Method for producing an electrically fused sealing tube and electrically fused sealing tube

Abstract

This invention discloses a method for preparing an electrofusion sealing tube and the electrofusion sealing tube itself, belonging to the field of pipeline connection technology. The preparation method includes: preparing an inner tube blank with axial and circumferential reinforcing ribs on the outer wall, the reinforcing ribs intersecting to form nodes; winding a resistance wire on the outer wall of the inner tube blank, spanning the nodes, and controlling the tension and fitting gap using a tension sensor and a laser displacement sensor; injecting a second fluoropolymer with a lower melting point to form an inner positioning layer to fix the resistance wire; machining an annular sealing groove on the outer wall of the inner positioning layer and embedding an elastic sealing ring with a cross-sectional diameter greater than the groove depth; winding continuous reinforcing fibers around the sealing ring to form a circumferential winding layer, applying radial preload to the sealing ring; injecting a third fluoropolymer with an even lower melting point to form an outer layer and fill the gaps between the fiber bundles; and cooling and inspection. This invention achieves long-term reliable sealing under high-pressure alternating and vibration conditions through the synergistic effect of precise positioning of the resistance wire, preload of the elastic sealing ring, and the fiber reinforcement layer, significantly improving axial pull-out resistance.

Description

Technical Field

[0001] This invention relates to the field of pipeline connection technology, and in particular to a method for preparing an electrofusion sealing pipe and the electrofusion sealing pipe itself. Background Technology

[0002] Pipeline connection technology is one of the most critical aspects of fluid transportation systems, as the sealing performance of the joints directly determines the safety and service life of the entire pipeline system. Among various pipeline connection methods, electrofusion connection has been widely used in gas transmission, water supply and drainage, and chemical pipelines due to its advantages such as convenient operation, high connection strength, and no need for open flame operations. The basic principle of electrofusion connection is to pre-embed a resistance wire in the inner wall of the pipe fitting. During construction, the pipe to be connected is inserted into the inner cavity of the fitting, and electricity is applied to the resistance wire to heat it up. Utilizing the Joule effect, the inner wall material of the fitting and the outer wall material of the pipe around the resistance wire melt simultaneously and fuse together. After cooling, an integrated connection joint is formed.

[0003] In existing technologies, Chinese Patent CN101025245A discloses a fully enclosed electrofusion pipe fitting and its manufacturing method, employing a secondary injection molding process to completely enclose the resistance wire inside the fitting body, solving the problem of oxidation caused by exposed resistance wires. Chinese Patent CN203395471U discloses a sealing-enhanced electrofusion pipe fitting, featuring a boss on the inner wall of the fitting body, with metal resistance wires for electrofusion heating located inside the side walls of the boss, enhancing the sealing effect through the boss structure. Chinese Patent CN213361481U discloses a composite electrofusion pipe fitting, with a resistance wire in the inner layer and annular cooling grooves on both sides of the resistance wire, utilizing the sealing ring formed by the molten material passing through the cooling grooves to improve the sealing effect. Furthermore, existing technologies disclose technical solutions for integrated connection of cooling water pipes using PFA welding joints via hot-melt methods.

[0004] However, the aforementioned existing technologies still reveal several shortcomings in practical engineering applications.

[0005] First, most existing electrofusion fittings are integrally injection molded from a single homogeneous polymer material (such as polyethylene PE). When subjected to internal pressure, the radial expansion deformation of such fittings is primarily constrained by the elastic modulus of the fitting's body material. When the pressure of the transported medium fluctuates periodically, the fitting's connection interface is subjected to alternating tensile stress. After long-term service, microcracks are prone to form at the connection interface and gradually propagate, eventually leading to seal failure. This problem is particularly prominent in pipeline systems transporting high-pressure fluids or where water hammer effects exist.

[0006] Secondly, the sealing of conventional electrofusion fittings relies on the interfacial bonding strength of the molten material after cooling, resulting in a relatively simple sealing structure. When the pipeline system is subjected to axial tensile loads, the tensile force is entirely borne by the fusion interface between the pipe and the fitting. If human factors such as scratches on the pipe surface, incomplete removal of the oxide layer, or improper socket insertion occur during construction, the actual bonding area and bonding strength of the fusion interface will be lower than the design value, significantly reducing the axial tensile strength.

[0007] Third, in industrial pipeline environments with high-frequency mechanical vibration, such as cooling water circulation pipelines and chemical pumping pipelines, pipeline joints are subjected to vibration loads for extended periods. The fretting wear and stress relaxation effects caused by vibration accelerate the degradation of the sealing interface, and existing electrofusion fittings do not have specific structural designs for vibration conditions.

[0008] Fourth, in the conventional manufacturing process of electrofusion fittings, the outer layer material is directly injection molded into the mold after the resistance wire is wired. Due to the scouring force generated by the flow of molten material on the resistance wire during injection molding, the resistance wire is prone to displacement, causing the actual embedded position of the resistance wire in the finished fitting to deviate from the design position. This displacement of the resistance wire position will cause uneven distribution of the molten zone during electrofusion, with insufficient or excessive melting in some areas, thus affecting the uniformity and reliability of the joint sealing quality.

