A shrinking and sealing assembly method for a precision hydraulic pipe

Abstract

The present invention discloses a shrinking sealing assembly method for a precision hydraulic pipe, which relates to the technical field of sealing assembly. A mold is used to shrink and seal a pole shoe and a copper pipe, and the method comprises the following steps: S01: obtaining a curvature radius R of a mold groove in the mold; ρ ; S02: obtain the pole shoe tube wall height h and the tube wall initial thickness t0; S03: determine the number of shrinkages according to the empirical rule of shrinkage times; S04: squeeze shrinkage. The present invention obtains the most reasonable mold groove curvature radius and pole shoe tube wall height, manufactures accurate and mutually matching molds and pole shoes, and adopts a secondary shrinkage method, thereby improving the sealing assembly qualification rate and process stability of hydraulic pipe micro devices.

Description

Technical Field

[0001] The invention relates to the technical field of sealing assembly, in particular to a shrinking sealing assembly method for a precision hydraulic pipe. Background Art

[0002] The hydraulic tube micro-device is a component that is assembled by assembling a pole shoe and a copper tube. It contains a certain amount of liquid inside, so the hydraulic tube assembly is required to have a certain degree of sealing. At present, the domestic assembly technology of hydraulic tubes can be divided into resistance welding and laser welding, but both welding methods have defects: since the pole shoe is electrical pure iron and the copper tube is brass, the thermal expansion coefficients of the two materials are different. After high-temperature resistance welding, the welding interface will produce a shrinkage stress difference, which affects the sealing of the assembly; during laser welding, when the laser output power is kept unchanged, the welding speed increases, and the sealing yield rate shows a downward trend, so it is difficult to balance the welding quality and efficiency. The effective assembly qualification rate of the above two methods is about 97%, and it is difficult to guarantee the efficiency of the sealing assembly and the process stability.

[0003] In order to avoid defects in the welding process, the existing technology uses shrinking and sealing assembly to process, but the existing shrinking and sealing assembly technology for hydraulic pipe micro devices is still immature, the size of the parts and the size of the inner mold groove of the mold are not supported by experimental data, and the use of the shrinking technology is still imperfect. The mismatch between the inner mold groove of the mold and the tube wall of the pole shoe will result in poor sealing of the shrinking. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a shrinking sealing assembly method for a precision hydraulic pipe, by obtaining the most reasonable radius of curvature of the inner mold groove and the pole shoe tube wall height of the mold, to produce precise and mutually matching molds and pole shoes, and at the same time adopting a secondary shrinking method, thereby improving the sealing assembly qualification rate and process stability of hydraulic pipe micro-devices.

[0005] To achieve the above purpose, the present invention is implemented by the following technical scheme: a shrinking and sealing assembly method for a precision hydraulic pipe, characterized in that a mold is used to shrink and seal the pole shoe and the copper pipe, comprising the following steps:

[0006] S01: Obtain the curvature radius R of the mold groove ρ

[0007] (1)

[0008] Where: α1 is the die-out angle, r0 is the tube wall radius after shrinking and r0 is a given value;

[0009] S02: Obtain the pole shoe wall height h and the initial wall thickness t0

[0010] Tube wall height h:

[0011] (2)

[0012] Where: R0 is the initial tube wall radius and is a given value, h1 is the side plate wall thickness of the copper tube and is a given value;

[0013] Initial thickness of pipe wall t0:

[0014] By establishing the relationship curve between the necking angle α, the initial thickness t0 of the tube wall and the axial compressive stress F, the initial thickness t0 of the tube wall is determined;

[0015] S03: Determine the number of shrinkages based on the empirical rule of shrinkage times

[0016] When m>0.75, use one shrinkage; when m is 0.64~0.74, use two shrinkages;

[0017] (3)

[0018] Wherein, m is the shrinkage coefficient;

[0019] S04: Squeeze the compression port

[0020] Preferably, the demoulding angle α1 in step S01 is obtained according to the actual assembly requirements of the project:

[0021] When shrinking, the pole piece tube wall radius decreases from R0 to r0, that is, it increases from the die entry angle α0 to the die exit angle α1. The die entry angle α0 is the initial value of the shrinking angle α, and the die exit angle α1 is the terminal value of the shrinking angle α.

