A gas-inflated sealing fixture and its sealing method

Abstract

This invention discloses a sealing fixture and sealing method for air-expanded forming, comprising an upper mold and a lower mold for forming a cavity, and pushers disposed at both ends of the cavity. The cavity is used to accommodate a tube blank. The pushers are provided with an air inlet channel and a sealing cavity communicating with the tube blank. One end of the sealing cavity is open and fits against the end of the tube blank. The sealing cavity is provided with a sealing section and a sealing extension section in sequence with respect to the tube's elongation direction. Both the sealing section and the sealing extension section are conical, and the maximum end of the sealing extension section connects to the minimum end of the sealing section. The tube blank elongates under high temperature, so that both ends of the tube blank fully cooperate with the sealing section. After the tube blank elongates, a new sealing area is formed at the sealing section, thereby maintaining the sealing performance. At the same time, the sealing extension section extends the sealing section, preventing the tube blank from being too long and damaging the air inlet, thus ensuring the reliability of the seal on the elongated tube blank.

Description

Technical Field

[0001] This invention relates to the field of high-pressure forming technology for pipes, specifically to a sealing fixture and sealing method for gas-expanded forming. Background Technology

[0002] Gas forming uses gas as a force transmission medium under heating. Under the combined action of internal pressure and axial feeding by the pusher, the tube blank can fit into the cavity surface of the mold. Therefore, the sealing between the tube blank and the pusher is particularly important during the forming process. Once the seal fails, the tube blank cannot fit fully into the cavity, resulting in unqualified products.

[0003] Existing air-forming technologies typically employ sealing rings on the pusher head for sealing. However, since air-formed tube blanks are usually internally pressurized after heating, soft rubber sealing rings are at risk of deformation and seal failure at high temperatures. While existing rubber sealing rings can withstand temperatures up to 200 degrees Celsius, they still cannot meet the forming temperatures of most tube blanks. Another existing air-forming sealing method uses rigid seals, which can solve the problem of sealing ring deformation at high temperatures. However, during the heating and internal pressing process, the length and diameter of the tube blank undergo changes in elongation or shortening. Rigid seals lack the ability to compensate for these changes in tube blank dimensions, resulting in seal failure or poor sealing performance. In particular, under heating conditions, the ends of the tube blank elongate to both sides under high temperatures, which is one of the main reasons for rigid seal failure.

[0004] Furthermore, existing air-forming pushers typically use plug-in connectors, where the pusher is directly inserted into the openings at both ends of the tube blank to achieve a preliminary seal. The rigid-sealing plug-in connectors generally used have the following main problems:

[0005] 1. Due to thermal expansion and contraction during the forming process, the tube blank tends to elongate when heated, which can cause gaps to appear between the inner wall of the tube blank and the outer wall of the pusher, resulting in sealing failure.

[0006] 2. After the tube blank is heated, the material softens and is easily stuck inside the pusher, forming flash at the sealing point. The sealing point cannot form effective support, causing the seal to fail.

[0007] 3. During air expansion molding, as the internal air pressure of the tube blank rises, the tube blank is formed in the cavity, causing the tube blank material to shrink inward and become shorter. If the left and right pushers cannot keep up with the material supply, a gap will be formed between the pusher and the tube end, causing the seal to fail.

[0008] This invention is a sealing method based on air expansion forming, mainly to overcome the above-mentioned shortcomings of air expansion forming sealing, and to provide a safe, reliable air expansion forming sealing tooling and method that can automatically adapt to changes in the length of the tube blank. Summary of the Invention

[0009] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a gas-inflated sealing fixture and its sealing method.

[0010] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a sealing fixture for air expansion forming, comprising an upper mold and a lower mold for forming a cavity, and pushers disposed at both ends of the cavity, wherein the cavity is used to accommodate a tube blank, and the pushers are provided with an air inlet channel and a sealing cavity communicating with the tube blank, one end of the sealing cavity being open and fitting against the end of the tube blank, and the sealing cavity being provided with a sealing section and a sealing extension section in sequence with respect to the elongation direction of the tube body, wherein both the sealing section and the sealing extension section are conical, and the maximum end of the sealing extension section is connected to the minimum end of the sealing section.

