GIS large-size tank composite casting mold low-pressure casting device and method

Abstract

The invention belongs to the technical field of low-pressure casting, and particularly relates to a GIS large-size tank composite casting mold low-pressure casting device and method.The GIS large-size tank is formed by connecting three cylindrical tank bodies, a plurality of bosses are arranged on the two sides of each cylindrical tank body, and flanges are designed on the top, the bottom and the end face of each cylindrical tank body according to requirements; a hemispherical part is arranged at the rear end of the tank body, the whole part in the tank body is formed by an inner cavity sand core, composite forming is conducted on the outer surface of the tank body in the mode that a metal outer mold system, an outer mold sand mold assembly and an outer mold metal movable insert assembly are combined, the proportion of a metal mold forming face is 70%, the surface precision of a casting is improved, internal defects are effectively controlled, and the casting quality is improved. The production efficiency and the product percent of pass are improved, and the production cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of low-pressure casting technology for large-size aluminum alloy structural parts, specifically relating to a low-pressure casting device and method for composite molds of GIS large-size tanks. It is particularly suitable for producing thin-walled cavity castings with high precision, high density, and zero leakage requirements through composite casting of metal outer mold, sand mold, and metal inserts. Background Technology

[0002] Large-size aluminum alloy shell structural components are widely used in aerospace, shipbuilding, and power equipment industries due to their lightweight, high strength and toughness, and excellent corrosion resistance. In the power equipment sector, GIS (Gas Insulated Metal Enclosed Switchgear) is a core high-voltage power distribution device in power systems. Its tank components, as key parts for sealing, insulating, and supporting internal conductive elements, directly determine the reliability, safety, and service life of the GIS equipment. Large GIS tanks, designed to meet the requirements of ultra-high voltage (110kV and above) or extra-high voltage (500kV and above) power systems, exhibit significant unique characteristics in structural design, material selection, and manufacturing processes.

[0003] Currently, the production methods for GIS tanks include casting, welding, and spinning. Among these, welding is highly adaptable and can be applied to the forming of tanks of various shapes and structures, with high material utilization. However, it has many welds (longitudinal and circumferential), resulting in insufficient sealing performance. Furthermore, it is difficult to control deformation after welding and there is residual stress, which increases production costs. Spinning, on the other hand, has extremely high dimensional accuracy and good inner wall surface quality, making it particularly advantageous for thin-walled designs. However, it is only suitable for cylindrical structures, and complex structures within these structures require welding. Moreover, it involves high equipment investment and its production efficiency needs to be improved.

[0004] In the production of GIS tank castings, the resulting castings have excellent airtightness due to their integral molding and lack of weld seams. They also have good structural integrity and strong impact resistance, making them suitable for integrated designs with complex components (flanges, lugs), and therefore have a very promising future.

[0005] However, the traditional sand casting gravity or sand casting low-pressure casting processes for GIS tanks still have certain problems. Sand casting is more flexible in design and suitable for castings with complex cavities, but due to its high surface roughness (Ra>12.5μm) and low dimensional accuracy (usually CT9~CT11 grade), the castings have poor airtightness and cannot meet the zero-leakage requirements of SF6 equipment. While pure metal mold casting can accelerate the solidification rate, thereby refining the grain structure and improving the surface quality, it is limited by the difficulty of demolding complex internal cavities, especially for deep cavity / negative angle structures, and is difficult to apply to castings with complex cavities.

[0006] Furthermore, GIS tank castings, characterized by thin walls and large cavities, are typical high-difficulty parts in casting production. Their height and length are large, and the flow of molten aluminum is extremely long. If the gating system is not designed properly, it is very easy for it to solidify prematurely due to excessive temperature drop, resulting in incomplete filling or cold shuts. This leads to a chain reaction of defects such as porosity, shrinkage, deformation, and inclusions, which seriously restricts the reliability of the tank and the batch pass rate (industry average <85%). In severe cases, the scrap rate can be as high as 30-50%. Summary of the Invention

[0007] This invention addresses the stringent requirements for surface density and dimensional accuracy in large-size GIS tank castings by proposing a composite casting scheme with a metal outer mold as the main component. The core breakthrough lies in minimizing the non-metallic forming area: through analysis of actual production processes, a universal composite forming method was obtained, increasing the proportion of the metal outer mold forming area to over 70%, significantly enhancing the solidification density and dimensional stability of the casting's outer surface.