[0009] Therefore, there is a need for an electrofusion sealing tube and its preparation method that can maintain long-term reliable sealing performance under high-pressure alternating loads, axial tensile loads, and vibration conditions. Summary of the Invention

[0010] To achieve the above objectives, the present invention provides a method for preparing an electrofusion sealing tube, comprising the following steps:

[0011] Step 1: The first fluoropolymer is injection molded into an inner tube blank. The inner wall diameter of the inner tube blank is larger than the outer wall diameter of the tube to be connected. The outer wall surface of the inner tube blank has axial reinforcing ribs and circumferential reinforcing ribs. The axial reinforcing ribs and circumferential reinforcing ribs intersect to form a node.

[0012] Step 2: Spiral wind resistance wire is made on the outer wall of the inner tube blank, so that the resistance wire crosses the node, and the two ends of the resistance wire are connected to the electrode terminals.

[0013] Step 3: Place the inner tube blank with the resistance wire wound into the mold, and inject the second fluoropolymer to form the inner positioning layer. The inner positioning layer fixes the resistance wire and fills the gap between the turns of the resistance wire. The melting point of the second fluoropolymer is lower than that of the first fluoropolymer.

[0014] Step 4: Machining an annular sealing groove on the outer wall surface of the inner positioning layer;

[0015] Step 5: Embed the annular sealing ring into the annular sealing groove. The cross-sectional diameter of the annular sealing ring is greater than the depth of the annular sealing groove, and the outer surface of the annular sealing ring protrudes from the outer wall of the inner positioning layer.

[0016] Step 6: Continuous reinforcing fibers are wound around the outer wall of the inner positioning layer containing the annular sealing ring to form a circumferential winding layer. The circumferential winding layer covers the outer surface of the annular sealing ring and applies radial preload.

[0017] Step 7: Place the component obtained in Step 6 into the mold, inject the third fluoropolymer to form the outer layer, the third fluoropolymer covers the circumferential winding layer and fills the gaps between the fiber bundles in the circumferential winding layer, the melting point of the third fluoropolymer is lower than that of the second fluoropolymer;

[0018] Step 8: Cool to obtain an electrofusion sealed tube.

[0019] Preferably, in step 1, the number of axial reinforcing ribs on the outer wall surface of the inner tube blank is 4 to 8, the height of each axial reinforcing rib is 0.3 to 0.6 times the wall thickness of the inner tube blank, and the width of each axial reinforcing rib is 0.5 to 1.0 times the wall thickness of the inner tube blank; the number of circumferential reinforcing ribs is 3 to 5, the height of each circumferential reinforcing rib is the same as the height of the axial reinforcing rib, and the width of each circumferential reinforcing rib is the same as the width of the axial reinforcing rib; the inner wall diameter of the inner tube blank is 1.0 mm to 2.5 mm larger than the outer wall diameter of the tube to be connected.

[0020] Preferably, in step 2, when winding the resistance wire, a tension sensor is used to measure the tension value applied to the resistance wire, so that the tension value is maintained in the range of 3N to 8N; a laser displacement sensor is used to measure the bonding gap between the resistance wire and the outer wall surface of the inner tube blank along the axial and circumferential directions, respectively, and the bonding gap at any measurement point does not exceed 0.3mm.

[0021] Preferably, in step 3, the design thickness of the inner positioning layer is such that the distance between the outer wall surface of the inner tube blank and the inner wall surface of the mold cavity is 1.5 mm to 4.0 mm; the melting point of the second fluoropolymer is 10°C to 30°C lower than the melting point of the first fluoropolymer; the injection pressure when injecting the second fluoropolymer is 5 MPa to 15 MPa, and the holding time is 10 s to 60 s.

[0022] Preferably, in step 4, two to four annular sealing grooves are machined on the outer wall surface of the inner positioning layer. Each annular sealing groove has a depth of 1.0 mm to 3.0 mm and a width of 2.0 mm to 6.0 mm. The axial spacing between adjacent annular sealing grooves is 8.0 mm to 20.0 mm. The surface roughness of the bottom of the annular sealing groove is measured using a surface roughness meter to ensure that the surface roughness Ra value of the bottom of the annular sealing groove does not exceed 3.2 μm. The annular sealing groove is formed by turning.

[0023] Preferably, in step 5, the material of the annular sealing ring is fluororubber or perfluoroether rubber; the cross-sectional diameter of the annular sealing ring is 0.2 mm to 0.8 mm greater than the depth of the annular sealing groove; after the annular sealing ring is embedded, the height of the outer surface of the annular sealing ring protruding from the outer wall of the inner positioning layer is 0.2 mm to 0.8 mm. The height of the outer surface of the annular sealing ring protruding from the outer wall of the inner positioning layer is measured using a height gauge, so that the radial compression ratio ε of the annular sealing ring is controlled within the range of 10% to 30%, and the radial compression ratio ε is calculated by the following formula:

[0024]

[0025] in, The diameter of the annular sealing ring is [diameter]. This refers to the depth of the annular sealing groove.

[0026] Preferably, in step 6, the material of the continuous reinforcing fiber is any one of glass fiber, carbon fiber or basalt fiber; the tension applied when winding the continuous reinforcing fiber is 20N to 60N, and the tension value applied to the continuous reinforcing fiber during the winding process is measured in real time using a tension sensor; the number of winding layers in the circumferential winding layer is 2 to 6 layers.

