A casting resin sand mixing apparatus
By enabling rapid switching between the grinding wheel sand mixing mode and the rotor sand mixing mode in the same equipment, the problem of low production efficiency caused by the complexity of switching in existing equipment is solved, the uniformity of sand mixing and the applicability of the equipment are improved, and the cost is reduced.
Patent Information
- Application Number
- CN202610413365.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-03-31
- Publication Date
- 2026-06-02
AI Technical Summary
Existing casting resin sand mixing equipment has overly complex switching between different processes, resulting in decreased production efficiency.
Design a casting resin sand mixing device that enables rapid switching between roller mixing mode and rotor mixing mode within the same device. Utilizes the combined motion of the first and second mixing plates to achieve powerful shearing, scattering, and crushing functions, thereby reducing the number of devices required.
It improves production efficiency, reduces equipment costs, enhances the uniformity and applicability of sand mixing, and avoids the problem of insufficient sand mixing caused by material agglomeration.
Smart Images
Figure CN122125164A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of casting technology, specifically a casting resin sand mixing device. Background Technology
[0002] Intelligent casting islands are the technological foundation for resin sand mixing equipment to solve the pain points of traditional sand mixing and adapt to the full-process control of intelligent casting. Resin sand mixing is the core process of resin sand casting, and traditional equipment mainly consists of rotor sand mixers and roller sand mixers.
[0003] Existing casting resin sand mixing equipment is generally divided into two types: roller type and rotor type. The roller type sand mixer relies on the roller to crush and knead to achieve sand coating and compaction, but its shearing and dispersing ability is weak, making it difficult to quickly break up the lumpy sand. The rotor type sand mixer achieves strong shearing and dispersing mixing through the mixing blades, but its crushing and kneading effect is insufficient, and the uniformity of resin coating and molding sand strength are poor. The existing casting resin sand mixing equipment has problems such as premixing, dispersing and final grinding need to be completed by different equipment, which makes the process switching between different equipment too complicated and time-consuming, resulting in a decrease in production efficiency.
[0004] Therefore, the present invention provides a casting resin sand mixing device. Summary of the Invention
[0005] In order to overcome the shortcomings of existing technologies and solve the problem mentioned in the background technology that the process switching between different equipment is too complicated and time-consuming, resulting in a decrease in production efficiency.
[0006] The technical solution adopted by the present invention to solve its technical problem is as follows: The casting resin sand mixing equipment of the present invention includes a shell, a first rotating rod rotatably connected to the upper surface of the shell, a second rotating rod rotatably connected to the surface of the first rotating rod, a first moving rod slidably and rotatably connected inside the second rotating rod, a fixed plate fixedly connected to one end of the first moving rod, a first stirring plate fixedly connected to one end of the fixed plate, a second moving rod slidably connected inside the first moving rod, a second stirring plate fixedly connected to one end of the second moving rod, and toothed grooves provided on the surfaces of the first stirring plate and the second stirring plate, the toothed grooves meshing with each other.
[0007] Preferably, a motor is provided on one side of the first rotating rod, and pulleys are fixedly connected to the output end of the motor and one end of the first rotating rod. The two pulleys are connected by belt drive.
[0008] Preferably, a fixing rod is fixedly connected inside the housing, the fixing rod is located inside the first rotating rod, a first bevel gear is fixedly connected to the surface of the fixing rod, and a second bevel gear is fixedly connected to one end of the second rotating rod, the second bevel gear and the first bevel gear meshing.
[0009] Preferably, a drive rod is slidably connected inside the second rotating rod, a first spring is fixedly connected between the surface of the drive rod and the inner wall of the second rotating rod, and a third spring is fixedly connected between the inner wall of the first moving rod and the second rotating rod.
[0010] Preferably, one end of the drive rod is provided with a first conical surface, and one end of the first moving rod is in contact with the surface of the drive rod.
[0011] Preferably, a kit is fixedly connected to the surface of the second rotating rod, and a limiting rod is fixedly connected inside the kit. A spiral groove is provided on the surface of the first moving rod, and the inner side of the spiral groove is slidably connected to one end of the limiting rod.
