SAND CASTING EQUIPMENT.

The valve body castings produced by Xinyuanzhu Group are used for loaders and other construction machinery. They mainly include distribution valves, flow amplification valves, and fork-lift valves. They are all made of HT250, weigh between 16 and 52 kg, and have an annual output of about 50,000 pieces. Production process: The sand core is a coated sand shot core, and the sand mold is a furan resin self-hardening sand model, which is smelted in an intermediate frequency furnace. The overall rejection rate is about 13%. When the rejection rate of individual castings is high, it can reach more than 20%, and 85% of the defects are pores. Stomatal defects are mainly located at the root of the outlet riser. The holes are relatively large, and it is easy to find after removing the outlet riser. Some are found under the skin of the casting, and some are found in the inner cavity of the casting, which need to be examined with an endoscope.
The quality of castings mainly depends on the production process of casting. If the process quality is controlled properly, the probability of casting defects will be greatly reduced. When casting defects occur, we mainly look for problems in the main production processes such as core making, modeling, smelting, pouring, etc., and analyze the influencing factors such as man, machine, material, method, ring, and measurement one by one to find a breakthrough to solve the problem .
1. Control measures to prevent air holes in the valve body during the core making process
1.1 Gas generation volume of coated sand
Under normal circumstances, when producing valve body castings, the smaller the amount of gas generated by the coated sand, the better, but the lower the amount of gas generated by the coated sand, the lower the strength of the sand core. Coated sand with good properties in both aspects is more expensive, and for mass production, the cost is higher. Based on many years of production experience, Xinyuanzhu Group generally uses coated sand with gas generation ≤20 ml/g and tensile strength ≥3.0MPa, which can basically meet the quality requirements of conventional valve body castings.
1.2 Baking of sand core
In order to reduce the porosity of castings, xinyuanzhu group has made strict requirements on the baking of coated sand cores. After the core shooting is completed, the sand cores are pre-baked before coating to make the thick parts of the sand cores inside The coated sand is completely solidified, and the generated gas can be emitted. After pre-baking, the sand core is trimmed and coated, and the coating is allowed to dry naturally. The air-dried sand cores are then baked. The baking process is to keep the temperature at 170℃～200℃ for 1.5～2.5 hours (1.5 hours for thin cores and 2.5 hours for thick cores), and then air-cooled out of the furnace. After the sand core is dried, it has not been used for more than 48 hours under normal weather conditions, and it must be dried back before use. Sand cores that have not been used for more than 24 hours in southerly weather must also be back-baked before use. The back-bake temperature is 170°C to 200°C, and the heat preservation is about 1 hour to prevent the sand core from absorbing moisture due to excessive storage time and causing porosity in the casting.
1.3 Exhaust of sand core
As we all know, the venting capacity of the sand core has a great influence on the occurrence of porosity in the casting. The gas emitted by the sand core is easily discharged, and the probability of porosity defects in the casting is small. For many years, Xinyuanzhu Group has used drills to drill the center or thick parts of the sand core of the valve body to improve the exhaust ability of the sand core. However, due to the long length of the sand core, even if an extended drill bit is used, the vent holes at both ends of the sand core cannot be formed through holes. In this way, the exhaust ability of the sand core is greatly reduced, and when the sand core is coated (dip coating method), the coating is easy to accumulate in the exhaust hole and cause the exhaust hole to be blocked. In order to reduce the impact of this problem on product quality, we use thin iron rods to dredge the vent holes of each valve body sand core before the core distribution box, but this increases the labor intensity of the workers and the work efficiency is also low. Later, we added a vent needle to the core box mold, that is, put the vent needle in the left or right half of the mold before closing the mold. After the sand is shot, the vent needle is contained in the center of the sand core. After the core is demolded The air outlet needle is brought out with the sand core, and then the air needle is pulled out to form an air outlet. The length of the designed vent needle is the same as the length of the sand core, and the formed vent hole runs through the center of the entire sand core, so that the venting capacity of the sand core is greatly enhanced. After comparison, the incidence of porosity in castings after the sand core is increased by the air outlet needle is reduced by nearly 60% compared with that before the increase. We believe that it is very important whether the vent holes at both ends of the sand core are formed with through holes, which should be met when designing and manufacturing the mold.
In addition, the following issues should be paid attention to when designing and using the vent needle:
1.31 The diameter of the air outlet needle is determined according to the diameter of the sand core. It is required to ensure the overall strength of the sand core. The larger the vent hole, the better. Avoid the excessively high waste rate of the sand core during the manufacturing process due to the too thin wall thickness of the sand core or the molten iron penetrates the sand core into the vent hole during pouring, causing iron formation in the inner cavity.
1.32 The sand core after adding the air outlet needle. For castings with high inner cavity size requirements, the castings should be dissected for size verification to check whether the sand core is deformed after pouring.
1.4 Requirements for the order of use of sand cores
Based on many years of production experience, Xinyuanzhu Group has made clear requirements on the order of use of sand cores, requiring the sand cores manufactured first to be used first. Mainly considering the use of water-based coatings and the use of viscose for core assembly. If the coating is placed for a long time, the moisture of the viscose and coating is easy to volatilize, and the gas generated when the sand core is used is reduced. Generally, it is required to put the coating on the sand core for at least 3 days before baking. In March 2009, there was a quality accident in our factory. Because the operator violated the operating rules, the sand cores that were assembled and coated on the same day were used in circulation on the same day, resulting in batches of castings that were scrapped. The scrap rate was as high as 32% that month. In the production process, in order to better distinguish which sand cores are produced first and which sand cores are produced later, the operator is required to write the production date with chalk on the core head part of the sand core or the tooling rack. The production date on the sand core is selected for baking according to the principle of first production, first use.
