Injection molding method and foam

Abstract

The application discloses an injection molding processing method and a foamed body. The method comprises the following steps: placing a polymer material in a reaction container, introducing a first inert gas to perform pre-impregnation treatment, and obtaining a pre-impregnated material; adding the pre-impregnated material into an injection molding machine and heating and melting; introducing a second inert gas into the injection molding machine, mixing the second inert gas with the melted pre-impregnated material, and obtaining a mixed melt; injecting the mixed melt into a mold, and foaming and forming to obtain the foamed body. By adding the pre-impregnation step in the reaction container before mixing in the injection molding machine, the inert gas is pre-penetrated into the intermolecular gap of the polymer material under sufficient time and space conditions, and then cooperates with the secondary mixing in the injection molding machine, so that the distribution uniformity of the gas in the polymer material is significantly improved, and the problem of uneven foaming of the product is improved.

Description

Technical Field

[0001] This application relates to the field of polymer material foaming technology, and in particular to an injection molding method and a foam. Background Technology

[0002] In-mold injection physical foaming technology is a process in which supercritical inert gas is mixed with polymer melt in an injection molding machine and then directly injected into the mold cavity for foaming and molding. Because it can achieve the integration of foaming and molding in a single process and does not use chemical foaming agents, it has the advantages of being environmentally friendly, producing high dimensional accuracy of products, and being able to mold complex shapes. It has been widely used in lightweight manufacturing in fields such as sports shoe soles and automotive interior parts.

[0003] However, existing in-mold injection molding physical foaming processes have significant shortcomings in terms of the uniformity of gas and polymer material mixing. In typical processes, polymer particles are directly melted after being added to the injection molding machine. Supercritical inert gas mixes with the polymer melt inside the injection molding machine screw. However, the mixing space inside the screw is narrow and the mixing time is short, making it difficult for the gas to fully and uniformly penetrate into the intermolecular spaces of the polymer material within the limited time and space. This uneven gas distribution directly leads to inconsistent cell nucleation density during the foaming process. Ultimately, this manifests in the product as higher hardness and lower foaming ratio near the injection port and in thinner areas, while other areas are over-foamed or have large cells. The overall foaming is uneven, severely affecting the consistency of the product's mechanical properties and user experience. Summary of the Invention

[0004] One objective of this application is to provide an injection molding process and a foam, which aims to solve the technical problem of uneven foaming of products caused by insufficient and uneven mixing of gas and polymer materials in the existing in-mold injection physical foaming process.

[0005] To achieve the above objectives, in a first aspect, this application provides an injection molding method, comprising the following steps:

[0006] The polymer material is placed in a reaction vessel and pre-impregnated by a first inert gas to obtain a pre-impregnated material. The prepreg material is added to the injection molding machine and heated to melt; A second inert gas is introduced into the injection molding machine to mix the second inert gas with the molten prepreg material to obtain a mixed melt; The mixed melt is injected into a mold and foamed to obtain a foamed body.

[0007] In the above injection molding process, the polymer material is first pre-impregnated with a first inert gas in a reaction vessel. Under the ample time and space provided by the reaction vessel, the first inert gas can slowly and fully diffuse and penetrate into the intermolecular spaces of the polymer material, ensuring that the polymer material has pre-absorbed a certain amount of inert gas before entering the injection molding machine. Subsequently, the pre-impregnated material is added to the injection molding machine and heated to melt. During the melting process, the first inert gas, which has pre-penetrated into the polymer material, is further uniformly dispersed in the melt along with the movement of the polymer chain segments. Based on this, a second inert gas is introduced into the injection molding machine. The second inert gas undergoes a secondary mixing with the molten pre-impregnated material already containing the first inert gas. Under the shearing action of the screw, the gas content is further replenished and the uniformity of gas distribution is improved, resulting in a mixed melt. Finally, the mixed melt is injected into the mold. After the pressure is released within the mold cavity, the gas uniformly nucleates and expands, foaming to form a foamed body.

[0008] The injection molding method of this application optimizes the overall uniformity of gas distribution in the polymer material through a two-step impregnation process involving pre-impregnation and secondary mixing within the injection molding machine. Compared to the existing process route where polymer particles are directly added to the injection molding machine and mixed with inert gas in the screw once, this application adds a pre-impregnation step in a reaction vessel before mixing in the injection molding machine. This allows sufficient time and conditions for the gas to penetrate into the polymer material, resulting in a more uniform gas distribution within the material. Consequently, the cell nucleation becomes denser and more uniform, the foaming ratio in different areas of the product tends to be consistent, and the overall mechanical properties and production stability of the product are effectively improved.

[0009] In conjunction with the first aspect, according to one embodiment of this application, the temperature inside the reaction vessel is a first temperature and the pressure is a first pressure, and the temperature inside the injection molding machine is a second temperature and the pressure is a second pressure, wherein the first temperature is lower than the second temperature and the first pressure is lower than the second pressure.

[0010] In the above embodiments, a lower first temperature and first pressure are used for pre-impregnation treatment in the reaction vessel. At this time, the polymer material remains in a solid particle form, and the first inert gas slowly penetrates into the molecular gaps of the particles under pressure. The lower temperature helps maintain the integrity of the solid structure of the polymer material and avoids the particles from melting and sticking together during the pre-impregnation stage. At the same time, the lower pressure conditions have relatively mild requirements on the pressure resistance of the reaction vessel, which helps to reduce equipment costs and safety risks. A higher second temperature and second pressure are used in the injection molding machine. The higher second temperature allows the pre-impregnated material to melt fully, and the polymer chain segments gain sufficient mobility, which is conducive to the dissolution and diffusion of the second inert gas in the melt. The higher second pressure ensures that the second inert gas can fully dissolve in the melt under high pressure, maintaining the uniform dispersion of the gas until it is injected into the mold cavity.

