Plastic bottle multi-layer continuous forming process

Abstract

The invention discloses a plastic bottle multi-layer continuous forming process and relates to the technical field of plastic processing. According to the process, continuous production is achieved through the steps of raw material pretreatment, multi-layer co-extrusion blank making, pipe blank preheating, stretching, blowing and forming, gradient cooling and sizing and continuous rolling. The design of a dynamic co-extrusion die head and a variable cross-section spiral runner is adopted, so that the wall thickness uniformity deviation of a pipe blank is smaller than or equal to + / -0.03 mm; the preheating temperature difference is controlled to be smaller than or equal to 10 DEG C through the four-temperature-zone annular heating cavity and the graphene heating film; and a double-station rotary blowing device and time-pressure double closed-loop control are adopted, so that the wall thickness deviation of the bottle body is less than or equal to + / -0.1 mm. The process can realize continuous packaging production with the rolling diameter of less than or equal to 1.5 m, and is suitable for the field of food and medicine packaging.

Description

Technical Field

[0001] This invention belongs to the field of plastic processing technology, and specifically relates to a multi-layer continuous molding process for plastic bottles. Background Technology

[0002] Multilayer molding technology for plastic bottles has been widely used in the packaging industry in recent years, especially in fields requiring high barrier properties, chemical resistance, or special optical properties. Traditional multilayer bottle production processes typically employ a two-step injection-blow molding process or intermittent co-extrusion blow molding technology. The injection-blow molding process requires first creating a preform and then performing a second heating and blow molding, which suffers from high energy consumption, long production cycles, and a tendency for delamination at the joints. While intermittent co-extrusion blow molding technology can achieve multilayer structures, its production efficiency is low, and the unstable melt flow results in poor uniformity of interlayer thickness, making it difficult to meet the demands of high-end packaging.

[0003] In continuous production, existing technologies mostly employ single-layer or double-layer co-extrusion blow molding processes. For example, a parallel extruder with a simple die is used to produce a tube blank, which is then preheated in an infrared heating furnace before being blown into shape. However, due to the significant differences in the rheological properties of multilayer melts, the simple die structure easily leads to instability at the interlayer interfaces, resulting in eddies or delamination. In addition, traditional preheating methods often use single- or dual-temperature zone heating, making it difficult to achieve uniform temperature inside and outside the tube blank, which can easily lead to localized cracking or uneven thickness during stretching and blow molding.

[0004] In existing technologies, cooling processes often employ single-mode water or air cooling. The mismatch between cooling rate and shrinkage rate can lead to bottle deformation or residual stress concentration. The winding process lacks coordinated tension and correction control, making continuous winding prone to wrinkles or uneven edges. Therefore, it is necessary to develop a continuous forming process capable of achieving high-precision layer thickness control, uniform temperature regulation, optimized cooling gradient, and stable winding. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide a multi-layer continuous molding process for plastic bottles, which solves the problems of poor uniformity of wall thickness of multi-layer tube blanks, uneven distribution of preheating temperature field leading to high bottle breakage rate, uneven cooling shrinkage causing warping deformation, and insufficient continuous winding accuracy in the prior art.

[0006] To address the above problems, the present invention provides the following technical solution:

[0007] A multi-layer continuous molding process for plastic bottles includes the following steps:

[0008] S1 Raw material pretreatment: The inner layer raw material, middle layer raw material, and outer layer raw material are respectively fed into independent drying units. The drying units adopt a hot air circulation system with a hot air temperature of 80℃ to 120℃ and an air velocity controlled at 1.5m / s to 2.5m / s. The drying time is 2 hours to 4 hours, so that the moisture content of the raw materials is ≤0.02%. The dried raw materials are respectively sent into the corresponding temporary storage bins. The bottom of the bins is equipped with a screw metering feeder, and the feeding accuracy is controlled within ±0.5%.

[0009] S2 Multilayer Co-extrusion Preformation: The three layers of raw material after pretreatment in step S1 are fed into three two-stage extruders. The first stage extruder is a single-screw extruder with a screw diameter of 60mm, an L / D ratio of 32:1, and a rotation speed of 80-100 rpm, used for raw material melting and preliminary mixing. The second stage extruder is a twin-screw extruder with a screw diameter of 75mm, an L / D ratio of 40:1, and a rotation speed of 100-120 rpm, used for melt homogenization and venting. The third stage extruder is a planetary gear extruder with a screw diameter of 90mm, an L / D ratio of 48:1, and a rotation speed of 120-150 rpm, used for high-pressure conveying of high-viscosity melts. The melts from the three extruders are then further processed... After being pressurized by the bulk pump, the melt enters the dynamic co-extrusion die. The dynamic co-extrusion die is equipped with three layers of variable cross-section spiral flow channels. The cross-section of the flow channel gradually changes from a rectangle with a length of 12mm and a width of 8mm at the inlet end to a trapezoid with an upper bottom of 10mm, a lower bottom of 14mm, and a height of 18mm at the outlet end. The spiral angle of the spiral flow channel is 15° to 20°. The die temperature is controlled in zones: 200° to 220° at the inlet section, 230° to 250° at the middle section, and 240° to 260° at the outlet section. The melt flows into the die to form a three-layer tube blank. The uniformity deviation of the tube blank wall thickness is ≤ ±0.03mm, and the outer diameter ranges from 50mm to 100mm. It is continuously output at a linear velocity of 2m / min to 5m / min.

[0010] S3. Preheating of the tube blank: The tube blank continuously output from step S2 enters the annular heating chamber. The annular heating chamber is divided into four temperature zones along the conveying direction: the first temperature zone, with a feed end temperature of 80℃ to 100℃; the second temperature zone, a transition zone temperature of 120℃ to 140℃; the third temperature zone, the main heating zone temperature of 160℃ to 180℃; and the fourth temperature zone, the heat preservation zone temperature of 140℃ to 160℃. A graphene heating film is installed on the inner wall of the heating chamber, with a power density of 60W / cm² to 90W / cm². The distance between the tube blank and the heating film is 60mm to 90mm. The tube blank conveying speed is synchronized with the tube blank output speed in step S2. The heating time is 20 seconds to 30 seconds. The surface temperature of the tube blank is monitored in real time by an infrared thermal imager, and the heating power of each temperature zone is adjusted accordingly to ensure that the temperature difference between the inside and outside of the tube blank is ≤10℃.

