A dual density foamed midsole, an athletic shoe comprising the midsole, and a method of making the same

Abstract

The application belongs to the technical field of supercritical foaming processing of shoe materials, and discloses a double-density foamed insole, sports shoes containing the insole and a preparation method of the insole. The preparation method comprises the following steps: a double-color double-screw injection molding system is used, hard layer slurry added with a nucleating agent and soft layer slurry without the nucleating agent are injected into a mold cavity at a time difference of 0.2s-2s; a layered pressure maintaining, cooling and shaping, mold opening and demolding are performed to obtain an integrated dense embryo; and the embryo is gradient-foamed in CO2 / N2 mixed supercritical gas to obtain the double-density foamed insole. The application realizes interlayer molecular fusion and eliminates the delamination cracking defect by means of differential nucleating agent matching of the two layers of slurry, supercritical gradient foaming and staggered injection molding. The two structures of soft inside and hard outside and hard inside and soft outside can be switched, and support and buffer rebound are simultaneously considered. The whole process is one-time injection molding + one-time foaming molding, the production efficiency is improved by more than 35%, the yield is high, and the application is suitable for various high-end professional sports shoes.

Description

Technical Field

[0001] This invention belongs to the field of supercritical foaming processing technology for footwear materials, specifically relating to a dual-density foamed midsole with integrated gradient hardness, a sports shoe containing the midsole, and a method for preparing the same. Background Technology The midsole is a core functional component of athletic and casual shoes, directly determining the shoe's support, cushioning, and comfort. Currently, most midsoles on the market are made of a single material and with a single hardness, making it difficult to simultaneously achieve both cushioning and support performance: when the material is too soft, the midsole is prone to collapse under external force and deforms severely with long-term use; when the material is too hard, there is insufficient cushioning for the foot, resulting in a stiff feel and poor comfort.

[0002] To address these issues, existing technologies have developed double-layer midsoles, primarily relying on a combination of processes such as secondary injection molding, adhesive bonding, and thermoforming. However, these processes are complex, have low production efficiency, and the bonding between the two layers relies solely on physical adhesion, resulting in weak interlayer bonding strength. Consequently, the products are prone to delamination and cracking after repeated bending and pressure, leading to poor durability.

[0003] Currently, conventional two-color injection molding processes mainly fall into two categories: simultaneous injection and partitioned injection. Simultaneous injection results in large-scale mixing of inner and outer layers, blurring the hardness gradient boundaries and making precise control of mechanical properties impossible. Partitioned injection increases mold costs, and even after the partitions are removed, weak interfacial bonding remains a problem. Furthermore, traditional processes add nucleating agents as a whole to all raw materials, making it impossible to differentiate the crystallization rate and grain size of the inner and outer layers. This results in a product with a single cell structure, and the inability to simultaneously optimize resilience, lightweighting, and dimensional stability.

[0004] Furthermore, conventional supercritical foaming processes can only achieve homogeneous foaming, producing homogeneous foamed materials with a single density and single cell structure, and cannot form a differentiated gradient microporous structure between layers. When applied to the production of composite midsoles with soft and hard layers, it is difficult to meet the high-end application requirements of integrated molding, flexible adjustment of soft and hard parameters, and high-strength bonding between interlayer interfaces. Based on the numerous shortcomings of existing technologies, developing a simple, interface-strong, and high-performance dual-density foamed midsole preparation method has become a research focus in this field. Summary of the Invention

[0005] This invention addresses the shortcomings of existing double-layer midsole processes, such as cumbersome manufacturing processes, susceptibility to delamination and cracking, uncontrollable hardness gradients in two-color injection molding, and the inability of conventional supercritical foaming to achieve gradient molding. It provides a dual-density foamed midsole and its preparation method. This invention combines three core processes: staggered two-color injection molding, differentiated nucleating agent addition, and supercritical mixed gas gradient foaming, achieving integrated molding that balances support and cushioning, improving product durability and production efficiency.