[0011] In conjunction with the first aspect, according to one embodiment of this application, the first temperature is 70-120°C; and / or, the first pressure is 5-20 MPa; and / or, the second temperature is 150-230°C; and / or, the second pressure is 5-40 MPa.

[0012] In conjunction with the first aspect, according to one embodiment of this application, the pre-impregnation treatment time is 2-8 hours; and / or, before injecting the mixed melt into the mold, the mixture is further subjected to a process of allowing it to stand in the injection molding machine for 5-10 minutes.

[0013] In conjunction with the first aspect, according to one embodiment of this application, injecting a mixed melt into a mold includes: dividing the injection stroke of the mixed melt into the mold into multiple sub-strokes, and sequentially executing each sub-stroke; wherein the injection speed and injection length of each sub-stroke decrease as the injection proceeds.

[0014] Considering that this application employs a two-step impregnation method—pre-impregnation treatment and secondary mixing within the injection molding machine—the total gas content in the mixed melt is significantly higher than that obtained through a single-step mixing process in the prior art. Under conditions of higher gas content, the gas nucleation and expansion behavior triggered by pressure release after the mixed melt is injected into the mold cavity is more intense. In the initial stage of injection, a large amount of mixed melt rapidly enters the cavity and experiences a sudden pressure drop, causing the high concentration of dissolved gases to concentrate, nucleate, and expand violently within a short period. This application further divides the injection stroke of the mixed melt into the mold into multiple sub-strokes, with the injection speed and injection length of each sub-stroke decreasing as the injection progresses.

[0015] In the initial stage of injection, a high injection speed and a long injection length are used to quickly fill the central area of ​​the cavity. The filling pressure maintained by the high injection speed inhibits gas expansion in the area near the injection port, preventing excessive foaming in this area due to high gas content and rapid pressure release. As injection progresses, the injection speed and injection length are gradually reduced, allowing areas farther from the injection port more time for gas nucleation and cell growth at lower injection speeds. This compensates for insufficient foaming capacity in these areas due to decreased melt temperature and weakened gas expansion driving force. Thus, this segmented, decreasing injection method matches the injection parameters at each stage of the injection stroke with the foaming characteristics of the mixed melt at different locations within the cavity, achieving precise control over the foaming process of high-gas-content mixed melts.

[0016] In conjunction with the first aspect, according to one embodiment of this application, the injection length of the first sub-stroke is 10-60 mm; and / or, the injection speed of the first sub-stroke is 10-30 mm / s; and / or, the injection length of each sub-stroke is reduced by 10-30 mm compared to the previous sub-stroke; and / or, the injection speed of each sub-stroke is reduced by 5-10 mm / s compared to the previous sub-stroke.

[0017] In conjunction with the first aspect, according to one embodiment of this application, the mold is divided into multiple annular regions from the inside out with the injection port as the center, and the number of annular regions corresponds to the number of segments of the sub-stroke; wherein, the temperature of the central region of the multiple annular regions is the same as the nozzle temperature, and the temperature of each region decreases sequentially from the inside out.

[0018] The above implementation method employs a zoned temperature control approach in conjunction with a segmented, decreasing injection method. After the mixed melt enters the mold from the nozzle, it first reaches the central region near the injection port. The temperature in this region is consistent with the nozzle temperature, ensuring sufficient fluidity of the melt in the central area, allowing it to flow smoothly from the central region to the outer regions and fill them. In the central region, due to the higher injection pressure, gas expansion is effectively suppressed, and the bubbles are in a controlled growth state. As the melt flows outward, the injection pressure gradually decreases along the flow path, weakening the suppression of gas expansion. If the outer regions maintain the same high temperature as the central region, the melt viscosity is low, and with the weakening pressure suppression, there is insufficient viscosity resistance to constrain bubble expansion, easily leading to excessive growth, merging, or even collapse of bubbles in the outer regions. Therefore, the temperature of each annular region decreases sequentially from the inside out. By gradually reducing the temperature, the melt viscosity is increased to compensate for the reduced expansion constraint force due to the decrease in injection pressure, ensuring that bubbles grow uniformly under appropriate expansion constraint conditions in each region. The number of annular regions corresponds to the number of sub-stroke segments, which allows the injection parameters of each sub-stroke segment to match the mold temperature of the corresponding region. The injection speed, injection pressure and mold temperature decrease synchronously, achieving precise control of the mixed melt throughout the filling and foaming process.

[0019] In conjunction with the first aspect, according to one embodiment of this application, in two adjacent annular regions, the temperature of the outer annular region is 5-20°C lower than the temperature of the inner annular region.

[0020] In conjunction with the first aspect, according to one embodiment of this application, the first inert gas and / or the second inert gas is carbon dioxide and / or nitrogen; and / or, the mass ratio of the preimpregnated material to the second inert gas is (1-20):1.

[0021] To achieve the above objectives, in a second aspect, this application provides a foam body prepared by the method of any embodiment of the first aspect.

[0022] The beneficial effects of the second aspect described above can be referenced to the first aspect or any possible implementation thereof, and will not be elaborated upon here. Based on the implementations provided in the above aspects, this application can also be further combined to provide more implementations.

