A co-extrusion process for cable plastic outer layer

Abstract

The present application relates to the technical field of plastic waste recycling, and discloses a co-extrusion forming process for cable plastic outer layers, which comprises the following steps: heating and melting recycled plastic waste to output homogeneous melt; applying transient pulse shear in a high-shear flow channel to output disentangled melt and locally overheated melt; guiding the melt into a stress relaxation section of a nonlinear shrinkage topology flow channel, using reverse cooling heat flux of a cooling jacket to offset local overheating and output stress relaxation melt; and guiding the stress relaxation melt and virgin melt into a co-extrusion converging die and coating the surface of the cable core wire, wherein the present application causes the disentanglement of the entangled network inside the recycled plastic, eliminates anisotropic elastic memory, realizes rheological matching, prevents interface flow instability and interlayer slip, eliminates bamboo joint defects, and improves the insulation protection performance and mechanical compactness of the outer sheath.

Description

Technical Field

[0001] This invention relates to a co-extrusion molding process for the plastic outer layer of cables, belonging to the field of plastic waste recycling technology. Background Technology

[0002] Recycling plastic waste into recycled materials for the production of cable plastic outer sheaths is an important way to improve the utilization value of waste plastics. Current processes typically involve first crushing the plastic waste, then feeding it into an extrusion channel for heating and melting, and finally co-coating it with virgin plastic onto the outer surface of the cable core to form a composite sheath. The plastic waste has already been subjected to heat and mechanical shearing during previous processing, and its molecular chains retain a certain degree of elastic memory. The degree of entanglement between chain segments is also uneven. Therefore, during the flow of the recycled melt, the stress relaxation rate varies at different locations and directions. This can lead to problems during the high-speed co-extrusion process of cables. Shear and temperature changes within the flow channel can easily cause the residual elastic stress in the regenerated melt to rebound. When the regenerated melt and the primary melt merge at the interface, if the first normal stress difference between the two phases is inconsistent and the shear rate near the interface changes abruptly, interface flow instability and interlayer slippage are likely to occur. As a result, bamboo-like rough defects will form on the surface of the cable outer sheath. When interlayer peeling occurs, it will also cause insulation failure, leading to cable scrapping. In addition, residual moisture in the waste material is prone to hydrolysis under high temperature and high shear conditions, causing defects such as pores inside the coating layer.

[0003] In existing production processes, the flow state of recycled materials is usually improved by increasing the extrusion temperature or extending the screw mixing time. However, this can easily lead to thermal degradation and localized coking of the waste material, and it cannot solve the problem of inconsistent stress relaxation rates at different locations and directions within the recycled melt. Some processes incorporate differential pressure regulating mechanisms to reduce melt pressure fluctuations, but these mechanisms can only smooth out overall pressure pulses and cannot change the original molecular chain entanglement state and its uneven distribution in the recycled melt. Therefore, it is difficult to ensure that the rheological properties of the two-phase melts match at the confluence interface. For example, Chinese invention patent application CN120481106A discloses an improvement method for injection molding of polypropylene composite materials. The method for controlling tiger-skin pattern defects in molded products involves applying a combined rotational and reciprocating oscillating shear field to the plasticized melt to achieve deentanglement control. However, this control method is suitable for long-term steady-state shear environments and sufficient relaxation time, and is applicable to non-continuous pre-injection granule modification. The objective working conditions of high-speed continuous co-extrusion of cables are characterized by extremely short interfacial spatiotemporal scales and high-frequency transient rheological instability. This macroscopic slow deentanglement control method cannot match the microsecond-level dynamic stress relaxation at the co-extrusion confluence front in real time, and the long-term accumulation of thermomechanical energy is prone to causing irreversible degradation of the material. It is difficult to suppress the abrupt change in interlayer shear tension gradient caused by complex component fluctuations, and thus cannot effectively eliminate bamboo-like defects and interfacial flow instability.

[0004] Therefore, how to provide a co-extrusion molding process for the plastic outer layer of cables to solve the interfacial flow instability, sheath delamination and surface defects caused by elastic memory and molecular chain entanglement of recycled plastic waste melt during cable co-extrusion remains a technical problem that needs to be solved in this field. Summary of the Invention

[0005] To address the problems in the background art, the technical solution of the present invention is as follows: A co-extrusion molding process for the plastic outer layer of cables, comprising the following steps:

[0006] Step S1: Heat and melt the recycled plastic waste, control the flow field shear rate to 350 / s to 500 / s, and output homogenized recycled melt;

[0007] Step S2: The homogenized regenerated melt is introduced into the high-shear flow channel, transient pulse shear is applied, the shear rate of the transient pulse shear is controlled to be 1200 / s to 1500 / s and maintained for 3s to 5s of flow delay, and the untangled regenerated melt and the locally overheated melt generated by shear dissipation are output.

[0008] Step S3: The detangled regenerated melt and the locally overheated melt are introduced into the stress relaxation section with a nonlinear contraction topology flow channel. The melt temperature in the stress relaxation section is controlled to be 5°C to 10°C lower than the melt temperature in the high shear flow channel. The relaxation residence time of the detangled regenerated melt in the nonlinear contraction topology flow channel is controlled to be 6s to 10s. The melt pressure oscillation frequency at the co-extrusion die head is collected using a pressure sensor and the standard deviation of the melt pressure oscillation frequency is calculated. The reverse cooling heat flow of the cooling jacket at the flow channel wall is controlled to increase with the increase of the standard deviation to offset the locally overheated melt and output the stress-relaxed regenerated melt.