[0011] Furthermore, the sealing section is provided with multi-level annular grooves, and the cross-sectional area of ​​the multi-level annular grooves decreases with respect to the elongation direction of the tube blank.

[0012] Furthermore, the sealing cavity also includes a guide section, which is located at the opening end of the sealing cavity and is coplanar with the sealing section.

[0013] Furthermore, the sealing section is at a 60-degree angle.

[0014] Furthermore, a stop section is provided inside the sealing cavity. The stop section is located at the minimum end of the sealing extension section, and the stop section blocks the air intake channel and the end of the tube blank.

[0015] A method for sealing by air inflation includes the following steps:

[0016] S1. Place the tube blank between the lower mold and the upper mold, wherein the upper mold and the lower mold have a lower cavity for forming the tube blank, and insert the pusher according to any one of claims 1 to 5 into both ends of the tube blank, wherein the two ends of the tube blank are corrected for the tube end centering by the guide section.

[0017] S2. The pushers at both ends of the tube blank move relative to each other, and at least part of both ends of the tube blank enter the sealing section of the pusher.

[0018] S3, Tube blank heating stage: After heating, the tube blank elongates, and the two ends of the tube blank further cooperate with the sealing section under the heating action. At this time, the two pushers remain fixed, and the two ends of the tube blank further cooperate with the multi-level annular groove on the sealing section. The two ends of the tube blank form a multi-level sealing part on the sealing section that matches the multi-level annular groove.

[0019] S4, Blank pressurization stage: The blank is pressurized through the air intake channel inside the pusher. Under the action of internal air pressure, the blank adheres tightly to the surface of the cavity and exhausts the air inside the cavity.

[0020] S5. Tube blank replenishment: The tube blank shortens under the action of internal air pressure, and the pushers at both ends continue to move relative to each other, with both ends of the tube blank keeping in contact with the sealing section or sealing extension section.

[0021] Furthermore, in step S3, both ends of the tube blank are further elongated under heating and enter the sealing extension section.

[0022] Furthermore, in step S5, if the pusher feeding distance is less than the tube blank shortening distance, the two ends of the tube blank move away from the sealing cavity along the shortening direction, and a compensation gap is formed between the two ends of the tube blank and the outer wall of the sealing cavity. The two ends of the tube blank are radially compensated under the action of internal air pressure and tube material rebound and keep in contact with the sealing section or sealing extension section.

[0023] Furthermore, it also includes step S6, cavity depressurization: the air pressure output of the air intake channel is released, the two ends of the tube blank retract, and the multi-stage sealing parts at both ends of the tube blank gradually move away from the sealing section, forming a depressurization gap with the sealing section.

[0024] Furthermore, the hardness of the multi-level annular groove is higher than that of the tube blank material.

[0025] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0026] 1. By setting pushers with end openings at both ends of the tube blank, the sealing cavity can be covered on the ends of the tube blank, thereby providing support and limiting for the deformed part during the process of thermal elongation and radial expansion under high pressure at both ends of the tube blank. Moreover, the cross-section of each section in the sealing cavity is a conical surface to form a good self-locking and ensure the sealing performance of the tube blank during gas expansion molding. The sealing cavity consists of a guide section, a sealing section, and a sealing extension section. Among them, the sealing extension section has a larger slope than the sealing section to further ensure the sealing performance at both ends of the tube blank.

[0027] 2. During the placement of the tube blank, the guide section covers the end of the tube blank and corrects both ends of the tube blank, ensuring that it is aligned with the axis of the cavity, improving the centering accuracy, and the end of the tube blank forms a preliminary seal with the sealing section.

[0028] 3. During the heating process, the tube blank elongates under high temperature, which allows the two ends of the tube blank to fully cooperate with the sealing section. After the tube blank elongates, a new sealing area is formed at the sealing section, thereby maintaining the sealing performance. At the same time, the setting of the sealing extension section extends the sealing section and prevents the tube blank from being damaged by excessive length, thus ensuring the reliability of the seal on the elongated tube blank.

[0029] 4. During the internal high-pressure forming process, the tube blank shortens under high pressure. At this time, the two ends of the tube blank are radially compensated to the inner wall of the sealing cavity under the action of internal pressure and material springback, forming a good self-locking.