[0008] The complete technical solution of this invention includes:

[0009] A low-pressure casting device for composite molds of large-size GIS tanks.

[0010] The large-size GIS tank is composed of three cylindrical sub-tanks connected together. Each cylindrical sub-tank has multiple bosses on both sides, and each cylindrical sub-tank has flanges on its top, bottom, and end faces, including side flanges, a top flange, a first bottom flange, and a second bottom flange. The rear end of each cylindrical sub-tank has a hemispherical section.

[0011] The main body wall thickness of the GIS large-size tank shall not exceed 15mm, the wall thickness at the flange shall not exceed 35mm, and the wall thickness at the boss shall not exceed 50mm; the lateral distance between the front and rear ends of the GIS large-size tank shall not be less than 1650mm.

[0012] The low-pressure casting device uses a composite mold forming process. The mold structure includes a metal outer mold system, an outer mold sand mold assembly, an inner cavity sand core, and an outer mold metal movable insert assembly.

[0013] The inner surface of the large-size GIS tank is entirely formed by inner cavity sand core molding, while the outer surface of the large-size GIS tank is composite-molded using a combination of a metal outer mold system, an outer mold sand mold component, and an outer mold metal movable insert component.

[0014] The low-pressure casting device includes a control system, which designs the composite molding method for the outer surface of the GIS large-size tank based on the structural characteristics of each component on the GIS large-size tank.

[0015] The structural features include: the continuous distribution area of ​​the difficult-to-demold blocks, the minimum distance between adjacent difficult-to-demold blocks, the maximum distance between adjacent difficult-to-demold blocks, the sum of the projected areas of all difficult-to-demold blocks on the vertical plane of the demolding direction, and the area ratio of the difficult-to-demold blocks.

[0016] Furthermore, the outer mold sand mold assembly includes an upper sand mold and a lower sand mold. The top flange is formed using the upper sand mold, while the bottom first flange and bottom second flange are formed using the lower sand mold.

[0017] Furthermore, the outer mold metal movable insert assembly includes multiple metal movable inserts, which are machined from H13 steel.

[0018] Furthermore, the metal outer mold system adopts a segmented combination design, including a bottom mold that provides the bottom contour forming of the casting, 2-4 hydraulically driven side molds that adapt to the part picking trajectory of the low-pressure casting device in the direction of movement, and a top mold.

[0019] Furthermore, a gating gate is arranged below the center of the large GIS tank. The molten aluminum rising from the gating gate flows through two branch gating channels to the horizontal gating channels on both sides. The horizontal gating channels distribute the molten aluminum to the bottom of three cylindrical sub-tanks.

[0020] Furthermore, after the molten aluminum is diverted to the bottom of the three cylindrical sub-tanks, it rises and enters the mold cavity through the ingate module.

[0021] Furthermore, at the connection point of the three cylindrical sub-tanks, a space is reserved on the inner cavity sand core for attaching metal chills.

[0022] Furthermore, a low-pressure casting method for large-size GIS tanks using the aforementioned low-pressure casting apparatus includes the following steps:

[0023] 1) Liquid rising stage: The molten aluminum in the holding furnace is transported to the gate through the liquid rising pipe under the pressure of compressed air;

[0024] 2) Filling stage: The filling pressure is dynamically adjusted according to the height of the molten aluminum to complete the filling of the cavity;

[0025] 3) Shelling stage: Increase the compressed air pressure to cause the molten aluminum to solidify and form a shell on the forming surface;

[0026] 4) Pressurization stage: Continue to increase the compressed air pressure. The pressure acts on the front of the aluminum liquid solidification interface to increase the material density.

[0027] 5) Holding pressure stage: Maintain compressed air pressure to ensure solidification from the far end of the casting to the gate, thus achieving feeding of the casting;

[0028] 6) Decompression stage: Release the compressed air pressure in the holding furnace to atmospheric pressure, and the residual aluminum liquid in the gating channel and riser pipe will automatically flow back under the action of gravity.

