A large-diameter, small-bending-radius thin-walled bend integral forming mold and method thereof

Abstract

A method for integral forming of a large-diameter, small-radius thin-walled bend is disclosed, which relates to a test bench. This invention addresses the problems in existing technologies where large-diameter, small-radius thin-walled bend blanks are difficult to form straight-wall sections, and the internal high-pressure forming process cannot provide axial material replenishment, leading to severe thickness reduction on the outer side of the bend and even cracking. The forming apparatus of this invention includes a pre-forming device and an internal high-pressure forming device. The forming method is as follows: Step 1: The bent blank is placed into the pre-forming mold to form a pre-formed blank with a straight-wall section; Step 2: The open ends on both sides of the pre-formed blank are cut into bevels with a certain angle, effectively suppressing excessive thinning of the outer wall during bulging and effectively controlling the overall wall thickness uniformity; Step 3: The cut pre-formed blank is placed into the internal high-pressure forming mold to obtain a large-diameter, small-radius thin-walled bend. This invention is used for integral forming of large-diameter, small-radius thin-walled bends.

Description

Technical Field

[0001] This invention relates to a bend integral forming mold and method thereof, specifically to a large-diameter, small-bending-radius thin-walled bend integral forming mold and method thereof. Background Technology

[0002] Thin-walled metal bends are widely used in industries such as aerospace, automotive, and shipbuilding, including piping systems for aerospace engines and large gas turbines. To meet requirements for weight reduction, improved space utilization, and high structural strength, integrally formed, large-diameter, small-radius thin-walled bends have long been the preferred solution for designers. However, when these bends are integrally bent, defects such as excessive thinning on the outer side and wrinkling on the inner side are prone to occur, making processing extremely difficult.

[0003] Existing technologies involve bending a tube blank before performing internal high-pressure forming. The main methods for obtaining the bent tube blank are push bending or CNC bending. Push bending tube blanks lack straight-wall sections, making the sealing during subsequent internal high-pressure forming extremely difficult. Furthermore, axial feeding cannot effectively suppress internal high-pressure forming, resulting in severe thinning of the outer wall. While CNC bending tube blanks have straight-wall sections, the diameter of the tube blanks formed by this method is typically limited to φ100mm or less, making it impossible to form large-diameter thin-walled bends.

[0004] The invention patent with publication number CN 102554009 A proposes a fluid pressure forming method for small-radius elbows. As described in the specification, after selecting a CNC bending or push-bending elbow, a straight-wall section is welded, followed by bulging to increase its diameter and decrease its bending radius, thereby reducing the relative bending radius of the elbow and achieving the forming of a thin-walled, small-radius elbow. As the specification states, the pipe blank needs to be welded with a straight-wall section. During bulging, due to the poor tensile strength of the weld joint, cracks are prone to occur in the weld and heat-affected zone, increasing the sealing difficulty of the forming process. The weld also affects the wall thickness distribution and dimensional accuracy of the formed pipe fitting. Therefore, this method has a complex process and a poor yield.

[0005] Chinese patent application CN 112091048 A discloses a small-radius bent pipe with straight sections at both ends, along with its hydraulic forming device and method. As described in the specification, a CNC bent pipe with straight sections is selected as the blank for hydraulic expansion forming, avoiding the problems of outer cracking and inner wrinkling that occur with traditional methods of forming small-radius bent pipes with straight sections at both ends. However, the diameter of the CNC bent pipe blank is usually limited to φ100mm or less, making it difficult to solve the problem of forming straight-walled sections from large-diameter pipe blanks.

[0006] Therefore, existing technologies have problems such as difficulty in forming straight-walled sections from large-diameter, small-bending-radius thin-walled bend tube blanks, and the inability to axially feed material during internal high-pressure forming, resulting in severe thickness reduction on the outer side of the bend, exceeding tolerances, or even cracking. Summary of the Invention

[0007] The purpose of this invention is to solve the problems in existing technologies where it is difficult to form straight-walled sections from large-diameter, small-bending-radius thin-walled bends, and the inability to axially replenish material during internal high-pressure forming leads to severe thinning of the outer side of the bend, exceeding tolerances, or even cracking. Therefore, this invention provides a mold and method for integral forming of large-diameter, small-bending-radius thin-walled bends.