[0009] Step S4: Stress-relaxed recycled melt and virgin plastic melt are introduced into the co-extrusion confluence die. By adjusting the internal volume of the front pressure regulating cavity, the absolute value of the pressure difference between the two phases at the confluence interface is controlled to be less than 0.3 MPa. After co-extrusion, they are coated onto the surface of the cable core.

[0010] Preferably, before heating and melting the recycled plastic waste in step S1, the following sub-steps are included: Step S11, using a forced hot air dryer to remove the free moisture from the recycled plastic waste, controlling the overall moisture content of the recycled plastic waste to be below 0.02%, and inhibiting the high-temperature hydrolysis reaction.

[0011] Preferably, step S1 includes the following sub-step: step S12, controlling the homogenized regenerated melt to flow through a continuous automatic screen-changing filter to intercept heterogeneous cross-linked substances and solid particulate impurities mixed in the homogenized regenerated melt.

[0012] Preferably, step S2 includes the following sub-steps: Step S21, online acquisition of the apparent viscosity of the homogenized regenerated melt, when the apparent viscosity is lower than a preset viscosity threshold, the upper limit of the screw speed of the high shear channel is reduced to limit the shear rate.

[0013] Preferably, step S3 includes the following sub-steps: Step S31, controlling the cross-sectional area of ​​the nonlinear contraction topology channel to gradually decrease from the high shear channel to the stress relaxation section, controlling the flow field shear rate to decrease in a gradient along the channel extension direction, and releasing the anisotropic structural stress of the unentangled regenerated melt.

[0014] Preferably, step S3 includes the following sub-steps: Step S32, obtaining the local apparent viscosity change rate of the untangled regenerated melt flowing through the nonlinear contraction topology channel online, adjusting the inner wall temperature distribution of the nonlinear contraction topology channel according to the local apparent viscosity change rate, and controlling the stress relaxation progress of the central flow field and the inner wall flow field of the nonlinear contraction topology channel to be synchronized.

[0015] Preferably, in step S4, adjusting the internal volume of the front-end pressure regulating cavity includes the following sub-steps: Step S41, obtaining the change in plunger displacement inside the front-end pressure regulating cavity through a ranging sensor, changing the internal volume of the front-end pressure regulating cavity according to the change in plunger displacement, absorbing the high-frequency pressure transients caused by material supply fluctuations, and maintaining the interlayer shear tension gradient at the confluence interface in a stable range.

[0016] Preferably, after the co-extrusion and bonding in step S4, the coating is applied to the surface of the cable core, the following sub-steps are included: Step S42, the cable core coated with the virgin plastic melt and stress-relaxed regenerated melt is guided to pass continuously through the cooling water tank, and the coating layer outside the cable core is cooled according to the preset temperature drop gradient to solidify the interlayer interface structure.

[0017] Preferably, step S42 includes the following sub-steps: Step S421, monitoring the water temperature of multiple sections of the cooling water tank, controlling the water temperature to decrease gradually along the direction of travel of the cable core, and eliminating the internal stress of cold shrinkage inside the sheathing layer.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1. In the co-extrusion molding process of the plastic outer layer of cables, the homogenized recycled melt is subjected to transient pulse shearing in the high-shear channel. The pulse holding time and the upper limit of the shear rate are adjusted according to the standard deviation of the melt pressure oscillation frequency and the apparent viscosity. This can untangle the molecular chain entanglement accumulated in the recycled melt of plastic waste and release the thermomechanical stress left by the previous processing of the material. This can make the degree of untangling of molecular chains more uniform, reduce the viscoelastic differences in different directions, avoid local overheating and degradation caused by excessive shearing, and reduce the impact of waste composition fluctuations on the flow state of the recycled melt.

[0020] 2. The nonlinear contraction topology of the flow channel gradually reduces the shear force on the melt, allowing the untangled regenerated melt to smoothly enter the stress relaxation zone. The heat generated by the high shear helps the molecular chains recover, while the cooling jacket can offset local overheating and reduce the difference in stress relaxation progress between the center of the flow channel and the melt near the wall. As a result, the untangled molecular chains can fully release stress under relatively stable temperature conditions, eliminating residual elastic memory and anisotropic distribution in the regenerated melt.

[0021] 3. After stress relaxation treatment, the first normal stress difference between the recycled melt and the virgin plastic melt is better matched. When the two-phase melt enters the co-extrusion confluence die, the front pressure regulating cavity can absorb pressure fluctuations and keep the pressure difference at the confluence interface stable. This can reduce the sudden change in shear rate at the interface and the flow difference between the two-phase melt, prevent interlayer slippage and delamination, avoid bamboo-like rough defects in the outer layer of the composite plastic, and improve the interfacial shear failure strength of the coating layer. Attached Figure Description

[0022] Figure 1 This is a flowchart of the plastic outer layer co-extrusion molding process steps of the present invention;

[0023] Figure 2 This is a melt state diagram of the plastic outer layer co-extrusion molding process of the present invention.

[0024] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0025] The technical solutions in the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0026] A co-extrusion molding process for the plastic outer layer of cables includes the following steps:

[0027] Step S1: Heat and melt the recycled plastic waste, control the flow field shear rate to 350 / s to 500 / s, and output homogenized recycled melt;

[0028] Step S2: The homogenized regenerated melt is introduced into the high-shear flow channel, transient pulse shear is applied, the shear rate of the transient pulse shear is controlled to be 1200 / s to 1500 / s and maintained for 3s to 5s of flow delay, and the untangled regenerated melt and the locally overheated melt generated by shear dissipation are output.