[0022] Preferably, in step S02, the process of obtaining the initial thickness t0 of the tube wall is specifically as follows:

[0023] Determine the shrinkage coefficient m according to formula (3):

[0024] According to the characteristics of bidirectional compressive stress in the shrinkage deformation zone, according to the Mises yield criterion

[0025] (4)

[0026] Among them: β is the main stress influence coefficient, σT is the true stress;

[0027] Power function relationship of true stress and strain

[0028] (5)

[0029] Where: k is the material strength coefficient, n is the material strain hardening exponent;

[0030] The shrinkage deformation is controlled by compression, and the wall thickness of the deformation zone increases significantly. In this case, the general equilibrium equation of the thickness change of the axisymmetric shell along the generatrix is

[0031] (6)

[0032] Where: ρ, α are the parallel circle radius and shrinking angle of any point in the deformation zone, R ρ , R θ are the curvature radii of the radial section and the axial section, σ ρ , σ θ are radial stress and axial stress respectively, μ and t are friction coefficient and wall thickness respectively. The equations of the shrinking angle α, initial wall thickness t0 and axial compressive stress F in the shrinking process are derived from formulas (3), (4), (5) and (6):

[0033] (7)

[0034] Among them: when the axial compressive stress is the minimum, it is the optimal solution for the necking angle α, thereby determining the optimal solution for the initial thickness t0 of the tube wall.

[0035] Preferably, in step S03, when two shrinking operations are adopted, the following steps are adopted: the process is started for the first time, and the pressure of the first shrinking assembly is applied. When the pressure reaches F1, the pressure is maintained for 2 seconds; then the pressure of the second shrinking is applied, and when the pressure reaches F2, the pressure is maintained for 5 seconds, and then the shrinking assembly process is ended.

[0036] The present invention provides a method for assembling a precision hydraulic pipe by sealing the necking, which has the following beneficial effects:

[0037] 1. The present invention obtains the most reasonable mold groove curvature radius and pole shoe tube wall height to manufacture accurate and mutually matching molds and pole shoes, and adopts a secondary shrinking method to improve the sealing assembly qualification rate and process stability of hydraulic pipe micro devices;

[0038] 2. In the present invention, the die entry angle, the size of the extrusion external force and the number of extrusions are all accurately designed, thereby ensuring the feasibility and accuracy of the shrinkage sealing assembly of the hydraulic pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the relationship between the die entry angle and the maximum axial compressive stress of the present invention;

[0040] Figure 2 This is a cross-sectional view of the arc groove of the tire of the present invention;

[0041] Figure 3 It is a schematic diagram before and after the assembly of the present invention;

[0042] Figure 4 It is a schematic diagram of the assembly steps of the present invention;

[0043] In the figure: 1. copper tube; 2. inner mold groove; 3. tube wall; 4. mold; 5. through hole; 6. pole shoe; 7. side edge plate. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] like Figure 1-4 As shown, a method for assembling a shrink seal of a precision hydraulic pipe is characterized in that a mold 4 is used to shrink and seal a pole shoe 6 and a copper pipe 1, and the method comprises the following steps:

[0046] S01: Obtain the curvature radius R of the inner mold groove 2 of the mold 4 ρ

[0047] (1)

[0048] Where: α1 is the die-out angle, r0 is the tube wall radius after shrinking and r0 is a given value;

[0049] S02: Obtain the height h of the tube wall of the pole shoe 6 and the initial thickness t0 of the tube wall

[0050] Tube wall height h:

[0051] (2)

[0052] Where: R0 is the initial tube wall radius and is a given value, h1 is the side plate wall thickness of the copper tube and is a given value;

[0053] Initial thickness of pipe wall t0:

[0054] By establishing the relationship curve between the necking angle α, the initial thickness t0 of the tube wall and the axial compressive stress F, the initial thickness t0 of the tube wall is determined;

[0055] S03: Determine the number of shrinkages based on the empirical rule of shrinkage times

[0056] When m>0.75, use one shrinkage; when m is 0.64~0.74, use two shrinkages;

[0057] (3)

