Large-diameter rivet pin cold extrusion forming die and forming method thereof

Abstract

The invention discloses a large-diameter rivet pin cold extrusion forming die and a forming method thereof. The die is matched with a part forming machine for extrusion forming. The device is composed of a shearing die, an end face leveling die, a forward extrusion die, a step surface forming die, a pre-pressing die, an initial pressing die, a final pressing and slender rod forward extrusion composite forming die and clamps matched with all the dies and used for feeding. According to the large-diameter rivet pin cold extrusion forming die and the forming method thereof, automatic production is achieved, and manual operation is reduced; a heating procedure is omitted, and energy consumption is reduced; according to the method, aquadag is not needed, the field operation environment is improved, and the environment-friendly requirement is met; the production efficiency is about seven times that of a warm upsetting forming method; the method has good process stability, the size precision and strength of the mold meet the product requirements, and the procedures of shearing and blanking, end face leveling, forward extrusion, step surface forming, pre-pressing, initial pressing, final pressing, slender rod forward extrusion composite forming and the like can be continuously and automatically completed.

Description

Technical Field

[0001] This invention belongs to the field of fastener rivet processing technology, and relates to the forming and processing technology of rivets, specifically to a large-diameter rivet cold extrusion forming mold and its forming method. Background Technology

[0002] Rivet pins are widely used in railway freight car manufacturing. Made of 40Cr steel, they weigh 970 grams and feature a thicker rod (φ36mm) and a thinner rod (φ22.2mm), with a cross-sectional shrinkage rate of 62%. The mushroom-shaped cap is 15mm thick and φ58mm in diameter, making them difficult to form and suitable for hot forming. The current rivet pin manufacturing process is: bar sawing → heating → warm upsetting → machining the short tail → wire making → heat treatment and surface treatment. The warm upsetting process uses a two-station mechanical press: heating the billet to 750℃ → positive extrusion at station one → forging the cap and stepped surface at station two. Station one is hot extrusion, requiring graphite emulsion lubrication; the graphite emulsion spraying and billet transfer between stations are manual operations, with a production efficiency of approximately four pieces / min. While the warm upsetting process provides good billet plasticity and facilitates forming, it suffers from low automation, high labor intensity, low production efficiency, high heating energy consumption, and environmental pollution from graphite emulsion.

[0003] In view of the shortcomings of the warm upsetting process, and in order to improve production efficiency, reduce labor intensity, reduce energy consumption and eliminate environmental pollution while ensuring the forming quality of rivet pins, it is necessary to study a new forming process for rivet pins.

[0004] Patent document CN208895093U discloses a cold heading forming die for hexagonal flange stepped bolts, comprising four sets of male dies (first, second, third, and fourth station male dies) and four sets of female dies (first, second, third, and fourth station female dies). The die sequentially performs strengthening of the shank, head balling and upsetting, head hexagonal cutting, and head rounding, ensuring the product's dimensions and appearance meet requirements and reducing processing costs. However, this cold heading forming die for hexagonal flange stepped bolts can only be used for cold heading of hexagonal flange stepped bolts and cannot be used for manufacturing other special fasteners. The structure and shape of hexagonal flange stepped bolts differ significantly from those of large-diameter rivet pins, necessitating the development of a specialized cold extrusion forming method for manufacturing large-diameter rivet pins.

[0005] Patent document CN111872295A discloses a cold heading die assembly and cold heading process for manufacturing engine center-mounted bolts. The cold heading die assembly includes a first cold heading die, a second cold heading die, a third cold heading die, a fourth cold heading die, a fifth cold heading die, and a sixth cold heading die. By using six cold heading dies, the wire blank is cold-headed six times, which improves material utilization, reduces costs, and increases production efficiency. Furthermore, when cold-heading metal materials, appropriate plastic deformation can be produced, which increases the strength of the metal material more than the single-stage upsetting process of hydraulic pressing. However, the above-mentioned cold heading die assembly and cold heading process are only applied to engine center-mounted bolts and cannot be used to manufacture other products, such as large-diameter rivet pins.

[0006] Therefore, there is an urgent need to develop a cold extrusion molding method specifically for manufacturing large-diameter rivet pins. Summary of the Invention

[0007] This invention discloses a cold extrusion molding die and its molding method for large-diameter rivet pins, addressing the shortcomings of existing technologies. The purpose of this invention is to provide a cold extrusion molding die and its molding method for large-diameter rivet pins that features high production efficiency, low labor intensity, low energy consumption, no environmental pollution, and meets molding quality requirements.

[0008] The present invention is achieved through the following technical solution.

[0009] This invention first discloses a large-diameter rivet pin cold extrusion forming die, which is used in conjunction with a parts forming machine for extrusion forming. The die is characterized by being composed of a shearing die, an end face leveling die, a positive extrusion die, a stepped surface forming die, a pre-pressing die, an initial pressing die, a final pressing and thin rod positive extrusion composite forming die, and clamps for feeding each die.

[0010] Shearing dies are used to cut processed materials into fixed-length bars by shearing, and include shearing dies and shears;

[0011] The end face leveling mold is used to level the inclined end face after large diameter blanks are cut and blanked. It includes a punch at one station, a front core for one station, a rear core for one station, and an ejector pin for one station.

[0012] A positive extrusion die is used for extruding and forming the thin rod at the front of a bar stock, including a two-station punch, a two-station front die core, a two-station rear die core, and a two-station ejector pin;

[0013] The stepped surface forming mold is used for pre-forming the stepped surface and chamfer at the transition between the thick and thin rods, and at the same time extruding and forming the tapered positioning groove on the end face of the thin rod. It includes a three-station punch, a three-station die-making front core, a three-station die-making rear core, a three-station die-making ejector pin, a positioning pad, and a positioning spring.

[0014] Pre-pressing mold, used for pre-pressing the cap head and forming the chamfer of the thick rod, includes a four-station punch core, a four-station front punch core, a four-station middle punch core, a four-station rear punch core, a four-station ejector pin, a positioning pad, and a positioning spring;

[0015] The initial pressing mold is used for the initial pressing and forming of the cap head, including a five-station punch core, a five-station front punch core, a five-station middle punch core, a five-station rear punch core, a five-station ejector pin, a positioning pad, and a positioning spring.

[0016] The final pressing and thin rod positive extrusion composite molding die is used for the final forming of the cap head and the positive extrusion forming of the thin rod. It includes a six-station punch core, a six-station front die core, a six-station middle die core, a six-station rear die core, a six-station ejector pin, a positioning pad, and a positioning spring.

[0017] Furthermore, in the end-face leveling mold, the punch at one station and the ejector pin at one station are cylindrical flat-end ejector pin structures; a through hole of equal diameter is set at the center of the front core of the ejector pin at one station; a variable-diameter through hole with a stepped structure is set at the center of the rear core of the ejector pin at one station, the cone angle formed by the variable-diameter through hole is α1=120°, the radius of the corner of the variable-diameter through hole is R1=(D1a-D1b) / 2, the large inner hole of the mold cavity is D1a=D1+0.15mm, and the small inner hole of the mold cavity is D1b=0.6×D1, where D1 is the diameter of the bar stock. Because the raw material of the rivet pin has a large wire diameter, after the large-diameter wire is cut, there is an obvious shearing slope on the end face, which cannot be eliminated in subsequent forming. This defect can be eliminated after the end-face leveling process.

