Micro / nano foamed light diffuser plate with ultra-fine uniform pore structure and its preparation method

Abstract

This invention relates to the field of optical materials and discloses a micro / nano foamed light diffuser plate with an ultrafine uniform pore structure and its preparation method. The method includes a chemical foaming agent, which is a segmented decomposition foaming agent with a decomposition temperature range between 190°C and 250°C. By mass percentage, the chemical foaming agent comprises: 30%–90% carrier resin, 10%–50% main foaming agent component, 5%–40% auxiliary foaming agent, 0.2%–5% activator, 0.5%–30% nucleating agent, 1%–5% dispersant or lubricant, and 2%–5% processing aid. This invention eliminates the need for expensive diffusion particles and achieves a haze of over 98% solely through precisely controlled micron-sized pores.

Description

Technical Field

[0001] This invention relates to the field of optical materials, and more particularly to micro / nano-foamed light diffusers with ultrafine uniform pore structures and their preparation methods. Background Technology

[0002] Currently, light diffusion plates are widely used in various display devices, but they suffer from problems such as insufficient performance stability, poor environmental friendliness, and high cost. Improper matching of the matrix material and foaming agent in traditional light diffusion plates leads to unstable cell structure, affecting optical performance.

[0003] Existing light diffusion plates face core challenges in terms of performance stability, environmental friendliness, and production cost control, particularly the difficulty in controlling the cell structure, which affects light scattering efficiency and material stability.

[0004] While existing technologies have attempted to improve the cell structure by adding fillers, this approach increases costs, reduces environmental friendliness, and makes it difficult to achieve a uniform distribution of cells. Excessively large cell sizes and extremely uneven cell distribution, resulting in large or merged bubbles, can lead to visible crystal points or shadows on the surface of the light diffuser plate, severely impacting the image quality and detail of high-end displays.

[0005] With the increasing demand for green materials and high-efficiency display technologies, there is a need to develop high-performance light diffusion plates. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides: 1. A micro / nano foamed light diffuser plate with an ultra-fine, uniform pore structure, comprising a three-layer co-extrusion structure; the three-layer co-extrusion structure includes a foamed core layer and a first skin layer and a second skin layer respectively bonded to opposite sides of the foamed core layer; the first and second skin layers are prepared from modified polymethyl methacrylate or polystyrene; the foamed core layer is formed by extrusion foaming of a raw material comprising a modified general-purpose polystyrene matrix and a chemical foaming agent, and the raw material does not contain light-diffusing particle fillers; The chemical foaming agent is a segmented decomposition foaming agent with a decomposition temperature range between 190°C and 250°C; by mass percentage, the chemical foaming agent comprises: 30%~90% carrier resin, 10%~50% main foaming agent component, 5%~40% auxiliary foaming agent, 0.2%~5% activator, 0.5%~30% nucleating agent, 1%~5% dispersant or lubricant, and 2%~5% processing aid; The main component of the foaming agent is selected from at least one of sodium citrate, disodium citrate, potassium bicarbonate, and sodium benzoate. The foamed core layer has a closed-cell foam structure, wherein the average pore diameter of the pores in the closed-cell foam structure is no greater than 150 μm and the maximum pore diameter is no greater than 200 μm.

[0007] This invention also relates to a method for preparing a micro / nano-foamed light diffuser plate with an ultrafine uniform pore structure, comprising the following steps: Step 1: Mix the chemical foaming agent with the modified general-purpose polystyrene matrix to obtain the core layer raw material; Step 2: The materials for preparing the first and second skin layers are added to the skin extruder of the multilayer co-extrusion equipment for melt plasticization; the core layer raw material is added to the single-screw core layer extruder of the multilayer co-extrusion equipment for melt plasticization and foaming reaction; Step 3: The materials for preparing the first and second skin layers after melting and plasticizing, together with the core layer raw material after melting and foaming reaction, are fed into the three-layer co-extrusion die to form a three-layer co-extrusion melt arranged in the form of "skin layer-core layer-skin layer". Step 4: The three-layer co-extruded melt extruded from the three-layer co-extruded die is shaped and cooled, and then pulled and cut to obtain the micro-nano foamed light diffusion plate; In step 2, the foaming temperature of the single-screw core extruder is constantly controlled at 220℃~225℃, and the screw speed is controlled at 250rpm~320rpm; in step 4, the cooling rate during the entire shaping and cooling process is constantly controlled at 6.5℃ / min~8.0℃ / min.