[0058] Wherein, m is the shrinkage coefficient;

[0059] S04: Extrusion shrinkage: The pole shoe 6 and the copper tube 1 matched therewith are installed on the prepared mold. According to the determined shrinkage times, an external force is applied to vertically press the top of the pole shoe downward to move the pole shoe downward, and the tube wall is rolled inward along the inner mold groove 2 of the mold 4 until it wraps around the side edge of the upper end of the copper tube; the demoulding angle α1 in step S01 is obtained according to the actual assembly requirements of the project:

[0060] During the shrinking, the tube wall radius of the pole shoe 6 decreases from R0 to r0, that is, it increases from the die entry angle α0 to the die exit angle α1, the die entry angle α0 is the initial value of the shrinking angle α, and the die exit angle α1 is the terminal value of the shrinking angle α; in step S02, the process of obtaining the initial tube wall thickness t0 is specifically as follows:

[0061] Determine the shrinkage coefficient m according to formula (3):

[0062] According to the characteristics of bidirectional compressive stress in the shrinkage deformation zone, according to the Mises yield criterion

[0063] (4)

[0064] Among them: β is the main stress influence coefficient, σT is the true stress;

[0065] Power function relationship of true stress and strain

[0066] (5)

[0067] Where: k is the material strength coefficient, n is the material strain hardening exponent;

[0068] The shrinkage deformation is controlled by compression, and the wall thickness of the deformation zone increases significantly. In this case, the general equilibrium equation of the thickness change of the axisymmetric shell along the generatrix is

[0069] (6)

[0070] Where: ρ, α are the parallel circle radius and shrinking angle of any point in the deformation zone, R ρ , R θ are the curvature radii of the radial section and the axial section, σ ρ , σ θ are radial stress and axial stress respectively, μ and t are friction coefficient and wall thickness respectively. The equations of the shrinking angle α, initial wall thickness t0 and axial compressive stress F in the shrinking process are derived from formulas (3), (4), (5) and (6):

[0071] (7)

[0072] Among them: when the axial compressive stress is the minimum, it is the optimal solution for the shrinking angle α, thereby determining the optimal solution for the initial thickness t0 of the tube wall; in step S03, when the shrinking is performed twice, the following steps are adopted: the process is started for the first time, and the pressure of the first shrinking assembly is applied. When the pressure reaches F1, the pressure is maintained for 2s; then the pressure of the second shrinking is applied. When the pressure reaches F2, the pressure is maintained for 5s, and the shrinking assembly process is ended; according to actual sealing assembly experience, the axial compressive stress F2≈1.05F1 during the second shrinking.

[0073] The detailed connection means are well-known in the art. The following mainly introduces the working principle and process, which are as follows:

[0074] The assembly of the present invention requires mutually matching pole shoes and copper tubes and a jig for shrinking; wherein the copper tube 1 is processed in advance, and the outer diameter of the copper tube 1, the outer diameter of the side plate 7 of the copper tube 1 and the thickness h1 of the side plate of the copper tube 1 are determined. Since the pole shoe 6 needs to match the copper tube 1, a part of the pole shoe 6 can be numerically determined, including the initial radius R0 of the tube wall 3 and the radius r0 of the tube wall after shrinking. Therefore, we also need to calculate the initial thickness t0 of the tube wall of the pole shoe 6 and the height h of the tube wall of the shoe tube 6 to determine the design of the pole shoe. It should be noted that the initial radius R0 of the tube wall and the radius r0 of the tube wall after shrinking are slightly larger than the outer diameter of the side plate 7 of the copper tube and the outer diameter of the copper tube 1 respectively; as for the jig 4, the through hole radius of the jig 4 is slightly larger than the outer diameter of the copper tube. In addition, the curvature radius R of the inner mold groove 2 is calculated. ρ The jig 4 for shrinking assembly can be produced; then the shrinking times are judged and the shrinking process is performed.

[0075] First, according to formula (1) Calculate the curvature radius of the mold groove in the jig and make the jig. The demoulding angle α1 is based on the actual assembly requirements of the project. The radius of the tube wall after shrinking r0 is a given value, and the through hole radius of the jig is r0. Thus, the curvature radius R of the mold groove in the jig can be calculated. ρ , and then a mold for assembly can be made.

