Physical foaming method of high polymer material

Abstract

The invention provides a high polymer material physical foaming method which comprises the following steps: S10, dividing a target finished product into at least two functional zones with different thicknesses in the length direction on the basis of thickness distribution of the target finished product in the length direction; s20, respectively setting a local foaming ratio for each functional zone; wherein the local foaming ratio is in direct proportion to the thickness distribution of the corresponding functional zones in the target finished product, and the local foaming ratio level difference between the adjacent functional zones in the length direction is 0.05-0.15; s30, forming a foamed green body based on each local foaming ratio, wherein the thickness of each position of the foamed green body in the length direction is equal to the ratio of the thickness of the target finished product in the corresponding position to the local foaming ratio corresponding to the position; and S40, carrying out supercritical physical foaming by taking the foamed green body as a raw material to obtain a target finished product. According to the foaming method, the problems of uneven hardness and poor dimensional stability of a finished product prepared by an existing process can be solved.

Description

Technical Field

[0001] This invention relates to the field of physical foaming technology, specifically to a method for physical foaming of polymer materials. Background Technology

[0002] Supercritical physical foaming technology for polymer materials utilizes gases such as carbon dioxide or nitrogen as foaming agents in a supercritical state to create a microporous structure in polymer materials. This technology is currently widely used in the production of lightweight, high-resilience polymer foamed parts, such as athletic shoe soles and shock-absorbing components. In existing foamed part manufacturing processes, a foam preform (small blank) is typically first formed through injection molding or compression molding, and then placed in an autoclave for impregnation and depressurization foaming. However, when the target finished product has a large thickness gradient along its length (such as the heel and forefoot of a shoe sole), existing processes result in uneven hardness and poor dimensional stability in the finished product. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned defects or problems in the prior art and to provide a physical foaming method for polymer materials. This foaming method can improve the problems of uneven hardness and poor dimensional stability of finished products prepared by existing processes.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: Technical Solution 1: A method for physical foaming of polymer materials, comprising the following steps: S10: Based on the thickness distribution of the target finished product along its length, the target finished product is divided into at least two functional zones with different thicknesses along its length; S20: A local foaming ratio is set for each functional zone; wherein the local foaming ratio is proportional to the thickness distribution of the corresponding functional zone in the target finished product, and the difference in local foaming ratio between adjacent functional zones along the length is 0.05-0.15; S30: A foamed preform is formed based on each local foaming ratio, wherein the thickness of the foamed preform at each position along its length is equal to the ratio of the thickness of the target finished product at the corresponding position to the local foaming ratio corresponding to that position; S40: The foamed preform is used as raw material for supercritical physical foaming to obtain the target finished product.

[0005] Technical Solution 2 based on Technical Solution 1: In step S10, at least the areas with a thickness greater than 15mm and the areas with a thickness less than 8mm in the target finished product are divided into different functional zones.

[0006] Technical Solution 3 based on Technical Solution 1: In step S10, the area of ​​the target finished product with a thickness variation rate of less than 0.1 along the length direction is divided into the same functional zone.

[0007] Technical solution four based on technical solution three: In step S10, a transition zone is further divided between adjacent functional zones in the length direction, and the transition zone is the area where the thickness change rate of the target finished product along the length direction is greater than 0.1.

[0008] Technical solution five based on technical solution four: In step S20, a transition foaming ratio is set for the transition zone, and the transition foaming ratio is the arithmetic mean of the local foaming ratios of the two functional zones adjacent to it in the length direction.

[0009] Technical Solution Six based on Technical Solution One: In step S20, a benchmark ratio is determined based on the ratio of the original density of the polymer material to the preset average target density of the target finished product; the local foaming ratio of at least one of the functional zones is set to the benchmark ratio, and the local foaming ratio of the remaining functional zones is increased or decreased based on the benchmark ratio by increasing or decreasing the local foaming ratio increment.

[0010] Technical solution seven based on technical solution five: In step S20, the functional partition with the local foaming ratio of the reference ratio is a functional partition with the thickness centered along the length direction, or when the number of functional partitions is two, it is a functional partition with the smallest thickness.

[0011] Technical solution eight based on technical solution six: The polymer material is one of thermoplastic polyurethane, polyether block amide, thermoplastic polyolefin elastomer or ethylene-vinyl acetate copolymer.

[0012] Technical solution nine based on technical solution eight: In step S20, the baseline foaming ratio of the thermoplastic polyurethane material is 2.2-2.3, and the local foaming ratio increment is 0.05.