[0023] Other advantages, objectives and features of this application will be partly apparent from the description below, and partly understood by those skilled in the art through study and practice of this application. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0025] Figure 1 This is one of the schematic flowcharts of the injection molding method provided in the embodiments of this application; Figure 2 This is a second schematic flowchart of the injection molding method provided in the embodiments of this application; Figure 3 This is a cross-sectional schematic diagram of the foam processed by injection molding according to Embodiment 1 of this application; Figure 4 This is a cross-sectional schematic diagram of the injection-molded foam of Comparative Example 1 of this application; Figure 5 This is a cross-sectional schematic diagram of the injection-molded foam of Comparative Example 2 of this application; Figure 6 This is a cross-sectional schematic diagram of the foam processed by injection molding in Comparative Example 3 of this application. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0027] In-mold injection physical foaming technology is a process in which supercritical inert gas is mixed with polymer melt in an injection molding machine and then directly injected into the mold cavity for foaming and molding. Because it can achieve the integration of foaming and molding in a single process and does not use chemical foaming agents, it has the advantages of being environmentally friendly, producing high dimensional accuracy of products, and being able to mold complex shapes. It has been widely used in lightweight manufacturing in fields such as sports shoe soles and automotive interior parts.

[0028] To improve the foaming uniformity of in-mold injection molded physically foamed products, some improvements have been attempted in existing technologies. For example, optimizing the screw structure to enhance the mixing effect between gas and melt, or adjusting injection process parameters such as injection speed and holding pressure to improve gas distribution during the filling process. In addition, some researchers have tried adding multiple gas injection points to the injection molding machine barrel to improve the uniformity of gas dispersion in the melt.

[0029] However, the effects of the aforementioned improvements remain limited. Due to the confined space and short time frame within the injection molding machine screw, the mixing process between the gas and the polymer material is consistently insufficient. The mixing time from gas injection to completion is typically only a few minutes, making it difficult to achieve sufficient and uniform gas penetration into the intermolecular spaces of the polymer material. This is particularly true for polymers with long molecular chains and high crystallinity; the limited residence time of the gas within the screw is far from sufficient for adequate diffusion and penetration. This results in higher hardness and lower foaming ratios near the injection port and in thin-walled areas of the product, while over-foaming or large pores are still prevalent in other areas.

[0030] To solve the above-mentioned technical problems, in a first aspect, this application provides an injection molding method, comprising the following steps: S101. Place the polymer material in a reaction vessel and introduce a first inert gas for pre-impregnation treatment to obtain a pre-impregnated material. S103. Add the prepreg material to the injection molding machine and heat it to melt; S105. Introduce a second inert gas into the injection molding machine to mix the second inert gas with the molten prepreg material to obtain a mixed melt; S107. Inject the mixed melt into the mold and foam it to obtain a foamed body.

[0031] In the above injection molding process, the polymer material is first pre-impregnated with a first inert gas in a reaction vessel. Under the ample time and space provided by the reaction vessel, the first inert gas can slowly and fully diffuse and penetrate into the intermolecular spaces of the polymer material, ensuring that the polymer material has pre-absorbed a certain amount of inert gas before entering the injection molding machine. Subsequently, the pre-impregnated material is added to the injection molding machine and heated to melt. During the melting process, the first inert gas, which has pre-penetrated into the polymer material, is further uniformly dispersed in the melt along with the movement of the polymer chain segments. Based on this, a second inert gas is introduced into the injection molding machine. The second inert gas undergoes a secondary mixing with the molten pre-impregnated material already containing the first inert gas. Under the shearing action of the screw, the gas content is further replenished and the uniformity of gas distribution is improved, resulting in a mixed melt. Finally, the mixed melt is injected into the mold. After the pressure is released within the mold cavity, the gas uniformly nucleates and expands, foaming to form a foamed body.

[0032] It should be noted that the first and second inert gases referred to in this application are chemically stable gaseous media that do not chemically react with the base material or other components during the foaming process. Their naming only characterizes the chemical inertness of the gas and does not limit its physical state during use. In actual foaming processes, the gaseous medium is usually in a supercritical state when mixed with the melt, meaning its temperature and pressure both exceed its critical temperature and critical pressure. At this state, the gas possesses both the high density of a liquid and the high diffusivity of a gas, enabling it to dissolve and penetrate the polymer melt more efficiently. Of course, it is understood that this application does not exclude the possibility of inert gases being mixed with the melt in other physical states, such as high-pressure liquid or high-pressure gaseous states, and then injected into the mold for foaming. As long as the inert gas can achieve pressure release and nucleation expansion within the mold after mixing with the melt, it falls within the scope of protection of this application.

[0033] The injection molding method of this application optimizes the overall uniformity of gas distribution in the polymer material through a two-step impregnation process involving pre-impregnation and secondary mixing within the injection molding machine. Compared to the existing process route where polymer particles are directly added to the injection molding machine and mixed with inert gas in the screw once, this application adds a pre-impregnation step in a reaction vessel before mixing in the injection molding machine. This allows sufficient time and conditions for the gas to penetrate into the polymer material, resulting in a more uniform gas distribution within the material. Consequently, the cell nucleation becomes denser and more uniform, the foaming ratio in different areas of the product tends to be consistent, and the overall mechanical properties and production stability of the product are effectively improved.