[0029] Step S3: The detangled regenerated melt and the locally overheated melt are introduced into the stress relaxation section with a nonlinear contraction topology flow channel. The melt temperature in the stress relaxation section is controlled to be 5°C to 10°C lower than the melt temperature in the high shear flow channel. The relaxation residence time of the detangled regenerated melt in the nonlinear contraction topology flow channel is controlled to be 6s to 10s. The melt pressure oscillation frequency at the co-extrusion die head is collected using a pressure sensor and the standard deviation of the melt pressure oscillation frequency is calculated. The reverse cooling heat flow of the cooling jacket at the flow channel wall is controlled to increase with the increase of the standard deviation to offset the locally overheated melt and output the stress-relaxed regenerated melt.

[0030] Step S4: Stress-relaxed recycled melt and virgin plastic melt are introduced into the co-extrusion confluence die. By adjusting the internal volume of the front pressure regulating cavity, the absolute value of the pressure difference between the two phases at the confluence interface is controlled to be less than 0.3 MPa. After co-extrusion, they are coated onto the surface of the cable core.

[0031] Preferably, before heating and melting the recycled plastic waste in step S1, the following sub-steps are included: Step S11, using a forced hot air dryer to remove the free moisture from the recycled plastic waste, controlling the overall moisture content of the recycled plastic waste to be below 0.02%, and inhibiting the high-temperature hydrolysis reaction.

[0032] Preferably, step S1 includes the following sub-step: step S12, controlling the homogenized regenerated melt to flow through a continuous automatic screen-changing filter to intercept heterogeneous cross-linked substances and solid particulate impurities mixed in the homogenized regenerated melt.

[0033] Preferably, step S2 includes the following sub-steps: Step S21, online acquisition of the apparent viscosity of the homogenized regenerated melt, when the apparent viscosity is lower than a preset viscosity threshold, the upper limit of the screw speed of the high shear channel is reduced to limit the shear rate.

[0034] Preferably, step S3 includes the following sub-steps: Step S31, controlling the cross-sectional area of ​​the nonlinear contraction topology channel to gradually decrease from the high shear channel to the stress relaxation section, controlling the flow field shear rate to decrease in a gradient along the channel extension direction, and releasing the anisotropic structural stress of the unentangled regenerated melt.

[0035] Preferably, step S3 includes the following sub-steps: Step S32, obtaining the local apparent viscosity change rate of the untangled regenerated melt flowing through the nonlinear contraction topology channel online, adjusting the inner wall temperature distribution of the nonlinear contraction topology channel according to the local apparent viscosity change rate, and controlling the stress relaxation progress of the central flow field and the inner wall flow field of the nonlinear contraction topology channel to be synchronized.

[0036] Preferably, in step S4, adjusting the internal volume of the front-end pressure regulating cavity includes the following sub-steps: Step S41, obtaining the change in plunger displacement inside the front-end pressure regulating cavity through a ranging sensor, changing the internal volume of the front-end pressure regulating cavity according to the change in plunger displacement, absorbing the high-frequency pressure transients caused by material supply fluctuations, and maintaining the interlayer shear tension gradient at the confluence interface in a stable range.

[0037] Preferably, after the co-extrusion and bonding in step S4, the coating is applied to the surface of the cable core, the following sub-steps are included: Step S42, the cable core coated with the virgin plastic melt and stress-relaxed regenerated melt is guided to pass continuously through the cooling water tank, and the coating layer outside the cable core is cooled according to the preset temperature drop gradient to solidify the interlayer interface structure.

[0038] Preferably, step S42 includes the following sub-steps: Step S421, monitoring the water temperature of multiple sections of the cooling water tank, controlling the water temperature to decrease gradually along the direction of travel of the cable core, and eliminating the internal stress of cold shrinkage inside the sheathing layer.

[0039] Example 1: This example applies the plastic waste recycling process to the co-extrusion molding of the outer sheath of power cables. The recycled polyethylene crushed material is dried by forced hot air, reducing the moisture content to 0.015%. The dried recycled polyethylene crushed material is fed into the auxiliary extrusion channel of the co-extrusion system. A temperature field is applied in sections to control the melting progress of the material in each section, and the flow field shear rate is controlled at 350 / s to obtain a homogenized recycled melt. The homogenized recycled melt enters the high-shear channel and undergoes transient pulse shearing at a shear rate of 1200 / s. The flow time is set to 3.5s, causing the entangled polymer molecular chains in the melt to dissociate. The output is a de-entangled recycled melt and a locally overheated melt formed by shear dissipation. Among them, the melt near the wall of the high-shear channel forms a locally overheated melt due to continuous shear dissipation, and flows together with the de-entangled recycled melt to the stress relaxation section.

[0040] Within the high-shear flow channel, the control unit uses pressure sensors to collect melt pressure signals at the co-extrusion die head, extracts the melt pressure oscillation frequency from continuous statistical cycles, and calculates the standard deviation of the melt pressure oscillation frequency sequence. When the standard deviation exceeds a preset benchmark value, the control unit linearly extends the transient pulse shearing delay time within a range of 3 to 5 seconds. The control unit also collects the apparent viscosity of the homogenized regenerated melt online. When the apparent viscosity is lower than a preset viscosity threshold, the upper limit of the screw speed in the high-shear flow channel is reduced to limit the shear rate and reduce thermomechanical degradation caused by material batch fluctuations.

[0041] The de-entangled regenerated melt and the locally overheated melt enter a stress relaxation section equipped with a nonlinear contraction topology flow channel. The cross-sectional area of ​​the nonlinear contraction topology flow channel gradually decreases from the high-shear flow channel to the stress relaxation section. The cross-sectional profile at each axial position is determined according to the shear rate distribution obtained from rheological calculations, so that the shear rate of the flow field decreases in a gradient along the extension direction of the flow channel. The shear field intensity of the de-entangled regenerated melt gradually weakens with the flow process, and the anisotropic structural stress is released according to the logarithmic decay trend of the polymer long chain relaxation spectrum. The melt temperature in the stress relaxation section is 6°C lower than the melt temperature in the high-shear flow channel. The relaxation residence time is set to 6.5s. After processing, the stress-relaxed regenerated melt is output.