[0011] S4 Stretch Blow Molding: The preheated tube blank from step S3 enters the stretch blow molding station, employing a dual-station rotary blow molding device, including a mandrel stretching unit and a high-pressure blow molding unit. The mandrel surface of the mandrel stretching unit is polished to Ra≤0.1μm, and chilled brine at -5℃ to 5℃ is introduced internally at a flow rate of 2L / min to 4L / min. The mandrel is axially inserted into the tube blank at a speed of 8m / min to 12m / min, with a stretching ratio of 3.0 to 4.0 times and a stretching time of 1 second to... 2 seconds; The high-pressure inflation unit synchronously injects compressed air into the stretched tube blank. The inflation process adopts time-pressure dual closed-loop control. In the initial inflation stage, from 0 seconds to 3 seconds, the pressure linearly increases from 0.5MPa to 1.5MPa. In the intermediate pressure holding stage, from 3 seconds to 8 seconds, the pressure is maintained at 1.2MPa. In the later pressure relief stage, from 8 seconds to 12 seconds, the pressure linearly decreases from 1.2MPa to 0.3MPa. After inflation is completed, the tube blank expands into a bottle body, and the uniformity deviation of the bottle body wall thickness is ≤±0.1mm.

[0012] S5 Gradient cooling and shaping: After inflation in step S4, the bottle enters a cooling tunnel containing three gradient cooling units. The first unit, in the high-temperature zone, uses water mist spray at 8°C to 12°C, with a spray angle of 70° to 90°, a particle size of 20μm to 60μm, and an action time of 10 to 15 seconds, reducing the bottle surface temperature to 60°C to 70°C. The second unit, in the medium-temperature zone, uses water mist spray at 4°C to 6°C, with a spray angle of 60° to 80°, a particle size of 10μm to 40μm, and an action time of 8 to 12 seconds, reducing the surface temperature to 40°C to 50°C. The third unit, in the low-temperature zone, uses cold water spray at 0°C to 2°C, with a spray angle of 50° to 70°, a particle size of 5μm to 30μm, and an action time of 5 to 8 seconds, reducing the surface temperature to 25°C to 35°C. During cooling, circulating air at 10°C to 15°C is introduced into the bottle at a speed of 0.5m / s to 1.5m / s to aid in balancing the internal and external temperature differences.

[0013] S6 Continuous winding: After cooling and shaping in step S5, the bottle body is conveyed to the winding station via a star-shaped dial wheel. The winding device is set with dual-station alternating winding. The surface of the winding roller is covered with a polyurethane elastic layer with a Shore A hardness of 60 to 70. The winding process adopts a tension-vision dual feedback control system. The tension sensor monitors the winding tension in real time, with a set range of 8N to 18N. When the tension deviation is ≥±2N, the speed of the winding roller is adjusted by the servo motor. The vision inspection camera simultaneously monitors the lateral offset of the bottle body. When the offset is ≥±3mm, the lateral correction cylinder is triggered to adjust the position of the conveyor belt to ensure that the alignment error of the winding edge is ≤±1mm. Finally, a continuous roll of plastic bottle with a roll diameter of ≤1.5m is formed.

[0014] In the aforementioned continuous molding process, preferably, in step S1, the inner layer material is food-grade low-density polyethylene (LDPE), the middle layer material is barrier polyamide (PA), and the outer layer material is UV-resistant polycarbonate (PC). In this invention, LDPE (inner layer) provides good flexibility and food contact safety; PA (middle layer) provides excellent gas barrier properties (such as oxygen and moisture barrier), which is the core function of multilayer bottles; and PC (outer layer) provides high strength, high rigidity, and UV aging resistance, protecting the inner layer.

[0015] In the aforementioned continuous molding process, preferably, the inner wall of the spiral flow channel of the dynamic co-extrusion die in step S2 is provided with micron-level grooves, with a depth of 0.5 μm to 1.5 μm and a spacing of 2 μm to 5 μm. The groove direction is consistent with the spiral direction of the spiral flow channel, which is used to enhance the shear flow of the melt. In this invention, the micron-level grooves can induce the melt to generate directional and enhanced shear flow, thereby effectively disrupting the laminar flow state near the melt interface, promoting the mutual diffusion and entanglement of different melt layers at the molecular level, improving the interlayer adhesion strength, and preventing delamination during subsequent processing or use, which is key to ensuring product quality.

[0016] In the aforementioned continuous forming process, preferably, in step S2, an online wall thickness detector with an accuracy of ±0.005mm is installed at the exit end of the dynamic co-extrusion die. The detection data is fed back to the die flow channel adjustment system in real time, and the opening of each flow channel is adjusted by an electric servo valve to dynamically compensate for wall thickness deviations. This invention achieves dynamic thickness control by monitoring the billet wall thickness and adjusting the die in real time via a servo valve.

[0017] In the aforementioned continuous molding process, preferably, a ceramic fiber insulation layer with a thickness of 5 mm to 10 mm is disposed above the graphene heating film in the annular heating cavity in step S3, and the surface temperature of the insulation layer is ≤50℃, thereby reducing heat loss to the external environment. The ceramic fiber insulation layer of this invention can, on the one hand, reflect and retain heat within the heating cavity, improving thermal efficiency; on the other hand, it helps maintain a stable and uniform temperature field within the cavity, avoiding interference from external environmental fluctuations on the preheating process.

[0018] In the aforementioned continuous forming process, preferably, the temperature zone division of the annular heating chamber in step S3 is adaptively adjusted according to the outer diameter of the tube blank. When the outer diameter of the tube blank increases by 10mm, the overall temperature of each temperature zone decreases by 5℃ to 8℃, ensuring the heating uniformity of tube blanks of different specifications. This invention, through the adaptive adjustment function of the heating system parameters, enables the equipment to quickly and accurately set the preheating temperature when switching production specifications, ensuring the stability and uniformity of the heating effect for products of different specifications.

[0019] In the aforementioned continuous forming process, preferably, a temperature sensor with an accuracy of ±0.5℃ is installed inside the mandrel of the mandrel stretching unit in step S4 to provide real-time feedback on the temperature of the chilled brine. The brine flow rate is adjusted by a PID controller to ensure that the mandrel surface temperature fluctuation is ≤±1℃. The stability of the mandrel temperature is crucial in this invention. Excessive temperature may cause melt adhesion, while excessively low temperature may cause premature cooling leading to tensile fracture. Through real-time feedback control using a high-precision sensor and PID algorithm, the mandrel surface temperature is precisely maintained within an extremely narrow range, thereby ensuring highly consistent process conditions for each stretching operation.