[0027] Preferably, in step 7, the melting point of the third fluoropolymer is 10°C to 30°C lower than that of the second fluoropolymer; the injection temperature when injecting the third fluoropolymer is 210°C to 250°C, the injection pressure is 10MPa to 20MPa, and the holding time is 20s to 120s; the inner surface of the outer layer formed by cooling and curing is embedded in the gaps between the fiber bundles in the circumferential winding layer, and forms a bond with the inner positioning layer.

[0028] Preferably, in step 8, the cooling time is 30 min to 120 min. After cooling, the resistance value between the two electrode terminals is detected, and the resistance value deviation does not exceed 5% of the design value. The airtightness of the electrofusion sealing tube is tested by filling the inner cavity of the electrofusion sealing tube with compressed air at a pressure of 0.6 MPa to 1.2 MPa and holding the pressure for 10 min to 30 min. The pressure change in the inner cavity is monitored by a pressure sensor, and the pressure drop does not exceed 2% of the filling pressure.

[0029] Accordingly, the present invention provides an electrofusion sealing tube, which is prepared by the method for preparing an electrofusion sealing tube according to any one of the embodiments of the present invention.

[0030] The beneficial effects of this invention are:

[0031] 1. This invention integrates axial and circumferential reinforcing ribs on the outer wall of the inner tube blank to form intersecting nodes. The resistance wire is spirally wound across the nodes, while the inner positioning layer fixes the resistance wire and fills the gap between turns. Combined with precise control of the resistance wire tension (3N to 8N) and the bonding gap (not exceeding 0.3mm) during the winding process, it effectively avoids the influence of subsequent turning, winding, and injection molding processes on the position of the resistance wire. This ensures uniform heating of the resistance wire during energized welding, fundamentally solving the problem of uneven melting zone caused by deviation in the wiring position of the resistance wire, and significantly improving the reliability and consistency of the internal welding quality of the joint.

[0032] 2. This invention involves machining an annular sealing groove on the outer wall of the inner positioning layer, embedding an elastic annular sealing ring with a cross-sectional diameter larger than the groove depth, and directly winding continuous reinforcing fibers onto its outer surface to form a circumferential winding layer. This applies a continuous and uniform radial preload stress to the annular sealing ring, ensuring it remains in a controlled compression state. When the pipeline is subjected to alternating internal pressure loads, the circumferential winding layer strongly constrains the radial expansion of the pipe fitting. When subjected to axial tensile loads, the annular sealing ring provides additional frictional resistance under preload stress, significantly improving its pull-out resistance. When subjected to long-term vibration, the elastic preload of the annular sealing ring dynamically compensates for the micro-displacement and stress relaxation of the sealing interface, preventing leakage. This synergistic mechanism of mechanical constraint and elastic compensation enables the joint to maintain long-term reliable sealing performance under high pressure, alternating, and vibration conditions.

[0033] 3. This invention employs a gradient design where the melting points of the first, second, and third fluoropolymers decrease sequentially by 10°C to 30°C, combined with a multi-layer composite structure, achieving precise control over the manufacturing process and sequential melting during the welding process. During electrofusion welding, the inner positioning layer, with its lower melting point, reaches complete melting before the inner tube blank, fully filling the socket gap between the tube and fitting. Meanwhile, the inner tube blank, with its higher melting point, maintains structural strength, preventing overall softening and deformation of the fitting. During injection molding of the outer layer, the third fluoropolymer's melting point is lower than that of the inner positioning layer, avoiding thermal shock to the inner positioning layer and the positioned resistance wire, ensuring the stability of the resistance wire's position. Under pressure, the outer layer material penetrates into the fiber bundle gaps and tightly bonds with the inner positioning layer, completely encapsulating the circumferential winding layer to form an integrated protective layer. This gives the electrofusion sealed tube excellent mechanical strength, corrosion resistance, and long-term service reliability. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely example drawings of this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a flowchart of the steps of the method of the present invention. Detailed Implementation

[0036] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. It should also be noted that, to make the embodiments more comprehensive, the following embodiments are the best and preferred embodiments, and those skilled in the art can use other alternative methods to implement some well-known technologies; moreover, the accompanying drawings are only for more specific description of the embodiments and are not intended to specifically limit the present invention.

[0037] Please see Figure 1 The present invention provides a method for preparing an electrofusion sealing tube and an electrofusion sealing tube. The core concept is to construct an electrofusion sealing tube with a multi-layer composite structure. Through the synergistic cooperation of the inner tube blank, the inner positioning layer, the annular sealing ring, the circumferential winding layer and the outer layer, the technical effects of multiple sealing and reinforcement are achieved.

[0038] In step 1, the axial and circumferential reinforcing ribs on the outer wall of the inner tube blank intersect to form nodes. These nodes provide a positioning reference for the winding of the resistance wire in the subsequent step 2. The resistance wire is wound across the nodes, allowing the heat generated during energized welding to be preferentially and uniformly conducted radially to the fitting through the reinforcing rib structure, avoiding localized overheating or insufficient melting. The inner diameter of the inner tube blank is 1.0 mm to 2.5 mm larger than the outer diameter of the tube to be connected. This gap is used to accommodate the insertion of the tube and is filled by the molten material during the welding process.