[0018] Furthermore, in the stepped surface forming mold, the three-station punch is a cylindrical flat-end ejector pin structure, and the three-station die-forming ejector pin is a cylindrical ejector pin structure with a flat side and a central cone at the end face; both the front die-forming core and the rear die-forming core of the three-station die-forming core are provided with through holes of equal diameter at their centers, with the inner diameter of the front die-forming core cavity being D3a=D2a+0.1mm and the inner diameter of the rear die-forming core cavity being D3b=D2b+0.05mm; a floating material support device is provided in the rear die-forming core of the three-station die-forming ejector pin; the diameter of the bottom surface of the central cone at the end face of the three-station die-forming ejector pin is D3d=3.5mm, the height of the cone is H3a=1mm, and the thickness of the side plane of the ejector pin is H3b=D3f-1mm; where D2a is the inner diameter of the cavity of the front die-forming core of the two-station die-forming core in the positive extrusion mold, D2b is the inner diameter of the reducing band at the rear end of the reducing through hole of the rear die-forming core of the two-station die-forming core, D3d is the diameter of the positioning spring, and D3f is the diameter of the ejector pin. Typically, the process of forming the stepped surface at the abrupt change in cross-section is combined with the pre-pressing process of the cap. However, the deformation of the stepped surface is insufficient, resulting in poor forming at the chamfer, which is difficult to eliminate in subsequent processes. This invention eliminates this defect by setting a separate forming process for the stepped surface at the abrupt change in cross-section.

[0019] Furthermore, in the pre-compression mold, the cavity of the four-station punch is shaped like a pre-compression cap, and the four-station ejector pin is a cylindrical ejector rod structure with a flat side and a central conical end face. The front and rear cores of the four-station punch have through holes of equal diameter at their centers. The inner diameter of the front core cavity is D4a = D3a + 0.08mm, and the inner diameter of the rear core cavity is D4c = D3b + 0.05mm. The middle core of the four-station punch has a variable diameter through hole with a stepped structure, and the inner diameter of the middle core cavity is D4b = D3b + 0.05mm. The four-station punch is trumpet-shaped. The cone angle of the die cavity formed by the expansion structure is α2=40°. The inner diameter of the die cavity front guide is D4=D3a+10mm, and the length is W4=1.5mm. The height of the die cavity is determined according to the volume of the initial upsetting cap V1=V2, where V2 is the volume of the product cap. A floating material support device is set after the four-station die-making process. The diameter of the bottom surface of the central cone of the ejector pin end face of the four-station die-making process is D3d=3.5mm, the height of the cone is H3a=1mm, and the thickness of the ejector pin side plane is H4b=D4f-1mm, where D4f is the diameter of the ejector pin.

[0020] Furthermore, in the aforementioned initial pressing mold, the five-station punch core cavity is shaped like an initial pressing cap, and the five-station ejector pin is a cylindrical ejector rod structure with flat sides and a central conical end face. The front and rear cores of the five-station ejector pin have through holes of equal diameter at their centers. The inner diameter of the front core cavity is D5a = D4a + 0.15mm, and the inner diameter of the rear core cavity is D5c = D4b + 0.05mm. The middle core of the five-station ejector pin has a variable diameter through hole with a stepped structure, and the inner diameter of the middle core cavity is D5b = D4b + 0.05mm; The dome arc of the mushroom-shaped die cavity of the five-station punch is R5a=0.85×D1, the opening diameter of the die cavity is D5=D0a-2mm, and the height of the die cavity is determined according to the volume of the initial upsetting head V1=V2; A floating material support device is set after the five-station die-making process; The bottom diameter of the central cone of the ejector pin end face of the five-station die-making process is D3d=3.5mm, the height of the cone is H3a=1mm, and the thickness of the ejector pin side plane is H5b=D5f-1mm, where D5f is the diameter of the ejector pin.

[0021] In the further described final pressing and thin rod positive extrusion composite molding die, the six-station punch cavity is shaped like a rivet cap, the six-station ejector pin is a cylindrical ejector pin structure with a flat side and a central conical end face, the front and rear punches of the six-station punch have through holes of equal diameter, the inner diameter of the front punch cavity is D6a=D5a+0.1mm, the inner diameter of the rear punch cavity is D6c=D5b+0.05mm, the middle punch of the six-station punch has a variable diameter through hole with a stepped structure, the variable diameter of the middle punch cavity with hole diameter D6b=D0d, where D0d is the diameter of the rivet pin thin rod; the rear punch of the six-station punch has a floating material support device; the diameter of the bottom surface of the central conical body of the six-station ejector pin end face is D3d=3.5mm, the height of the conical body is H3a=1mm, and the thickness of the side plane of the ejector pin is H6b=D6f-1mm, where D6f is the ejector pin diameter.

[0022] The floating material support device further includes: two opposing positioning spring mounting holes are provided for each die-casting core, with an inner diameter D3c = D3d + 0.2mm; ejector pins, positioning pads, and positioning springs are installed in the mounting holes and limited by end caps. Because the rivet pin rod is relatively long, during pre-pressing at the workstation, the clamps cannot be released before the blank is fed into the die-casting core cavity. However, the continuous movement of the punch at this time will impact the clamps. This invention provides a floating material support device, where the tip is inserted into the tip groove on the end face of the tip rod for positioning. Two springs apply pressure to the upper and lower planes of the ejector pin, generating a certain frictional resistance. Even after the clamps are released in advance, the blank can still be stably supported, ensuring accurate feeding of the blank into the die-casting core cavity while avoiding impact on the clamps.

[0023] This invention features a tapered positioning groove on the end face of a thin rod. The third station uses a top-tipped ejector pin to press out the tapered positioning groove on the end face of the thin rod during the forming of the blank at the third station. Subsequent fourth, fifth, and sixth stations are also equipped with top-tipped ejector pins to facilitate insertion into the tapered positioning groove on the end face of the thin rod of the blank. This achieves accurate positioning of the workpiece during floating material handling and allows it to be smoothly guided into the mold cavity of the next station.

[0024] The clamp thickness is the length of the thin rod plus 10mm. This invention uses a thick clamp to hold the thick rod of the billet. The clamp thickness is the length of the thin rod plus 10mm. The thick rod is held throughout the billet ejection process. When the billet is completely ejected, the clamp can still hold the thick rod. The clamping part is close to the center of gravity of the workpiece, which prevents the billet from tilting and avoids the defect of the billet being easily bitten when it is pushed into the mold cavity at the next station.

