Cold heading forming method of oil cylinder for progressive gas spring after upsetting and gradual extension

Abstract

The invention discloses a cold heading forming method for an oil cylinder for a progressive gas spring by upsetting firstly and then gradually extending. The cold heading forming method comprises the following steps: (1) preparing and shaping raw materials; (2) upsetting; (3) indenting; 4) backward extrusion stretching; (5) thin-wall reducing extension one-time forming is carried out; and (6) thin-wall reducing extension secondary forming is carried out. In combination with the characteristics of small overall size, large length-diameter ratio and thin wall and batch production of the oil cylinder for the gas spring, a part with a complex structure is divided into a plurality of simple stations by adopting a method of upsetting first and then gradual extension, and through a process of upsetting, combined extrusion and multi-step diameter-reducing stretching, the length-diameter ratio of the oil cylinder for the gas spring is greatly reduced. And a reducing amount distribution formula based on a strain hardening index and a flow stress model is introduced, so that gradual reduction of the inner and outer diameters of the cylinder body and wall thickness homogenization control are realized. According to the method, on one hand, the product wall thickness uniformity is improved, and the local wall thickness abnormity condition is avoided; and on the other hand, due to the fact that the five-station progressive one-time forming technology is adopted, the auxiliary clamping time is greatly shortened, and therefore the forming efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of mechanical processing, in particular to a cold upsetting forming method of an oil-gas cylinder for a progressive gas spring by upsetting first and then gradually extending. Background Art

[0002] Gas spring is an industrial accessory with functions such as support, buffering, braking, height adjustment and angle adjustment. The present invention focuses on the research of the molding method of its oil and gas cylinder part. The traditional manufacturing process of oil and gas cylinder for gas spring is generally welding type split molding, that is, the plug and the cylinder body are processed separately and then welded. This method has low pipe utilization rate, long production cycle, low product quality and short service life. In response to the above problems, after literature search, there are several improved thin-walled tube parts processing processes: warm forging process, warm forging cold finishing process, and cold heading process.

[0003] Warm forging, for example: In 2020, Xu Yingquan and others from Jiangsu Enno Axle Research Technology Co., Ltd. proposed a warm forging production process for bearing rollers (authorization announcement number: CN 111531116B). The invented process heats the rods at low temperature and then performs multi-station step-forging, which greatly reduces costs and improves production efficiency. However, when using warm forging, the temperature of the billet is difficult to control and is prone to uneven heating, especially the mold cavity is difficult to cool, which can easily damage the mold.

[0004] Warm forging and cold finishing process, for example: In 2020, Xu Xiangqian and others from Jiangsu Senwei Precision Forging Co., Ltd. proposed a forging method for precision deep-hole thin-walled tubes (authorization announcement number: CN 107414416B). The invention uses warm forging forward extrusion and reverse extrusion for preliminary forging, and optimizes the dimensional accuracy through cold finishing after cooling, thereby improving material utilization and product strength. However, the combination of warm and cold upsetting greatly increases production costs and reduces production efficiency.

[0005] Cold heading process For example: In 2021, Weng Xuanzhang and others from Ningbo Huahong Machinery Co., Ltd. proposed a production process for shock absorber bushing blanks (authorization announcement number: CN 112247053B). This invention process improves the production efficiency of shock absorber bushing blanks by adopting multi-station cold heading forming, reduces the loss of material heads and tails, and greatly saves production costs, effectively proving that multi-station cold heading process is of great significance in the mass production of small parts. However, for the forming of thin-walled tube parts with large aspect ratio, it is also necessary to consider the defects such as uneven wall thickness, irregular cross section and even hole wall cracks caused by the forming of the two characteristics of deep holes and thin walls.

[0006] For this purpose, the cold heading forming methods of related thin-walled deep-hole parts were searched. In 2020, Li T et al. from Guizhou University proposed a multi-stage cold heading process for thin-walled A-286 superalloy tubes in "Analyzing Forged Quality of Thin-Walled A-286Superalloy Tube under Multi-Stage Cold Forging Processes" (included in the journal "Materials", Volume 16). With the help of finite element analysis software, two forming processes with opposite forging orders were compared, and the method of first heading and then reverse extrusion was determined. The thin-walled tube has higher hardness and quality, and is more suitable for actual production applications.

