Plastic pipe forming apparatus and forming method

Abstract

The application discloses a plastic pipe forming device and a forming method thereof, and relates to the technical field of plastic pipe processing. The device comprises an extrusion die head, an outer wall cooling cover, a conical hole expanding die, a first traction mechanism, a vacuum shaping box and a second traction mechanism arranged in sequence along the extrusion direction of the pipe blank. An air inlet mechanism is arranged at the conical hole expanding die, which can send cooling air into the pipe blank to cool the inner wall. A return air mechanism is arranged at the extrusion die head, which collects the secondary cooling air after heat exchange with the inner wall and guides the secondary cooling air to the outer wall cooling cover to air cool the outer wall of the pipe blank, so that the cooling rates of the inner and outer walls are adaptively balanced. A bidirectional auxiliary spraying mechanism is arranged before the hole expanding to accurately calibrate the temperature of the pipe blank and eliminate residual temperature difference. The application realizes continuous online hole expanding without completely cooling and then heating the pipe blank again by gradient cooling the inner and outer walls of the pipe wall with the same airflow, and has the advantages of uniform cooling, accurate temperature control, low energy consumption and the like, and is suitable for efficient continuous production of large-diameter plastic pipes.

Description

Technical Field

[0001] This invention relates to the field of plastic processing technology, and specifically to a plastic pipe forming apparatus and its forming method. Background Technology

[0002] Polyvinyl chloride (PVC) pipes are widely used in municipal water supply and drainage, building sewage, and other fields due to their excellent corrosion resistance, mechanical properties, and cost advantages. Among them, the market demand for large-diameter PVC pipes is increasing year by year. Due to the limitations of PVC melt characteristics and molding process, the traditional direct extrusion process is difficult to produce large-diameter, high-rigidity PVC pipes. The industry generally adopts a segmented production process that first extrudes a small-diameter, thick-walled pipe blank, and then heats it again to expand the hole. The specific process is as follows: the pipe blank is extruded through a twin-screw extruder, and after being sprayed and vacuum-shaped and completely cooled and hardened, it is transferred to the hole-expanding station. The cold and hardened pipe blank is heated as a whole again through oven, hot air, etc., so that it softens to a high-elasticity state. Then, it is radially expanded and axially stretched through a conical die to finally form a large-diameter pipe.

[0003] While the existing process can meet the performance requirements of large-diameter pipes, it has many unavoidable drawbacks: the entire production process is divided into multiple independent steps such as extrusion, cooling, transfer, secondary heating, hole expansion, and re-cooling, resulting in a long production chain and low production efficiency due to intermittent operation; the pipe blank needs to undergo two heating and two cooling processes, and the secondary heating process significantly increases energy consumption and production costs, while multiple transfers can easily cause scratches and deformation of the pipe, affecting the quality of the finished product.

[0004] Ultimately, the core technological bottleneck of the aforementioned segmented process lies in the challenge of ensuring uniform cooling of the enclosed cylindrical PVC pipe blank after extrusion. After the pipe blank is extruded from the die, conventional cooling processes only target the outer wall with spraying and forced air cooling. However, the interior of the pipe is a closed cavity with poor airflow, preventing rapid heat dissipation from the inner wall. This results in the outer wall cooling much faster than the inner wall, creating a significant temperature gradient and causing the pipe blank to exhibit an uneven hardness, with a hard outer surface and a soft inner surface. In this state, the pipe blank cannot be directly enlarged, easily leading to problems such as wall wrinkles, eccentricity, tearing, and uneven wall thickness, compromising molding quality. Therefore, the industry must first completely cool and solidify the pipe blank before secondary heating to ensure uniform wall temperature before enlargement can begin. This is the core reason why current technologies cannot achieve continuous online enlargement.

[0005] Currently, existing production equipment and processes have not effectively addressed the problem of excessively different cooling rates between the inner and outer walls of the tube blank. They can only rely on "complete cooling + secondary heating" to compensate for the defects caused by uneven cooling, and have been unable to escape the industry's predicament of high energy consumption, low efficiency, and cumbersome processes. Summary of the Invention

[0006] The purpose of this invention is to provide a plastic pipe forming device and method that can achieve high-precision temperature control and energy saving.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is: a plastic pipe forming device, comprising an extrusion die head, an outer wall cooling cover, a conical expanding die, a first traction mechanism, a vacuum shaping box, and a second traction mechanism arranged sequentially along the pipe blank extrusion direction;

[0008] The conical expanding die is equipped with an air intake mechanism that can supply external cooling air into a section of tube blank between the extrusion die and the conical expanding die. The extrusion die is equipped with a return air mechanism that collects and guides the secondary cooling air after heat exchange with the inner wall of the tube blank to the outside of the extrusion die. An outer wall cooling cover is also fitted over the section of tube blank between the extrusion die and the conical expanding die. The outer wall cooling cover is used to receive the secondary cooling air returned by the return air mechanism and to exchange heat with the outer wall of the tube blank through the secondary cooling air.