[0008] The technical solution of this invention is as follows: A large-diameter, small-bending-radius thin-walled bend integral forming mold includes an upper pad, a lower pad, a lower mold base, a forming hydraulic cylinder, a filling hydraulic cylinder, a forming cylinder seat, a filling cylinder seat, and a mold body. The upper and lower pads are arranged parallel to each other, the lower pad is installed on the lower mold base, the mold body is installed between the upper and lower pads, the forming cylinder seat and the filling cylinder seat are installed between the upper and lower pads, the forming hydraulic cylinder is installed on the forming cylinder seat, the filling hydraulic cylinder is installed on the filling cylinder seat, the forming hydraulic cylinder is connected to the forming punch of the mold body, and the filling hydraulic cylinder is connected to the filling punch of the mold body. The mold body includes an upper die, a lower die, a forming punch, and a filling punch. The upper and lower dies are interlocked, forming a U-shaped groove between them. The forming punch and the filling punch are respectively sealed and inserted into the straight wall section of the U-shaped groove.

[0009] Furthermore, the two ends of the U-shaped groove are straight-walled sections, and the middle cavity of the U-shaped groove is in the shape of a tube blank. At this time, the mold body is a pre-forming mold.

[0010] Furthermore, the diameter of the head of the forming punch and the liquid-filling punch that are sealed and extended into the straight wall section gradually decreases.

[0011] Furthermore, the upper die in the preforming mold is provided with a boss.

[0012] Furthermore, the two ends of the U-shaped groove are straight-walled sections, and the middle cavity of the U-shaped groove is a designed bend. At this time, the mold body is an internal high-pressure forming mold.

[0013] Furthermore, both the forming punch and the liquid filling punch are stepped punches.

[0014] Furthermore, both the forming punch and the filling punch have embedded sealing rings at their heads.

[0015] This invention also provides a forming method for an integral forming mold for a thin-walled bend with a large diameter and small bending radius, which includes the following steps:

[0016] Step 1: Place the bent tube blank into the preforming mold and form a preformed tube blank with a straight wall section.

[0017] Step 2: Cut the open ends on both sides of the precast tube blank from Step 1 into bevels with a certain angle;

[0018] Step 3: Place the cut precast tube blank into the inner high-pressure forming mold to form a thin-walled bend with a large diameter and small bending radius.

[0019] Furthermore, in step two, the method for determining the cutting angle of the open ends on both sides of the precast tube blank is as follows:

[0020] The preformed tube blank is cut at both ends to obtain the designed angle α. The formula for selecting α is:

[0021]

[0022] In the formula: D 0 represents the diameter of the tube blank; D 1 represents the design bend diameter; R 0 represents the bending radius of the tube blank; R 1 represents the bending radius of the design bend.

[0023] Furthermore, the forming of the preformed tube blank in the inner high-pressure forming mold B in step three includes the following steps:

[0024] Step 31: Control the forming hydraulic cylinder and the filling hydraulic cylinder to advance the forming punch and the filling punch, so that the outer walls of the inclined ports on both sides deform and store material first;

[0025] Step 32: Control the forming punch and the filling punch to continue advancing, and control the filling hydraulic cylinder to fill and pressurize for internal high-pressure forming;

[0026] Step 33: Control the filling hydraulic cylinder to depressurize;

[0027] Steps 3 and 4: Control the retraction of the filling punch;

[0028] Step 35: Lift the upper die and remove the part;

[0029] Step 36: Cut the bulging part to obtain the designed pipe.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] 1. This invention directly obtains straight-walled sections from large-diameter bends through pre-forming, which facilitates sealing during bulging. Simultaneously, during bulging, the straight-walled section allows the tube material to be fed axially by the punch, increasing the material in the deformation zone and promoting material flow during deformation. This facilitates axial feeding and avoids excessive thinning of the outer wall during bending (this application employs the following technical means: first, increasing the material in the deformation zone through axial feeding of the tube material during straight-walled section bulging; second, using ports with beveled sides, the punch preferentially advances the outer tube material storage in the early stage of bulging, increasing the material in the outer deformation zone and avoiding excessive thinning of the outer wall) or even cracking. Furthermore, the internal high-pressure bulging technology avoids the problem of wrinkling and instability during bending (because the deformation zone during bulging in this application is under bidirectional tensile stress, it effectively suppresses wrinkling compared to other processes).