[0029] The beneficial effects of this invention are as follows:

[0030] 1. A reasonable method for dividing the molding area in composite molding is proposed, taking into account the coupling of demolding difficulty, spatial distribution and size factors, and determining the reasonable application range of the metal outer mold system, outer mold sand mold component and outer mold metal movable insert component. This method systematically solves the problems of mold design and demolding difficulty for complex multi-cavity castings such as triple cans.

[0031] 2. Based on the design results of the above method, local external mold sand molds or metal movable inserts are introduced only in complex structures with high demolding difficulty (such as deep cavities and negative angles), so that the forming surface of the metal external mold accounts for 70%, which is 40% higher than the traditional composite process. The flatness error of the metal external mold forming is ≤0.1mm / m (GB / T1184), and the shrinkage area ratio detected by X-ray is ≤0.02% (EN1369 standard).

[0032] 3. Addressing the challenge of coexisting thin-walled and thick-walled structures, the gating system is creatively integrated into the tank cavity. Internal gates are directly opened in thicker areas such as the flange neck to achieve directional feeding (replacing the chiller solution). Simultaneously, runners are arranged in layers along the tank height to ensure complete filling of ultra-thin-walled sections (≥6mm). The mold structure employs a modular metal outer mold system highly adaptable to low-pressure equipment, consisting of a top mold (which can be omitted based on top surface geometry), a bottom mold, and 2-4 hydraulically driven side molds. This ensures precise closure of the segmented metal cavity while achieving automatic mold opening and closing, increasing production cycle efficiency by over 50%.

[0033] 4. During the filling stage, a dynamic filling method is adopted, which adjusts the filling pressure and speed in real time based on the real-time cross-sectional area change during the rise of the aluminum liquid. This avoids the problems of drastic changes in the flow velocity of aluminum liquid that are prone to occur during the filling process of large-sized castings with complex internal cavity shapes and abrupt changes in cross-sectional area, which can lead to air entrapment, porosity, oxide inclusions, and cold shuts. Attached Figure Description

[0034] Figure 1 This is an exploded view of the composite mold low-pressure casting device provided by the present invention.

[0035] Figure 2 This is a diagram of a large-size GIS tank casting and its gating system.

[0036] Figure 3 for Figure 2 The bottom view.

[0037] Figure 4This is a schematic diagram of the metal insert molding process.

[0038] Figure 5 This is a schematic diagram of the ingate module.

[0039] In the diagram, 1-bottom mold, 2-top mold, 3-side mold, 4-upper sand mold, 5-inner cavity sand core, 6-lower sand mold, 7-metal chill, 8-riser, 9-gate, 10-sprue, 11-runner, 12-ingate module, 13-casting, 14-boob, 15-side flange, 16-metal insert, 17-first flange ingate unit, 18-second flange ingate unit, 19-third flange ingate unit, 20-fourth flange ingate unit, 21-upper secondary ingate, 22-lower secondary ingate. Detailed Implementation

[0040] The present invention will now be described in detail with reference to embodiments and accompanying drawings. However, it should be understood that the embodiments and drawings are for illustrative purposes only and do not constitute any limitation on the scope of protection of the present invention. All reasonable modifications and combinations included within the inventive spirit of the present invention fall within the scope of protection of the present invention.

[0041] A low-pressure casting device for large-size GIS tank composite molds, such as Figure 1 As shown, the main structure of the tank involved in this invention consists of three cylindrical sub-tanks connected together, also known as a triple-tank assembly, with overall dimensions of 1650mm × 1110mm × 640mm and a weight of 230kg. The main body wall thickness is approximately 13mm. The tank body has bosses 14 of varying sizes and numbers on both sides. Each cylindrical sub-tank has flanges designed at its top, bottom, and end faces, including side flanges 15, a top flange, a first bottom flange, and a second bottom flange. The rear end of the tank also has a near-hemispherical section. The wall thickness at the flanges is 20mm~35mm, and the wall thickness at the bosses is 25mm~50mm. Furthermore, the thickness increases at the connection points of the cylindrical sub-tanks.

[0042] The present invention discloses a low-pressure casting device for composite mold of large-size GIS tank. In terms of mold composition, a composite molding method is adopted. The mold structure includes a metal outer mold system, an outer mold sand mold assembly, an inner cavity sand core, and an outer mold metal movable insert assembly.