[0007] In 2023, Jiang Yulian and others from Guizhou University published "Optimization of Upsetting Process of A286 High-temperature Alloy Thin-walled Pipe Fittings" (included in the journal "Forging Technology", pages 71-80). In order to solve the problems of folding and incomplete filling in the thin-walled parts during the upsetting forming process of thin-walled pipe fittings, they used Deform simulation to conduct comparative tests on the forming methods of the inner hole. They concluded that reverse extrusion to form the inner hole is more conducive to the axial flow and stress state distribution of the metal, and improves the stability of the thin-wall feature forming.

[0008] Both of the above methods form thin-walled deep holes through multiple reverse extrusions. However, as the number of reverse extrusions increases, the thin-walled cavity becomes narrower and narrower, the metal flow space is extremely limited, and the friction with the cavity wall increases significantly, which can easily lead to uneven material flow and cause problems such as local thin walls and low hole wall quality. Summary of the invention

[0009] The purpose of the present invention is to overcome the defects of the prior art and provide a cold upsetting forming method for a progressive oil-gas cylinder for gas springs by first upsetting and then gradually extending. The method is based on a progressive five-station cold upsetting near-net forming process and the concept of "first upsetting and then gradually extending + cold upsetting". An external force is applied to a metal blank by a mold to repeatedly extrude and stretch the metal material. A roughing-then-fine processing scheme is adopted as an overall method. The metal blank is first upset to improve the forging ratio during reverse extrusion and stretching and the lateral mechanical properties of the forging, and to reduce anisotropy. Then, a concave die is used to progressively reduce the wall thickness of the blind hole of the oil-gas cylinder for the gas spring, and at the same time, the diameter of the punch is reduced to refine the inner diameter size of the oil-gas cylinder for the gas spring, so as to achieve a gradual reduction in the inner and outer diameters of the cylinder body, improve the product dimensional accuracy and material utilization rate, and avoid the low quality of local thin walls.

[0010] The object of the present invention is achieved by: a method for cold upsetting a hydraulic cylinder for a progressive gas spring by upsetting first and then gradually extending, comprising the following steps:

[0011] S1: Raw material preparation and shaping: according to the shape and volume of the final product, select the long cylindrical billet of appropriate size, and use the bar cutting machine to evenly cut it into short bars of equal length; shape the cut billet; pickle and phosphate the billet;

[0012] S2: Upsetting, the short bar material after shaping is conveyed to the first station of the multi-station cold heading machine through the feeder, and the short bar material is fixed by the clamping nut; the punch part of the first station of the multi-station cold heading machine is provided with an upsetting pressure rod, and the metal blank is upset by pressing the upsetting pressure rod downward; the upsetting die of the first station is in the shape of a boss, and when the upsetting pressure rod is pressed downward, the lower end of the extruded blank is in the shape of a boss. After the upsetting process is completed, the short bar material is fed to the second station of the multi-station cold heading machine through the clamping rotary mechanism;

[0013] S3: Concave pressing. The second station punch of the multi-station cold heading machine is equipped with a short thick punch. The short thick punch is pressed down to punch out a center top hole at the upper end of the short bar. After the concave pressing process is completed, the short bar is sent to the third station of the multi-station cold heading machine through the clamping rotary mechanism.

[0014] S4: Reverse extrusion stretching. The third station punch of the multi-station cold heading machine is provided with a short punch. The short punch is used to punch the center top hole, and the short bar is initially stretched to process the transition area between the inner wall and the outer wall of the cylinder. After the initial stretching is completed, the short bar is sent to the fourth station of the multi-station cold heading machine through the clamping rotary mechanism.

[0015] S5: Thin-walled shrinkage and extension one-step forming. The fourth station punch of the multi-station cold heading machine is provided with a medium-long punch. The medium-long punch is pressed down to make the medium-long punch perform the first punching on the depth of the cylinder body, further extending the depth of the cylinder body. At the same time, the thin-walled shrinkage and extension one-step forming die is used to reduce the diameter, so that the wall thickness of the cylinder body is reduced. After the thin-walled shrinkage and extension one-step forming, the short bar material is sent to the fifth station of the multi-station cold heading machine through the clamping rotary mechanism;

[0016] S6: Thin-walled shrinkage and extension secondary molding. The fifth station punch of the multi-station cold heading machine is equipped with a long punch. The long punch is pressed down to perform a second punch on the inside of the cylinder body, stretching the short bar into a cylinder body with a standard inner diameter. At the same time, the thin-walled shrinkage and extension secondary molding die is used to reduce the diameter, so that the wall thickness of the cylinder body can be reduced to meet the standard wall thickness requirements.