[0042] The flow channel wall in the stress relaxation section is equipped with a cooling jacket. The control unit uses the standard deviation of the melt pressure oscillation frequency as the adjustment amount of the overall cooling load. When the standard deviation increases, the reverse cooling heat flow of the cooling jacket is increased accordingly to offset the excessive heat brought in by the locally overheated melt. At the same time, the local apparent viscosity change rate of the untangled regenerated melt flowing through the nonlinear contraction topology flow channel is acquired online. This change rate is used to characterize the stress relaxation progress of the melt. The control unit compares the local apparent viscosity change rates of the central flow field and the inner wall flow field, and adjusts the inner wall temperature distribution at the corresponding position according to the difference between the two to synchronize the stress relaxation progress of the central flow field and the inner wall flow field.

[0043] The shear dissipation heat generated by the high-shear flow channel wall layer enters the stress relaxation zone with the melt, keeping the melt near the wall within the temperature range where the polymer chain segments can undergo conformational transformation. This heat also serves as thermal activation energy to lower the conformational transformation energy barrier when the molecular chains undergo Brownian motion. This process involves the disordered thermal motion generated by shear dissipation participating in the thermal relaxation and conformational entropy increase of the molecular chains after detangling, rather than the ordered work being directly converted into internal energy. Under this temperature condition, the molecular chains overcome the conformational transformation energy barrier and return to an isotropic equilibrium state. Simultaneously, the cooling jacket removes local heat exceeding the stress relaxation requirement, reducing the interference of local temperature rise on the molecular chain relaxation process.

[0044] Stress-relaxed regenerated melt and virgin plastic melt enter a co-extrusion confluence die. A front-end pressure regulating chamber is set before the inlet of the co-extrusion confluence die. By adjusting the internal volume of the front-end pressure regulating chamber, the pressure pulses of the virgin plastic melt and the stress-relaxed regenerated melt are absorbed, and the absolute value of the pressure difference between the two phases at the confluence interface is controlled to 0.12 MPa. After the two phases merge in the co-extrusion confluence die, they coat the surface of the cable core to form a composite plastic outer layer. The cable core continuously passes through a cooling water tank and cools its outer coating layer according to a preset temperature drop gradient, solidifying the interlayer interface structure. The interfacial shear failure strength of the composite plastic outer layer is tested to be 46.5 MPa.

[0045] Example 2: This example is used to verify the actual effect of eliminating the viscoelastic memory effect of recycled plastic waste melt through transient pulse shearing and stress relaxation treatment on a high-speed cable sheath co-extrusion production line. The test platform uses a co-rotating twin-screw extruder as an auxiliary extrusion channel with a length-to-diameter ratio of 40:1 and a temperature control accuracy of 1℃. The end is connected to the co-extrusion confluence die and the core wire traction device. The test material is recycled polyethylene crushed material with a melt index of 1.2g / 10min and an initial moisture content of 0.025%. The test environment temperature is maintained at 25℃. At the same time, power cable electromagnetic interference with a frequency of 50Hz is introduced, and random material pressure pulsation with an amplitude of 0.05MPa is applied at the feed port.

[0046] Both the experimental and control groups used the same basic extrusion process. In the experimental group, transient pulse shearing was applied to the homogenized regenerated melt in the high-shear channel, and then the detangled regenerated melt was introduced into the stress relaxation section with a nonlinear contraction topology channel. In the control group, no transient pulse shearing or stress relaxation treatment was performed, and the melt output from the auxiliary extrusion channel was introduced into the co-extrusion confluence die. During the experiment, the melt pressure in the auxiliary extrusion zone was set to 12 MPa. In the experimental group, which was used to determine the process parameters, the transient pulse shear rate was set to 1350 / s, the casting time was set to 4.2s, the melt temperature in the stress relaxation section was controlled to be 8°C lower than the melt temperature in the high-shear channel, and the relaxation residence time was set to 8.5s.

[0047] The control system uses pressure sensors to collect the melt pressure oscillation frequency at the co-extrusion confluence die and calculates the standard deviation of the melt pressure oscillation frequency. When the standard deviation exceeds the preset benchmark value, the transient pulse shearing delay time is linearly extended within the range of 3s to 5s. The control system also collects the apparent viscosity of the homogenized regenerated melt online. When the apparent viscosity is lower than the preset viscosity threshold, the upper limit of the screw speed in the high-shear flow channel is reduced to limit the shear rate and reduce thermomechanical degradation caused by material batch fluctuations.

[0048] In the control group, the transient pulse shear rate was 0, the casting relaxation time was 0, and the melt output from the auxiliary extrusion channel directly entered the co-extrusion confluence die. The absolute value of the pressure difference at the confluence interface was 0.85 MPa. The outer layer of the resulting composite plastic had multiple bamboo-like rough defects, the interface shear failure strength was 28.4 MPa, and the defect rate was 12.5%.

[0049] In test group A, the transient pulse shear rate was set to 1200 / s and the relaxation residence time was set to 6.0s. After transient pulse shear and stress relaxation treatment, the absolute value of the pressure difference at the confluence interface decreased to 0.28MPa, the interfacial shear failure strength of the composite plastic outer layer was 41.2MPa, and the defect rate was 2.1%. In test group B, the transient pulse shear rate was increased to 1350 / s and the relaxation residence time was extended to 8.5s. The absolute value of the pressure difference at the confluence interface further decreased to 0.18MPa, the interfacial shear failure strength increased to 45.8MPa, and the defect rate decreased to 0.4%. In test group C, the transient pulse shear rate was set to 1500 / s and the relaxation residence time was set to 10.0s. The absolute value of the pressure difference at the confluence interface was 0.15MPa, the interfacial shear failure strength was 44.5MPa, and the defect rate was 0.3%.

