Compression mold and compression method for sintered NdFeB (neodymium iron boron)

Abstract

The invention discloses a profiling die and a profiling method for sintered neodymium iron boron, and belongs to the field of magnetic material preparation. The working surfaces of the upper pressure head and the lower pressure head of the mold are concave spherical surfaces, the front and rear working surfaces of the pressure heads are convex cylindrical surfaces, and the front and rear surfaces of a mold cavity are designed by adopting concave cylindrical surfaces with the same radian and matched with each other; the pressing method comprises the steps that neodymium iron boron magnetic powder is pre-pressed to the set density, orientation pressing is conducted under the conditions of a 1.52.0 T magnetic field and the pressure of 610 MPa, an arc-shaped pressed blank with the surface parallel to the direction of the magnetic field is obtained, and after the blank is sintered and shrunk, the cuboid magnet with the regular geometrical shape and the remarkably-reduced edge magnetic declination angle is finally formed. The easy magnetization axis of the magnetic powder is highly consistent with the direction of an external magnetic field, the magnetic declination angle is effectively reduced, and the material utilization rate is increased to 98% or above from traditional about 94%.

Description

Technical Field

[0001] This invention relates to the field of magnetic material preparation technology, and in particular to a molding die and molding method for sintering NdFeB. Background Technology

[0002] Sintered NdFeB permanent magnets are core functional materials in modern high-tech industries, and their performance consistency and production cost control are of paramount importance. Currently, the industry commonly uses magnetic field orientation molding technology, which typically employs a planar indenter and a cuboid mold cavity. Its design is based on the ideal assumption that the magnetic field distribution within the cavity is uniform and parallel.

[0003] However, under actual working conditions, due to the physical limitations of the magnetic pole structure, the magnetic field lines in the edge region of the mold cavity will inevitably bend, resulting in a significant magnetic declination. An excessively large magnetic declination means that the magnetization direction at the edge of the magnet is not parallel to the geometric edge, which will directly lead to magnetic field distortion in applications such as precision sensors and motors, reducing the accuracy and performance of the equipment.

[0004] To address this problem, existing technologies mainly employ two process paths, but both have significant drawbacks: High magnetic field / high pressure forming: While using a high magnetic field (e.g., >2T) and high pressure can produce regular cuboid green bodies, it cannot correct the orientation deviation of the magnetic powder at the edges. After sintering, the magnetic declination in the edge region of the magnet is still very large. To meet the requirements of high-end applications, these poorly oriented edge parts must be removed by machining, resulting in a significant reduction in material utilization, typically only around 90%, and serious waste of raw materials.

[0005] Weak magnetic field / low pressure molding: Using a weaker magnetic field and lower pressure, the surface tension during sintering is utilized to achieve better orientation in the central region of the magnet, but this results in surface depressions after sintering. To obtain a rectangular finished product, grinding is still required to remove a large amount of material, leading to even lower material utilization.

[0006] Therefore, how to improve material utilization while ensuring low magnetic declination of the magnet has become a technical bottleneck that the sintered NdFeB industry urgently needs to overcome. Summary of the Invention

[0007] To address the technical problem of large magnetic declination and low material utilization caused by the mismatch between the actual magnetic field and the edge direction of the magnet in the powder pressing process of sintered NdFeB magnets, this invention provides a pressing mold and pressing method for sintered NdFeB magnets.

[0008] The specific technical solution of the present invention is as follows: In a first aspect, the present invention provides a molding die for sintering NdFeB magnets, the die being disposed between an N-pole magnetic head 1 and an S-pole magnetic head 2, comprising: The mold cavity is formed by a left mold cavity magnetic plate 3, a right mold cavity magnetic plate 4, a front mold cavity non-magnetic plate 11, and a rear mold cavity non-magnetic plate 10. The front mold cavity non-magnetic plate (11) and the rear mold cavity non-magnetic plate (10) are arranged opposite each other along a direction parallel to the line connecting the magnetic pole heads. The inner surface of the front mold cavity non-magnetic plate (11) facing the mold cavity is a first concave cylindrical surface, and the inner surface of the rear mold cavity non-magnetic plate (10) facing the mold cavity is a second concave cylindrical surface. Both the first concave cylindrical surface and the second concave cylindrical surface are cylindrical surfaces, and the first concave cylindrical surface and the second concave cylindrical surface are arranged opposite each other. The inner surfaces of the left mold cavity magnetic plate 3 and the right mold cavity magnetic plate 4, which are arranged opposite each other perpendicular to the magnetic field orientation direction, are planes. The upper pressure head 5 and the lower pressure head 6 are driven by the upper pressure rod 8 and the lower pressure rod 9 respectively, and are positioned opposite each other above and below the mold cavity; The upper pressure head 5 has a first concave spherical surface facing the mold cavity for pressing powder, a first convex cylindrical surface facing the non-magnetic plate 11 of the mold cavity in front, and a second convex cylindrical surface facing the non-magnetic plate 10 of the mold cavity in the rear. The pressing head 6 has a second concave spherical surface facing the mold cavity for pressing powder, a third convex cylindrical surface facing the non-magnetic plate 11 of the mold cavity in front, and a fourth convex cylindrical surface facing the non-magnetic plate 10 of the mold cavity in the rear. The first and second concave spherical surfaces are oppositely arranged concave spherical surfaces used to press the upper and lower surfaces of the compact into convex spherical surfaces. The working surfaces of the first, second, third, and fourth convex cylindrical surfaces are all cylindrical arc surfaces, i.e., cylindrical surfaces. The curvature of the first and third convex cylindrical surfaces is equal to and closely fits the curvature of the first concave cylindrical surface. The curvature of the second and fourth convex cylindrical surfaces is equal to and closely fits the curvature of the second concave cylindrical surface, thereby preventing powder leakage during pressing.

[0009] Preferably, the transverse arc of the first concave spherical surface satisfies the following relation: Where K1 is a relational constant with a value ranging from 50 to 100; a1 is the chord length of the transverse arc of the first concave spherical surface; X is the final target height of the green blank pressed using the mold; and b1 is the bow height corresponding to the transverse arc.

[0010] K1 is a dimensionless design constant with a value ranging from 50 to 100. Using this formula, given the target product height X and mold size a1, the optimal bow height b1 can be determined, thus matching the curved surface of the compact with the curvature of the magnetic field edge.

[0011] Preferably, the arc height c of the transverse arcs of the first concave cylindrical surface, the second concave cylindrical surface, the first convex cylindrical surface, the second convex cylindrical surface, the third convex cylindrical surface, and the fourth convex cylindrical surface satisfies the following relationship: .