[0009] Preferably, the extrusion die includes a die body, and a central fixed shaft and a flow divider cone are sequentially arranged at the center of the die body along the direction close to the outlet. An extrusion annular gap is formed between the inner wall of the outlet of the die body and the outer wall of the flow divider cone.

[0010] The end face of the diverting cone is provided with an axially extending mounting rod, and the conical expanding die is mounted on the mounting rod to form a fixed connection with the extrusion die head.

[0011] Preferably, the return air mechanism includes a central return air hole that passes through the central fixed shaft and the flow divider cone, a side return air hole that is fixedly disposed on the central fixed shaft and communicates with the central return air hole, and a return air duct cover that is fixedly disposed on the mounting rod near one end of the flow divider cone; the return air duct cover communicates with the central return air hole; the return air mechanism also includes an outlet pipe and an outlet air pump, one end of the outlet pipe communicates with the side return air hole, and the other end extends to the outside of the mold head body and communicates with the outer wall cooling cover; the outlet pipe is provided with an outlet air pump.

[0012] Preferably, the inner walls of the central return air hole, the side return air hole, and the outlet pipe are all provided with a heat insulation layer.

[0013] Preferably, a porous material distribution plate is provided between the central fixed shaft in front of the annular gap and the inner wall of the die head body.

[0014] Preferably, the air intake mechanism includes an air intake cover disposed at one end of the mounting rod near the conical expansion mold, the air intake cover being connected to one end of an air intake hole disposed at the center of the conical expansion mold, and an air intake pump being disposed at the other end of the air intake hole.

[0015] Preferably, both the return air duct cover and the air inlet duct cover include a cylindrical body and a number of air holes disposed on the circumference of the body.

[0016] Preferably, the outer wall cooling shroud includes a main body section and an access section disposed at one end of the main body section near the extrusion die; the main body section is conical and gradually decreases in size along the direction away from the extrusion die.

[0017] The access section has an I-shaped cylindrical structure and includes a protrusion, a neck, and an ejector section in sequence along the direction close to the main body section. The outlet pipe is connected to the peripheral sidewall of the protrusion. A porous screen ring with an L-shaped cross-section is provided between the protrusion and the neck. The secondary cooling air is distributed in a secondary uniform manner through the porous screen ring. An ejector cone that gradually decreases in size along the tube blank extrusion direction is provided inside the ejector section. A cooling airflow channel is formed between the ejector cone and the outer surface of the tube blank. A compensation airflow channel is provided outside the ejector cone. A compensation air port controlled by an electromagnetic valve is provided on the end face of the ejector section.

[0018] Preferably, the inner surface of the main body section of the outer wall cooling cover and the outer surface of the mounting rod are provided with several sets of temperature sensors for detecting the temperature of the tube blank along the length direction; the mounting rod is also provided with a pressure sensor for detecting the internal air pressure of one end of the tube blank between the extrusion die and the conical expansion die.

[0019] Preferably, the end of the outer wall cooling cover and the position on the mounting rod opposite to this position are both provided with auxiliary spraying mechanisms for spraying cooling mist onto the tube blank; the auxiliary spraying mechanism can perform bidirectional temperature calibration of the tube blank that is about to enter the conical expansion mold, eliminate the residual temperature difference between the inside and outside of the tube wall, and make the temperature of the inner and outer walls of the tube blank uniform.

[0020] The tube blank forming method is as follows:

[0021] S1. Equipment Debugging and Preparation: Check the connection status of each component of the device to ensure that the extrusion die, fixed conical expansion die, and traction mechanism are set coaxially; debug the working parameters of the inlet pump and outlet pump, and set the delivery rate of the cooling airflow; debug the spray volume of the auxiliary spray mechanism and the opening and closing status of the solenoid valve to ensure that the compensation port can work normally; start the temperature sensor and pressure sensor to ensure that the temperature inside and outside of the tube blank and the air pressure inside the tube can be monitored in real time; check the integrity of the heat insulation layer to avoid temperature loss of the cooling airflow.

[0022] S2. Melt extrusion and tube blank forming: Plastic raw materials are added to the extrusion equipment, melted and plasticized, and then fed into the extrusion die. After the plastic melt is evenly distributed by the porous material distribution plate, it is extruded through the extrusion annular gap between the die body outlet and the flow divider cone to form a continuous tubular tube blank. The tube blank moves along the mounting rod towards the conical expansion die.

[0023] S3. Cooling of the inner wall of the tube blank: Start the air pump, and the outside cooling air enters the air inlet cover through the air inlet hole in the center of the conical expansion die. It is then evenly delivered into the inside of the tube blank through the air holes on the periphery of the air inlet cover. The cooling air flows along the tube blank axis towards the extrusion die head, and fully exchanges heat with the high-temperature inner wall of the tube blank, carrying away the heat of the inner wall of the tube blank, realizing forced convection cooling of the inner wall of the tube blank, and forming secondary cooling air.

