1. Sand sticking
The research shows that the sand sticking defects of general green sand castings, whether cast steel or cast iron, belong to mechanical sand sticking, not chemical sand sticking. There are many reasons for mechanical sand sticking, and the most common examples are as follows:
1. The sand grains are too coarse and the air permeability is too high, so that molten metal can easily drill into the pores between the sand grains, making the casting surface rough, or wrapping and fixing the sand grains on the surface. A large amount of 3/5 mesh coarse-grained core sand is continuously mixed into the used cast iron sand of a foreign-funded factory in Jiangsu, so that the permeability of molding sand reaches above 22, and the casting surface is extremely rough. The pneumatic microseismic molding machine in the casting workshop of a factory in Inner Mongolia produces medium and small castings. Using 4/7 coarse-grained quartz sand, which is mainly concentrated in 4 meshes, to make molding sand, the surface of the casting is seriously stuck. Usually, the permeability of molding sand is not tested, and it is considered that it has met the requirements of process regulations ≥8. In order to find out the cause of sand sticking, the air permeability was found to be as high as 17, which indicated that this was the cause of sand sticking. Therefore, the permeability of molding sand must have an upper limit, and the particle size and permeability of molding sand should be within a suitable range. In general, the most suitable molding sand particle size for molding single sand by vibro-compaction machine is mostly 7/14 mesh, and the air permeability is roughly 7~1. The particle size of high-density molding sand is preferably 5/14 or 1/5, and the air permeability is 8~14. Some foundries that produce engines use 5/1 mesh coarse sand to make sand cores. When the sand falls, it is continuously mixed into the old sand, so that the permeability of molding sand may reach more than 18. Therefore, 1/14 mesh fine sand should be added, or the fine particles in the cyclone separator should be returned to the old sand to correct the particle size of molding sand.
2. The content of pulverized coal in cast iron molding sand is insufficient or the quality of pulverized coal is poor. A foundry in Beijing produces brake discs for high-speed trains. The casting material meets the requirements, but there is serious sand sticking on the surface, which needs to be polished as a whole before delivery. The coal powder used in molding sand comes from a small private supplier with close ties in the suburbs. The cause of sand sticking may be that the quality of pulverized coal is too poor, or that the effective amount of pulverized coal in molding sand is not enough. The "pulverized coal" used in a valve general factory in Anhui Province is the waste from coke washing, and its ash content is as high as 76%. After use, the performance of the whole molding sand was destroyed, and more than half of the castings were rejected. The foundry should strengthen the inspection of the quality of the purchased pulverized coal. High-quality pulverized coal requires ash ≤1%, volatile matter 3~37%, and coke residue characteristics 4~5. The effective pulverized coal content of molding sand can be detected by gas generation. The gas generation of molding sand with ordinary coal powder per 1g is about 22~26mL, which is equivalent to 6 ~ 7% of ordinary quality effective coal powder, or 5 ~ 6% of high-quality coal powder or 4 ~ 5% of synergistic coal powder. The gas production of high compactness molding sand is about 18~24mL, which is equivalent to 3~4% of synergistic pulverized coal content. Most foreign foundries in China use LOI to estimate the sand adhesion resistance of green sand for cast iron. For example, the static pressure molding line of an automobile casting factory in Jiangsu stipulates that the ignition loss of surface sand is 4.1 .3%. There are many modeling materials companies in China that supply all kinds of "coal powder substitutes". The foundry should first carry out pouring test, compare the surface effect of castings, the change of molding sand performance and the production cost of castings with high-quality pulverized coal or synergistic pulverized coal, and then decide whether to choose them.