[0012] Preferably, an electric cylinder is fixedly connected to the upper surface of the housing, a conical block is fixedly connected to the telescopic end of the electric cylinder, one end of the conical block is slidably connected to the inside of the fixed rod, a protrusion is fixedly connected to the surface of the conical block, and a second conical surface is provided at the end of the drive rod away from the first conical surface, the second conical surface and the surface of the conical block are in contact.
[0013] Preferably, a concave surface is provided on one side of the first conical surface, and a pressing surface is provided on one side of the concave surface. Both the concave surface and the pressing surface are provided on the surface of the drive rod, and the size of the concave surface matches the size of the second moving rod.
[0014] Preferably, a second spring is fixedly connected between the lower surface of the second stirring plate and the fixed plate, and the two sides of the second stirring plate and the fixed plate are slidably connected.
[0015] Preferably, a hinge plate is fixedly connected to the surface of the first rotating rod, and the upper surface of the hinge plate is shaped as an inclined surface with the inner side higher than the outer side.
[0016] The beneficial effects of this invention are as follows: 1. The casting resin sand mixing equipment of the present invention, by means of a first stirring rod and a second stirring rod, can drive the driving rod to move when it is necessary to perform strong shearing and sand dispersion mixing of resin sand. The driving rod squeezes the first moving rod and the second moving rod to move synchronously. The first moving rod drives the second moving rod to move outward of the second rotating rod. At the same time, the spiral groove on the surface of the first moving rod is subject to the action of the limiting rod, so that the first moving rod and the second moving rod rotate synchronously. The first moving rod drives the first stirring plate to rotate synchronously, and the second moving rod drives the second stirring plate to rotate synchronously. Then, the first rotating rod and the second rotating rod are driven to rotate. The first rotating rod drives the second rotating rod to rotate, and the second rotating rod drives several first stirring plates and second stirring plates to rotate synchronously. Thus, the resin sand can be strongly sheared and sand dispersion mixed. Under normal conditions, the first stirring plate and the second stirring plate can form a grinding wheel. This device can quickly switch between grinding wheel sand mixing mode and rotor sand mixing mode, without the need to equip two independent sand mixers, effectively reducing equipment costs and improving the applicability of the device.
[0017] 2. The casting resin sand mixing equipment of the present invention, when the first and second stirring plates are used to shear and disperse the sand, the second moving rod enters the concave surface of the driving rod, so that a crushing space is formed between the first and second stirring plates. When the first and second stirring plates rotate, the lumpy material accumulates between the first and second stirring plates. By driving the second stirring plate to reciprocate, the tooth grooves on the second stirring plate and the first stirring plate mesh, and the lumpy material can be crushed. This device can shear and disperse the resin sand while the first and second stirring plates have the functions of mixing and crushing, which improves the mixing uniformity and avoids the problem of insufficient sand mixing caused by material agglomeration. Attached Figure Description
[0018] The invention will now be further described with reference to the accompanying drawings.
[0019] Figure 1 This is a schematic diagram of the overall structure of the present invention; Figure 2 This is a cross-sectional view of the overall structure of the present invention; Figure 3 This is a schematic diagram of the structure of the first rotating rod of the present invention; Figure 4 This is a cross-sectional view of the structure of the first rotating rod of the present invention; Figure 5 This is a cross-sectional view of the structure of the second rotating rod of the present invention; Figure 6 This is a schematic diagram of the structure of the drive rod, the first moving rod, and the second moving rod of the present invention; Figure 7 This is a schematic diagram of the structure of the kit and the spiral groove of the present invention; Figure 8This is a schematic diagram of the structure of the first stirring plate and the second stirring plate forming a grinding wheel according to the present invention; In the diagram: 1. Housing; 11. First rotating rod; 111. Flip plate; 12. Motor; 121. Pulley; 13. Fixed rod; 131. First bevel gear; 2. Second rotating rod; 21. Second bevel gear; 22. Drive rod; 221. First conical surface; 222. Concave surface; 223. Extrusion surface; 224. Second conical surface; 225. First spring; 23. Kit; 231. Limiting rod; 24. First moving rod; 241. Fixed plate; 242. First stirring plate; 243. Spiral groove; 25. Second moving rod; 251. Second stirring plate; 252. Second spring; 26. Third spring; 3. Electric cylinder; 31. Conical block; 311. Protrusion. Detailed Implementation
[0020] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.