[0024] S4. Cooling of the outer wall of the tube blank: Start the exhaust air pump. The secondary cooling air, after heat exchange in the tube blank, enters the return air shroud through the air holes on the circumference of the return air shroud, and then enters the exhaust pipe through the central return air hole and the side return air hole in sequence. The secondary cooling air is transported in the exhaust pipe (the heat insulation layer reduces temperature loss) and finally sent to the bulge of the outer wall cooling shroud. After being evenly distributed twice by the porous screen ring, the secondary cooling air enters the neck and ejector section, flows along the cooling airflow channel between the ejector cone and the outer surface of the tube blank, and exchanges heat with the outer wall of the tube blank to achieve cooling of the outer wall of the tube blank. According to the tube blank temperature monitoring results, the opening and closing of the compensation air port is controlled by the solenoid valve to introduce compensation airflow into the compensation airflow channel, and the cooling intensity of the outer wall is finely adjusted to ensure uniform temperature of the outer wall of the tube blank.

[0025] S5. Precise calibration of tube blank temperature: When the tube blank moves to the end of the outer wall cooling shroud and is about to enter the conical expansion die, the auxiliary spray mechanism on the mounting rod and the end of the outer wall cooling shroud is activated to spray cooling mist onto the inner and outer walls of the tube blank simultaneously. The inner wall spray vaporizes rapidly after contacting the inner wall of the tube blank, absorbing the residual heat of the inner wall. The vaporized steam is discharged through the return air mechanism along with the cooling airflow inside the tube. The outer wall spray performs fine-tuning cooling on the outer wall of the tube blank. Through the synergistic effect of the inner and outer sprays, the residual temperature difference between the inner and outer walls of the tube blank is eliminated, ensuring that the temperature of the inner and outer walls of the tube blank is uniform and in a softened state suitable for expansion.

[0026] S6. Online Continuous Hole Expansion and Shaping: After temperature calibration, the tube blank is pulled at a constant speed through a fixed conical pore-expanding die under the traction of the traction mechanism. Under the radial constraint of the conical pore-expanding die and the axial tension of the traction mechanism, radial pore expansion and axial tension are carried out simultaneously to form a tube of the target diameter. The expanded tube continues to move and, after subsequent cooling and shaping treatment, completes the final forming, realizing continuous online production.

[0027] S7. Real-time monitoring and parameter adjustment: During the molding process, the temperature inside and outside of the tube blank is monitored in real time by a temperature sensor, and the internal air pressure of the tube blank is monitored in real time by a pressure sensor. If the temperature difference between the inside and outside of the tube blank exceeds the set range, the air intake rate of the air intake pump, the exhaust rate of the exhaust pump, or the spray volume of the auxiliary spray mechanism and the compensation airflow of the compensation air port can be adjusted to ensure that the temperature of the tube blank remains uniform. If the internal air pressure of the tube blank is abnormal, the working parameters of the air intake pump and the exhaust pump are adjusted in time to avoid the tube blank from collapsing or bulging.

[0028] The beneficial effects of this invention are mainly reflected in:

[0029] 1. This invention adopts a series airflow structure with the inner wall cooled first and the outer wall cooled later. The same air first forces heat exchange on the inner wall of the tube blank, and then cools the outer wall by air. By utilizing the limited heat exchange capacity of the air, the internal and external cooling rates are adaptively balanced, which reduces the temperature difference between the inside and outside of the tube wall from the source and solves the problem of the traditional method of only forcing cooling the outer wall, which results in the outer wall being hard and the inner wall being soft, and the inability to directly expand the hole.

[0030] 2. After the cooling airflow absorbs heat on the inner wall, its temperature rises, and the cooling intensity on the outer wall naturally weakens. This can match the heat dissipation of the outer wall itself, achieving gentle, uniform, and synchronous cooling without complex control. Compared with independent internal and external cooling methods, the temperature field is easier to control, resulting in higher molding stability.

[0031] 3. By recovering the air after heat exchange on the inner wall and guiding it to the outer wall for cooling, the cooling airflow is utilized in stages, which significantly improves the utilization rate of the cooling medium and reduces cooling energy consumption. At the same time, it eliminates the need for an independent outer wall cooling air supply system, making the equipment structure simpler and the layout more compact.

[0032] 4. Before expanding the hole, set up internal and external bidirectional auxiliary sprays to perform final temperature fine calibration on the tube blank, further eliminate residual temperature difference, and ensure that the internal and external temperatures are uniform when the tube blank enters the conical expanding die. The tube blank can be directly expanded online while maintaining an overall softened state, completely eliminating the complete cooling and secondary heating process in the traditional process, greatly shortening the process and improving production efficiency.

[0033] 5. The steam generated by the spray vaporization on the inner wall can be discharged through the same return air channel along with the cooling airflow, without taking up extra space or causing liquid accumulation and pressure buildup. With the fixed conical expansion mold, the overall structure has good rigidity and high coaxiality, which can stably achieve continuous online expansion forming. The expanded pipe has good roundness and uniform wall thickness, and is not prone to wrinkling, eccentricity or tearing defects. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of the present invention;

[0035] Figure 2 for Figure 1 Enlarged view of section A in the middle;

[0036] Figure 3 for Figure 1 Enlarged view of section B in the middle.