[0008] Compared with the prior art, the beneficial effects of the present invention are as follows: In this invention, the bubble size is reduced from hundreds of micrometers to less than 150 μm. This micro-nano-level scattering unit makes the light emission extremely delicate and uniform, completely solving the "graininess" problem of the display screen, and is particularly suitable for ultra-thin displays with high pixel density.

[0009] This invention achieves over 98% haze without the need for expensive diffusion particles, relying solely on precisely controlled micron-level pores. This not only reduces material costs but also significantly lightens the substrate weight, aligning with the trend towards thinner and lighter display devices.

[0010] This invention establishes a structure-property relationship model based on matrix rheology, foaming kinetics, and optical scattering mechanism, resulting in extremely low coefficient of variation in cell size distribution during the production process and excellent consistency of mass-produced finished products.

[0011] The entire process of this invention does not involve harmful chemical fillers, and the foaming agent has a high thermal weight loss rate and no residual odor or toxicity. Attached Figure Description

[0012] Figure 1 This is a bubble diagram of the basic embodiment 1 of the present invention.

[0013] Figure 2 This is a bubble diagram of the basic embodiment 2 of the present invention.

[0014] Figure 3 This is a bubble diagram of the basic embodiment 3 of the present invention.

[0015] Figure 4 The bubble pattern of the optimized embodiment 1 of the present invention is shown.

[0016] Figure 5 The bubble diagram is for an extended embodiment 1 of the present invention.

[0017] Figure 6 The bubble diagram is for an extended embodiment 2 of the present invention.

[0018] Figure 7 This is a bubble diagram of control group 1 of the present invention.

[0019] Figure 8 This is a bubble diagram of control group 2 of the present invention.

[0020] Figure 9 The figure shows the thermal analysis test results of the basic embodiment 1 of the present invention.

[0021] Figure 10 The graph shows the thermal analysis test results of the second basic embodiment of the present invention.

[0022] Figure 11 The figure shows the thermal analysis test results of the basic embodiment 3 of the present invention.

[0023] Figure 12 The image shows the thermal analysis test results of the optimized embodiment 1 of the present invention.

[0024] Figure 13 The graph shows the thermal analysis test results of control group 1 in this invention.

[0025] Figure 14 The graph shows the thermal analysis test results of control group 2 in this invention. Detailed Implementation

[0026] The terms “first,” “second,” “third,” “fourth,” etc. (if present) in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms “comprising” or “having,” and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0027] A micro / nano foamed light diffusion plate with an ultra-fine uniform pore structure has a three-layer co-extruded structure; The three-layer co-extruded structure includes a foamed core layer and a first skin layer and a second skin layer respectively bonded to opposite sides of the foamed core layer. The materials used to prepare the first and second skin layers include modified polymethyl methacrylate or polystyrene; The foamed core layer is formed by extrusion foaming of raw materials including modified general-purpose polystyrene matrix and chemical foaming agent, and the raw materials do not contain light-diffusing particle fillers. The chemical foaming agent is a segmented decomposition foaming agent with a decomposition temperature range between 190°C and 250°C; by mass percentage, the chemical foaming agent comprises: 30%~90% carrier resin, 10%~50% main foaming agent component, 5%~40% auxiliary foaming agent, 0.2%~5% activator, 0.5%~30% nucleating agent, 1%~5% dispersant or lubricant, and 2%~5% processing aid; The main component of the foaming agent is selected from at least one of sodium citrate, disodium citrate, potassium bicarbonate, and sodium benzoate. The foamed core layer has a closed-cell foam structure, wherein the average pore diameter of the pores in the closed-cell foam structure is no greater than 150 μm and the maximum pore diameter is no greater than 200 μm.

[0028] The carrier resin is selected from at least one of ethylene-octene copolymer, hydrogenated styrene-butadiene block copolymer, polystyrene, ethylene-vinyl acetate copolymer and polyolefin elastomer; The auxiliary foaming agent is selected from at least one of sodium bicarbonate, citric acid and 4,4'-oxobis(benzenesulfonyl hydrazine); The activator is selected from at least one of zinc stearate and borax; The nucleating agent is selected from at least one of talc, montmorillonite, kaolin, silica, and calcium oxide; The dispersant or lubricant is selected from at least one of white oil, silane coupling agent and stearic acid; The processing aid is selected from at least one of polyethylene wax, ethylene bis-stearamide, and microcrystalline wax.