[0034] It should be noted that the two-step impregnation method of this application should not be understood as a simple combination of two impregnation steps. The technical effect of this application is not only superior to the existing process route of mixing only once in the injection molding machine screw, but also superior to the process route of directly injecting into the mold for foaming after pre-impregnation in the reaction vessel. If the polymer material is pre-impregnated only in the reaction vessel without a second inert gas being introduced into the injection molding machine for secondary mixing, although the pre-impregnation stage allows the gas to penetrate into the molecular gaps of the polymer material for a relatively long time, the gas penetration during the pre-impregnation process is limited by the dense structure of the solid particles and the limited molecular chain segment mobility. There is an upper limit to the amount of gas that can penetrate, and the gas concentration gradient between the particle surface and the core area is difficult to completely eliminate. The gas content and distribution uniformity obtained by pre-impregnation alone may not be sufficient to meet the requirements of high foaming ratio products. If the inert gas is only introduced into the injection molding machine screw for mixing with the melt once without pre-impregnation, as mentioned above, the mixing space in the screw is narrow and the mixing time is short, making it difficult for the gas to fully and uniformly penetrate into the molecular gaps of the polymer material.

[0035] This application combines pre-impregnation in the reaction vessel with secondary mixing in the injection molding machine. The pre-impregnation stage establishes a basic gas distribution for the polymer material under ample time and mild conditions, pre-filling the intermolecular gaps of the polymer material with a certain amount of inert gas. On this basis, the secondary mixing in the injection molding machine is carried out in a high-temperature molten state. At this time, the polymer chain segments move violently and the intermolecular gaps are fully opened. The second inert gas can quickly dissolve and diffuse into the melt under the strong shearing action of the screw, superimposing with the first inert gas that has already penetrated in the pre-impregnation stage, so that the total gas content and distribution uniformity in the melt significantly exceed the level that any single step can achieve.

[0036] In conjunction with the first aspect, according to one embodiment of this application, the temperature inside the reaction vessel is a first temperature and the pressure is a first pressure, and the temperature inside the injection molding machine is a second temperature and the pressure is a second pressure, wherein the first temperature is lower than the second temperature and the first pressure is lower than the second pressure.

[0037] In the above embodiments, a lower first temperature and first pressure are used for pre-impregnation treatment in the reaction vessel. At this time, the polymer material remains in a solid particle form, and the first inert gas slowly penetrates into the molecular gaps of the particles under pressure. The lower temperature helps maintain the integrity of the solid structure of the polymer material and avoids the particles from melting and sticking together during the pre-impregnation stage. At the same time, the lower pressure conditions have relatively mild requirements on the pressure resistance of the reaction vessel, which helps to reduce equipment costs and safety risks. A higher second temperature and second pressure are used in the injection molding machine. The higher second temperature allows the pre-impregnated material to melt fully, and the polymer chain segments gain sufficient mobility, which is conducive to the dissolution and diffusion of the second inert gas in the melt. The higher second pressure ensures that the second inert gas can fully dissolve in the melt under sufficient pressure, maintaining the uniform dispersion of the gas until it is injected into the mold cavity.

[0038] In conjunction with the first aspect, according to one embodiment of this application, the first temperature is 70-120°C; and / or, the first pressure is 5-20 MPa; and / or, the second temperature is 150-230°C; and / or, the second pressure is 5-40 MPa.

[0039] For example, the first temperature can be 70°C, 80°C, 90°C, 100°C, 110°C, or 120°C; the first pressure can be 5MPa, 8MPa, 10MPa, 15MPa, or 20MPa; the second temperature can be 150°C, 170°C, 200°C, or 230°C; and the second pressure can be 5MPa, 10MPa, 20MPa, 30MPa, or 40MPa.

[0040] The first temperature range of 70-120℃ and the first pressure range of 5-20MPa ensure that the first inert gas fully penetrates into the interior of the polymer particles within a reasonable time. This temperature range is lower than the melting point or softening temperature of common foaming polymers, ensuring that the polymer maintains its solid particle morphology during the pre-impregnation process. The second temperature range of 150-230℃ and the second pressure range of 5-40MPa cover the processing window of common foaming polymers such as thermoplastic polyester elastomers and thermoplastic polyurethane elastomers, ensuring that the melt has good fluidity and gas dissolving ability.

[0041] In conjunction with the first aspect, according to one embodiment of this application, the pre-impregnation treatment time is 2-8 hours; and / or, before injecting the mixed melt into the mold, the mixture is further subjected to a process of allowing it to stand in the injection molding machine for 5-10 minutes.

[0042] For example, the pre-impregnation treatment time can be 2 hours, 3 hours, 4 hours, 5 hours, 6 hours or 8 hours; the settling time can be 5 minutes, 6 minutes, 7 minutes, 8 minutes or 10 minutes.

[0043] The pre-impregnation treatment time is 2-8 hours. This time range provides ample time for the first inert gas to diffuse and penetrate from the surface of the polymer particles inward, allowing the gas to penetrate to the core area of ​​the particles rather than just remaining on the surface, thus ensuring sufficient and uniform pre-impregnation. If the pre-impregnation time is too short, the gas may only penetrate to the surface area of ​​the particles, resulting in a large difference in gas content between the inside and outside of the particles, affecting the uniformity of subsequent foaming. If the pre-impregnation time is too long, although the gas penetration is more complete, the production efficiency is reduced, and the gas content may tend to saturate, so the marginal benefit of further extending the time is limited. Letting the mixed melt stand in the injection molding machine for 5-10 minutes allows the second inert gas to further diffuse and homogenize in the melt, reducing local gas concentration differences caused by screw shearing and mixing, and ensuring that the mixed melt achieves a more uniform gas distribution state before being injected into the mold.