[0037] Figure label:

[0038] 1—Extrusion die head, 11—Die head body, 12—Central fixed shaft, 13—Flow divider cone, 14—Extrusion annular gap, 15—Mounting rod, 16—Temperature sensor, 17—Pressure sensor, 18—Auxiliary spray mechanism, 2—Outer wall cooling shroud, 21—Main body section, 22—Inlet section, 221—Protrusion, 222—Neck, 223—Ejector section, 224—Porous screen ring, 225—Ejector cone, 226—Cooling airflow channel 227—Compensation airflow channel, 228—Solenoid valve, 229—Compensation air port, 3—Conical expansion mold, 4—First traction mechanism, 5—Vacuum shaping box, 6—Second traction mechanism, 7—Air intake mechanism, 71—Air intake cylinder cover, 72—Air intake hole, 73—Air intake pump, 8—Return air mechanism, 81—Central return air hole, 82—Side return air hole, 83—Return air cylinder cover, 84—Outlet pipe, 85—Outlet air pump, 9—Multi-hole material equalization tray; Detailed Implementation

[0039] The following is in conjunction with the appendix Figure 1-3 The specific embodiments of the present invention will be further described in detail below. This embodiment provides a complete, clear, and repeatable description of the device structure, working principle, airflow path, temperature control, pressure coordination, and molding process. Conventional modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the core concept of the present invention are all within the protection scope of the present invention.

[0040] like Figures 1 to 3 As shown, a plastic pipe forming device is mainly used for online extrusion, synchronous temperature-controlled cooling, and continuous pore-expanding forming of large-diameter PVC pipes. The device consists of an extrusion die 1, an outer wall cooling shroud 2, a conical pore-expanding die 3, a first traction mechanism 4, a vacuum shaping box 5, and a second traction mechanism 6, arranged coaxially along the pipe blank extrusion direction. The high coaxiality and rigid connection of all components ensure that the pipe blank remains symmetrical, stable, and undeformed throughout the extrusion, cooling, pore-expanding, and traction processes, significantly improving forming accuracy and stability.

[0041] The extrusion die 1 includes a die body 11. A central fixing shaft 12 and a flow divider cone 13 are sequentially arranged at the center of the die body 11 along the direction near the outlet. An annular extrusion gap 14 is formed between the inner wall of the die body 11 at the outlet and the outer wall of the flow divider cone 13. A porous material distribution disk 9 is arranged upstream of the extrusion gap 14, between the central fixing shaft 12 and the inner wall of the die body 11, so that the plastic melt is evenly distributed circumferentially before entering the extrusion gap 14, eliminating problems such as melt flow deviation, uneven wall thickness, and circumferential density differences, and providing a uniform and stable foundation for the initial forming of the tube blank. An axially forward extending mounting rod 15 is provided on the end face of the flow divider cone 13. The conical expanding die 3 is fixedly installed at the end of the mounting rod 15, so that the conical expanding die 3 and the extrusion die 1 form a rigid integrated fixed structure with no relative movement and no radial runout. Structurally, this ensures the stability and reliability of the expanding process and avoids defects such as expanding eccentricity, wrinkling, tearing, and wall thickness fluctuation.

[0042] An air intake mechanism 7 is provided at the conical expanding die 3 to supply external cooling air into the tube blank between the extrusion die 1 and the conical expanding die 3. The air intake mechanism 7 includes an air intake cover 71 installed on the mounting rod 15 near one end of the conical expanding die 3. The air intake cover 71 is connected to the air intake hole 72 at the center of the conical expanding die 3, and the other end of the air intake hole 72 is connected to an air intake pump 73. The air intake cover 71 is a cylindrical body with several air holes evenly distributed around its circumference, so that the cooling air is evenly supplied into the interior along the circumference of the tube blank, achieving uniform forced convection cooling of the entire inner wall. After the external cooling air enters the interior of the tube blank through the air intake mechanism 7, it flows along the axial direction of the tube blank towards the extrusion die 1, fully exchanging heat with the high-temperature inner wall of the tube blank, and quickly removing the heat from the inner wall, thus solving the core problems of the tube blank inner wall being a closed cavity, slow heat dissipation, high temperature, and large temperature difference between the inside and outside in the traditional process. After absorbing heat from the inner wall, the air temperature rises, forming secondary cooling air. This part of the air is collected and discharged by the return air mechanism 8 at the extrusion die 1.