[0029] The raw material of the foamed core layer also includes nano-silica particles, and the mass ratio of the modified general-purpose polystyrene matrix to the nano-silica particles is 100:1. Alternatively, the raw materials of the foamed core layer may also include polymethyl methacrylate, and the mass ratio of the modified general-purpose polystyrene matrix to the polymethyl methacrylate is 9:1.

[0030] The modified general-purpose polystyrene matrix is ​​a polystyrene resin with a long-chain branched structure and a bimodal molecular weight distribution.

[0031] The chemical foaming agent has a microcapsule encapsulation structure; wherein, the main component of the foaming agent and the auxiliary foaming agent together constitute the core layer of the microcapsule, the dispersant or lubricant coats the surface of the core layer to form a first coating layer, the activator and the nucleating agent are attached to the outside of the first coating layer, and the carrier resin constitutes the outermost shell layer.

[0032] The method for preparing a micro / nano-foamed light diffuser plate with an ultrafine uniform pore structure in this invention includes the following steps: Step 1: Mix the chemical foaming agent with the modified general-purpose polystyrene matrix to obtain the core layer raw material; Step 2: The materials for preparing the first and second skin layers are added to the skin extruder of the multilayer co-extrusion equipment for melt plasticization; the core layer raw material is added to the single-screw core layer extruder of the multilayer co-extrusion equipment for melt plasticization and foaming reaction; Step 3: The materials for preparing the first and second skin layers after melting and plasticizing, together with the core layer raw material after melting and foaming reaction, are fed into the three-layer co-extrusion die to form a three-layer co-extrusion melt arranged in the form of "skin layer-core layer-skin layer". Step 4: The three-layer co-extruded melt extruded from the three-layer co-extruded die is shaped and cooled, and then pulled and cut to obtain the micro-nano foamed light diffusion plate; In step 2, the foaming temperature of the single-screw core extruder is constantly controlled at 220℃~225℃, and the screw speed is controlled at 250rpm~320rpm; in step 4, the cooling rate during the entire shaping and cooling process is constantly controlled at 6.5℃ / min~8.0℃ / min.

[0033] In steps 2 and 4, the extrusion and shaping cooling parameters are selected from one of the following parameter groups: Parameter group one: The foaming temperature of the single screw core extruder is 220℃, the screw speed is 250rpm, and the cooling rate of the shaping and cooling is 7.0℃ / min; Parameter group two: The foaming temperature of the single screw core extruder is 224℃, the screw speed is 300rpm, and the cooling rate of the shaping and cooling is 7.0℃ / min; Parameter group three: The foaming temperature of the single-screw core extruder is 225℃, the screw speed is 320rpm, and the cooling rate of the shaping and cooling process is 6.5℃ / min or 7.0℃ / min.

[0034] In steps 3 and 4: When the co-extrusion process is adjusted to increase the thickness ratio of the first and second skin layers in the three-layer co-extrusion melt, the screw speed of the single-screw core extruder is simultaneously adjusted to 300 rpm, and the cooling rate of the shaping and cooling process is adjusted to 8.0 °C / min.

[0035] In step 2: The single-screw core extruder has multiple heating zones on the outside of the barrel, with the temperature increasing in a continuous gradient along the material conveying direction; The multiple heating zones are sequentially divided into a preheating zone, a compression zone, and a foaming reaction zone. The heating temperature of the preheating zone is set between 150°C and 190°C, the heating temperature of the compression zone is set between 190°C and 215°C, and the heating temperature of the foaming reaction zone is set to a foaming temperature of 220°C to 225°C.

[0036] In step 4: The shaping and cooling process is performed using a calendering roll group consisting of a first calendering cooling roll, a second calendering cooling roll, and a third calendering cooling roll arranged in sequence. Along the travel direction of the three-layer co-extrusion melt, the set temperatures of the roll surfaces of the first calendering cooling roll, the second calendering cooling roll, and the third calendering cooling roll are distributed in a stepwise decreasing manner. As the three-layer co-extrusion melt passes through the roll surfaces of each calendering cooling roll in sequence, its surface temperature decreases linearly over time, thereby maintaining the stringent cooling rate of 6.5℃ / min to 8.0℃ / min.

[0037] The following are specific embodiments of the present invention, with appendices. Figure 1-14 The image shows the test results.

[0038] The wide-temperature-range segmented decomposition chemical foaming agent includes: 80% carrier resin, 10% sodium citrate, 5% sodium bicarbonate, 1% zinc stearate, 1% talc, 2% stearic acid, and 1% ethylene bis-stearamide.

[0039] In the following text, the carrier resin is EVA.

