A two-stage draw down forming die for preventing cracking of a workpiece

Abstract

The present application belongs to the field of stamping die, and particularly relates to a two-stage lower punch forming die for preventing workpiece cracking, which comprises a female die, an upper punch, a core rod, an inner lower punch, an outer lower punch, an outer lower punch seat, an inner lower punch seat, an inner driving rod and an outer driving rod, further comprises an elastic pre-pressing assembly and a rigid follow-up limiting assembly. The elastic pre-pressing assembly is arranged between the outer lower punch seat and the inner lower punch seat, and always applies a downward pre-pressing force to the inner lower punch seat; the rigid follow-up limiting assembly is connected between the outer lower punch seat and the inner lower punch seat, and has a preset movement gap in the axial direction; when the demolding is started, the pre-pressing force of the elastic pre-pressing assembly offsets the initial thrust of the inner driving rod, and prevents the inner lower punch from moving first due to small frictional resistance at the small cross-section part. The synchronization of the inner and outer lower punches during the demolding starting is forcibly realized through a pure mechanical structure, the micro-cracks caused by asynchronous shearing impact at the step root are eliminated, and the product yield is improved.

Description

Technical Field

[0001] This invention belongs to the field of stamping die technology, specifically a two-stage bottom punch forming die for preventing workpiece cracking. Background Technology

[0002] Existing dual-stage punch dies commonly employ independent drive systems, with two drive sources controlling the outer and inner punches separately. Each punch allows for the setting of its own displacement and speed curves, thus resolving the issues of uneven density in the step area and improper preset demolding timing caused by the synchronous movement of the inner and outer punches from a static design perspective. However, when pressing I-shaped or base-type inductor cores with copper coils, scattered and difficult-to-eliminate micro-cracks still appear at the root of the step during the demolding stage. After the pressure holding ends and the upper punch retracts, the large cross-section of the workpiece corresponding to the outer lower punch forms a high static friction self-locking force with the inner wall of the die due to radial residual stress; the friction force of the small cross-section corresponding to the inner lower punch is much smaller; when the control system issues an upward demolding command at the same time, the inner lower punch can respond to the movement immediately, while the outer lower punch must first establish a thrust peak sufficient to overcome the high static friction force in an instant, resulting in an uncontrollable lag in its actual displacement starting point relative to the inner lower punch; this asynchronous start caused by the dynamic difference in friction force causes the root of the step to be subjected to shear impact exceeding its green strength at the beginning of demolding, resulting in microcracks; this lag is sensitive to variables such as powder batch characteristics and mold temperature changes, and cannot be compensated by a preset fixed demolding sequence.

[0003] Therefore, a two-stage bottom punch forming die is proposed to address the above problems and prevent workpiece cracking. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of the prior art by providing a two-stage downward forming die to prevent workpiece cracking, thereby solving the technical problems mentioned in the background art.

[0005] To address the above technical problems, the following technical solution is adopted: A two-stage downward forming die for preventing workpiece cracking, comprising: A female mold, wherein an axially penetrating cavity is provided inside the female mold; An upper punch, which is axially movable and positioned above the cavity; A mandrel, which is axially movable and passes through the center of the cavity; An inner lower punch is axially movable and sleeved on the outside of the mandrel, with the top of the inner lower punch extending into the cavity. An outer lower punch is axially movable and sleeved on the outside of the inner lower punch, with the top of the outer lower punch extending into the cavity; An outer lower punch seat, which is fixedly connected to the bottom end of the outer lower punch; An inner lower punch seat is fixedly connected to the bottom end of the inner lower punch, and the inner lower punch seat is slidably disposed in the internal cavity of the outer lower punch seat along the axial direction; An inner drive rod, the top end of which is fixedly connected to the inner lower punch seat; An external drive rod, the top end of which is fixedly connected to the external lower punch seat; Its characteristic is that it further includes: An elastic preload assembly is disposed between the outer lower punch and the inner lower punch, and always applies a downward preload to the inner lower punch. A rigid follower limiting component is connected between the outer lower punch and the inner lower punch. When the inner lower punch is pressed to its lowest position by the elastic pre-compression component, the rigid follower limiting component leaves a preset movement gap in the axial direction. Only when the outer lower punch moves upward and overcomes the movement gap will the rigid follower limiting component forcibly connect the outer lower punch and the inner lower punch into a whole and move upward synchronously.