[0043] The entire inner surface of the large-size GIS tank is formed by the inner cavity sand core 5, preferably by one-time forming of a high-strength resin sand core.

[0044] For the outer surface of the large-size GIS tank, a composite molding process is used, consisting of a metal outer mold system, an outer mold sand mold assembly, and an outer mold metal movable insert assembly. For the outer wall of the casting, the molding area is subdivided based on demolding feasibility. The average local wall thickness (the ratio of its volume to its opening area) is obtained for each surface component type, including bosses, grooves, and flanges. For bosses, grooves, and flanges, their maximum depth along the demolding direction is obtained (for bosses and flanges, this is the distance from their outer surface to the main outer surface of the large-size GIS tank; for grooves, it is the distance from their deepest inner surface to the main outer surface of the tank), as well as their opening width. This opening corresponds to the outer surface of the boss and flange, and for the groove, it corresponds to the deepest inner surface. For circular openings, the opening width is the diameter of the outer or inner surface; for irregularly shaped openings, the opening width is... , where S is the area of ​​the opening, and is its projected area on the vertical plane in the demolding direction (for bosses and flanges, it is the projected area of ​​the outer surface on the vertical plane in the demolding direction; for grooves, it is the projected area of ​​the inner surface on the vertical plane in the demolding direction).

[0045] The surface of the casting is divided into a difficult-to-demold area and an easy-to-demold area. The easy-to-demold area needs to meet the following two conditions:

[0046]

[0047]

[0048] in The depth-to-width ratio is the ratio of the maximum depth of the flange or groove along the demolding direction to the width of the opening. This is the difference between the local average wall thickness and the main wall thickness of the casting (13mm). Conversely, it indicates a difficult-to-demold area.

[0049] The above parameters take into account important factors affecting the demolding process. Regarding wall thickness differences, the shrinkage of thicker walls during solidification is less than that of thinner walls, creating tensile stress within the casting. This, along with warpage, causes the thicker walls to adhere tightly to the mold cavity, forming a wedge-shaped jam and increasing demolding resistance. Furthermore, grooves or deep cavities with excessively large depth-to-width ratios can form a structure similar to an undercut. Therefore, based on actual production data analysis, thresholds for the above parameters are set, and areas meeting these conditions are defined as difficult-to-demold zones and easy-to-demold zones.

[0050] Based on the above definitions, the metal outer mold forming area, sand mold forming area, and metal movable insert forming area on the surface of the casting are determined.

[0051] Areas formed using metal molds must meet the following requirements:

[0052]

[0053]

[0054] The continuous distribution area of ​​the difficult-to-demold block, i.e. or The continuous distribution area of ​​the region. This is the minimum distance between two adjacent, difficult-to-demold blocks.

[0055] Areas using sand molding must meet the following requirements:

[0056]

[0057]

[0058]

[0059]

[0060] This represents the maximum distance between two adjacent, difficult-to-demold blocks. It is the sum of the projected areas of all difficult-to-demold blocks within the sand mold forming area on the vertical plane in the demolding direction. This represents the area percentage of the difficult-to-demold block within the sand mold forming area.

[0061] Based on the above principles, the metal outer mold forming area and the sand mold forming area are determined. For the remaining isolated and difficult-to-demold blocks, the metal movable insert method is used for forming.

[0062] The calculation module, which designs the composite molding method of the outer surface based on the structural characteristics of each component on the large-size GIS tank, is deployed in the control system of the composite casting low-pressure casting device of this invention. Before casting, the specific composite molding method can be obtained by inputting the three-dimensional model of the casting for measurement and calculation.

[0063] The above-mentioned molding area division is based on demolding feasibility, and fully considers factors such as demolding difficulty (γ, Spatial distribution factors , , ) and size factor ( This approach utilizes multi-dimensional collaborative constraints to determine the appropriate application range of metal outer molds, sand molds, and metal movable inserts. It systematically solves the problems of mold design and demolding difficulty for complex multi-cavity castings such as large-size GIS tanks. It shifts from traditional experience-based design to a self-controlled design approach, defining critical thresholds for key parameters and employing multi-parameter collaborative constraints to avoid the limitations of single-parameter judgments. For large-area continuous areas difficult to demold, sand molding is prioritized due to its non-structural interference and strong collapse resistance, avoiding the rigid clamping of the metal mold. For isolated, small-sized features difficult to demold, metal movable inserts are used, employing a step-by-step demolding method where the outer mold is removed first, followed by the insert, solving the demolding difficulty of isolated features and avoiding the insufficient precision of sand molding. For easily demolded areas, metal outer molds are used.