[0017] As a further limitation of the present invention, in step 4), the ratio of the diameter of the short punch needle to the diameter of the short thick punch is in the range of 1.03 to 1.05.

[0018] As a further limitation of the present invention, the ratio of the diameter of the medium-long punch needle to the diameter of the short punch needle in step 5) is in the range of 0.98 to 0.99, and the outer diameter reduction amount Δd1 of the first diameter reduction of the oil and gas cylinder is specifically as shown in formulas (1) to (3):

[0019]

[0020] Where: Δd1 is the outer diameter reduction of the first shrinkage, D3 is the outer diameter after reverse extrusion, α is the strain distribution weight coefficient, ε total is the total equivalent plastic strain of the two reductions, β is the process correction factor, A represents the initial yield strength of the material, B is the hardening coefficient of the material, is the equivalent strain of the material, n is the strain hardening exponent of the material, C is the strain rate sensitivity coefficient of the material, and D5 is the final target outer diameter.

[0021] As a further limitation of the present invention, the ratio of the diameter of the long punch needle to the diameter of the medium-long punch needle in step 6) is in the range of 0.99 to 0.995, which is consistent with the inner diameter of the oil cylinder for the finished gas spring. The outer diameter of the oil cylinder is reduced by the thin-walled shrinkage extension secondary molding die. The outer diameter reduction amount Δd2 of the second shrinkage of the oil cylinder is specifically shown in formula (4):

[0022]

[0023] The present invention adopts the above technical scheme, and compared with the prior art, the beneficial effects are as follows: 1) The present invention proposes a method for cold upsetting of a progressive oil-gas cylinder for gas springs by upsetting first and then gradually extending. Based on the progressive five-station cold upsetting near-net forming process, through the process of upsetting + composite extrusion + multi-step diameter reduction and stretching, the wall thickness can be accurately controlled, and the inner and outer diameters of the cylinder body can be gradually reduced, thereby avoiding the phenomenon of local metal accumulation, improving the product dimensional accuracy, and solving the problem that thin-wall features are difficult to form and local thin walls and metal accumulation are prone to occur during the cold upsetting forming process of the oil-gas cylinder for gas springs; at the same time, compared with the multiple forming process, the five-station progressive one-time forming process can save auxiliary clamping time, thereby improving the forming efficiency.

[0024] 2) Based on the idea of ​​"first upsetting and then gradual extension + cold upsetting", the wall thickness is precisely controlled by using the synergistic effect of the progressive shrinkage of the die and the synchronous extension of the inner hole of the punch. And combined with the material strain hardening index n and the flow stress model σ=A+Bε n , introduced an empirical formula for the distribution of reduction amount, scientifically distributed the two reduction amounts, ensured that the single deformation amount was within the allowable deformation range of the material, avoided uneven wall thickness or cracks caused by local stress exceeding the limit, and provided theoretical guidance for the cold heading of the same type of thin-walled tubes. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a flow chart of the present invention.

[0026] Figure 2 It is a simple schematic diagram of the upsetting forming die structure of the present invention.

[0027] Figure 3It is a simple schematic diagram of the concave forming mold structure of the present invention.

[0028] Figure 4 It is a simple schematic diagram of the reverse extrusion stretching die structure of the present invention.

[0029] Figure 5 It is a simple schematic diagram of the thin-walled, diameter-reducing and one-step forming mold structure of the present invention.

[0030] Figure 6 It is a simple schematic diagram of the thin-walled, diameter-reducing and stretching secondary molding die structure of the present invention.

[0031] Figure 7 It is a simulation diagram of the damage values ​​at each workstation of the hydraulic cylinder cold upsetting forming of the progressive gas spring of the present invention which is first upsetting and then gradually extending.