[0043] The return air mechanism 8 includes a central return air hole 81 passing through the central fixed shaft 12 and the diversion cone 13, a side return air hole 82 opened on the central fixed shaft 12 and communicating with the central return air hole 81, and a return air duct cover 83 fixed to one end of the mounting rod 15 near the diversion cone 13. The return air duct cover 83 is a cylindrical body with several air holes evenly opened around its circumference, so that the secondary cooling air in the tube blank can enter the return air duct cover 83 evenly and smoothly, and then flow through the central return air hole 81 and the side return air hole 82 in sequence. The side return air hole 82 is connected to the outlet pipe 84, and an outlet air pump 85 is installed on the outlet pipe 84 to provide stable power for the airflow return. The inner walls of the central return air hole 81, the side return air hole 82, and the outlet pipe 84 are all provided with a heat insulation layer. This heat insulation layer has a dual heat insulation function. On the one hand, it prevents the airflow from exchanging heat with the outside cold air, avoiding excessive cooling of the airflow during the transportation process. On the other hand, it prevents the airflow from exchanging heat with the high-temperature die head body, preventing the heat of the die head from being carried away by the airflow and affecting the melt extrusion state. At the same time, it prevents the airflow from being reheated by the die head, which would cause the outer wall cooling to fail. Through dual heat insulation, the secondary cooling air temperature is kept stable and the cooling capacity is controllable, realizing the efficient tiered utilization of the cooling airflow.

[0044] The outlet pipe 84 delivers secondary cooling air to the outer wall cooling shroud 2, which is fitted over the outside of the tube blank. The outer wall cooling shroud 2 includes a main body section 21 and an inlet section 22 near the extrusion die 1. The main body section 21 is a tapered structure that gradually narrows along the extrusion direction. This tapered structure allows the cooling air to gradually converge and be directed during flow, resulting in a more uniform and closer fit to the outer wall surface of the tube blank. This prevents airflow diffusion and escape, improves the heat exchange efficiency of the outer wall, and ensures circumferential cooling uniformity. The inlet section 22 is an I-shaped cylindrical structure, which is divided into a protrusion 221, a neck 222, and an ejector section 223 along the direction near the main body section 21. The outlet pipe 84 communicates with the side wall of the protrusion 221. A porous screen ring 224 with an L-shaped cross-section is provided between the protrusion 221 and the neck 222 to achieve secondary circumferential uniform distribution of the secondary cooling air, preventing airflow deviation and excessive local wind speed that could lead to uneven cooling of the outer wall. An ejector cone 225 is installed inside the ejector section 223. The ejector cone 225 can directionally, centrally, and uniformly guide the uniformly distributed cooling airflow to the outer wall surface of the tube blank, forming a stable, thin-layer, high-speed cooling air film. This allows the airflow to fully contact and exchange heat with the outer wall, avoiding airflow turbulence, eddies, and excessively strong or weak local cooling. A uniform cooling airflow channel 226 is formed between the ejector cone 225 and the outer surface of the tube blank, allowing the uniformly distributed secondary cooling air to flow stably along the outer wall of the tube blank, achieving gentle air cooling of the outer wall.

[0045] This invention employs a series structure where the same airflow first cools the inner wall and then the outer wall. Its temperature control logic possesses natural self-balancing properties: the inner wall temperature is higher, and the airflow absorbs a large amount of heat on the inner wall, increasing its temperature and correspondingly weakening its cooling capacity. Therefore, when cooling the outer wall, the cooling intensity naturally decreases, matching the outer wall's own radiative and natural heat dissipation. This creates a matching state where the inner wall cools faster and the outer wall cools slower. Without a complex control system, the internal and external cooling rates can be made consistent from a physical principle, eliminating the internal and external temperature difference at its source. This ensures the tube blank remains in a uniformly softened state, providing the core conditions for direct online pore enlargement. This is fundamentally different from existing technologies that only force cooling the outer wall, passively dissipate heat from the inner wall, and result in a large internal and external temperature difference. A compensating airflow channel 227 is provided on the outside of the ejector cone 225, and a compensating air port 229, controlled by a solenoid valve 228, is provided on the end face of the ejector part 223. Based on real-time temperature monitoring results, compensating airflow can be introduced into the compensating airflow channel 227 to finely adjust the cooling intensity of the outer wall, further improving temperature uniformity.

[0046] To achieve real-time monitoring of all parameters during the forming process, several sets of temperature sensors 16 are installed along the length of the inner surface of the main body section 21 of the outer wall cooling cover 2 and the outer surface of the mounting rod 15 to monitor the outer and inner wall temperatures of the tube blank in real time. Pressure sensors 17 are also installed on the mounting rod 15 to monitor the internal air pressure of the tube blank in real time. The pressure sensors 17 have a dual function: firstly, they monitor the internal air pressure to prevent excessive pressure from causing bulging of the tube blank and excessive pressure from causing collapse; secondly, they work in conjunction with the first traction mechanism 4 and the second traction mechanism 6 to form a coordinated internal support. By controlling the intake and return air volumes, a stable, slightly positive pressure is maintained inside the tube, providing flexible internal support for the tube blank. Combined with the axial tension of the traction mechanism, this ensures that the tube blank maintains good roundness, does not collapse, and does not deform during hole expansion, significantly improving the expansion accuracy and wall thickness uniformity. This coordinated internal support method eliminates the need for additional internal support devices, resulting in a simpler structure and more stable control.