[0032] 2. This invention pre-forms the two ends of the cut strip into an α-angle bevel. During the internal high-pressure forming, the outer tube material is preferentially deformed and stored first (the outer tube material preferentially contacts and deforms with the flat stepped punch, increasing the outer deformation zone of the tube material, and the outer tube material has good fluidity during subsequent expansion). This helps to control the uniformity of the overall wall thickness of the bend and further reduces the risk of thinning or even cracking of the outer wall.

[0033] 3. This invention selects a large-diameter push-bending tube blank to directly form a straight-wall section, which overcomes the problem that it is difficult to form a straight-wall section from a large-diameter bent tube blank. At the same time, it reduces the difficulty of forming a large-diameter, small-bending-radius thin-walled bend as a whole. It has the advantages of high forming quality, easy tube blank preparation, low process difficulty, and simple operation.

[0034] 4. The high forming quality of this invention is specifically reflected in the following aspects: First, the integral forming of the pipe avoids weld seams (traditional manufacturing methods involve half-pipe deep drawing and welding, which results in poor overall performance and low forming quality due to the presence of weld seams). Second, the formed pipe has a uniform wall thickness distribution, avoiding excessive thinning and wrinkling thickening, which is beneficial for meeting the service environment with strict requirements for wall thickness. Furthermore, precise forming can reduce material usage and contribute to overall lightweighting. Third, the formed pipe has high surface quality. During hydraulic bulging, the pipe material is subjected to bidirectional tensile stress, avoiding wrinkling during forming and also avoiding the problem of scratches easily formed on the surface when bending pipes. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the present invention. Figure 2 This is a schematic diagram illustrating the changes in the state of the bend during preparation.

[0036] Figure 3 This is a schematic diagram of a preforming mold. Figure 4 This is a schematic diagram of an internal high-pressure forming mold. Figure 5 This is a schematic diagram of the overall structure of a thin-walled bend forming mold with a large diameter and small bending radius. Detailed Implementation

[0037] Specific implementation method one: Combining Figures 3 to 5 This embodiment describes a large-diameter, small-radius, thin-walled bend integral forming mold, comprising an upper pad 1, a lower pad 2, a lower mold base 3, a forming hydraulic cylinder 4, a filling hydraulic cylinder 5, a forming cylinder base 6, a filling cylinder base 7, and a mold body. The upper pad 1 and lower pad 2 are arranged parallel vertically. The lower pad 2 is mounted on the lower mold base 3. The mold body is mounted between the upper pad 1 and lower pad 2. The forming cylinder base 6 and the filling cylinder base 7 are mounted between the upper pad 1 and lower pad 2. Hydraulic cylinder 4 is mounted on forming cylinder seat 6, and filling hydraulic cylinder 5 is mounted on filling cylinder seat 7. Forming hydraulic cylinder 4 is connected to forming punch of mold body, and filling hydraulic cylinder 5 is connected to filling punch of mold body. Mold body includes upper die 8, lower die 9, forming punch 10 and filling punch 11. Upper die 8 and lower die 9 are snapped together, and a U-shaped groove is formed between upper die 8 and lower die 9. Forming punch 10 and filling punch 11 are respectively sealed and inserted into the straight wall section of U-shaped groove.

[0038] Specific Implementation Method Two: Combining Figure 3 In this embodiment, the two ends of the U-shaped groove are straight-walled sections, and the middle cavity of the U-shaped groove is in the shape of a tube blank. In this case, the mold body is a pre-forming mold A.

[0039] This setup is used for preforming the tube blank. Other components and connections are the same as in Specific Embodiment One.

[0040] In this embodiment, the upper backing plate 1 is connected to the single-action press via a T-slot and bolts, and the upper die 8 is bolted to the underside of the upper backing plate 1. The cavity design of the upper die 8 and the lower die 9 is mainly as follows: Figure 3 The schematic U-shaped groove has a tube blank-shaped cavity in the middle section and straight wall sections at both ends. The filling punch 11 and forming punch 10 are bolted to the forming hydraulic cylinder 4 and filling hydraulic cylinder 5, respectively. The filling pipe is bolted to the filling punch 11. The forming hydraulic cylinder 4 is bolted to the forming cylinder seat 6, and the filling hydraulic cylinder 5 is bolted to the filling cylinder seat 7. The forming cylinder seat 6 and filling cylinder seat 7 are connected and fixed to the lower pad 2 via T-slots and bolts. The lower die 9 is mounted on the lower pad 2, which is connected to the lower die base 3 via T-slots and bolts. The lower die base 3 is connected to the slide block of the single-action press via T-slots and bolts.