[0021] Example 1: As Figures 1 to 8 As shown in the embodiment of the present invention, a casting resin sand mixing device includes a housing 1. A first rotating rod 11 is rotatably connected to the upper surface of the housing 1. A second rotating rod 2 is rotatably connected to the surface of the first rotating rod 11. A first moving rod 24 is slidably and rotatably connected inside the second rotating rod 2. A fixed plate 241 is fixedly connected to one end of the first moving rod 24. A first stirring plate 242 is fixedly connected to one end of the fixed plate 241. A second moving rod 25 is slidably connected inside the first moving rod 24. A second stirring plate 251 is fixedly connected to one end of the second moving rod 25. The surfaces of the first stirring plate 242 and the second stirring plate 251 are provided with toothed grooves that mesh with each other. A lifting plate 111 is fixedly connected to the surface of the first rotating rod 11. The upper surface of the lifting plate 111 is shaped as an inclined surface with a higher inner surface and a lower outer surface.
[0022] Specifically, existing foundry resin sand mixing equipment is generally divided into two types: roller type and rotor type. The roller type sand mixer relies on the roller to crush and knead the sand to achieve coating and compaction, but its shearing and dispersing ability is weak, making it difficult to quickly break up lumpy sand. The rotor type sand mixer achieves strong shearing and dispersing mixing through the stirring blades, but its crushing and kneading effect is insufficient, resulting in poor uniformity of resin coating and poor strength of molding sand. Since premixing, dispersing and final grinding need to be completed by separate equipment, the process switching between different equipment is too complicated and time-consuming, thus causing a decrease in production efficiency. To solve the above problems, when using this device, if the grinding wheel sand mixing device is required, resin sand is introduced into the interior of the housing 1, and then the first rotating rod 11 is driven to rotate. The first rotating rod 11 drives the second rotating rod 2 to rotate. While the second rotating rod 2 rotates with the first rotating rod 11, it can also be driven to rotate on its own axis. The second rotating rod 2 drives the first moving rod 24 and the second moving rod 25 to rotate synchronously, so that the first stirring plate 242 and the second stirring plate 251 rotate synchronously while maintaining engagement. At this time, multiple sets of first stirring plates 242 and second stirring plates 251 cooperate to form a grinding wheel, which can perform grinding and kneading operations on the resin sand. When a rotor-type mixing device is required, the first moving rod 24 and the second moving rod 25 can be driven to slide synchronously inside the second rotating rod 2, so that the first moving rod 24 and the second moving rod 25 move synchronously away from the surface of the second rotating rod 2. At the same time, the first moving rod 24 drives the second moving rod 25 to rotate synchronously, so that the first stirring plate 242 and the second stirring plate 251 change from radial to axial. The second rotating rod 2 drives the first stirring plate 242 and the second stirring plate 251 to rotate. The first stirring plate 242 and the second stirring plate 251 cooperate to perform shearing and dispersion operations on the material. At the same time, when the first rotating rod 11 rotates, the first rotating rod 11 drives the lifting plate 111 to rotate synchronously. The rotation of the lifting plate 111 can push the material that has moved to the inside to the outside, which is convenient for shearing and crushing operations. By changing the first mixing plate 242 and the second mixing plate 251 from radial to axial, the material receiving area of the axial blades is larger, which can improve the shearing and scattering efficiency of the material and thus improve the uniformity of sand mixing.
[0023] This device can quickly switch between grinding wheel sand mixing mode and rotor sand mixing mode without the need for two independent sand mixers, effectively reducing equipment costs and improving the applicability of the device.
[0024] like Figure 1 As shown, a motor 12 is provided on one side of the first rotating rod 11. The output end of the motor 12 and one end of the first rotating rod 11 are both fixedly connected to pulleys 121, and the two pulleys 121 are connected by belt drive.
[0025] Specifically, when it is necessary to drive the first rotating rod 11 to rotate, the motor 12 can be turned on. The motor 12 drives the pulley 121 at its output end to rotate synchronously. The rotation of the pulley 121 and the transmission through the belt cause the pulley 121 on the surface of the first rotating rod 11 to rotate synchronously, thereby driving the first rotating rod 11 to rotate synchronously.