[0020] In the aforementioned continuous molding process, preferably, the compressed air in the high-pressure blowing unit in step S4 is treated by a three-stage filtration system: the first-stage filtration accuracy is 5μm, the second-stage filtration accuracy is 1μm, and the third-stage filtration accuracy is 0.01μm; the dew point temperature is ≤-50℃, and the oil content is ≤0.01mg / m³. The multi-stage precision filtration and deep drying (low dew point) in this invention ensure the high cleanliness of the blowing medium.

[0021] In the aforementioned continuous molding process, preferably, the water mist spraying system of the gradient cooling unit in step S5 adopts a pulse spraying mode with a pulse frequency of 5Hz to 10Hz and a duty cycle of 30% to 50%, reducing the amount of water mist residue on the bottle surface. In this invention, pulse spraying can provide the same total cooling capacity intermittently, but gives surface water droplets more time to flow and evaporate, thus effectively reducing surface residue. In the aforementioned continuous molding process, preferably, the star-shaped derailleur of the winding device in step S6 is equipped with an elastic derailleur made of nitrile rubber with a Shore A hardness of 50 to 60. The contact pressure between the derailleur and the bottle is controlled at 0.1N to 0.3N to avoid scratching the bottle surface. In this invention, hard derailleurs or excessive pressure can cause indentations, scratches, or friction marks on the soft plastic bottle surface, affecting the product appearance and even causing stress concentration. Using soft, elastic materials and precisely controlling the contact pressure can provide surface protection while reliably conveying the bottle.

[0022] The continuous molding process of this invention employs a dynamic co-extrusion die with a variable cross-section spiral flow channel and micron-level grooves. By optimizing the flow channel geometry and surface characteristics, it guides the melt to flow stably in layers, reducing interlayer disturbances and ensuring the uniformity of the tube blank's layer thickness and the strength of the interface bonding from the source. A ring-shaped heating chamber with precise temperature control in zones, combined with real-time infrared monitoring, achieves uniform regulation of the axial and radial temperature fields of the tube blank, providing uniform material conditions for subsequent stretching and blowing. A time-pressure dual closed-loop control blowing strategy, coordinated with low-temperature mandrel stretching, precisely manages the orientation and crystallization process of the polymer chains, ensuring the bottle's dimensional accuracy and morphological stability. A pulsed gradient cooling system, by controlling the cooling rate and directionality, reduces internal stress and deformation caused by non-uniform shrinkage. Finally, a tension-visual dual feedback winding system maintains the tension stability and edge alignment of the continuous roll. Through the precise design and coupling of control parameters at each stage, the entire process ultimately achieves the continuous and stable molding of high-quality multilayer plastic bottles.

[0023] Compared with the prior art, the advantages of the present invention are as follows:

[0024] (1) The present invention enhances melt shear flow and reduces interlayer interface disturbance by using a dynamic co-extrusion die head with a variable cross-section spiral flow channel design and a micron-level groove structure.

[0025] (2) The present invention uses a combination of a four-temperature zone annular heating cavity and a graphene heating film, combined with real-time feedback from an infrared thermal imager, to make the temperature difference between the inside and outside of the tube blank ≤10℃;

[0026] (3) The present invention controls the inflation process through a time-pressure dual closed loop and, in conjunction with low-temperature mandrel stretching, avoids bottle shrinkage or deformation;

[0027] (4) The gradient cooling system of the present invention, combined with pulsed water mist spraying, reduces residual water marks on the surface;

[0028] (5) The dual-station winding and tension-vision dual feedback control of the present invention achieves an edge alignment error of ≤±1mm;

[0029] (6) The present invention can be adapted to the production of tube blanks with an outer diameter of 50mm to 100mm by adaptive adjustment of the die flow channel and adjustment of temperature zone parameters. Detailed Implementation

[0030] To facilitate understanding of the present invention, the invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0031] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0032] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0033] Example 1

[0034] The multi-layer continuous molding process for plastic bottles in this embodiment includes the following steps:

[0035] S1 Raw material pretreatment: Take 200 kg of inner layer raw material low-density polyethylene (LDPE), 150 kg of middle layer raw material polyamide (PA), and 180 kg of outer layer raw material polycarbonate (PC), and put the three raw materials into independent hot air circulating drying units. Set the hot air temperature to 100℃, the air velocity to 2.0 m / s, and the drying time to 3 hours. After drying, the moisture content of the raw materials is reduced to 0.018%. Then, each raw material is transported to its corresponding temporary storage bin and accurately fed at a rate of 50 kg per hour through the screw metering feeder at the bottom of the bin.

[0036] The S2 multi-layer co-extrusion process involves feeding the pretreated three-layer raw materials into three two-stage extruders: a first-stage single-screw extruder with a screw diameter of 60 mm and a rotation speed of 90 rpm; a second-stage twin-screw extruder with a screw diameter of 75 mm and a rotation speed of 110 rpm; and a third-stage planetary gear extruder with a screw diameter of 90 mm and a rotation speed of 135 rpm. The melt, pressurized by a melt pump, is then conveyed to a dynamic co-extrusion die. The die inlet temperature is set at 220℃, the middle section at 240℃, and the outlet temperature at 255℃. The melt flows through an internal variable cross-section spiral channel with a spiral angle of 18°, forming a three-layer tube blank with an outer diameter of 60 mm. This blank is continuously output at a linear velocity of 3.5 m / min, and the online wall thickness gauge shows a wall thickness uniformity deviation of ±0.028 mm.

[0037] S3 billet preheating: The continuously output billet is introduced into the annular heating chamber. The temperature of the four temperature zones of the heating chamber is set as follows: Zone 1: 90℃, Zone 2: 130℃, Zone 3: 170℃, and Zone 4: 155℃. The billet passes through the heating chamber at a speed of 3.5 m / min and the heating time is 25 seconds. The maximum temperature difference between the inside and outside of the billet is 8℃, as monitored by an infrared thermal imager.