[0012] Preferably, the first concave spherical surface is cut along the longitudinal central axis of the upper pressure head along the direction of the magnetic pole head connection, and a vertical section is made. The section length is the chord length of the lower edge arc, i.e., the transverse chord length is defined as a1, and the bow height is the vertical distance from the vertex of the arc to the chord, defined as b1. The first concave spherical surface is cut along the direction perpendicular to the line connecting the magnetic pole heads, taking the transverse central axis of the upper pressure head as the cutting path, and a vertical section is made. The section length is the chord length of the lower edge arc, that is, the longitudinal chord length is defined as a2, and the bow height is the vertical distance from the vertex of the arc to the chord is defined as b1. Along the direction of the magnetic pole head connection, the second concave spherical surface is cut along the longitudinal central axis of the pressure head as the cutting path, and a vertical section is made. The section length is the chord length of the lower edge arc, that is, the transverse chord length is defined as a3, and the bow height is the vertical distance from the vertex of the arc to the chord is defined as b2. The second concave spherical surface is cut along the direction perpendicular to the line connecting the magnetic pole heads, with the transverse central axis of the pressure head as the cutting path, and a vertical section is made. The section length is the chord length of the lower edge arc, i.e., the longitudinal chord length is defined as a4, and the bow height is the vertical distance from the vertex of the arc to the chord is defined as b2, a3 = a1, a4 = a2, and bow height b2 = b1.

[0013] The chord length of the first concave cylindrical surface is defined as m1, and the bow height is defined as c1; The chord length of the second concave cylindrical surface is defined as m2, and the bow height is defined as c2; The chord length of the first convex cylindrical surface is defined as m3, and the corresponding bow height is c3; The chord length of the second convex cylindrical surface is defined as m4, and the corresponding bow height is c4; The chord length of the third convex cylindrical surface is defined as m5, and the corresponding bow height is c5; The chord length of the fourth convex cylindrical surface is defined as m6, and the corresponding bow height is c6; Where a1 = a3 = m1 = m2 = m3 = m4 = m5 = m6, a2 = a4, b1 = b2, .

[0014] Preferably, the chord lengths a1 and a2 of the first concave spherical surface are both in the range of 30-70 mm, and the bow height... 。

[0015] Preferably, the chord lengths a3 and a4 of the second concave spherical surface are both in the range of 30-70 mm, and the bow height... .

[0016] The chord length m1 of the first concave cylindrical surface is 30-70mm, and the bow height is... ; The chord length m2 of the second concave cylindrical surface is 30-70mm, and the bow height is... ; The chord length m3 of the first convex cylindrical surface is 30-70mm, and the bow height... ; The chord length m4 of the second convex cylindrical surface is 30-70mm, and the bow height is... ; The chord length m5 of the third convex cylindrical surface is 30-70mm, and the bow height is... ; The chord length m6 of the fourth convex cylindrical surface is 30-70mm, and the bow height is... .

[0017] Preferably, X The final target height of the pressed billet is calculated using the following formula: ; M represents the powder mass, and ρ1 represents the final density of the pressed compact, initially set at 4 g / cm³. 3 Make an estimate. S The projected areas of the first and second concave spherical surfaces onto the horizontal plane are calculated using the following formula: .

[0018] Preferably, the upper pressure head 5 has two planar sides facing the left mold cavity magnetic guide plate 3 and the right mold cavity magnetic guide plate 4, and is in close contact with the inner surface of the planar surfaces of the left mold cavity magnetic guide plate 3 and the right mold cavity magnetic guide plate 4, respectively; the lower pressure head 6 has two planar sides facing the left mold cavity magnetic guide plate 3 and the right mold cavity magnetic guide plate 4, and is in close contact with the inner surface of the planar surfaces of the left mold cavity magnetic guide plate 3 and the right mold cavity magnetic guide plate 4, respectively.

[0019] Preferably, the structure of the lower pressure head 6 is arranged opposite to that of the upper pressure head 5.

[0020] Preferably, the top of the upper pressure head 5 and the bottom of the lower pressure head 6 have horizontal cross sections to ensure stable contact with the flat platen of the press.

[0021] Preferably, the upper pressure head 5, the lower pressure head 6, the left mold cavity magnetic plate 3, the right mold cavity magnetic plate 4, the front mold cavity non-magnetic plate 11, and the rear mold cavity non-magnetic plate 10 are all made of non-magnetic materials to ensure that the mold itself is not magnetized and thus does not interfere with the magnetic field distribution in the cavity when an orientation magnetic field is applied.

[0022] Secondly, the present invention provides a method for molding sintered NdFeB using the above-mentioned molding die. This method combines "precise pre-pressing control" with "arc-shaped structure adaptation," and specifically includes the following steps: S1. Pre-compression parameter calculation: Determine the final target height X of the green compact; measure the loose packing density ρ of the NdFeB magnetic powder. 松装 The vibration density ρvibration density is determined by setting a proportionality coefficient K3, where K3 ranges from 20% to 50%. The formula ρ0 = (1-K3) × ρvibration density is used. 松装 + K3 × ρ 振实 Calculate the target pre-compression density ρ; then, based on the powder mass M, the target pre-compression density ρ, and the bottom area S of the mold cavity, calculate the target pre-compression height. H = M / (ρ0 × S); The base area S can be approximately calculated as S = a1 × a2.

[0023] S2. Powder Loading and Pre-compression: The NdFeB magnetic powder of mass M is loaded into the mold cavity. Under no magnetic field conditions, the upper pressure head 5 and the lower pressure head 6 are controlled to move towards each other, applying a pressure of 1-3 MPa to compress the powder to the pre-compression target height. H This forms a pre-pressed blank; Preferably, M is 400-600g, and the particle size of the neodymium iron boron magnetic powder is 2-4μm.

[0024] Preferably, a mechanical pressure of 2 MPa is applied during the pre-compression process, and the pressure is maintained for 10-20 seconds to allow the loose magnetic powder to achieve initial densification and uniform distribution, forming a pre-compressed blank. No magnetizing magnetic field is applied during this stage.

[0025] S3. Magnetic field orientation forming: An orientation magnetic field of 1.5T-2.0T is applied to the pre-pressed billet, and the pressure is increased to 6-10MPa for orientation pressing. After holding the pressure, a green billet with four arc-shaped surfaces is obtained. The height of the green billet is the final target height X, and X... <H; S4. Sintering and processing: The green blank is sintered to obtain a sintered blank, and then machined to obtain the final cuboid magnet.