[0076] Secondly, according to formula (2) Calculate the pole shoe tube wall height h, where R0 is the initial tube wall radius and is a given value, h1 is the side plate wall thickness of the copper tube and is a given value, and the tube wall radius r0 after shrinking is a given value, thereby calculating the pole shoe tube wall height h; by establishing a relationship curve between the shrinking angle α, the initial tube wall thickness t0 and the axial compressive stress F, determine the initial tube wall thickness t0, thereby determining various values ​​of the pole shoe tube wall, and then manufacturing the pole shoe.

[0077] Then, according to the empirical rule of shrinking times, the shrinking times are determined according to formula (3) , determine the value of the shrinkage coefficient m. When m>0.75, one shrinkage is used; when m is 0.64~0.74, two shrinkages are used. The shrinkage coefficient indicates the degree of deformation of the pipe diameter reduction, and its value is the ratio of the diameter of the pipe before and after shrinkage.

[0078] Finally, the pole shoe prepared above and the matching copper tube are installed on the prepared mold. According to the determined number of shrinkages, external force is applied to press vertically downward on the top of the pole shoe to move the pole shoe downward. The tube wall rolls inward along the inner mold groove of the mold until it wraps around the side edge of the upper end of the copper tube.

[0079] Specifically, when determining the initial thickness t0 of the tube wall, according to the characteristics of the shrinkage deformation zone being bidirectional compressive stress, the Mises yield criterion formula (4) is used: , where the principal stress influence coefficient β≈1.15; the power function relationship of the true stress-strain is formula (5) , where k is the material strength coefficient and n is the material strain hardening exponent. Since the shrinkage deformation is controlled by compression, the wall thickness of the deformation zone increases significantly. In this case, the general equilibrium equation for the thickness change of the axisymmetric shell along the generatrix is ​​formula (6) , where ρ and α are the parallel circle radius and shrinkage angle of any point in the deformation zone, respectively, and R ρ , R θ are the curvature radii of the radial section and the axial section, σ ρ , σ θ are radial stress and axial stress respectively, μ and t are friction coefficient and wall thickness respectively. Combining formulas (3), (4), (5) and (6) with the geometric relationship model of the shrinking process and the details of the published literature, the formula (7) for the axial compressive stress F during the shrinking process can be calculated: , formula (7) can be used to calculate the corresponding axial compressive stress F value for different shrinking angles α, and the optimal shrinking angle α when the axial compressive stress load is the smallest can be determined. At this time, the die entry angle α0 is determined to be this value. The die entry angle is the angle between the tangent line of the point where the pole shoe tube wall and the inner wall of the inner mold groove just contact and the axial direction. The perpendicular line of this point intersects with the perpendicular line of the ejection position point. The intersection point is the center point of Rρ. According to the principle of minimum axial compressive stress and considering the size matching problem before shrinking, the initial thickness of the tube wall is t0.

[0080] Specifically, the demolding angle α1 is obtained according to the actual assembly needs of the project. When shrinking, the radius of the pole piece tube wall decreases from R0 to r0, that is, it increases from the entry angle α0 to the demolding angle α1, and the change amount is the shrinking angle α. The terminal value of the shrinking angle α is the demolding angle α1.

[0081] Specifically, when adopting the second shrinking, the basic principle for determining the values ​​of the second shrinking coefficients m1 and m2 is: the total deformation of the shrinking is mainly borne by the first shrinking, and the deformation of the second shrinking is relatively small in order to ensure the appearance of the parts; the following steps are adopted: the process is started for the first time, and the pressure of the first shrinking assembly is applied. When the pressure reaches F1, the pressure is maintained for 2s; then the pressure of the second shrinking is applied. When the pressure reaches F2, the pressure is maintained for 5s, and the compression assembly process is ended; among them, according to actual sealing assembly experience, the axial compressive stress F2≈1.05F1 during the second shrinking. Example

[0082] This implementation case adopts a circular necking design method. According to the design requirements of the matching between the tube wall 3 of the pole shoe 6 and the copper tube 1, it can be determined that the initial tube wall radius R0=3.5mm, the tube wall radius r0 after necking=2.4mm, and the necking coefficient m=r0 / R0=0.68.