[0025] In the further preliminary pressing mold, final pressing mold, and thin rod positive extrusion composite molding mold, a pre-set mold compression elastic deformation amount Δ is provided. The compression elastic deformation amount Δ is 0.25% of the sum of the lengths L2 of the middle core and the rear core, and the length L1 of the front core is the length L-Δ of the rivet pin thick rod. This invention sets the mold compression elastic deformation amount so that during molding at the fifth and sixth stations, due to the large molding force, the step surface at the connection between the thick rod and the thin rod will first bear a large pressure. This pressure will be sequentially transmitted to the middle core and the rear core of the mold, thus generating a certain amount of compression elastic deformation. This value is approximately 0.25% of the sum of the lengths L2 of the middle core and the rear core. For example, if the length of the rivet pin thick rod is L, the length L1 of the front core should be L-Δ; otherwise, the length of the thick rod after molding will exceed the tolerance.

[0026] This invention relates to a mushroom-shaped cap head formed through three cold pressing processes. The mushroom-shaped cap head with rivets is relatively thin with a large diameter; the thickness of its circumferential edge is only 3-4 mm. If the traditional two-stage pre-pressing and final pressing process is used during cap head forming, excessive deformation will lead to significant tensile stress on the circumferential edge of the cap head, causing cracks. The three-stage forming process for the mushroom-shaped cap head, consisting of pre-pressing, initial pressing, and final pressing (stations four, five, and six), reduces the deformation at each station and avoids cracking defects on the circumferential edge of the cap head.

[0027] This invention also discloses a method for cold extrusion forming of large-diameter rivet pins. The method includes shearing, end face leveling, forward extrusion, stepped surface forming, pre-pressing, initial pressing, final pressing, and thin rod forward extrusion steps. The punches at each step are mounted on the main slide of the part forming machine and reciprocate. The die cores, middle cores, rear cores, ejector pins, positioning pads, and positioning springs at each step are mounted on the fixed base of the part forming machine. The blanks are automatically transferred between each step using clamps. The specific process includes:

[0028] 1) The cutting and blanking are completed by the shearing die and the shears. The size of the positioning stop is adjusted according to the blanking length. The straightened processing material is fed into and passes through the inner hole of the shearing die. When it contacts the front positioning stop, the shears move to complete the cutting of the fixed length bar.

[0029] 2) The cut bar stock is conveyed by clamps to the end face leveling station. The punch at station 1 pushes the bar stock into the cavity of the die core at station 1. After that, the clamps are released. The punch at station 1 continues to push the bar stock forward to the front dead center position. The end face leveling is completed in the cavity formed by the die core and ejector pin at station 1. The punch at station 1 retracts. The ejector pin at station 1 ejects the workpiece. When the workpiece is ejected 50mm out of the die core cavity at station 1, the clamps at station 2 close to clamp the bar stock until the workpiece is completely ejected.

[0030] 3) The flattened blank is transferred to the positive extrusion station 2 by the clamp. The punch at station 2 pushes the blank into the cavity of the die-cutting mold at station 1 by 30mm. Then the clamp is released and the punch at station 2 continues to push the blank forward to the front dead center position. The thin rod positive extrusion is completed in the cavity of the die-cutting mold at station 2. The punch at station 2 retracts and the ejector pin at station 2 ejects the workpiece. When the workpiece is ejected 50mm out of the cavity of the die-cutting mold at station 2, the clamp at station 3 closes to clamp the thick rod until the workpiece is completely ejected.

[0031] 4) The extruded billet is conveyed to the three-station step surface forming station. The three-station punch pushes the billet into the front core cavity of the three-station die-making station by 70mm. Then the clamps are released, and the three-station punch continues to push the billet forward to the front dead center position. The step surface is formed in the rear core cavity of the three-station die-making station. At the same time, the three-station ejector pin presses out a conical positioning groove on the end face of the thin rod of the billet. The three-station punch retracts, and the three-station die-making ejector pin ejects the workpiece. When the workpiece is ejected 50mm out of the front core cavity of the three-station die-making station, the four-station clamps close to clamp the thick rod until the workpiece is completely ejected.

[0032] 5) After the stepped surface is formed, the blank is transferred to the pre-pressing station 4. At the same time, the conical tip of the ejector pin of the station 4 pushes the blank and inserts into the conical positioning groove on the end face of the thin rod of the blank. After the blank is pushed into the cavity of the die-cutting core of the station 4 by 20mm, the clamp is released and the blank is stably clamped and will not fall. The die-cutting core of the station 4 continues to push the blank forward to the front dead point position to complete the pre-pressing of the head. The die-cutting core of the station 4 retracts and the ejector pin of the die-cutting core of the station 4 ejects the workpiece. When the workpiece is ejected 65mm out of the cavity of the die-cutting core of the station 4, the clamp of the station 5 closes to clamp the thick rod until the workpiece is completely ejected.

[0033] 6) The pre-pressed blank is transferred to the fifth station of the initial pressing. At the same time, the conical tip of the fifth station ejector pin is inserted into the conical positioning groove on the end face of the thin rod of the blank while the blank is pushed into the cavity of the fifth station die-cutting core 20mm. Then the clamp is released and the blank is stably clamped and will not fall. The fifth station punch continues to push the blank forward to the front dead point position to complete the initial pressing of the head. The fifth station punch retracts and the fifth station die-cutting ejector pin ejects the workpiece. When the workpiece is ejected 65mm out of the cavity of the fifth station die-cutting core, the sixth station clamp closes to clamp the thick rod until the workpiece is completely ejected.

[0034] 7) After initial pressing, the billet is transferred to the final pressing and fine rod positive extrusion six-station station. While the six-station punch pushes the billet, the conical tip of the six-station ejector pin is inserted into the conical positioning groove on the end face of the fine rod of the billet. After the billet is pushed into the cavity of the six-station die-making core 20mm, the clamp is released, and the billet is stably clamped and will not fall. The six-station punch continues to push the billet forward to the front dead center position to complete the final pressing of the cap head. At the same time, the positive extrusion of the fine rod is completed in the cavity of the six-station die-making core. The six-station punch retracts, and the six-station die-making ejector pin ejects the workpiece until the workpiece is completely ejected and falls into the discharge port; the cold extrusion forming of the rivet pin is completed.

[0035] For a long time, large-diameter rivet pins have been produced using the traditional warm upsetting method. This requires heating the blank, and the transfer of blanks between stations is all manual, resulting in high labor intensity and a production efficiency of approximately four pieces / min, which is inefficient. Existing production processes require the application of graphite emulsion for lubrication, leading to poor working conditions and severe environmental pollution. This invention employs a cold extrusion forming method, using wire rod on a six-station parts forming machine with cold extrusion dies to achieve the cold extrusion forming of large-diameter rivet pins.

[0036] This invention relates to a large-diameter rivet pin cold extrusion forming die and its forming method. During the cold extrusion process, the wire rod is automatically straightened and fed. Through the cold extrusion die and clamps, processes such as shearing, end face leveling, positive extrusion, stepped surface forming, pre-pressing, initial pressing, final pressing, and fine rod positive extrusion composite forming are completed. This realizes continuous and automated cold heading forming of rivet pins. Each wire rod weighs about two tons and can be continuously produced for about two hours.