[0026] like Figure 2As shown, a fixing rod 13 is fixedly connected inside the housing 1. The fixing rod 13 is located inside the first rotating rod 11. A first bevel gear 131 is fixedly connected to the surface of the fixing rod 13. A second bevel gear 21 is fixedly connected to one end of the second rotating rod 2. The second bevel gear 21 and the first bevel gear 131 mesh with each other.
[0027] Specifically, when the first rotating rod 11 rotates, the first rotating rod 11 drives the second rotating rod 2 to rotate synchronously. The second rotating rod 2 drives the second bevel gear 21 to mesh on the surface of the first bevel gear 131, so that the second bevel gear 21 drives the second rotating rod 2 to rotate synchronously. That is, while the first rotating rod 11 drives the second rotating rod 2 to rotate, the second rotating rod 2 drives the first stirring plate 242 and the second stirring plate 251 to rotate to perform shearing and scattering operations.
[0028] like Figures 3 to 5 As shown, a drive rod 22 is slidably connected inside the second rotating rod 2. A first spring 225 is fixedly connected between the surface of the drive rod 22 and the inner wall of the second rotating rod 2. A third spring 26 is fixedly connected between the first moving rod 24 and the inner wall of the second rotating rod 2. A first conical surface 221 is provided at one end of the drive rod 22. One end of the first moving rod 24 is in contact with the surface of the drive rod 22.
[0029] like Figure 7 As shown, a kit 23 is fixedly connected to the surface of the second rotating rod 2, and a limiting rod 231 is fixedly connected inside the kit 23. A spiral groove 243 is provided on the surface of the first moving rod 24, and the inner side of the spiral groove 243 is slidably connected to one end of the limiting rod 231.
[0030] Specifically, when the first moving rod 24 and the second moving rod 25 need to slide inside the rotating rod, under normal conditions, both the first moving rod 24 and the second moving rod 25 are in contact with the first conical surface 221 of the drive rod 22, controlling the drive rod 22 to slide inside the second rotating rod 2. The drive rod 22 compresses the first spring 225, and at the same time, the first conical surface 221 pushes the first moving rod 24 and the second moving rod 25, causing the first moving rod 24 and the second moving rod 25 to slide towards the outer wall of the rotating rod. The spiral groove 243 on the surface of the first moving rod 24 slides at one end of the limiting rod 231, causing the first moving rod 24 to rotate during its ascent inside the kit 23. The first moving rod 24 drives the second moving rod 25 to rotate synchronously, causing the first stirring plate 242 and the second stirring plate 251 to change from radial to axial, and expanding the shear area of the first stirring plate 242 and the second stirring plate 251.
[0031] Example 2: Figures 5 to 8As shown in the comparative embodiment one, another embodiment of the present invention is as follows: an electric cylinder 3 is fixedly connected to the upper surface of the housing 1, a cone block 31 is fixedly connected to the telescopic end of the electric cylinder 3, one end of the cone block 31 is slidably connected to the inside of the fixed rod 13, a protrusion 311 is fixedly connected to the surface of the cone block 31, and a second cone surface 224 is provided at the end of the drive rod 22 away from the first cone surface 221, and the second cone surface 224 is in contact with the surface of the cone block 31.
[0032] like Figure 6 As shown, a concave surface 222 is provided on one side of the first conical surface 221, and a pressing surface 223 is provided on one side of the concave surface 222. Both the concave surface 222 and the pressing surface 223 are provided on the surface of the drive rod 22. The size of the concave surface 222 matches the size of the second moving rod 25. A second spring 252 is fixedly connected between the lower surface of the second stirring plate 251 and the fixed plate 241. The two sides of the second stirring plate 251 and the fixed plate 241 are slidably connected.