[0039] In step 2, real-time monitoring and control of the tension and bonding gap during the resistance wire winding process is a crucial technical means to ensure the positioning accuracy of the resistance wire wiring. The tension sensor controls the tension value within the range of 3N to 8N, ensuring a tight fit between the resistance wire and the outer wall of the inner tube blank without causing the resistance wire to embed into the inner tube blank material due to excessive tension, thus altering the cross-sectional shape of the resistance wire. The laser displacement sensor scans and measures the bonding gap along the axial and circumferential directions, comprehensively detecting the bonding state of the resistance wire at various points and controlling the bonding gap to within 0.3mm, thereby ensuring the uniformity of the heat field distribution during energized welding.

[0040] In step 3, the inner positioning layer serves a dual function. First, it fixes the precisely wound resistance wire from step 2 to the outer wall of the inner tube blank, preventing the wire from shifting position during subsequent processing. Second, it fills the gaps between the turns of the resistance wire, partially embedding it within the layer and increasing the heat transfer area between the wire and the surrounding material. The melting point of the second fluoropolymer is 10°C to 30°C lower than that of the first fluoropolymer. This difference ensures that during the electrofusion process, when the temperature reaches the melting point of the first fluoropolymer, the inner positioning layer is completely melted and has good fluidity, effectively filling the gaps between the tube and the inner tube blank. Simultaneously, the inner tube blank, composed of the first fluoropolymer, maintains sufficient structural strength to prevent overall deformation of the tube.

[0041] In step 4, an annular sealing groove is machined on the outer wall of the inner positioning layer. The surface roughness Ra value of the bottom of the annular sealing groove does not exceed 3.2μm. The smooth bottom surface of the groove can fit tightly with the annular sealing ring, reducing the microscopic leakage channels at the interface.

[0042] In step 5, after the annular sealing ring is embedded in the annular sealing groove, because the cross-sectional diameter of the annular sealing ring is larger than the depth of the annular sealing groove, the outer surface of the annular sealing ring protrudes from the outer wall of the inner positioning layer by 0.2 mm to 0.8 mm. This protrusion provides deformation space for the circumferential winding layer to apply radial preload to the annular sealing ring in the subsequent step 6.

[0043] In step 6, the circumferential winding layer formed by the continuous reinforcing fiber winding directly covers the outer surface of the annular sealing ring, applying a continuous and uniform radial preload stress to the annular sealing ring. Under the action of radial preload, the annular sealing ring is always in a radially compressed state, with the compression rate ε controlled within the range of 10% to 30%. Even if the pipeline system is subjected to alternating internal pressure loads or vibration loads for a long time, the annular sealing ring can still maintain a stable sealing contact pressure on the outer wall of the pipe, effectively compensating for the stress relaxation effect at the sealing interface. The circumferential winding layer of continuous reinforcing fibers also provides circumferential restraint for the entire pipe fitting, significantly improving the internal pressure resistance of the fitting.

[0044] In step 7, the outer layer is injection molded onto the outside of the circumferentially wound layer. Driven by injection pressure, the third fluoropolymer penetrates into the gaps between the fiber bundles in the circumferentially wound layer. After cooling and curing, the outer layer and the inner positioning layer achieve structural integration through the third fluoropolymer filling the gaps between the fiber bundles. The outer layer protects the circumferentially wound layer, preventing the continuous reinforcing fibers from being eroded by the external environment or mechanically damaged. The melting point of the third fluoropolymer is 10°C to 30°C lower than that of the second fluoropolymer, ensuring that subsequent injection molding of the outer layer will not cause softening or deformation of the inner positioning layer.

[0045] In step 8, the finished electrofusion-sealed tube undergoes resistance and airtightness testing. Resistance testing verifies the integrity of the resistance wire and the reliability of the wiring; a resistance deviation of no more than 5% of the design value indicates that the resistance wire did not break or short-circuit during manufacturing. Airtightness testing involves filling the tube with compressed air and monitoring pressure changes; a pressure drop of no more than 2% of the filling pressure indicates that the tube body and sealing interface have good airtightness, meeting the airtightness requirements for engineering applications.

[0046] Example

[0047] This embodiment uses the pipeline connection of an industrial cooling water circulation system with high pressure, temperature fluctuations, and vibration conditions as the application scenario. In this application scenario, the pipeline system needs to withstand a design internal pressure of 1.6MPa, the medium temperature fluctuates periodically between 20℃ and 60℃, and there is continuous low-frequency mechanical vibration caused by the operation of the circulating water pump.

[0048] Step 1: Prepare the inner tube blank;

[0049] A melt-processable tetrafluoroethylene-perfluoroalkoxy vinyl ether copolymer with a melting point of 305°C was selected as the first fluoropolymer. The first fluoropolymer was melted and plasticized at 220°C and injected into the cavity of a first mold. The core diameter of the first mold cavity was 52.5 mm, and grooves for forming axial and circumferential reinforcing ribs were provided on the inner wall surface of the cavity. After cooling to room temperature within the mold cavity, the first fluoropolymer was demolded to obtain the inner tube preform.

[0050] The inner tube blank has an inner diameter of 52.5 mm, a wall thickness of 6.0 mm, and a length of 120 mm. The pipe to be connected is a stainless steel pipe with an outer diameter of 50.0 mm. Therefore, the inner diameter of the inner tube blank is 2.5 mm larger than the outer diameter of the pipe to be connected.