[0047] Auxiliary spray mechanisms 18 are installed at the end of the outer wall cooling shroud 2 and in the area corresponding to the mounting rod 15. The auxiliary spray mechanisms 18 simultaneously spray cooling mist onto the inner and outer walls of the tube blank before it enters the conical expansion mold 3, performing final temperature calibration on the tube blank, completely eliminating residual internal and external temperature differences, ensuring that the tube blank enters the expansion area with completely uniform internal and external temperatures and consistent hardness, achieving optimal expansion conditions. The water vapor generated by the spray vaporization on the inner wall can be discharged through the same return air channel along with the cooling airflow inside the tube, preventing liquid accumulation, pressure buildup, and additional structural complexity, resulting in simple and smooth system operation. This fine-tuning method is completely different from existing technologies that rely on overall secondary heating to achieve temperature uniformity; it eliminates the need for repeated heating and cooling, resulting in more precise temperature control and lower energy consumption.

[0048] Based on the above apparatus, the plastic tube forming method of the present invention is carried out continuously according to the following steps:

[0049] S1. Equipment Debugging and Preparation: Check the overall coaxiality of the device to ensure that the extrusion die 1, conical expansion die 3, first traction mechanism 4, and second traction mechanism 6 are coaxial; Debug the air volume and air pressure of the inlet pump 73 and the outlet pump 85; Debug the spray volume, atomization particle size, and opening and closing parameters of the auxiliary spray mechanism 18 and the solenoid valve 228; Start the temperature sensor 16 and pressure sensor 17 and confirm that the monitoring is normal; Check the integrity of the thermal insulation layer to avoid heat loss from airflow.

[0050] S2. Melt extrusion and tube blank forming: PVC raw material is added to the extruder, and after being melted and plasticized, it is fed into the extrusion die 1; after the melt is evenly distributed around the periphery by the porous material distribution plate 9, it is extruded through the extrusion annular gap 14 to form a continuous and uniform tubular tube blank, and the tube blank moves forward smoothly along the mounting rod 15.

[0051] S3. Forced cooling of the inner wall of the tube blank: Start the air pump 73, and the external cooling air is evenly sent into the interior of the tube blank through the air inlet 72 and the air inlet cover 71. It flows axially towards the extrusion die 1 and fully forces convection heat exchange with the inner wall to quickly reduce the temperature of the inner wall and form secondary cooling air with increased temperature.

[0052] S4. Staged cooling of the outer wall of the tube blank: Start the exhaust air pump 85, and the secondary cooling air in the tube blank is delivered to the outer wall cooling cover 2 through the return air shroud 83, the central return air hole 81, the side return air hole 82, and the exhaust pipe 84. After being evenly distributed twice by the porous screen ring 224, the outer wall is gently cooled along the cooling airflow channel 226. According to the real-time monitoring results of the temperature sensor 16, the cooling intensity of the outer wall is finely adjusted by the compensation air port 229 and the compensation airflow channel 227 to make the outer wall temperature uniform and stable.

[0053] S5. Precise calibration of tube blank temperature: When the tube blank moves to the end of the outer wall cooling cover 2 and is about to enter the conical expansion mold 3, the internal and external bidirectional auxiliary spray mechanism 18 is activated to spray cooling mist on the inner and outer walls at the same time, quickly eliminating the residual temperature difference, so that the temperature inside and outside the tube blank is completely consistent, maintaining a uniform softening state, and meeting the requirements for direct expansion.

[0054] S6. Online continuous pore enlargement and shaping: Under the synchronous traction of the first traction mechanism 4 and the second traction mechanism 6, the uniformly heated tube blank passes through the fixed conical pore enlargement die 3 at a constant speed. Under the combined action of radial constraint and axial tension, the pore enlargement is completed to obtain the target diameter tube. The enlarged tube enters the vacuum shaping box 5 for further cooling and shaping, realizing the integrated continuous online production of extrusion-cooling-pore enlargement-shaping.

[0055] S7. Real-time monitoring and closed-loop regulation: During the production process, temperature sensor 16 and pressure sensor 17 continuously collect the internal and external temperatures and internal air pressure of the tube blank; when the internal and external temperature difference exceeds the set range, the air intake volume, return air volume, spray volume and compensation air flow are automatically or manually adjusted; when the air pressure inside the tube is abnormal, the working parameters of the air intake pump 73 and the air outlet pump 85 are adjusted in time to ensure the stability of the hole expansion process and the consistency of product quality.

[0056] Compared with existing technologies, this invention addresses the issue that existing technologies require the tube blank to be completely cooled and hardened before being transported and reheated to a softened state for pore expansion. This process is lengthy, energy-intensive, and inefficient. This invention, through an integrated design of forced internal air cooling, tiered airflow reuse, adaptive external cooling, fine-tuning of internal and external spraying, and coordinated internal air pressure support, enables the tube blank to achieve uniform internal and external temperatures immediately after extrusion. This allows for online pore expansion while maintaining a continuously softened state, completely eliminating the need for cooling, transporting, and reheating. This represents a technological breakthrough from intermittent segmented production to continuous integrated online production, offering significant advantages such as more precise temperature control, lower energy consumption, more compact structure, more stable molding quality, and higher production efficiency.