[0040] Basic Example 1 Preparation method: Modified GPPS (Dushanzi Petrochemical GPPS-500N) with regulated molecular weight distribution is used as the core foaming matrix, combined with a wide-temperature-range segmented decomposition chemical foaming agent. The single-screw core extrusion process parameters are set as follows: foaming temperature 220℃, screw speed 250rpm, and setting cooling rate 7℃ / min.

[0041] Test results: The melt strength of the modified matrix effectively locked in the gas, reducing the average pore size to 300-400 μm. The density measured was 0.8926 g / cm³. 3 The measured value was 0.8953 g / cm³. 3 The average density decreased significantly to 0.8940 g / cm³. 3 Light transmittance decreased to 38.0%, while haze increased to 94.0%.

[0042] Basic Implementation Example 2 Preparation method: Based on the basic example 1, the extrusion ratio of PMMA skin material (Evonik Degussa PMMA 8N) was increased to optimize the multilayer co-extrusion process. Simultaneously, process parameters were adjusted: the core layer screw speed was adjusted to 300 rpm, and the shaping and cooling rate was increased to 8°C / min.

[0043] Test results: Enhanced surface cooling and skin confinement resulted in more uniform cell size. Density measurement 1 showed a value of 0.8966 g / cm³. 3 The measured value was 0.8991 g / cm³. 3 The average density is 0.8979 g / cm³. 3 Light transmittance further decreased to 35.0%, while haze increased to 96.0%.

[0044] Basic Example 3 Preparation method: Based on the basic example 1, the proportion and formulation of the chemical foaming agent system were adjusted. The core layer single-screw extrusion process parameters were adjusted as follows: the foaming temperature was slightly increased to 224℃, the screw speed was adjusted to 300rpm, and the shaping and cooling rate was maintained at 7℃ / min.

[0045] Chemical foaming agent: 80% carrier resin, 12% sodium citrate, 3% sodium bicarbonate, 1% zinc stearate, 1% talc, 2% stearic acid, and 1% ethylene bis-stearamide.

[0046] Test results: Cell merging was effectively suppressed, with the average size controlled below 300 μm. Density measurement 1: 0.8989 g / cm³. 3 The measured value was 0.9001 g / cm³. 3 The average density is 0.8995 g / cm³. 3 Light transmittance decreased to 34.2%, while haze significantly increased to 97.5%.

[0047] Optimized Example 1 Preparation method: The same raw material system as in basic Example 1 was used. Rigorous optimization of process parameters was conducted: the single-screw core layer foaming temperature was precisely controlled at 225℃, the screw speed was increased to 320rpm to provide high-frequency shearing, and the shaping and cooling rate was optimized and slowed down to 6.5℃ / min.

[0048] Chemical foaming agent: 80% carrier resin, 13% sodium citrate, 2% sodium bicarbonate, 1% zinc stearate, 1% talc, 2% borax, and 1% ethylene bis-stearamide.

[0049] Test results: A perfect balance was achieved between the release of the foaming agent and the solidification of the high-strength melt, resulting in an excellent micro / nano closed-cell structure with an extremely narrow average size, all within 150 μm. The density measurement value was 0.8944 g / cm³. 3 The measured value was 0.8921 g / cm³. 3 The average density is as low as 0.8933 g / cm³. 3 The dense micropore array brings about an extreme micropore scattering effect, significantly reducing light transmittance to 32.5% and haze to 98.5%.

[0050] Extended Example 1 (High Strength and Weather Resistance Type) Preparation method: Based on the basic example 1, nano-silica was introduced as a reinforcing material and heterogeneous nucleating agent, and added to the modified GPPS matrix at a mass ratio of 1:100. The mixture was then extruded using a modified foaming process.

[0051] Chemical foaming agent: 80% carrier resin, 13% sodium citrate, 2% sodium bicarbonate, 1% zinc stearate, 1% talc, 1% borax, 1% nano silica and 1% ethylene bis-stearamide.

[0052] Test results: The material's mechanical strength and weather resistance were significantly improved while maintaining its excellent micro / nanoporous structure. The density measurement value was 0.8992 g / cm³. 3 The measured value was 0.8979 g / cm³. 3 The average density is stable at 0.8986 g / cm³. 3 The light transmittance is 32.6%, and the haze is maintained at 98.4%.

[0053] Extended Example 2 (Core Layer Resin Alloy Type) Preparation method: Based on the basic example 1, the single core matrix was replaced with a blend of modified GPPS and PMMA mixed at a mass ratio of 9:1. The extrusion foaming process parameters were controlled as follows: foaming temperature 225℃, screw speed 320rpm, and setting cooling rate 7℃ / min.