[0051] The outer wall surface of the inner tube blank has six axial reinforcing ribs evenly distributed circumferentially. Each axial reinforcing rib extends from one end of the inner tube blank to the other along the axial direction. The height of each axial reinforcing rib is 2.4 mm (i.e., 0.4 times the wall thickness of the inner tube blank), and the width of each axial reinforcing rib is 4.2 mm (i.e., 0.7 times the wall thickness of the inner tube blank).

[0052] Four circumferential reinforcing ribs are evenly distributed along the axial direction on the outer wall surface of the inner tube blank, with an axial spacing of 30 mm between any two adjacent circumferential reinforcing ribs. Each circumferential reinforcing rib extends along the entire circumference of the inner tube blank to form a closed loop. The height of each circumferential reinforcing rib is the same as the height of the axial reinforcing rib, which is 2.4 mm; the width of each circumferential reinforcing rib is the same as the width of the axial reinforcing rib, which is 4.2 mm.

[0053] Six axial stiffeners and four circumferential stiffeners form a total of 24 nodes at their intersections.

[0054] Step 2: Winding the resistance wire;

[0055] A copper-nickel alloy resistance wire with a diameter of 0.8 mm was selected. The resistance wire was wound in a circumferential spiral along the outer wall of the inner tube blank, with a pitch of 5.0 mm. The winding path was designed so that the resistance wire successively crossed each node formed by the intersection of the axial and circumferential reinforcing ribs, that is, the resistance wire passed over the outer top surface of the axial reinforcing rib at each node.

[0056] During the winding process, a tension sensor was used to measure the tension applied to the resistance wire in real time, and the resistance of the pay-off wheel was adjusted by a tension controller to maintain the tension at 5N. Simultaneously, a laser displacement sensor was used to take 10 equidistant measurement sections along the axial direction of the inner tube blank. Within each measurement section, 8 measurement points were taken circumferentially to measure the contact gap between the resistance wire and the outer wall of the inner tube blank. The measurements showed that the maximum contact gap among the 80 measurement points was 0.18mm, meeting the requirement of not exceeding 0.3mm.

[0057] The two ends of the resistance wire are fixedly connected to two copper electrode terminals by spot welding. The two electrode terminals are located on the tube end faces near the axial ends of the inner tube blank.

[0058] Step 3: Form the inner positioning layer;

[0059] A tetrafluoroethylene-hexafluoropropylene copolymer was selected as the second fluoropolymer, which has a melting point of 275°C. The melting point of the second fluoropolymer is 30°C lower than that of the first fluoropolymer.

[0060] The inner layer tube blank, with resistance wire wound around it, is placed inside the second mold cavity, ensuring a uniform distance of 3.0 mm between the outer wall surface of the inner layer tube blank and the inner wall surface of the second mold cavity. This 3.0 mm distance is the designed thickness of the inner positioning layer. The outer wall diameter of the inner layer tube blank, including the height of the axial and circumferential reinforcing ribs, is measured to be 58.8 mm. Therefore, the inner wall diameter of the second mold cavity is 64.8 mm.

[0061] The second fluoropolymer is melted and plasticized at 220°C and then injected into the cavity of the second mold at an injection pressure of 10 MPa for 40 seconds. The molten second fluoropolymer fills all the gaps between the outer wall of the inner tube blank and the inner wall of the second mold cavity, including the gaps between axial reinforcing ribs, the gaps between circumferential reinforcing ribs, and the gaps between the turns of the resistance wire. After cooling and solidifying in the mold, the second fluoropolymer forms an inner positioning layer.

[0062] The inner positioning layer fixes the resistance wire to the outer wall of the inner tube blank. The spaces between each turn of the resistance wire are filled with a second fluoropolymer. Approximately half the thickness of the outer surface of the resistance wire is embedded in the inner positioning layer. The outer wall diameter of the inner positioning layer is 64.8 mm, and the thickness of the inner positioning layer is 3.0 mm (measured from the outermost edge of the outer wall of the inner tube blank).

[0063] Step 4: Machining the sealing structure;

[0064] The tube blank with the inner positioning layer obtained in step 3 is clamped on a lathe, and the outer wall surface of the inner positioning layer is machined. Two annular sealing grooves are machined circumferentially on the outer wall surface of the inner positioning layer. The two annular sealing grooves are located 20 mm away from the two axial end faces of the inner positioning layer, and the axial distance between two adjacent annular sealing grooves is 16.0 mm.

[0065] Each annular sealing groove has a depth of 2.0 mm and a width of 4.0 mm. The cross-sectional shape of the annular sealing groove is rectangular. After turning, the surface roughness Ra value is measured at 6 points along the bottom circumference of each annular sealing groove using a surface roughness tester. The maximum Ra value among the 12 measurement points is 2.1 μm, which meets the requirement of not exceeding 3.2 μm.

[0066] Step 5: Install the sealing ring;

[0067] Two annular sealing rings made of perfluoroelastomer rubber were selected. Perfluoroelastomer rubber has excellent chemical corrosion resistance and high temperature resistance, making it suitable for the cooling water conditions in this embodiment. Each annular sealing ring has a cross-sectional diameter of 2.6 mm and an inner diameter of 62.0 mm.