[0038] S4 stretch blow molding: The preheated tube blank enters a dual-station rotary blow molding device. The mandrel stretching mechanism polishes the surface to Ra 0.1μm and passes through a mandrel with 3℃ chilled brine at a flow rate of 3 L / min. Axial stretching is performed at a speed of 10 m / min, with a stretching ratio of 3.2 times. Simultaneously, the high-pressure blow molding mechanism injects compressed air according to a time-pressure dual closed-loop control program. The pressure linearly increases from 0.5MPa to 1.5MPa in 0-3 seconds, maintains 1.2MPa in 3-8 seconds, and linearly decreases to 0.3MPa in 8-12 seconds. After blow molding, the bottle body is obtained.

[0039] The S5 gradient cooling and shaping process involves sending the molded bottle into a cooling tunnel. The first cooling zone uses 10°C water mist spray for 12 seconds to reduce the bottle surface temperature to 65°C. The second cooling zone uses 5°C water mist spray for 10 seconds to reduce the temperature to 45°C. The third cooling zone uses 1°C cold water spray for 6 seconds to finally reduce the surface temperature to 30°C. During the cooling process, 12°C circulating air is introduced into the bottle at a speed of 1.0 m / s.

[0040] The S6 continuously winds up the cooled bottle body, which is then conveyed to the winding device via a star-shaped dial. The winding tension is controlled by a sensor and maintained at 12N. The visual correction system ensures that the edge alignment error is less than ±1mm, ultimately resulting in a continuous roll of plastic bottles with a roll diameter of 1.2m.

[0041] Testing revealed that the multi-layer continuous molding process for plastic bottles in this embodiment produced a three-layer plastic bottle with strong interlayer bonding, an average peel strength of 28.5 N / cm, uniform bottle wall thickness, and an overall deviation controlled within ±0.028 mm. This demonstrates the advantages of the dynamic die head spiral flow channel design. The product gloss is 89 GU, and the production energy consumption is low, with a total energy consumption of 0.35 kWh / kg for pretreatment and preheating, achieving a balance between high quality and low energy consumption.

[0042] Example 2

[0043] The multi-layer continuous molding process for plastic bottles in this embodiment includes the following steps:

[0044] S1 Raw material pretreatment: Take 200 kg of inner layer raw material low-density polyethylene (LDPE), 150 kg of middle layer raw material polyamide (PA), and 180 kg of outer layer raw material polycarbonate (PC), and put the three raw materials into independent hot air circulating drying units. Set the hot air temperature to 100℃, the air velocity to 2.0 m / s, and the drying time to 3 hours. After drying, the moisture content of the raw materials is reduced to 0.018%. Then, each raw material is transported to its corresponding temporary storage bin and accurately fed at a rate of 50 kg per hour through the screw metering feeder at the bottom of the bin.

[0045] The S2 multi-layer co-extrusion process involves feeding the pretreated three-layer raw materials into three two-stage extruders: a first-stage single-screw extruder with a screw diameter of 60 mm and a rotation speed of 90 rpm; a second-stage twin-screw extruder with a screw diameter of 75 mm and a rotation speed of 110 rpm; and a third-stage planetary gear extruder with a screw diameter of 90 mm and a rotation speed of 135 rpm. The melt, pressurized by a melt pump, is then conveyed to a dynamic co-extrusion die. The die inlet temperature is set at 220℃, the middle section at 240℃, and the outlet temperature at 255℃. The melt flows through an internal variable cross-section spiral channel with a spiral angle of 18°, forming a three-layer tube blank with an outer diameter of 60 mm. This blank is continuously output at a linear velocity of 3.5 m / min, and the online wall thickness gauge shows a wall thickness uniformity deviation of ±0.028 mm.

[0046] S3 billet preheating: The continuously output billet is introduced into the annular heating chamber. The temperature of the four temperature zones of the heating chamber is set as follows: Zone 1: 85℃, Zone 2: 125℃, Zone 3: 170℃, and Zone 4: 150℃. The billet passes through the heating chamber at a speed of 3.5 m / min and the heating time is 25 seconds. The maximum temperature difference between the inside and outside of the billet is 7℃, as monitored by an infrared thermal imager.

[0047] S4 stretch blow molding: The preheated tube blank enters a dual-station rotary blow molding device. The mandrel stretching mechanism polishes the surface to Ra 0.1μm and passes through a mandrel with 3℃ chilled brine at a flow rate of 3 L / min. Axial stretching is performed at a speed of 10 m / min, with a stretching ratio of 3.2 times. Simultaneously, the high-pressure blow molding mechanism injects compressed air according to a time-pressure dual closed-loop control program. The pressure linearly increases from 0.5MPa to 1.5MPa in 0-3 seconds, maintains 1.2MPa in 3-8 seconds, and linearly decreases to 0.3MPa in 8-12 seconds. After blow molding, the bottle body is obtained.

[0048] The S5 gradient cooling and shaping process involves sending the molded bottle into a cooling tunnel. The first cooling zone uses 10°C water mist spray for 12 seconds to reduce the bottle surface temperature to 65°C. The second cooling zone uses 5°C water mist spray for 10 seconds to reduce the temperature to 45°C. The third cooling zone uses 1°C cold water spray for 6 seconds to finally reduce the surface temperature to 30°C. During the cooling process, 12°C circulating air is introduced into the bottle at a speed of 1.0 m / s.

[0049] The S6 continuously winds up the cooled bottle body, which is then conveyed to the winding device via a star-shaped dial. The winding tension is controlled by a sensor and maintained at 12N. The visual correction system ensures that the edge alignment error is less than ±1mm, ultimately resulting in a continuous roll of plastic bottles with a roll diameter of 1.2m.

[0050] Testing revealed that the multi-layer continuous molding process for plastic bottles in this embodiment, by optimizing the temperature zone setting of the annular heating chamber, further reduced production energy consumption to 0.35 kWh / kg while ensuring sufficient preheating of the tube blank. The tube blank exhibited a uniform temperature distribution during preheating, with a maximum internal and external temperature difference of only 7°C. The bottle body was neatly rolled up without wrinkles, demonstrating the stability of the entire process control.

