High-density polyethylene shell prefabricated direct-buried heat-insulating reducing pipe hot-melt welding process

Abstract

The application relates to a high-density polyethylene shell prefabricated direct-buried heat-preservation variable-diameter pipe hot melting welding process and relates to the prefabricated direct-buried heat-preservation pipe construction technical field. The high-density polyethylene shell prefabricated direct-buried heat-preservation variable-diameter pipe hot melting welding process comprises the following steps: S1, construction early preparation; S2, end face pretreatment; S3, accurate alignment correction; S4, gradient hot melting heating; S5, pressure melting forming; S6, constant-temperature pressure maintaining cooling; and S7, finished product acceptance and protection. The application has the effects that through gradient heating and pressurizing parameters, the problems of uneven melting, over-melting and virtual welding caused by the wall thickness difference of the large and small ends of the variable-diameter pipe are solved, high-temperature resistant heat-insulating protective pads are pre-installed to prevent the damage of welding high temperature to the polyurethane heat-preservation layer from the source, the whole process is standardized controlled, the hidden defects such as slag inclusion, cracks and eccentricity are reduced, and the operation and maintenance cost is greatly reduced.

Description

Technical Field

[0001] This application relates to the field of prefabricated direct-buried insulated pipe construction technology, and in particular to a hot-melt welding process for a high-density polyethylene shell prefabricated direct-buried insulated reducing pipe. Background Technology

[0002] Pre-insulated direct-buried reducing pipes are core supporting pipe fittings for heating pipe network systems. They are mainly used for key nodes such as pipe diameter transitions, branch connections, and switching between high and low zone pipe networks. They consist of a three-layer structure: an inner steel pipe, a middle rigid polyurethane foam insulation layer, and an outer HDPE high-density polyethylene protective shell. The HDPE outer shell possesses excellent corrosion resistance, impact resistance, and waterproof and seepage-proof performance. It is a crucial structure protecting the inner insulation layer and the steel pipe, and isolating them from soil and external environmental erosion. Its welding quality directly determines the airtightness and durability of the entire direct-buried insulated pipe network. With the large-scale advancement of municipal heating pipe network construction in my country, the number of old pipe network renovations and new regional heating pipe network projects is increasing year by year. The requirements for the sealing, stability, and long-term effectiveness of pipe connections during pipe network construction are continuously increasing.

[0003] Currently, the industry has established certain standards for the hot-melt welding process of equal-diameter prefabricated direct-buried insulated pipes. The appendix of the national standard atlas "Direct Burial Installation of Insulated Plastic Pipes for Heating" (24R426) provides the hot-melt butt welding parameters and process requirements for PE-RTⅡ and PB pipes, and the appendix of the local standard "Technical Specification for Application of Prefabricated Direct-Buried Composite Plastic Insulated Pipes" (DB64 / T1056-2014) also involves the hot-melt butt welding method for pipes. However, the above standards mainly target the conventional hot-melt butt welding of equal-diameter pipes and do not cover the special process requirements of reducing pipes due to the gradual change in cross-section and wall thickness. The diameter and wall thickness gradient of reducing pipes are significant. Unlike the regular butt joint structure of equal-diameter straight pipes, the conventional hot-melt welding process cannot accurately match the heating, pressurization, and welding requirements of the gradual cross-section of the reducing section. This easily leads to problems such as uneven welding temperature at the large and small ends, inconsistent weld edge width, and excessive deviation in coaxiality of the butt joint.

[0004] During the welding of reducing pipes, factors such as high-temperature radiation from the heating plate, misalignment of welding pressure, and uneven cooling and shrinkage can easily cause charring, collapse, and delamination of the polyurethane insulation layer inside the pipe, disrupting the continuity of the insulation layer. Outdoor construction conditions such as dust, moisture, low temperatures, and strong winds can cause impurities to adhere to the welding surfaces of the HDPE outer sheath, resulting in incomplete welds and slag inclusions. Traditional welding processes lack specific standards for reducing pipe pressure holding parameters; construction workers often apply welding parameters for equal-diameter pipes, leading to incomplete fusion at the larger end of the reducing pipe, over-melting and collapse at the smaller end, and insufficient tensile and settlement resistance at the weld joint. Summary of the Invention

[0005] To address the aforementioned technical issues, this application provides a hot-melt welding process for prefabricated direct-buried insulated variable-diameter pipes with high-density polyethylene shells.