[0006] In a first aspect, the present invention provides a method for preparing a dual-density foamed insole, comprising the following steps: using a two-color twin-screw injection molding system, a hard layer slurry and a soft layer slurry are respectively delivered and injected into a mold cavity with a time difference of 0.2s to 2s; the hard layer slurry contains a nucleating agent, and the soft layer slurry does not contain a nucleating agent; the materials in the mold cavity are sequentially subjected to layered pressure holding, cooling and shaping, followed by mold opening and demolding to obtain an integrated dense preform; the integrated dense preform is placed in a supercritical mixed gas environment for gradient foaming treatment to obtain a dual-density foamed insole.

[0007] In a preferred embodiment, the time difference of the staggered injection is set to 0.5s to 1.5s.

[0008] As a preferred embodiment, the specific process parameters for the supercritical gradient foaming are as follows: in a supercritical gas environment, the ambient pressure is controlled at 30MPa~45MPa and the temperature at 130℃~140℃, and the heat and pressure holding treatment is carried out for 120min~180min; after the pressure holding is completed, the pressure is rapidly released at a pressure release rate of 5MPa / s~9MPa / s to complete the gradient foaming, with the preferred pressure release rate being 7MPa / s.

[0009] In a preferred embodiment, the supercritical gas is a mixture of carbon dioxide and nitrogen, with a volume ratio of 7-8:2-3.

[0010] In one preferred embodiment, the rigid layer slurry is composed of a solvent, a nucleating agent, and unfoamed granules; the soft layer slurry is composed of a solvent and unfoamed granules, and the performance difference between the two slurries is achieved by whether or not a nucleating agent is added.

[0011] In a preferred embodiment, the unfoamed granules are selected from unfoamed thermoplastic elastomers, preferably one or more of polypropylene (PP), nylon, thermoplastic polyurethane (TPU), and ethylene-vinyl acetate copolymer (EVA); the nucleating agent is selected from one or more of β-crystalline polypropylene nucleating agents, talc, and organophosphate nucleating agents.

[0012] In a preferred embodiment, the amount of nucleating agent added is 1wt% to 2wt%, based on the total mass of unfoamed granules in the hard layer slurry.

[0013] Secondly, the present invention also provides a dual-density foamed midsole, which is prepared by the preparation method described in any of the above embodiments.

[0014] Thirdly, the present invention also provides a sports shoe, including the above-mentioned dual-density foam midsole.

[0015] The present invention has the following beneficial effects: 1. High interface bonding strength and excellent durability. This invention employs a dual-color, twin-screw, staggered injection molding process, in which two layers of slurry are fused together at high temperature within the mold, eliminating the need for glue bonding and secondary injection molding. The interlayer has a molecular-level fusion interface, resulting in a bonding strength far exceeding that of traditional bonding processes. This completely eliminates the problems of delamination and cracking during long-term product use, significantly improving the product's fatigue resistance and bending resistance.

[0016] 2. Adjustable performance, adaptable to a wide range of scenarios. This invention allows for flexible adjustment of the injection sequence: injecting the soft layer first and then the hard layer can create a midsole with an inner soft and outer hard structure, balancing cushioning and external support performance, and offering advantages such as good shock absorption, strong grip, excellent wear resistance, and controllable propulsion; alternatively, injecting the hard layer first and then the soft layer can create a midsole with an inner hard and outer soft structure, balancing structural rigidity and a soft feel; it can be adapted to various sports and usage scenarios such as professional racing shoes, everyday casual shoes, and training shoes.

[0017] 3. Simplified process and high production efficiency This invention requires only one injection molding and one supercritical foaming to complete the finished product preparation, eliminating the need for multiple processes such as secondary processing, bonding, and trimming in traditional double-layer midsoles. Compared with traditional processes, production efficiency is increased by more than 35%. The simplified production process reduces human intervention, significantly improves the product yield, and effectively reduces the overall production cost of enterprises.