[0013] As can be seen from the above description of the present invention, compared with the prior art, the present invention has the following beneficial effects: The reason why foamed preforms with varying thicknesses exhibit uneven hardness and poor dimensional stability after foaming is that during the physical foaming process, thicker areas have higher thermal inertia and cool more slowly during the depressurization cooling phase. This results in significantly greater secondary shrinkage stress within the material compared to thinner areas. Conventional physical foaming processes use a uniform foaming ratio, leading to dimensional shrinkage or surface collapse in thicker areas, while thinner areas may have higher hardness due to limited foaming. Furthermore, the difference in secondary shrinkage stress can cause uneven stress distribution, leading to warping and deformation during placement and affecting dimensional stability.

[0014] To address this, the present invention provides a method for physical foaming of polymer materials. This method first divides the target finished product into different functional zones and sets a local foaming ratio proportional to the thickness distribution. This results in a higher local foaming ratio in the thicker functional zones, allowing these zones to gain stronger expansion kinetic energy during the pressure relief process of physical foaming. The increased expansion displacement offsets the subsequent shrinkage energy caused by slow heat dissipation, thereby improving the problems of uneven hardness and poor dimensional stability caused by thickness differences. However, because unequal expansion spaces are artificially introduced within the same continuous material, during the physical foaming stage, adjacent functional zones along the length direction will experience kinetic energy imbalance at the physical boundaries due to the difference in local foaming ratios. Therefore, this solution uses step S30 to proportionally convert the thickness of each position in the foamed preform, transforming this artificially set ratio difference into accurate material distribution in the spatial dimension of the foamed preform. This not only compensates for macroscopic shrinkage but also absorbs the interfacial kinetic energy imbalance caused by the difference in local foaming ratios.

[0015] In technical solution two, areas with a thickness greater than 15mm and areas with a thickness less than 8mm are divided into different functional zones. Two specific threshold areas are set for independent segmentation and assigned different local foaming ratios to ensure precise expansion kinetic energy compensation at the parts with the most significant shrinkage energy deviation, thereby minimizing the performance gap between the thick and thin areas of the target finished product.

[0016] In technical solution three, areas with a thickness variation rate of less than 0.1 are divided into the same functional zone, ensuring the uniformity of the cell structure within a single functional zone.

[0017] In technical solution four, a transition zone is divided between adjacent functional zones, and a thickness change rate greater than 0.1 is used as the criterion to establish a buffer zone between adjacent functional zones. This further improves the kinetic energy imbalance at the physical boundary caused by the inconsistent expansion rates between adjacent functional zones, and can effectively prevent the accumulation of kinetic energy between adjacent functional zones during the foaming process, which would lead to disordered cell structure in this area. In technical solution five, the arithmetic mean of the local foaming ratio of adjacent functional zones is set for the transition zone. This prevents interface kinetic energy conflicts caused by drastic changes in ratio when the foaming kinetic energy evolves from one functional zone to another, smooths the stress gradient, and solves the problem of boundary cell collapse or structural delamination caused by excessive differences in local expansion forces.

[0018] In technical solution six, the benchmark ratio is determined based on the ratio of the original density of the polymer material to the preset average target density of the target finished product. The benchmark ratio is used as the calculation origin, and the ratio of the remaining zones is determined by increasing or decreasing the ratio difference, so as to ensure that the setting of the local foaming ratio is accurate and reasonable.

[0019] In technical solution seven, the reference magnification is set at the section with the central thickness or the smallest thickness. By adjusting the magnification from the central thickness position to both ends, or by compensating upwards step by step from the thinnest position, the magnification growth trend and the thermal shrinkage growth trend caused by the increase in thickness are synchronized in direction. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are preferred embodiments of the present invention and should not be considered as excluding other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] In the claims and description of this invention, the terms "comprising," "having," and variations thereof are used to mean "including but not limited to."

[0022] This invention relates to a physical foaming method for polymer materials, which mainly includes the following steps: S10: Based on the thickness distribution of the target finished product along the length direction, the target finished product is divided into at least two functional zones with different thicknesses along the length direction; S20: Set a local foaming ratio for each functional zone; wherein the local foaming ratio is proportional to the thickness distribution of the corresponding functional zone in the target finished product, and the local foaming ratio difference between adjacent functional zones along the length direction is 0.05-0.15; S30: Based on each of the aforementioned local foaming ratios, a foamed preform is formed, wherein the thickness of the foamed preform at each position along its length is equal to the ratio of the thickness of the target finished product at the corresponding position to the local foaming ratio corresponding to that position; S40: Supercritical physical foaming is performed using the foamed preform as raw material to obtain the target finished product.

