Uniform density curved polyurethane foam core, its rotational foaming preparation method and antenna sandwich structure

Abstract

This invention relates to a uniformly dense curved polyurethane foam core material, its rotational foaming preparation method, and an antenna sandwich structure, belonging to the technical field of polymer material foaming molding and composite material structure manufacturing. In the key stage of the polyurethane mixture foaming reaction, this invention drives the mold to rotate continuously and uniformly around the central axis of the antenna surface, placing the liquid in a microgravity state where centrifugal force and gravity are balanced, eliminating density stratification and preparing a curved polyurethane foam core material with a density difference ≤5%. This core material is then composited with a fiber-reinforced resin composite skin to obtain the antenna curved sandwich structure. This invention achieves near-net-shape forming through the conformal design of the mold cavity and the antenna reflector surface, and strengthens the interface bonding by setting a film reinforcement layer between the skin and the core material. This invention effectively improves the surface accuracy stability and electrical performance consistency of the antenna reflector surface, and has the advantages of simple process, low cost, and wide applicability.

Description

Technical Field

[0001] This invention relates to the field of polymer foaming molding process and composite material structure manufacturing technology, specifically to a uniform density curved polyurethane foam core material, its rotational foaming preparation method, and an antenna sandwich structure. Background Technology

[0002] Polyurethane foam is widely used as the core material for high-performance sandwich structures such as antenna reflectors, radomes, and aerospace components due to its advantages such as light weight, high specific strength, and ease of molding. Sandwich structures typically consist of two layers of high-modulus skins (such as carbon fiber or glass fiber composites) and a low-density foam core in the middle, exhibiting extremely high flexural stiffness and strength-to-weight ratio.

[0003] In traditional polyurethane foaming processes (static casting foaming), under the influence of gravity, the bubbles generated by the foaming agent tend to migrate upwards, resulting in a significant density gradient in the molded foam block: lower density at the top and higher density at the bottom. For curved sandwich structures used in high-precision antennas (especially high-frequency antennas), this non-uniformity in core material density can lead to a series of serious problems: 1. Non-uniform thermal expansion: The thermal conductivity and thermal expansion coefficient of different density regions are different. When the ambient temperature changes, it causes non-uniform thermal deformation of the structure, which seriously affects the surface accuracy of the antenna. 2. Fluctuations in mechanical properties: Density gradients cause uneven spatial distribution of the compressive modulus and shear modulus of the core material, affecting the overall mechanical properties and dimensional stability of the sandwich structure; 3. During subsequent CNC machining of curved surfaces, uneven density will cause changes in cutting resistance, affecting machining accuracy and surface quality.

[0004] Currently, to solve the density gradient problem, the common approach is to cut, test, screen, and splice large foam blocks. This method results in low material utilization, high cost, and difficulty in achieving overall uniformity of large-sized, complex curved core materials.

[0005] Furthermore, Chinese patent application CN120620544A discloses a process and apparatus for improving the compressibility of polyurethane foam. This apparatus uses a lever structure formed by a motor-driven mold rotation combined with a counterweight plate to achieve a composite motion of rotation and reciprocating swaying. While this solution can improve the compressibility of the foam, its motion mechanism is complex, it does not involve the application of curved sandwich structures, and it does not solve the problem of antenna profile accuracy caused by density inhomogeneity. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a simple rotary foaming process that can significantly improve the density consistency of polyurethane foam core material. This process is then applied to the fabrication of high-precision antenna reflector curved sandwich structures, thereby improving the surface accuracy, stability, and electrical performance consistency of the antenna reflector.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention proposes a method for preparing a uniformly dense curved polyurethane foam core material by rotational foaming, comprising the following steps: a. Mold preparation steps: Prepare a mold with a curved cavity that matches the reflective surface of the target antenna, and install the mold on a drive device that can rotate around the central axis of the antenna surface; b. Pouring steps: Pour the polyurethane mixture into the cavity of the mold; c. Rotational foaming step: During the period from the start of the foaming reaction of the polyurethane mixture to the basic gelation and shaping of the foam, the drive device is controlled to drive the mold to rotate continuously and uniformly around the central axis of the antenna surface, so that the polyurethane mixture is in a microgravity state in which centrifugal force and gravity are balanced during the foaming process. d. Demolding step: After the foam has cured and formed, demold to obtain curved polyurethane foam core material.