[0044] It should be noted that the pre-impregnation time and settling time can be adjusted adaptively according to factors such as the type of polymer material, particle size, and type of inert gas. For example, for polymer materials with longer molecular chain segments and higher crystallinity, the pre-impregnation time can be appropriately extended to ensure sufficient gas penetration; for polymer materials with smaller particle sizes, the pre-impregnation time can be appropriately shortened due to the shorter gas diffusion path.

[0045] In conjunction with the first aspect, according to one embodiment of this application, injecting a mixed melt into a mold includes: dividing the injection stroke of the mixed melt into the mold into multiple sub-strokes, and sequentially executing each sub-stroke; wherein the injection speed and injection length of each sub-stroke decrease as the injection proceeds.

[0046] Considering that this application employs a two-step impregnation method—pre-impregnation treatment and secondary mixing within the injection molding machine—the total gas content in the mixed melt is significantly higher than that obtained through a single-step mixing process in the prior art. Under conditions of higher gas content, the gas nucleation and expansion behavior triggered by pressure release after the mixed melt is injected into the mold cavity is more intense. In the initial stage of injection, a large amount of mixed melt rapidly enters the cavity and experiences a sudden pressure drop, causing the high concentration of dissolved gases to concentrate, nucleate, and expand violently within a short period. This application further divides the injection stroke of the mixed melt into the mold into multiple sub-strokes, with the injection speed and injection length of each sub-stroke decreasing as the injection progresses.

[0047] In the initial stage of injection, a high injection speed and a long injection length are used to quickly fill the central area of ​​the cavity. The filling pressure maintained by the high injection speed inhibits gas expansion in the area near the injection port, preventing excessive foaming in this area due to high gas content and rapid pressure release. As injection progresses, the injection speed and injection length are gradually reduced, allowing areas farther from the injection port more time for gas nucleation and cell growth at lower injection speeds. This compensates for insufficient foaming capacity in these areas due to decreased melt temperature and weakened gas expansion driving force. Thus, this segmented, decreasing injection method matches the injection parameters at each stage of the injection stroke with the foaming characteristics of the mixed melt at different locations within the cavity, achieving precise control over the foaming process of high-gas-content mixed melts.

[0048] It should be noted that the number of segments in a multi-stage injection stroke can be flexibly set according to the size and shape complexity of the mold cavity and the foaming uniformity requirements of the product. For example, for mold cavities with smaller size and simpler shape, the injection stroke can be divided into 2-3 segments; for mold cavities with larger size and more complex shape, the injection stroke can be divided into 4-6 or more segments to achieve more precise injection control.

[0049] In conjunction with the first aspect, according to one embodiment of this application, the injection length of the first sub-stroke is 10-60 mm; and / or, the injection speed of the first sub-stroke is 10-30 mm / s; and / or, the injection length of each sub-stroke is reduced by 10-30 mm compared to the previous sub-stroke; and / or, the injection speed of each sub-stroke is reduced by 5-10 mm / s compared to the previous sub-stroke.

[0050] For example, the injection length of the first sub-stroke can be 10mm, 20mm, 30mm, 40mm, 50mm or 60mm; the injection speed of the first sub-stroke can be 10mm / s, 15mm / s, 20mm / s, 25mm / s or 30mm / s; the decrease in injection length of each sub-stroke can be 10mm, 15mm, 20mm, 25mm or 30mm; and the decrease in injection speed of each sub-stroke can be 5mm / s, 6mm / s, 7mm / s, 8mm / s or 10mm / s.

[0051] The first sub-stroke has an injection length of 10-60 mm and an injection speed of 10-30 mm / s, setting a reasonable initial parameter range for the beginning of the injection process. The injection length of each sub-stroke is reduced by 10-30 mm compared to the previous sub-stroke, and the injection speed of each sub-stroke is reduced by 5-10 mm / s compared to the previous sub-stroke. This uniform reduction ensures smooth and controllable changes in speed and stroke during the injection process, avoiding melt flow instability and abrupt changes in cell structure caused by sudden parameter changes.

[0052] It should be noted that the specific values ​​of the injection length and injection speed, as well as the reduction amount, can be adaptively adjusted according to the volume of the mold cavity, the wall thickness distribution of the product, and the flow characteristics of the polymer material. For example, when the product wall thickness is relatively thin, the injection speed of the first sub-stroke can be appropriately increased to ensure the filling capacity of the melt; when the mold cavity volume is large, the injection length of the first sub-stroke can be appropriately increased to improve the filling efficiency.

[0053] In conjunction with the first aspect, according to one embodiment of this application, the mold is divided into multiple annular regions from the inside out with the injection port as the center, and the number of annular regions corresponds to the number of segments of the sub-stroke; wherein, the temperature of the central region of the multiple annular regions is the same as the nozzle temperature, and the temperature of each region decreases sequentially from the inside out.

[0054] The above embodiment employs a zoned temperature control method in conjunction with a segmented, decreasing injection technique. After the mixed melt enters the mold from the nozzle, it first reaches the central region near the injection port. The temperature in this region is consistent with the nozzle temperature, ensuring sufficient fluidity of the melt in the central area, allowing it to flow smoothly from the central region to the outer regions. In the central region, due to the higher injection pressure, gas expansion is effectively suppressed, and the bubbles are in a controlled growth state. As the melt flows outward, the injection pressure gradually decreases along the flow path, weakening the suppression of gas expansion. If the outer regions maintain the same high temperature as the central region, the melt viscosity is low, and with the weakening pressure suppression, there is insufficient viscosity resistance to constrain bubble expansion, easily leading to excessive bubble growth, merging, or even collapse in the outer regions. Therefore, the temperature of each annular region decreases sequentially from the inside out. By gradually lowering the temperature, the melt viscosity is increased to compensate for the reduced expansion constraint force due to the decrease in injection pressure, ensuring that the bubbles grow uniformly under appropriate expansion constraint conditions in each region. The number of annular regions corresponds to the number of sub-stroke segments, which allows the injection parameters of each sub-stroke segment to match the mold temperature of the corresponding region. The injection speed, injection pressure and mold temperature decrease synchronously, achieving precise control of the mixed melt throughout the filling and foaming process.