[0050] In the out-of-range control group, the transient pulse shear rate increased to 1650 / s, and the relaxation residence time was extended to 11.5s. Although the absolute value of the pressure difference at the confluence interface decreased to 0.14MPa, the interfacial shear failure strength decreased to 39.2MPa, and the defect rate rose to 1.8%. The shearing action experienced by the melt in this group exceeded the stable treatment range, and the material underwent irreversible thermomechanical degradation. The mechanical density of the outer layer of the composite plastic and the interfacial bonding state decreased accordingly.

[0051] During the operation of the test group, pressure sensors placed before and after the front pressure regulating chamber were used to monitor the pressure status of the two-phase melt. When the absolute value of the pressure difference at the confluence interface exceeded 0.3 MPa, the internal volume of the front pressure regulating chamber was adjusted to absorb the pressure pulses generated by the feeding fluctuations, so that the absolute value of the pressure difference was stabilized at 0.18 MPa.

[0052] The data from each group show that when the transient pulse shear rate increases from 1200 / s to 1350 / s and the relaxation residence time increases from 6s to 8.5s, the interfacial shear failure intensity increases as the pressure difference and defect rate at the confluence interface decrease. When the transient pulse shear rate continues to increase to 1500 / s and the relaxation residence time is extended to 10s, the pressure difference and defect rate at the confluence interface still decrease, but the interfacial shear failure intensity is already below the peak value. Thus, the transient pulse shear rate of 1200 / s to 1500 / s and the relaxation residence time of 6s to 10s form a stable working range.

[0053] When the transient pulse shear rate is below 1200 / s or the casting time is below 3.0s, the mechanical shear input is insufficient to overcome the long-range non-covalent entanglement barrier between the long chain segments of recycled polyethylene, and the physical entanglement network cannot be fully disintegrated. When the transient pulse shear rate is above 1500 / s or the casting time is longer than 5.0s, the frictional heating and shearing inside the melt intensify, the polymer backbone undergoes irreversible breakage, and the mechanical density and interfacial shear failure strength of the outer layer of the composite plastic decrease accordingly. Therefore, controlling the transient pulse shear rate between 1200 / s and 1500 / s and the casting time between 3.0s and 5.0s allows the polymer molecular chain network to complete disentanglement without excessive thermomechanical degradation. Subsequently, the anisotropic structural stress is released within a relaxation residence time of 6s to 10s, reducing the flow difference between the recycled melt and the virgin plastic melt at the confluence interface.

[0054] Example 3: The current co-extrusion molding system for the outer plastic layer of power cables is equipped with an auxiliary extrusion channel for melting and processing crushed plastic waste. The auxiliary extrusion channel has two heating temperature control zones inside and a pressure sensor at the outlet. The crushed plastic waste is processed by a forced hot air dryer, and the moisture content is controlled to 0.015%. Then it enters the auxiliary extrusion channel. The temperatures of the first and second temperature control zones are set to 180℃ and 195℃ respectively, and the screw speed is set to 280rpm to obtain a homogenized recycled melt with an apparent viscosity of 1500Pa·s.

[0055] The homogenized regenerated melt enters the high-shear flow channel and undergoes transient pulse shearing at a shear rate of 1200 to 1500 s. The output is detangled regenerated melt and locally overheated melt due to shear dissipation. The control unit uses a pressure sensor located on the inner wall of the co-extrusion die to collect melt pressure signals. A sampling period of 0.05 s is used, and every 100 consecutive sampling periods form a statistical window. Within this window, the melt pressure oscillation frequency is identified, and the pressure fluctuation amplitudes corresponding to each oscillation are compiled into a sequence. The standard deviation of this sequence is then calculated. It is used as a standard deviation of the melt pressure oscillation frequency, with units of MPa. When the pressure exceeds the preset reference threshold of 0.02 MPa, the system adjusts the transient pulse shear hold time Δt, and the control unit adjusts accordingly. The variation range is calculated using linear interpolation. ;when At MPa s, when At MPa s, the calculation formula is: ,in, For the transient pulse shearing hold time, the control unit according to The increased flow rate of the cooling circulating water increases the reverse cooling heat flow of the cooling jacket at the high shear channel wall, correcting the temperature distribution caused by continuous shear dissipation in the wall layer within a delay of several seconds to tens of seconds. This counteracts the influence of locally overheated melt on the molecular chain recovery process. The system also collects the apparent viscosity of the homogenized regenerated melt online. When the apparent viscosity is below 1300 Pa·s, the upper limit of the screw speed in the high shear channel is reduced by 10% to limit the shear rate and reduce the thermomechanical degradation of the material.

[0056] The unentangled regenerated melt and the locally overheated melt enter the stress relaxation zone with a nonlinear contraction topology flow channel. The cross-sectional area of ​​the nonlinear contraction topology flow channel is... axial position of the flow channel Change according to the following formula: ,in, Axial position The cross-sectional area at that point, Let be the cross-sectional area at the entrance. The attenuation constant is set to 0.25 per centimeter. The nonlinear contraction topology of the flow channel is configured to reduce the shear rate of the flow field in a gradient manner along the extension direction of the channel. The cross-sectional profile is checked through rheological simulation to allow the detangled regenerated melt to gradually release the anisotropic structural stress. The melt temperature in the stress relaxation section is controlled at 185℃, which is 10℃ lower than the melt temperature of the high-shear channel. The attenuation constant is determined based on the evolution law of the linear relaxation spectrum of recycled polyethylene crushed material at 185℃. When the attenuation constant is lower than 0.10 per centimeter, the channel contraction rate is slower, the flow time of the detangled regenerated melt in the channel is prolonged, and the polymer long chains are more likely to reform disordered entanglement. When the attenuation constant is higher than 0.45 per centimeter, the channel cross-sectional contraction rate is faster, and the downstream extension flow generates additional nonlinear elastic stress. After multiple sets of flow channel rheological simulations and experimental tests, when the attenuation constant is 0.25 per centimeter, the shear field intensity in the channel corresponds to the logarithmic decay trend of the polyethylene long chain relaxation spectrum, which can reduce the elastic memory of the regenerated melt.