[0006] This application provides a hot-melt welding process for a prefabricated direct-buried insulated reducing pipe with a high-density polyethylene shell, which adopts the following technical solution: A hot-melt welding process for a prefabricated direct-buried insulated reducing pipe with a high-density polyethylene shell includes the following steps: S1: Pre-construction preparation, complete the verification of personnel, equipment, materials and environment, control the error of equipment parameters within ±2℃ and ±0.01MPa, check the appearance quality of the reducer, and build a windproof, dustproof and rainproof work shed. S2: End face pretreatment, use pipe cutting equipment to vertically cut the welding end face of the reducer, ensure that the flatness and perpendicularity error of the end face does not exceed 0.5mm, remove burrs and debris, wipe the welding interface and surrounding area with anhydrous ethanol, and install high temperature resistant heat insulation protective pads on the inside and outside of the reducer. S3: Precise alignment and correction. Use the adjustable clamps of the welding machine to fix the large and small ends of the reducing pipe. Fine-tune the clamp spacing according to the diameter gradient, calibrate the pipe coaxiality, ensure that the misalignment does not exceed 10% of the pipe wall thickness, and lock the clamp position. S4: Gradient hot melt heating, the heating temperature is set according to the characteristics of HDPE material, and the heat absorption time is adjusted differently for the difference in wall thickness between the large end and the small end of the variable diameter. The heat absorption time of the large end is extended by 20%-30%, and the heat absorption time of the small end is controlled. During the heating process, the end face is in close contact with the heating plate. S5: Pressure welding and forming, gradient pressure is applied after the heating plate is removed, the welding pressure is increased at the large end and the pressure is stabilized and controlled at the small end, and the pressure application process is uniform and stable. S6: Constant temperature and pressure cooling, maintain constant welding pressure, implement natural cooling, and the cooling time at room temperature shall not be less than 30 minutes. Extend the cooling time in low temperature environment. S7: Finished product acceptance and protection, inspect welding edges, conduct appearance inspection and airtightness spot checks, and wrap the joints with protective materials.

[0007] By adopting the above technical solutions, in the early preparation stage of construction, the personnel, equipment, materials, and environment are checked, and the error of equipment parameters is controlled within a very small range, ensuring the precise operation of construction equipment and laying the foundation for high-quality welding. Checking the appearance quality of the reducer can detect pipe problems in advance, avoiding the use of defective pipes that affect the welding effect. Constructing a windproof, dustproof, and rainproof work shed can effectively reduce the interference of harsh outdoor environments such as dust, water vapor, and strong winds on welding, improving welding quality. In the end face pretreatment step, the welding end face of the reducer is cut vertically with pipe cutting equipment, which can make the welding end face fit better and facilitate molecular fusion. Removing burrs and debris can prevent these impurities from affecting the fusion effect. Wiping the welding interface and surrounding area with anhydrous ethanol can further clean the welding surface and prevent impurities from causing problems such as incomplete welding. Adding high-temperature resistant heat insulation protective pads to the inside and outside of the reducer can effectively isolate the radiation damage of the internal polyurethane insulation layer caused by the high temperature of welding, ensuring the continuity and integrity of the insulation layer and reducing the heat loss of the pipeline network. In the precise alignment and correction stage, adjustable clamps on the welding machine are used to fix the large and small ends of the reducer. The clamp spacing is finely adjusted according to the diameter gradient to adapt to the special structure of the reducer. Calibration of pipe coaxiality and strict control of misalignment prevent eccentric stress at the welded joint, enhance the pipe's resistance to deformation, and improve its stability under soil pressure and settlement stress. In the gradient hot-melt heating step, an appropriate heating temperature is set based on the characteristics of HDPE material. The heat absorption time is adjusted differently for the wall thickness difference between the large and small ends of the reducer. Extending the heat absorption time at the large end ensures full melting, while strictly controlling the heat absorption time at the small end prevents over-melting and collapse. During heating, the end face is kept in close contact with the heating plate to form a uniform and regular weld ring that meets the thickness requirements, ensuring the weld depth meets the standard and promoting full molecular fusion. During pressure welding, after the heating plate is removed, gradient pressure is applied. The pressure is increased at the larger end to ensure deep fusion, while the pressure is stabilized at the smaller end to avoid excessive compression and deformation. The pressure application process is uniform, stable, and without impact or deviation, allowing the molecules at the welding interface to fully diffuse, entangle, and fuse together, enhancing the mechanical properties of the weld joint. In the constant temperature and pressure cooling stage, a constant welding pressure is maintained for natural cooling, with the cooling time adjusted according to the ambient temperature. This avoids problems such as internal micro-cracks and weld ring cracking caused by forced cooling, ensuring complete curing and improving the stability and durability of the joint. This ensures the entire welding process meets engineering quality requirements, thereby guaranteeing the long-term stable operation of the heating network, reducing subsequent maintenance costs, and improving the safety and energy efficiency of the network.