[0032] Among them, 1 buffer block, 2 fastening nut, 3 punch sleeve, 4 positioning fastening ring, 5A upsetting pressure rod, 6 prestressing ring, 7A upsetting die, 8 unloading ejector, 5B short thick punch, 7B embossing die, 5C short punch needle, 7C reverse extrusion stretching die, 5D medium and long punch needle, 7D thin-walled reduction and extension primary forming die, 5E long punch needle, 7E thin-walled reduction and extension secondary forming die. DETAILED DESCRIPTION

[0033] like Figure 1 The method for cold upsetting a hydraulic cylinder for a progressive gas spring by upsetting first and then gradually extending comprises the following steps:

[0034] S1: Raw material preparation and shaping. According to the shape and volume of the final product, select the appropriate size of long cylindrical 45 steel forming blanks, and use a bar cutting machine to evenly cut them into short bars of equal length; shape the cut blanks to ensure that the appearance of the blanks is neat and has no burrs, and the blank diameter meets the specified requirements; pickle and phosphate the blanks; select the blanks The constitutive equation of 45 steel is shown in Formula 5 and Table 1.

[0035]

[0036] Where: σ is the material flow stress; ε is the plastic strain; A, B, C, n, m are all material constants; is the plastic strain rate, which is 10s -1 ; is the dimensionless temperature, T, T r 、T m They represent material temperature, ambient temperature and material melting point temperature respectively.

[0037] Table 1 Constitutive model parameters of 45 steel

[0038]

[0039] like Figure 2 As shown, the upsetting forming die includes a buffer block 1, a fastening nut 2, a punch sleeve 3, a positioning fastening ring 4, an upsetting pressure rod 5A, a prestressing ring 6, an upsetting die 7A, and a discharge ejector 8; the punch portion: the buffer block 1 is installed above the upsetting pressure rod 5A; the fastening nut 2 is installed on the outside of the punch sleeve 3; the punch sleeve 3 is fixed to the outside of the positioning fastening ring 4 by the fastening nut 2; the positioning fastening ring 4 is located between the punch sleeve 3 and the upsetting pressure rod 5A; the upsetting pressure rod 5A is fixed inside the punch sleeve 3 by the cooperation of the fastening nut 2 and the positioning fastening ring 4. The die portion: an integral structure is adopted, the prestressing ring 6 is sleeved on the outside of the upsetting die 7A to clamp it; the discharge ejector 8 is installed below the upsetting die 7A.

[0040] S2: Upsetting, the short bar material after shaping is conveyed to the first station of the multi-station cold heading machine through the feeder, and the short bar material is fixed by the clamping nut 2; the punch part of the first station of the multi-station cold heading machine is provided with an upsetting pressure rod 5A, and the metal blank is upset by pressing the upsetting pressure rod 5A downward; the upsetting die 7A of the first station is in the shape of a boss, and when the upsetting pressure rod 5A is pressed downward, the lower end of the extruded blank is in the shape of a boss. After the upsetting process is completed, the short bar material is fed into the second station of the multi-station cold heading machine through the clamping rotary mechanism; the length of the short bar material is reduced and the diameter is increased, so as to increase the cross section and flatten the end face of the blank before punching, which is beneficial to improve the forging ratio during reverse extrusion stretching; the diameter after upsetting is 22.00mm and the length is 17.44mm.

[0041] S3: Depression, such as Figure 3 As shown, the second station punch part of the multi-station cold heading machine is provided with a short thick punch 5B. By pressing down with the short thick punch 5B, a large diameter center top hole is punched out at the upper end of the short bar. After the embossing process is completed, the short bar is sent to the third station of the multi-station cold heading machine through the clamping rotary mechanism for embossing. The purpose of pre-embossing the pit is to pre-distribute the material for reverse extrusion and stretching, and to avoid the breakage of the punch. The diameter after embossing is 22.20mm, the diameter of the concave hole is 17.20mm, and the hole depth is 1.00mm.

[0042] S4: Anti-extrusion stretching, such as Figure 4 As shown, the third station punch part of the multi-station cold heading machine is provided with a short punch needle 5C, which is used to punch the center top hole, and the short bar material is preliminarily stretched to process the transition area between the inner wall and the outer wall of the cylinder. After the preliminary stretching is completed, the short bar material is sent to the fourth station of the multi-station cold heading machine through the clamping rotary mechanism; the diameter of the short punch needle is 0.60mm larger than the diameter of the short thick punch; the diameter after reverse extrusion stretching is 22.60mm, the inner diameter of the cylinder body is 17.80mm, and the depth of the cylinder body is 30.16mm.