[0023] The following provides a detailed explanation of each of the above steps. In step S10, the geometric model data of the target finished product is first obtained, and its thickness distribution along the length direction is accurately measured and analyzed. Based on the measured thickness distribution data, the target finished product is divided into at least two functional zones with different thickness attributes along the length direction. The division process is completed by identifying the geometric features of each part of the target finished product, aiming to establish the physical basis for subsequent differentiated foaming ratio settings. In the specific division process, considering the difference in thermal inertia of the material during the physical foaming and cooling stage, there is a fundamental difference in the cooling rate between areas with larger thickness and areas with smaller thickness. Therefore, it is necessary to divide the areas with a thickness greater than 15 mm and areas with a thickness less than 8 mm in the target finished product into different functional zones to ensure that these two parts with significant thermal inertia deviations can be independently controlled.

[0024] To ensure the uniformity of physical properties within each functional zone, the classification criteria also reference the thickness variation rate of the target finished product along its length. The thickness variation rate refers to the change in thickness of the target finished product per unit length, where both unit length and thickness change are measured in millimeters (mm). For regions in the target finished product with a thickness variation rate less than 0.1, due to their relatively smooth thickness transition and more uniform physical stress distribution, these regions with stable thickness evolution are classified as the same functional zone. This approach ensures a consistent benchmark for the physical foaming process within each functional zone, thereby improving the uniformity of the cell structure within a single zone.

[0025] Between two adjacent functional zones along the length direction, there are often areas with drastic thickness fluctuations. When the thickness variation rate of the target finished product along the length direction is greater than 0.1, this area is defined as a transition zone. The transition zone is located between two functional zones with significantly different thickness properties, serving as a geometric connection and physical buffer. By defining an independent transition zone along the length direction, a spatial carrier can be provided for the subsequent stepwise evolution of foaming kinetic energy.

[0026] After completing the functional partitioning, step S20 is executed to set a precise local foaming ratio for each partition. The setting process first requires determining a baseline ratio, which is obtained by calculating the ratio of the original density of the polymer material to the preset average target density of the finished product. In specific implementations, the baseline ratio is assigned to specific functional partitions. If the target product is divided into only two functional partitions, the baseline ratio is set to the local foaming ratio of the functional partition with the smallest thickness. If there are more than two functional partitions, the baseline ratio is set to the local foaming ratio of the functional partition located in the middle along the length direction. The local foaming ratios of the remaining functional partitions are determined based on this baseline ratio by increasing or decreasing the local foaming ratio increments.

[0027] The local foaming ratio is set according to the principle of being proportional to the thickness distribution of the target finished product. This means that functional zones with greater thickness in the target finished product will be assigned a higher local foaming ratio, while functional zones with less thickness will be assigned a lower local foaming ratio. To ensure the smoothness of stress transition between adjacent zones, the difference in local foaming ratio between adjacent functional zones along the length direction is strictly controlled within the range of 0.05 to 0.15. For a specific transition zone, its transition foaming ratio is set as the arithmetic mean of the local foaming ratios of the two functional zones adjacent to it along the length direction. Through this median setting, the foaming kinetic energy can form a smooth, step-like evolution between functional zones, thereby eliminating the interfacial kinetic energy imbalance caused by abrupt changes in ratio and avoiding damage to the cell structure at the boundary.

[0028] This method is applicable to a variety of polymer materials, including thermoplastic polyurethane, polyether block amide, thermoplastic polyolefin elastomers, or ethylene-vinyl acetate copolymers. Specific parameter settings will be adjusted according to different material properties. Taking thermoplastic polyurethane as an example, its baseline foaming ratio is typically set in the range of 2.2 to 2.3, while the local foaming ratio difference between adjacent zones is fixed at 0.05.

[0029] After setting the local foaming ratio in step S20, step S30 is executed to form the foamed preform. In the specific design process, the thickness of the foamed preform at each position along its length is strictly calculated according to a preset ratio, that is, the thickness at that position is equal to the ratio of the thickness of the target finished product at the corresponding position to its corresponding local foaming ratio.