[0008] Furthermore, in step c, the rotational speed of the continuous uniform rotational motion is 10 to 15 rpm.

[0009] Furthermore, in step c, when the driving device drives the mold to rotate at a constant speed around the central axis of the antenna surface, the rotation axis passes through or approaches the geometric center of the mold cavity.

[0010] Furthermore, the density difference of the curved polyurethane foam core material is ≤5%.

[0011] Secondly, this invention proposes a method for manufacturing an antenna curved sandwich structure, comprising the following steps: a. Curved polyurethane foam core material was prepared using the method described above; b. Reinforcing fiber prepreg or dry fiber fabric is laid on the upper and lower surfaces of the curved polyurethane foam core material, respectively; c. The upper skin, lower skin and the curved polyurethane foam core material are compositely molded into an integral curved sandwich structure through a co-curing process or a secondary bonding process. d. The edges of the formed curved sandwich structure are trimmed and post-processed to obtain a high-precision antenna reflector.

[0012] Furthermore, in step b, the reinforcing fiber prepreg is one or more of carbon fiber prepreg, glass fiber prepreg, or aramid fiber prepreg, and the number of layers is at least two, with the fiber directions of adjacent layers being orthogonal or cross-laid.

[0013] Furthermore, in step c, the co-curing process uses an autoclave for curing, with a curing pressure of 0.3–0.5 MPa, a curing temperature of 100–130°C, and a curing time of 60–120 minutes.

[0014] Thirdly, the present invention proposes an antenna curved sandwich structure, comprising: Curved polyurethane foam core material prepared by the above method; The upper skin is composited on the upper surface of the curved polyurethane foam core material; The lower skin is composited on the lower surface of the curved polyurethane foam core material; Both the upper and lower skins are fiber-reinforced resin composite laminates.

[0015] Furthermore, the curved polyurethane foam core material is a parabolic or hyperboloid shape conforming to the reflective surface of the target antenna.

[0016] Furthermore, an adhesive film reinforcement layer is provided between the upper skin and the lower skin and the curved polyurethane foam core material.

[0017] The beneficial effects of this invention are as follows: (1) The method for preparing uniformly dense curved polyurethane foam core material by rotational foaming provided by the present invention drives the mold to rotate continuously and uniformly around the central axis of the antenna surface during the key stage of the foaming reaction of the polyurethane mixture. This puts the mixture in a microgravity state in which centrifugal force and gravity are balanced during the foaming process, effectively offsetting the unidirectional influence of gravity on bubble migration and avoiding the density gradient problem caused by bubble floating in the traditional static foaming process. The curved polyurethane foam core material prepared by this method has a highly uniform overall density distribution, and the density difference can be controlled within 5%. The core material has consistent thermal conductivity, thermal expansion coefficient, and mechanical properties such as compression modulus and shear modulus in the thickness and radial directions, laying a material foundation for the performance consistency of the subsequent sandwich structure.

[0018] (2) This invention applies the above-mentioned rotary foaming process to the manufacturing of high-precision antenna curved sandwich structures. By designing the mold cavity to conform to the antenna reflector, near-net-shape forming of complex curved foam core material is achieved. Core material that precisely matches the antenna reflector can be obtained without or with only a small amount of subsequent machining, which greatly improves material utilization, reduces cutting processes, and lowers manufacturing costs. The uniformly dense curved polyurethane foam core material and fiber-reinforced resin composite skin are composite molded through co-curing or secondary bonding processes. The resulting sandwich structure has a consistent thermal conductivity and coefficient of thermal expansion due to the uniform density of the core material when the ambient temperature changes. Uniform thermal deformation occurs in each area, which effectively avoids the problem of non-uniform thermal deformation caused by density gradient in traditional sandwich structures and significantly improves the surface accuracy and stability of the antenna reflector under the working temperature field.