[0006] Preferably, the elastic preload assembly includes at least one disc spring, which is disposed between the top wall of the inner cavity of the outer lower punch and the top surface of the inner lower punch, and is always in a compressed state.

[0007] Preferably, the downward preload applied by the elastic preload assembly is greater than the sum of the initial frictional resistance experienced by the small cross-section of the inner lower punch when it starts demolding and the weight of the inner lower punch and its connected components.

[0008] Preferably, the rigid follower limiting component includes: A radially inwardly protruding annular step is disposed on the inner cavity wall of the outer lower punch seat; A radially outwardly protruding annular flange is disposed on the outer peripheral surface of the inner lower punch and located below the annular step; The movable gap is the axial distance between the upper end face of the annular flange and the lower end face of the annular step when the inner lower punch is pressed into its lowest position by the elastic pre-compression component.

[0009] Preferably, the annular step and the outer lower punch are integrally formed, and the annular flange and the inner lower punch are integrally formed.

[0010] Preferably, a sliding guide sleeve is provided between the outer peripheral surface of the inner lower punch and the inner cavity wall of the outer lower punch.

[0011] Preferably, the inner cavity of the outer lower punch has an opening at the bottom, through which the inner drive rod passes and is fixedly connected to the bottom end face of the inner lower punch.

[0012] Preferably, the inner drive rod and the outer drive rod are driven by independent servo electric cylinders.

[0013] Preferably, the outer lower punch is used to form the large cross-section of the workpiece, and the inner lower punch is used to form the small cross-section of the workpiece. The workpiece is an integrally formed inductor core with a pre-embedded coil and a stepped structure.

[0014] Preferably, the preload provided by the elastic preload assembly is set as follows: At the moment of demolding start, when the outer lower punch has not yet moved due to overcoming the high static friction self-locking force of its corresponding large cross-section, the pre-pressure counteracts all the upward thrust applied by the inner drive rod, forcing the inner lower punch and the outer lower punch to remain synchronously stationary. Until the upward thrust on the outer lower punch accumulates to a level sufficient to overcome the high static friction self-locking force and begins to move upward, after the outer lower punch has completed the movement gap, the rigid follower limiting component forces the inner lower punch to move upward together.

[0015] The beneficial effects of this invention are: An elastic preload assembly is placed between the outer and inner lower punches to continuously apply downward preload to the inner lower punch. At the moment of demolding, this preload counteracts the initial upward thrust applied to the inner lower punch by the inner drive rod, preventing the inner lower punch from moving independently first due to the low frictional resistance at its small cross-section, thus causing the inner lower punch to lose the mechanical condition of asynchronous advance.

[0016] An axial clearance is pre-set between the outer and inner lower punches by a rigid follower limiting component. This allows the outer lower punch to complete the clearance first during demolding to accumulate the thrust required to overcome the high static friction self-locking force. The rigid follower limiting component then forces the outer and inner lower punches together to move upward synchronously, thereby eliminating the asynchronous start of the inner and outer lower punches caused by the dynamic difference in friction force at the moment of demolding. This prevents the workpiece step root from being subjected to shear impact, thus preventing the generation of microcracks. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] In the attached diagram: Figure 1 This is a front view of the overall structure of the mold in this embodiment; Figure 2 This is a side view of the overall structure of the mold in this embodiment; Figure 3 This is a schematic cross-sectional view of the mold in this embodiment; Figure 4 This is the embodiment. Figure 3 A magnified schematic diagram of a portion of the middle mold structure.

[0019] Legend: 1. Female mold; 2. Upper punch; 3. Core rod; 4. Inner lower punch; 5. Outer lower punch; 6. Outer lower punch seat; 7. Inner lower punch seat; 8. Inner drive rod; 9. Outer drive rod; 10. Elastic preload assembly; 10a. Disc spring; 10b. Helical compression spring; 11. Rigid follower limiting assembly; 11a. Annular step; 11b. Annular flange; δ. Movement clearance. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0021] Specific implementation examples are given below.

[0022] Example 1 Please see Figures 1-4 This invention provides a double-stage bottom punch forming die for preventing workpiece cracking. This embodiment provides a double-stage bottom punch forming die for preventing workpiece cracking. The die is used to press an integrally formed inductor core with a pre-embedded coil and a stepped structure. The inductor core has a large cross-section portion and a small cross-section portion. The large cross-section portion is, for example, a flange or a base, and the small cross-section portion is, for example, a magnetic column.