[0057] The system acquires in real time the local apparent viscosity change rate of the unentangled regenerated melt flowing through the nonlinear contraction topology channel. The calculation relationship is as follows: ,in, This refers to the change in apparent viscosity at the same measuring point between adjacent sampling times. For the corresponding relaxation time interval, miniature pressure sensors are installed at different axial and radial positions of the nonlinear contraction topology flow channel. The control system calculates the local apparent viscosity change rate of the central flow field and the inner wall flow field based on these sensors. When the local apparent viscosity change rate of the inner wall flow field is greater than that of the central flow field, the control system increases the circulating water flow rate of the cooling jacket in the corresponding area proportionally according to the difference between the two. This reduces the local temperature of the inner wall flow field by 1.5℃ to 3.0℃, slows down the relaxation rate of the molecular chains in the wall layer, and synchronizes the stress relaxation progress of the central flow field and the inner wall flow field. The shear heat dissipation generated by the high shear channel wall layer enters the stress relaxation zone with the melt, providing thermal activation energy for the isotropic recovery of the molecular chains after deentanglement. The cooling jacket simultaneously removes excess local heat and outputs stress-relaxed regenerated melt.

[0058] The stress-relaxed recycled melt and the virgin plastic melt enter the co-extrusion confluence die. A front-end pressure regulating chamber is provided at the inlet of the co-extrusion confluence die. The front-end pressure regulating chamber includes a first pressure regulating chamber communicating with the stress-relaxed recycled melt channel and a second pressure regulating chamber communicating with the virgin plastic melt channel. Both the first and second pressure regulating chambers are equipped with overflow plungers driven by a hydraulic mechanism and movable axially. A distance sensor continuously acquires the displacement change of the plungers. This is used as feedback of the internal volume of the front-end pressure regulating chamber. When the pressure sensor detects a pressure pulse exceeding 0.1 MPa caused by feeding fluctuations, the main control unit sends an electrical signal to the hydraulic drive mechanism, based on... Adjusting the axial position of the overflow plunger in the first or second pressure regulating chamber changes the internal volume of the front pressure regulating cavity with the displacement of the plunger. This adjustment changes the flow channel volume and flow resistance of the two-phase melt, absorbs the high-frequency pressure transients caused by the feeding fluctuations, and regulates the pressure of the two-phase melt before entering the co-extrusion merging die. The absolute value of the pressure difference at the merging interface is stably controlled within 0.2 MPa, so that the interlayer shear tension gradient is kept in a stable range.

[0059] After the two-phase melt merges in the co-extrusion die, it coats the surface of the cable core to form a composite plastic outer layer. The cable core continuously passes through the cooling water tank and is cooled in stages according to the preset temperature drop gradient to solidify the interlayer interface structure. The test results show that the interfacial shear failure strength of the composite plastic outer layer is 47.2 MPa and the defect rate of the finished product is 0.2%.

[0060] Example 4: This example combines Figures 1 to 2 A description of a co-extrusion molding process for the plastic outer layer of cables, such as... Figure 1As shown, step S1 involves heating and melting recycled plastic waste, controlling the flow field shear rate to be 350s to 500s, and outputting a homogenized regenerated melt; step S2 involves introducing the homogenized regenerated melt into a high-shear channel, applying transient pulse shear, controlling the transient pulse shear rate to be 1200s to 1500s and maintaining it for a flow delay of 3s to 5s, and outputting a detangled regenerated melt and a locally overheated melt due to shear dissipation; step S3 involves introducing the detangled regenerated melt and the locally overheated melt into a stress relaxation section equipped with a nonlinear contraction topology flow channel, controlling the melt temperature in the stress relaxation section to be 5°C to 10°C lower than the melt temperature in the high-shear channel. The relaxation residence time of the untangled regenerated melt in the nonlinear contraction topology flow channel is controlled to be 6s to 10s. The melt pressure oscillation frequency at the co-extrusion confluence die is collected by a pressure sensor and the standard deviation of the melt pressure oscillation frequency is calculated. The reverse cooling heat flow of the cooling jacket at the flow channel wall is controlled to increase with the increase of the standard deviation to offset the local overheated melt and output stress-relaxed regenerated melt. In step S4, the stress-relaxed regenerated melt and the virgin plastic melt are introduced into the co-extrusion confluence die. By adjusting the internal volume of the front pressure regulating cavity, the absolute value of the pressure difference between the two phases at the confluence interface is controlled to be less than 0.3MPa. After co-extrusion, they are coated on the surface of the cable core.