[0018] 4. Performance is precise and controllable, with good product consistency. This invention can precisely control the crystallinity, hardness, pore size, foaming ratio, and rebound performance of the two layers of the midsole by adjusting parameters such as the amount of nucleating agent added, injection sequence, foaming temperature, foaming pressure, and depressurization rate. The entire process adopts closed-loop parameter control, resulting in small performance differences between batches of products and excellent product consistency, which can meet the stringent quality requirements of high-end professional sports shoes.

[0019] 5. Balanced overall performance Relying on supercritical gradient foaming technology, the two layers of the midsole form a gradient of cell density, pore size and foaming ratio. While achieving a lightweight design, it also takes into account wearing comfort and overall dimensional stability. Its comprehensive performance is superior to traditional single-layer foamed midsoles and composite bonded midsoles. Attached Figure Description

[0020] This specification includes two accompanying drawings to explain the product structure of this invention: Figure 1 is a schematic diagram of the layered structure of the inner soft and outer hard dual-density foamed midsole described in Embodiment 1 of the present invention; Figure 2 is a schematic diagram of the layered structure of the dual-density foamed midsole with an inner hard and outer soft structure as described in Embodiment 7 of the present invention.

[0021] Explanation of reference numerals in the attached figures: 1-Hard layer; 2-Soft layer. Detailed Implementation

[0022] The technical solution of the present invention will be described in detail below with reference to specific embodiments. The following embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Simple modifications and equivalent substitutions made by those skilled in the art without departing from the core concept of the present invention all fall within the protection scope of the present invention.

[0023] Unless otherwise specified, all raw materials and equipment used in this invention are commercially available conventional products; cyclohexane is uniformly selected as the slurry solvent, with 80mL of cyclohexane used for every 100g of unfoamed granules; the injection molding, holding pressure, and cooling parameters are uniform throughout the invention; the solvent cyclohexane can be replaced with other alkane solvents depending on the type of resin, and is not limited to this embodiment.

[0024] Example 1 A method for preparing a dual-density foamed midsole, comprising the following steps: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 1g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0025] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively; first, 60mL of soft slurry is injected into the mold to form the inner layer, and after a delay of 0.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure. Figure 1 As shown, it has the advantages of good cushioning, strong grip, good wear resistance, and controllable propulsion, making it suitable for professional racing shoes, training shoes, and sports and usage scenarios.

[0026] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0027] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0028] Example 2 A method for preparing a dual-density foamed midsole is identical to that of Example 1 in terms of raw materials, injection sequence, injection time difference, and holding pressure and cooling parameters. The only difference lies in the supercritical foaming parameters: the supercritical gas is a mixture of CO2 and N2 with a volume ratio of 8:2; the foaming pressure is 45 MPa; the temperature is 130°C; and the holding pressure and temperature are maintained for 180 min. The depressurization rate remains constant at 7 MPa / s. The remaining steps are exactly the same as in Example 1. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 1g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0029] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 0.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0030] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0031] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 8:2, and the pressure was set at 45MPa and the temperature at 130℃ for 180min. Then, the pressure was released at a rate of 7MPa / s to obtain a dual-density foamed midsole.

[0032] Example 3 A method for preparing a dual-density foamed midsole, the basic steps of which are the same as in Example 1, the difference being: the amount of nucleating agent added to the rigid layer slurry is 1.5g; the staggered injection time difference is adjusted to 1.5s; and the foaming process parameters are the same as in Example 1. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 1.5g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0033] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 1.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0034] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0035] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0036] Example 4 A method for preparing a dual-density foamed midsole, the basic steps of which are the same as in Example 1, the difference being: the amount of nucleating agent added to the rigid layer slurry is 2g; the staggered injection time difference is adjusted to 0.2s; and the foaming process parameters are the same as in Example 1. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 2g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0037] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 0.2s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0038] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0039] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0040] Example 5 A method for preparing a dual-density foamed midsole is disclosed. The injection molding sequence, injection time difference, foaming parameters, and holding and cooling parameters are consistent with those in Example 1, except that the unfoamed granules are replaced with thermoplastic polyurethane (TPU), the nucleating agent is replaced with talc, and cyclohexane remains the solvent. The remaining operational steps are unchanged. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g TPU and 1g nucleating agent talc, add to 80mL cyclohexane and stir until well mixed; adding a nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g TPU and add to 80mL cyclohexane and stir until well mixed; without adding a nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent cushioning and resilience.