[0030] Because the local foaming ratio of each functional zone and transition zone is set according to the thickness distribution differences, the final foamed preform exhibits a non-proportional change in geometry relative to the target finished product. For example, in functional zones where the target finished product is thicker and a higher local foaming ratio is allocated, the corresponding foamed preform thickness will be reduced accordingly, thus reserving sufficient space for the subsequent dramatic expansion during the foaming process. In the specific process implementation of molding the foamed preform, injection molding or compression molding can be used. The cavity dimensions of the molding die are precisely made according to the thickness distribution calculated above, ensuring that every coordinate point along the length of the foamed preform can accurately bear the set amount of material.

[0031] Step S40 is the final foaming and molding stage of this method. In this step, the foamed preform formed in step S30 is used as the raw material and placed in a supercritical foaming device for physical foaming treatment to finally obtain the target finished product. In specific implementation, the foamed preform is placed in a pressure vessel, and a physical foaming agent such as carbon dioxide or nitrogen is injected to allow the foaming agent to penetrate into the interior of the polymer material in a supercritical state and reach a wetting saturation state.

[0032] In the subsequent depressurization phase, with the instantaneous release of pressure, the foaming agent permeating the foamed preform rapidly expands and forms a uniform microporous structure. Since the foamed preform has already undergone a non-proportional thickness distribution based on the local foaming ratio in step S30, each functional zone and transition zone will generate expansion kinetic energy matching its geometric thickness. The larger functional zones, due to the lower relative thickness and higher local foaming ratio reserved in the preform stage, can precisely offset the larger contraction displacement caused by thermal inertia during the subsequent cooling stage with their expansion displacement.

[0033] This foaming step is applicable to the molding of various polymer materials, including thermoplastic polyurethane, polyether block amide, thermoplastic polyolefin elastomer, and ethylene-vinyl acetate copolymer. Depending on the physical properties of different materials, the corresponding temperature and pressure parameters are adjusted in step S40.

[0034] To further illustrate the technical effects of this solution, the following embodiments and comparative examples are designed.

[0035] The specific grades and manufacturers of the raw materials used in the embodiments and comparative examples of this invention are as follows: The first raw material is a thermoplastic polyurethane elastomer produced by Wanhua Chemical Group Co., Ltd., with the grade WHT-1192, a hardness of Shore 92A, and an original density of 1.20 g / cm³.

[0036] The second raw material is a polyether block amide produced by Arkema, France, with the brand name Pebax 5533 and an original density of 1.01 g / cm³.

[0037] The physical foaming agent uses high-pressure nitrogen gas with a purity of not less than 99.9%.

[0038] To verify the beneficial effects of the preparation method of this invention, the performance of the obtained target product was tested. The testing standards and descriptions are as follows: Hardness testing was conducted according to the national standard GB / T 531.1-2008, "Test Method for Indentation Hardness of Vulcanized Rubber or Thermoplastic Rubber - Part 1: Shore Hardness Tester Method." Due to the softness of the foamed material, a Shore C hardness tester (Asker C hardness) was specifically used. The test locations were the center point of the heel (thickest part) and the center point of the forefoot (thinnest part) in the target finished product. Five measurements were taken, and the average value was calculated. The hardness deviation between the two locations was then calculated, i.e., the hardness range.

[0039] The density test was conducted in accordance with the national standard GB / T 533-2008 Determination of density of vulcanized rubber or thermoplastic rubber, using the impregnation method therein.

[0040] Dimensional stability is assessed by measuring the 24-hour shrinkage rate. The target finished product is removed from the foaming mold and left at room temperature for 24 hours. The actual length of the finished product is then measured using precision calipers. The shrinkage rate is equal to the difference between the mold cavity length and the actual length of the finished product, divided by the mold cavity length. The result is expressed as a percentage. The lower the shrinkage rate, the better the dimensional stability of the finished product.

[0041] Visual evaluation involves inspecting the surface of the finished product. Special attention should be paid to areas with greater thickness to check for localized depressions, obvious shrinkage marks, or overall geometric distortion.

[0042] Example 1 Example 1 uses the aforementioned thermoplastic polyurethane elastomer with the grade WHT-1192 as raw material, and the target finished product is a midsole for an athletic shoe. The maximum thickness of the heel portion of the midsole is 30mm, and the minimum thickness of the forefoot portion is 8mm.