[0019] (3) This invention further strengthens the interfacial bonding strength between the skin and the core material by setting an adhesive film reinforcement layer between the upper skin, lower skin, and curved polyurethane foam core material. During the curing process, the adhesive film forms chemical bonds and mechanical interlocks with the skin resin and the core material surface. At the same time, with its moderate modulus and good toughness, it absorbs and disperses the thermal stress between the skin and the core material, effectively preventing the decrease in stiffness and loss of surface accuracy of the sandwich structure caused by interface debonding. The antenna curved sandwich structure prepared by the above process has a uniform core material density and reliable interfacial bonding between the skin and the core material, which makes the antenna reflector surface have stable electrical performance consistency in complex service environments, meeting the strict requirements of high-frequency antennas for the geometric accuracy and electrical performance of the reflector surface.

[0020] (4) The rotary foaming process equipment used in this invention is simple and easy to operate. It can be achieved by simply adding a rotary drive device to the existing foaming mold. Compared with the existing technology, which improves foam performance by a combination of rotation and reciprocating shaking motion, the single uniform rotation motion control of this invention is simpler and the process parameters are easier to stabilize. It is suitable for the manufacturing needs of antenna reflectors with different curvatures and sizes, and is especially suitable for the preparation of large, high-precision hyperboloid or parabolic antennas.

[0021] To make the above description of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the rotary foaming process equipment of the present invention.

[0023] Figure 2 This is a schematic diagram of the rotating foaming feed port configuration of the present invention.

[0024] Figure 3 This is a schematic diagram of the parting surface of the rotary foamed polyurethane mold of the present invention.

[0025] Explanation of reference numerals in the attached figures: 1. Protective cover; 2. Mold; 201. Upper mold; 202. Lower mold; 203. Cavity; 204. Upper mold mounting hole; 205. Lower mold mounting hole; 206. Mold fitting surface; 3. Drive unit; 4. Motor control console; 5. Plug; 6. Rubber pad; 7. Feed port. Detailed Implementation

[0026] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0027] Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the drawings is not intended to limit the invention. In the drawings, the same units / elements are referred to by the same reference numerals.

[0028] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0029] This embodiment relates to a method for preparing a uniformly dense curved polyurethane foam core material by rotational foaming, comprising the following steps: a. Mold preparation steps: Prepare a mold 2 with a curved cavity that matches the reflective surface of the target antenna. The cavity 203 of the mold 2 is used to form a curved shape that conforms to the reflective surface of the antenna, so as to achieve near-net-shape molding of the foam core material, that is, directly molding it into the required curved shape without or with only a small amount of subsequent machining, thereby reducing machining allowance and improving material utilization. Install the mold 2 on a drive device 3 that can rotate around the central axis of the antenna surface. The drive device 3 is used to provide rotational power to the mold 2 to ensure that the rotational motion can be carried out around the central axis of the antenna surface, so that the material in the mold 2 is subjected to uniform force during rotation.

[0030] b. Pouring steps: The polyurethane mixture is poured into the cavity 203 of the mold 2 through the feeding port 7. The feeding port 7 is set on the mold 2 to guide the polyurethane mixture to be smoothly injected into the cavity 203, so as to avoid air bubbles or uneven pouring during the pouring process.

[0031] c. Rotational foaming step: During the period from the start of the foaming reaction of the polyurethane mixture to the basic gelation and solidification of the foam, the drive device 3 drives the mold 2 to rotate continuously and uniformly around the central axis of the antenna surface. This continuous and uniform rotational motion is used to continuously change the spatial orientation of the polyurethane mixture and the growing foam in the mold 2 during the rotation, thereby counteracting the unidirectional influence of gravity on bubble migration. By controlling the rotation speed, the polyurethane mixture is kept in a microgravity state in which centrifugal force and gravity are balanced during the foaming process, eliminating the density stratification phenomenon caused by gravity, making the bubbles evenly distributed inside the foam, and avoiding the formation of a density gradient with low density at the top and high density at the bottom.

[0032] d. Demolding step: After the foam has solidified and formed, it is demolded to obtain a curved polyurethane foam core material. The curved polyurethane foam core material has a curved shape consistent with the cavity 203 of mold 2, and the overall density distribution is uniform with the density difference controlled within 5%. It is used as the core material of the antenna curved sandwich structure to ensure that the sandwich structure has uniform thermal expansion characteristics when the temperature changes, thereby ensuring the surface accuracy and stability of the antenna reflector.