[0030] 5. After the molding is completed, the internal pressure is released. After the two ends of the tube blank lose the internal pressure, they move away from the sealing section, thus forming a pressure relief gap. This allows the mold cavity to gradually release pressure, thereby forming a stable self-pressure relief effect.

[0031] 6. The same type of pusher can be used to seal pipe blanks of different diameters, and has good versatility. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the sealing fixture of the present invention;

[0033] Figure 2 This is a schematic diagram showing the fit between the pusher and the end of the tube blank according to the present invention;

[0034] Figure 3 For the present invention Figure 2 Enlarged view of point A in the middle;

[0035] Figure 4 This is a schematic diagram of the tube blank heating stage of the present invention;

[0036] Figure 5 This is a schematic diagram of the tube blank end pressurization stage of the present invention;

[0037] Figure 6 This is a schematic diagram of the depressurization stage of the present invention;

[0038] In the diagram: 1. Upper mold; 2. Lower mold; 3. Cavity; 3.1. First cavity; 3.2. Second cavity; 4. Push head; 4.1. Sealing section; 4.2. Sealing extension section; 4.3. Guide section; 4.4. Multi-stage annular groove; 4.5. Stop section; 4.6. Sealing cavity; 4.7. Air inlet channel; 5. Tube blank; 6. Compensation gap; 7. Pressure relief gap; 8. Pre-forming space; Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] It should be understood that although the terms upper, middle, lower, top, one end, etc., appear in this document to describe various elements, these elements are not limited by these terms. These terms are only used to distinguish the elements from each other for ease of understanding, and are not used to define any directional or sequential restrictions.

[0041] like Figure 1-6As shown, a sealing fixture for air-expansion forming includes an upper mold 1 and a lower mold 2 for forming a cavity 3, and pushers 4 disposed at both ends of the cavity 3. The cavity 3 is used to accommodate a tube blank 5. Specifically, the cavity 3 includes a first cavity 3.1 and a second cavity 3.2 connecting the two ends of the first cavity 3.1. The first cavity 3.1 has the final profile of the tube blank 5, and the second cavity 3.2 is used for positioning with the tube blank 5 and the axial portions of the two ends of the formed tube.

[0042] The pusher 4 has an air inlet channel and a sealing cavity that connects to the tube blank 5. The sealing cavity has a tapered cross-section and one end is open and fits against the end of the tube blank 5, so that the sealing cavity covers both ends of the tube blank 5 and supports the length elongation and radial expansion of the tube blank 5 during the air expansion molding process, so that both ends of the tube blank 5 can fit against the sealing cavity and form a good self-locking.

[0043] As a further explanation of the pusher 4, the sealing cavity is provided with a sealing section 4.1 and a sealing extension section 4.2 in sequence with respect to the elongation direction of the tube body. Both the sealing section 4.1 and the sealing extension section 4.2 are tapered, and the maximum end of the sealing extension section 4.2 connects to the minimum end of the sealing section 4.1. The sealing section 4.1 is provided with an annular groove, which is mainly used to abut against the end of the tube blank 5. During the air expansion molding process, under the combined action of the pusher 4's side thrust and internal air pressure, the end of the tube blank 5 is forced to undergo plastic deformation, so that the surface of the end of the tube blank 5 is tightly attached to the sealing cavity, achieving a good seal. The sealing extension section 4.2 is provided with an annular groove on its upper part, which is mainly used to extend the sealing section 4.1 to ensure that the end of the tube blank 5 will not fail to seal due to excessive elongation.

[0044] In the above embodiment, a sealing extension section 4.2 with a larger slope is adopted. After the end of the tube blank 5 extends to the sealing extension section 4.2, it enters the sealing extension section 4.2 with a gradually decreasing cross-sectional area. The extended part of the end of the tube blank 5 abuts against the sealing extension section 4.2 under the guidance of the conical surface, which is beneficial to maintaining the sealing effect between the end of the tube blank 5 and the sealing cavity.