[0041] Specific implementation method three: Combining Figure 3In this embodiment, the forming punch 10 and the liquid-filling punch 11 have gradually narrowing head diameters as they extend into the straight-wall section. This design allows the forming punch 10 and the liquid-filling punch 11 to quickly penetrate into the tube blank. Furthermore, under the action of stamping, the outer walls of the forming punch 10 and the liquid-filling punch 11 form the cornered straight-wall section of the tube blank located between the straight-wall section and the forming punch 10 and the liquid-filling punch 11, facilitating forming in the internal high-pressure forming die. Other components and connections are the same as in specific embodiments one or two.

[0042] Specific implementation method four: Combination Figure 3 To illustrate this embodiment, the upper concave mold 8 in the preforming mold A of this embodiment is provided with a boss 12.

[0043] With this configuration, the surface of boss 12 is the shape of the inner wall of the preformed tube blank. The size of boss 12 is related to the size of the straight wall section and the final formed tube. The arc size of the surface of boss 12 should be larger than the arc size of the inner wall of the final formed tube. The sum of the arc size of the surface of boss 12 and the length of the straight wall sections on both sides is approximately the arc size of the inner wall of the tube blank. Therefore, after meeting the size requirements of the final formed tube, the size of boss 12 should be reduced as much as possible. The purpose is to increase the size of the straight wall section, which is to facilitate axial feeding and expansion sealing.

[0044] Other components and connections are the same as in specific implementation methods one, two, or three.

[0045] Specific Implementation Method Five: Combining Figure 4 In this embodiment, the two ends of the U-shaped groove are straight-walled sections, and the middle cavity of the U-shaped groove is a designed bend. In this case, the mold body is an internal high-pressure forming mold B.

[0046] This configuration facilitates the forming of tube blanks that have been cut into bevels at a certain angle. Other components and connections are the same as in specific embodiments one, two, three, or four.

[0047] Specific Implementation Method Six: Combination Figure 4 In this embodiment, both the forming punch 10 and the filling punch 11 are stepped punches. This design facilitates the production of bent pipes with wall dimensions that meet design specifications. Other components and connections are the same as in any of the specific embodiments one through five.

[0048] Specific implementation method seven: Combination Figure 4 In this embodiment, both the forming punch 10 and the filling punch 11 have a sealing ring embedded in their heads.

[0049] With this configuration, the sealing ring ensures a seal during the stamping process. Other components and connections are the same as in any of the specific embodiments one through six.

[0050] Specific implementation methods one, five, and seven together form an internal high-pressure forming mold B, wherein the upper pad 1 is connected to the single-action press via a T-slot and bolts, and the upper concave mold 8 is installed under the upper pad 1 via bolts.

[0051] The cavity design of the upper die 8 and the lower die 9 is as follows: Figure 4 The diagram illustrates a U-shaped channel. The cavity shape in the middle of the U-shaped channel is the design shape of the pipe material, and the two ends of the U-shaped channel are the design straight wall sections.

[0052] Specific implementation method eight: Combination Figure 1 and Figure 2 This embodiment, specifically the forming method of this embodiment, is characterized by comprising the following steps:

[0053] Step 1: Place the bent tube blank into the preforming mold A and form a preformed tube blank with a straight wall section.

[0054] Step 2: Cut the open ends on both sides of the precast tube blank from Step 1 into bevels with a certain angle;

[0055] Step 3: Place the cut precast tube blank into the inner high-pressure forming mold B to form a thin-walled bend with a large diameter and small bending radius.

[0056] In this embodiment, the purpose of the first step of preforming is to obtain a preformed tube blank with a straight wall section, which is beneficial for sealing and axial feeding during subsequent internal high-pressure expansion, suppresses the thinning of the outer wall during internal high-pressure forming, and overcomes the problem that it is difficult to seal large-diameter pipes without a straight wall section during internal high-pressure forming.