[0008] Optionally, in step S1, the equipment uses an adjustable hot melt welding machine adapted to reduce pipe diameter, and the heating plate temperature and hydraulic pressure accuracy are calibrated in advance; the material verification includes checking that the HDPE outer shell is free from cracks, dents, and damage, that the internal insulation layer is free from hollows and peeling, and that the pipe diameter and wall thickness meet the design standards; the heating equipment is turned on when the ambient temperature is below 5℃, and shading and cooling are implemented when the ambient temperature is above 35℃.

[0009] By adopting the above technical solution and using an adjustable hot-melt welding machine adapted to reducer pipe fittings, and by pre-calibrating the heating plate temperature and hydraulic pressure accuracy, the special welding requirements caused by the differences in pipe diameter and wall thickness at the large and small ends of the reducer pipe can be accurately met. This avoids problems such as uneven fusion temperature and inconsistent weld edge width at the large and small ends due to inaccurate equipment parameters, thereby improving the quality and mechanical properties of the weld joint. Environmental temperature control is also crucial. When the ambient temperature is below 5℃, the heating equipment is turned on; when it is above 35℃, shading and cooling are implemented. Low temperatures cause the weld end face to cool too quickly, resulting in an incomplete weld; high temperatures may alter material properties and affect the welding effect. Through environmental regulation, the welding process can be carried out under suitable temperature conditions, ensuring the reliability and stability of the weld joint and improving the construction quality and long-term operational safety of the entire prefabricated direct-buried insulated reducer pipe.

[0010] Optionally, in step S2, the anhydrous ethanol wiping area is the welding interface and a surrounding 5cm area, and it is allowed to air dry naturally after wiping; the high-temperature resistant heat insulation protective pad is used to isolate the high temperature of welding from radiation damage to the internal polyurethane insulation layer.

[0011] By adopting the above technical solution, the wiping range of anhydrous ethanol is set to the welding interface and the surrounding 5cm area. This can remove impurities such as oil, dust, and moisture in this area. Natural air drying after wiping ensures that the welding interface is dry and clean, creating a good molecular fusion environment for subsequent welding. At the same time, adding high-temperature resistant heat insulation pads to the inside and outside of the reducing pipe can isolate the high temperature of welding from radiation damage to the internal polyurethane insulation layer, avoiding charring, collapse, delamination, etc. of the insulation layer, and fully preserving the continuity and integrity of the insulation layer. This reduces the heat loss of the pipeline network and improves the energy-saving performance and heating efficiency of the pipeline network.

[0012] Optionally, in step S2, the pipe cutting equipment is an electric pipe cutting machine. During cutting, the rough surface of the pipe opening is first quickly removed with high lateral pressure, and then the pipe opening is slowly smoothed with low lateral pressure to ensure that the end face is flat. After cutting, a special scraper is used to remove burrs from the inner and outer walls of the pipe opening, and compressed air is used to blow away debris from the end face.

[0013] By adopting the above technical solution, when cutting the welded end face of a reducer, the electric pipe cutter first applies greater lateral pressure, which can quickly remove the rough, uneven surface of the pipe end, improving cutting efficiency. Then, a smaller lateral pressure is used for a slow, leveling operation, making the cut surface smoother and more delicate, ensuring the flatness of the end face. Next, a special scraper is used to remove burrs from the inner and outer walls of the pipe end, making the surface more uniform and avoiding any adverse effects of burrs on subsequent welding operations, such as potential problems like localized overheating or incomplete welds.

[0014] Optionally, in step S3, the coaxiality calibration includes two stages: coarse adjustment and fine adjustment. In the coarse adjustment stage, the two end faces are basically aligned by visual inspection. In the fine adjustment stage, the deviation of the pipe wall circumference at multiple points is measured by dial indicator, and the fixture is finely adjusted point by point until the misalignment meets the requirements.

[0015] By adopting the above technical solution, the rough adjustment stage allows construction personnel to quickly align the two ends of the reducer by visual inspection, placing them in a relatively suitable position. The fine adjustment stage uses a dial indicator to measure the deviation at multiple points on the pipe wall circumference. This multi-point measurement method comprehensively and accurately reflects the coaxiality of the two ends, ensuring high precision in the reducer connection. After the pipeline is buried, this effectively reduces the continuous shear force exerted on the joint by soil pressure and settlement stress, minimizing the risk of weld damage and improving the stability and reliability of the weld joint.