[0045] As a further explanation of the sealing section 4.1, the sealing section 4.1 is provided with multi-level annular grooves 4.4. The cross-sectional area of ​​the multi-level annular grooves 4.4 decreases with respect to the elongation direction of the tube blank 5, so that the multi-level annular grooves 4.4 in the sealing section 4.1 are connected to each other and form a sawtooth shape. The connecting surface of the multi-level annular grooves 4.4 is preferably perpendicular to the center line of the pusher 4. With this structure, the end of the tube blank 5 undergoes plastic deformation, thereby forming a multi-level sealing ring.

[0046] The elongation phenomenon at the end of the tube blank 5 mentioned above is mainly due to the effect of high temperature. During this process, the pusher 4 does not need to move, but only needs to apply a lateral thrust to keep it in the current position, so that the end of the tube blank 5 naturally elongates in the sealing cavity and is tightly attached to the sealing section 4.1 or the sealing extension section 4.2, making the seal more reliable.

[0047] Optionally, the conical surface of the sealing section 4.1 forms a 60-degree angle with respect to the centerline of the pusher head 4.

[0048] Specifically, the sealing cavity also includes a guide section 4.3, which is located at the opening end of the sealing cavity and is coplanar with the sealing section 4.1. During the placement of the tube blank 5, the guide section 4.3 covers the end of the tube blank 5 and corrects both ends of the tube blank 5 to ensure that it is aligned with the axis of the cavity 3, thereby improving the centering accuracy. The end of the tube blank 5 forms a preliminary seal with the sealing section 4.1. At the same time, the guide section 4.3 also guides the end of the tube blank 5 into the sealing section 4.1.

[0049] Specifically, the sealing cavity is also provided with a stop section 4.5, which is located at the smallest end of the sealing extension section 4.2, and the stop section 4.5 blocks the air intake channel and the end of the tube blank 5.

[0050] The sealing fixture described above achieves a rigid seal at the end of the tube blank 5. The sealing section 4.1 forms a multi-stage rigid sealing ring at both ends of the tube blank 5, which is beneficial for sealing the end of the tube blank 5 during subsequent processing. Furthermore, during the left and right movement of the pusher 4, the uniformity of the axial force on the end of the tube blank 5 can be improved, which is beneficial for the forming effect of the tube blank 5.

[0051] From a cost perspective, by using the pusher head 4 with a sealed cavity, the pusher head 4 does not need to enter the upper mold 1 and the lower mold 2. The corresponding mold can be set according to the actual length of the tube blank 5. For tube blanks with a longer length, the pusher head 4 can be set outside the upper and lower molds, thereby reducing the manufacturing cost of the upper and lower molds.

[0052] If the pusher 4 is placed inside the mold, a sliding gap is inevitably required between the outer surface of the pusher 4 and the upper and lower molds 2. Therefore, the flatness requirement between the outer surface of the pusher 4 and the upper and lower molds 2 is also high. During the heating process, the elongated part of the tube blank 5 end is also easy to get stuck in the sliding gap, affecting the normal progress of the forming process. The sealing tooling of the present invention can avoid the above problems and ensure the reliable sealing and normal operation of the tube blank 5 forming.

[0053] A method for sealing by air inflation includes the following steps:

[0054] S1. Place the tube blank 5 between the lower mold 2 and the upper mold 1. The upper mold 1 and the lower mold 2 have a lower cavity 3 for forming the tube blank 5. Insert the pusher 4 as described in any one of claims 1 to 5 into both ends of the tube blank 5. The two ends of the tube blank 5 are corrected for the tube end centerness through the guide section 4.3 to complete the initial positioning of the tube blank 5.

[0055] S2. The pushers 4 at both ends of the tube blank 5 move relative to each other. At least part of both ends of the tube blank 5 enter the sealing section 4.1 of the pusher 4. Under the action of the guide section 4.3, the tube blank 5 is aligned with the cavity 3, completing the positioning of the tube blank 5. Both ends of the tube blank 5 enter part of the sealing section 4.1 and rigidly abut against the multi-stage annular groove 4.4.