[0026] During sintering, the green blank shrinks uniformly in all directions, and its convex spherical surface tends to flatten, thus obtaining a blank that is approximately rectangular. Then, conventional machining (such as grinding) is performed on the blank to obtain the rectangular magnet of the final specifications.

[0027] Because the main working surfaces of the upper pressure head 5 and the lower pressure head 6, as well as the non-magnetic plate 11 of the front mold cavity and the non-magnetic plate 10 of the rear mold cavity, are all arc-shaped, and the curvature is parallel to the bending direction of the actual magnetic field edge, the easy magnetization axis (c-axis) of the magnetic powder particles can be fully arranged and oriented along the real magnetic field lines. Ultimately, the resulting green blank is a hexahedron with arc-shaped upper, lower, front, and rear surfaces, replacing the traditional cuboid green blank with flat upper and lower surfaces.

[0028] Preferably, the entire molding process (S2 and S3) is carried out under controlled oxygen in a protective atmosphere, preferably using nitrogen as the protective atmosphere, with an oxygen content not exceeding 0.5%.

[0029] Preferably, in step S3, the holding time of the orientation pressing is 5-20 seconds to allow stress relaxation, full interlocking between powder particles, and improved uniformity of green body density and strength.

[0030] Preferably, the formula for calculating the bottom area S of the mold cavity is S = a1 × a2, where a1 is the transverse chord length of the first concave sphere and a2 is its longitudinal chord length.

[0031] Preferably, in step S2, the specific method for controlling the upward movement of the upper pressure head 5 and the lower pressure head 6 is as follows: based on the pre-pressure target height... H The initial position height h of the upper pressure head 5 and the height n of the mold cavity are used to calculate the descending distance D1 of the upper pressure head 5 and the rising distance D2 of the lower pressure head 6, and the upper pressure head 5 and the lower pressure head 6 are controlled to move the distances D1 and D2 respectively; wherein, the initial position height h is the vertical distance between the highest point of the arc surface of the upper pressure head 5 and the top of the mold, and the value range is 90-120mm; the preferred range of the height n of the mold cavity is 180-220mm.

[0032]

[0033]

[0034] Preferably, after the S3 orientation pressing is completed, subsequent steps such as demagnetization, demolding, and rapid vacuum packaging of the green body are also included to protect the green body and facilitate its transfer.

[0035] Preferably, the demagnetization uses a reverse magnetic field and a demagnetization current of 12-18A.

[0036] Preferably, the demolding process involves the upper pressure head moving upwards and the lower pressure head moving downwards to eject the green blank.

[0037] The removed green blank needs to be quickly vacuum-sealed. The sealing is done in two layers: the inner layer is wrapped with a polyethylene film of about 0.03mm to protect the edges and corners of the blank; the outer layer is put into a polyethylene film bag of about 0.08mm. Then it is put into a vacuum sealing machine for degassing and heat sealing to ensure a tight seal.

[0038] Thirdly, the present invention provides a sintered NdFeB magnet prepared by the above method.

[0039] The magnet is characterized by its uniform shrinkage during sintering, resulting in a regular cuboid shape. Thanks to the use of an arc-shaped mold adapted to the magnetic field and a precise pre-pressing process, this magnet exhibits extremely low edge magnetic declination and extremely high material utilization. Specifically, by cutting and sampling measurements from the eight vertices of the final cuboid magnet, the average magnetic declination (Var.) is less than 1°. Furthermore, by weighing the sintered blank (M1) and the final machined cuboid magnet (M2), its material utilization rate is greater than 97%, significantly higher than that of traditional processes.

[0040] Material utilization rate The calculation formula is:

[0041] Compared with the prior art, the beneficial technical effects of this invention are reflected in: 1. Significantly reduce magnetic declination and improve magnet performance consistency: By designing the surface of the mold cavity and the contact head with the magnetic powder as an arc that matches the bending direction of the magnetic field, the easy magnetization axis of the powder particles can be more fully aligned along the actual magnetic field lines during pressing. After the arc-shaped green blank shrinks evenly during sintering, it is easier to obtain a blank that is close to a parallelepiped. The edge magnetic field lines are reoriented and aligned with the edge shape, thereby significantly reducing the magnetic declination of the magnet edge after sintering.

[0042] 2. Significantly improved material utilization: The magnet with a lower magnetic declination greatly reduces the amount of poorly oriented scrap that must be removed by machining in order to obtain a regular cuboid and remove the large magnetic declination at the edges, thus increasing the material utilization rate from the traditional 90-93% to over 97%.

[0043] 3. High process controllability and high green quality: The pre-calculation process enables precise control over the initial densification of the powder, ensuring uniform density of the pre-pressed green and laying the foundation for subsequent high-quality gravity orientation molding. The non-magnetic mold material ensures the purity of the magnetic field. Attached Figure Description

[0044] Figure 1 This is a front view of the mold as a whole; Figure 2 This is a top view of the mold cavity; Figure 3 This is a 3D view of the upper pressure head; Figure 4 Here are the cross-sectional views of the upper pressure head: (1) is a cross-sectional view with a vertical plane along the longitudinal centerline of the upper pressure head; (2) is a cross-sectional view with a vertical plane along the transverse centerline of the upper pressure head. Figure 5 The three views of the upper pressure head are: (1) front view, (2) side view, and (3) top view. Figure 6 This is a 3D view of the downward pressing head; Figure 7 A schematic diagram showing the chord length m and bow height c of the non-magnetic plate in the front / rear mold cavity; Figure 8 Three views of the non-magnetic plate of the front / rear mold cavity: (1) front view, (2) side view, (3) top view; Figure 9 This is a schematic diagram showing the shape of the NdFeB alloy powder from the pressed blank to the finished blank. Figure 10 The three views of the pressed blank are: (1) the front view, (2) the side view, and (3) the top view.

[0045] In the diagram: 1-N pole magnetic head, 2-S pole magnetic head, 3-left mold cavity magnetic plate, 4-right mold cavity magnetic plate, 5-upper pressure head, 6-lower pressure head, 7-actual magnetic field distribution, 8-upper pressure rod, 9-lower pressure rod, 10-rear mold cavity non-magnetic plate, 11-front mold cavity non-magnetic plate. Detailed Implementation

[0046] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0047] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0048] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0049] I. Introduction to the Overall Structure of the Mold Please see Figure 1 It shows a schematic diagram of the overall assembly structure of the forming mold of the present invention.