[0064] Existing composite processes attempt to combine the advantages of metal molds and sand cores, but their metal molds are mainly limited to the bottom contour of the casting and the liquid lifting system (covering only about 10% to 30% of the outer surface). The remaining outer walls still rely on sand core forming, resulting in dimensional deviations (above ±0.8mm) at key assembly interfaces due to sand mold shrinkage fluctuations. Furthermore, micro-shrinkage porosity frequently occurs in the thickness transition areas due to insufficient solidification control. This invention increases the metal forming area ratio to over 70%, a 40% improvement over traditional composite processes, and adapts to different precision requirements. It avoids over-design and complex core-pulling mechanisms (such as hydraulic core pulling and mechanical core pulling), significantly shortening the mold design and manufacturing cycle. Moreover, it is highly versatile. When the casting structure is appropriately adjusted (such as modifying the number of bosses, flange thickness, cavity dimensions, etc.), only the parameter values ​​need to be re-submitted to quickly determine the new forming and demolding methods, without the need to reconstruct the mold design.

[0065] Based on the above principles and considering the structural features of the tank body of this invention, the top flange is formed using an upper sand mold 4, and the two bottom flanges are formed using a lower sand mold 6. These sand molds simultaneously form the thicker wall sections of the connecting parts between the tank bodies. The upper and lower sand molds are prefabricated together with the sand cores used for forming the inner cavity of the casting. For the side bosses, those meeting design requirements are formed using metal movable inserts, while the remaining parts are formed using metal outer molds, improving the surface accuracy of the casting, with a roughness Ra ≤ 6.3 μm. The embedded metal movable inserts 16 are machined from H13 steel and are reusable. Figure 4 As shown.

[0066] The metal outer mold adopts a segmented combination design, including a bottom mold 1 that provides a reference for the bottom contour forming of the casting, a top mold 2, and 2 to 4 hydraulically driven side molds 3 that are adapted to the improved equipment picking trajectory in the direction of movement. The top mold 2 can be selectively omitted depending on the top structure of the product.

[0067] like Figure 2 , Figure 3As shown, the gating system is as follows: the liquid riser is placed in the bottom mold, and the gating gate 9 is arranged in the center of the tank. The aluminum liquid rising in the gating gate 9 flows through two branch gating channels 11 to the horizontal gating channels 10 on both sides. The horizontal gating channels divide the aluminum liquid to the bottom of the three tanks. Then the aluminum liquid rises and enters the cavity through the inner gating module 12.

[0068] The inner gating section of the tank is circular to facilitate molding in resin sand core making, and the inner gating is designed according to the tank structure features and molding method of the present invention.

[0069] Structurally, the casting 13 of the present invention is a large thin-walled hollow casting, including three cylindrical sub-tanks. Each sub-tank includes four flange ends, namely two at the bottom, one at the top, and one at the front. The rear end of the sub-tank also has a near-hemispherical part. The lateral distance between the front and rear ends of the tank is extremely large, and the length of the tank reaches 1650mm.

[0070] The top flange is formed by upper sand mold 4, the two bottom flanges are formed by lower sand mold 6, and the cylindrical main body of the tank is formed by metal outer mold.

[0071] Regarding the specific structure of the gating system, the inventor's prior patent application CN111673072B discloses a wheel forming device and method based on multi-lifting liquid pipe central pressurization. This method uses multiple lifting liquid pipes and gates for filling, solving the problems of long filling distances and difficulties in forming and feeding of aluminum alloy wheel castings. However, the aluminum alloy shell casting involved in this invention has a larger overall size and a more complex internal structure, and cannot use an all-metal mold structure. It requires the formation of a composite internal cavity structure using sand cores. Therefore, the aforementioned prior application's gating method cannot be used. Therefore, this invention designs an ingate module composed of multiple ingates, which branches out to each flange end where the solidification speed is slower. After reaching the flange end, the solidification speed is slower, ensuring continuous flow. Meanwhile, the tank body at the far end is formed by a metal outer mold, resulting in faster cooling and solidification, ensuring sequential solidification from the far end to the ingate.