[0016] Optionally, in step S4, the heating temperature of the PE100 outer shell is controlled at 225±10℃, and the heating temperature of the PE80 outer shell is controlled at 210±10℃. The specific adjustment of the heat absorption time is as follows: due to the larger wall thickness at the large end of the variable diameter, the heat absorption time is extended by 20%-30%, while the heat absorption time at the small end is strictly controlled to prevent over-melting and collapse. During the heating process, a uniform and regular weld ring is formed, with a uniform width and a thickness controlled within 1 / 3 to 1 / 2 of the pipe wall thickness.

[0017] By adopting the above technical solution, the heating temperature is precisely set according to different HDPE materials. The heating temperature of the PE100 outer shell is controlled at 225±10℃, and the heating temperature of the PE80 outer shell is controlled at 210±10℃. This allows the outer shells of different materials to reach the optimal heat fusion state, providing a good foundation for subsequent welding. Regarding the heat absorption time, for cases where the wall thickness of the large end of the reducer is large, extending the heat absorption time by 20%-30% ensures that the large end can fully absorb heat, allowing the thick-walled part to melt completely, enhancing the depth and strength of the weld, and ensuring a tight weld at the large end. Meanwhile, strictly controlling the heat absorption time at the small end effectively prevents over-melting and collapse, avoiding material waste and joint quality problems. During the heating process, continuous heating causes the two ends of the reducing pipe to fit tightly against the heating plate, thereby forming a uniform and regular fusion ring with a consistent width and a thickness ranging from 1 / 3 to 1 / 2 of the pipe wall thickness. This allows the molecules at the fusion interface at both ends to diffuse and entwine more fully, thereby improving the sealing, tensile strength, and anti-settling properties of the welded joint and achieving precise fitting of the reducing pipe fitting.

[0018] Optionally, in step S4, the heating plate is preheated to the set temperature and kept at a constant temperature for at least 10 minutes to ensure a uniform temperature field on the heating plate; during the heating process, the contact pressure between the two ends of the reducing tube and the heating plate remains consistent, and the contact time is controlled by the heat absorption time.

[0019] By adopting the above technical solution, preheating the heating plate to the set temperature and maintaining it at a constant temperature for at least 10 minutes can make the temperature field of the heating plate uniform. In the subsequent heating process, the uniform temperature field can ensure that the two ends of the reducing pipe are heated more evenly, avoiding the problem of uneven welding temperature at the large and small ends due to temperature differences, and improving the welding quality. Keeping the contact pressure between the two ends of the reducing pipe and the heating plate consistent can ensure that the two ends of the pipe are in full contact with the heating plate.

[0020] Optionally, in step S5, the gradient pressurization specifically involves: increasing the welding pressure at the large end to ensure deep fusion based on the specified dragging pressure, and stabilizing and controlling the pressure at the small end to avoid excessive extrusion and deformation. The pressurization process is free from impact and deviation, ensuring that the molecules at the welding interface of both ends are fully diffused and entangled.

[0021] By adopting the above technical solution, for the large end of the variable diameter, due to its thicker wall, increasing the welding pressure can allow the deep material to fully melt and fuse, ensuring the welding depth and stability of the large end interface and avoiding the situation of loose welding; while the small end adopts a pressure stabilization and control method, because the wall thickness of the small end is thinner, which can prevent excessive extrusion and deformation, and ensure the shape and structure integrity of the small end.

[0022] Optionally, in step S5, the heating plate is pulled out for no more than 3 seconds, and the total conversion time from pulling out the heating plate to completing the butt joint pressure application is no more than 5 seconds, so as to prevent excessive heat dissipation from the end face from affecting the welding quality.

[0023] By adopting the above technical solution, strict operation control can avoid excessive heat dissipation at the end face during the critical conversion stage. Since the large and small ends of the reducer are in a suitable fusion state after gradient thermal melting heating, if the conversion time is too long, the end face temperature will drop rapidly, resulting in insufficient fluidity and activity required for fusion. However, by completing the removal of the heating plate and the butt pressure application operation in a short time, the two ends of the reducer can always maintain a high temperature level, allowing the molecules at the fusion interface at both ends to still have sufficient energy for diffusion and entanglement, thereby ensuring that the molecules can fully fuse during the fusion process and form a tight and firm bond.

[0024] Optionally, in step S6, the natural cooling process must not involve watering or forced cooling by blowing air. The cooling time is extended by 50% in low-temperature environments. The process proceeds to the next step after the interface is fully cured and the temperature drops to room temperature.

