Semi enclosed castings are more common in casting products, and they are also one of the main parts in machinery, mainly used for load-bearing and accommodating purposes. The structural feature is the use of thin walls to enclose cavities of different shapes, with complex shapes, thin and uneven walls, and an internal cavity shape. Semi closed castings are commonly produced using sand casting technology, but due to the complexity of the manufacturing process, the need for sand cores, and high costs, they are gradually shifting towards lost foam casting. Worldwide, lost foam casting technology has been maturely applied in materials such as cast iron, cast steel, and aluminum alloy parts. Currently, technological innovation is rapidly developing in automotive parts, aerospace parts, and alloying production. Lost foam casting is suitable for producing structurally complex parts, with the advantages of small batch production, simplified processes, and significant environmental improvement. The production process does not involve parting or demoulding, and there are no complex processes such as sand mixing or core making. It is a precise casting process that simplifies the production process, shortens the process flow, improves production efficiency, and reduces labor intensity. Its advantages are obvious, so a certain company adopts the lost foam production process for such castings. This article explores and practices the lost foam casting process technology for a type of semi enclosed casting produced by a certain company, focusing on the mold making and casting processes.
The internal cavity of semi enclosed castings has uneven wall thickness, and it is difficult to form a vacuum degree inside the mold cavity when using lost foam casting. If the negative pressure does not meet the standard, it is easy to cause box collapse and expansion defects. The four key technical points of model, coating, modeling, and process methods are indispensable. Improper foaming density, particle size, curing time, and drying temperature of the model can easily lead to deformation of the pattern, size deviation, and poor appearance quality defects; Poor coating strength, breathability, fire resistance, and thickness can easily lead to sand sticking, surface porosity, and slag hole defects in castings; The design is not compact enough and the vacuum negative pressure control is insufficient, which can easily cause the box to collapse or expand due to unevenness; Poor process methods make it difficult to successfully cast castings and prone to defects such as shrinkage, porosity, porosity, and slag inclusion, which run through the entire production process. At present, the qualification rate is only 40%. In order to improve the uniformity of negative pressure by addressing the problem of difficulty in forming vacuum inside semi enclosed castings and inadequate negative pressure, combined with the method of making internal negative pressure tubes based on the shape of castings, the yield and surface quality of castings can be increased.
1) The process principle is the lost foam casting process, which adopts a dual cycle separation line design layout on site. It mainly includes four parts: casting mold making process, casting model bonding and coating process, casting sand treatment, casting cleaning, electric furnace melting process, and machining process;
2) Foam plate with specific gravity of 10kg/m3 is selected for the slag box of this casting. The slag box body and sprue are manually molded, and the cutting model is made without any gaps or pits. For size deviations, burn marks, poor local continuity, damage, etc. during cutting, trimming burrs can be repaired with a repair knife, and concave areas can be repaired with adhesive coating. After the model is formed, it is dried to reduce the moisture content of the mold to below 0.8%, and the casting system is used;
3) Water based coatings are selected for this type of casting, and the ingredients are mixed according to the required ratio of the process. After starting the machine and stirring for a certain period of time, testing is carried out to determine the viscosity. After meeting the requirements, the body is coated three times, and the casting system is coated four times. The position is changed during each baking to prevent deformation.
4) Production selects raw materials such as carbon steel heads, carbon steel plates, and heads, and strictly controls their composition. Priority should be given to adding clean furnace materials, preheating the furnace materials with low power, gradually adding large materials for high-power melting, and adding slag making materials during the process to cover the molten steel with slag and prevent oxidation. Before the molten steel is completely melted and cast, the slag in the furnace must be cleaned up, and samples must be taken and analyzed according to the requirements. After adjustment and qualification, it can be cast out of the furnace.
5) According to the principle of one box, one piece, the box is buried and poured. The placement of the yellow mold in the sand box is determined based on the size of the sand box and the size of the yellow mold model, as shown in Figure 3. Firstly, lay the bottom sand to fix the model. After the model is fixed, clamp the sand box on the vibration table for bottom vibration first, and then add sand while compacting. Stop adding sand when the molding sand reaches the middle position of the mold cavity, vibrate and compact it, and then continue adding sand. Stop adding sand when the molding sand covers about 80mm above the top, with a total vibration time of not less than 600s and a vibration frequency of 40-50 Hz.
6) The temperature of the molten steel after discharge is controlled at around 1650 ℃. After the molten steel is discharged and poured, it must be allowed to settle for 2-3 minutes before discharging again. The pouring temperature is 1550-1600 ℃, and the pouring time is controlled within 5 minutes. After the casting is poured, ensure that each box is pressurized for 10 minutes.
Adopting the lost foam casting process to produce semi enclosed castings requires strict quality control for each process step. So, the occurrence of casting defects can occur throughout the entire process, and if a detail is not controlled, the final result will be the scrapping of the casting. In the production of lost foam casting, the role of negative pressure throughout the casting process includes: ① compacting dry sand to prevent mold punching, mold collapse, and mold wall movement; ② Accelerating the exhaust speed, reducing the interface pressure, accelerating the metal front speed, and improving the filling ability are beneficial for reducing surface defects Make the contour of the casting clearer; ④ Casting under sealed conditions to improve the working environment. The magnitude of negative pressure for semi enclosed castings is particularly important and is the key to the success of lost foam casting; Due to the hollow structure of the casting, the internal cavity relies solely on the negative pressure at the bottom and four sides of the sand box, which cannot achieve the required negative pressure degree. This can easily result in insufficient compaction of the sand in the cavity, making it unable to resist the erosion and buoyancy of the metal liquid. Tilt the casting at a certain angle for box molding to ensure that the molding sand is easy to fill, compact and effective; The degree of inclination needs to be verified through testing. When inclined at 21 degrees, the amount of sand consumed in the upper part is insufficient and does not meet the conditions for burying the box; When tilted at 30 °, the bottom position of the casting is not conducive to the natural flow of molding sand, and the space for manual operation is small. After casting, this position will form bulging and bulging defects; When tilted at 45 °, the sand flows evenly and the gaps in each part are consistent, which is also conducive to manual operation and stable casting; When tilted at 60 °, the gap between the casting and the lower side wall is small, which is not conducive to the flow of molding sand and uneven compaction.
By summarizing and analyzing the defects that occurred during the lost foam casting process of semi enclosed parts, taking measures and tracking improvements, it is concluded that
Conclusion drawn:
1) Tilt the upper surface of the semi closed component at a 45 ° angle for box embedding, which is conducive to filling and compacting the molding sand;
2) Create an external negative pressure tube and connect it to the main negative pressure tube using a throat clamp. During casting, both the internal and external negative pressures are opened to ensure a balanced negative pressure inside and outside the mold cavity, thereby improving the casting qualification rate.