[0054] Test results: The introduction of PMMA further improved the interfacial rheological properties of the system, and the cell morphology exhibited extremely regular spherical shapes. The density measurement value was 0.8974 g / cm³. 3 The measured value was 0.8987 g / cm³. 3 The average density is 0.8981 g / cm³. 3 The light transmittance is 32.9%, and the haze is 98.0%. Due to the introduction of PMMA, the material's weather resistance is improved by approximately 15% compared to the basic Example 1.

[0055] Control group 1 Preparation method: Unmodified conventional GPPS (general-purpose polystyrene) matrix and conventional chemical foaming agent formulation are used. Single-screw extrusion process is adopted, with the foaming temperature set at 220℃, the single screw speed at 200rpm, and the cooling rate during the traction and shaping stage controlled at 7℃ / min.

[0056] Test Results: Microscopic observation showed that the average pore size was between 330 and 530 μm, with obvious large bubbles present. The density was measured to be 0.9199 g / cm³. 3 The measured value was 0.9042 g / cm³. 3 (The large density difference between the two locations indicates extremely uneven foaming), the average density is 0.9121 g / cm³. 3 In terms of optical performance, the light transmittance is as high as 43.5%, the haze is only 93.2%, and visible crystal points and shadows appear on the surface of the board.

[0057] Control group 2 Preparation method: The raw material formulation was the same as that of control group 1. The single-screw extrusion process parameters were adjusted as follows: the foaming temperature was increased to 225℃, the screw speed was increased to 250rpm, and the setting and cooling rate was maintained at 7℃ / min.

[0058] Test results: Due to insufficient melt strength of the unmodified matrix, high temperature and high shear caused severe cell merging, increasing the average cell size to 450-600 μm. The density measurement value was 0.9088 g / cm³. 3 The measured value was 0.9148 g / cm³. 3 The average density is 0.9118 g / cm³. 3 The light transmittance is 41.6%, and the haze is reduced to 92.9%.

[0059] The specific results are shown in Table 1.

[0060] Table 1 Experimental Results The density data in the table are based on ASTM D792-20 and were determined using the Archimedes displacement method. Measurements were performed using a DahoMeter DE-120M solid density meter, with each sample measured twice in parallel, and the results were averaged.

[0061] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A micro / nano-foamed light diffuser plate with an ultra-fine, uniform pore structure, characterized in that, It has a three-layer co-extrusion structure; The three-layer co-extruded structure includes a foamed core layer and a first skin layer and a second skin layer respectively bonded to opposite sides of the foamed core layer. The materials used to prepare the first and second skin layers include modified polymethyl methacrylate or polystyrene; The foamed core layer is formed by extrusion foaming of raw materials including modified general-purpose polystyrene matrix and chemical foaming agent, and the raw materials do not contain light-diffusing particle fillers. The chemical foaming agent is a segmented decomposition foaming agent with a decomposition temperature range between 190°C and 250°C. The chemical foaming agent comprises, by weight percentage: 30% to 90% carrier resin, 10% to 50% main foaming agent component, 5% to 40% auxiliary foaming agent, 0.2% to 5% activator, 0.5% to 30% nucleating agent, 1% to 5% dispersant or lubricant, and 2% to 5% processing aid. The main component of the foaming agent is selected from at least one of sodium citrate, disodium citrate, potassium bicarbonate, and sodium benzoate. The foamed core layer has a closed-cell foam structure, wherein the average pore diameter of the pores in the closed-cell foam structure is no greater than 150 μm and the maximum pore diameter is no greater than 200 μm.

2. The micro / nano foamed light diffuser plate with an ultra-fine uniform pore structure according to claim 1, characterized in that: The carrier resin is selected from at least one of ethylene-octene copolymer, hydrogenated styrene-butadiene block copolymer, polystyrene, ethylene-vinyl acetate copolymer and polyolefin elastomer; The auxiliary foaming agent is selected from at least one of sodium bicarbonate, citric acid and 4,4'-oxobis(benzenesulfonyl hydrazine); The activator is selected from at least one of zinc stearate and borax; The nucleating agent is selected from at least one of talc, montmorillonite, kaolin, silica, and calcium oxide; The dispersant or lubricant is selected from at least one of white oil, silane coupling agent and stearic acid; The processing aid is selected from at least one of polyethylene wax, ethylene bis-stearamide, and microcrystalline wax.