[0033] The polyurethane mixture used in the casting step can be commercially available polyurethane foaming raw materials, prepared according to the manufacturer's recommended ratio. For example, the polyurethane A and B component formula of Wanhua Chemical Co., Ltd. can be used and mixed according to the manufacturer's recommended ratio.

[0034] In some preferred embodiments, the rotational speed of the continuous uniform rotation in step c is 10-15 rpm. This speed range ensures that the polyurethane mixture and the growing foam in the mold 2 are subjected to appropriate centrifugal force during rotation, achieving a precise balance between centrifugal force and gravity, thus creating a microgravity environment. When the rotational speed is below 10 rpm, the centrifugal force is insufficient to counteract the unidirectional effect of gravity on bubble migration, and the bubbles will still migrate upwards, resulting in a density gradient in the foam core material with lower density at the top and higher density at the bottom. When the rotational speed is above 15 rpm, excessive centrifugal force will generate a new density gradient, where the density in the region far from the rotation axis is higher than that in the region near the rotation axis. By controlling the rotational speed within the range of 10-15 rpm, the bubbles are evenly distributed inside the foam, neither migrating upwards nor gathering towards the outer edge, ensuring that the overall density of the molded curved polyurethane foam core material is uniform, with the density difference controlled within 5%.

[0035] In some preferred embodiments, when the drive device 3 (motor) drives the mold 2 to rotate at a constant speed around the central axis of the antenna surface, the rotation axis passes through or approaches the geometric center of the cavity 203 of the mold 2. This rotation axis position ensures that the polyurethane mixture and the growing foam within the mold 2 rotate around the center of the cavity 203 during rotation, ensuring that the material at each point within the cavity 203 has the same trajectory and stress state. When the rotation axis passes through the geometric center of the cavity 203, the distance from each point within the cavity 203 to the rotation axis is symmetrically distributed, the centrifugal force is symmetrically distributed radially, the foam is subjected to uniform stress, and the bubble migration behavior in each region is consistent. When the rotation axis approaches the geometric center of the cavity 203, i.e., there is a slight offset but the offset is much smaller than the size of the cavity 203, the stress distribution remains basically symmetrical, and no significant difference in centrifugal force is generated. This setting avoids uneven centrifugal force distribution caused by deviation of the rotation axis, ensures consistent stress state in all areas of the foam, and makes the density distribution of the molded curved polyurethane foam core material uniform in the radial direction, with the overall density difference controlled within 5%.

[0036] This embodiment relates to a method for manufacturing an antenna curved sandwich structure, including the following steps: a. Foam core material preparation steps: The curved polyurethane foam core material is prepared by the above-mentioned method of preparing uniformly dense curved polyurethane foam core material through rotational foaming. The curved polyurethane foam core material has a curved shape that matches the reflective surface of the target antenna, and the overall density distribution is uniform with the density difference controlled within 5%. It is used as the core material of the sandwich structure to provide a lightweight and high-strength support foundation for the sandwich structure, while ensuring that the overall structure has uniform thermal expansion characteristics when the temperature changes after subsequent skin composite.

[0037] b. Laying steps: Reinforcing fiber prepreg or dry fiber fabric is laid on the upper and lower surfaces of the curved polyurethane foam core material to form the upper and lower skins of the sandwich structure. This gives the skins high modulus and high strength characteristics, allowing them to work together with the foam core material to withstand bending loads. To enhance the interfacial bonding between the foam core material and the skin, an adhesive film can be used to reinforce the bonding between the foam core material and the reinforcing fiber prepreg. The adhesive film is laid between the foam core material and the prepreg and cures simultaneously during the subsequent curing process, forming a reinforced interfacial bonding layer to prevent the skin from debonding from the core material during use.