[0037] This invention relates to a large-diameter rivet cold extrusion forming mold and its forming method, which realizes automated continuous production of rivets, replacing the existing warm upsetting forming method, and realizing the cold extrusion forming of large-size fasteners.

[0038] This invention relates to a large-diameter rivet pin cold extrusion forming die and its forming method, which achieves automated production and reduces manual operation; it eliminates the heating process, reducing energy consumption; it eliminates the need for graphite emulsion, improving the on-site working environment and meeting environmental protection requirements; it significantly improves production efficiency, approximately seven times that of the warm upsetting method; it exhibits good process stability, with die dimensional accuracy and strength meeting product quality and usage requirements, and can continuously and automatically complete processes such as shearing, end face leveling, positive extrusion, stepped surface forming, pre-pressing, initial pressing, final pressing, and thin rod positive extrusion composite forming, significantly improving production efficiency. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the rivet pin structure made according to the present invention; in the figure, a is the cap, b is the thick rod, c1 is the transition step rod, c2 is the thin rod, D0a is the diameter of the cap, D0b is the diameter of the thick rod, D0c is the diameter of the transition step rod, D0d is the diameter of the thin rod, W0 is the thickness of the cap, R0 is the radius of the cap arc, L is the length of the thick rod, and L0 is the length of the thin rod.

[0040] Figure 2 This is a schematic diagram of the workpiece formed by cold extrusion of the rivet pin at each station according to the present invention; in the figure, d is the sheared blank, e is the workpiece at station one, f is the workpiece at station two, g is the workpiece at station three, h is the workpiece at station four, i is the workpiece at station five, j is the workpiece at station six, W1 is the length of the sheared blank, D1 is the diameter of the sheared blank, V1 is the volume of the pre / initial pressing cap at station two, and V2 is the volume of the rivet pin cap.

[0041] Figure 3 This is a schematic diagram of the assembly of the dies for each station of the cold extrusion of the rivet pin in this invention.

[0042] Figure 4 This is a schematic diagram of the cross-sectional structure of the cold extrusion end face leveling die of the present invention; in the figure, A1a is a punch at a station, B1a is the front core of the die at a station, B1b is the rear core of the die at a station, α1 is the cone angle formed by the variable diameter through hole, R1 is the radius of the fillet of the variable diameter through hole, D1a is the large inner hole of the die cavity, and D1b is the small inner hole of the die cavity.

[0043] Figure 5 This is a schematic diagram of the cross-sectional structure of the cold extrusion two-station positive extrusion die of the present invention; in the figure, A2a is the two-station punch, B2a is the two-station front die core, B2b is the two-station rear die core, D2a is the inner diameter of the die cavity of the front die core, and D2b is the inner diameter of the reducing band at the rear end of the reducing through hole of the rear die core.

[0044] Figure 6This is a schematic diagram of the cross-sectional structure of the cold extrusion three-station stepped surface forming die of the present invention; in the figure, A3a is the three-station punch, B3a is the three-station front die core, B3b is the three-station rear die core, C3 is the three-station ejector pin, D3a is the inner diameter of the front die core cavity, D3b is the inner diameter of the rear die core cavity, and D3f is the diameter of the three-station ejector pin.

[0045] Figure 7 This is a schematic diagram of the cross-sectional structure of the four-station pre-pressing die for cold extrusion of the present invention; in the figure, A4a is the four-station punch core, B4a is the four-station front die core, B4b is the four-station middle die core, B4c is the four-station rear die core, C4 is the four-station ejector pin, D4a is the inner diameter of the front die core cavity, D4b is the inner diameter of the middle die core cavity, D4c is the inner diameter of the rear die core cavity, D4 is the inner diameter of the front guide of the four-station punch core cavity, W4 is the guide length, α2 is the cone angle of the punch core cavity, and D4f is the diameter of the four-station ejector pin.

[0046] Figure 8 This is a schematic diagram of the cross-sectional structure of the five-station cold extrusion initial pressing die of the present invention; in the figure, A5a is the five-station punch core, B5a is the five-station front die core, B5b is the five-station middle die core, B5c is the five-station rear die core, C5 is the five-station ejector pin, D5a is the inner diameter of the front die core cavity, D5b is the inner diameter of the middle die core cavity, D5c is the inner diameter of the rear die core cavity, R5a is the radius of the arc of the mushroom-shaped cavity dome of the punch core, D5 is the opening diameter of the punch core cavity, and D5f is the diameter of the five-station ejector pin.

[0047] Figure 9 This is a schematic diagram of the cross-sectional structure of the composite molding die for cold extrusion six-station final pressing and fine rod positive extrusion of the present invention; in the figure, A6a is the six-station punch core, B6a is the six-station front die core, B6b is the six-station middle die core, B6c is the six-station rear die core, C6 is the six-station ejector pin, D6a is the inner diameter of the front die core cavity, H6 is the depth of the punch core cavity, R6a is the radius of the arc of the dome surface of the mushroom-shaped die core cavity, D6b is the inner diameter of the middle die core cavity, D6c is the inner diameter of the rear die core cavity, D6 is the opening diameter of the punch core cavity, and D6f is the diameter of the five-station ejector pin.

[0048] Figure 10 This is a cross-sectional schematic diagram of the floating material support device of the present invention; in the figure, E1 is the ejector pin, E2 is the positioning pad, E3 is the positioning spring, E4 is the end cap, D3c is the inner diameter of the spring mounting hole, D3d is the diameter of the bottom surface of the central cone of the ejector pin end face of the three-station die-making ejector pin, H3a is the height of the cone, and H3 / 4 / 5 / 6b are the thickness of the ejector pin side plane of the three to six stations.

[0049] Figure 11 This is a schematic diagram of the ejector pin structure of the floating material support device of the present invention.

[0050] Figure 12 yes Figure 11Enlarged schematic diagram of part I in the middle.

[0051] Figure 13 yes Figure 11 Schematic diagram of the AA section structure.

[0052] Figure 14 This is a schematic diagram of the front outline of the thick clamp of the present invention.

[0053] Figure 15 This is a side view of the thick-type clamp of the present invention, i.e. Figure 14 A schematic diagram of the profile in direction A; in the diagram, H0 is the clamp thickness.

[0054] Figure 16 This is a schematic diagram of the cross-sections of the front kernel, middle kernel, and rear kernel at the fifth and sixth stations of the present invention, which are configured with elastic compression. In the diagram, L1 is the length of the front kernel, and L2 is the sum of the lengths of the middle kernel and the rear kernel. Specific Implementation

[0055] The present invention will be further described below with reference to specific embodiments. These specific embodiments are further explanations of the principles of the present invention and are not intended to limit the present invention in any way. Any technology that is the same as or similar to the present invention does not exceed the scope of protection of the present invention.

[0056] Refer to the attached diagram.