[0050] The mold system is positioned between the N-pole magnetic head 1 and the S-pole magnetic head 2, which are used to generate a horizontally oriented magnetic field. The core of the mold includes a mold cavity for containing neodymium iron boron magnetic powder, which is enclosed by four side plates: a left-side magnetically conductive plate 3 and a right-side magnetically conductive plate 4, both facing each other with flat inner surfaces. They are located on both sides perpendicular to the magnetic field direction, i.e., perpendicular to the line connecting the magnetic heads 1 and 2. A front-side non-magnetically conductive plate 11 and a rear-side non-magnetically conductive plate 10, both facing each other with concave cylindrical inner surfaces of a specific curvature, are located on both sides parallel to the magnetic field direction, i.e., parallel to the line connecting the magnetic heads 1 and 2.

[0051] An upper pressure head 5 and a lower pressure head 6 are respectively provided above and below the mold cavity. The upper pressure head 5 is driven by an upper pressure rod 8, and the lower pressure head 6 is driven by a lower pressure rod 9. The two can move in opposite directions to complete the powder loading, pre-compression and final compression actions.

[0052] The design of the working surfaces of the upper and lower pressure heads is key to this invention: the main working surface facing the mold cavity and used for directly pressing powder is a concave spherical surface; the first pressure head has a first concave spherical surface, and the lower pressure head has a second concave spherical surface. The sides facing the front and rear non-magnetic plates 11 and 10 are convex cylindrical surfaces; the pressure head has a first and second convex cylindrical surface, and the lower pressure head has a third and fourth convex cylindrical surface. The curvature of these convex cylindrical surfaces is equal to that of the concave cylindrical surface of the front non-magnetic plate 11 and the rear non-magnetic plate 10, respectively, and they fit tightly together during the pressing process, serving to guide and prevent powder leakage.

[0053] Figure 1 The diagram also schematically illustrates the curved shape of the actual magnetic field distribution 7. It can be seen that the arc-shaped working surface of the mold of the present invention is designed to conform to this curved magnetic field line.

[0054] II. Definition and Design Relationship of Key Parameters Regarding the reference numerals in the accompanying drawings: For clarity, different components with the same geometric features (such as chord length, bow height) are represented by the same abbreviated numerals (e.g., m, c). It should be understood that these numerals represent the corresponding features of the component. In the specific description of the claims and specification of this invention, precise symbols with subscripts (e.g., m1, c1, m2, c2...) are used to distinguish different components. The 'm' numeral in the drawings corresponds to the chord length parameter (m1, m2,...) of the component, and the 'c' numeral corresponds to the bow height parameter (c1, c2,...) of the component. Specific correspondences and dimensions are as described in the claims and specification.

[0055] Combination Figures 3-8 The key geometric parameters of the mold are defined and explained, among which, Figure 8 (2) In the side view, the gray dashed line represents the inward concave arc surface of the mold cavity.

[0056] 1. Parameters of the concave spherical surfaces of the upper and lower pressure heads (taking the first concave spherical surface as an example): Horizontal arc: Along the direction of the magnetic pole head connection (i.e., the mold "horizontal"), take the longitudinal centerline of the pressure head as the cutting path and make a vertical section. The length of the section, i.e. the chord length of the lower edge arc, is defined as a1, and its arc height is defined as the vertical distance from the vertex of the arc to the chord, which is defined as b1.

[0057] Longitudinal arc: Along the direction perpendicular to the line connecting the magnetic pole heads (i.e., the "longitudinal" of the mold), take the transverse centerline of the pressure head as the cutting path and make a vertical section. The length of the section, i.e. the chord length of the lower edge arc, is defined as a2, and the arc height, i.e. the vertical distance from the vertex of the arc to the chord, is defined as b1.

[0058] The spherical curvature design relationship satisfies the following formula: .

[0059] Where K1 is a relational constant, ranging from 50 to 100; X is the final target height of the green billet. This formula is used to determine the optimal bow height b1 based on the product dimensions (a1, X), so that the convex upper and lower surfaces of the pressed billet are compatible with the magnetic field curvature.

[0060] The dimensions of the second concave spherical surface (lower pressure head) are essentially the same as those of the first concave spherical surface (upper pressure head). The chord length of the transverse arc of the lower pressure head is defined as a3, the chord length of the longitudinal arc is defined as a4, and the arc height of both the transverse and longitudinal arcs is defined as b2. That is, a3 = a1, a4 = a2, and its arc height b2 = b1.

[0061] 2. Parameters of the concave cylindrical surfaces of the front and rear non-magnetic plates and the corresponding convex cylindrical surfaces of the indenter: The chord length of the non-magnetic plate 11 concave cylindrical surface, i.e. the first concave cylindrical surface, is defined as m1, and the bow height is defined as c1.

[0062] The chord length of the concave cylindrical surface of the non-magnetic plate 10 at the rear, i.e. the second concave cylindrical surface, is defined as m2, and the bow height is defined as c2.

[0063] The chord lengths of the four convex cylindrical surfaces (first to fourth convex cylindrical surfaces) corresponding to the upper and lower pressure heads are defined as m3, m4, m5, and m6, respectively, and the corresponding bow heights are c3, c4, c5, and c6.

[0064] Dimensional uniformity: In mold design, these cylindrical surfaces together enclose the front and rear boundaries of the cavity and mate with the pressure head. Therefore, their chord lengths are equal, i.e., m1 = m2 = m3 = m4 = m5 = m6 = a1. Their values ​​all range from 30-70mm.

[0065] Cylindrical curvature design relationship: Its curvature height is determined by the following formula:

[0066] That is, c1 = c2 = c3 = c4 = c5 = c6 = .

[0067] This formula, through the same design constant K1, links the cylindrical curvature with the main dimensions (a1, a2) of the indenter spherical surface, ensuring a unified design logic for all components of the mold. Based on this relationship, the values ​​of the arc height c and arc height b1 are different.

[0068] 3. Other structural parameters: n: The height of the mold cavity when it is not filled with powder.

[0069] h: The vertical distance between the highest point of the concave spherical surface of the upper pressure head 5 and the top of the mold when the upper pressure head 5 is in its initial position.

[0070] All mold components (pressure head, magnetic plate, non-magnetic plate) are made of non-magnetic materials (such as stainless steel, hard alloy, etc.) to avoid interfering with the magnetic field distribution within the cavity.

[0071] III. Molding Method and Examples The following detailed description of the molding method using the aforementioned mold, with reference to specific embodiments, illustrates the method in detail. The method mainly includes four steps: pre-pressing parameter calculation, powder loading and pre-pressing, magnetic field orientation molding, and sintering and processing.