[0056] S3, Heating stage of tube blank 5: After heating, tube blank 5 elongates, and both ends of tube blank 5 further cooperate with sealing section 4.1 under the action of heating. At this time, the two pushers 4 remain fixed, and both ends of tube blank 5 further cooperate with the multi-stage annular groove 4.4 on sealing section 4.1, so that both ends of tube blank 5 undergo plastic deformation, and a multi-stage sealing part matching the multi-stage annular groove 4.4 is formed on sealing section 4.1. The multi-stage sealing part is in close contact with the multi-stage annular groove 4.4, keeping the pusher 4 and tube blank 5 sealed.

[0057] S4, tube blank 5 pressurization stage: The tube blank 5 is pressurized through the air inlet channel in the pusher 4. Under the action of internal air pressure, the tube blank 5 is tightly attached to the surface of the cavity 3. The tube blank 5 is formed under the action of internal air pressure and the air in the cavity 3 is discharged. At this time, the multi-stage sealing part is further tightly attached to the multi-stage annular groove 4.4 under the action of internal air pressure.

[0058] S5, Tube blank 5 feeding: Tube blank 5 expands under the action of internal air pressure and shortens in the length direction. The pushers 4 at both ends continue to move relative to each other. The two ends of tube blank 5 are in contact with the sealing section 4.1 or the sealing extension section 4.2.

[0059] Specifically, in step S3, both ends of the tube blank 5 are extended under heating and enter the sealing extension section 4.2.

[0060] Specifically, in step S5, if the material feeding distance of the pusher 4 is less than the shortening distance of the tube blank 5, the two ends of the tube blank 5 move away from the sealing cavity along the shortening direction, and a compensation gap 6 is formed between the two ends of the tube blank 5 and the outer wall of the sealing cavity. The two ends of the tube blank 5 are radially compensated under the action of internal air pressure and tube material rebound and remain in contact with the sealing section 4.1 or the sealing extension section 4.2. In other words, after the two ends of the tube blank 5 are shortened, they are still located in the sealing cavity. Under the above-mentioned internal air pressure and rebound action, they can still be in contact with the inner wall of the sealing cavity to achieve compensation.

[0061] Specifically, it also includes step S6, depressurizing the cavity 3: releasing the air pressure output of the air inlet channel, the two ends of the tube blank 5 retract, and the multi-stage sealing parts at both ends of the tube blank 5 gradually move away from and exit the sealing section 4.1, forming a pressure relief gap with the sealing section 4.1, so that the mold cavity 3 gradually depressurizes, forming a stable self-depressurization effect, without the need to set up a pressure relief component.

[0062] Optionally, in step S5, the two ends of the tube blank 5 further engage with the multi-stage annular groove 4.4, the front end of the multi-stage sealing part enters the sealing extension section 4.2, and the axial rear part of the multi-stage sealing part enters the sealing section 4.1, forming a new multi-stage sealing part, so that the seal is stable and reliable.

[0063] Specifically, the multi-stage annular groove 4.4 is heat-treated to make its hardness higher than that of the tube blank 5 material, causing the tube blank 5 to undergo plastic deformation and form a reliable seal.

[0064] Specifically, a pre-forming space 8 is provided between the cavity wall of the first cavity 3.1 of the upper mold 1 and the lower mold 2 and the tube blank 5. The tube blank 5 expands under the action of internal air pressure and discharges the air in the pre-forming space 8, so that after the tube blank 5 is formed, the uniformity of the wall thickness in the first cavity and the second cavity is improved.

[0065] During air expansion molding, the formed tube body softens due to the loss of high temperature. Thanks to the sealing cavity effect of the cover method, the two ends of the tube blank 5 can further adhere to the inner surfaces of the sealing cavity under the dual action of internal air pressure and its own elastic rebound. Furthermore, the higher the internal gas pressure, the tighter the outer surface of the tube blank 5 fits with the inner conical annular groove, the more reliable the seal, and a very good high-pressure sealing effect can be formed.