[0057] This embodiment discloses a cold extrusion forming die for large-diameter rivet pins and its forming process. It utilizes ML40Cr wire and a forming machine with a forging pressure ≥8000KN and six stations to achieve cold extrusion forming.

[0058] The process of this invention includes: placing ML40Cr wire on the feeding shaft of a six-station parts forming machine for automatic feeding; completing the blank shearing, end face leveling, positive extrusion, stepped surface forming, pre-pressing, initial pressing, final pressing, and thin rod positive extrusion composite forming processes through cold extrusion dies and clamps; continuously and automatically realizing the cold extrusion forming of large-diameter rivet pins; the cold extrusion die is designed to be six-station, and each station die mainly includes punches, die-cutting components, and ejector pins, etc.; the punches are installed on the main slide of the equipment and can reciprocate; the die-cutting components and ejector pins are installed on the fixed base of the equipment; and the blanks are automatically transferred between each station using clamps.

[0059] The raw material is ML40Cr wire with a diameter of φ35mm.

[0060] The cutting and blanking are completed by the shearing die and the shears. The size of the positioning stop is adjusted according to the blanking length. The straightened wire rod is fed in and passes through the inner hole of the shearing die. When it contacts the front positioning stop, the shears move to complete the cutting of the fixed length bar.

[0061] The sheared bar stock is conveyed by clamps to the end face leveling station. The punch at station 1 pushes the bar stock into the cavity of the die-casting core at station 1. After that, the clamps are released, and the punch at station 1 continues to push the bar stock forward to the front dead center position. The end face leveling is completed in the cavity formed by the die-casting core and ejector pin at station 1. The punch at station 1 retracts, and the ejector pin at station 1 ejects the workpiece. When the workpiece is ejected 50mm out of the die-casting core cavity at station 1, the clamps at station 2 close to clamp the bar stock until the workpiece is completely ejected. The flattened blank is conveyed by clamps to the second extrusion station. The second station punch pushes the blank 30mm into the cavity of the first station die-making core. Then the clamps are released, and the second station punch continues to push the blank forward to the front dead center position. The thin rod is extruded in the cavity of the second station die-making core. The second station punch retracts, and the second station die-making ejector pin ejects the workpiece. When the workpiece is ejected 50mm out of the cavity of the second station die-making core, the third station clamps close to clamp the thick rod until the workpiece is completely ejected. The extruded billet is conveyed to the three-station step surface forming station. The three-station punch pushes the billet 70mm into the front core cavity of the three-station die-making station. Then the clamps are released, and the three-station punch continues to push the billet forward to the front dead center position. The step surface is formed in the rear core cavity of the three-station die-making station. At the same time, the three-station ejector pin presses out a conical positioning groove on the end face of the thin rod of the billet. The three-station punch retracts, and the three-station die-making ejector pin ejects the workpiece. When the workpiece is ejected 50mm out of the front core cavity of the three-station die-making station, the four-station clamps close to clamp the thick rod until the workpiece is completely ejected. After the stepped surface is formed, the blank is transferred to the pre-pressing fourth station. At the same time, the conical tip of the fourth station ejector pin pushes the blank and inserts into the conical positioning groove on the end face of the thin rod of the blank. After the blank is pushed into the cavity of the fourth station die-cutting core 20mm, the clamps are released and the blank is stably clamped and will not fall. The fourth station die-cutting core continues to push the blank forward to the front dead point position to complete the pre-pressing of the head. The fourth station die-cutting core retracts and the fourth station die-cutting ejector pin ejects the workpiece. When the workpiece is ejected 65mm out of the cavity of the fourth station die-cutting core, the fifth station clamp closes to clamp the thick rod until the workpiece is completely ejected. The pre-pressed blank is conveyed to the fifth station of the initial pressing. At the same time, the conical tip of the fifth station ejector pin is inserted into the conical positioning groove on the end face of the thin rod of the blank while the blank is pushed into the cavity of the fifth station die-cutting core 20mm. Then the clamps are released and the blank is stably held and will not fall. The fifth station die-cutting core continues to push the blank forward to the front dead point position to complete the initial pressing of the head. The fifth station die-cutting core retracts and the fifth station die-cutting ejector pin ejects the workpiece. When the workpiece is ejected 65mm out of the cavity of the fifth station die-cutting core, the sixth station clamps close to clamp the thick rod until the workpiece is completely ejected.After initial pressing, the blank is transferred to the final pressing and thin rod positive extrusion composite forming station 6. At the same time, the conical tip of the ejector pin at station 6 pushes the blank, and the blank is inserted into the conical positioning groove on the end face of the thin rod of the blank. After the blank is pushed into the cavity of the front core of the die at station 6 by 20mm, the clamp is released, and the blank is stably clamped and will not fall. The front core of the die at station 6 continues to push the blank forward to the front dead point position, completing the final pressing forming of the cap head. At the same time, the positive extrusion of the thin rod is completed in the cavity of the middle core of the die at station 6. The front core of the die at station 6 retracts, and the ejector pin of the die at station 6 ejects the workpiece until the workpiece is completely ejected and falls into the discharge port. Thus, the cold extrusion forming of the rivet pin is completed.

[0062] like Figure 1 As shown, rivet pins are important fasteners on railway freight cars. Rivet pins consist of a cap, a thick rod, a transition step rod, and a thin rod. The diameter of the thick rod is significantly larger than that of conventional fasteners, classifying it as a large-diameter fastener. The cross-sectional shrinkage rate of the thick and thin rods is relatively large. The mushroom-shaped cap has a small thickness and a large diameter, resulting in a large degree of deformation during forming, which is not conducive to cold extrusion forming. For a long time, it has been manufactured using the warm upsetting method.

[0063] like Figure 2 , Figure 3 As shown, the cold extrusion process for rivet pins includes: shearing (d), end face leveling (e), forward extrusion (f), step surface forming (g), pre-pressing (h), initial pressing (i), final pressing, and composite forming of thin rod forward extrusion (j); forming is completed by the cold extrusion die (k). Shearing is the process of cutting the product according to its volume to obtain a fixed-length bar. End face leveling eliminates the shearing bevel after shearing large-diameter bars. Forward extrusion is used because the cross-sectional shrinkage rate of the thick rivet pin and the transition step bar is relatively large, ranging from 30% to 75%, resulting in a large degree of deformation. This method is a closed-die forming, where the blank is in a closed state and subjected to triaxial pressure, allowing for a large degree of deformation to complete the forming of the thin rod. Step surface forming is used when the step surface at the abrupt change in cross-section is combined with the cap pre-pressing process. Insufficient deformation of the step surface can lead to defects in the chamfer, which are difficult to eliminate in subsequent processes. The defect was eliminated by a separate forming process for the stepped surface at the abrupt change. Pre-pressing was adopted because direct forming of the cap head resulted in a large deformation, which could easily lead to defects such as folding, incomplete metal filling in some areas, and cap head eccentricity. Therefore, a gradual transition deformation method was used to initially forge the billet into a cone. According to the law of "constant volume", the volume of the cone is equal to the volume of the rivet pin cap head, which can eliminate the above-mentioned defects. Initial pressing was used to reduce the deformation of the cap head and to initially form the cap head, preventing the cap head from cracking at the circumferential edge due to excessive deformation in a single process. Final pressing and thin rod positive extrusion composite forming completed the final forming of the cap head, while the thin rod positive extrusion achieved the final size.