ii. sand holes
sand holes and slag holes on the surface of castings are usually collectively called "sand holes". Slag holes are mostly formed by using rice straw ash or dry sand as slag aggregating agent. The reasons for the formation of sand holes are as follows:
1. A joint venture foundry in Tianjin produces medium and small gray iron castings such as motor shells by hand molding. The main defect is that dispersed sand particles can be seen on the upper surface of the whole casting. It is analyzed that the reason for the formation of this sand hole is sand washing, which is formed by the scattered sand particles falling from the gating system and cavity washed by molten iron floating on the liquid surface. The factory usually does not control the quality of molding sand. According to the cloud, the wet compressive strength was only 25kPa. The best wet compaction strength of molding sand for manual molding is about 7~8kPa, and the molding of vibration compaction machine should be 9 ~ 12 kPa. For high-density molding, the green compressive strength of molding sand can be 14~18 kPa. Large pieces can be higher. In order to improve the green compressive strength of molding sand, inferior bentonite should be avoided, and the blue absorption capacity of .2g bentonite should be above 35mL. Molding sand also needs to contain enough effective bentonite. For example, the blue absorption of high-density molding sand 5g is mostly 55~65mL. 6-7% of high-quality effective bentonite.
2. There is only one vibro-compression molding machine in a foundry in Shandong. After going to work, the lower mold is made to cover the ground and the lower core. After half a day, change the template to make the upper mold and the closed mold, and prepare for casting. Defects such as sand holes often appear in castings. The reason is that the surface strength of the green sand mold decreases sharply after dehydration and drying, and the surface sand particles are easily eroded and fall into molten iron. The phenomenon of "air drying" is more serious in dry season. The opening time of the green sand lower box is preferably not more than half an hour. If there are signs of drying and dehydration on the surface of the sand mold, a sprayer should be used to spray water on the surface of the sand mold before molding to restore the wet state. A Japanese-funded automobile engine factory in Tianjin used to spray the surface of the cavity with imported surface enhancer, but now it also uses water spray.
3. An automobile parts foundry in Sichuan uses hydrostatic molding machine to produce cylinder block and cylinder head, and there are many sand holes on the surface of castings. The molding sand in this factory uses bentonite and pulverized coal with low quality in this province, and the used sand is not regularly dusted, resulting in the mud content in the used sand sometimes rising to 24%. In order to keep the water content of molding sand about 4.% and prevent the formation of porosity defects, the compaction rate of molding sand has to be kept within the range of 27 ~ 32%. The green compressive strength of molding sand is not low, ranging from 17 to 21 kPa, which is not the cause of sand holes. Due to the high ash content and low compaction rate of molding sand, the toughness is insufficient, and the crushing index is only about 65 ~ 75%. The performance of this kind of molding sand is too brittle, the demoulding performance is poor, and the edges and corners of the sand mold are easily broken, thus causing sand hole defects. The factory should switch to high-quality bentonite and pulverized coal; Dust removal equipment for used sand should also be used to control the mud content of used sand below 12% and that of molding sand below 13%. Control the crushing index of molding sand to 8 ~ 85%. The compaction rate of molding sand at the molding site is increased to 35-38%, and the water content is 3.2-3.6%, so that the ratio of (compaction rate)/(water content) is in the range of 1-12. This can improve the toughness of molding sand and reduce sand hole defects. Several factories in Shanghai, Beijing and Harbin add a small amount of α -starch to sand to improve the toughness of molding sand, reduce the friction resistance of mold drawing and enhance the surface air-drying strength. It is beneficial to prevent sand hole defects and improve the surface smoothness of castings.
4. The engine foundry branch of a tractor factory in Henan province has a lot of cold box sand cores mixed in, which makes the molding sand brittle and the demoulding performance worse and worse. Not only the edge of sand mold is fragile, but also the hanging sand is easy to break. According to the factory regulations, the cycle time of the grinding wheel sand mixer is only 3 minutes, so the sand mixing time cannot be extended to avoid affecting the sand demand of the molding machine. Later, the sand mixing period was extended by 1min as much as possible, and it was found that the feel of molding sand changed and the demoulding property was improved. This shows that the original sand mixing time is too short to produce excellent molding sand performance.
iii. sand inclusion (scarring and peeling)
since many domestic companies supplied high-quality activated bentonite, the sand inclusion defects on the surface of green castings have been greatly reduced. However, some green mold casting plants also accidentally produce sand inclusion defects.