[0023] The mold in this embodiment includes a female mold 1, an upper punch 2, a mandrel 3, an inner lower punch 4, an outer lower punch 5, an outer lower punch seat 6, an inner lower punch seat 7, an inner drive rod 8, an outer drive rod 9, an elastic preload assembly 10, a rigid follow-up limiting assembly 11, and a sliding guide sleeve.

[0024] The female mold 1 is an integral structure made of high-strength mold steel. The female mold 1 has an axially penetrating cavity. The cross-sectional shape of the cavity matches the outer contour of the inductor core to be formed. The inner wall of the cavity has a high surface finish in the area corresponding to the large cross-section of the workpiece to withstand the high pressure friction during pressing and demolding. The female mold 1 is fixedly installed on the lower template of the mold frame.

[0025] The upper punch 2 is axially movable above the cavity; the upper end of the upper punch 2 is connected to the upper drive cylinder. In the pressing process, the upper punch 2 is pressed into the cavity from top to bottom, applying downward pressure to the soft magnetic metal powder in the cavity, and pressing the powder into a green blank; after the pressure holding process is completed and before the demolding process begins, the upper punch 2 first retracts upward and completely leaves the cavity, making room for the green blank to be ejected upward.

[0026] The mandrel 3 is axially movable and inserted into the center of the cavity; the mandrel 3 is used to form the central through hole of the inductor core, and its outer diameter is consistent with the diameter of the central through hole; the bottom end of the mandrel 3 is fixed on the mandrel seat, which is controlled by an independent drive mechanism and can move axially as needed during pressing and demolding.

[0027] The inner lower punch 4 is axially movable and sleeved on the outside of the mandrel 3; the inner lower punch 4 is a hollow cylindrical structure, and its inner hole slides in fit with the outer circumferential surface of the mandrel 3; the top of the inner lower punch 4 extends into the cavity, and the end face of the top is used to form the small cross-section of the workpiece, that is, the lower end face of the magnetic column of the inductor core; the bottom of the inner lower punch 4 is fixedly connected to the inner lower punch seat 7.

[0028] The outer lower punch 5 is axially movable and sleeved on the outside of the inner lower punch 4; the outer lower punch 5 is a hollow cylindrical structure, and its inner hole slides in fit with the outer peripheral surface of the inner lower punch 4; the top of the outer lower punch 5 extends into the cavity, and the end face of the top is used to form the large cross-section of the workpiece, namely the flange or the lower end face of the base of the inductor core; the bottom of the outer lower punch 5 is fixedly connected to the outer lower punch seat 6.

[0029] The outer lower punch 6 is fixedly connected to the bottom end of the outer lower punch 5. The outer lower punch 6 and the outer lower punch 5 are fastened together by bolts, and the connecting surfaces are precision ground to ensure their coaxiality and perpendicularity. The outer lower punch 6 has an internal cavity, which is an upward-opening cylindrical space enclosed by a bottom wall and a peripheral wall. A through hole is opened in the center of the bottom wall of the internal cavity for the inner drive rod 8 to pass through. The bottom end face of the outer lower punch 6 is fixedly connected to the top end of the outer drive rod 9.

[0030] The inner lower punch 7 is fixedly connected to the bottom end of the inner lower punch 4; the inner lower punch 7 and the inner lower punch 4 are fastened together by bolts, and the connecting surfaces are also precision ground; the inner lower punch 7 is slidably disposed in the inner cavity of the outer lower punch 6 along the axial direction; a sliding guide sleeve is provided between the outer peripheral surface of the inner lower punch 7 and the peripheral wall of the inner cavity of the outer lower punch 6. The sliding guide sleeve is made of self-lubricating copper alloy material and is fitted into the peripheral wall of the inner cavity of the outer lower punch 6 by interference fit; the sliding guide sleeve and the outer peripheral surface of the inner lower punch 7 are in a small clearance sliding fit, which ensures that the inner lower punch 7 slides smoothly and without obstruction in the axial direction in the inner cavity of the outer lower punch 6, and also ensures that its radial positioning is accurate and without shaking, thereby ensuring that the coaxiality of the inner lower punch 4 and the outer lower punch 5 is always maintained in dynamic movement.