[0025] By adopting the above technical solutions, natural cooling is strictly prohibited from watering or forced cooling by blowing air, which can avoid problems such as internal micro-cracks and weld ring cracking caused by rapid cooling. Extending the cooling time by 50% in low-temperature environments allows the interface to have enough time to cool slowly under low-temperature conditions, ensuring that the interface is fully cured and further improving the strength and sealing of the interface.

[0026] In summary, this application includes at least one of the following beneficial technical effects: To address the gradual change in diameter and wall thickness of reducing pipes, gradient heating, pressure stabilization, and heat absorption parameters are adopted to completely solve problems such as uneven welding, over-melting, and incomplete welding caused by uniform parameters in traditional processes. This achieves precise adaptation of reducing pipe welding, full molecular fusion at the welding interface, significantly improved mechanical properties, and strong process adaptability. By adding a pre-insulation protection process, the damage to the internal polyurethane insulation layer caused by high welding temperatures is prevented from the source, the continuity and integrity of the insulation layer are fully preserved, the heat loss of the pipeline network is significantly reduced, and the insulation protection effect is excellent. The entire process is standardized and controlled, with clear operating standards for end face treatment, alignment calibration, parameter control and cooling and curing. This effectively reduces hidden defects such as slag inclusions, cracks, eccentricity and leakage, resulting in strong consistency in interface quality and high stability in construction quality. The welded joints offer excellent sealing, durability, and fatigue resistance, with a service life synchronized with the main pipe body. This significantly reduces subsequent joint leakage, damage, insulation failure, and other malfunctions, resulting in substantial reductions in maintenance costs. Attached Figure Description

[0027] Figure 1 This is a flowchart of the hot-melt welding process of the high-density polyethylene shell prefabricated direct-buried insulated reducing pipe in the embodiments of this application. Detailed Implementation

[0028] The following is in conjunction with the appendix Figure 1 This application will be described in further detail.

[0029] This application discloses a hot-melt welding process for a prefabricated direct-buried insulated reducer with a high-density polyethylene shell. (Refer to...) Figure 1 The hot-melt welding process for prefabricated direct-buried insulated reducer pipes with high-density polyethylene shells includes the following steps: Step S1: Pre-construction preparation. First, conduct a comprehensive check of personnel, equipment, materials, and the environment. Regarding equipment, an adjustable hot-melt welding machine compatible with reducing pipe fittings is used. High-precision calibration instruments are used beforehand to calibrate the heating plate temperature and hydraulic system pressure, ensuring that equipment parameter errors are strictly controlled within ±2℃ and ±0.01MPa. The accuracy of the heating temperature is crucial in determining whether HDPE material can reach the ideal melting state; even slight temperature deviations can lead to insufficient melting or overheating and degradation of PE100 or PE80 materials. Precise control of hydraulic pressure directly affects the degree of molecular diffusion and entanglement at the weld interface, preventing incomplete welds due to insufficient pressure or excessive material extrusion due to excessive pressure, significantly improving the consistency and reliability of the weld joint.

[0030] During the material verification process, construction personnel meticulously inspected each HDPE outer casing for cracks, dents, and damage. They gently tapped the casing to confirm the internal polyurethane insulation layer was free of hollow areas and detachment. Calipers and other measuring tools were used to verify that the pipe diameter and wall thickness met design standards. This inspection eliminates defective pipe fittings at the source, preventing the use of reducers with inherent quality issues during construction and ensuring the long-term airtightness of the pipeline system.

[0031] In terms of environmental control, a windproof, dustproof, and rainproof work shed is erected on site. When the ambient temperature is below 5℃, in addition to turning on heating equipment such as warm air blowers, the ends of the pipes are also locally heated and insulated to ensure that the initial temperature of the welding interface meets the process requirements and to prevent the cold pipe wall from absorbing too much heat during heating, resulting in an excessively thin molten layer. When the ambient temperature is above 35℃, measures such as shade nets and spray cooling are taken. During construction in winds of level 4 or above, the enclosure of the work shed is reinforced to prevent the "wind-cooling effect" from causing uneven heat dissipation from the heating plate or end face, thus eliminating the problem of inconsistent molten edge width.

[0032] Step S2: End face pretreatment. An electric pipe cutter is used to vertically cut the welding ends of the large and small ends of the reducer. The cutting process consists of two steps: first, a large lateral pressure is applied to quickly remove the rough surface from the pipe end at the factory or the scratches generated during transportation; then, a small lateral pressure is applied to slowly smooth the surface. This meticulous operation ensures that the flatness and perpendicularity error of the end face does not exceed 0.5mm. After cutting, a special scraper is used to carefully remove small burrs from the inner and outer walls of the pipe end. Then, anhydrous alcohol or compressed air is used to blow away debris from the end face to prevent foreign matter from entering the molten pool and forming slag inclusions.