[0043] S5: Thin-wall shrinkage and extension molding, such as Figure 5 As shown in the figure, the fourth station punch of the multi-station cold heading machine is provided with a medium-long punch needle 5D. The medium-long punch needle 5D is pressed down to make the first punching of the cylinder body depth. At the same time, the thin-walled shrinkage extension one-time forming die 7D is used to reduce the wall thickness of the cylinder body and further extend the depth. After the thin-walled shrinkage extension one-time forming, the short bar material is sent to the fifth station of the multi-station cold heading machine through the clamping rotary mechanism; the diameter of the medium-long punch needle is 0.20 mm shorter than the diameter of the short punch needle. The shrinkage formula (1) combined with the 45 steel material constitutive model formula (5) and Table 1 calculates ε total =0.148, α=0.79, Δd1=2.00, the wall thickness is reduced by about 1.00mm, and the process correction factor β is 0.80. The diameter of the thin-walled shrinkage and extension after one-time molding is 20.60mm, the inner diameter of the cylinder is 17.60mm, and the depth of the cylinder is 54.87mm.

[0044] S6: Thin-wall shrinkage and extension secondary molding, such as Figure 6 As shown, the fifth station punch of the multi-station cold heading machine is provided with a long punch needle 5E. The long punch needle 5E is pressed down to perform a second punching on the inside of the cylinder body, and the short bar material is stretched into a cylinder body with a standard inner diameter. At the same time, the thin-walled shrinkage extension secondary forming die 7E is used to reduce the diameter, so that the wall thickness of the cylinder body is reduced to meet the standard wall thickness requirement. The diameter of the long punch needle is 0.10mm shorter than the diameter of the medium-long punch needle. The outer diameter reduction of the oil and gas cylinder after the second shrinkage is calculated by the shrinkage formula (4) to be Δd2 = 1.1mm, and the wall thickness is reduced by about 0.55mm; the diameter after the thin-walled shrinkage extension secondary forming is 19.50mm, the inner diameter of the cylinder body is 17.50mm, and the cylinder body depth is 97.45mm.

[0045] like Figure 7 As shown in the figure, the cold upsetting process is simulated by finite element method, and the damage value simulation diagram of each station of the cold upsetting forming of the oil cylinder for gas spring is obtained. The material is 45 steel (critical damage value 0.6232). The deformation in the upsetting process is uniform and the damage value is low; the deformation in the dent process is concentrated near the pit, and the damage value is low; the material flows violently during the reverse extrusion, and the stress concentration in the punch fillet and other areas is easy, and the damage value increases; the thin-wall extension is easy to be unstable and deformed due to the thin wall thickness, and the damage value increases; the second thin-wall extension has experienced a thin-wall extension, and the internal stress becomes complicated, and the damage value is higher. The maximum damage value of each station is less than the critical value, and the process deformation is reasonably distributed to ensure the forming quality.

[0046] Based on the cold upsetting method, the present invention proposes a cold upsetting method for a progressive gas spring oil cylinder, which is first upsetting and then gradually extending. The method combines the characteristics of the gas spring oil cylinder parts, which are small in overall size, have many forming steps, and require large-scale production. Based on the idea of ​​"first upsetting and then gradually extending + cold upsetting", the method uses the synergistic effect of the progressive shrinkage of the die and the synchronous extension of the inner hole of the punch to accurately control the wall thickness. The material strain hardening index n is combined with the flow stress model σ=A+Bε n , introduced an empirical formula for the distribution of the reduction amount, scientifically distributed the two reduction amounts, ensured that the single deformation amount was within the allowable deformation range of the material, avoided uneven wall thickness or cracks caused by local stress exceeding the limit, and provided theoretical guidance for the cold heading of the same type of thin-walled tubes. The forming method proposed in the present invention reduces the forming steps on the basis of the original production process, adopts progressive forming, and reduces the load of each forming step. It not only has the advantages of high dimensional accuracy, good surface quality, continuous metal streamlines, good organizational properties, strong bearing capacity and high material utilization of cold heading parts, but also has the characteristics of high-speed forming, which is conducive to improving efficiency and reducing production costs, and is suitable for batch production of small parts.