[0072] As described in the setting of the inner gating module 12, Figure 5As shown, the system includes a primary ingate and a primary ingate. Approaching the relevant flanges, it connects directly to the four flange openings via a first flange ingate unit 17, a second flange ingate unit 18, a third flange ingate unit 19, and a fourth flange ingate unit 20. Each flange ingate unit includes multiple secondary flange ingates that branch off to the inside of the flange. For the hemispherical section at the rear of the tank, due to its distance from adjacent flange ingate units and its relatively large lateral distance, an upper secondary ingate 21 and a lower secondary ingate 22 are provided for filling to avoid cold shuts. The diameter of the primary ingate is 40mm~50mm, and the diameter of the secondary ingate is 30mm~40mm.

[0073] The ingate module is completely built into the casting cavity to avoid interference with the outer surface; the feeding channel goes directly to the thick part: the diameter of the ingate opened in the flange cavity area is Φ30~60mm, which forms a directional feeding path with the gating system.

[0074] In addition, since the wall thickness increases at the connection point of the three tanks, hot spots are easily generated. A space can be reserved at the corresponding sand core position to attach metal chills 7. If the thickness of the boss is less than twice the thickness of the main tank wall, a space can be reserved on the inner side of the boss at the corresponding sand core position to attach metal chills, with the metal chills covering the thick area. If the thickness of the boss is greater than twice the thickness of the main tank wall, a space for risers 8 can be machined at the corresponding position of the metal mold on the outer side of the boss. To improve the feeding capacity of the risers, riser sleeves can be added. The pressure of the gating system with the inner gate and the connecting riser can be used to synergistically feed and eliminate shrinkage cavities and porosity.

[0075] The method for low-pressure casting of the large-size GIS tank of the present invention using the above-described apparatus includes the following steps:

[0076] 1) Liquid Lifting Stage: The goal of this stage is to smoothly deliver molten aluminum to the gate. This is achieved by increasing the pressure from atmospheric pressure to 220-250 mbar (15-25 mbar / s) within 10-15 seconds, thus ensuring a smooth rise of the molten aluminum within the riser tube and avoiding turbulence.

[0077] 2) Filling Stage: The goal is to completely fill the mold cavity. Starting from the end of the previous stage, the pressure platform continuously increases the pressure, causing the molten aluminum to fill the mold cavity.

[0078] In low-pressure casting, existing technologies typically employ a fixed pressure increase rate during filling. However, large-sized castings often have complex internal cavity shapes with multiple abrupt changes in cross-sectional area and relatively thin walls. As the molten aluminum rises, its flow velocity changes drastically at these abrupt changes in cross-section. A rapid, short-term increase in velocity can impact the cavity interior and sand core, easily causing air entrapment, porosity, sand holes, or rupture of the oxide film on the liquid surface, leading to oxide inclusions. Conversely, when the molten aluminum enters a cavity with a suddenly enlarged cross-section, the flow velocity drops rapidly, resulting in excessively slow flow and temperature decreases, potentially leading to cold shut defects.

[0079] To address the above problems, the equipment of this invention employs a dynamic control method for filling the mold through a control system. Based on the casting process conditions, a reasonable baseline filling pressure and rate of increase are first determined. Then, the filling pressure and rate of increase are dynamically adjusted according to the real-time changes in the cross-sectional area of ​​the molten aluminum during the actual filling process. Specifically, this includes:

[0080] First, based on process conditions such as pouring temperature, mold preheating temperature, and thermal properties of molten aluminum, simulation and experimental verification were conducted. Combined with the minimum wall thickness of the casting, the minimum flow rate of molten aluminum to avoid cold shuts and the maximum flow rate to prevent air entrapment were determined. Based on this, a baseline pressure ramp rate of 8 mbar / s to 12 mbar / s was obtained to ensure stable filling.

[0081] Subsequently, real-time dynamic adjustments are made during the filling process. This process is the core step to cope with sudden changes in the filling cross section and achieve flow stability. The sudden changes in the cross section are identified by the real-time height position of the molten aluminum, and the pressurization rate is dynamically adjusted.