[0041] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 0.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0042] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0043] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0044] Example 6 A method for preparing a dual-density foamed midsole, with process parameters and injection molding sequence consistent with Example 1, differing in that: the unfoamed granules are replaced with ethylene-vinyl acetate copolymer (EVA) of DuPont grade V5110J, the nucleating agent is replaced with organophosphate nucleating agent NA-11, and cyclohexane is still used as the solvent; the remaining operation steps remain unchanged. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g EVA and 1g organophosphate nucleating agent NA-11, add to 80mL cyclohexane and stir until well mixed; adding nucleating agent can improve the crystallinity of the material, resulting in finer crystals, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g EVA and add to 80mL cyclohexane and stir until well mixed; without adding nucleating agent, the material crystallizes slowly, has larger crystal size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0045] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 0.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0046] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0047] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0048] Example 7 A method for preparing a dual-density foamed midsole, with the same raw material ratio, foaming process parameters, and pressure holding and cooling parameters as in Example 1, differs in the injection sequence: first, 40 mL of hard layer slurry is injected into the mold to form the inner layer; after a 0.5-second delay, 60 mL of soft layer slurry is injected to form the outer layer, thus preparing a dual-density foamed midsole with a hard inner and soft outer structure, suitable for everyday casual shoe use. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 1g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0049] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively; first, 40mL of hard slurry is injected into the mold to form the inner layer, and after a delay of 0.5s, 60mL of soft slurry is injected to form the outer layer, thus preparing a preform with a hard inner and soft outer structure, such as... Figure 2 As shown.

[0050] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0051] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0052] Comparative Example 1 This comparative example uses the same raw materials, foaming parameters, and pressure holding and cooling parameters as Example 1. The difference lies in that the rigid layer slurry and the soft layer slurry are injected into the mold simultaneously, without any time difference. The remaining steps are exactly the same. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 1g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0053] 2. Synchronous injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively; at the same time, 60mL of soft slurry is injected into the mold to form the inner layer and 40mL of hard slurry to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0054] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0055] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0056] Comparative Example 2 This comparative example follows the same injection molding, pressure holding, and cooling processes as Example 1, except that ordinary air is used instead of supercritical mixed gas for foaming, while the foaming pressure, temperature, processing time, and depressurization rate remain consistent with Example 1. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 1g of nucleating agent Huafeng NC60A, add to 80mL of cyclohexane and stir until well mixed; adding the nucleating agent can improve the crystallinity of the material, resulting in finer grains, higher hardness, and better dimensional stability of the finished product. Soft layer slurry: Take 100g of nylon DRS93 and add to 80mL of cyclohexane and stir until well mixed; without the addition of the nucleating agent, the material crystallizes slowly, has larger grain size, and the finished product has a soft texture and excellent buffering and rebound properties.

[0057] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 0.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0058] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0059] 4. Supercritical gradient foaming The integrated dense preform was placed in a normal air environment with a pressure of 30 MPa and a temperature of 140°C, and kept at the same temperature and pressure for 120 minutes. Then, the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0060] Comparative Example 3 This comparative example is classified as the control group, and its process steps, injection molding parameters, and foaming parameters are consistent with those of Example 1. The difference is that the nucleating agent added to the rigid layer slurry is 2 wt%, and the nucleating agent added to the soft layer slurry is 0.5 wt%. The specific process is as follows: 1. Slurry preparation Hard layer slurry: Take 100g of nylon DRS93 and 2g of nucleating agent Huafeng NC60A, add them to 80mL of cyclohexane and stir until well mixed. Soft layer slurry: Take 100g of nylon DRS93 and 0.5g of nucleating agent Huafeng NC60A, add them to 80mL of cyclohexane and stir until well mixed.