[0033] Specifically, when the binders such as resin and curing agent are unevenly distributed in a local area, the resin sand will undergo a local curing reaction in advance, causing the sand particles to stick together and form clumps. When the first stirring plate 242 and the second stirring are used, they can only achieve overall turning and kneading, which makes it difficult to completely break up the tightly bound clumps inside, resulting in insufficient sand mixing and thus causing uneven material mixing. To avoid the above problems, when using this device: when it is necessary to control the movement of the drive rod 22, the electric cylinder 3 can be activated. The electric cylinder 3 drives the cone block 31 to move upward, and the cone block 31 presses against the second cone surface 224, causing the drive rod 22 to press against the first spring 225 and slide away from the cone block 31. The drive rod 22 drives the first cone surface 221 to slide, and the first cone surface 221 can press against the first moving rod 24 and the second moving rod 25. When the first moving rod 24 and the second moving rod 25 move from the first cone surface 221 to the surface of the drive rod 22, because the size of the concave surface 222 matches the size of the second moving rod 25, the second spring 252 drives the second moving rod 25 to slide towards the inside of the concave surface 222. The first moving rod 24 remains in contact with the surface of the drive rod 22, and the second... The moving rod 25 drives the second stirring plate 251 to slide stably on the fixed plate 241, so that a crushing space is formed between the first stirring plate 242 and the second stirring plate. Then, the second cone surface 224 rotates around the surface of the cone block 31 and contacts the protrusion 311. The protrusion 311 squeezes the second cone surface 224, causing the driving rod 22 to move again, so that the second moving rod 25 moves to the squeezing surface 223. The second moving rod 25 drives the second stirring plate 251 to move upward. The tooth groove on the second stirring plate 251 meshes with the tooth groove on the surface of the first stirring plate 242, so that the agglomerated material in the crushing space can be crushed. Then, the second cone block 31 moves away from the protrusion 311, the second stirring plate 251 descends and forms a crushing area again. By repeating the above operation, the agglomerated material can be crushed.
[0034] During the resin sand mixing process, this device can crush agglomerated materials, enabling the first mixing plate 242 and the second mixing plate 251 to perform multiple functions such as mixing, grinding, shearing and crushing.
[0035] Working principle: When using this device, resin sand is introduced into the interior of the housing 1. When it is necessary to grind and mix the material, the motor 12 can be turned on. The motor 12 drives the pulley 121 at its output end to rotate synchronously. The rotation of the pulley 121 and the transmission through the belt cause the pulley 121 on the surface of the first rotating rod 11 to rotate synchronously, thereby driving the first rotating rod 11 to rotate synchronously. The first rotating rod 11 drives the second rotating rod 2 to rotate synchronously. The second rotating rod 2 drives the second bevel gear 21 to mesh on the surface of the first bevel gear 131, so that the second bevel gear 21 drives the second rotating rod 2 to rotate synchronously. The second rotating rod 2 can then drive the grinding wheel to perform the grinding operation. When the rotor-type mixing equipment is needed, the electric cylinder 3 can be turned on. The electric cylinder 3 drives the cone block 31 to move upward. The cone block 31 squeezes the second cone surface 224, causing the drive rod 22 to squeeze the first spring 225 and slide away from the cone block 31. The drive rod 22 drives the first cone surface 221 to slide. The first cone surface 221 pushes the first moving rod 24 and the second moving rod 25, causing the first moving rod 24 and the second moving rod 25 to slide towards the outer wall of the rotating rod. The spiral groove 243 on the surface of the first moving rod 24 slides at one end of the limiting rod 231, causing the first moving rod 24 to rotate during the upward movement inside the kit 23. The first moving rod 24 drives the second moving rod 25 to rotate synchronously, causing the first stirring plate 242 and the second stirring plate 251 to change from radial to axial, and expanding the shear area of the first stirring plate 242 and the second stirring plate 251. When the first moving rod 24 and the second moving rod 25 move from the first conical surface 221 to the surface of the drive rod 22, because the size of the concave surface 222 matches the size of the second moving rod 25, the second spring 252 causes the second moving rod 25 to slide towards the inside of the concave surface 222. The first moving rod 24 remains in contact with the surface of the drive rod 22, and the second moving rod 25 causes the second stirring plate 251 to slide stably on the fixed plate 241, so that a breaking space is formed between the first stirring plate 242 and the second stirring plate. Subsequently, the second conical surface 224 rotates around the surface of the cone block 31. The conical surface 224 contacts the protrusion 311, which presses against the second conical surface 224, causing the drive rod 22 to move again. This causes the second moving rod 25 to move onto the pressing surface 223. The second moving rod 25 drives the second stirring plate 251 to move upward. The grooves on the second stirring plate 251 mesh with the grooves on the surface of the first stirring plate 242, thus crushing the agglomerated material in the crushing space. Subsequently, the second conical block 31 moves away from the protrusion 311, and the second stirring plate 251 descends, forming a crushing area again. Repeating the above operation allows for the crushing of agglomerated material. The above description shows and illustrates the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.