[0033] Next, use a lint-free cloth soaked in anhydrous ethanol to vigorously wipe the welding interface and the surrounding 5cm area to thoroughly remove invisible contaminants such as oil and moisture. After wiping, let it air dry to ensure that the interface is absolutely clean and dry.

[0034] Crucially, high-temperature resistant heat-insulating protective pads are installed on both the inner and outer sides of the reducing pipe. During the subsequent heating step S4, the high-temperature radiation from the heating plate will be conducted into the pipe body, which can easily cause the polyurethane insulation layer tightly attached to the HDPE outer shell to char and collapse. Adding this protective pad effectively isolates the high-temperature radiation, protecting the continuity and integrity of the insulation layer structure from the source. Compared to traditional processes, this pre-welding protective measure fundamentally reduces heat loss during later operation of the pipeline network and improves heating efficiency.

[0035] Step S3: Precise alignment and correction. This step is for the reducing pipe structure and uses the adjustable fixture of the welding machine for precise operation. The adjustable fixture of the welding machine can independently adjust the height and spacing of the fixtures used to fix the large end and the small end according to the diameter gradient, so as to achieve a perfect fit to the gradual cross-section.

[0036] Coaxiality calibration is divided into two stages: coarse adjustment and fine adjustment. In the coarse adjustment stage, the operator visually aligns the end faces of the large and small ends to achieve preliminary visual coaxiality. In the fine adjustment stage, the probe of a high-precision dial indicator is placed on the circumference of the pipe wall, and the clamp is slowly rotated to measure the radial runout deviation at multiple points on the circumference. Based on this, the corresponding clamp knobs are finely adjusted point by point until the misalignment is controlled within 10% of the pipe wall thickness. This coarse-to-fine calibration process ensures extremely high coaxial accuracy of the reducing pipe interface. When pipelines are buried underground, they are subjected to complex stresses generated by uneven soil settlement and ground loads. A highly coaxial interface can evenly transfer these stresses, effectively avoiding stress concentration and shear failure caused by interface eccentricity, and significantly improving the long-term operational reliability of the pipeline network under harsh geological conditions.

[0037] Step S4: Gradient heat fusion heating. First, preheat the heating plate to the set temperature and keep it constant for at least 10 minutes. For example, for HDPE outer shell made of PE100 material, the temperature is set to 225℃; for PE80 material, it is set to 210℃. The constant temperature process ensures the uniformity of the temperature field on the surface of the heating plate.

[0038] During heating, the end faces of the large and small ends of the reducer are tightly bonded to the heating plate under uniform pressure. The key difference lies in the heat absorption time. Since the wall thickness of the large end of the reducer is typically much greater than that of the small end, if the same heat absorption time is used, the small end will collapse due to over-melting, while the large end will not melt sufficiently. Therefore, this process extends the heat absorption time of the large end by 20%-30% beyond the standard time to ensure that the inner layer material in the thick-walled area also fully absorbs heat and melts. The heat absorption time of the small end is strictly controlled; once a uniform and regular weld ring is formed, the bonding time is considered complete. During this process, uniform and regular weld rings are formed at both ends, with their thickness controlled within 1 / 3 to 1 / 2 of the pipe wall thickness.

[0039] Step S5: Pressure welding and forming. After step S4 is completed, quickly remove the heating plate and immediately perform the butt welding. The entire operation requires that the heating plate be removed in no more than 3 seconds, and the total conversion time from removal to completion of butt welding and pressure application should not exceed 5 seconds, in order to prevent the high-temperature end face from dissipating too much heat in the air and causing a sharp drop in surface temperature.

[0040] The pressing process adopts a gradient pressurization strategy corresponding to the heating stage: for the large end of the reducing pipe with a larger wall thickness, the welding pressure is further increased on the basis of the specified drag pressure, and the strong pressure squeezes and mixes the deeply molten material at the large end, ensuring that molecules can fully diffuse and entangle to achieve deep fusion; for the small end of the reducing pipe with a thinner wall thickness, the pressure is stably controlled at a relatively low level, so as to avoid a large amount of molten material being extruded due to excessive pressure, which causes the wall thickness of the small end to be thinned or even collapsed. The entire pressing process is uniform, stable and non-impact. In this way, on the welding interfaces of the large end and the small end, the synergistic effect of temperature and pressure promotes the HDPE molecular chains on both sides to fully untie, diffuse and re-entangle into a whole, forming a firm bonding layer with mechanical properties far exceeding that of traditional equal-parameter welding.