[0068] One annular sealing ring is embedded in each annular sealing groove. Since the cross-sectional diameter of the annular sealing ring is 2.6 mm and the depth of the annular sealing groove is 2.0 mm, the cross-sectional diameter of the annular sealing ring is 0.6 mm greater than the depth of the annular sealing groove. After the annular sealing ring is embedded, the inner surface of the annular sealing ring fits tightly against the bottom of the annular sealing groove, and the outer surface of the annular sealing ring protrudes from the outer wall of the inner positioning layer.

[0069] Using a height gauge, the protrusion height was measured at eight points along the circumference of the annular sealing ring. The protrusion height values ​​at the 16 measurement points ranged from 0.5 mm to 0.6 mm, with an average protrusion height of 0.55 mm, which meets the design requirement of a protrusion of 0.2 mm to 0.8 mm.

[0070] The radial compressibility ε of the annular seal is calculated using the following formula:

[0071]

[0072] Substitution , The calculation yields:

[0073]

[0074] The compression rate is in the range of 10% to 30%.

[0075] Step 6: Winding reinforcing fibers;

[0076] T700 grade continuous carbon fiber is selected as the reinforcing fiber material. The diameter of the carbon fiber filament is 7μm, and each bundle of carbon fiber contains 12,000 filaments.

[0077] The inner positioning layer blank with an embedded annular sealing ring obtained in step 5 is clamped on the mandrel of the winding machine. The winding machine is started, and the continuous carbon fiber is wound circumferentially along the inner positioning layer. During the winding process, the tension sensor measures the tension value applied to the continuous carbon fiber in real time, and the braking torque of the unwinding wheel is adjusted by the tension control system to keep the tension value at 40N.

[0078] The circumferential winding consists of four layers. The first circumferential winding layer is directly wound onto the outer wall of the inner positioning layer and the outer surfaces of the two annular sealing rings, with a center-to-center spacing of 1.5 mm between each bundle of carbon fibers. The second, third, and fourth layers are wound sequentially over the previous layer. The total thickness of the four circumferential winding layers is approximately 1.2 mm.

[0079] After circumferential winding, the carbon fiber winding layer applies a uniform radial pre-compression stress to the two annular seals. This radial pre-compression stress further compresses each annular seal beyond its radial compression ratio of 23.1%, enhancing the contact pressure between the annular seal and the bottom of the annular sealing groove, as well as between the annular seal and the outer wall of the pipe to be inserted in subsequent steps.

[0080] Step 7: Injection molding of the outer layer;

[0081] A tetrafluoroethylene-perfluoromethyl vinyl ether copolymer was selected as the third fluoropolymer, which has a melting point of 260°C. The melting point of the third fluoropolymer is 15°C lower than that of the second fluoropolymer.

[0082] The inner positioning layer assembly with continuous carbon fiber wound in step 6 is placed in the third mold cavity. The inner wall diameter of the third mold cavity is set to 80.0 mm, and the distance between the inner wall of the third mold cavity and the outermost edge of the circumferential winding layer is 7.6 mm, which is the design thickness of the outer layer.

[0083] The third fluoropolymer is melted and plasticized at 230°C and then injected into the third mold cavity at an injection pressure of 15 MPa, with a holding time of 80 seconds. Driven by the injection pressure, the molten third fluoropolymer penetrates into the gaps between adjacent carbon fiber bundles in the circumferential winding layer, while simultaneously filling the entire space between the circumferential winding layer and the inner wall of the third mold cavity.

[0084] The third fluoropolymer is cooled and cured in the mold to form the outer layer. The inner surface of the outer layer is embedded in the gaps between the carbon fiber bundles in the circumferential winding layer. The inner surface of the outer layer and the outer wall of the inner positioning layer are in direct contact and bonded to each other through the third fluoropolymer at the gaps between the carbon fiber bundles. The circumferential winding layer is firmly encapsulated between the outer layer and the inner positioning layer.

[0085] Step 8: Post-processing and inspection;

[0086] After injection molding in step 7, allow the pipe fitting, along with the mold, to cool naturally at room temperature for 60 minutes. Once the pipe fitting temperature has cooled to no more than 30°C, demold and remove the electrofused sealed pipe product.

[0087] Resistance testing was performed on the finished electrofusion-sealed tube. A precision resistance meter was used to measure the resistance between the two electrode terminals. Three measurements were taken, and the average value was calculated. The measured resistance was 12.3Ω. The design resistance was 12.0Ω, with a deviation of 2.5%, meeting the requirement of not exceeding 5% of the design value.

[0088] An airtightness test was performed on the finished electrofusion sealing tube. One end of the electrofusion sealing tube was sealed with a special plug, and the other end was connected to a compressed air line via a quick-connect coupling. Dry compressed air at a pressure of 0.8 MPa was injected into the inner cavity of the electrofusion sealing tube, and the pressure was maintained for 20 minutes after the inflation valve was closed. A digital pressure sensor with an accuracy of 0.001 MPa was used to monitor the pressure change in the inner cavity. The measured pressure drop was 0.008 MPa, which is 1.0% of the inflation pressure, meeting the requirement of not exceeding 2%. Both the resistance test and the airtightness test were qualified, and the finished electrofusion sealing tube met the factory standards.