[0038] c. Composite Molding Steps: The upper and lower skins and the curved polyurethane foam core are compositely molded into a single curved sandwich structure using either a co-curing process or a two-stage bonding process. When using the co-curing process, the foam core material with the reinforcing fiber prepreg laid on it is encapsulated in a vacuum bag and vacuum-compressed, then placed in an autoclave for curing. During curing, the resin in the reinforcing fiber prepreg flows, impregnates, and cures under high temperature and pressure, while the adhesive film cures simultaneously, firmly bonding the skin and foam core material together. When using the two-stage bonding process, the cured upper and lower skins and foam core material are prepared separately, and then bonded together using structural adhesive. The co-curing process achieves one-time molding of the skin and core material, simplifying the process and improving interfacial bonding strength.

[0039] d. Post-processing steps: The formed curved sandwich structure undergoes edge trimming and post-processing. Edge trimming removes burrs, flash, and excess material generated during the forming process, ensuring neat edges and precise dimensions. Post-processing includes precision measurement and surface metallization. Precision measurement checks whether the antenna reflector's profile accuracy meets design requirements. Surface metallization forms a conductive layer on the skin surface to meet the antenna's electromagnetic wave reflection performance requirements. After these steps, a high-precision antenna reflector is obtained. This reflector exhibits consistent thermal conductivity and coefficient of thermal expansion due to the uniform density of the core material, ensuring profile accuracy stability and electrical performance consistency despite changes in ambient temperature.

[0040] In the layup step, the adhesive film can be an epoxy resin-based structural adhesive film with a thickness of 0.1 to 0.3 mm. The adhesive film is laid between the foam core material and the prepreg and is cured simultaneously during the subsequent curing process to form a reinforced interface bonding layer, preventing the skin and core material from debonding during use.

[0041] In some embodiments, the reinforcing fiber prepreg in step b is preferably one or more of carbon fiber prepreg, glass fiber prepreg, or aramid fiber prepreg. Carbon fiber prepreg has the highest specific strength and specific modulus, making it suitable for high-precision antenna reflectors with strict requirements for mechanical properties and weight; glass fiber prepreg has good dielectric properties and cost advantages, making it suitable for radome structures with electromagnetic wave transmission requirements; aramid fiber prepreg has excellent impact resistance and toughness, making it suitable for applications that need to withstand impact loads. At least two layers are laid to ensure the skin has sufficient thickness and load-bearing capacity, avoiding insufficient mechanical properties due to insufficient thickness in a single layer. Adjacent fiber layers are orthogonally or cross-laid. Orthogonal layups provide balanced mechanical properties in two mutually perpendicular directions, while cross-layups provide optimized mechanical properties in a specific direction, ensuring the skin provides sufficient strength and stiffness under different stress directions.

[0042] In some embodiments, the curing process in step c employs autoclave curing, with a curing pressure of 0.3–0.5 MPa, a curing temperature of 100–130°C, and a curing time of 60–120 minutes. The curing pressure applies uniform compressive force to the layup, ensuring tight adhesion between prepreg layers, expelling residual air and volatiles between layers, and reducing the porosity of the skin. Curing pressures below 0.3 MPa are prone to defects such as porosity and delamination in the skin, while pressures above 0.5 MPa may cause excessive compression or even crushing of the foam core material. The curing temperature activates the cross-linking reaction of the resin matrix, transforming the resin from a viscous flow state to a solid state. Temperatures below 100°C result in insufficient resin curing, while temperatures above 130°C may lead to thermal deformation or degradation of the foam core material. The curing time ensures sufficient resin cross-linking; times below 60 minutes result in insufficient curing, while times above 120 minutes extend the production cycle and increase costs. Through the synergistic combination of the above process parameters, a composite material skin with high density and high mechanical properties is formed. At the same time, the adhesive film is cured to form a reinforced interface bonding layer, ensuring that the sandwich structure has excellent overall mechanical properties and dimensional stability.

[0043] This embodiment relates to an antenna curved sandwich structure, including a curved polyurethane foam core material, an upper skin composited on the upper surface of the core material, and a lower skin composited on the lower surface of the core material.