[0041] Step S6: Constant temperature and pressure maintaining cooling. After welding and forming, the constant welding pressure must be maintained to enter the cooling stage. Any forced cooling methods such as watering and blowing are strictly prohibited in this stage, as forced cooling will cause a huge temperature difference between the weld surface and the interior, forming internal microcracks and shrinkage stress, which seriously reduces the long-term strength of the joint. This process implements static natural cooling, allowing the HDPE macromolecular chains to slowly crystallize and solidify under the support of pressure. Under normal temperature environment, the natural cooling time is not less than 30 minutes; in winter or low temperature environment, the cooling time is extended by 50% accordingly, to ensure that the entire welding zone is fully and uniformly solidified from the inside to the outside and from the large end to the small end, and the pressure is not released until the joint temperature drops to be consistent with the natural environment. This constant-pressure slow cooling system is the key to ensuring that the joint obtains high strength and toughness and dimensional stability.

[0042] Step S7: Finished product acceptance and protection. After cooling and solidification, 100% visual inspection of the joint is carried out first. The formed welding bead shall be uniform, smooth, surround the entire circumference, and be free of defects such as cracks, depressions and inclusions. The uniformity of the bead is the external manifestation of the internal welding quality. Then, the air tightness sampling inspection is carried out, using the gas charging and pressure maintaining method, compressed air is charged into the closed cavity of the welded joint to the set pressure, and the pressure is maintained for observation for at least 15 minutes. If the reading of the pressure gauge does not drop, it is judged as qualified, which confirms that the joint has complete sealing performance. The joints that have passed the acceptance shall be immediately wrapped tightly with protective materials such as flexible rubber and plastic sponge, so as to prevent them from being impacted by sand and stone or damaged by collision during the subsequent trench backfilling and compaction operations.

[0043] The implementation principle of the hot-melt welding process for a prefabricated direct-buried insulated reducer with a high-density polyethylene shell in this application embodiment is as follows: This process, from a systems engineering perspective, constructs a refined control system covering the entire welding process of the reducer. Before construction, strict checks on personnel, machinery, materials, and environment are conducted to eliminate potential quality hazards. In the welding preparation stage, an innovative high-temperature resistant heat-insulating protective pad is added, achieving pre-protection of the polyurethane insulation layer and solving the problem of high-temperature welding damaging the insulation structure. In the core welding stage, considering the inherent characteristics of the wall thickness difference between the large and small ends of the reducer, the traditional extensive mode of uniform parameters for equal-diameter pipes is abandoned. Instead, a coordinated control of "gradient hot-melt heating" and "pressure welding forming" is implemented: the heat absorption time is extended and the welding pressure is increased at the large end to ensure deep fusion, while the heat absorption time is strictly controlled and the pressure is stabilized at the small end to prevent over-melting and collapse. This differentiated and parameterized operation precisely matches the physical properties of the gradient cross-section, allowing the HDPE molecular chains to fully diffuse and entangle within the optimal temperature and pressure window. After slow, natural cooling under constant pressure, a uniform and dense integral weld zone is formed. Finally, rigorous finished product inspection and protection ensure the delivery of welded joints with high sealing performance, durability, and fatigue resistance. This process significantly improves the mechanical properties and long-term service life of the reducing pipe joint under complex underground burial conditions, achieving a design lifespan identical to the main pipe body. This greatly reduces the later-stage operation and maintenance costs of the heating network and ensures the safe and efficient operation of the municipal heating system.