[0061] like Figure 2 As shown, recycled plastic waste is heated and melted, and the flow field shear rate is controlled at 350s to 500s to form a homogenized recycled melt. The homogenized recycled melt is introduced into a high-shear channel and subjected to transient pulse shearing. Under the conditions of a flow rate of 1200s to 1500s and a flow delay of 3s to 5s, an unentangled recycled melt and a locally overheated melt are formed. The unentangled recycled melt and the locally overheated melt are introduced into a nonlinear contraction topology channel. After controlling the relaxation residence time to 6s to 10s and using a cooling jacket for reverse cooling, a stress-relaxed recycled melt is output. The stress-relaxed recycled melt is introduced into a co-extrusion confluence die along with the virgin plastic melt. Under the conditions of adjusting the internal volume of the front pressure regulating cavity and controlling the absolute value of the interface pressure difference to be less than 0.3MPa, a co-extruded confluence melt is output. The co-extruded confluence melt is coated on the surface of the cable core and guided to continuously pass through a cooling water tank. After cooling according to a preset temperature drop gradient, the interlayer interface structure is solidified.

[0062] Example 5: When power cable manufacturers use recycled plastic waste to prepare the plastic outer layer of cables, they need to maintain the rheological matching state between the stress-relaxed recycled melt and the virgin plastic melt under varying ambient temperature conditions. In order to correct the extrusion pressure drift caused by ambient temperature fluctuations, the co-extrusion molding system performs offline calibration before being put into operation.

[0063] With the machine stopped, recycled polyethylene crushed material samples with melt flow indexes of 0.8 g / 10 min, 1.2 g / 10 min, and 1.5 g / 10 min were selected and placed in the auxiliary extrusion channel. By adjusting the temperature settings and screw speed of each zone in the auxiliary extrusion channel, the corresponding melt pressure and apparent viscosity data were collected to construct the apparent viscosity. With screw speed Functional relationship: ,in, For apparent viscosity, Zero shear viscosity The screw speed is... The viscosity-speed sensitivity coefficient is used to determine the screw speed working window for different material batches, so that the apparent viscosity of the homogenized regenerated melt output from the auxiliary extrusion channel is maintained between 1500 Pa·s and 1800 Pa·s.

[0064] The viscosity-speed sensitivity coefficient reflects the degree of nonlinear shear thinning of recycled plastic melt in a twin-screw shear flow field and is related to the flow activation energy and deentanglement sensitivity of polymer chain segments. Before actual production, online rheological calibration of the current batch of recycled plastic crushed material samples is performed using a twin-screw compressor. At the standard production temperature, the screw speed is set to 100 rpm, 200 rpm, and 300 rpm in sequence. The apparent viscosity of the melt corresponding to each speed is simultaneously measured using an online rheometer. Then, each set of apparent viscosity and screw speed is substituted into the above-mentioned exponential decay function, and nonlinear data fitting is performed using the least squares method to obtain the viscosity-speed sensitivity coefficient of the current batch of material.

[0065] During production, the control system performs dynamic compensation based on real-time pressure conditions, and the pressure sensor at the extruder outlet acquires the real-time melt pressure. And compare it with the preset pressure obtained from offline calibration. The pressure deviation is calculated using the following formula for comparison: in, For pressure deviation, when When the absolute value exceeds the preset range of 0.05 MPa for five consecutive sampling cycles, the control unit fine-tunes the screw speed of the auxiliary extrusion channel according to the PID control strategy, changes the melt shear rate, and corrects the apparent viscosity of the homogenized regenerated melt online, so that... Returning to the preset pressure range, the control system simultaneously collects the apparent viscosity of the homogenized regenerated melt online; when the apparent viscosity is lower than the preset viscosity threshold, the upper limit of the screw speed in the high-shear channel is reduced to limit the shear rate of transient pulse shearing.

[0066] A pressure regulating cavity is installed at the inlet of the co-extrusion confluence die, and a distance sensor acquires the plunger displacement change in real time. When the pressure sensor detects a pressure pulse exceeding 0.1 MPa caused by fluctuations in the feed, the system drives the plunger mechanism to operate, and according to... Adjusting the plunger position changes the internal volume of the front pressure regulating chamber, absorbing the high-frequency pressure transients between the virgin plastic melt and the stress-relaxed regenerated melt, keeping the absolute value of the pressure difference between the two phases at the confluence interface within 0.3 MPa, thus maintaining the structural density of the composite plastic outer layer during long-term continuous production.

[0067] Example 6: This example addresses the problem of uneven and fluctuating viscoelastic rheological behavior of recycled melt caused by heterogeneous components in plastic waste. It combines offline rheological calibration with online adjustment to establish a baseline library of rheological parameters, and adjusts the front-end pressure regulating chamber according to the melt pressure state during continuous co-extrusion.

[0068] In the initial stage of the experiment, plastic waste components with melt flow indices of 0.5 g / 10 min, 1.0 g / 10 min, and 2.0 g / 10 min were selected for rheological calibration on a twin-screw extrusion test bench. Dynamic viscosity and storage modulus data were collected at shear rates ranging from 100 s to 2000 s. The collected data were then subjected to regression processing to obtain the stress relaxation time constant. With melt temperature Functional relationship: ,in, The stress relaxation time constant is Reference temperature The stress relaxation time constant under the given conditions For flow activation energy, Let be the ideal gas constant. The melting temperature is... As a reference temperature, the functional relationship and corresponding parameters obtained from the regression are entered into the rheological parameter baseline library. This is used to determine the relaxation response of the stress relaxation section based on the melt temperature, and to verify the relaxation residence time of the unentangled regenerated melt after transient pulse shearing.

[0069] During continuous extrusion, the system collects the melt pressure oscillation frequency at the inner wall of the co-extrusion confluence die in real time and obtains the standard deviation of the melt pressure oscillation frequency as a characterization quantity. ,when When the pressure exceeds a preset threshold of 0.03 MPa, the system initiates volume adjustment of the front-end pressure regulating chamber, and the control system adjusts accordingly. Calculate the piston displacement adjustment amount : ,in, This is the adjustment amount for the plunger displacement. This is the proportional feedback coefficient. For integral feedback coefficients, The time for integration.