[0089] Effect comparison:

[0090] To verify the technical effectiveness of the electrofusion sealing tube in this embodiment, a comparative test was conducted using a control example. The control example consisted of a conventional electrofusion fitting injection molded from a single homogeneous polyethylene material, with a resistance wire pre-embedded in the inner wall, and without an inner positioning layer, annular sealing ring, or circumferential winding layer structure. The specifications of the control example fitting were identical to those of the electrofusion sealing tube in this embodiment, with an inner diameter of 52.5 mm, used to connect to a stainless steel tube with an outer diameter of 50.0 mm.

[0091] The electrofusion sealing tube of this embodiment and the conventional electrofusion fitting of the comparative example were respectively subjected to electrofusion welding operations. The connected pipes were all 304 stainless steel pipes with an outer diameter of 50.0 mm and a wall thickness of 3.0 mm. After welding, internal pressure alternating test, axial pull-out force test and vibration fatigue test were carried out respectively. The test results are shown in Table 1.

[0092] Internal pressure alternating test conditions: The medium is demineralized water at room temperature, the pressure cycling range is 0.2MPa to 1.6MPa, the frequency is 0.2Hz, and the number of cycles is 50,000.

[0093] Axial pull-out force test conditions: Axial tension is performed on a universal testing machine at a tensile speed of 10 mm / min, and the maximum tensile force value when the joint separates is recorded.

[0094] Vibration fatigue test conditions: vibration frequency of 50 Hz, amplitude of 0.5 mm, and vibration time of 1000 h.

[0095] Table 1. Performance comparison between the electrofusion sealing tube of this embodiment and the conventional electrofusion fittings of the comparative example.

[0096] Test Project This embodiment features an electrofusion sealing tube. Comparative example of conventional electrofusion fittings Sealing status after internal pressure alternation test No leakage after 50,000 cycles Leakage appeared at the joint after 12,000 cycles. Axial pull-out force (kN) 18.6 9.2 Sealing condition after vibration fatigue test No leakage after 1000 hours Leakage appeared at the joint after 280 hours. Axial pull-out force (kN) after vibration fatigue test 17.8 5.4

[0097] As can be seen from the data in Table 1, the sealing durability of the electrofusion sealing tube in this embodiment under alternating internal pressure conditions is significantly better than that of the conventional electrofusion fitting in the comparative example, maintaining a good seal even after 50,000 internal pressure cycles. Regarding axial pull-out force, the pull-out force of the electrofusion sealing tube in this embodiment reaches 18.6 kN, more than twice that of the comparative example. This is attributed to the constraint effect of the circumferential winding layer on the radial expansion of the fitting and the additional frictional resistance provided by the annular sealing ring. In the vibration fatigue test, the electrofusion sealing tube in this embodiment remained sealed after 1000 hours of vibration, while the comparative example leaked after 280 hours, indicating that the continuous elastic compensation capability of the annular sealing ring under radial preload effectively resisted the degradation of the sealing interface caused by vibration. After the vibration test, the axial pull-out force of the electrofusion sealing tube in this embodiment decreased by only 4.3%, while that of the comparative example decreased by 41.3%, further confirming the structural stability of the multilayer composite structure of this invention under long-term vibration conditions.

[0098] It is understood that the first fluoropolymer, second fluoropolymer, and third fluoropolymer described in this invention refer to a class of high-performance thermoplastic polymers containing fluorine atoms in their molecular chains. These materials generally possess excellent chemical stability, resistance to high and low temperatures, low coefficient of friction, and good electrical insulation, making them particularly suitable for pipeline systems operating under harsh conditions such as cooling water and chemical fluid transportation.

[0099] The first, second, and third fluoropolymers may be of the same or different types, but they must satisfy the melting point gradient relationship described in the claims, i.e., the melting point of the second fluoropolymer is lower than that of the first fluoropolymer, and the melting point of the third fluoropolymer is lower than that of the second fluoropolymer. Preferably, the melting point difference between adjacent fluoropolymer layers is 10°C to 30°C. This melting point gradient design is crucial to ensuring the structural stability of the electrofusion sealing tube during manufacturing and to achieving sequential melting and full filling during subsequent electrofusion connection.

[0100] Exemplary fluoropolymers include, but are not limited to, polytetrafluoroethylene (PTFE), melt-processable tetrafluoroethylene-perfluoroalkoxy vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoromethyl vinyl ether copolymer (MFA), and polyvinylidene fluoride (PVDF). In embodiments of the present invention, the first fluoropolymer is preferably PFA, the second fluoropolymer is preferably FEP, and the third fluoropolymer is preferably MFA. They belong to the same fluoropolymer series, have similar chemical properties and good interlayer compatibility, and can precisely meet the above-mentioned melting point gradient requirements, thereby synergistically achieving the multiple sealing and reinforcement effects described in the present invention.