Claims
1. A casting resin sand mixing device, characterized in that: The device includes a housing (1), on the upper surface of which a first rotating rod (11) is rotatably connected. A second rotating rod (2) is rotatably connected to the surface of the first rotating rod (11). A first moving rod (24) is slidably and rotatably connected inside the second rotating rod (2). A fixed plate (241) is fixedly connected to one end of the first moving rod (241). A first stirring plate (242) is fixedly connected to one end of the fixed plate (241). A second moving rod (25) is slidably connected inside the first moving rod (24). A second stirring plate (251) is fixedly connected to one end of the second moving rod (25). The surfaces of the first stirring plate (242) and the second stirring plate (251) are provided with toothed grooves that mesh with each other.
2. The casting resin sand mixing equipment according to claim 1, characterized in that: A motor (12) is provided on one side of the first rotating rod (11). The output end of the motor (12) and one end of the first rotating rod (11) are both fixedly connected to pulleys (121). The two pulleys (121) are connected by belt drive.
3. The casting resin sand mixing equipment according to claim 2, characterized in that: A fixing rod (13) is fixedly connected inside the housing (1). The fixing rod (13) is located inside the first rotating rod (11). A first bevel gear (131) is fixedly connected to the surface of the fixing rod (13). A second bevel gear (21) is fixedly connected to one end of the second rotating rod (2). The second bevel gear (21) and the first bevel gear (131) mesh with each other.
4. The casting resin sand mixing equipment according to claim 3, characterized in that: The second rotating rod (2) is slidably connected to a drive rod (22), and a first spring (225) is fixedly connected between the surface of the drive rod (22) and the inner wall of the second rotating rod (2). A third spring (26) is fixedly connected between the first moving rod (24) and the inner wall of the second rotating rod (2).
5. A casting resin sand mixing device according to claim 4, characterized in that: One end of the drive rod (22) is provided with a first conical surface (221), and one end of the first moving rod (24) is in contact with the surface of the drive rod (22).
6. The casting resin sand mixing equipment according to claim 5, characterized in that: The surface of the second rotating rod (2) is fixedly connected to a kit (23), and the inside of the kit (23) is fixedly connected to a limiting rod (231). The surface of the first moving rod (24) is provided with a spiral groove (243), and the inner side of the spiral groove (243) is slidably connected to one end of the limiting rod (231).
7. A casting resin sand mixing device according to claim 6, characterized in that: An electric cylinder (3) is fixedly connected to the upper surface of the housing (1). A cone block (31) is fixedly connected to the telescopic end of the electric cylinder (3). One end of the cone block (31) is slidably connected to the inside of the fixed rod (13). A protrusion (311) is fixedly connected to the surface of the cone block (31). A second cone surface (224) is provided at the end of the drive rod (22) away from the first cone surface (221). The second cone surface (224) and the surface of the cone block (31) are in contact.
8. The casting resin sand mixing equipment according to claim 7, characterized in that: A concave surface (222) is provided on one side of the first conical surface (221), and a pressing surface (223) is provided on one side of the concave surface (222). Both the concave surface (222) and the pressing surface (223) are provided on the surface of the drive rod (22). The size of the concave surface (222) matches the size of the second moving rod (25).
9. A casting resin sand mixing device according to claim 1, characterized in that: A second spring (252) is fixedly connected between the lower surface of the second stirring plate (251) and the fixed plate (241), and the two sides of the second stirring plate (251) and the fixed plate (241) are slidably connected.
10. A casting resin sand mixing device according to claim 9, characterized in that: A hinge (111) is fixedly connected to the surface of the first rotating rod (11), and the upper surface of the hinge (111) is shaped as an inclined surface with the inner side higher than the outer side.