[0070] The control system calculates the product of the absolute value of the plunger displacement adjustment and the directional correction coefficient to determine the plunger's movement direction and adjustment range. The sign of the directional correction coefficient is determined by a differential pressure sensor located at the inlet of the two-phase melt channel before the merging. When the pressure of the stress-relaxed regenerated melt is greater than the pressure of the virgin plastic melt, the directional correction coefficient is positive, driving the plunger to move outward and increasing the internal volume of the front-end pressure regulating chamber. When the pressure of the stress-relaxed regenerated melt is less than the pressure of the virgin plastic melt, the directional correction coefficient is negative, driving the plunger to move inward and decreasing the internal volume of the front-end pressure regulating chamber.

[0071] During the piston's movement, the ranging sensor acquires the piston displacement change in real time. The control system is based on The feedback adjustment plunger position ensures that the internal volume of the front-end pressure regulating chamber reaches the corresponding adjustment amount. This adjustment absorbs the pressure pulses of the two-phase melt and stabilizes the absolute value of the pressure difference at the confluence interface at 0.15MPa. After long-term continuous production testing, the interfacial shear failure strength of the outer plastic layer of the cable remains above 45MPa, and the interlayer peeling defect rate is controlled below 0.1%.

[0072] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A co-extrusion molding process for the plastic outer layer of cables, characterized in that, Includes the following steps: Step S1: Heat and melt the recycled plastic waste, control the flow field shear rate to 350 / s to 500 / s, and output homogenized recycled melt; Step S2: The homogenized regenerated melt is introduced into the high-shear flow channel, transient pulse shear is applied, the shear rate of the transient pulse shear is controlled to be 1200 / s to 1500 / s and maintained for 3s to 5s of flow delay, and the untangled regenerated melt and the locally overheated melt generated by shear dissipation are output. Step S3: The detangled regenerated melt and the locally overheated melt are introduced into the stress relaxation section with a nonlinear contraction topology flow channel. The melt temperature in the stress relaxation section is controlled to be 5°C to 10°C lower than the melt temperature in the high shear flow channel. The relaxation residence time of the detangled regenerated melt in the nonlinear contraction topology flow channel is controlled to be 6s to 10s. The melt pressure oscillation frequency at the co-extrusion die head is collected using a pressure sensor and the standard deviation of the melt pressure oscillation frequency is calculated. The reverse cooling heat flow of the cooling jacket at the flow channel wall is controlled to increase with the increase of the standard deviation to offset the locally overheated melt and output the stress-relaxed regenerated melt. Step S4: Stress-relaxed recycled melt and virgin plastic melt are introduced into the co-extrusion confluence die. By adjusting the internal volume of the front pressure regulating cavity, the absolute value of the pressure difference between the two phases at the confluence interface is controlled to be less than 0.3 MPa. After co-extrusion, they are coated onto the surface of the cable core.

2. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, Before heating and melting the recycled plastic waste in step S1, the following sub-steps are included: Step S11, using a forced hot air dryer to remove the free moisture from the recycled plastic waste, controlling the overall moisture content of the recycled plastic waste to be below 0.02%, and inhibiting the high-temperature hydrolysis reaction.

3. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, Step S1 includes the following sub-steps: Step S12, controlling the homogenized regenerated melt to flow through a continuous automatic screen-changing filter to intercept heterogeneous cross-linked substances and solid particulate impurities mixed in the homogenized regenerated melt.

4. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, Step S2 includes the following sub-steps: Step S21, the apparent viscosity of the homogenized regenerated melt is collected online. When the apparent viscosity is lower than the preset viscosity threshold, the upper limit of the screw speed of the high shear channel is reduced to limit the shear rate.

5. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, Step S3 includes the following sub-steps: Step S31, controlling the cross-sectional area of ​​the nonlinear contraction topology channel to gradually decrease from the high shear channel to the stress relaxation section, controlling the flow field shear rate to decrease in a gradient along the channel extension direction, and releasing the anisotropic structural stress of the unentangled regenerated melt.

6. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, Step S3 includes the following sub-steps: Step S32, online acquisition of the local apparent viscosity change rate of the untangled regenerated melt flowing through the nonlinear contraction topology channel, adjustment of the inner wall temperature distribution of the nonlinear contraction topology channel according to the local apparent viscosity change rate, and control of the stress relaxation progress of the central flow field and the inner wall flow field of the nonlinear contraction topology channel to be synchronized.

7. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, In step S4, the internal volume of the front pressure regulating chamber is adjusted, including the following sub-steps: Step S41, the displacement change of the plunger inside the front pressure regulating chamber is obtained by the ranging sensor, and the internal volume of the front pressure regulating chamber is changed according to the displacement change of the plunger to absorb the high-frequency pressure transients caused by the feeding fluctuations and maintain the interlayer shear tension gradient at the confluence interface in a stable range.

8. The co-extrusion molding process for the plastic outer layer of cables according to claim 1, characterized in that, After being co-extruded and coated onto the surface of the cable core in step S4, the following sub-steps are included: Step S42, the cable core coated with the virgin plastic melt and stress-relaxed regenerated melt is guided to continuously pass through the cooling water tank, and the coating layer outside the cable core is cooled according to the preset temperature drop gradient to solidify the interlayer interface structure.

9. The co-extrusion molding process for the plastic outer layer of cables according to claim 8, characterized in that, Step S42 includes the following sub-steps: Step S421, monitor the water temperature of multiple sections of the cooling water tank, control the water temperature to decrease gradually along the direction of the cable core wire, and eliminate the internal stress of cold shrinkage inside the sheathing layer.

Images