[0072] Example 1 1. Mold preparation and parameter determination: Using a set of arc-shaped molds of this invention, the key design constant K1 is selected as 50, and the final target height of the green blank is X = 52 mm. The chord length of the spherical indenter: a1 = 40 mm, a2 = 60 mm.

[0073] Calculate the bow height using the spherical design formula: = √(40 × 52) / 50 ≈ 0.91mm.

[0074] The chord lengths of the mold cavity and the side cylindrical surfaces of the pressure head are uniformly set as follows: m1 = m2 = m3 = m4 = m5 = m6 = a1 = 40 mm.

[0075] Calculate the bow height using the cylindrical design formula: c = ≈ 0.98 mm. That is, c1 = c2 = c3 = c4 = c5 = c6 = 0.98 mm.

[0076] Other structural dimensions: cavity height n = 200 mm, initial height of upper pressure head h = 100 mm.

[0077] 2. Calculation of preload parameters (S1): Weigh out 500 g of NdFeB fine magnetic powder (average particle size 2.3 μm).

[0078] The loose packing density ρ of the magnetic powder was measured. 松装 = 1.63 g / cm³, tap density ρ 振实 = 2.58 g / cm³.

[0079] Set the proportionality coefficient K3 = 50%, and calculate the target preload density: ρ0 = (1-0.5)×1.63 + 0.5×2.58 = 2.11 g / cm³.

[0080] Calculate the approximate bottom area of ​​the mold cavity: S = a1 × a2 = 4.0 cm × 6.0 cm = 24.0 cm 2 .

[0081] Calculate the target height for preloading: H = M / (ρ0 × S) = 500 / (2.11 × 24.0) ≈ 9.87 cm = 98.7 mm.

[0082] 3. Powder loading and pre-compression (S2): 500g of magnetic powder is loaded into the mold cavity. Under the condition of no magnetic field, the upper and lower pressure heads are controlled to move in opposite directions and a pressure of 2 MPa is applied to compress the powder to a height of about 98.7 mm to form a pre-compressed blank.

[0083] 4. Magnetic Field Orientation Forming (S3): A steady-state orientation magnetic field of 2.0 T is applied to the pre-pressed billet. Under the action of the magnetic field, the pressing pressure is increased to 10 MPa for orientation pressing, and the pressure is held for 20 seconds. After the pressure holding is completed, the magnetization is removed and the billet is demolded to obtain a hexahedral green billet with dimensions of approximately 40 mm × 60 mm × 52 mm. The upper and lower surfaces of this green billet are convex spherical surfaces (complementary to the concave spherical surface of the press head), and the front and rear surfaces are convex cylindrical surfaces (complementary to the concave cylindrical surface of the mold), as shown below. Figure 9 and Figure 10 As shown.

[0084] 5. Sintering and Machining (S4): The green blank is sintered. Due to uniform shrinkage in all directions, the curved surface tends to be flat, resulting in a near-rectangular blank. The blank is then subjected to conventional machining (grinding) to obtain the final regular rectangular magnet.

[0085] Performance testing: The mass of the sintered blank is M1 = 499.65 g, and the final mass of the magnet after machining is M2 = 491.16 g. The material utilization rate is α = (M2 / M1) × 100% = 98.3%.

[0086] The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the cuboid blank. The test results are shown in Table 1.

[0087] To concisely demonstrate the effects of the present invention under different parameters, Examples 2 to 8 and Comparative Examples 1 to 9 will be described in a general manner, highlighting the key process parameters, mold dimensions (the arch heights b1 and c are calculated strictly according to the aforementioned formulas), and final performance results that differ from Example 1. The specific process steps are similar to those in Example 1.

[0088] Example 2 Fine magnetic powder with an average particle size of 2.3 μm was obtained by air jet milling, and the loose packing density of the fine magnetic powder was measured to be 1.63 g / cm³. 3 The tap density is 2.58 g / cm³. 3 450g of fine magnetic powder was weighed into the glove box and poured into the designed molding die cavity. A press head with K1=50 and a mold cavity with two curved surfaces were used, where: X=47mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.87mm, c1=c2=c3=c4=0.98mm. Pre-pressing was performed until the pre-pressed green body achieved a powder loose density to tapped density difference ratio K3 of 50%. At this point, the pre-pressed density ρ0 was calculated to be 2.11g / cm³. 3 The pre-pressing height of the compact was calculated to be 89mm. At this time, the upper pressure head descended by 155.5mm and the lower pressure head rose by 55.5mm. After the pre-pressing was completed, orientation was carried out under a constant magnetic field of 2.0T. Under the condition of orientation pressure of 10MPa, the pressure was held for 20s. After that, the upper pressure head moved upward and the lower pressure head moved downward relative to each other to demold. The orientation molding produced a hexahedron with four arc-shaped surfaces parallel to the direction of the magnetic field, measuring 40×60×47mm. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.76g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 489.27g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated to be α = (489.27 / 499.76) × 100% = 97.9%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0089] Example 3 Fine magnetic powder with an average particle size of 2.3 μm was obtained by air jet milling, and the loose packing density of the fine magnetic powder was measured to be 1.63 g / cm³. 3 The tap density is 2.58 g / cm³. 3 500g of fine magnetic powder was weighed into the glove box and poured into the designed molding die cavity. A press head with K1=50 and a mold cavity with two curved surfaces were used, where: X=52mm, m=30mm, n=200mm, h=100mm, a1=30mm, a2=70mm, b1=0.79mm, and c=0.92mm. Pre-pressing was performed until the pre-pressed green body achieved a powder loose density to tapped density difference ratio K3 of 50%. At this point, the pre-pressed density ρ0 was calculated to be 2.11g / cm³. 3 The pre-pressing height of the compact was calculated to be 112.8 mm. At this time, the upper pressure head descended by 143.6 mm and the lower pressure head rose by 43.6 mm. After the pre-pressing was completed, orientation was carried out under a steady magnetic field of 2.0 T. Under the condition of orientation pressure of 10 MPa, the pressure was held for 20 seconds. The upper pressure head moved upward and the lower pressure head moved downward relative to each other to demold. The orientation molding produced a hexahedron with four arc-shaped surfaces parallel to the direction of the magnetic field, with a length of 30×70×52 mm. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.91g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 490.41g that was strictly flat, smooth and without missing corners. The material utilization rate before and after processing was calculated to be α = (490.41 / 499.91) × 100% = 98.1%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0090] Example 4 Fine magnetic powder with an average particle size of 2.3 μm was obtained by air jet milling, and the loose packing density of the fine magnetic powder was measured to be 1.63 g / cm³. 3 The tap density is 2.58 g / cm³. 3 500g of fine magnetic powder was weighed into the glove box and poured into the designed molding die cavity. A press head with K1=50 and a mold cavity with two curved surfaces were used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.91mm, c1=c2=c3=c4=0.98mm. Pre-pressing was performed until the pre-pressed green body achieved a powder loose density to tapped density difference ratio K3 of 20%. At this point, the pre-pressed density ρ0 was calculated to be 1.82 g / cm³. 3The height of the pre-pressed blank was calculated to be 114.5 mm. At this time, the upper pressure head descended by 157.25 mm and the lower pressure head rose by 57.25 mm. After the pre-pressing was completed, orientation was carried out under a constant magnetic field of 2.0 T. Under the condition of orientation pressure of 10 MPa, the pressure was held for 20 seconds. The upper pressure head moved upward and the lower pressure head moved downward relative to each other to demold. The orientation molding produced a hexahedron with four arc-shaped surfaces parallel to the direction of the magnetic field, measuring 40×60×52 mm. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.86g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 490.36g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated to be α = (490.36 / 499.86) × 100% = 98.1%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0091] Example 5 Fine magnetic powder with an average particle size of 2.3 μm was obtained by air jet milling, and the loose packing density of the fine magnetic powder was measured to be 1.63 g / cm³. 3 The tap density is 2.58 g / cm³. 3 500g of fine magnetic powder was weighed into the glove box and poured into the designed molding die cavity. A press head with K1=50 and a mold cavity with two curved surfaces were used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.91mm, c1=c2=c3=c4=0.98mm. Pre-pressing was performed until the pre-pressed green body achieved a powder loose density to tapped density difference ratio K3 of 30%. At this point, the pre-pressed density ρ0 was calculated to be 1.97 g / cm³. 3 The height of the pre-pressed blank was calculated to be 105.7 mm. At this time, the upper pressure head descended by 147.2 mm and the lower pressure head rose by 47.2 mm. After the pre-pressing was completed, orientation was carried out under a steady magnetic field of 2.0 T. Under the condition of orientation pressure of 10 MPa, the pressure was held for 20 s. After that, the upper pressure head moved downward and the lower pressure head moved upward relative to each other to demold. The orientation molding produced a hexahedron with four arc-shaped surfaces parallel to the direction of the magnetic field, measuring 40×60×52 mm. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.68g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 489.69g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated as α = (489.69 / 499.68) × 100% = 98%. The eight vertices of the rectangular blank were cut off and the magnetic declination Var.1-8 and the average magnetic declination were measured. The test results are shown in Table 1.