[0055] It should be noted that the above-mentioned division of the annular region is not limited to a strictly concentric ring shape; its specific shape can be adaptively adjusted according to the actual shape of the mold cavity. For example, for a non-circular mold cavity, the annular region can be an area enclosed by equidistant contour lines centered on the injection port, and its shape varies with the contour of the mold cavity. Furthermore, temperature control between each annular region can be achieved by setting independent heating and cooling channels inside the mold, with each channel independently controlling its temperature, thereby achieving a temperature gradient distribution in each region.

[0056] In conjunction with the first aspect, according to one embodiment of this application, in two adjacent annular regions, the temperature of the outer annular region is 5-20°C lower than the temperature of the inner annular region.

[0057] For example, the temperature difference between adjacent annular regions can be 5°C, 8°C, 10°C, 12°C, 15°C, or 20°C.

[0058] The temperature difference between two adjacent annular regions is 5-20℃. This temperature range is sufficient to create an effective temperature gradient to accommodate the decrease in injection pressure during melt flow, while preventing abrupt viscosity changes and foaming behavior abrupt changes at the region boundary due to excessive temperature differences between adjacent regions. If the temperature difference is too small, the temperature gradient is not obvious, and the effect of zoned temperature control on constraining and compensating for cell expansion is limited. If the temperature difference is too large, the melt at the boundary between adjacent regions may experience significant temperature jumps, leading to discontinuities in the cell structure at that location.

[0059] In conjunction with the first aspect, according to one embodiment of this application, the first inert gas and / or the second inert gas is carbon dioxide and / or nitrogen; and / or, the mass ratio of the preimpregnated material to the second inert gas is (1-20):1.

[0060] For example, the mass ratio of the preimpregnated material to the second inert gas can be 1:1, 5:1, 8:1, 10:1, 15:1 or 20:1.

[0061] Carbon dioxide and nitrogen are both common inert gases used in physical foaming, possessing advantages such as good chemical inertness, wide availability, low cost, and environmental friendliness. Carbon dioxide has high solubility in polymer materials, which is beneficial for increasing gas penetration and foaming ratio; nitrogen diffuses rapidly, which is conducive to forming a fine and uniform cell structure. The first and second inert gases can be of the same type or different types, flexibly selected according to the type of polymer material and foaming requirements. The mass ratio of prepreg material to second inert gas is (1-20):1, providing a reasonable range for the amount of gas added when mixing the second inert gas with the molten prepreg material in the injection molding machine.

[0062] It should be noted that the first and second inert gases are not limited to carbon dioxide and nitrogen. Other chemically inert gases, such as argon, can also be used in the pre-impregnation treatment and in-injection molding mixing steps of this application, as long as they can penetrate into the polymer material or dissolve in the polymer melt under the corresponding temperature and pressure conditions. This embodiment is only a preferred gas example.

[0063] To address the aforementioned technical problems, in a second aspect, this application provides a foam material prepared by the method of any embodiment in the first aspect.

[0064] The foam prepared by the injection molding method of any of the above embodiments exhibits characteristics of uniform cell distribution, good cell size consistency, and uniform foaming ratio in all regions due to the significantly improved uniformity of inert gas distribution in the polymer material during the preparation process. Its mechanical property consistency and appearance quality are superior to those of foams prepared by existing in-mold injection molding physical foaming processes. This foam can be applied to fields requiring lightweight and uniform mechanical properties, such as sports shoe soles, automotive interior parts, and electronic product housings.

[0065] The beneficial effects of the second aspect described above can be referenced to the first aspect or any possible implementation thereof, and will not be elaborated upon here. Based on the implementations provided in the above aspects, this application can also be further combined to provide more implementations.

[0066] The technical solution of this application will be further described below through specific embodiments.

[0067] In the following examples and comparative examples, unless otherwise specified, the polymer material used is thermoplastic polyurethane elastomer (TPU) with a particle size of 2-3 mm; the inert gas used is supercritical nitrogen; the injection molding machine screw diameter is 45 mm; the mold cavity is rectangular, 10 cm × 20 cm, and 10 mm thick. All images of the actual objects involved were taken under a 4x optical microscope.

[0068] Example 1 (Please refer to) Figure 3 ) S201. Place TPU granules in a sealed autoclave, introduce nitrogen gas, and pre-impregnate for 4 hours at a first temperature of 90℃ and a first pressure of 10MPa to obtain the pre-impregnated material.

[0069] S203. Add the pre-impregnated material to the injection molding machine and heat it to melt under the conditions of a second temperature of 220°C and a second pressure of 20MPa.

[0070] S205. Supercritical nitrogen gas is introduced into the injection molding machine. The mass ratio of preimpregnated material to nitrogen gas is 10:1. After the screw shears and mixes, a mixed melt is obtained and allowed to stand for 8 minutes.