[0051] Example 3

[0052] The multi-layer continuous molding process for plastic bottles in this embodiment includes the following steps:

[0053] S1 Raw material pretreatment: Take 200 kg of inner layer raw material high-density polyethylene (HDPE), 160 kg of middle layer raw material ethylene-vinyl alcohol copolymer (EVOH), and 190 kg of outer layer raw material polypropylene (PP). Put the three raw materials into independent hot air circulating drying units. Set the hot air temperature to 100℃, the air velocity to 2.0 m / s, and the drying time to 3 hours. After drying, the moisture content of the raw materials will be reduced to 0.018%. Then, each raw material will be transported to its corresponding temporary storage bin and accurately fed at a rate of 50 kg per hour through the screw metering feeder at the bottom of the bin.

[0054] The S2 multi-layer co-extrusion process involves feeding the pretreated three-layer raw materials into three two-stage extruders: a first-stage single-screw extruder with a screw diameter of 60 mm and a rotation speed of 90 rpm; a second-stage twin-screw extruder with a screw diameter of 75 mm and a rotation speed of 110 rpm; and a third-stage planetary gear extruder with a screw diameter of 90 mm and a rotation speed of 135 rpm. The melt, pressurized by a melt pump, is then conveyed to a dynamic co-extrusion die. The die inlet temperature is set at 220℃, the middle section at 240℃, and the outlet temperature at 260℃. The melt flows through an internal variable cross-section spiral channel with a spiral angle of 18°, forming a three-layer tube blank with an outer diameter of 80 / 100 mm. This blank is continuously output at a linear velocity of 3.0 m / min, and the online wall thickness gauge shows a wall thickness uniformity deviation of ±0.028 mm.

[0055] S3 billet preheating: The continuously output billet is introduced into the annular heating chamber. The temperature of the four temperature zones of the heating chamber is set as follows: Zone 1: 90℃, Zone 2: 130℃, Zone 3: 170℃, and Zone 4: 155℃. The billet passes through the heating chamber at a speed of 3.5 m / min and the heating time is 25 seconds. The maximum temperature difference between the inside and outside of the billet is 8℃, as monitored by an infrared thermal imager.

[0056] S4 stretch blow molding: The preheated tube blank enters a dual-station rotary blow molding device. The mandrel stretching mechanism polishes the surface to Ra 0.1μm and passes through a mandrel with 3℃ chilled brine at a flow rate of 3 L / min. Axial stretching is performed at a speed of 9 m / min, with a stretching ratio of 3.5 times. Simultaneously, the high-pressure blow molding mechanism injects compressed air according to a time-pressure dual closed-loop control program. The pressure linearly increases from 0.5MPa to 1.5MPa in 0-3 seconds, maintains 1.2MPa in 3-10 seconds, and linearly decreases to 0.3MPa in 10-12 seconds. After blow molding, the bottle body is obtained.

[0057] The S5 gradient cooling and shaping process involves sending the molded bottle into a cooling tunnel. The first cooling zone uses 10°C water mist spray for 12 seconds to reduce the bottle surface temperature to 65°C. The second cooling zone uses 5°C water mist spray for 10 seconds to reduce the temperature to 45°C. The third cooling zone uses 1°C cold water spray for 6 seconds to finally reduce the surface temperature to 30°C. During the cooling process, 12°C circulating air is introduced into the bottle at a speed of 1.0 m / s.

[0058] The S6 continuously winds up the cooled bottle body, which is then conveyed to the winding device via a star-shaped dial. The winding tension is controlled by a sensor and maintained at 12N. The visual correction system ensures that the edge alignment error is less than ±1mm, ultimately resulting in a continuous roll of plastic bottles with a roll diameter of 1.2m.

[0059] Testing revealed that, in this embodiment, the multi-layer continuous molding process for plastic bottles, when the raw material was changed to an HDPE / EVOH / PP system and two bottle sizes with outer diameters of 80mm and 100mm were produced, had ellipticity deviations of only 0.12mm and 0.14mm, respectively, and volume errors that remained stable at ±1.2% and ±1.4%, verifying the excellent versatility of the process.

[0060] Comparative Example 1

[0061] The plastic bottle molding process in this comparative example includes the following steps.

[0062] S1 Raw material pretreatment: Take 200 kg of inner layer raw material low-density polyethylene (LDPE), 150 kg of middle layer raw material polyamide (PA), and 180 kg of outer layer raw material polycarbonate (PC), and put the three raw materials into independent hot air circulating drying units. Set the hot air temperature to 100℃, the air velocity to 2.0 m / s, and the drying time to 3 hours. After drying, the moisture content of the raw materials is reduced to 0.018%. Then, each raw material is transported to its corresponding temporary storage bin and accurately fed at a rate of 50 kg per hour through the screw metering feeder at the bottom of the bin.

[0063] S2 multilayer co-extrusion preform preparation involves feeding the pretreated three-layer raw materials into three two-stage extruders: a first-stage single-screw extruder with a screw diameter of 60 mm and a rotation speed of 90 rpm; a second-stage twin-screw extruder with a screw diameter of 75 mm and a rotation speed of 110 rpm; and a third-stage planetary gear extruder with a screw diameter of 90 mm and a rotation speed of 135 rpm. The melt is pressurized by a melt pump and then conveyed to a single-layer flat straight-channel die. The die channel is a straight cylindrical shape without spiral structures or micron-level grooves. The die temperature is uniformly set to 240℃. After the melt flows together, a three-layer preform with an outer diameter of 60 mm is formed and output at a linear velocity of 3.5 m / min. Online monitoring shows that the wall thickness uniformity deviation of the preform is ±0.048 mm, and interlayer interface fluctuations are visible.

[0064] S3 billet preheating: The continuously output billet is introduced into the annular heating chamber. The temperature of the four temperature zones of the heating chamber is set as follows: Zone 1: 90℃, Zone 2: 130℃, Zone 3: 170℃, and Zone 4: 155℃. The billet passes through the heating chamber at a speed of 3.5 m / min and the heating time is 25 seconds. The maximum temperature difference between the inside and outside of the billet is 8℃, as monitored by an infrared thermal imager.

[0065] S4 stretch blow molding: The preheated tube blank enters a dual-station rotary blow molding device. The mandrel stretching mechanism polishes the surface to Ra 0.1μm and passes through a mandrel with 3℃ chilled brine at a flow rate of 3 L / min. Axial stretching is performed at a speed of 10 m / min, with a stretching ratio of 3.2 times. Simultaneously, the high-pressure blow molding mechanism injects compressed air according to a time-pressure dual closed-loop control program. The pressure linearly increases from 0.5MPa to 1.5MPa in 0-3 seconds, maintains 1.2MPa in 3-8 seconds, and linearly decreases to 0.3MPa in 8-12 seconds. After blow molding, the bottle body is obtained.