[0092] Example 6 The difference from Example 1 lies in the intensity of the magnetic field applied after pre-compression. Specifically, fine magnetic powder with an average particle size of 2.3 μm was obtained through an air jet milling process, and the loose packing density of the fine magnetic powder was measured to be 1.63 g / cm³. 3 The tap density is 2.58 g / cm³. 3 500g of fine magnetic powder was weighed into the glove box and poured into the designed molding die cavity. A press head with K1=50 and a mold cavity with two curved surfaces were used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.91mm, c1=c2=c3=c4=0.98mm. Pre-pressing was performed until the pre-pressed green body achieved a powder loose density to tapped density difference ratio K3 of 50%. At this point, the pre-pressed density ρ0 was calculated to be 2.11 g / cm³. 3 The pre-pressing height of the compact was calculated to be 99mm. At this time, the upper pressure head descended by 150.5mm and the lower pressure head rose by 150.5mm. After the pre-pressing was completed, orientation was carried out under a constant magnetic field of 1.5T. Under the condition of orientation pressure of 10MPa, the pressure was held for 20s. Then, the upper pressure head moved upward and the lower pressure head moved downward relative to each other to demold. The orientation molding produced a hexahedron with four arc-shaped surfaces parallel to the direction of the magnetic field, measuring 40×60×52mm. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.67g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 489.18g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated to be α = (489.18 / 499.67) × 100% = 97.9%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0093] Example 7 The difference from Example 1 lies in the curvature of the arc-shaped indenter and the front and rear surfaces of the mold cavity. Specifically, an indenter with a proportionality coefficient K1=75 and a mold cavity with arc-shaped front and rear surfaces are used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.61mm, c1=c2=c3=c4=0.65mm. Everything else is the same as in Example 1. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.77g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 490.77g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated to be α = (490.77 / 499.77) × 100% = 98.2%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0094] Example 8 The difference from Example 1 lies in the curvature of the arc-shaped pressure head and the front and rear surfaces of the mold cavity. Specifically, the curvature of the pressure head and the front and rear surfaces of the mold cavity are different using a proportionality coefficient K1=100, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.46mm, c1=c2=c3=c4==0.49mm, and the rest are the same as in Example 1. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.84g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 492.34g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated to be α = (492.34 / 499.84) × 100% = 98.5%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0095] Comparative Example 1 The difference from Example 1 lies in the curvature of the arc-shaped indenter and the front and rear surfaces of the mold cavity. Specifically, an indenter with K1=25 and an arc-shaped mold cavity with front and rear surfaces are used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=1.82mm, c1=c2=c3=c4=1.96mm. Everything else is the same as in Example 1. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.67g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 470.68g that was perfectly flat and smooth without any missing corners. The material utilization rate before and after processing was calculated to be α = (470.68 / 499.67) × 100% = 94.2%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0096] Comparative Example 2 The difference from Example 1 lies in the curvature of the arc-shaped pressure head and the front and rear surfaces of the arc-shaped mold cavity. Specifically, a pressure head with K1=150 and two arc-shaped mold cavities are used, where X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.3mm, c1=c2=c3=c4=0.32mm, and everything else is the same as in Example 1. The pressed blank was sintered and shrunk uniformly to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.07g. After grinding, it was processed into a rectangular magnetic block with a mass of M2 = 472.12g that was strictly flat, smooth and without missing corners. The material utilization rate before and after processing was calculated to be α = (472.12 / 499.07) × 100% = 94.6%. The magnetic declination Var.1-8 and the average magnetic declination were measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0097] Comparative Example 3 The difference from Example 1 is that the mold cavity is planar. Specifically, a pressure head with K1=50 and a planar mold cavity are used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, b1=0.91mm. Everything else is the same as in Example 1. The pressed blank is sintered, and after uniform shrinkage, a rectangular sintered NdFeB magnet with a mass M1=498.66g is obtained. After grinding, it is processed into a strictly flat, smooth, and corner-free rectangular magnetic block with a mass M2=476.22g. The material utilization rate before and after processing is calculated as α=(476.22 / 498.66)×100%=95.5%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0098] Comparative Example 4 The difference from Example 1 is that the pressure head is flat and the front and rear surfaces of the mold cavity are arc-shaped. Specifically, a mold cavity with K1=50 is used, where: X=52mm, m=40mm, n=200mm, h=100mm, a1=40mm, a2=60mm, c1=c2=c3=c4=0.98mm. Everything else is the same as in Example 1. The pressed blank is sintered, and after uniform shrinkage, a rectangular sintered NdFeB magnet with a mass M1=498.58g is obtained. After grinding, it is processed into a strictly flat, smooth, and corner-free rectangular magnetic block with a mass M2=473.15g. The material utilization rate before and after processing is calculated as α=(473.15 / 498.58)×100%=94.9%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0099] Comparative Example 5 The only difference between Comparative Example 5 and Example 1 is that the pre-pressing process is not used. The powder poured into the mold is pressed and simultaneously oriented by a strong magnetic field of 2.0T. Everything else is the same as Example 1. The pressed blank is sintered and uniformly shrunk to obtain a rectangular sintered NdFeB magnet with a mass of M1 = 499.01g. After grinding, it is processed into a strictly flat, smooth, and corner-free rectangular magnetic block with a mass of M2 = 472.06g. The material utilization rate before and after processing is calculated to be α = (472.06 / 499.01) × 100% = 94.6%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0100] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that Comparative Example 6 uses the molding process of a traditional powder molding machine, where the upper and lower pressure heads and the mold are all flat. It does not use the powder molding machine and pre-pressing process of this invention. The powder poured into the mold is pressed while a strong magnetic field of 2.0T is applied for orientation molding. Everything else is the same as in Example 1. The pressed blank is sintered, and after uniform shrinkage, a rectangular sintered NdFeB magnet with a mass M1 = 499.27g is obtained. After grinding, it is processed into a strictly flat, smooth, and corner-free rectangular magnetic block with a mass M2 = 465.3g. The material utilization rate before and after processing is calculated as α = (465.3 / 499.27) × 100% = 93.2%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0101] Comparative Example 7 The difference from Example 1 is that a traditional planar pressure head and planar mold cavity are used, but the same pre-pressing height as in Example 1 is adopted in the pre-pressing stage. All other process parameters are consistent with Example 1. The pressed blank is sintered and uniformly shrunk to obtain a rectangular sintered NdFeB magnet with a mass M1 = 499.19g. After grinding, it is processed into a strictly flat, smooth, and corner-free rectangular magnetic block with a mass M2 = 470.74g. The material utilization rate before and after processing is calculated to be α = (470.74 / 499.19) × 100% = 94.3%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0102] Comparative Example 8 The difference from Example 1 is that the traditional powder molding machine is used, where the upper and lower pressure heads and the mold are all flat. The powder molding machine and pre-pressing process of this invention are not used. The powder poured into the mold is oriented by applying a weak magnetic field of 1T. Everything else is the same as in Example 1. The sintered NdFeB blank has a mass of M1=499.23g after shrinking on all six sides and forming a concave arc shape in the middle. After grinding, it is made into a strictly flat and smooth rectangular magnetic block with a mass of M2=462.79g. The material utilization rate before and after processing is calculated to be α=(462.79 / 499.23)×100%=92.7%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0103] Comparative Example 9 The difference between Comparative Example 9 and Example 1 is that Comparative Example 9 uses the molding process of a traditional powder molding machine, where the upper and lower pressure heads and the mold are all flat. It does not use the arc-shaped mold and arc-shaped pressure head and innovative process of this invention. A strong magnetic field of 2.5T is applied simultaneously during a single pressing process. The remaining mold structure and process parameters are the same as in Example 1. The pressed blank is sintered, and after uniform shrinkage, a rectangular sintered NdFeB magnet with a mass M1 = 499.38g is obtained. After grinding, it is processed into a strictly flat, smooth, and corner-free rectangular magnetic block with a mass M2 = 464.92g. The material utilization rate before and after processing is calculated as α = (464.92 / 499.38) × 100% = 93.1%. The magnetic declination Var.1-8 and the average magnetic declination are measured by cutting off the eight vertices of the rectangular blank. The test results are shown in Table 1.