[0057] Step 2, cutting the open ends on both sides along the bevel, is intended to facilitate the preferential deformation and material storage of the outer wall in the early stage of internal high-pressure forming, further suppress the thinning of the outer wall during internal high-pressure forming, and control the overall wall thickness uniformity.

[0058] Step 3 involves internal high-pressure forming of the precast tube blank. The purpose is to increase the diameter of the deformed part and reduce the bending radius to form a target bend with a large diameter and a small bending radius.

[0059] This method effectively avoids the sealing problem during high-pressure bulging of large-diameter push-bending tube blanks, effectively solves the problem of axial material feeding during internal high-pressure forming, effectively controls excessive thinning of the outer wall during forming, effectively reduces the working pressure of internal high-pressure bulging, and effectively suppresses wrinkling and instability during tube bending. It has advantages such as good forming quality, high workpiece lightweighting, high controllability, and good process stability, which can significantly improve the overall performance of parts, reduce manufacturing costs, and shorten production cycles.

[0060] In step one of this embodiment: the preforming hydraulic cylinder is controlled to advance the preforming punch and the preforming liquid-filling punch, while the preforming liquid-filling tube is filled and pressurized. The hydraulic pressure inside the tube provides support for the tube blank, preventing instability and wrinkling of the tube blank during preforming. The purpose is to directly form a straight-walled section on a large-diameter tube blank. The straight-walled section facilitates sealing during liquid-filling expansion and axial feeding, avoids excessive thinning and cracking of the outer wall during expansion, and improves the uniformity of the part's wall thickness.

[0061] Specific Implementation Method Nine: Combining Figure 1 and Figure 2 In this embodiment, the method for determining the cutting angle of the open ends on both sides of the precast tube blank in step two is as follows:

[0062] The preformed tube blank is cut at both ends to obtain the designed angle α. The formula for selecting α is:

[0063]

[0064] In the formula: D 0 represents the diameter of the tube blank; D 1 represents the design bend diameter; R 0 represents the bending radius of the tube blank; R 1 represents the bending radius of the design bend.

[0065] This configuration suppresses the thinning of the outer wall during internal high-pressure forming, controlling the overall wall thickness uniformity. Because the wall thickness distribution of the tube blank is uneven, and the thickness distribution of the push-bending tube blank varies depending on the material, friction coefficient, die clearance, and relative bending radius, different cutting angles α can be calculated and selected for different tube blanks. By controlling the preferential deformation and material storage of the outer wall, the wall thickness distribution required by the final formed part design can be met. Controlling the angle allows for control of the thickness distribution of different tubes, avoiding the problem of low ductility and easy breakage of difficult-to-deform materials, reducing the forming difficulty of difficult-to-form parts, and meeting the high uniformity requirements of tubes in extreme service environments. Other components and connections are the same as any one of the specific embodiments one to eight.

[0066] Specific Implementation Method Ten: Combining Figure 1 and Figure 2 This embodiment describes the following steps in which the preformed tube blank is formed in the inner high-pressure forming mold B during step three:

[0067] Step 31: Control the forming hydraulic cylinder 4 and the filling hydraulic cylinder 5 to advance the forming punch 10 and the filling punch 11, so that the outer walls of the inclined ports on both sides deform and store material first.

[0068] Step 32: Control the forming punch 10 and the filling punch 11 to continue advancing, and control the filling hydraulic cylinder 5 to fill and pressurize for internal high-pressure forming;

[0069] Step 33: Control the hydraulic cylinder 5 to depressurize;

[0070] Steps 3 and 4: Control the retraction of the filling punch 11;

[0071] Step 35: Lift the upper die 8 and remove the part;

[0072] Step 36: Cut the bulging part to obtain the designed pipe.

[0073] With this setup, the propulsion speed is related to the hydraulic pressure, which in turn is related to the deformation of the tube. In general, during bulging, the punch propulsion speed needs to be controlled for axial material feeding to promote tube flow, improve overall wall thickness uniformity, and prevent excessive thinning and breakage. Other components and connections are the same as in any of the specific implementation methods one through nine.

[0074] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make other changes within the spirit of the invention and apply it to fields not mentioned in the invention. Of course, all such changes made in accordance with the spirit of the invention should be included within the scope of protection claimed by the invention.