[0066] The S5 gradient cooling and shaping process involves sending the molded bottle into a cooling tunnel. The first cooling zone uses 10°C water mist spray for 12 seconds to reduce the bottle surface temperature to 65°C. The second cooling zone uses 5°C water mist spray for 10 seconds to reduce the temperature to 45°C. The third cooling zone uses 1°C cold water spray for 6 seconds to finally reduce the surface temperature to 30°C. During the cooling process, 12°C circulating air is introduced into the bottle at a speed of 1.0 m / s.

[0067] The S6 continuously winds up the cooled bottle body, which is then conveyed to the winding device via a star-shaped dial. The winding tension is controlled by a sensor and maintained at 12N. The visual correction system ensures that the edge alignment error is less than ±1mm, ultimately resulting in a continuous roll of plastic bottles with a roll diameter of 1.2m.

[0068] Testing revealed that the plastic bottle molding process in this comparative example, using a traditional flat straight-channel die, exhibited significant disturbance at the melt interlayer interface. The interlayer peel strength was only 20.3 N / cm, and the uniformity deviation of the tube blank wall thickness increased to ±0.048 mm, resulting in uneven distribution of the bottle wall thickness. The difference between the thinnest and thickest parts in some areas exceeded 0.15 mm, failing to meet the consistency requirements of high-end packaging containers.

[0069] Comparative Example 2

[0070] The plastic bottle molding process in this comparative example includes the following steps.

[0071] S1 Raw material pretreatment: Take 200 kg of inner layer raw material low-density polyethylene (LDPE), 150 kg of middle layer raw material polyamide (PA), and 180 kg of outer layer raw material polycarbonate (PC), and put the three raw materials into independent hot air circulating drying units. Set the hot air temperature to 100℃, the air velocity to 2.0 m / s, and the drying time to 3 hours. After drying, the moisture content of the raw materials is reduced to 0.018%. Then, each raw material is transported to its corresponding temporary storage bin and accurately fed at a rate of 50 kg per hour through the screw metering feeder at the bottom of the bin.

[0072] The S2 multi-layer co-extrusion process involves feeding the pretreated three-layer raw materials into three two-stage extruders: a first-stage single-screw extruder with a screw diameter of 60 mm and a rotation speed of 90 rpm; a second-stage twin-screw extruder with a screw diameter of 75 mm and a rotation speed of 110 rpm; and a third-stage planetary gear extruder with a screw diameter of 90 mm and a rotation speed of 135 rpm. The melt, pressurized by a melt pump, is then conveyed to a dynamic co-extrusion die. The die inlet temperature is set at 220℃, the middle section at 240℃, and the outlet temperature at 255℃. The melt flows through an internal variable cross-section spiral channel with a spiral angle of 18°, forming a three-layer tube blank with an outer diameter of 60 mm. This blank is continuously output at a linear velocity of 3.5 m / min, and the online wall thickness gauge shows a wall thickness uniformity deviation of ±0.028 mm.

[0073] S3 billet preheating: The continuously output billet is passed through a dual-temperature zone heating furnace with temperatures set at 130℃ and 170℃ respectively. The heating time is about 25 seconds. The temperature is detected intermittently using a contact thermocouple. The maximum temperature difference between the inside and outside of the billet is measured to be 18℃.

[0074] S4 stretch blow molding: The preheated tube blank enters a dual-station rotary blow molding device. The mandrel stretching mechanism polishes the surface to Ra 0.1μm and passes through a mandrel with 3℃ chilled brine at a flow rate of 3 L / min. Axial stretching is performed at a speed of 10 m / min, with a stretching ratio of 3.2 times. Simultaneously, the high-pressure blow molding mechanism injects compressed air according to a time-pressure dual closed-loop control program. The pressure linearly increases from 0.5MPa to 1.5MPa in 0-3 seconds, maintains 1.2MPa in 3-8 seconds, and linearly decreases to 0.3MPa in 8-12 seconds. After blow molding, the bottle body is obtained.

[0075] The S5 gradient cooling and shaping process involves sending the molded bottle into a cooling tunnel. The first cooling zone uses 10°C water mist spray for 12 seconds to reduce the bottle surface temperature to 65°C. The second cooling zone uses 5°C water mist spray for 10 seconds to reduce the temperature to 45°C. The third cooling zone uses 1°C cold water spray for 6 seconds to finally reduce the surface temperature to 30°C. During the cooling process, 12°C circulating air is introduced into the bottle at a speed of 1.0 m / s.

[0076] The S6 continuously winds up the cooled bottle body, which is then conveyed to the winding device via a star-shaped dial. The winding tension is controlled by a sensor and maintained at 12N. The visual correction system ensures that the edge alignment error is less than ±1mm, ultimately resulting in a continuous roll of plastic bottles with a roll diameter of 1.2m.

[0077] Testing revealed that the plastic bottle molding process in this comparative example used a simple dual-temperature zone heating furnace, which resulted in uneven preheating of the tube blank, with an internal and external temperature difference of up to 18°C. Local overheating caused slight thermal oxidation of the outer layer material, and the unit energy consumption reached 0.45 kWh / kg. The large temperature gradient also caused stress concentration during subsequent inflation, resulting in a decrease in drop resistance of about 15%.

[0078] Comparative Example 3

[0079] The plastic bottle molding process in this comparative example includes the following steps.

[0080] S1 Raw material pretreatment: Take 200 kg of inner layer raw material low-density polyethylene (LDPE), 150 kg of middle layer raw material polyamide (PA), and 180 kg of outer layer raw material polycarbonate (PC), and put the three raw materials into independent hot air circulating drying units. Set the hot air temperature to 100℃, the air velocity to 2.0 m / s, and the drying time to 3 hours. After drying, the moisture content of the raw materials is reduced to 0.018%. Then, each raw material is transported to its corresponding temporary storage bin and accurately fed at a rate of 50 kg per hour through the screw metering feeder at the bottom of the bin.