2. Control measures to prevent air holes in the valve body during the modeling process
After analyzing the pores of the casting, most of the pores produced by Valve body casting are intrusive pores, and the shape is mostly round or oblate. The principle of intrusion porosity is that due to the heat between the molten metal and the mold during pouring, the moisture in the mold evaporates, the combustion or volatilization of organic matter, and the chemical reaction between the molten metal and the mold occurs, etc., on the interface between the molten metal and the mold. Produce a lot of gas. When the gas exiting the sand mold is slower than the gas generation rate, the gas accumulates in the cavity to generate pressure. After the pressure reaches a certain value, the gas will invade the molten metal to form bubbles, and form pores after cooling.
There are two main factors affecting the porosity of castings during the molding process: one is the rationality of the process design and whether the sand mold exhausts smoothly; the second is the drying degree of the sand mold.
In terms of process design, a gas riser is designed on the casting body, and a gas hole is designed on the core head, so that the gas in the cavity can be smoothly discharged. The other is to strengthen the baking of the sand molds. Our factory uses a through-type drying furnace to dry the sand molds at a temperature of 200 to 250 ℃. The sand molds are required to stay in the furnace for no less than 15 minutes. Because the coating is sprayed, the coating is easy to accumulate in the outlet riser. When the outlet riser is deep, the inside of the outlet riser is not easy to dry. When the iron liquid rises to the outlet riser, body fluid encounters unbaked The dry paint will react violently, and sometimes the molten iron will spray out from the gas outlet riser, and sometimes even fry the box. After tracking, if this phenomenon occurs during pouring, most of the castings will have pores. Later we made improvements:
2.1 Before the paint is sprayed, use a wooden stopper to plug the air outlet riser to prevent the paint from flowing into the air riser during spray coating and causing the paint to accumulate or dry out.
2.2 Check the inner cavity of the sand mold before setting the core. If it is found that the deep concave part has not been dried, use a diesel blowtorch to repair it, and then set the core after drying.
2.3 After the core setting is completed, it is required to use a diesel torch to bake the upper and lower models again. The baking time is generally about 30 seconds. After the implementation of the above measures, the phenomenon of molten iron spraying during pouring is almost eliminated, and the porosity of the casting is also significantly reduced.
3. Control measures to prevent pores in the valve body during smelting and pouring processes
3.1 Control of smelting
In order to effectively reduce the generation of pores, xinyuanzhu group mainly controls the quality of raw materials and smelting process in the smelting process. The raw material scrap steel, pig iron, alloy, etc. must be dry, non-moist, and the scrap steel and pig iron must be free from corrosion and other inclusions. A large amount of slag was generated during smelting due to the inclusion of aluminum alloy and other inclusions in the briquette steel. Although the slag was removed at that time, there were still a lot of pores in the casting, and the pores were small. , Most of them are revealed after processing. In terms of smelting process control, the smelting temperature is generally required to be between 1480 and 1500 ℃, and it is stipulated that the iron can be tapped after all melting is allowed to stand for at least 15 minutes, so that the slag in the molten iron can precipitate and float.
3.2 Pouring control
The pouring process mainly controls the pouring temperature. When pouring valve body castings, the pouring temperature is required to be 1330～1390℃. According to the field follow-up test, the liquidity of the molten iron is poor when a ladle of molten iron is poured until the temperature is lower than 1320℃. The molten iron cannot even be poured during pouring. Out of the gas riser, in this case, the porosity defects of the casting increase significantly.
In order to improve this problem, the pouring temperature was appropriately increased and controlled between 1375 and 1405°C. As in the past, one ladle of molten iron can be used to pour all valve body castings. Later, according to the temperature of the molten iron, the caster is required to first pour the valve body castings for each ladle of molten iron, and then pour other unimportant castings with cold molten iron. It is stipulated that the castings poured in the last box of each ladle cannot be valve body castings. After this is implemented, the phenomenon that the gas outlet riser cannot be taken up due to the low temperature of the molten iron is gone, and the porosity of the casting is also significantly reduced.
In addition, the drying degree of the ladle is also very important, and the ladle that has not been thoroughly dried can not be used. If some qualified manufacturers require that the ladle must be used to pass the ladle with molten iron, the effect will be better.
4. Conclusion
The sand core has a large amount of gas generation, does not dry out, poor exhaust ability of the sand core, unreasonable process design, poor quality of molten iron, and low pouring temperature are the main factors that cause porosity in castings. Xinyuanzhu Group improves the above influencing factors, pays attention to the process and pays attention to details, and can effectively reduce the porosity of castings. The casting process is a very complex production process. From core making, styling, core assembly to melting and pouring, each process must ensure the quality of the process, as long as there is a quality problem in a process, it will cause defects in the casting. Scrap, no matter how good the quality of other processes is, it is futile. In casting quality management, the quality of each process must be grasped together to ensure that good castings can be obtained.