[0082] Using a 3D CAD model of the casting, the casting is sliced ​​along its height and discretized into n height steps, preferably n = 50-120. The real-time equivalent cross-sectional area is obtained by dividing the volume within each step by its height, and a lookup table corresponding to the filling height h and the cavity cross-sectional area A is established. During the actual filling process, a reference filling pressure is initially used for increasing the filling speed. Simultaneously, the control system uses real-time h(t) (which can be obtained through multiple pre-installed non-contact liquid level sensors in the mold or through pressure feedback calculation) to interpolate from this table to obtain the real-time equivalent cross-sectional area A(h). A threshold for the area change rate is defined. =20%. The rate of change of the equivalent cross-sectional area between two adjacent step sizes (height intervals). , For the next step, the equivalent cross-sectional area, This is the equivalent cross-sectional area of ​​the current step size. When this threshold is exceeded, dynamic adjustments are made.

[0083]

[0084] To accelerate the pressure in the next long period, The pressure increase rate is set for the current step size. This is an adjustment coefficient, with a value range of 0.5–0.7.

[0085] The control module for real-time dynamic adjustment of the filling process is also deployed in the control system of the low-pressure casting device. During actual casting, the filling process is dynamically controlled based on the calculation results of the filling pressure rise rate.

[0086] The filling method of this invention, compared to the prior art, first predicts the shape of the cavity ahead through a pre-stored lookup table, and then combines this with real-time liquid level for feedforward-feedback composite control. This achieves a shift from passive execution to active adaptation, allowing pressure to be adjusted in advance before the molten aluminum reaches the abrupt change region, rather than afterward, greatly improving control accuracy.

[0087] Secondly, a balance between rapid and stable filling is achieved by setting a reasonable baseline filling pressure and rate of increase for initial filling. In areas with wide cross-sections, the pressure and rate of increase are dynamically adjusted to compensate for filling time, thereby optimizing the overall filling speed efficiency while ensuring no air entrapment or scouring. Furthermore, all parameters of the above-mentioned filling method have clear physical meanings and are directly related to the actual physical processes (solidification, flow resistance, cross-sectional changes), exhibiting strong predictability. They can be pre-calibrated and optimized through numerical simulation, reducing the cost and risk of on-site trial and error. Moreover, the combination of lookup tables and model adjustments achieves complete digitization of the filling process scheme. Once successfully debugged for a certain type of casting, the method can be quickly replicated in the development of similar new products, significantly improving the level of process standardization.

[0088] Through the above-mentioned filling method, under stable pressure, the aluminum liquid fills the entire mold cavity in a laminar flow manner. Precise filling speed control is used to obtain castings with clear contours and smooth surfaces, and to prevent casting defects.

[0089] 3) Shell formation stage: The pressure is gradually increased to 500mbar~550mbar over a period of 8s~10s at a rate of 2~3mbar / s. During this slow pressure increase, the molten aluminum solidifies and forms a shell on the forming surface of the inner cavity sand core, preventing the sand core from cracking due to excessive pressure, thus avoiding surface defects in the casting.

[0090] 4) Pressurization stage: To cope with solidification shrinkage, the pressure is further intensified. Within 10-15 seconds, the pressure jumps to 650-750 mbar above atmospheric pressure at a rate of 10-12 mbar / s. This pressure can effectively act on the solidification front and increase the density of the material.

[0091] 5) Holding pressure stage: To achieve sequential solidification, pressure needs to be maintained continuously. The system maintains a pressure of 650mbar~750mbar for 1100~1200 seconds to ensure sequential solidification from the far end of the casting to the gate, and to achieve sufficient feeding of the entire casting;

[0092] 6) Depressurization stage: After all processes are completed, the pressure inside the furnace is released to atmospheric pressure, and the residual aluminum liquid in the gating system and riser pipe automatically flows back under gravity, completing the entire cycle.

[0093] The foregoing has only described preferred embodiments of the present invention in detail and is not intended to limit the invention. Those skilled in the art will readily conceive of other embodiments of this disclosure upon considering the disclosure in the specification and embodiments. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and embodiments are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the claims.

