Compared with other casting methods, wet castings are more prone to defects such as sand, sand, sand, and pores. If the foundry takes care to control the quality of the wet sand, these defects are likely to be reduced or avoided. The following examples illustrate the relationship between sand properties and surface defects of castings.
One. Sticky sand
Research work shows that, in general wet sand castings, whether cast steel or cast iron, sand-bonding defects are mechanical sand, not chemical sand. There are many reasons for the occurrence of mechanical sanding, the most common examples of which are as follows:
1. The sand is too thick and the gas permeability is too high, and the molten metal is easy to be drilled into the pores between the sand grains, so that the surface of the casting is rough, or the sand is wrapped and fixed on the surface. A large amount of 30/50 mesh coarse core sand is continuously mixed into the cast iron old sand of a foreign-funded factory in Jiangsu, so that the permeability of the molding sand reaches 220 or more, and the surface of the casting is extremely rough. The pneumatic micro-vibration molding machine of a steel casting workshop in a factory in Inner Mongolia produces medium and small castings. The use of 40/40 coarse-grained quartz sand mixed sand mainly concentrated on the surface of the casting produces severe sand adhesion. Normally, the permeability of the sand is not detected, and it is considered to have met ≥80 as specified in the technical regulations. In order to find the cause of sand sticking, it was specifically tested once, and it was found that the gas permeability was as high as about 1070, which indicates that this is the cause of sand sticking. Therefore, the sand permeability must have an upper limit, and the grit size and gas permeability should be within a suitable range. Generally, the most suitable type of sand for a single type of seismic equipment is 70/140 mesh, and the gas permeability is about 70-100. The high-density molding sand is preferably 50/140 or 100/50, and the gas permeability is 80-140. . Some foundries that produce engines use 50/100 mesh coarse sand to make sand cores. When sand is added, the sand is continuously mixed into the old sand, so that the sand permeability can reach 180 or more. It should be added with 100/140 mesh fine sand, or it will be rotated. The fine particle fraction in the flow separator is returned to the old sand to correct the sand particle size.
2. Insufficient coal powder content in cast iron sand or poor quality coal powder. A foundry in Beijing produces brake discs for high-speed trains. The castings meet the requirements, and the surface has severe sanding. It needs to be polished before delivery. The pulverized coal used in the molding sand comes from a close private small supplier in the suburbs. The cause of the sticking sand may be that the quality of the coal powder is too poor, and it may be that the amount of effective coal powder in the molding sand is not enough. The “pulverized coal†used by a valve factory in Anhui is the waste that is produced by coke washing, and the ash content is as high as 76%. After use, the performance of the entire molding sand is destroyed, and the casting waste is more than half. The foundry should strengthen the inspection of the quality of the purchased coal powder. High-quality pulverized coal requires ash content ≤10%, volatile matter 30~37%, and coke slag characteristics 4~5. The effective pulverized coal content of the molding sand can be detected by the amount of gas generated. The vibration of the medium and small gray iron castings is about 22~26mL per 1g of the type sand of ordinary coal powder, which is equivalent to 6~7% of ordinary quality effective coal powder, or 5~6% of high quality coal powder, or increase Coal powder 4~5%. The high-tightness molding sand has a gas volume of 18~24mL, which is equivalent to 3~4% of the synergistic coal powder. In some foreign-funded foundries in China, the COI is used to estimate the anti-sticking performance of wet sand for cast iron. For example, the static pressure molding line of Jiangsu No. 1 Automobile Casting Factory stipulates that the amount of surface sand reduction is 4.10±0.30%. There are a number of domestic modeling materials companies supplying a variety of "pulverized coal substitutes." The foundry should first carry out the casting test, compare the surface effect of the casting, the change of the sand molding performance and the production cost of the casting with the high-quality coal powder or the synergistic coal powder, and then determine whether to use.