[0064] like Figure 4As shown, end face leveling is accomplished by a first-station punch A1a, a first-station die-forming front core B1a, and a first-station die-forming rear core B1b. The sheared bar stock is conveyed to the end face leveling station by clamps. The first-station punch pushes the bar stock into the die-forming front core cavity of the first-station station. After that, the clamps release, and the first-station punch continues to push the bar stock forward to the front dead center position. End face leveling is completed in the die cavity formed by the first-station die-forming rear core and ejector pin. The first-station punch retracts, and the first-station die-forming ejector pin ejects the workpiece. When the workpiece is ejected 50mm out of the die-forming front core cavity of the first-station station, the second-station clamps close to hold the bar stock until the workpiece is completely ejected.

[0065] like Figure 5 As shown, the forward extrusion is completed by the two-station punch A2a, the two-station die-forming front core B1a, and the two-station die-forming rear core B2b. The blank with its end face flattened is conveyed to the two-station forward extrusion station by clamps. The two-station punch pushes the blank into the die-forming front core cavity of the two-station station by 30mm, then the clamps release. The two-station punch continues to push the blank forward to the front dead center position, where the forward extrusion of the thin rod is completed in the die-forming rear core cavity of the two-station station. The two-station punch retracts, and the two-station die-forming ejector pin ejects the workpiece. When the workpiece is ejected 50mm out of the die-forming front core cavity of the two-station station, the three-station clamps close to clamp the thick rod until the workpiece is completely ejected.

[0066] like Figure 6 As shown, the stepped surface forming is accomplished by a three-station punch A3a, a three-station front die B3a, a three-station rear die B3b, and a three-station ejector pin C3. The extruded blank is conveyed to the three-station stepped surface forming station. The three-station punch pushes the blank into the cavity of the front die 70mm, then the clamps release. The three-station punch continues to push the blank forward to the front dead center position, where the stepped surface is formed in the cavity of the rear die. Simultaneously, the three-station ejector pin presses out a conical positioning groove on the end face of the thin rod of the blank. The three-station punch retracts, and the three-station ejector pin ejects the workpiece. When the workpiece is ejected 50mm out of the front die cavity, the four-station clamps close to clamp the thick rod until the workpiece is completely ejected.

[0067] like Figure 7 As shown, the pre-compression is completed by the four-station punch core A4a, the four-station die-forming front core B4a, the four-station die-forming middle core B4b, the four-station die-forming rear core B4c, and the four-station die-forming ejector pin C4. After the stepped surface is formed, the blank is transferred to the four-station pre-compression station. While the four-station punch core pushes the blank, the conical tip of the four-station ejector pin is inserted into the conical positioning groove on the end face of the thin rod of the blank. After the blank is pushed into the die cavity of the four-station die-forming front core 20mm, the clamps are released, and the blank is stably clamped and will not fall. The four-station punch core continues to push the blank forward to the front dead center position to complete the pre-compression forming of the cap head. The four-station punch core retracts, and the four-station die-forming ejector pin ejects the workpiece. When the workpiece is ejected 65mm out of the die cavity of the four-station die-forming front core, the five-station clamps close to clamp the thick rod until the workpiece is completely ejected.

[0068] like Figure 8 As shown, the initial pressing is completed by the five-station punch core A5a, the five-station front die core B5a, the five-station middle die core B5b, the five-station rear die core B54c, and the five-station ejector pin C5. The pre-pressed blank is transferred to the five-station initial pressing station. While the five-station punch core pushes the blank, the conical tip of the five-station ejector pin is inserted into the conical positioning groove on the end face of the thin rod of the blank. After the blank is pushed into the cavity of the five-station front die core 20mm, the clamps are released, and the blank is stably held and will not fall. The five-station punch core continues to push the blank forward to the front dead center position, completing the initial pressing of the cap head. The five-station punch core retracts, and the five-station ejector pin ejects the workpiece. When the workpiece is ejected 65mm out of the cavity of the five-station front die core, the six-station clamps close to clamp the thick rod until the workpiece is completely ejected.

[0069] like Figure 9 As shown, the final pressing and fine rod positive extrusion composite forming is completed by a six-station punch core A6a, a six-station die-forming front core B6a, a six-station die-forming middle core B6b, a six-station die-forming rear core B6c, and a six-station die-forming ejector pin C6. The initial pressing blank is transferred to the six-station final pressing and fine rod positive extrusion composite forming station. While the six-station punch core pushes the blank, the conical tip of the six-station ejector pin inserts into the conical positioning groove on the end face of the fine rod. After the blank is pushed 20mm into the die cavity of the six-station die-forming front core, the clamps release, and the blank is stably held and will not fall. The six-station punch core continues to push the blank forward to the front dead center position, completing the final pressing forming of the cap head. Simultaneously, the positive extrusion of the fine rod is completed in the die cavity of the six-station die-forming middle core. The six-station punch core retracts, and the six-station die-forming ejector pin ejects the workpiece until it is completely ejected and falls into the discharge port. Thus, the cold extrusion forming of the rivet pin is completed.

[0070] like Figure 10 , Figure 11 , Figure 12 , Figure 13 As shown, the floating material support device consists of an ejector pin E1, a positioning pad E2, a positioning spring E3, and an end cap E4. Two sets of floating material support devices are symmetrically arranged. Due to the large length of the rivet pin rod, during the pre-pressing at the fourth station, the clamps cannot be released before the blank is fed into the cavity of the die-casting mold. However, the continuous movement of the punch at this time will cause the clamps to collide. To solve this problem, a floating material support device is set up. The tip is inserted into the tip groove on the end face of the rod for positioning. Two springs apply pressure to the upper and lower planes of the ejector pin to generate a certain frictional resistance. Even after the clamps are released in advance, the blank can still be stably supported to ensure that the blank is accurately fed into the cavity of the die-casting mold, while avoiding collisions with the clamps.

[0071] like Figure 14 , Figure 15As shown, thick-type clamps are used for transferring blanks between stations. The thicker clamping rod holds the blank, with a thickness equal to the length of the thinner rod plus 10mm. This thicker rod is held throughout the blank ejection process, and even when the blank is fully ejected, the clamp still holds the thicker rod. The clamping point is close to the workpiece's center of gravity, preventing blank skewing and avoiding the defects of being easily damaged when the blank is pushed into the mold cavity at the next station. This avoids the problems of impact, blank swaying, and blank center of gravity shift caused by traditional clamps during the sequential clamping of the thicker and thinner rods, which can lead to workpiece damage and jamming during mold cavity insertion.