Claims

1. A plastic tube forming device, characterized in that: It includes an extrusion die (1), an outer wall cooling cover (2), a conical expanding die (3), a first traction mechanism (4), a vacuum shaping box (5), and a second traction mechanism (6) arranged sequentially along the tube blank extrusion direction; The conical expanding die (3) is provided with an air intake mechanism (7) that can send external cooling air into a section of tube blank between the extrusion die (1) and the conical expanding die (3). The extrusion die (1) is provided with a return air mechanism (8) for collecting and guiding the secondary cooling air after heat exchange with the inner wall of the tube blank to the outside of the extrusion die (1). An outer wall cooling cover (2) is also provided on the outside of the section of tube blank between the extrusion die (1) and the conical expanding die (3). The outer wall cooling cover (2) is used to receive the secondary cooling air sent back by the return air mechanism (8) and exchange heat with the outer wall of the tube blank through the secondary cooling air.

2. The plastic tube forming apparatus according to claim 1, characterized in that: The extrusion die (1) includes a die body (11), and a central fixed shaft (12) and a flow divider cone (13) are sequentially arranged at the center of the die body (11) along the direction close to the outlet. An extrusion annular gap (14) is formed between the inner wall of the outlet of the die body (11) and the outer wall of the flow divider cone (13). The end face of the diverting cone (13) is provided with an axially extending mounting rod (15), and the conical expanding die (3) is mounted on the mounting rod (15) to form a fixed connection with the extrusion die (1).

3. The plastic tube forming apparatus according to claim 2, characterized in that: The return air mechanism (8) includes a central return air hole (81) that runs through the central fixed shaft (12) and the diversion cone (13), a side return air hole (82) that is fixedly installed on the central fixed shaft (12) and communicates with the central return air hole (81), and a return air shroud (83) that is fixedly installed on the mounting rod (15) near the diversion cone (13); the return air shroud (83) communicates with the central return air hole (81); the return air mechanism (8) also includes an outlet pipe (84) and an outlet air pump (85), one end of the outlet pipe (84) communicates with the side return air hole (82), and the other end extends to the outside of the mold head body (11) and communicates with the outer wall cooling cover (2); the outlet pipe (84) is provided with an outlet air pump (85).

4. The plastic tube forming apparatus according to claim 3, characterized in that: A porous material distribution plate (9) is provided between the central fixed shaft (12) in front of the extrusion annular gap (14) and the inner wall of the die body (11).

5. The plastic tube forming apparatus according to claim 4, characterized in that: The air intake mechanism (7) includes an air intake cover (71) disposed on the mounting rod (15) near the end of the conical expansion mold (3). The air intake cover (71) is connected to one end of an air intake hole (72) disposed at the center of the conical expansion mold (3). An air intake pump (73) is disposed at the other end of the air intake hole (72).

6. The plastic tube forming apparatus according to claim 5, characterized in that: Both the return air duct cover (83) and the air inlet duct cover (71) include a cylindrical body and several air holes on the circumference of the body.

7. The plastic tube forming apparatus according to claim 6, characterized in that: The outer wall cooling cover (2) includes a main body section (21) and an access section (22) disposed at one end of the main body section (21) near the extrusion die (1); the main body section (21) is conical and gradually decreases in size along the direction away from the extrusion die (1); The access section (22) has an I-shaped cylindrical structure and includes a protrusion (221), a neck (222), and an ejector section (223) in sequence along the direction close to the main body section (21). The outlet pipe (84) is connected to the circumferential sidewall of the protrusion (221). A porous screen ring (224) with an L-shaped cross-section is provided between the protrusion (221) and the neck (222). The secondary cooling air is distributed in a secondary uniform manner through the porous screen ring (224). An ejector cone (225) that gradually decreases in size along the tube blank extrusion direction is provided inside the ejector section (223). A cooling airflow channel (226) is formed between the ejector cone (225) and the outer surface of the tube blank. A compensation airflow channel (227) is provided outside the ejector cone (225). A compensation air port (229) controlled by an electromagnetic valve (228) is provided on the end face of the ejector section (223).

8. The plastic tube forming apparatus according to claim 7, characterized in that: The inner surface of the main body section (21) of the outer wall cooling cover (2) and the outer surface of the mounting rod (15) are provided with several sets of temperature sensors (16) for detecting the temperature of the tube blank along the length direction; the mounting rod (15) is also provided with a pressure sensor (17) for detecting the internal air pressure of a section of tube blank between the extrusion die (1) and the conical expansion die (3).

9. The plastic tube forming apparatus according to claim 8, characterized in that: At the end of the outer wall cooling cover (2) and at the position on the mounting rod (15) opposite to the end of the cooling cover (2), there is an auxiliary spray mechanism (18) for spraying cooling mist onto the tube blank; the auxiliary spray mechanism (18) can perform bidirectional temperature calibration on the tube blank that is about to enter the conical expansion mold (3), eliminate the residual temperature difference between the inside and outside of the tube wall, and make the temperature of the inner and outer walls of the tube blank uniform.