two. Sand hole
The sand holes and slag holes on the surface of the casting are commonly referred to as "sand eyes". Most of the slag holes are formed by using straw ash or dry sand as a slag agent. The reasons for the formation of sand holes are as follows:
1. A joint venture foundry in Tianjin produced small and medium-sized gray iron castings such as motor casings by hand. The main drawback is that the dispersed sand is visible on the upper surface of the entire casting. The reason for the analysis of this sand hole formation is sand washing, which is formed by the floating system in which the casting system and the cavity are washed by the molten iron and floated on the liquid surface. The factory usually does not control the quality of the sand. According to the cloud, the wet compressive strength was only 25 kPa. The wet compressive strength of the sand for manual modeling is preferably around 70~80kPa, and the shape of the vibration machine should be 90~120kPa. If it is a high-density shape, the wet compressive strength of the molding sand can be 140~180 kPa. Large pieces can be increased. In order to improve the wet compressive strength of the molding sand, inferior bentonite should be avoided, and the amount of blue absorption of 0.2 g of bentonite is preferably 35 mL or more. The molding sand also needs to contain enough effective bentonite. For example, the high-density molding sand 5g is mostly in the range of 55~65mL. The amount of high-quality effective bentonite is 6~7%.
2. There is only one vibration molding machine in a foundry in Shandong. After work, the first type is built to cover the ground and the lower core. After half a day, the template was replaced and the top and the buckle were assembled to prepare for casting. Castings often have defects such as sand holes. The reason is that the surface strength of the wet sand type is degraded and dried, and the surface sand is easily washed away into the molten iron. The phenomenon of “drying†in the dry weather is more serious. It is best not to open the wet sand box for less than half an hour. If it is found that there is any sign of dry dehydration on the sand surface, spray the water onto the sand surface to restore the moisture state before the mold is combined. In the past, a Japanese-owned automobile engine factory in Tianjin used imported surface intensifiers to spray the surface of the cavity, and now it has also switched to water spray.
3. An auto parts foundry in Sichuan uses a static pressure molding machine assembly line to produce cylinder blocks and cylinder heads. There are many different sand holes on the surface of the casting. The plant type sand adopts the low-quality bentonite and pulverized coal in the province, and the old sand is not subjected to frequent dust removal treatment, so that the mud content in the old sand sometimes rises to 24%. In order to maintain the moisture content of the molding sand to about 4.0% to prevent the occurrence of porosity defects, the compaction rate of the molding sand has to be lowered in the range of 27 to 32%. The wet compressive strength of the molding sand is not low, and it is not the cause of sand holes at 170 to 210 kPa. Due to the high ash content and low compaction rate of the molding sand, the toughness is insufficient, and the fracture index is only about 65 to 75%. The sand performance is too brittle, the mold release property is poor, and the edges and edges of the sand mold are easily broken, thereby causing sand hole defects. The plant should be replaced with high-quality bentonite and coal powder; old sand dust removal equipment should also be used to control the mud content of the old sand to less than 12%, the mud content of the sand to not exceed 13%, and the crushing index of the sand to be controlled to 80-85%. . The compacting rate of the molding sand in the molding is increased by 35 to 38%, the water content is 3.2 to 3.6%, and the ratio of (compacting rate) / (water content) is in the range of 10 to 12. This will increase the toughness of the sand and reduce sand hole defects. Several factories in Shanghai, Beijing, and Harbin added a small amount of α-starch to the sand to improve the toughness of the sand, reduce the frictional resistance of the die, and enhance the surface air drying strength. It is beneficial to prevent sand hole defects and improve the smoothness of the surface of the casting.
4. The engine casting branch of a tractor factory in Henan has made the molding sand brittle due to the incorporation of a large number of cold core box sand cores, and the die-forming performance is getting worse. Not only the sand-shaped edges are fragile, but also the hanging sand is easy to break. According to the factory regulations, the cycle time of the roller mixer is only 3 minutes, and the mixing time cannot be lengthened to avoid affecting the sand used in the molding machine. Later, I tried my best to extend the sand mixing cycle by 1 min. I found that the feel of the sand changed and the moldability improved. This shows that the original sand mixing time is too short to mix excellent sand performance.
three. Sand (scarring, peeling)
Since many domestic companies have supplied high-quality activated bentonite, the sand inclusion defects on wet castings have been greatly reduced. However, individual wet casting plants can also accidentally create sand inclusion defects.