[0072] like Figure 16 As shown, the elastic compression amount is set, where L1 is the length of the front core and L2 is the sum of the lengths of the middle core and the rear core. During molding at the fifth and sixth stations, due to the large molding force, the step surface at the connection between the thick and thin rods will first bear a large pressure. This pressure will be transmitted to the middle core and the rear core of the mold in sequence, thereby generating a certain amount of compressive elastic deformation Δ. This value is approximately 0.25% of the sum of the lengths of the middle core and the rear core, L2. For example, if the length of the rivet pin is L, the length of the front core L1 should be L-Δ, which solves the defect of the thick rod length being out of tolerance after molding.

Claims

1. A large diameter rivet pin cold extrusion forming die, the die being adapted to be used in conjunction with a part forming machine to extrude form, characterised in that: The utility model relates to a kind of die sets for the production of bar, which is composed of shearing die, end face flattening die, direct extrusion die, step surface forming die, pre-pressing die, initial pressing die, final pressing and fine rod direct extrusion composite forming die, and each die is matched with clamping for feeding. The shearing die is used to make the processing material into a fixed-size bar by shearing, which includes a shearing die and a shearing cutter. The end face flattening die is used to flatten the inclined end face after the large-diameter blank is sheared, which includes a one-station punch, a one-station pre-punch, a one-station post-punch, and a one-station ejector pin. The direct extrusion die is used to extrude the front fine rod of the bar, which includes a two-station punch, a two-station pre-punch, a two-station post-punch, and a two-station ejector pin. The step surface forming die is used to pre-form the step surface and chamfer at the transition between the coarse rod and the fine rod, and to extrude the tapered positioning groove at the end face of the fine rod, which includes a three-station punch, a three-station pre-punch, a three-station post-punch, a three-station ejector pin, a positioning pad, and a positioning spring. The pre-pressing die is used to pre-press the cap head and form the chamfer of the coarse rod, which includes a four-station punch, a four-station pre-punch, a four-station post-punch, a four-station ejector pin, a positioning pad, and a positioning spring. The initial pressing die is used to form the cap head, which includes a five-station punch, a five-station pre-punch, a five-station post-punch, a five-station ejector pin, a positioning pad, and a positioning spring. The final pressing and fine rod direct extrusion composite forming die is used to form the cap head and extrude the fine rod, which includes a six-station punch, a six-station pre-punch, a six-station post-punch, a six-station ejector pin, a positioning pad, and a positioning spring.

2. The cold extrusion forming die for large diameter rivet pins according to claim 1, characterized in that: In the end face flattening die, the one-station punch and the one-station ejector pin are cylindrical flat-end top rod structures; the one-station pre-punch is provided with a through hole with a constant diameter at the center; the one-station post-punch is provided with a variable-diameter through hole with a stepped structure at the center, the taper angle α1 formed by the variable-diameter through hole is 120°, the round corner radius R1 of the variable-diameter through hole is (D1a-D1b) / 2, the large inner hole diameter D1a of the die cavity is D1+0.15 mm, and the small inner hole diameter D1b of the die cavity is 0.6×D1, where D1 is the diameter of the bar.

3. The cold extrusion forming die for large diameter rivet pin as claimed in claim 1, wherein: In the step surface forming die, the three-station punch is a cylindrical flat-end top rod structure, and the three-station ejector pin is a cylindrical top rod structure with a flat surface on one side and a tapered surface at the center; the three-station pre-punch and the three-station post-punch are both provided with a through hole with a constant diameter at the center, the inner hole diameter D3a of the pre-punch die cavity is D2a+0.1 mm, and the inner hole diameter D3b of the post-punch die cavity is D2b+0.05 mm; the three-station post-punch is provided with a floating material supporting device; the diameter D3d of the bottom surface of the conical body at the center of the end face of the three-station ejector pin is 3.5 mm, the height H3a of the conical body is 1 mm, and the thickness H3b of the flat surface on one side of the ejector pin is D3f-1 mm; where D2a is the inner diameter of the two-station pre-punch of the direct extrusion die, D2b is the inner diameter of the reduced-diameter part at the rear end of the variable-diameter through hole of the two-station post-punch, D3d is the diameter of the positioning spring, and D3f is the diameter of the ejector pin.

4. The cold extrusion forming die for large diameter rivet pins according to claim 3, characterized in that: In the pre-pressing die, the four-station punch pin die cavity is in the shape of a pre-pressing cap head, the four-station punch pin is in the structure of a cylindrical top rod with a flat and a center conical end face on the opposite side, the four-station punch pin die cavity has a through hole with an equal diameter at the center of the front pin and the rear pin, the inner diameter of the four-station punch pin die cavity of the front pin is D4a=D3a+0.08mm, the inner diameter of the four-station punch pin die cavity of the rear pin is D4c=D3b+0.05mm, the four-station punch pin die cavity of the middle pin has a stepped structure, and the inner diameter of the four-station punch pin die cavity of the middle pin is D4b=D3b+0.05mm; the die cavity of the punch pin formed by the trumpet-shaped expansion structure of the four-station punch pin has a conical angle of 40°, the front guide inner diameter of the punch pin die cavity is D4=D3a+10mm, the length W4 is 1.5mm, and the die cavity height is determined according to the initial upsetting cap head volume V1=V2, wherein V2 is the product cap head volume; the four-station punch pin die cavity of the rear pin is provided with a floating material supporting device; the diameter of the bottom surface of the conical body at the center of the end face of the four-station punch pin is D3d=3.5mm, the height of the conical body is H3a=1mm, and the thickness of the flat on the opposite side of the punch pin is H4b=D4f-1mm, wherein D4f is the diameter of the punch pin.

5. The cold extrusion forming die for large diameter rivet pins according to claim 4, characterized in that: In the pre-pressing die, the four-station punch pin die cavity is in the shape of a pre-pressing cap head, the four-station punch pin is in the structure of a cylindrical top rod with a flat and a center conical end face on the opposite side, the four-station punch pin die cavity has a through hole with an equal diameter at the center of the front pin and the rear pin, the inner diameter of the four-station punch pin die cavity of the front pin is D4a=D3a+0.08mm, the inner diameter of the four-station punch pin die cavity of the rear pin is D4c=D3b+0.05mm, the four-station punch pin die cavity of the middle pin has a stepped structure, and the inner diameter of the four-station punch pin die cavity of the middle pin is D4b=D3b+0.05mm; the die cavity of the punch pin formed by the trumpet-shaped expansion structure of the four-station punch pin has a conical angle of 40°, the front guide inner diameter of the punch pin die cavity is D4=D3a+10mm, the length W4 is 1.5mm, and the die cavity height is determined according to the initial upsetting cap head volume V1=V2, wherein V2 is the product cap head volume; the four-station punch pin die cavity of the rear pin is provided with a floating material supporting device; the diameter of the bottom surface of the conical body at the center of the end face of the four-station punch pin is D3d=3.5mm, the height of the conical body is H3a=1mm, and the thickness of the flat on the opposite side of the punch pin is H4b=D4f-1mm, wherein D4f is the diameter of the punch pin.