10. A molding method using the plastic pipe molding apparatus of claim 9, characterized in that, Includes the following steps: S1. Equipment debugging and preparation: Check the connection status of each component of the device to ensure that the extrusion die (1), the fixed conical expansion die (3), and the traction mechanism (4, 6) are set coaxially; debug the working parameters of the air inlet pump (73) and the air outlet pump (85) and set the delivery rate of the cooling airflow; debug the spray volume of the auxiliary spray mechanism (18) and the opening and closing status of the solenoid valve (228) to ensure that the compensation air port (229) can work normally; start the temperature sensor (16) and the pressure sensor (17) to ensure that the temperature inside and outside of the tube blank and the air pressure inside the tube can be monitored in real time; S2, Melt extrusion and tube blank forming: Plastic raw materials are added to the extrusion equipment, melted and plasticized, and then fed into the extrusion die (1); after the plastic melt is evenly distributed by the porous material distribution plate (9), it is extruded through the extrusion annular gap (14) between the die body (11) outlet and the flow divider cone (13) to form a continuous tubular tube blank, and the tube blank moves along the mounting rod (15) towards the conical expansion die (3); S3. Cooling of the inner wall of the tube blank: Start the air pump (73), and the external cooling air enters the air inlet cover (71) through the air inlet hole (72) in the center of the conical expansion die (3), and is evenly sent into the inside of the tube blank through the air holes on the circumference of the air inlet cover (71); the cooling air flows along the tube blank axis towards the extrusion die (1), and fully exchanges heat with the inner wall of the high-temperature tube blank, taking away the heat of the inner wall of the tube blank, realizing forced convection cooling of the inner wall of the tube blank, and forming secondary cooling air; S4. Cooling of the outer wall of the tube blank: Start the exhaust air pump (85). The secondary cooling air, after heat exchange in the tube blank, enters the return air shroud (83) through the air holes on the circumference of the return air shroud (83), and then enters the exhaust pipe (84) through the central return air hole (81) and the side return air hole (82) in sequence. The secondary cooling air is transported in the exhaust pipe (84) and finally sent to the bulge (221) of the outer wall cooling shroud (2). The secondary cooling air passes through the porous screen ring (224). After secondary uniform distribution, it enters the neck (222) and ejector section (223), flows along the cooling airflow channel (226) between the ejector cone (225) and the outer surface of the tube blank, and exchanges heat with the outer wall of the tube blank to achieve cooling of the outer wall of the tube blank; according to the tube blank temperature monitoring results, the opening and closing of the compensation air port (229) is controlled by the solenoid valve (228) to introduce compensation airflow into the compensation airflow channel (227), finely adjust the cooling intensity of the outer wall, and ensure that the temperature of the outer wall of the tube blank is uniform; S5. Precise calibration of tube blank temperature: When the tube blank moves to the end of the outer wall cooling cover (2) and is about to enter the conical expansion mold (3), the auxiliary spray mechanism (18) on the mounting rod (15) and the end of the outer wall cooling cover (2) is activated to spray cooling mist onto the inner and outer walls of the tube blank at the same time; the inner wall spray vaporizes rapidly after contacting the inner wall of the tube blank, absorbing the residual heat of the inner wall, and the vaporized steam is discharged through the return air mechanism (8) along with the cooling airflow in the tube; the outer wall spray performs fine-tuning cooling on the outer wall of the tube blank, and through the synergistic effect of the inner and outer bidirectional sprays, the residual temperature difference between the inner and outer walls of the tube blank is eliminated, ensuring that the inner and outer walls of the tube blank are uniform in temperature and in a softened state suitable for expansion. S6. Online continuous expansion and shaping: After temperature calibration, the tube blank passes through the fixed conical expansion mold (3) at a constant speed under the traction action of the traction mechanism (4, 6). Under the radial constraint of the conical expansion mold (3) and the axial tension of the traction mechanism (4, 6), the radial expansion and axial tension are carried out simultaneously to form a tube of the target diameter. The expanded tube continues to move and, after subsequent cooling and shaping treatment, completes the final forming, realizing continuous online production. S7. Real-time monitoring and parameter adjustment: During the molding process, the temperature inside and outside of the tube blank is monitored in real time by temperature sensor (16), and the internal air pressure of the tube blank is monitored in real time by pressure sensor (17). If the temperature difference between the inside and outside of the tube blank exceeds the set range, the air intake rate of the air intake pump (73) and the exhaust rate of the exhaust pump (85) can be adjusted, or the spray volume of the auxiliary spray mechanism (18) and the compensation airflow of the compensation port (229) can be adjusted to ensure that the temperature of the tube blank remains uniform. If the internal air pressure of the tube blank is abnormal, the working parameters of the air intake pump (73) and the exhaust pump (85) can be adjusted in time.

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