[0104] Performance testing The morphology of the sintered NdFeB magnets prepared in the examples and comparative examples was evaluated according to the magnetic test method of permanent magnet (hard magnet) materials in GB / T3217. The evaluation results are shown in Table 1, including the magnetic declination of the eight vertices of the magnet and the utilization rate of the material to evaluate the effect.

[0105] Table 1. Performance Comparison of Neodymium Iron Boron Permanent Magnets After Treatment Under Different Conditions

[0106] From the performance data of sintered NdFeB magnets in Examples 1-8 and Comparative Examples 1-9 in Table 1, it can be concluded that: by comparing different upper and lower pressure heads, the curvature of the two sides of the mold cavity, the orientation magnetic field strength, the pressing pressure, and different pre-pressing positions, the magnetic declination and utilization rate of the sintered NdFeB magnets in the examples are superior to those of the traditional planar pressure head and planar pressure head process. A comparison of Example 1 with Comparative Examples 1 and 2 shows that the arc proportion coefficient K1 = 50~100 can most effectively guide the powder to align as close as possible to the actual magnetic field orientation. After uniform shrinkage during liquid phase sintering, a product with the smallest magnetic declination and the highest utilization rate is obtained. The low magnetic declination of the final product indicates that this process can maintain good parallelism between the magnetization direction inside the magnet and the geometric edge of the outer shape, thereby producing a cuboid hexahedron through uniform shrinkage during subsequent sintering. A comparison of Example 1 with Comparative Example 6 shows that when using an arc-shaped pressure head and pre-pressing process, the average magnetic declination is 2.4° lower than that of the traditional pressing mold and pressing process, and the material utilization rate is increased by 5.1%. By comparing Example 1 and Comparative Examples 1-4, it was determined that when the ratio of chord length to bow height of the arc is 50-100 and both working surfaces of the upper and lower pressure heads and the front and rear surfaces of the mold cavity are arc surfaces, the lowest average magnetic declination angle and the highest material processing utilization rate are achieved.