Claims

1. A large diameter, small bend radius, thin wall, bend through integral forming die characterized by: It includes upper pad (1), lower pad (2), lower die seat (3), forming hydraulic cylinder (4), liquid filling hydraulic cylinder (5), forming cylinder seat (6), liquid filling cylinder seat (7) and die body, The upper pad (1) and the lower pad (2) are arranged in parallel, the lower pad (2) is installed on the lower die seat (3), the die body is installed between the upper pad (1) and the lower pad (2), the forming cylinder seat (6) and the liquid filling cylinder seat (7) are installed between the upper pad (1) and the lower pad (2), the forming hydraulic cylinder (4) is installed on the forming cylinder seat (6), the liquid filling hydraulic cylinder (5) is installed on the liquid filling cylinder seat (7), the forming hydraulic cylinder (4) is connected with the forming punch (10) of the die body, and the liquid filling hydraulic cylinder (5) is connected with the liquid filling punch (11) of the die body; The die body includes upper concave die (8), lower concave die (9), forming punch (10) and liquid filling punch (11), the upper concave die (8) and the lower concave die (9) are buckled in upper and lower, the U-shaped groove is formed between the upper concave die (8) and the lower concave die (9), the forming punch (10) and the liquid filling punch (11) are respectively sealingly inserted into the straight wall section of the U-shaped groove, and the die body is a preforming die (A) and an internal high pressure forming die (B); The two ends of the U-shaped groove are in the shape of straight wall section, the middle cavity of the U-shaped groove is in the shape of pipe blank, at this time, the die body is the preforming die (A); the upper concave die (8) in the preforming die (A) is provided with a boss (12); The two ends of the U-shaped groove are in the shape of straight wall section, the middle cavity of the U-shaped groove is in the shape of design elbow, at this time, the die body is the internal high pressure forming die (B).

2. A large diameter, small bend radius, thin wall elbow integrally formed die according to claim 1, wherein: The head diameter of the forming punch (10) and the liquid filling punch (11) gradually shrinks when sealingly inserted into the straight wall section.

3. A large diameter, small bend radius, thin wall elbow integrally formed die according to claim 2, wherein: The forming punch (10) and the liquid filling punch (11) are both stepped punches.

4. A large diameter, small bend radius, thin wall elbow integrally formed die according to claim 3 wherein: The head of the forming punch (10) and the liquid filling punch (11) is embedded with a sealing ring.

5. A method of forming a large diameter, small bend radius, thin wall elbow using the integral forming die of any one of claims 1 to 4, characterized by: It includes the following steps: Step one: put the bent pipe blank into the preforming die (A) and form the preformed pipe blank with straight wall section; Step two: cut the opening ends of the preformed pipe blank in step one into inclined surfaces with a certain angle; Step three: put the cut preformed pipe blank into the internal high pressure forming die (B) to form, and a large-diameter, small-bending-radius thin-wall elbow can be obtained.

6. A method of integrally forming a large diameter, small bend radius, thin wall elbow as defined in claim 5 wherein: In step two, the cutting angle of the opening ends of the preformed pipe blank is determined as follows: The two ends of the preformed pipe blank are cut to process a design angle α, and the selection formula of α is as follows: wherein: D 0 is the tube blank diameter; D 1 is the design bend pass diameter; R 0 is the tube blank bend radius; R 1 is the design bend pass bend radius.

7. A method of integrally forming a large diameter, small bend radius, thin wall elbow as defined in claim 6 wherein: The forming of the preformed pipe blank in the internal high pressure forming die (B) in step three includes the following steps: Step three one: control the forming hydraulic cylinder (4) and the liquid filling hydraulic cylinder (5) to make the forming punch (10) and the liquid filling punch (11) advance, so that the outer side wall of the inclined surface port on both sides is deformed preferentially to store materials; Step three two: control the forming punch (10) and the liquid filling punch (11) to continue to advance, control the liquid filling hydraulic cylinder (5) to perform liquid filling and pressurization to perform internal high pressure forming; Step three three: control the liquid filling hydraulic cylinder (5) to perform pressure relief; Step three four: control the liquid filling punch (11) to retreat; Step three five: lift the upper concave die (8) and take out the part; Step three six: cut the expanded part to obtain a design pipe.

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