1. A small automobile repair shop in Jiangxi hopes to produce aluminum castings for motorcycle engines in green form. At the beginning, I borrowed two brown paper bags of Qiushan "clay" for sand mixing. Later, I went to the material department to buy two clay bags. However, it is found that the newly purchased clay molding sand has low adhesion, and the sand mold cracks and peels after baking by the fire, resulting in serious sand inclusion defects in the casting. At that time, it was checked whether the mud of two kinds of clay could be thickened by alkali activation under extremely simple conditions. It is proved that the sack is not bentonite, but real clay, which can not be used for green mold casting. The reason for the problem is that the weak acidic calcium-based bentonite was called "acidic clay" by the geological department. Many foundries refer to "acid clay" as "clay" for short. Results Bentonite with montmorillonite as the main mineral component is confused with real clay with kaolinite as the main mineral component (that is, ordinary clay). Real clay is mainly used for firing ceramics and is not suitable for green mold casting. Foundry plants can also identify two different clay minerals by the blue absorption capacity. .2g bentonite absorbs 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. Using high-quality activated bentonite to mix sand, the green compressive strength of molding sand is 2 ~ 25 kPa, the compactness is 35 ~ 38%, and the water content is about 3%. However, the sand inclusion defect occurred near the inner runner of the casting, and it was suspected that there was a problem with the well water used for sand mixing. The well pipe of deep well water used for sand mixing in this factory was blocked. In order to save money, the boss drilled a 2m shallow well for mixing sand and adding water. Workers found that the water in this well was salty and could not be drunk, and washing hands and rubbing soap did not foam. This shallow well water contains a lot of sodium, magnesium and chloride ions, which has a strong anti-activation effect on activated bentonite, and it is easy to produce sand inclusion defects when it is used to mix sand to produce castings. After drinking water mixed with sand was introduced from a nearby factory, the influence of the original water quality could not be completely eliminated. There is a factory in Jiangsu province that produces refrigerator compressor castings by extrusion molding. River water flowing through a small river outside the factory is used to mix sand, which coincides with the fact that a chemical plant in the upper reaches of the river discharges wastewater into the water, causing sand inclusion defects in the castings, and the reason is also due to the anti-activation effect of wastewater. If you doubt whether the water quality is suitable for sand mixing, you can take 2g or 3g bentonite and use purified water and the water to be tested to determine the swelling value, or the swelling multiple and free swelling amount. If the test result of the water to be tested is much lower than that of purified water, it indicates that the quality of the water to be tested is not available.
iv. blowhole
The blowhole defects of castings are mainly divided into four types: wrapped blowhole, invaded blowhole, precipitated blowhole and reaction blowhole. The following examples illustrate the causes and preventive measures of common blowholes in factories.
1. It is not easy to identify the types and causes of stomata. According to the production experience, increasing the pouring temperature by 3~5℃ can often reduce the occurrence of any type of blowhole defects. Attention should be paid to the temperature of pouring the last one or two sand molds in each ladle, because the temperature of molten iron in the ladle has dropped at this time, which is easy to cause blowhole defects. A Taiwan-funded foundry in Tianjin produces industrial sewing machine shells. Each molten iron can be poured with 7 sand molds, but only 5 sand molds are poured. The molten iron left in the ladle is poured back into the electric furnace, and then a full ladle of molten iron is reconnected to pour 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 blowholes. After sawing, it was found that the blowholes were floating continuously. It is estimated that the back pressure exceeds the static pressure of molten iron at the interface where the pores are generated, which leads to the invasion of pores, but it is impossible to judge what the gas source is. Some factories treat old sand piles as garbage dumps, and cigarette butts, popsicles, waste paper balls, melon seed skins ... are thrown into old sand and mixed with molding sand, which may form pore defects. Smoking is strictly prohibited in some foreign foundry factories, which is also one of the effective measures to prevent blowholes.