[0044] The curved polyurethane foam core material is prepared using the aforementioned rotational foaming method to produce a uniformly dense curved polyurethane foam core material. Used as the core material in sandwich structures, it exhibits consistent thermal conductivity and coefficient of thermal expansion in both the thickness and radial directions. When the ambient temperature changes, all parts of the core material undergo uniform thermal deformation, avoiding non-uniform thermal deformation caused by density gradients. Simultaneously, the compressive modulus and shear modulus of the core material are spatially consistent, ensuring stable overall mechanical properties and dimensional stability of the sandwich structure. The curved shape of the polyurethane foam core material conforms to the reflector surface of the target antenna, allowing it to be directly used in sandwich structure assembly with little or no subsequent machining, achieving near-net-shape forming.

[0045] Both the upper and lower skins are fiber-reinforced resin composite laminates. The upper skin forms the upper high-modulus panel of the sandwich structure, sharing bending loads with the lower skin, protecting the internal core material from external environmental corrosion, and providing the surface function for electromagnetic wave reflection or transmission for the antenna reflector. The lower skin forms the lower high-modulus panel of the sandwich structure, sharing bending loads with the upper skin, and providing structural support and protection for the back of the sandwich structure. The fiber-reinforced resin composite laminate is composed of reinforcing fibers and a resin matrix. The reinforcing fibers provide the main mechanical properties of the skin, while the resin matrix bonds the reinforcing fibers together, transfers loads, and protects the fibers, giving the laminate excellent specific strength and specific modulus. Working in conjunction with the curved polyurethane foam core material, it forms a sandwich structure with high bending stiffness and strength-to-weight ratio.

[0046] In some embodiments, the curved polyurethane foam core material is a parabolic or hyperboloid shape conforming to the reflector surface of the target antenna. The parabolic core material is used to reflect electromagnetic waves from the antenna focal point to form a parallel beam, or conversely, to converge the parallel beam to the focal point, suitable for satellite communication antennas, radar antennas, and other scenarios. The hyperboloid core material is used to form the sub-reflector support structure of Cassegrain or Gregorian antennas, working in conjunction with the main reflector surface to achieve a compact antenna system design, suitable for large-aperture antennas, radio telescopes, and other scenarios. Through conformal design, the core material in the sandwich structure not only serves as structural support but also directly forms the geometric contour basis of the antenna reflector surface, avoiding dimensional errors and surface damage introduced by subsequent machining. Simultaneously, it ensures a tight fit between the core material and the skin interface, improving the overall integrity of the sandwich structure and the strength of the interface bonding.

[0047] In some embodiments, a film reinforcement layer is further provided between the upper and lower skins and the curved polyurethane foam core material. The film reinforcement layer is a polymeric structure film laid between the core material and the skin. During the curing process of the sandwich structure, the resin matrix in the film melts, flows, and undergoes a cross-linking curing reaction under high temperature and pressure, forming chemical bonds and mechanical interlocks with the skin resin and the core material surface, firmly bonding the skin and core material together. The film reinforcement layer compensates for the interfacial bonding strength between the skin and the core material. The core material surface has a porous structure, and relying solely on the penetration of the skin resin is insufficient to guarantee the stability of the interfacial bonding strength. After curing, the film forms a continuous and uniform interfacial layer, significantly improving the interfacial bonding strength and avoiding failure modes such as skin peeling and core material delamination. Simultaneously, the film reinforcement layer buffers the thermal stress between the skin and the core material. Since the core material and skin have different coefficients of thermal expansion, the film has a moderate modulus and good toughness, enabling it to absorb and disperse interfacial thermal stress through elastic deformation, improving the durability and reliability of the sandwich structure under varying temperature environments.

[0048] In some specific embodiments, the adhesive film reinforcement layer can be an epoxy resin-based structural adhesive film with a thickness of 0.1 to 0.3 mm, which is laid between the core material and the skin. During the curing and molding process of the sandwich structure, the resin matrix in the adhesive film melts, flows and undergoes a cross-linking curing reaction under high temperature and high pressure, forming chemical bonds and mechanical interlocks with the skin resin and the surface of the core material, firmly bonding the skin and the core material into one piece.

[0049] Example 1 This embodiment relates to a method for preparing a uniformly dense curved polyurethane foam core material using rotational foaming, comprising the following steps: like Figure 1 As shown, the rotary foaming equipment used in this embodiment mainly includes components such as mold 2, drive device 3, and motor control console 4. Mold 2 is mounted on drive device 3, which can rotate around the central axis of the antenna surface, and drive device 3 is controlled by motor control console 4.