[0043] In step S10, the heel portion with a thickness greater than 15mm is divided into functional zone A, the forefoot portion with a thickness less than 8mm is divided into functional zone B, and the middle portion with a thickness between 8mm and 15mm is divided into functional zone C. Areas with a thickness variation rate greater than 0.1 are identified as transition zones D.

[0044] In step S20, a baseline expansion ratio is calculated based on the raw material density and the preset target average density of the finished product. The local foaming ratio of functional zone C is set to the baseline expansion ratio of 2.25. Based on the principle that thickness distribution is proportional, the local foaming ratio of functional zone A (with the largest thickness) is set to 2.30, and the local foaming ratio of functional zone B (with the smallest thickness) is set to 2.20. The transition foaming ratio of transition zone D is set to 2.275, with the increment controlled within 0.05.

[0045] In step S30, the preform thickness is calculated and the foamed preform is injection molded. The preform thickness at the location corresponding to functional zone A is 13.04 mm, and the preform thickness at the location corresponding to functional zone B is 3.64 mm.

[0046] In step S40, the foamed preform is placed in a high-pressure autoclave, nitrogen gas is introduced to bring the pressure to 18 MPa, the temperature is controlled at 150°C, and after wetting equilibrium is achieved, the pressure is released instantaneously within 100 ms to obtain the finished product.

[0047] Example 2 Example 2 uses the above-mentioned polyether block amide with the brand name Pebax 5533 as a raw material.

[0048] In step S10, the partitioning logic is the same as in Example 1.

[0049] In step S20, the base expansion ratio is set to 2.80. The local foaming ratio of the thick functional zone A is set to 2.90, the local foaming ratio of the thin functional zone B is set to 2.70, and the transition zone is set to 2.80. The increment is 0.10.

[0050] In step S30, the thickness of the blank at each position is calculated and the foamed blank is formed.

[0051] In step S40, the immersion pressure is set to 20 MPa, the temperature is set to 155 °C, and the pressure relief time is set to 100 ms.

[0052] Comparative Example 1 Comparative Example 1 uses the exact same materials and target finished product shape as Example 1, but the foamed preform adopts a traditional design method of being scaled down proportionally as a whole.

[0053] The foaming ratio was set to a uniform 2.25 for the entire palm. The corresponding thickness of the foamed preform at the heel was 13.33 mm, and the thickness at the forefoot was 3.56 mm. The process parameters such as pressure, temperature, and time for physical foaming were completely consistent with those in Example 1.

[0054] Comparative Example 2 Comparative Example 2 used the same polyether block amide material as Example 2.

[0055] The foaming ratio was set to a uniform 2.80 for the entire foam. The corresponding foamed preform was scaled down proportionally. The physical foaming process parameters were exactly the same as in Example 2.

[0056] The performance test results are shown below: In Example 1, the heel hardness was 52 Asker C, the forefoot hardness was 53 Asker C, and the hardness difference was 1 degree. The 24-hour shrinkage rate was 0.5%. The appearance was smooth with no collapse.

[0057] Comparative Example 1 showed a heel hardness of 47 Asker C and a forefoot hardness of 55 Asker C, with a hardness range of 8. The 24-hour shrinkage rate was 2.8%. Visually, this manifested as noticeable surface collapse at the heel.

[0058] In Example 2, the heel hardness was 45 Asker C, the forefoot hardness was 46 Asker C, and the hardness difference was 1 degree. The 24-hour shrinkage rate was 0.4%. The surface appeared smooth.

[0059] Comparative Example 2 showed a hardness of 40 Asker C at the heel and 49 Asker C at the forefoot, with a hardness range of 9. The 24-hour shrinkage rate was 3.2%. Visually, shrinkage lines were observed at the heel.

[0060] Based on the test results above, regarding hardness uniformity, in Comparative Example 1 and Comparative Example 2, when using a uniform foaming ratio commonly used in existing technologies, the hardness at the heel of the finished product was significantly lower than that at the forefoot. This is because during the supercritical physical foaming process, areas with greater thickness have higher thermal inertia, resulting in slower internal cooling after pressure release, longer cell growth time, and accompanied by severe secondary shrinkage, leading to a loose microstructure and decreased macroscopic hardness. However, Examples 1 and 2 of this invention, by setting the local foaming ratio to be proportional to the thickness distribution, pre-impart higher expansion kinetic energy to the thicker areas. This differentiated energy distribution utilizes the increased expansion displacement to offset the subsequent thermal shrinkage stress, reducing the hardness difference between the heel and forefoot of the finished product to approximately 1 degree, achieving a high degree of consistency in the physical properties of the entire foot.