[0031] The top end of the inner drive rod 8 passes through the through hole on the bottom wall of the inner cavity of the outer lower punch 6 and is fixedly connected to the bottom end face of the inner lower punch 7 by threads; the bottom end of the inner drive rod 8 is connected to the piston rod of the inner lower punch drive cylinder; the inner drive rod 8 is used to transmit the power of the inner lower punch drive cylinder to the inner lower punch 7 to drive the inner lower punch 4 to move axially; a bushing is provided between the inner drive rod 8 and the through hole on the bottom wall of the outer lower punch 6 to ensure smooth axial sliding of the inner drive rod 8.

[0032] The top end of the outer drive rod 9 is fixedly connected to the bottom end face of the outer lower punch seat 6 by a thread; the bottom end of the outer drive rod 9 is connected to the piston rod of the outer lower punch drive cylinder; the outer drive rod 9 is used to transmit the power of the outer lower punch drive cylinder to the outer lower punch seat 6, so as to drive the outer lower punch 5 to move axially.

[0033] In this embodiment, both the inner and outer lower punch drive cylinders are servo electric cylinders, and the inner drive rod 8 and the outer drive rod 9 are driven by independent servo electric cylinders. During the pressing stage, the two servo electric cylinders independently control the movement of the inner lower punch 4 and the outer lower punch 5 according to the preset displacement and pressure curves, so as to achieve precise adjustment of the powder filling amount and compression ratio of the large cross-section and small cross-section parts, and ensure that the density of each part of the green body is uniform.

[0034] The elastic preload assembly 10 is disposed between the outer lower punch 6 and the inner lower punch 7, and always applies a downward preload to the inner lower punch 7; specifically, the elastic preload assembly 10 includes a disc spring 10a. The disc spring 10a is disposed between the bottom wall of the inner cavity of the outer lower punch 6 and the bottom end face of the inner lower punch 7; the lower end of the disc spring 10a abuts against the upper end face of the bottom wall of the inner cavity, and the upper end abuts against the bottom end face of the inner lower punch 7, and is always in a compressed state. Its initial compression is preset by the axial installation position of the inner lower punch 7 in the inner cavity of the outer lower punch 6; the disc spring 10a has the mechanical characteristic of providing a large elastic force with a small deformation, and can provide a sufficient and stable preload within the limited axial installation space of the mold.

[0035] The preload applied by the elastic preload assembly 10 must be set to be greater than the sum of the initial frictional resistance of the small cross-section of the inner lower punch 4 when demolding starts and the weight of the inner lower punch 4 and its connected components. The components connected to the inner lower punch 4 include the inner lower punch seat 7 and the inner drive rod 8. The initial frictional resistance is the component of the static friction force between the small cross-section of the workpiece and the inner wall of the cavity of the female mold 1 in the demolding direction. By ensuring that the preload is greater than the sum of these two forces, it can be guaranteed that at the moment of demolding start, the upward thrust applied by the inner drive rod 8 is completely offset by the preload, and the inner lower punch seat 7 and the inner lower punch 4 are forced to remain stationary and cannot move upward independently.

[0036] A rigid follower limiting assembly 11 is connected between the outer lower punch 6 and the inner lower punch 7. The rigid follower limiting assembly 11 includes a radially inwardly protruding annular step 11a and a radially outwardly protruding annular flange 11b. The annular step 11a is located on the peripheral wall of the inner cavity of the outer lower punch 6, slightly above the center of the cavity. The annular step 11a and the outer lower punch 6 are integrally formed, machined by stepped boring of the inner cavity of the outer lower punch 6, resulting in extremely high structural rigidity and impact resistance. The annular flange 11b is located on the outer peripheral surface of the inner lower punch 7, at the center of the inner lower punch 7. The annular flange 11b and the inner lower punch 7 are integrally formed, machined by turning, also exhibiting extremely high structural rigidity. In the assembled state, the annular flange 11b is located below the annular step 11a.

[0037] When the inner lower punch 7 is pressed upward to the highest position by the disc spring 10a of the elastic preload assembly 10 (that is, the inner lower punch 7 is at the highest position relative to the outer lower punch 6), there is a preset distance in the axial direction between the upper end face of the annular flange 11b and the lower end face of the annular step 11a. This distance is the movable clearance δ. The value of the movable clearance δ is determined according to the mold specifications and product size, and is generally in the range of 0.1mm to 1mm. The function of the clearance δ is to allow the outer lower punch 6 to move upward a small, preset distance relative to the inner lower punch 7 during the demolding start-up phase, so as to complete the process of accumulating thrust and overcoming static friction. Only when the distance the outer lower punch 6 moves upward is equal to the clearance δ will the lower end face of the annular step 11a make rigid contact with the upper end face of the annular flange 11b. The annular step 11a and the annular flange 11b form a rigid axial limit, forcibly connecting the outer lower punch 6 and the inner lower punch 7 into one unit. The two can only move upward synchronously, and the inner lower punch 7 cannot produce any axial lag or advance relative to the outer lower punch 6.