[0101] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0102] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing an electrofusion sealing tube, characterized in that, Includes the following steps: Step 1: The first fluoropolymer is injection molded into an inner tube blank. The inner wall diameter of the inner tube blank is larger than the outer wall diameter of the tube to be connected. The outer wall surface of the inner tube blank has axial reinforcing ribs and circumferential reinforcing ribs. The axial reinforcing ribs and circumferential reinforcing ribs intersect to form a node. Step 2: Spiral wind resistance wire is made on the outer wall of the inner tube blank, so that the resistance wire crosses the node, and the two ends of the resistance wire are connected to the electrode terminals. Step 3: Place the inner tube blank with the resistance wire wound into the mold, and inject the second fluoropolymer to form the inner positioning layer. The inner positioning layer fixes the resistance wire and fills the gap between the turns of the resistance wire. The melting point of the second fluoropolymer is lower than that of the first fluoropolymer. Step 4: Machining an annular sealing groove on the outer wall surface of the inner positioning layer; Step 5: Embed the annular sealing ring into the annular sealing groove. The cross-sectional diameter of the annular sealing ring is greater than the depth of the annular sealing groove, and the outer surface of the annular sealing ring protrudes from the outer wall of the inner positioning layer. Step 6: Continuous reinforcing fibers are wound around the outer wall of the inner positioning layer containing the annular sealing ring to form a circumferential winding layer. The circumferential winding layer covers the outer surface of the annular sealing ring and applies radial preload. Step 7: Place the component obtained in Step 6 into the mold, inject the third fluoropolymer to form the outer layer, the third fluoropolymer covers the circumferential winding layer and fills the gaps between the fiber bundles in the circumferential winding layer, the melting point of the third fluoropolymer is lower than that of the second fluoropolymer; Step 8: Cool to obtain an electrofusion sealed tube.

2. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 1, the number of axial reinforcing ribs on the outer wall of the inner tube blank is 4 to 8, the height of each axial reinforcing rib is 0.3 to 0.6 times the wall thickness of the inner tube blank, and the width of each axial reinforcing rib is 0.5 to 1.0 times the wall thickness of the inner tube blank; the number of circumferential reinforcing ribs is 3 to 5, the height of each circumferential reinforcing rib is the same as the height of the axial reinforcing rib, and the width of each circumferential reinforcing rib is the same as the width of the axial reinforcing rib; the inner wall diameter of the inner tube blank is 1.0 mm to 2.5 mm larger than the outer wall diameter of the tube to be connected.

3. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 2, when winding the resistance wire, a tension sensor is used to measure the tension applied to the resistance wire, and the tension is maintained in the range of 3N to 8N. A laser displacement sensor is used to measure the contact gap between the resistance wire and the outer wall of the inner tube blank along the axial and circumferential directions, respectively, and the contact gap at any measurement point does not exceed 0.3mm.

4. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 3, the design thickness of the inner positioning layer makes the distance between the outer wall of the inner tube blank and the inner wall of the mold cavity 1.5mm to 4.0mm; the melting point of the second fluoropolymer is 10℃ to 30℃ lower than that of the first fluoropolymer; the injection pressure when injecting the second fluoropolymer is 5MPa to 15MPa, and the holding time is 10s to 60s.

5. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 4, two to four annular sealing grooves are machined on the outer wall of the inner positioning layer. The depth of each annular sealing groove is 1.0 mm to 3.0 mm, the width is 2.0 mm to 6.0 mm, and the axial spacing between adjacent annular sealing grooves is 8.0 mm to 20.0 mm. The surface roughness of the bottom of the annular sealing groove is measured using a surface roughness tester to ensure that the surface roughness Ra value of the bottom of the annular sealing groove does not exceed 3.2 μm. The annular sealing groove is formed by turning.

6. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 5, the material of the annular sealing ring is fluororubber or perfluoroether rubber; the cross-sectional diameter of the annular sealing ring is 0.2 mm to 0.8 mm greater than the depth of the annular sealing groove; after the annular sealing ring is embedded, the height of the outer surface of the annular sealing ring protruding from the outer wall of the inner positioning layer is 0.2 mm to 0.8 mm. The height of the outer surface of the annular sealing ring protruding from the outer wall of the inner positioning layer is measured using a height gauge, so that the radial compression ratio ε of the annular sealing ring is controlled within the range of 10% to 30%. The radial compression ratio ε is calculated according to the following formula: in, The diameter of the annular sealing ring is [diameter]. This refers to the depth of the annular sealing groove.

7. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 6, the material of the continuous reinforcing fiber is any one of glass fiber, carbon fiber or basalt fiber; The tension applied during the winding of continuous reinforcing fibers is 20N to 60N, and a tension sensor is used to measure the tension applied to the continuous reinforcing fibers in real time during the winding process. The number of circumferential winding layers ranges from 2 to 6.

8. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 7, the melting point of the third fluoropolymer is 10°C to 30°C lower than that of the second fluoropolymer; the injection temperature when injecting the third fluoropolymer is 210°C to 250°C, the injection pressure is 10MPa to 20MPa, and the holding time is 20s to 120s; the inner surface of the outer layer formed by cooling and curing is embedded in the gaps between the fiber bundles in the circumferential winding layer, and forms a bond with the inner positioning layer.

9. The method for preparing the electrofusion sealing tube according to claim 1, characterized in that, In step 8, the cooling time is 30 min to 120 min. After cooling, the resistance value between the two electrode terminals is measured, and the resistance value deviation does not exceed 5% of the design value. The airtightness of the electrofusion sealing tube is tested by filling the inner cavity of the electrofusion sealing tube with compressed air at a pressure of 0.6 MPa to 1.2 MPa and holding the pressure for 10 min to 30 min. The pressure change in the inner cavity is monitored by a pressure sensor, and the pressure drop does not exceed 2% of the filling pressure.

10. An electrofusion sealing tube, characterized in that, The electrofusion sealing tube is prepared according to any one of claims 1 to 9.

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