[0061] 2. Staggered injection molding Two independent screws in a two-color injection molding machine are used to complete the plasticization and precise metering of the slurry respectively. First, 60mL of soft slurry is injected into the mold to form the inner layer. After a delay of 0.5s, 40mL of hard slurry is injected to form the outer layer, thus preparing a preform with a soft inner and hard outer structure.

[0062] 3. Pressure holding, cooling, and demolding The mold holding pressure delay time is 2s, the pressure ratio of the hard layer to the soft layer is 1.5:1, and the holding pressure duration is 15s; the mold is water-cooled, the cooling temperature is 35℃, and the cooling time is 60s; after completion, the mold is opened and demolded to obtain an integrated dense blank.

[0063] 4. Supercritical gradient foaming The integrated dense preform was placed in a supercritical gas environment of CO2 and N2 mixed in a volume ratio of 7:3, and the pressure was set at 30 MPa and the temperature at 140℃ for 120 min. Then the pressure was released at a rate of 7 MPa / s to obtain a dual-density foamed midsole.

[0064] The grades “Nylon DRS93”, “Huafeng NC60A”, “TPU 9075AU”, “EVA V5110J”, etc. mentioned above are only preferred examples. Commercially available resins and nucleating agents with equivalent performance can be substituted.

[0065] Experimental Example I. Testing Items, Equipment, and Testing Methods The performance of the dual-density foamed midsoles prepared in Examples 1-7 and Comparative Examples 1-3 were tested respectively. All tests were conducted at room temperature of 25°C and humidity of 50%, and the specific standards and methods are as follows: 1. Durability (bending resistance test) Execution standard: GB / T 1690-2010; Testing equipment: Shoe material bending tester; Testing method: Fix the sample on the tester, bend at an angle of 60°, bend at a frequency of 30 times / min, bend continuously for 100,000 times, and observe whether the sample delaminates or cracks. The sample is considered qualified if it is intact and undamaged.

[0066] 2. Hardness Execution standard: GB / T 531.1-2008; Testing equipment: Shore C hardness tester; Testing method: Test the surface hardness of the hard layer and soft layer separately, test 5 points for each group of samples, and take the average value.

[0067] 3. Rebound performance (rebound rate) Execution standard: GB / T 6670-2008; Testing equipment: Falling ball rebound tester; Testing method: A standard steel ball is dropped freely to impact the surface of the sample, and the ratio of the rebound height to the falling height is calculated, which is the rebound rate (%).

[0068] 4. Cell density and average pore size Testing equipment: Scanning electron microscope (SEM); Testing method: Cut a cross-section of the sample, scan it with an electron microscope, count the number of bubbles per unit area, measure the bubble diameter, and calculate the average value.

[0069] 5. Expansion ratio Test method: Test the volume of the sample before and after foaming respectively. Foaming ratio = volume after foaming / volume before foaming.

[0070] 6. Yield Statistical standard: For a single batch of 500 products, the number of qualified products is counted, and the yield rate is calculated as: (Number of qualified products / Total number of products) × 100%.

[0071] The standards for qualified products are: no bubbles, missing materials, delamination, or overflow in appearance; complete and seamless fusion of interlayer interfaces; and the hardness, resilience, and foaming ratio of the rigid layer falling within the preset qualified range. Specifically, the preset qualified range for the hardness of the rigid layer is 52-73 (Shore C), the preset qualified range for the resilience is 48%-67%, and the preset qualified range for the foaming ratio is 6.9-24.4.

[0072] II. Test Data The specific performance test data of the above embodiments and comparative examples are shown in Table 1 below.

[0073] Table 1 Performance test data of each sample

[0074] III. Experimental Conclusions 1. Interlayer bonding and durability Examples 1-7 of this invention showed no delamination or cracking after 100,000 bending tests; Comparative Example 1 (synchronous injection molding) showed obvious interlayer cracking, Comparative Example 2 (air foaming) showed local collapse, and Comparative Example 3 (both layers with nucleating agents added) showed slight delamination. This demonstrates that the method of staggered injection molding + supercritical mixed gas foaming + differentiated addition of nucleating agents can achieve high-strength interlayer fusion, with durability far superior to traditional processes.