[0038] During the powder loading stage, the upper punch 2 is positioned above the cavity, while the inner lower punch 4 and outer lower punch 5 are driven to their respective filling heights by their servo electric cylinders according to the preset powder loading positions, thereby forming a powder loading space within the cavity that matches the amount of powder in each part of the workpiece. The wound copper coil is placed inside the cavity, and then the soft magnetic metal powder is filled into the cavity, with the powder enveloping the coil and filling the entire powder loading space.

[0039] During the pressing stage, the upper punch 2 is pressed downward into the cavity; the inner drive rod 8 and the outer drive rod 9, driven by their respective servo electric cylinders, move downward independently or float according to the preset position and pressure curve, controlling the displacement and back pressure of the inner lower punch 4 and the outer lower punch 5, thereby adjusting the powder compression ratio of the small cross-section and the large cross-section of the workpiece respectively, so that the overall density of the green blank is uniform; during this stage, the disc spring 10a of the elastic pre-pressing component 10 is in a compressed state, and the disc spring 10a presses the inner lower punch seat 7 upward onto the top wall of the cavity inside the outer lower punch seat 6 (the upper limit surface formed by the annular step 11a or the upper end face of the cavity), and the upper end face of the annular flange 11b and the lower end face of the annular step 11a maintain an active gap δ; since both the inner and outer lower punches are subjected to downward force during the pressing process, the rigid follower limiting component 11 does not participate in the force transmission.

[0040] After pressing is completed, the pressure holding stage begins; the upper punch 2, inner lower punch 4 and outer lower punch 5 all remain in the same position to maintain the molding pressure in the cavity for a certain period of time in order to eliminate the elastic internal stress between powder particles.

[0041] After the pressure holding period, the demolding stage begins. First, the upper punch 2 retracts upwards, completely leaving the cavity, leaving the top of the cavity open to provide space for the green blank to be ejected. At this point, the green blank inside the cavity has been formed. The large cross-section of the outer lower punch 5 (flange or base) has accumulated a large amount of radial residual stress during the pressing process due to its large radial dimension. This residual stress creates a large radial pressure between the outer circumferential surface of the large cross-section of the workpiece and the inner wall of the cavity of the female mold 1, thereby generating a high static friction self-locking force. The small cross-section of the inner lower punch 4 (magnetic column) has a small radial dimension, and its contact area with the inner wall of the cavity and its radial residual stress are much smaller than those of the large cross-section. Therefore, the static friction resistance it experiences is also much smaller than the high static friction self-locking force of the large cross-section.

[0042] When demolding starts, the control system simultaneously sends an upward movement command to the outer lower punch drive cylinder and the inner lower punch drive cylinder; the outer drive rod 9 begins to push the outer lower punch seat 6 upward, but due to the abnormally large static friction self-locking force of the large cross-section part, the outer lower punch seat 6 and the outer lower punch 5 do not produce actual displacement for the time being, and the thrust of the outer drive rod 9 continues to accumulate; at the same time, the inner drive rod 8 also pushes the inner lower punch seat 7 upward. However, because the disc spring 10a of the elastic preload assembly 10 applies an upward preload to the bottom surface of the inner lower punch 7 (i.e., pushes the inner lower punch 7 downward relative to the outer lower punch 6), and this preload is greater than the sum of the initial frictional resistance of the small cross-section of the inner lower punch 4 and the weight of the inner lower punch 4 and its connected components (inner lower punch 7, inner drive rod 8), the initial thrust of the inner drive rod 8 is completely offset by the preload, and the inner lower punch 7 is firmly pressed against the top wall limiting surface of the inner cavity of the outer lower punch 6, and cannot produce any upward independent movement.