3. The micro / nano foamed light diffuser plate with an ultra-fine uniform pore structure according to claim 1, characterized in that: The raw material of the foamed core layer also includes nano-silica particles, and the mass ratio of the modified general-purpose polystyrene matrix to the nano-silica particles is 100:

1. Alternatively, the raw materials of the foamed core layer may also include polymethyl methacrylate, and the mass ratio of the modified general-purpose polystyrene matrix to the polymethyl methacrylate is 9:

1.

4. The micro / nano foamed light diffuser plate with an ultra-fine uniform pore structure according to claim 1, characterized in that: The modified general-purpose polystyrene matrix is ​​a polystyrene resin with a long-chain branched structure and a bimodal molecular weight distribution.

5. The micro / nano foamed light diffuser plate with an ultra-fine uniform pore structure according to claim 1, characterized in that: The chemical foaming agent has a microcapsule encapsulation structure; wherein, the main component of the foaming agent and the auxiliary foaming agent together constitute the core layer of the microcapsule, the dispersant or lubricant coats the surface of the core layer to form a first coating layer, the activator and the nucleating agent are attached to the outside of the first coating layer, and the carrier resin constitutes the outermost shell layer.

6. A method for preparing a micro / nano-foamed light diffuser plate with an ultrafine uniform pore structure as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: Mix the chemical foaming agent with the modified general-purpose polystyrene matrix to obtain the core layer raw material; Step 2: The materials for preparing the first and second skin layers are added to the skin extruder of the multilayer co-extrusion equipment for melt plasticization; the core layer raw material is added to the single-screw core layer extruder of the multilayer co-extrusion equipment for melt plasticization and foaming reaction; Step 3: The materials for preparing the first and second skin layers after melting and plasticizing, together with the core layer raw material after melting and foaming reaction, are fed into a three-layer co-extrusion die to form a three-layer co-extrusion melt arranged in "skin layer-core layer-skin layer"; Step 4: The three-layer co-extruded melt extruded from the three-layer co-extruded die is shaped and cooled, and then pulled and cut to obtain the micro-nano foamed light diffusion plate. In step 2, the foaming temperature of the single-screw core extruder is constantly controlled at 220℃~225℃, and the screw speed is controlled at 250rpm~320rpm; in step 4, the cooling rate during the entire shaping and cooling process is constantly controlled at 6.5℃ / min~8.0℃ / min.

7. The method according to claim 6, characterized in that, In steps 2 and 4, the extrusion and shaping cooling parameters are selected from one of the following parameter groups: Parameter group one: The foaming temperature of the single screw core extruder is 220℃, the screw speed is 250rpm, and the cooling rate of the shaping and cooling is 7.0℃ / min; Parameter group two: The foaming temperature of the single screw core extruder is 224℃, the screw speed is 300rpm, and the cooling rate of the shaping and cooling is 7.0℃ / min; Parameter group three: The foaming temperature of the single-screw core extruder is 225℃, the screw speed is 320rpm, and the cooling rate of the shaping and cooling process is 6.5℃ / min or 7.0℃ / min.

8. The method according to claim 6, characterized in that, In steps 3 and 4: When the co-extrusion process is adjusted to increase the thickness ratio of the first and second skin layers in the three-layer co-extrusion melt, the screw speed of the single-screw core extruder is simultaneously adjusted to 300 rpm, and the cooling rate of the shaping and cooling process is adjusted to 8.0 °C / min.

9. The method according to claim 6, characterized in that, In step 2: The single-screw core extruder has multiple heating zones on the outside of the barrel, with the temperature increasing in a continuous gradient along the material conveying direction; The multiple heating sections are sequentially divided into a preheating section, a compression section, and a foaming reaction section. The heating temperature of the preheating section is set between 150°C and 190°C, the heating temperature of the compression section is set between 190°C and 215°C, and the heating temperature of the foaming reaction section is set between 220°C and 225°C.

10. The method according to claim 6, characterized in that, In step 4: The shaping and cooling process is performed using a calendering roll group consisting of a first calendering cooling roll, a second calendering cooling roll, and a third calendering cooling roll arranged in sequence. Along the travel direction of the three-layer co-extrusion melt, the set temperatures of the roll surfaces of the first calendering cooling roll, the second calendering cooling roll, and the third calendering cooling roll are distributed in a stepwise decreasing manner. As the three-layer co-extrusion melt passes through the roll surfaces of each calendering cooling roll in sequence, its surface temperature decreases linearly over time, thereby maintaining the stringent cooling rate of 6.5℃ / min to 8.0℃ / min.

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