1. A small car repair and distribution plant in Jiangxi hopes to produce aluminum alloy castings for motorcycle engines in wet form. At the beginning, Qiushan “Clayâ€, which was borrowed from two kraft paper bags, was used for sand mixing. Later, I went to the materials department to buy two kinds of clay bags wrapped in sacks. However, it was found that the clay sand of the newly purchased clay was very low, and the sand type cracked and peeled after being baked by the fire, and the casting castings had serious sand inclusion defects. At that time, it was checked whether the mud of the two clays could be thickened by alkali activation with extremely simple conditions. Prove that the sack is not bentonite but real clay, not for wet casting. The reason for the problem was that the geological department used the slightly acidic calcium-based bentonite as “acidic clayâ€. Many foundries have referred to “acidic clay†as “ceramicâ€. As a result, bentonite with montmorillonite as the main mineral component was confused with real clay (or ordinary clay) with sorghum as the main mineral component. The real clay is mainly used for firing ceramics and is not suitable for wet casting. The foundry can also use the amount of blue to identify two different clay minerals. 0.2g of bentonite adsorbs methylene blue in 25~45mL, while ordinary clay absorbs only one-tenth of bentonite.
2. Impact of water quality: A Taiwan-funded foundry in Tianjin uses an extrusion molding machine to produce export cast iron frying pans. The sand is mixed with high-quality activated bentonite. The wet compressive strength of the molding sand is 200-250 kPa, the compacting rate is 35-38%, and the water content is about 3%. However, the castings were found to have sand-carrying defects near the inner runner, and it was suspected that there was a problem with the well water used for the sand mixing. The well pipe of the original mixed sand used in the plant was blocked. In order to save, the boss took a 20m shallow well for sand and water. The workers found that the water in this well could not be salted, and the soap in the hand soap did not foam. After testing, the shallow well water contains a large amount of sodium, magnesium and chloride ions, which has a strong anti-activation effect on the activated bentonite. After the mixed drinking water from the neighboring factories, the effect of the original water quality cannot be completely eliminated. Jiangsu has an extrusion molding production refrigerator compressor casting factory, which uses river water mixed with sand flowing through the small river outside the factory. It is caused by the chemical plant discharging water to the upstream of the river and causing sand casting defects. The reason is also due to waste water. Anti-activation effect. If it is suspected that the water quality is suitable for sand mixing, 2g or 3g bentonite can be used to determine the swelling value, or expansion ratio and free expansion amount, respectively, with pure water and water to be tested. If the test result of the water to be tested is much lower than that of pure water, it indicates The quality of the water to be tested is not available.
4. Porosity
The stomatal defects of castings mainly include four types: air bearing holes, intrusion air holes, precipitation air holes and reaction air holes. The following examples illustrate the causes and preventive measures of common pores in the factory.
1. It is not easy to identify the type of pores and the cause of the formation. According to production experience, increasing the pouring temperature of 30~50 °C can often reduce the occurrence of any type of porosity defects. Attention should be paid to the temperature of the last one or two sand molds per pack of iron, because the temperature of the molten iron in the package has dropped and the porosity defects are likely to occur. A Taiwan-based foundry produced an industrial sewing machine casing. Each of the irons could be cast with seven sand molds, but only five sand molds were poured. The molten iron left in the bag is returned to the electric furnace, and then a full package of molten iron is re-connected to cast the sand mold, in order to maintain the pouring temperature and reduce the porosity defects.
2. A Japanese factory in Beijing once found a casting with a porosity defect. After sawing, it saw that the pores were continuously floating. It is estimated that the back pressure at the interface where the pores are generated exceeds the static pressure of the molten iron to cause intrusion into the pores, but it is impossible to judge what the gas source is. Some factories use old sand piles as garbage dumps, cigarette heads, popsicles, waste paper balls, melon seed skins... thrown into old sand, which may form pore defects in the sand. Some foreign foundry factories strictly prohibit smoking in the factory area and are also one of the effective measures to prevent stomata.