[0043] When the thrust of the outer drive rod 9 continues to accumulate to the moment that it is sufficient to overcome the high static friction self-locking force of the large cross-section, the outer lower punch 6 and the outer lower punch 5 begin to move upward; at the beginning stage of the upward movement of the outer lower punch 6, since the inner lower punch 7 is still pressed in place by the disc spring 10a, the annular flange 11b remains stationary, the annular step 11a rises together with the outer lower punch 6, and the movement gap δ between the lower end face of the annular step 11a and the upper end face of the annular flange 11b gradually decreases. After completing the movement gap δ, the lower end face of the annular step 11a abuts against the upper end face of the annular flange 11b, and the rigid follower limiting component 11 closes. At this time, the outer lower punch 6 and the inner lower punch 7 are forcibly connected as one unit through the rigid contact between the annular step 11a and the annular flange 11b. The outer lower punch 6 continues to move upward, pushing the upper end face of the annular flange 11b upward through the lower end face of the annular step 11a, thereby forcibly driving the inner lower punch 7 and the inner lower punch 4 to move upward together. From this point on, the outer lower punch 5 and the inner lower punch 4 achieve absolutely synchronous demolding, and the two eject the green blank from the cavity at the same speed.

[0044] Throughout the entire demolding start-up process, the inner lower punch 4 never gets an opportunity to move independently ahead of the outer lower punch 5. The outer lower punch 5 and the inner lower punch 4 are completely synchronized from the very first moment they begin to move. The root of the step of the green blank no longer bears any shear impact or tensile stress caused by asynchronous movement, and microcracks are fundamentally eliminated. At the same time, since the elastic pre-compression component 10 and the rigid follow-up limit component 11 are both purely mechanical structures, their response speed is not limited by the sampling frequency and signal delay of the electronic control system. They can work reliably under any transient conditions during demolding start-up and have a natural adaptability to friction fluctuations caused by changes in powder batch characteristics and mold temperature. They do not need to rely on any preset demolding timing curve or real-time feedback control system.

[0045] Example 2 In this embodiment, the elastic preload assembly 10 includes four helical compression springs 10b. The four helical compression springs 10b are evenly distributed circumferentially, that is, the central angle between two adjacent helical compression springs 10b is 90 degrees. The upper end of each helical compression spring 10b abuts against the top wall of the inner cavity of the outer lower punch 6, and the lower end abuts against the top surface of the inner lower punch 7, and is always in a compressed state. Four spring positioning holes are correspondingly opened on the top wall of the inner cavity of the outer lower punch 6, and four spring positioning holes are correspondingly opened on the top surface of the inner lower punch 7. The two ends of the spiral compression spring 10b are respectively sunk into the corresponding spring positioning holes to prevent the spring from shifting laterally during compression. Four helical compression springs 10b together apply downward preload to the inner lower punch 7. Since the four springs are symmetrically distributed, the resultant force of the preload coincides with the axis of the mold and will not generate a deflection torque on the inner lower punch 7.

[0046] Compared with the disc spring 10a used in Embodiment 1, the advantage of using multiple helical compression springs 10b in this embodiment is that the helical compression springs 10b have a larger compression stroke, and under the same preload value requirement, the initial compression of the spring is larger, which makes the preload less sensitive to the spring manufacturing tolerance and installation position tolerance, and the preload value is easier to set and adjust precisely by adjusting the initial compression of the spring or replacing springs with different stiffnesses. This solution is suitable for production scenarios where product specifications change frequently. It allows for easy replacement of spring assemblies to meet the different frictional differences between large and small cross-section parts, based on the demolding requirements of inductor core products of different specifications.

[0047] The working process of the mold in this embodiment is exactly the same as that in embodiment one: at the moment of demolding, the downward pre-pressure provided by the four spiral compression springs 10b prevents the inner lower punch 4 from moving upward alone; after the outer lower punch seat 6 overcomes the high static friction self-locking force of the large cross section and completes the movement gap δ, the inner lower punch 4 is forced to demold synchronously by the annular step 11a of the rigid follower limit component 11 abutting against the annular flange 11b, thereby eliminating the micro-cracks at the root of the green blank step.

[0048] In the description of this invention, it should be understood that the terms "front and back", "left and right", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0049] Of course, those skilled in the art should understand that in this technical solution, the term "a" should be understood as "at least one" or "one or more," that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple. The term "a" should not be construed as a limitation on the quantity.