[0081] The S2 multi-layer co-extrusion process involves feeding the pretreated three-layer raw materials into three two-stage extruders: a first-stage single-screw extruder with a screw diameter of 60 mm and a rotation speed of 90 rpm; a second-stage twin-screw extruder with a screw diameter of 75 mm and a rotation speed of 110 rpm; and a third-stage planetary gear extruder with a screw diameter of 90 mm and a rotation speed of 135 rpm. The melt, pressurized by a melt pump, is then conveyed to a dynamic co-extrusion die. The die inlet temperature is set at 220℃, the middle section at 240℃, and the outlet temperature at 255℃. The melt flows through an internal variable cross-section spiral channel with a spiral angle of 18°, forming a three-layer tube blank with an outer diameter of 60 mm. This blank is continuously output at a linear velocity of 3.5 m / min, and the online wall thickness gauge shows a wall thickness uniformity deviation of ±0.028 mm.

[0082] S3 billet preheating: The continuously output billet is introduced into the annular heating chamber. The temperature of the four temperature zones of the heating chamber is set as follows: Zone 1: 90℃, Zone 2: 130℃, Zone 3: 170℃, and Zone 4: 155℃. The billet passes through the heating chamber at a speed of 3.5 m / min and the heating time is 25 seconds. The maximum temperature difference between the inside and outside of the billet is 8℃, as monitored by an infrared thermal imager.

[0083] S4 stretch blow molding: The preheated tube blank enters a dual-station rotary blow molding device. The mandrel stretching mechanism polishes the surface to Ra 0.1μm and passes through a mandrel with 3℃ chilled brine at a flow rate of 3 L / min. Axial stretching is performed at a speed of 10 m / min, with a stretching ratio of 3.2 times. Simultaneously, the high-pressure blow molding mechanism injects compressed air according to a time-pressure dual closed-loop control program. The pressure linearly increases from 0.5MPa to 1.5MPa in 0-3 seconds, maintains 1.2MPa in 3-8 seconds, and linearly decreases to 0.3MPa in 8-12 seconds. After blow molding, the bottle body is obtained.

[0084] S5 gradient cooling and shaping involves immersing the inflated, high-temperature bottle directly into a static cold water bath at 10°C for 30 seconds, and then removing it.

[0085] The S6 continuously winds up the cooled bottle body, which is then conveyed to the winding device via a star-shaped dial. The winding tension is controlled by a sensor and maintained at 12N. The visual correction system ensures that the edge alignment error is less than ±1mm, ultimately resulting in a continuous roll of plastic bottles with a roll diameter of 1.2m.

[0086] Testing revealed that the plastic bottle molding process in this comparative example, which uses a single water tank for cooling, caused the bottle surface to shrink rapidly, resulting in obvious watermarks and warping. Its surface gloss was only 77 GU. The non-uniform cooling caused large differences in crystallinity in different parts of the bottle, with a volume error of ±3.5%. Furthermore, after being stored at room temperature for 24 hours, it showed a shrinkage deformation of about 1.2%, which could not meet the requirements for container volume stability for precise filling.

[0087] In summary, the embodiments of this invention, through the synergistic effect of a dynamic co-extrusion die with a variable cross-section spiral flow channel and micron-level grooves, a four-temperature zone infrared feedback preheating system, time-pressure dual closed-loop blow-up control, and pulsed gradient cooling process, solve the core technical problems of low interlayer bonding strength, poor wall thickness uniformity, high energy consumption, and difficulty in controlling product consistency in traditional multi-layer bottle molding processes. Data from the embodiments show that, compared to Comparative Example 1, which suffers a 28.8% decrease in interlayer peel strength (20.3 N / cm) due to the use of a flat straight-flow die, Comparative Example 2, which suffers a 22.2% increase in preheating energy consumption (0.45 kWh / kg) due to the use of a heating furnace, and Comparative Example 3, which suffers a 13.5% decrease in bottle surface gloss (77 GU) and an increase in volumetric error to ±3.5% due to the use of single water cooling, the process of this invention significantly improves product quality and performance, achieving continuous and stable molding of high-quality multi-layer plastic bottles.