[0071] S207. The mixed melt is injected into the mold. The injection stroke is divided into three sub-strokes: the first segment has an injection speed of 20 mm / s and an injection length of 50 mm; the second segment has an injection speed of 12 mm / s and an injection length of 30 mm; and the third segment has an injection speed of 5 mm / s and an injection length of 10 mm. The mold is divided into three annular regions from the injection port outwards. The temperature of the innermost region is 220℃ (consistent with the nozzle temperature), the temperature of the second region is 210℃, and the temperature of the third region is 200℃. Foaming molding yields a foamed body.

[0072] Example 2 The first pressure in Example 1 was changed to 20 MPa, the second pressure was changed to 10 MPa, and the other conditions were the same as in Example 1.

[0073] Example 3 The pre-impregnation treatment time in Example 1 was changed to 2 hours, and the other conditions were the same as in Example 1.

[0074] Example 4 The pre-impregnation treatment time in Example 1 was changed to 8 hours, and the other conditions were the same as in Example 1.

[0075] Example 5 The pre-impregnation treatment time in Example 1 was changed to 0.5 hours, and the other conditions were the same as in Example 1.

[0076] Example 6 The pre-impregnation treatment time in Example 1 was changed to 12 hours, and the other conditions were the same as in Example 1.

[0077] Example 7 The settling time in Example 1 was changed to 5 minutes, and the other conditions were the same as in Example 1.

[0078] Example 8 The settling time in Example 1 was changed to 10 minutes, and the other conditions were the same as in Example 1.

[0079] Example 9 The settling time in Example 1 was changed to 1 minute, and the other conditions were the same as in Example 1.

[0080] Example 10 The settling time in Example 1 was changed to 20 minutes, and the other conditions were the same as in Example 1.

[0081] Comparative Example 1 (Please refer to) Figure 4 ) Step S201 in Example 1 is omitted. TPU particles are directly added to the injection molding machine. The amount of gas introduced in step S205 is the sum of the amounts of gas introduced in steps S201 and S205 in Example 1. All other conditions are the same as in Example 1.

[0082] Comparative Example 2 (please refer to) Figure 5 ) The operation of introducing supercritical nitrogen into the injection molding machine in step S205 of Example 1 is omitted. That is, after the prepreg material is added to the injection molding machine and heated to melt, no second inert gas is introduced. The melt is directly injected into the mold. The amount of gas introduced in step S201 is the sum of the amounts of gas introduced in steps S201 and S205 of Example 1. All other conditions are the same as in Example 1.

[0083] Comparative Example 3 (please refer to) Figure 6 ) In Example 1, the segmented injection in step S207 was changed to uniform injection, with an injection speed of 20 mm / s, an injection length of 90 mm, and a uniform mold temperature of 220°C. All other conditions were the same as in Example 1.

[0084] Performance testing The following performance tests were performed on the foams from the above embodiments and comparative examples: ① Density: Three standard samples with dimensions no smaller than 50mm × 50mm × 50mm were cut from the central region of the foam. The mass of each sample was weighed using an analytical balance, and the length, width, and height of the sample were measured using vernier calipers. The volume of the sample was calculated. The apparent density of the foam was calculated according to the formula, and the average value of the three samples was taken as the apparent density of the foam. This test was performed in accordance with GB / T 6343-2009 "Determination of Apparent Density of Foamed Plastics and Rubber".

[0085] ② Average cell diameter: The cross-section of the foam was observed using a scanning electron microscope (SEM). Five fields of view were randomly selected, and the diameter of 20 cells in each field of view was measured. The average value was calculated.

[0086] ③ Standard deviation of bubble diameter: Based on the above bubble diameter measurement data, the standard deviation is calculated to characterize the uniformity of bubble size.

[0087] ④ Hardness uniformity: Take 3 test points in the center, middle and edge areas of the foam, and determine the Shore C hardness according to GB / T 531.1-2008 "Test method for indentation hardness of vulcanized rubber or thermoplastic rubber - Part 1: Shore hardness test (Shore hardness)", denoted as HC, and calculate the maximum difference in hardness at each test point.

[0088] Test Results

[0089] Example 1 was compared with Comparative Examples 1 and 2. The total gas volume was the same for all three. The difference lay in the two-step impregnation method: Example 1 used pre-impregnation followed by secondary mixing within the injection molding machine; Comparative Example 1 omitted the pre-impregnation step and mixed all the gas in a single step within the injection molding machine; and Comparative Example 2 omitted the secondary mixing within the injection molding machine and permeated all the gas in a single step during the pre-impregnation stage. Example 1 outperformed Comparative Examples 1 and 2 in four indicators: density, average cell diameter, standard deviation of cell diameter, and maximum difference in hardness. This indicates that, under the same total gas volume, the two-step impregnation method significantly improved the uniformity of gas distribution in the polymer material compared to either individual step. In Comparative Example 1, all the gas mixed with the melt within the limited space and time of the injection molding machine screw, making it difficult for the gas to permeate sufficiently and uniformly. In Comparative Example 2, all the gas contacted the solid particles within the reaction vessel; however, due to the density of the solid structure, the gas permeation and uniformity were insufficient. The synergistic effect of the two-step impregnation method resulted in a much higher overall uniformity of gas distribution than either individual step.