[0083] When shrinking, the wall radius of the pole piece 6 decreases from R0 to r0, that is, it increases from the die entry angle α0 to the die exit angle α1, and the change amount is the shrinking angle α. The terminal value of the shrinking angle α is the die exit angle α1. Figure 2 As shown in the figure, in order to ensure the sealing quality after shrinking and the smoothness of deformation and shrinking, and according to the actual assembly of the project, the mold release angle α1 of this case is designed to be 90°. According to formula (1), when α1=90°, the curvature radius R of the mold groove is calculated. ρ =1.2mm; according to the inner die groove curvature radius R required for shrinking ρ =1.2mm, the corresponding mold can be made.

[0084] In this case, the side wall thickness h1 of the copper tube is 0.2 mm. The height h of the pole shoe tube wall can be calculated to be 1.5 mm according to the empirical formula (2). Figure 3 as shown in .

[0085] The principal stress influence coefficient β in the Mises yield criterion is ≈ 1.15. The pole shoe material used in the example is DT4E. In its material constitutive model, k=105 MPa, n=0.154, and the friction coefficient μ=0.15.

[0086] For the initial wall thickness t0 of the pole shoe tube wall, this implementation case designed three initial wall thicknesses of 0.51mm, 0.48mm, and 0.45mm. For different initial wall thicknesses, the axial compressive stress F value when the shrinkage angle α is 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, and 60° is calculated according to formula (7), and the plot is shown in the figure. Figure 1 The curve diagram shown by Figure 1It can be seen that when the shrinking angle is 30°, the axial compressive stress in the shrinking process is the smallest. In order to ensure that the initial axial compressive stress F of the shrinking is the smallest, and because the die angle α0 is the initial value of the shrinking angle α, the die angle α0 is set to 30°; when the initial wall thickness t0 of the tube wall decreases from 0.51mm and 0.48mm to 0.45mm, the minimum axial compressive stress change rate is 5%, and the impact is relatively small. Therefore, when selecting the initial wall thickness, the matching dimensions before shrinking are given priority. In this case, the initial wall thickness t0 is determined to be 0.50mm; the corresponding pole shoe tube wall can be made according to the pole shoe tube wall height h of 1.5mm, the initial wall thickness t0 of 0.50mm, the initial tube wall radius R0=3.5mm and the tube wall radius r0=2.4mm after shrinking.

[0087] According to the empirical principle of shrinkage times: when m>0.75, one shrinkage is used; when m is 0.64~0.74, two shrinkages are used. Since the shrinkage coefficient m=r0 / R0=0.68 in this implementation case, two shrinkages are selected; the design process of the two shrinkage coefficients m1 and m2 in this case is as follows:

[0088] The first shrinkage coefficient m1 is taken as 0.68, then the tube wall radius after the first shrinkage is R1=R0*m1=3.5 mm*0.68=2.4 mm, which bears the main shrinkage deformation; according to formula (7), it can be calculated that after the first shrinkage, the axial compressive stress load F1 is 2870 MPa, and the second shrinkage coefficient m2 is taken as 1, then the tube wall radius after the second shrinkage is r0=R1*m2=2.4 mm*1=2.4 mm, that is, no shrinkage deformation is implemented, only the outer dimensions of the part after shrinkage are guaranteed; according to actual sealing assembly experience, the axial compressive stress load F2≈1.05F1=3013 MPa is implemented to ensure the outer dimensions of the shrinkage for the second time.

[0089] According to the number of shrinking times, in order to ensure shrinking efficiency and process stability, this case designed the sealing process flow of precision hydraulic parts, as follows:

[0090] When the process is started for the first time, the pressure of the first shrinking assembly is applied. When the pressure reaches F1=2870 Mpa, the pressure is maintained for 2s. If the specified pressure F1 is not reached, the axial pressure is continued to be applied. Then the second shrinking pressure is applied. When the pressure reaches F2≈1.05F1=3013 Mpa, the pressure is maintained for 5s, and the crimping process is ended. If the set pressure F2 is not reached, the axial pressure is continued to be applied. Figure 4 shown.