Claims

1. A low-pressure casting device for composite molds of large-size GIS tanks, characterized in that, The large-size GIS tank is composed of three cylindrical sub-tanks connected together. Each cylindrical sub-tank has multiple bosses on both sides, and each cylindrical sub-tank has flanges on its top, bottom, and end faces, including side flanges, a top flange, a first bottom flange, and a second bottom flange. The rear end of each cylindrical sub-tank has a hemispherical section. The main body wall thickness of the GIS large-size tank shall not exceed 15mm, the wall thickness at the flange shall not exceed 35mm, and the wall thickness at the boss shall not exceed 50mm; the lateral distance between the front and rear ends of the GIS large-size tank shall not be less than 1650mm. The low-pressure casting device uses a composite mold forming process. The mold structure includes a metal outer mold system, an outer mold sand mold assembly, an inner cavity sand core, and an outer mold metal movable insert assembly. The inner surface of the large-size GIS tank is entirely formed by inner cavity sand core molding, while the outer surface of the large-size GIS tank is composite-molded using a combination of a metal outer mold system, an outer mold sand mold component, and an outer mold metal movable insert component. The low-pressure casting device includes a control system, which designs the composite molding method for the outer surface of the GIS large-size tank based on the structural characteristics of each component on the GIS large-size tank. The structural features include: the continuous distribution area of ​​the difficult-to-demold blocks, the minimum distance between adjacent difficult-to-demold blocks, the maximum distance between adjacent difficult-to-demold blocks, the sum of the projected areas of all difficult-to-demold blocks on the vertical plane of the demolding direction, and the area ratio of the difficult-to-demold blocks.

2. The low-pressure casting device for large-size GIS tank composite molds according to claim 1, characterized in that, The outer mold sand mold assembly includes an upper sand mold and a lower sand mold. The top flange is formed using the upper sand mold, while the bottom first flange and bottom second flange are formed using the lower sand mold.

3. The low-pressure casting device for large-size GIS tank composite molds according to claim 2, characterized in that, The outer mold metal movable insert assembly includes multiple metal movable inserts, which are machined from H13 steel.

4. The low-pressure casting device for large-size GIS tank composite molds according to claim 3, characterized in that, The metal outer mold system adopts a segmented combination design, including a bottom mold that provides the bottom contour forming of the casting, 2-4 hydraulically driven side molds that adapt to the part picking trajectory of the low-pressure casting device in the direction of movement, and a top mold.

5. The low-pressure casting device for large-size GIS tank composite molds according to claim 4, characterized in that, A gating gate is arranged below the center of the large GIS tank. The molten aluminum rising from the gating gate flows through two branch runners to the horizontal runners on both sides. The horizontal runners then distribute the molten aluminum to the bottom of three cylindrical sub-tanks.

6. The low-pressure casting device for large-size GIS tank composite molds according to claim 5, characterized in that, After the molten aluminum is diverted to the bottom of the three cylindrical sub-tanks, it rises and enters the mold cavity through the ingate module.

7. A low-pressure casting device for large-size GIS tank composite molds according to claim 6, characterized in that, At the connection point of the three cylindrical sub-tanks, a space is reserved on the inner cavity sand core for attaching metal chills.

8. A low-pressure casting method for a composite mold of a large-size GIS tank using the low-pressure casting apparatus described in claim 7, characterized in that, Includes the following steps: 1) Liquid rising stage: The molten aluminum in the holding furnace is transported to the gate through the liquid rising pipe under the pressure of compressed air; 2) Filling stage: The filling pressure is dynamically adjusted according to the height of the molten aluminum to complete the filling of the cavity; 3) Shelling stage: Increase the compressed air pressure to cause the molten aluminum to solidify and form a shell on the forming surface; 4) Pressurization stage: Continue to increase the compressed air pressure. The pressure acts on the front of the aluminum liquid solidification interface to increase the material density. 5) Holding pressure stage: Maintain compressed air pressure to ensure solidification from the far end of the casting to the gate, thus achieving feeding of the casting; 6) Decompression stage: Release the compressed air pressure in the holding furnace to atmospheric pressure, and the residual aluminum liquid in the gating channel and riser pipe will automatically flow back under the action of gravity.

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