3. A factory in Shandong produces a medium-sized export valve cast iron piece, which is shaped by a shock molding machine and a cold core box made of sand core. The factory adopts two-day continuous modeling and lower core and combined type, and the furnace is poured once every other day. The casting porosity of the castings produced is extremely high. The reason for the analysis is that the sand core absorbs moisture and enters the molten iron to invade the pores. The cold core box sand core is easy to absorb moisture when it is placed in a sand mold with a relatively high humidity for a long time. When pouring, not only the binder generates gas, but also the moisture absorbed by the core also emits a large amount of water vapor, so that pore defects are easily generated. The furnace should be opened every other day for the daily opening, or the shape should be empty first. After the opening day, the lower core and the box are poured. It can prevent the sand core from absorbing moisture, and can reduce sand-type air-drying and dehydration, so that the porosity defects are greatly reduced. Open the gas riser and increase the exhaust capacity. Properly increasing the pouring speed and quickly establishing static pressure resists intrusion of interfacial gas, which is also beneficial for preventing intrusion into the pores.
4. From the situation of the generation of stomata defects in engine foundries such as Henan, Shandong, Liaoning, Jilin, etc., the stomata encountered are still intrusive pore defects of the gas generated by the sand core, and are rarely precipitated nitrogen pinholes. Because the binder of the sand core used is changed to a resin with a lower nitrogen content, and if necessary, appropriate iron oxide is added to the core sand and the coating. Therefore, the exhaust capacity of the sand core should be strengthened first. A smooth vent hole should be opened in the middle of the sand core. For thick section sand cores, they can be hollowed out or subdivided into a mesh-shaped cavity and then bonded. The most common method of exhausting the resin from the hard sand core is to use a nylon braided tube, which can be easily embedded in the sand core in any shape along the core. The hot core box, the cold core box and the shell core are all integrally fired. The exhaust line cannot be embedded, and the vent needle or rod can be placed and taken out before or after the core is taken. But more is to use the carbide drill bit to drill the core to help vent after the sand core is hardened. For example, a hydraulic parts factory in Shanxi produces hydraulic valves with extremely complicated shapes, and all the cores of the shell core are drilled and drilled to help the exhaust. In Spain, there is a foundry that produces cars. The water jacket cores that make the cylinder heads use a special multi-head drilling machine to drill blind holes in the cooling water channel cores of the water jacket core from bottom to top. When the core of the larger core is lower, if the gap between the core and the core is too large, the phenomenon that the molten iron penetrates into the core vent will occur. The core of the sand core shall be enclosed with a sealing material such as refractory fiber mat, mud strip or asbestos rope. Also pay attention to the high temperature fast pouring, quickly establish the pressure of the molten iron beyond the back pressure of the gas generation point so that the gas can not be drilled into the molten iron to become a bubble. Even if the gas has been drilled into the molten iron, it can float and discharge as the molten iron enters the exhaust riser. In addition, the use of low-volume binder is necessary to prevent porosity defects. For example, a gas furnace in a gas plant produced in a factory in Beijing has only one core. If the exhaust is difficult, try to control the gas output of the shell to 12 mL/g. Below, and high temperature rapid pouring.