[0090] Example 1 and Example 2 are compared. In Example 1, the first pressure is 10 MPa and the second pressure is 20 MPa, satisfying the relationship that the first pressure is less than the second pressure. In Example 2, the first pressure is changed to 20 MPa and the second pressure is changed to 10 MPa, with the first pressure being greater than the second pressure. The various indicators of Example 2 are not significantly different from those of Example 1, indicating that even if the pressure relationship between the two steps is changed within the two-step impregnation framework of this application, a good foaming effect can still be obtained. However, from the perspective of process rationality, using a lower first pressure in the pre-impregnation stage and a higher second pressure in the mixing stage within the injection molding machine is a more preferred solution: a lower first pressure requires a milder pressure resistance rating for the reaction vessel, which helps reduce equipment costs and safety risks; a higher second pressure ensures that the second inert gas is fully dissolved in the melt in a high-temperature molten state, maintaining a uniform dispersion of the gas until it is injected into the mold cavity.

[0091] Example 1 was compared with Comparative Example 3. The mixed melts were the same in both examples; the difference lay in the injection speed: Example 1 used a segmented decreasing injection rate and zoned gradient temperature control, while Comparative Example 3 used uniform injection speed and a uniform mold temperature. The standard deviation of cell diameter and the maximum difference in hardness in Comparative Example 3 were significantly greater than those in Example 1, indicating that even with the same uniform gas distribution in the mixed melt, the injection method and mold temperature control significantly affect the foaming uniformity of the final product. The segmented decreasing injection speed combined with zoned temperature control that decreases from the inside out allows for precise control of the foaming behavior of the mixed melt at different locations within the mold cavity, further improving the uniformity of cell size and hardness in the product.

[0092] Examples 1 and 3-6 were compared, with pre-impregnation times of 4 hours, 2 hours, 8 hours, 0.5 hours, and 12 hours, respectively. The results showed that a pre-impregnation time within the range of 2-8 hours achieved good foaming effects. Longer pre-impregnation times resulted in more complete gas penetration and better cell uniformity, but the marginal benefits were limited after 8 hours. When the pre-impregnation time was too short, gas only penetrated to the surface of the particles, and foaming uniformity decreased significantly. Considering both foaming effect and production efficiency, a pre-impregnation time of 4-8 hours was the preferred range.

[0093] Examples 1 and 7-10 were compared, with settling times of 8 minutes, 5 minutes, 10 minutes, 1 minute, and 20 minutes, respectively. The results showed that a settling time within the range of 5-10 minutes yielded good foaming effects. Longer settling times resulted in more complete homogenization of gas diffusion in the melt and better cell uniformity. When the settling time was too short, the gas distribution was not sufficiently homogenized, and the foaming uniformity decreased significantly. When the settling time was too long, the polymer material underwent thermal degradation at high temperatures, leading to yellowing of the material, which was detrimental to product quality.

[0094] It should be noted that the order of description of the embodiments in this application is not intended to limit the priority of the embodiments.

[0095] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in this application and in its specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0096] The embodiments of this application have been described above with reference to the accompanying drawings. However, this application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many forms under the guidance of this application without departing from the spirit and scope of protection of the claims. All equivalent transformations made under the inventive concept of this application using the content of this application's specification and drawings, or direct / indirect applications in other related technical fields, are included within the patent protection scope of this application.

Claims

1. An injection molding process, characterized in that, Includes the following steps: The polymer material is placed in a reaction vessel and pre-impregnated by a first inert gas to obtain a pre-impregnated material. The prepreg material is added to the injection molding machine and heated to melt; A second inert gas is introduced into the injection molding machine to mix the second inert gas with the molten prepreg material to obtain a mixed melt; The mixed melt is injected into a mold and foamed to obtain a foamed body.

2. The method according to claim 1, characterized in that, The temperature inside the reaction vessel is a first temperature, and the pressure is a first pressure. The temperature inside the injection molding machine is a second temperature, and the pressure is a second pressure. The first temperature is lower than the second temperature, and the first pressure is lower than the second pressure.

3. The method according to claim 2, characterized in that, The first temperature is 70-120℃; and / or, the first pressure is 5-20MPa; and / or, the second temperature is 150-230℃; and / or, the second pressure is 5-40MPa.

4. The method according to claim 1, characterized in that, The pre-impregnation treatment time is 2-8 hours; And / or, before injecting the mixed melt into the mold, the process further includes: allowing the mixed melt to stand in the injection molding machine for 5-10 minutes.

5. The method according to claim 1, characterized in that, Injecting the mixed melt into a mold includes: dividing the injection stroke of the mixed melt into the mold into multiple sub-strokes, and executing each sub-stroke in sequence; wherein the injection speed and injection length of each sub-stroke decrease as the injection proceeds.

6. The method according to claim 5, characterized in that, The injection length of the sub-stroke described in the first paragraph is 10-60 mm; And / or, the injection speed of the sub-stroke described in the first paragraph is 10-30 mm / s; And / or, the injection length of each sub-stroke is reduced by 10-30 mm compared to the previous sub-stroke; And / or, the injection speed of each sub-stroke is reduced by 5-10 mm / s compared to the previous sub-stroke.

7. The method according to claim 5, characterized in that, The mold is divided into multiple annular regions from the inside out with the injection port as the center, and the number of annular regions corresponds to the number of segments of the sub-stroke; The temperature of the innermost region of the multiple annular regions is the same as the nozzle temperature, and the temperature of each region decreases sequentially from the inside to the outside.

8. The method according to claim 7, characterized in that, In two adjacent annular regions, the temperature of the outer annular region is 5-20°C lower than the temperature of the inner annular region.

9. The method according to any one of claims 1-8, characterized in that, The first inert gas and / or the second inert gas is carbon dioxide and / or nitrogen; And / or, the mass ratio of the preimpregnated material to the second inert gas is (1-20):

1.

10. A foam body, characterized in that, Prepared by the method according to any one of claims 1-9.

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