[0091] The present invention includes a pole shoe to be assembled, a copper tube with a side plate at the upper end, a jig for assembling the pole shoe and the copper tube, and a press for applying external force, wherein the tube wall is arranged on the lower bottom surface of the pole shoe, a through hole matching the outer diameter of the copper tube is vertically opened at the center of the jig, an inner mold groove is arranged on the upper edge of the through hole, the copper tube is inserted into the through hole, and the side plate at the upper end is inserted into the tube wall of the pole shoe, the lower end of the tube wall of the pole shoe abuts against the inner mold groove of the jig, and the angle formed with the inner mold groove is the die entry angle; during the extrusion process, the press applying external force vertically downwardly applies external force to the top of the pole shoe to move the pole shoe downward, and at the same time, the tube wall is rolled inward along the inner mold groove of the jig until it wraps the side plate at the upper end of the copper tube, and when the tube wall is completely abutted against the lower bottom surface of the side plate, the shrinking assembly process is completed.

[0092] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for sealing and assembling a precision hydraulic pipe, characterized in that: Using a mold (4) to shrink and seal the pole shoe (6) and the copper tube (1) comprises the following steps: S01: Obtain the curvature radius R of the inner mold groove (2) of the mold (4) ρ （1） Where: α1 is the die-out angle, r0 is the tube wall radius after shrinking and r0 is a given value; S02: Obtain the height h of the pole shoe (6) and the initial thickness t0 of the tube wall (3) Tube wall height h: （2） Where: R0 is the initial tube wall radius and is a given value, h1 is the side plate wall thickness of the copper tube (1) and is a given value; Initial thickness of pipe wall t0: By establishing the relationship curve between the necking angle α, the initial thickness t0 of the tube wall and the axial compressive stress F, the initial thickness t0 of the tube wall is determined; S03: Determine the number of shrinkages based on the empirical rule of shrinkage times When m>0.75, use one shrinkage; when m is 0.64~0.74, use two shrinkages; （3） Wherein, m is the shrinkage coefficient; The specific process of obtaining the initial thickness t0 of the tube wall is: Determine the shrinkage coefficient m according to formula (3): According to the characteristics of bidirectional compressive stress in the shrinkage deformation zone, according to the Mises yield criterion （4） Among them: β is the main stress influence coefficient, σT is the true stress; Power function relationship of true stress and strain （5） Where: k is the material strength coefficient, n is the material strain hardening exponent; The shrinkage deformation is controlled by compression, and the wall thickness of the deformation zone increases significantly. In this case, the general equilibrium equation of the thickness change of the axisymmetric shell along the generatrix is （6） Where: ρ, α are the parallel circle radius and shrinking angle of any point in the deformation zone, R ρ , R θ are the curvature radii of the radial section and the axial section, σ ρ , σ θ are radial stress and axial stress respectively, μ and t are friction coefficient and wall thickness respectively. The equations of the shrinking angle α, initial wall thickness t0 and axial compressive stress F in the shrinking process are derived from formulas (3), (4), (5) and (6): （7） Among them: when the axial compressive stress is the minimum, it is the optimal solution of the necking angle α, thus determining the optimal solution of the initial thickness t0 of the tube wall; S04: Squeeze the compression port.

2. The method for assembling a precision hydraulic pipe by shrinking and sealing according to claim 1, characterized in that: The demoulding angle α1 in step S01 is obtained according to the actual assembly requirements of the project: When shrinking, the radius of the tube wall (3) of the pole shoe (6) decreases from R0 to r0, that is, it increases from the die entry angle α0 to the die exit angle α1, the die entry angle α0 is the initial value of the shrinking angle α, and the die exit angle α1 is the terminal value of the shrinking angle α.

3. The method for sealing and assembling a precision hydraulic pipe according to claim 1, characterized in that: In step S03, when the second shrinking is adopted, the following steps are adopted: the process is started for the first time, the pressure of the first shrinking assembly is applied, and when the pressure reaches F1, the pressure is maintained for 2 seconds; then the pressure of the second shrinking is applied, and when the pressure reaches F2, the pressure is maintained for 5 seconds, and then the shrinking assembly process is ended.

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