[0075] 2. Comparison of yield rates The yield rate of all embodiments of the present invention reached over 94.9%, which is much higher than that of the three comparative examples (the highest being 88.1%), indicating that the integrated molding process of the present invention is simple, has less human error, and has stronger mass production stability.

[0076] 3. Hardness, cell structure, and foaming effect The embodiment achieves a gradient structure by adding nucleating agents differently, resulting in a hard layer with high hardness, small pores, and low foaming ratio, and a soft layer with low hardness, large pores, and high resilience. In contrast, the comparative example 3 has a blurred hardness gradient, uneven pore size, and a low foaming ratio, which fails to balance support and cushioning performance.

[0077] 4. Parameter Influence Analysis Data from the examples show that within the range of 1wt% to 2wt% nucleating agent, the higher the amount of nucleating agent added, the higher the hardness of the hard layer and the finer the pores. When the time difference and gas ratio fluctuate within the range defined by this invention, the overall performance of the product remains stable. The performance of the two structures, soft inside and hard outside, and hard inside and soft outside, is similar, and can be flexibly selected according to the application scenario.

[0078] In summary, this invention solves the problems of easy delamination, complicated process, and limited performance of traditional double-layer midsoles by using staggered two-color injection molding, differentiated nucleating agents, and supercritical gradient foaming. The product's support, cushioning, durability, and mass production yield are all significantly improved, making it highly valuable for market applications.

[0079] Obviously, those skilled in the art will understand that the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. For those skilled in the art, other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a dual-density foamed midsole, characterized in that, Includes the following steps: A dual-color twin-screw injection molding system is used to separately deliver hard layer slurry and soft layer slurry, injecting them into the mold cavity with a time difference of 0.2s to 2s. The hard layer slurry contains a nucleating agent, while the soft layer slurry does not. The materials in the mold cavity are sequentially subjected to layered pressure holding, cooling and shaping, followed by mold opening and demolding to obtain an integrated dense preform. The integrated dense preform is placed in a supercritical mixed gas environment for gradient foaming treatment to obtain a dual-density foamed insole.

2. The method for preparing the dual-density foamed midsole according to claim 1, characterized in that, The time difference for the staggered injection is 0.5s to 1.5s.

3. The method for preparing the dual-density foamed midsole according to claim 1, characterized in that, The gradient foaming process includes: placing an integrated dense preform in a supercritical gas atmosphere and holding it at a pressure of 30MPa to 45MPa and a temperature of 130℃ to 140℃ for 120min to 180min; then depressurizing at a depressurization rate of 5MPa / s to 9MPa / s to obtain a dual-density foamed midsole.

4. The method for preparing the dual-density foamed midsole according to claim 3, characterized in that, The supercritical gas is a mixture of carbon dioxide and nitrogen, with a volume ratio of 7-8:2-3.

5. The method for preparing the dual-density foamed midsole according to any one of claims 1 to 4, characterized in that, The raw materials for the rigid layer slurry consist of solvent, nucleating agent, and unfoamed granules; the raw materials for the soft layer slurry consist of solvent and unfoamed granules.

6. The method for preparing the dual-density foamed midsole according to claim 5, characterized in that, The unfoamed granules are selected from one or more of polypropylene, nylon, thermoplastic polyurethane, and ethylene-vinyl acetate copolymer.

7. The method for preparing the dual-density foamed midsole according to claim 5, characterized in that, The nucleating agent is selected from one or more of β-crystalline polypropylene nucleating agents, talc, and organophosphate nucleating agents.

8. The method for preparing the dual-density foamed midsole according to claim 5, characterized in that, Based on the total mass of unfoamed granules in the hard layer slurry, the amount of nucleating agent added is 1wt% to 2wt%.

9. A dual-density foamed midsole, characterized in that, The dual-density foamed midsole is prepared by the preparation method according to any one of claims 1 to 8.

10. A type of athletic shoe, characterized in that, It includes the dual-density foam midsole as described in claim 9.

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