[0047] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solution disclosed in the present invention, technicians in this field can make some substitutions and deformations to some technical features therein according to the disclosed technical content without creative labor, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A method for cold upsetting a hydraulic cylinder for a progressive gas spring by first upsetting and then gradually extending, characterized in that: The five-station progressive cold heading forming method comprises the following steps: S1: Raw material preparation and shaping: according to the shape and volume of the final product, select the appropriate size of long cylindrical billets and cut them into short bars of equal length; shape, pickle and phosphate the cut billets; S2: Upsetting, the short bar material after shaping is conveyed to the first station of the multi-station cold heading machine through the feeder, and the short bar material is fixed by the clamping nut; the punch part of the first station of the multi-station cold heading machine is provided with an upsetting pressure rod, and the metal blank is upset by pressing the upsetting pressure rod downward; the upsetting die of the first station is in the shape of a boss, and when the upsetting pressure rod is pressed downward, the lower end of the extruded blank is in the shape of a boss. After the upsetting process is completed, the short bar material is fed to the second station of the multi-station cold heading machine through the clamping rotary mechanism; S3: Concave pressing. The second station punch of the multi-station cold heading machine is equipped with a short thick punch. The short thick punch is pressed down to punch out a center top hole at the upper end of the short bar. After the concave pressing process is completed, the short bar is sent to the third station of the multi-station cold heading machine through the clamping rotary mechanism. S4: Reverse extrusion stretching. The third station punch of the multi-station cold heading machine is provided with a short punch. The short punch is used to punch the center top hole, and the short bar is initially stretched to process the transition area between the inner wall and the outer wall of the cylinder. After the initial stretching is completed, the short bar is sent to the fourth station of the multi-station cold heading machine through the clamping rotary mechanism. S5: Thin-walled shrinkage and extension one-step forming. The fourth station punch of the multi-station cold heading machine is provided with a medium-long punch. The medium-long punch is pressed down to make the medium-long punch perform the first punching on the depth of the cylinder body, further extending the depth of the cylinder body. At the same time, the thin-walled shrinkage and extension one-step forming die is used to reduce the diameter, so that the wall thickness of the cylinder body is reduced. After the thin-walled shrinkage and extension one-step forming, the short bar material is sent to the fifth station of the multi-station cold heading machine through the clamping rotary mechanism; S6: Thin-walled shrinkage and extension secondary molding. The fifth station punch of the multi-station cold heading machine is equipped with a long punch. The long punch is pressed down to perform a second punch on the inside of the cylinder body, stretching the short bar into a cylinder body with a standard inner diameter. At the same time, the thin-walled shrinkage and extension secondary molding die is used to reduce the diameter, so that the wall thickness of the cylinder body can be reduced to meet the standard wall thickness requirements.

2. The cold upsetting forming method of a hydraulic cylinder for a progressive gas spring by upsetting first and then gradually extending according to claim 1, characterized in that: In step 4), the ratio of the diameter of the short punch needle to the diameter of the short thick punch is in the range of 1.03 to 1.

05.

3. The cold upsetting forming method of a hydraulic cylinder for a progressive gas spring by upsetting first and then gradually extending according to claim 1, characterized in that: The ratio of the diameter of the medium-long punch needle to the diameter of the short punch needle in step 5) is in the range of 0.98 to 0.

99. The outer diameter reduction amount Δd1 of the first diameter reduction of the oil and gas cylinder is specifically as shown in formulas (1) to (3): Where: Δd1 is the outer diameter reduction of the first shrinkage, D3 is the outer diameter after reverse extrusion, α is the strain distribution weight coefficient, ε total is the total equivalent plastic strain of the two reductions, β is the process correction factor, A represents the initial yield strength of the material, B is the hardening coefficient of the material, is the equivalent strain of the material, n is the strain hardening exponent of the material, C is the strain rate sensitivity coefficient of the material, and D5 is the final target outer diameter.

4. The cold upsetting forming method of a hydraulic cylinder for a progressive gas spring by upsetting first and then gradually extending according to claim 3, characterized in that: The ratio of the diameter of the long punch needle to the diameter of the medium-long punch needle in step 6) is in the range of 0.99 to 0.

995. The outer diameter of the oil and gas cylinder is reduced by the thin-walled shrinkage extension secondary molding die. The outer diameter reduction amount Δd2 of the second shrinkage of the oil and gas cylinder is specifically shown in formula (4): Δd2=D3-D5-Δd1 (4).

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