3. A factory in Shandong produces medium-sized export valve iron castings, which are molded by a shock molding machine and sand cores are made by cold box. The factory adopts two-day continuous molding, core-dropping and molding, and the cupola is opened and poured every other day. The porosity rejection rate of that produce castings is extremely high. It is analyzed that the reason is that the sand core absorbs moisture and generates gas to enter the molten iron, which causes the invasion of pores. It is easy to absorb moisture when the cold box sand core is placed in a sand mold with high relative humidity for a long time. When pouring, not only the binder gives off gas, but also the water absorbed by the sand core gives off a lot of water vapor, so it is easy to produce blowhole defects. We should start the furnace every other day as daily, or close the mold after molding, and then open the box, core and box for pouring on the day of opening the furnace. It can not only prevent the sand core from absorbing moisture, but also reduce the air drying and dehydration of the sand mold, so that the porosity defects are greatly reduced. Open more gas risers to increase the exhaust capacity. Properly increasing the pouring speed and quickly establishing static pressure to resist the invasion of interface gas are also beneficial to prevent the invasion of pores.
4. judging from the occurrence of blowhole defects in engine casting factories in Henan, Shandong, Liaoning, Jilin, etc., most of the blowholes encountered still belong to the blowhole defects caused by gas generated in the sand core, and few are precipitated nitrogen pinholes. Because the binder of the used sand core is changed to resin with low nitrogen content, and appropriate iron oxide is added to the core sand and paint when necessary. Therefore, firstly, the exhaust capacity of sand core should be strengthened. An unobstructed vent should be opened in the middle of the sand core. For thick and large cross-section sand cores, they can be hollowed out or dug into grid-shaped cavities in half and then bonded. The most common exhaust method of resin self-hardening sand core is to use nylon braided pipe, which can be conveniently embedded in the sand core along any shape of the sand core when making the core. The hot core box, the cold core box and the shell core are all integrally shot, so the exhaust pipe cannot be embedded, and a ventilation needle or rod can be placed and extracted before or after coring. But more often, after the sand core is hardened, a cemented carbide drill is used to drill holes from the core head to help exhaust. For example, a hydraulic parts factory in Shanxi produces hydraulic valves with extremely complicated shapes, and all the core heads of the shell core are drilled with angle drills to help exhaust. There is a foundry in Spain that produces cars. The water jacket sand core of the cylinder head is made, and the core heads of each cooling water channel of the water jacket sand core are simultaneously drilled with a special multi-head drilling machine from bottom to top. When a large sand core is placed, if the gap between the core head and the core seat is too large, molten iron will drill into the sand core vent hole. The core head of the sand core should be enclosed with sealing materials such as fire-resistant fiber felt pad, mud strip and asbestos rope. We should also pay attention to high temperature and rapid pouring, and quickly establish that the pressure of molten iron exceeds the back pressure of the gas generating point so that gas can not drill into molten iron and become bubbles. Even if the gas has been drilled into the molten iron, it can float and be discharged with the molten iron entering the exhaust riser. In addition, it is necessary to use low gassing binder to prevent blowhole defects. For example, the combustion ring of a British gas stove produced by a factory in Beijing has only one core, so it is difficult to exhaust, so the gassing of the shell core should be controlled below 12mL/g as far as possible, and it should be poured quickly at high temperature.
5. A factory in Shanxi uses an extrusion molding machine to produce gray cast iron crankshafts, and small pores are formed on the surface and under the casting. Generally, it is a small hole with a diameter of 1~3mm, which mostly exists in the epidermis at 1 ~ 3 mm, and is exposed during shot blasting cleaning or rough machining. This factory does not use resin sand core, and there will be no nitrogen holes. The defects should belong to reaction holes. That is, the molten metal reacts with the mold at the interface, and the generated gas is dissolved in the molten metal. When cooling, the solubility decreases and bubbles are precipitated. The casting material is gray iron, which also excludes the reaction between magnesium or rare earth in molten iron and moisture in sand mold. It is suspected that the burden and inoculants may bring aluminum and titanium into the molten iron. Because aluminum and titanium react with water to release atomic state [H] which is easily dissolved in molten iron. When the layer solidifies, the solubility of hydrogen decreases, and it precipitates in molecular gas phase and grows into hydrogen bubbles. From the analysis report of ferrosilicon inoculant in this factory, it can be seen that the aluminum content reaches 1.86%, which may be the main cause of subcutaneous stomata. The aluminum content of ferrosilicon inoculant should be about 1.%, but not more than 1.5% at most. right