[0044] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A hot-melt welding process for a prefabricated direct-buried insulated reducing pipe with a high-density polyethylene shell, characterized in that, Includes the following steps: S1: Pre-construction preparation, complete the verification of personnel, equipment, materials and environment, control the error of equipment parameters within ±2℃ and ±0.01MPa, check the appearance quality of the reducer, and build a windproof, dustproof and rainproof work shed. S2: End face pretreatment, use pipe cutting equipment to vertically cut the welding end face of the reducer, ensure that the flatness and perpendicularity error of the end face does not exceed 0.5mm, remove burrs and debris, wipe the welding interface and surrounding area with anhydrous ethanol, and install high temperature resistant heat insulation protective pads on the inside and outside of the reducer. S3: Precise alignment and correction. Use the adjustable clamps of the welding machine to fix the large and small ends of the reducing pipe. Fine-tune the clamp spacing according to the diameter gradient, calibrate the pipe coaxiality, ensure that the misalignment does not exceed 10% of the pipe wall thickness, and lock the clamp position. S4: Gradient hot melt heating, the heating temperature is set according to the characteristics of HDPE material, and the heat absorption time is adjusted differently for the difference in wall thickness between the large end and the small end of the variable diameter. The heat absorption time of the large end is extended by 20%-30%, and the heat absorption time of the small end is controlled. During the heating process, the end face is in close contact with the heating plate. S5: Pressure welding and forming, gradient pressure is applied after the heating plate is removed, the welding pressure is increased at the large end and the pressure is stabilized and controlled at the small end, and the pressure application process is uniform and stable. S6: Constant temperature and pressure cooling, maintain constant welding pressure, implement natural cooling, and the cooling time at room temperature shall not be less than 30 minutes. Extend the cooling time in low temperature environment. S7: Finished product acceptance and protection, inspect welding edges, conduct appearance inspection and airtightness spot checks, and wrap the joints with protective materials.

2. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 1, characterized in that, In step S1, the equipment uses an adjustable hot melt welding machine adapted to reduce pipe diameter, and the heating plate temperature and hydraulic pressure accuracy are calibrated in advance; the material verification includes checking that the HDPE outer shell is free of cracks, dents, and damage, the internal insulation layer is free of voids and peeling, and the pipe diameter and wall thickness meet the design standards; the heating equipment is turned on when the ambient temperature is below 5℃, and shading and cooling are carried out when the ambient temperature is above 35℃.

3. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 1, characterized in that, In step S2, the anhydrous ethanol wiping area is the welding interface and the surrounding 5cm area, and it is allowed to air dry naturally after wiping; the high temperature resistant heat insulation protective pad is used to isolate the high temperature of welding from radiation damage to the internal polyurethane insulation layer.

4. The hot-melt welding process for prefabricated direct-buried insulated reducing pipe with high-density polyethylene shell according to claim 3, characterized in that, In step S2, the pipe cutting equipment is an electric pipe cutting machine. During cutting, the rough surface of the pipe opening is first quickly removed with high lateral pressure, and then the pipe opening is slowly smoothed with low lateral pressure to ensure that the end face is flat. After cutting, a special scraper is used to remove burrs from the inner and outer walls of the pipe opening, and compressed air is used to blow away debris from the end face.

5. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 1, characterized in that, In step S3, coaxiality calibration includes two stages: coarse adjustment and fine adjustment. In the coarse adjustment stage, the two end faces are basically aligned by visual inspection. In the fine adjustment stage, the deviation of the pipe wall circumference at multiple points is measured by dial indicator, and the fixture is finely adjusted point by point until the misalignment meets the requirements.

6. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 1, characterized in that, In step S4, the heating temperature of the PE100 outer shell is controlled at 225±10℃, and the heating temperature of the PE80 outer shell is controlled at 210±10℃. The specific adjustment of the heat absorption time is as follows: due to the larger wall thickness at the large end of the variable diameter, the heat absorption time is extended by 20%-30%, while the heat absorption time at the small end is strictly controlled to prevent over-melting and collapse. During the heating process, a uniform and regular weld ring is formed, with a uniform width and a thickness controlled within 1 / 3 to 1 / 2 of the pipe wall thickness.

7. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 6, characterized in that, In step S4, the heating plate is preheated to the set temperature and kept at a constant temperature for at least 10 minutes to ensure a uniform temperature field on the heating plate. During the heating process, the contact pressure between the two ends of the reducing tube and the heating plate remains consistent, and the contact time is controlled by the heat absorption time.

8. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 1, characterized in that, In step S5, the gradient pressurization specifically involves: increasing the welding pressure at the large end to ensure deep fusion based on the specified dragging pressure, and stabilizing and controlling the pressure at the small end to avoid excessive extrusion and deformation. The pressurization process is free from impact and deviation, ensuring that the molecules at the welding interface of both ends are fully diffused and entangled.

9. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 8, characterized in that, In step S5, the heating plate is pulled out for no more than 3 seconds, and the total conversion time from pulling out the heating plate to completing the docking and pressure application is no more than 5 seconds, so as to prevent excessive heat dissipation from the end face from affecting the welding quality.

10. The hot-melt welding process for high-density polyethylene shell prefabricated direct-buried insulated reducing pipe according to claim 1, characterized in that, In step S6, natural cooling is strictly prohibited from being forced by water or air blowing. The cooling time is extended by 50% in low-temperature environments. After the interface is completely cured and the temperature drops to room temperature, proceed to the next step.

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