[0107] The above data confirms that this invention optimizes the curvature of the arc-shaped indenter and the pre-pressing process to ensure that the arc-shaped surface of the pressed blank is consistent with the actual magnetic field direction. The four-sided arc-shaped pressed blank prepared by the innovative method, after uniform shrinkage after sintering, can effectively reduce the magnetic declination of the magnet edge and improve the processing utilization rate of the magnet. This reduces the magnetic declination of the sintered NdFeB magnet produced by the innovative mold and the innovative pressing process, thereby improving the material processing utilization rate. This is superior to products prepared by traditional rectangular molds and effectively solves the problem of low material processing utilization caused by the large magnetic declination of the edge after shrinkage of traditional flat indenters, resulting in a waste of cost resources.

[0108] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

[0109] It should be particularly noted that the various embodiments listed in this specification and accompanying drawings are intended to illustrate the technical solutions and advantages of the present invention, and not to limit the scope of protection of the present invention. Without departing from the core ideas and technical effects of the present invention, those skilled in the art can make any form of improvement, substitution, combination, or modification to the structural arrangement, process parameters, material selection, control logic, etc., of the described embodiments; any obvious changes based on the same concept should be considered equivalent solutions of the present invention and should be included within the scope of protection defined by the claims of the present invention. The actual scope of protection of the present invention is determined by the appended claims and should be correctly understood in conjunction with the specification and accompanying drawings.

Claims

1. A molding die for sintering NdFeB magnets, characterized in that, include: The mold cavity is formed by a left mold cavity magnetic plate (3), a right mold cavity magnetic plate (4), a front mold cavity non-magnetic plate (11), and a rear mold cavity non-magnetic plate (10). The front mold cavity non-magnetic plate (11) and the rear mold cavity non-magnetic plate (10) are arranged opposite each other along a direction parallel to the line connecting the magnetic pole heads. The inner surface of the front mold cavity non-magnetic plate (11) facing the mold cavity is a first concave cylindrical surface, and the inner surface of the rear mold cavity non-magnetic plate (10) facing the mold cavity is a second concave cylindrical surface. The first concave cylindrical surface and the second concave cylindrical surface are arranged opposite each other. The inner surfaces of the left mold cavity magnetic plate (3) and the right mold cavity magnetic plate (4) arranged opposite each other perpendicular to the magnetic field orientation direction are planes. The upper pressure head (5) and the lower pressure head (6) are driven by the upper pressure rod (8) and the lower pressure rod (9) respectively, and are positioned opposite each other above and below the mold cavity; The upper pressure head (5) has a first concave spherical surface for pressing powder facing the mold cavity, a first convex cylindrical surface facing the non-magnetic plate (11) of the mold cavity in front, and a second convex cylindrical surface facing the non-magnetic plate (10) of the mold cavity in the rear. The pressing head (6) has a second concave spherical surface for pressing powder facing the mold cavity, a third convex cylindrical surface facing the non-magnetic plate (11) of the mold cavity in front, and a fourth convex cylindrical surface facing the non-magnetic plate (10) of the mold cavity in the rear. Wherein, the first concave spherical surface and the second concave spherical surface are oppositely arranged concave spherical surfaces used to press the upper and lower surfaces of the blank into convex spherical surfaces; the first convex cylindrical surface and the third convex cylindrical surface are both convex cylindrical surfaces with the same curvature as the first concave cylindrical surface and closely fitted; the second convex cylindrical surface and the fourth convex cylindrical surface are both convex cylindrical surfaces with the same curvature as the second concave cylindrical surface and closely fitted.

2. The forming die according to claim 1, characterized in that, The transverse arc of the first concave spherical surface satisfies the following relation: Where K1 is a relational constant, with a value ranging from 50 to 100; a1 is the chord length of the transverse arc of the first concave spherical surface; X is the final target height of the green blank pressed using the mold; and b1 is the bow height corresponding to the transverse arc.

3. The forming die according to claim 2, characterized in that, The arc height *c* of the transverse arcs of the first concave cylinder, the second concave cylinder, the first convex cylinder, the second convex cylinder, the third convex cylinder, and the fourth convex cylinder satisfies the following relationship: .

4. The forming die according to any one of claims 1 to 3, characterized in that, The upper pressure head (5) and the lower pressure head (6) are respectively flat on the side of the left mold cavity magnetic guide plate (3) and the right mold cavity magnetic guide plate (4), and are closely attached to the inner surface of the plane of the left mold cavity magnetic guide plate (3) and the right mold cavity magnetic guide plate (4).

5. The forming die according to any one of claims 1 to 3, characterized in that, The top of the upper pressure head (5) and the bottom of the lower pressure head (6) have horizontal cross sections.

6. The forming die according to any one of claims 1 to 3, characterized in that, The upper pressure head (5), lower pressure head (6), left mold cavity magnetic plate (3), right mold cavity magnetic plate (4), front mold cavity non-magnetic plate (11) and rear mold cavity non-magnetic plate (10) are all made of non-magnetic materials.

7. A method for forming sintered NdFeB magnets using a forming die as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Pre-compression parameter calculation: Determine the final target height X of the green compact; measure the loose packing density ρ of the NdFeB magnetic powder. 松装 and the true density ρ 振实 Set a proportionality coefficient K3, with a value ranging from 20% to 50%. According to the formula ρ0 = (1-K3) × ρ 松装 + K3× ρ 振实 Calculate the target pre-compression density ρ0; based on the powder mass M, the target pre-compression density ρ0, and the bottom area S of the mold cavity, calculate the target pre-compression height. H = M / (ρ0 × S); S2. Powder loading and pre-compression: The NdFeB magnetic powder of mass M is loaded into the mold cavity. Under no magnetic field conditions, the upper pressure head (5) and the lower pressure head (6) are controlled to move towards each other, and a pressure of 1-3 MPa is applied to compress the powder to the pre-compression target height. H This forms a pre-pressed blank; S3. Magnetic field orientation forming: An orientation magnetic field of 1.5T-2.0T is applied to the pre-pressed billet, and the pressure is increased to 6-10MPa for orientation pressing. After holding the pressure, a green billet with four arc-shaped surfaces is obtained. The height of the green billet is the final target height X, and X < H. S4. Sintering and processing: The green blank is sintered to obtain a sintered blank, and then machined to obtain the final cuboid magnet.

8. The method according to claim 7, characterized in that, In step S3, the holding time for the orientation pressing is 5-20 seconds.

9. The method according to claim 7, characterized in that, The formula for calculating the bottom area S of the mold cavity is S = a1 × a2, where a1 is the transverse chord length of the first concave sphere and a2 is its longitudinal chord length.

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