5. A factory in Shanxi uses an extrusion molding machine to produce gray cast iron crankshafts, which form small pores on the surface of the casting and under the skin. Generally, the small holes with a diameter of 1 to 3 mm are mostly present in the skin 1 to 3 mm, and are exposed when shot blasting or roughing. This factory does not use a resin sand core and does not generate nitrogen pores. The defects should belong to the reaction pores. That is, the molten metal and the mold chemically react at the interface, and the generated gas is dissolved in the molten metal. When the cooling is cooled, the solubility is lowered to form bubbles. The casting material is gray iron, which also eliminates the reaction of magnesium or rare earth in the molten iron and the moisture in the sand type. It is suspected that the charge and the inoculant may bring aluminum and titanium into the molten iron. Because aluminum and titanium react with water to release an atomic state [H] which is easily dissolved into the iron layer. When the layer is solidified, the solubility of hydrogen is lowered to precipitate in the molecular state and grow into hydrogen bubbles. The analysis report of the ferrosilicon inoculant showed that the aluminum content reached 1.86%, which may be the main reason for the production of subcutaneous pores. The ferric inoculant preferably has an aluminum content of about 1.0% and a maximum of 1.5%. For ferrosilicon used in the flow, not only should the lower aluminum content be controlled, but the amount of addition should not exceed 0.08~0.10%. In order to prevent the reaction of elements such as aluminum and titanium with water, the moisture content of the extruded molding sand must also be controlled to not more than 4%.
6. After the molten iron of the ductile iron is poured into the wet sand type, the residual magnesium reacts strongly with oxygen in the water molecule to generate an atomic state [H], which is the main cause of subcutaneous pore defects. Both smelting and process measures must be taken to prevent the generation of reaction pores. A ductile iron factory in Hebei is a professional manufacturer of ductile iron castings such as clutch pressure plates for trucks. The castings have no subcutaneous pores. The reason for avoiding the pores from the perspective of the smelting of the plant is that the indicators are not beyond the conventional range. For example, using high-quality foundry coke, the cupola furnace temperature is above 1480 °C, the depth of the spheroidizing package is 1.5~2 times the diameter, the surface of the ladle and cavity is shaking cryolite powder, the molten iron contains 0.03~0.04% sulfur, and the residue The amount of magnesium is 0.40 to 0.46%. However, from the perspective of the process, the water content of the surface sand is as high as 7.0 to 7.6%, far exceeding the maximum of 5.5%. The reason for analyzing the non-venting holes may be: (1) The sand is added with a large amount of coal powder, and the amount of ignition is as high as 7.0 to 7.9%. More than the usual 4 to 5%. The amount of unmeasured sand gas is estimated to be above 35mL/5g. The surface of the casting exposed after pouring shows a dark blue color, indicating that there is a large amount of intense reducing atmosphere in the cavity during pouring, which dilutes the water vapor in the cavity. (2) The sand type has a gas permeability of 100, and a large number of vent holes are formed. Most of the water vapor generated by the pouring is discharged outside the cavity, and the water vapor which may participate in the reaction is reduced. (3) Although the water content of the surface sand is quite high, the mud content is as high as 21%. Therefore, the measured compaction rate is only about 36%, indicating that the molding sand is not wet. It is possible that the mud absorbs a lot of water and slows down the chemical reaction. It is therefore conceivable that the occurrence of reaction pore defects mainly depends on the compaction rate (i.e., the degree of dryness of the molding sand) rather than the water content.
V. Discussion
1. The conditions for obtaining high-quality sand are first of all to select high-quality raw materials, and also to apply an excellent sand mixing process. Some wet sand casting plants with strict technical management require that the sand in the sand mixer be completely removed before the end of each shift. Most of the major wet foundries in the United States require that the sander blade be at a coin thickness from the chassis. In Japan, several car casting factories using roller-type sand mixers have a sand mixing cycle of 6 minutes. However, some of the sand mills in the foundry of China and the sand on the drum are not cleaned for a long time, the scraper wear is not adjusted, and the sand mixing time is up to 3 min. How can this mix good quality sand?
2. The quality of the sand is based on the performance. The detection and control of the sand can be used to prepare the superior sand. There are more than 20 kinds of daily inspection items for surface sand used in the static molding line of a Japanese auto parts casting factory in Jiangsu, which does not include the back sand and raw material testing items. In some of China's foundry sand mills, the equipment is simple, and the daily testing of wet sand performance may be only three or four. How can we explain the cause of the surface defects of the casting based on the measured results? How can it be used to reduce the rate of casting scrap?
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