[0050] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art under the technical guidance of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A two-stage bottom punch forming die for preventing workpiece cracking, characterized in that, include: A female mold (1) having an axially penetrating cavity inside; The upper punch (2) is axially movable and disposed above the cavity; The mandrel (3) is axially movable and passes through the center of the cavity; Inner lower punch (4), the inner lower punch (4) is axially movable and sleeved on the outside of the mandrel (3), and the top of the inner lower punch (4) extends into the cavity; Outer lower punch (5), the outer lower punch (5) is axially movable and sleeved on the outside of the inner lower punch (4), the top of the outer lower punch (5) extends into the cavity; Outer lower punch seat (6), the outer lower punch seat (6) is fixedly connected to the bottom end of the outer lower punch (5); Inner lower punch seat (7), the inner lower punch seat (7) is fixedly connected to the bottom end of the inner lower punch (4), and the inner lower punch seat (7) is slidably disposed in the inner cavity of the outer lower punch seat (6) along the axial direction; The inner drive rod (8) is fixedly connected to the inner lower punch (7) at its top end. An external drive rod (9) is fixedly connected to the external lower punch (6) at its top end. Also includes: An elastic preload assembly (10) is disposed between the outer lower punch (6) and the inner lower punch (7) and always applies a downward preload to the inner lower punch (7); A rigid follower limiting component (11) is connected between the outer lower punch (6) and the inner lower punch (7). When the inner lower punch (7) is pressed to the lowest position by the elastic pre-pressing component (10), the rigid follower limiting component (11) leaves a preset movable gap (δ) in the axial direction. Only when the outer lower punch (6) moves upward and overcomes the movable gap (δ) will the rigid follower limiting component (11) force the outer lower punch (6) and the inner lower punch (7) to be connected as a whole and move upward synchronously.

2. A double-stage bottom punch forming die for preventing workpiece cracking according to claim 1, characterized in that: The elastic preload assembly (10) includes at least one disc spring (10a), which is disposed between the top wall of the inner cavity of the outer lower punch (6) and the top surface of the inner lower punch (7), and is always in a compressed state.

3. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 2, characterized in that: The downward preload applied by the elastic preload assembly (10) is greater than the sum of the initial frictional resistance of the small cross-section of the inner lower punch (4) when it starts demolding and the weight of the inner lower punch (4) and its connected components.

4. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 1, characterized in that: The rigid follower limiting component (11) includes: A radially inwardly protruding annular step (11a) is provided on the inner cavity wall of the outer lower punch (6); A radially outwardly protruding annular flange (11b) is disposed on the outer peripheral surface of the inner lower punch (7) and located below the annular step (11a); The movable gap (δ) is the axial distance between the upper end face of the annular flange (11b) and the lower end face of the annular step (11a) when the inner lower punch (7) is pressed into the lowest position by the elastic preload assembly (10).

5. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 4, characterized in that: The annular step (11a) and the outer lower punch (6) are integrally formed, and the annular flange (11b) and the inner lower punch (7) are integrally formed.

6. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 1, characterized in that: A sliding guide sleeve is provided between the outer peripheral surface of the inner lower punch (7) and the inner cavity wall of the outer lower punch (6).

7. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 1, characterized in that: The inner cavity of the outer lower punch (6) has an opening at the bottom, through which the inner drive rod (8) passes and is fixedly connected to the bottom end face of the inner lower punch (7).

8. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 1, characterized in that: The inner drive rod (8) and the outer drive rod (9) are driven by independent servo electric cylinders.

9. A two-stage bottom punch forming die for preventing workpiece cracking according to claim 1, characterized in that: The outer lower punch (5) is used to form the large cross-section of the workpiece, and the inner lower punch (4) is used to form the small cross-section of the workpiece. The workpiece is an integrally formed inductor core with a pre-embedded coil and a stepped structure.

10. A two-stage bottom punch forming die for preventing workpiece cracking according to any one of claims 1 to 9, characterized in that, The preload provided by the elastic preload assembly (10) is set as follows: At the moment of demolding start, when the outer lower punch (6) has not yet moved due to overcoming the high static friction self-locking force of its corresponding large cross-section, the pre-pressure counteracts all the upward thrust applied by the inner drive rod (8), forcing the inner lower punch (7) and the outer lower punch (6) to remain synchronously stationary. Until the upward thrust on the outer lower punch (6) accumulates to a point where it is sufficient to overcome the high static friction self-locking force and begins to move upward, after the outer lower punch (6) has completed the movement gap (δ), the inner lower punch (7) is forcibly driven to move upward together by the rigid follower limit component (11).

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