6. The cold extrusion forming die for large diameter rivet pins according to claim 5, characterized in that: In the pre-pressing die, the four-station punch pin die cavity is in the shape of a pre-pressing cap head, the four-station punch pin is in the structure of a cylindrical top rod with a flat and a center conical end face on the opposite side, the four-station punch pin die cavity has a through hole with an equal diameter at the center of the front pin and the rear pin, the inner diameter of the four-station punch pin die cavity of the front pin is D4a=D3a+0.08mm, the inner diameter of the four-station punch pin die cavity of the rear pin is D4c=D3b+0.05mm, the four-station punch pin die cavity of the middle pin has a stepped structure, and the inner diameter of the four-station punch pin die cavity of the middle pin is D4b=D3b+0.05mm; the die cavity of the punch pin formed by the trumpet-shaped expansion structure of the four-station punch pin has a conical angle of 40°, the front guide inner diameter of the punch pin die cavity is D4=D3a+10mm, the length W4 is 1.5mm, and the die cavity height is determined according to the initial upsetting cap head volume V1=V2, wherein V2 is the product cap head volume; the four-station punch pin die cavity of the rear pin is provided with a floating material supporting device; the diameter of the bottom surface of the conical body at the center of the end face of the four-station punch pin is D3d=3.5mm, the height of the conical body is H3a=1mm, and the thickness of the flat on the opposite side of the punch pin is H4b=D4f-1mm, wherein D4f is the diameter of the punch pin.

7. A cold extrusion forming die for large diameter rivet pins according to any one of claims 3 to 6, characterised in that: The floating material supporting device comprises: two positioning spring mounting holes opposite to each other after each die setting, the inner diameter of the mounting hole is D3c=D3d+0.2mm; a ejector pin, a positioning pad and a positioning spring are arranged in the mounting hole and are limited by an end cover.

8. The cold extrusion forming die for large diameter rivet pins according to claim 7, characterized in that: The thickness of the clamp is the length of the thin rod plus 10mm, which is used to clamp the thick rod part during the process of forming and conveying the blank in each station.

9. The cold extrusion forming die for large diameter rivet pins according to claim 7, characterized in that: The preset die compression elastic deformation Δ in the initial pressing die, final pressing and thin rod positive extrusion die, the compression elastic deformation Δ is 0.25% of the sum L2 of the lengths of the middle and rear stems, and the length L1 of the front stem is the length L of the pull rivet thick rod minus Δ.

10. A method of cold extrusion forming of a large diameter rivet pin, characterized by: The forming die extrusion forming method according to any one of claims 1 to 9 comprises the steps of shearing and cutting, end face flattening, positive extrusion, step surface forming, pre-pressing, initial pressing, final pressing and thin rod positive extrusion; the punch of each step station is reciprocally moved on the main slide block of the part forming machine, the front stem, the middle stem, the rear stem, the ejector pin, the positioning pad and the positioning spring of each station are mounted on the fixed seat of the part forming machine, the blank is automatically conveyed between each station by the clamp, and the specific process comprises: 1) the shearing and cutting is completed by a shear die and a shear, the positioning stop size is adjusted according to the length of the cut material, the straightened processed material is sent into and passes through the inner hole of the shear die, when the processed material contacts the front positioning stop, the shear moves to complete the shearing of the cut material; 2) the cut material is conveyed to the end face flattening station by the clamp, after the punch of the station pushes the cut material into the front stem die cavity of the station, the clamp is released, the punch continues to push the cut material to the front dead point position, the end face is flattened in the die cavity composed of the rear stem and the ejector pin of the station, the punch retreats, the ejector pin of the station ejects the workpiece, when the workpiece is ejected 50mm from the front stem die cavity of the station, the clamp of the second station closes to clamp the thick rod until the workpiece is completely ejected; 3) the blank after end face flattening is conveyed to the positive extrusion second station by the clamp, after the punch of the second station pushes the blank into the front stem die cavity of the station for 30mm, the clamp is released, the punch continues to push the blank to the front dead point position, the thin rod positive extrusion is completed in the rear stem die cavity of the second station, the punch retreats, the ejector pin of the second station ejects the workpiece, when the workpiece is ejected 50mm from the front stem die cavity of the second station, the clamp of the third station closes to clamp the thick rod until the workpiece is completely ejected; 4) the blank after positive extrusion is conveyed to the step surface forming third station, after the punch of the third station pushes the blank into the front stem die cavity of the third station for 70mm, the clamp is released, the punch continues to push the blank to the front dead point position, the step surface forming is completed in the rear stem die cavity of the third station, and the taper positioning groove is pressed on the end face of the thin rod of the blank by the ejector pin of the third station, the punch retreats, the ejector pin of the third station ejects the workpiece, when the workpiece is ejected 50mm from the front stem die cavity of the third station, the clamp of the fourth station closes to clamp the thick rod until the workpiece is completely ejected. 5) After the step surface is formed, the blank is conveyed to the pre-pressing four-station, the four-station punch pin pushes the blank, and the conical tip of the four-station punch pin is inserted into the conical positioning groove at the end surface of the thin rod of the blank. After the blank is pushed into the four-station die cavity by 20 mm, the clamp is released, the blank is stably clamped and will not fall off, the four-station punch pin continues to push the blank to the front dead point position, the pre-pressing forming of the cap head is completed, the four-station punch pin retreats, the four-station die pin ejects the workpiece, and when the workpiece is ejected from the four-station die cavity by 65 mm, the five-station clamp is closed to clamp the thick rod until the workpiece is completely ejected; 6) After the pre-pressing, the blank is conveyed to the initial pressing five-station, the five-station punch pin pushes the blank, and the conical tip of the five-station punch pin is inserted into the conical positioning groove at the end surface of the thin rod of the blank. After the blank is pushed into the five-station die cavity by 20 mm, the clamp is released, the blank is stably clamped and will not fall off, the five-station punch pin continues to push the blank to the front dead point position, the initial pressing forming of the cap head is completed, the five-station punch pin retreats, the five-station die pin ejects the workpiece, and when the workpiece is ejected from the five-station die cavity by 65 mm, the six-station clamp is closed to clamp the thick rod until the workpiece is completely ejected; 7) After the initial pressing, the blank is conveyed to the final pressing and thin rod positive extrusion six-station, the six-station punch pin pushes the blank, and the conical tip of the six-station punch pin is inserted into the conical positioning groove at the end surface of the thin rod of the blank. After the blank is pushed into the six-station die cavity by 20 mm, the clamp is released, the blank is stably clamped and will not fall off, the six-station punch pin continues to push the blank to the front dead point position, the final pressing forming of the cap head is completed, and the positive extrusion of the thin rod in the six-station die cavity is completed. The six-station punch pin retreats, the six-station die pin ejects the workpiece until the workpiece is completely ejected and falls into the discharge port; the cold extrusion forming of the pull rivet pin is completed.

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