Claims

1. A multi-layer continuous molding process for plastic bottles, characterized in that: Includes the following steps, S1 Raw material pretreatment: The inner layer raw material, middle layer raw material, and outer layer raw material are respectively fed into independent drying units. The drying units adopt a hot air circulation system with a hot air temperature of 80℃ to 120℃ and an air velocity controlled at 1.5m / s to 2.5m / s. The drying time is 2 hours to 4 hours, so that the moisture content of the raw materials is ≤0.02%. The dried raw materials are respectively sent into the corresponding temporary storage bins. The bottom of the bins is equipped with a screw metering feeder, and the feeding accuracy is controlled within ±0.5%. S2 Multilayer Co-extrusion Preformation: The three layers of raw material after pretreatment in step S1 are fed into three two-stage extruders. The first stage extruder is a single-screw extruder with a screw diameter of 60mm, an L / D ratio of 32:1, and a rotation speed of 80-100 rpm, used for raw material melting and preliminary mixing. The second stage extruder is a twin-screw extruder with a screw diameter of 75mm, an L / D ratio of 40:1, and a rotation speed of 100-120 rpm, used for melt homogenization and venting. The third stage extruder is a planetary gear extruder with a screw diameter of 90mm, an L / D ratio of 48:1, and a rotation speed of 120-150 rpm, used for high-pressure conveying of high-viscosity melts. The melts from the three extruders are then further processed... After being pressurized by the bulk pump, the melt enters the dynamic co-extrusion die. The dynamic co-extrusion die is equipped with three layers of variable cross-section spiral flow channels. The cross-section of the flow channel gradually changes from a rectangle with a length of 12mm and a width of 8mm at the inlet end to a trapezoid with an upper bottom of 10mm, a lower bottom of 14mm, and a height of 18mm at the outlet end. The spiral angle of the spiral flow channel is 15° to 20°. The die temperature is controlled in zones: 200° to 220° at the inlet section, 230° to 250° at the middle section, and 240° to 260° at the outlet section. The melt flows into the die to form a three-layer tube blank. The uniformity deviation of the tube blank wall thickness is ≤ ±0.03mm, and the outer diameter ranges from 50mm to 100mm. It is continuously output at a linear velocity of 2m / min to 5m / min. S3. Preheating of the tube blank: The tube blank continuously output from step S2 enters the annular heating chamber. The annular heating chamber is divided into four temperature zones along the conveying direction: the first temperature zone, with a feed end temperature of 80℃ to 100℃; the second temperature zone, a transition zone temperature of 120℃ to 140℃; the third temperature zone, the main heating zone temperature of 160℃ to 180℃; and the fourth temperature zone, the heat preservation zone temperature of 140℃ to 160℃. A graphene heating film is installed on the inner wall of the heating chamber, with a power density of 60W / cm² to 90W / cm². The distance between the tube blank and the heating film is 60mm to 90mm. The tube blank conveying speed is synchronized with the tube blank output speed in step S2. The heating time is 20 seconds to 30 seconds. The surface temperature of the tube blank is monitored in real time by an infrared thermal imager, and the heating power of each temperature zone is adjusted accordingly to ensure that the temperature difference between the inside and outside of the tube blank is ≤10℃. S4 Stretch Blow Molding: The preheated tube blank from step S3 enters the stretch blow molding station, employing a dual-station rotary blow molding device, including a mandrel stretching unit and a high-pressure blow molding unit. The mandrel surface of the mandrel stretching unit is polished to Ra≤0.1μm, and chilled brine at -5℃ to 5℃ is introduced internally at a flow rate of 2L / min to 4L / min. The mandrel is axially inserted into the tube blank at a speed of 8m / min to 12m / min, with a stretching ratio of 3.0 to 4.0 times and a stretching time of 1 second to... 2 seconds; The high-pressure inflation unit synchronously injects compressed air into the stretched tube blank. The inflation process adopts time-pressure dual closed-loop control. In the initial inflation stage, from 0 seconds to 3 seconds, the pressure linearly increases from 0.5MPa to 1.5MPa. In the intermediate pressure holding stage, from 3 seconds to 8 seconds, the pressure is maintained at 1.2MPa. In the later pressure relief stage, from 8 seconds to 12 seconds, the pressure linearly decreases from 1.2MPa to 0.3MPa. After inflation is completed, the tube blank expands into a bottle body, and the uniformity deviation of the bottle body wall thickness is ≤±0.1mm. S5 Gradient cooling and shaping: After inflation in step S4, the bottle enters a cooling tunnel containing three gradient cooling units. The first unit, in the high-temperature zone, uses water mist spray at 8°C to 12°C, with a spray angle of 70° to 90°, a particle size of 20μm to 60μm, and an action time of 10 to 15 seconds, reducing the bottle surface temperature to 60°C to 70°C. The second unit, in the medium-temperature zone, uses water mist spray at 4°C to 6°C, with a spray angle of 60° to 80°, a particle size of 10μm to 40μm, and an action time of 8 to 12 seconds, reducing the surface temperature to 40°C to 50°C. The third unit, in the low-temperature zone, uses cold water spray at 0°C to 2°C, with a spray angle of 50° to 70°, a particle size of 5μm to 30μm, and an action time of 5 to 8 seconds, reducing the surface temperature to 25°C to 35°C. During cooling, circulating air at 10°C to 15°C is introduced into the bottle at a speed of 0.5m / s to 1.5m / s to aid in balancing the internal and external temperature differences. S6 Continuous winding: After cooling and shaping in step S5, the bottle body is conveyed to the winding station via a star-shaped dial wheel. The winding device is set with dual-station alternating winding. The surface of the winding roller is covered with a polyurethane elastic layer with a Shore A hardness of 60 to 70. The winding process adopts a tension-vision dual feedback control system. The tension sensor monitors the winding tension in real time, with a set range of 8N to 18N. When the tension deviation is ≥±2N, the speed of the winding roller is adjusted by the servo motor. The vision inspection camera simultaneously monitors the lateral offset of the bottle body. When the offset is ≥±3mm, the lateral correction cylinder is triggered to adjust the position of the conveyor belt to ensure that the alignment error of the winding edge is ≤±1mm. Finally, a continuous roll of plastic bottle with a roll diameter of ≤1.5m is formed.

2. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S1, the inner layer material is food-grade low-density polyethylene (LDPE), the middle layer material is barrier polyamide (PA), and the outer layer material is UV-resistant polycarbonate (PC).

3. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S2, the inner wall of the spiral flow channel of the dynamic co-extrusion die is provided with micron-level grooves, with a depth of 0.5μm to 1.5μm and a spacing of 2μm to 5μm. The groove direction is consistent with the spiral direction of the spiral flow channel, which is used to enhance the melt shear flow.

4. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S2, an online wall thickness detector with an accuracy of ±0.005mm is installed at the outlet end of the dynamic co-extrusion die. The detection data is fed back to the die flow channel adjustment system in real time, and the opening of each flow channel is adjusted by an electric servo valve to dynamically compensate for the wall thickness deviation.

5. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S3, a ceramic fiber heat insulation layer with a thickness of 5mm to 10mm is set above the graphene heating film of the annular heating cavity, and the surface temperature of the heat insulation layer is ≤50℃ to reduce heat loss to the external environment.

6. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S3, the temperature zone division of the annular heating cavity is adaptively adjusted according to the outer diameter of the tube blank. When the outer diameter of the tube blank increases by 10mm, the temperature of each temperature zone decreases by 5℃ to 8℃, ensuring the heating uniformity of tube blanks of different specifications.

7. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S4, a temperature sensor with an accuracy of ±0.5℃ is installed inside the mandrel of the mandrel stretching unit to provide real-time feedback on the temperature of the frozen brine. The brine flow rate is adjusted by a PID controller to ensure that the temperature fluctuation of the mandrel surface is ≤±1℃.

8. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S4, the compressed air from the high-pressure blowing unit is processed by a three-stage filtration system. The first stage filtration accuracy is 5μm, the second stage filtration accuracy is 1μm, and the third stage filtration accuracy is 0.01μm. The dew point temperature is ≤-50℃ and the oil content is ≤0.01mg / m³.

9. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S5, the water mist spraying system of the gradient cooling unit adopts a pulse spraying mode with a pulse frequency of 5Hz to 10Hz and a duty cycle of 30% to 50%, which reduces the amount of water mist remaining on the bottle surface.

10. The multi-layer continuous molding process for plastic bottles according to claim 1, characterized in that: In step S6, the star-shaped dial of the winding device is equipped with an elastic paddle made of nitrile rubber with a Shore A hardness of 50 to 60. The contact pressure between the paddle and the bottle body is controlled at 0.1N to 0.3N to avoid scratching the bottle surface.