[0061] Regarding dimensional stability, the shrinkage rates of Comparative Example 1 and Comparative Example 2 both exceeded 2.5%, and shrinkage and collapse occurred in the thicker areas. This demonstrates that in parts with thickness gradients, a uniform ratio cannot suppress localized thermal shrinkage. The embodiments of this invention, through the non-proportional variation of the blank designed in step S30, relatively reduce the thickness of the blank in the thicker areas, allowing the expansion force generated during foaming to counteract the shrinkage stress during the subsequent cooling stage. Experimental data show that the shrinkage rate of the finished product using this method is controlled within 0.5%, solving the dimensional instability problem mentioned in the background art.

[0062] Furthermore, in Example 1, by dividing the transition zone in step S10 and setting the arithmetic mean scaling factor in step S20, a smooth, stepped transition of foaming kinetic energy is achieved between regions with different scaling factors. Observing the interface of the finished product of the example, its cell structure is intact and there are no stress marks. This indicates that the present invention not only solves the macroscopic difference in thickness performance, but also solves the problem of interface kinetic energy imbalance caused by scaling factor differences through the design of the transition zone.

[0063] The foregoing description of the specifications and embodiments is intended to explain the scope of protection of this invention, but does not constitute a limitation on the scope of protection of this invention. Modifications, equivalent substitutions, or other improvements to the embodiments of this invention or a portion thereof that can be obtained by those skilled in the art through logical analysis, reasoning, or limited experimentation, based on the teachings of this invention or the foregoing embodiments, in conjunction with common knowledge, general technical knowledge, and / or existing technology, should all be included within the scope of protection of this invention.

Claims

1. A physical foaming method for polymer materials, characterized in that, Includes the following steps: S10: Based on the thickness distribution of the target finished product along the length direction, the target finished product is divided into at least two functional zones with different thicknesses along the length direction; S20: Set a local foaming ratio for each functional zone; wherein the local foaming ratio is proportional to the thickness distribution of the corresponding functional zone in the target finished product, and the local foaming ratio difference between adjacent functional zones along the length direction is 0.05-0.15; S30: Based on each of the aforementioned local foaming ratios, a foamed preform is formed, wherein the thickness of the foamed preform at each position along its length is equal to the ratio of the thickness of the target finished product at the corresponding position to the local foaming ratio corresponding to that position; S40: Supercritical physical foaming is performed using the foamed preform as raw material to obtain the target finished product.

2. The physical foaming method for polymer materials as described in claim 1, characterized in that, In step S10, at least the areas with a thickness greater than 15 mm and the areas with a thickness less than 8 mm in the target finished product are divided into different functional zones.

3. The physical foaming method for polymer materials as described in claim 1, characterized in that, In step S10, the area where the thickness variation rate of the target finished product along the length direction is less than 0.1 is divided into the same functional zone.

4. The physical foaming method for polymer materials as described in claim 3, characterized in that, In step S10, a transition zone is further defined between adjacent functional zones along the length direction. The transition zone is the area where the thickness variation rate of the target finished product along the length direction is greater than 0.

1.

5. The physical foaming method for polymer materials as described in claim 4, characterized in that, In step S20, a transition foaming ratio is set for the transition zone, which is the arithmetic mean of the local foaming ratios of the two functional zones adjacent to it in the length direction.

6. The physical foaming method for polymer materials as described in claim 1, characterized in that, In step S20, a reference ratio is determined based on the ratio of the original density of the polymer material to the preset average target density of the target finished product; the local foaming ratio of at least one of the functional zones is set to the reference ratio, and the local foaming ratio of the remaining functional zones is increased or decreased based on the reference ratio by increasing or decreasing the local foaming ratio increment.

7. The physical foaming method for polymer materials as described in claim 5, characterized in that, In step S20, the functional partition with the local foaming ratio of the reference ratio is a functional partition with a thickness centered along the length direction, or a functional partition with the smallest thickness when the number of functional partitions is two.

8. The physical foaming method for polymer materials as described in claim 6, characterized in that, The polymer material is one of thermoplastic polyurethane, polyether block amide, thermoplastic polyolefin elastomer, or ethylene-vinyl acetate copolymer.

9. The physical foaming method for polymer materials as described in claim 8, characterized in that, In step S20, the baseline foaming ratio for thermoplastic polyurethane material is 2.2-2.3, and the local foaming ratio increment is 0.05.