[0066] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A sealing fixture for air-expansion forming, comprising an upper mold (1) and a lower mold (2) for forming a cavity (3), and pushers (4) disposed at both ends of the cavity (3), the cavity (3) being used to accommodate a tube blank (5), characterized in that, The pusher (4) is provided with an air intake channel and a sealing cavity that connects to the tube blank (5). One end of the sealing cavity is open and fits against the end of the tube blank (5). The sealing cavity is provided with a sealing section (4.1) and a sealing extension section (4.2) in sequence along the extension direction of the tube body. Both the sealing section (4.1) and the sealing extension section (4.2) are conical. The maximum end of the sealing extension section (4.2) connects to the minimum end of the sealing section (4.1). The sealing section (4.1) is provided with a multi-level annular groove (4.4). The cross-sectional area of ​​the multi-level annular groove (4.4) decreases along the extension direction of the tube blank (5). The tube blank (5) elongates after heating, so that both ends of the tube blank (5) fully cooperate with the sealing section (4.1). After the tube blank (5) elongates, a new sealing area is formed at the sealing section (4.1). At the same time, the sealing extension section (4.2) extends the sealing section (4.1) and prevents the tube blank (5) from being too long and damaging the air intake.

2. The air-inflated sealing fixture according to claim 1, characterized in that: The sealing cavity further includes a guide section (4.3), which is located at the opening end of the sealing cavity and is coplanar with the sealing section (4.1).

3. The air-inflated sealing fixture according to claim 1, characterized in that: The conical surface of the sealing section (4.1) forms a 60-degree angle with respect to the center line of the pusher (4).

4. The air-inflated sealing fixture according to claim 1, characterized in that: The sealed cavity is also provided with a stop section (4.5), which is located at the smallest end of the sealed extension section (4.2) and blocks the air intake channel and the end of the tube blank (5).

5. A sealing method using air inflation molding, characterized in that, Includes the following steps: S1. Place the tube blank (5) between the lower mold (2) and the upper mold (1). The upper mold (1) and the lower mold (2) have cavities (3) for forming the tube blank (5). Insert the pusher (4) as described in claim 2 into both ends of the tube blank (5). The two ends of the tube blank (5) are corrected for tube end centering by the guide section (4.3). S2, the pushers (4) at both ends of the tube blank (5) move relative to each other, and at least part of both ends of the tube blank (5) enter the sealing section (4.1) of the pusher (4); S3, Heating stage of tube blank (5): After heating, the tube blank (5) elongates, and the two ends of the tube blank (5) further cooperate with the sealing section (4.1) under the action of heating. At this time, the two pushers (4) remain fixed, and the two ends of the tube blank (5) further cooperate with the multi-level annular groove (4.4) on the sealing section (4.1). The two ends of the tube blank (5) form a multi-level sealing part on the sealing section (4.1) that matches the multi-level annular groove (4.4). S4, tube blank (5) pressurization stage: pressurize the tube blank (5) through the air inlet channel in the pusher (4), and the tube blank (5) adheres tightly to the surface of the cavity (3) under the action of internal air pressure and discharges the air in the cavity (3); S5, Tube blank (5) feeding: The tube blank (5) shortens under the action of internal air pressure, and the pushers (4) at both ends continue to move relative to each other. The two ends of the tube blank (5) are in contact with the sealing section (4.1) or the sealing extension section (4.2).

6. The air-inflated sealing method according to claim 5, characterized in that: In step S3, the two ends of the tube blank (5) are further elongated under heating and enter the sealing extension section (4.2).

7. A sealing method for air-inflated molding according to any one of claims 5 or 6, characterized in that: In step S5, if the material feeding distance of the pusher (4) is less than the shortening distance of the tube blank (5), the two ends of the tube blank (5) move away from the sealing cavity along the shortening direction, and a compensation gap (6) is formed between the two ends of the tube blank (5) and the outer wall of the sealing cavity. The two ends of the tube blank (5) are radially compensated under the action of internal air pressure and tube material rebound and keep in contact with the sealing section (4.1) or the sealing extension section (4.2).

8. The air-inflated sealing method according to claim 5, characterized in that: It also includes step S6, cavity (3) depressurization: release the air pressure output of the air inlet channel, the two ends of the tube blank (5) retract, and the multi-stage sealing parts at both ends of the tube blank (5) gradually move away from the sealing section (4.1) and form a depressurization gap with the sealing section (4.1).

9. A sealing method for air expansion forming according to claim 5, characterized in that: The hardness of the multi-level annular groove (4.4) is higher than that of the tube blank (5).

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