise the sand fever reduces the fluidity, the strength is affected instead.

3.2 modelling

Preheat the formwork before molding (especially in the cold winter season), so that the temperature is 5℃ higher than molding sand is appropriate, the mold is not sticky, the surface is not dry, the casting surface is clean.

3.3 Sand detection

First, strict incoming inspection of raw materials (including raw sand, bentonite, pulverized coal, etc.); The second is to strengthen the online testing of molding sand, sand performance regularly testing, timely detection of problems and timely solution.

3.4 Judgment of molding sand quality

Some foundry enterprises believe that "the value of wet compressive strength" can be used instead of "the value of effective bentonite" to determine the amount of bentonite added to the sand mixer. Sanzhuji, combined with the operating principle of sand treatment equipment and molding feedback from foundry enterprises, thinks that this view is not valid. To put it simply, both bentonite and dead clay in molding sand produce wet compressive strength, which increases the amount of dead clay, but the value of wet compressive strength also increases with the constant compacting rate, so the wet compressive strength will have a misleading effect. Other properties and the results of available clay and silt content should also be considered when considering wet compressive strength. The effective clay content test is the only method to determine the bentonite content in moulding sand.

On the premise of ensuring the effective clay content, the compaction rate of molding sand should be mainly controlled in the sand mixing site to make the proportion of bentonite and moisture in molding sand moderate, give full play to the bonding effect of bentonite, and avoid the phenomenon of free water caused by excessive moisture in molding sand.

3.5 pouring

Should be rapid, high temperature, continuous casting, rapid filling; Otherwise, it is easy to produce gas and sand washing, forming water migration (commonly known as shooting), resulting in sand mold damage, casting waste.

4. Analysis and countermeasures of major defects

4.1 sand holes
Sand hole is due to the casting system or mold there are sand particles or due to iron liquid scour wall, scattered sand particles fall. If the mixing position of molding sand is far away from the molding line, the surface dehydration strength of molding sand decreases sharply during the belt conveyance process of wet sand, and the "air drying" phenomenon becomes more serious in the dry season of northern weather, attention should be paid to it. In addition, if the ash content and silt content in the old sand are too high repeatedly used, it will lead to low toughness and brittle, poor molding performance, so that the edges and corners of the sand are broken, causing sand hole defects. Measures to prevent sand hole: (1) using high quality bentonite, according to the season (dry, wet, etc.) change, reasonable control sand water, improve sand strength; (2) Adjust the old sand dust removal equipment, control the silt content of the old sand below 12% (using the sand loosening machine or crusher combined with dust removal equipment), the molding part sand compacted rate is increased to 34% ~ 38%, the water content is 3.2% ~ 3.8%, make the ratio of compacted rate and water content within the range of 10 ~ 12; A small amount of -starch is added into molding sand to improve the toughness of molding sand, reduce the friction resistance of molding, and enhance the surface air-drying strength; (4) Timely adjust the old sand core sand content, appropriately extend the rolling time.

4.2 adhering sand
Clay sand is a surface defect caused by iron penetration into the mold. The countermeasures are as follows: (1) Adopt the fine particle size and moderate distribution of the original sand, improve the sand fluidity and compacted degree (the hardness of the casting mold is controlled at about 90), reduce the porosity of the casting surface; (2) The use of high efficiency and high quality pulverized coal, control the effective pulverized coal content in molding sand namely volatilization, reduce iron surface oxidation and wetting and infiltration of sand surface; (3) Properly reduce the pouring temperature, reduce the liquid iron pressure.

4.3 Prevention of porosity of nodular iron parts: (1) Improve the permeability of molding sand; (2) Control the amount of molding sand coal and water content and core sand content, reduce the molding sand gas volume; (3) The vent groove is arranged reasonably on the formwork, which is conducive to the outward escape of the gas inside the mold; (4) Properly raise the pouring temperature (& GT; At 1380℃) and speed, sprinkle ice crystal powder on the ladle.

Hope that the above content is helpful to the foundry enterprise, welcome the guests to visit the factory

Qingdao Sanzhuji Equipment Manufacturing Co., Ltd. specializes in the production of sand reclamation equipment,foundry machines,Sand casting equipment,GS high efficiency rotary mixer,no-bake resin sand mixer,Jolt squeeze molding machine/Jolt-squeezing moulding machines,Multi- Piston Moulding Machine/Hydraulic multi-piston moulding machine,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.