[0050] a. Mold preparation steps: Prepare mold 2 with a curved cavity that matches the reflective surface of the target antenna, such as... Figure 3 As shown, the mold 2 consists of an upper mold 201 and a lower mold 202 that cooperate with each other. The upper mold 201 and the lower mold 202 close at the mold mating surface 206 to form a cavity 203. The cavity 203 is used to form a curved surface shape that conforms to the antenna reflector, so as to achieve near-net-shape molding of the foam core material, that is, directly molding it into the required curved surface shape without or with only a small amount of subsequent machining, thereby reducing machining allowance and improving material utilization. The mold 2 is provided with an upper mold mounting hole 204 and a lower mold mounting hole 205 for fixing the mold 2 to the rotary drive device. The mold 2 is mounted on the drive device 3 that can rotate around the central axis of the antenna surface. The drive device 3 is controlled by the motor control console 4 to provide rotational power to the mold 2, ensuring that the rotational motion can be carried out around the central axis of the antenna surface, so that the material in the mold 2 is subjected to uniform force during rotation.

[0051] b. Pouring steps: as follows Figure 2 As shown, the polyurethane mixture is poured into the cavity 203 of the mold 2 through the inlet 7. The inlet 7 is located on the mold 2. After pouring, a plug 5 is used for sealing, and a rubber gasket 6 is placed between the plug 5 and the inlet 7 to ensure airtightness. The inlet 7 is used to guide the polyurethane mixture smoothly into the cavity 203. The plug 5 and the rubber gasket 6 are used to prevent leakage of the mixture and the entry of external air after pouring, avoiding air bubbles or uneven pouring during the pouring process. The polyurethane mixture can be made from commercially available polyurethane foaming raw materials, prepared according to the manufacturer's recommended ratio. For example, the polyurethane A and B component formula from Wanhua Chemical Co., Ltd. can be used, mixed according to the manufacturer's recommended ratio.

[0052] c. Rotational foaming step: During the period from the start of the foaming reaction of the polyurethane mixture to the basic gelation and solidification of the foam, the drive device 3 drives the mold 2 to rotate continuously and uniformly around the central axis of the antenna surface. This continuous and uniform rotational motion is used to continuously change the spatial orientation of the polyurethane mixture and the growing foam in the mold 2 during the rotation, thereby counteracting the unidirectional influence of gravity on bubble migration. By controlling the rotation speed, the polyurethane mixture is kept in a microgravity state in which centrifugal force and gravity are balanced during the foaming process, eliminating the density stratification phenomenon caused by gravity, making the bubbles evenly distributed inside the foam, and avoiding the formation of a density gradient with low density at the top and high density at the bottom.

[0053] d. Demolding step: After the foam has solidified and formed, it is demolded to obtain a curved polyurethane foam core material. The curved polyurethane foam core material has a curved shape consistent with the cavity 203 of mold 2, and the overall density distribution is uniform with the density difference controlled within 5%. It is used as the core material of the antenna curved sandwich structure to ensure that the sandwich structure has uniform thermal expansion characteristics when the temperature changes, thereby ensuring the surface accuracy and stability of the antenna reflector.

[0054] Example 2 This embodiment relates to the preparation of a foam sandwich core material for a parabolic antenna, specifically including the following steps: like Figure 3 As shown, an aluminum alloy parabolic cavity mold matching the parabolic antenna is manufactured. The mold consists of an upper mold 201 and a lower mold 202, which close at the mold closing surface 206 to form a cavity 203. The mold is mounted on the output shaft of the rotary table through the upper mold mounting hole 204 and the lower mold mounting hole 205. Based on 80kg / m 3 The filling density is used for ingredient mixing and foaming, such as Figure 2 As shown, the polyurethane mixture is poured into the mold cavity 203 through the feed port 7. After pouring, the feed port 7 is sealed with a plug 5, and a rubber gasket 6 is placed between the plug 5 and the feed port 7 to ensure sealing. like Figure 1 As shown, the program is set through the motor control console 4, and the drive device 3 is started to make the mold rotate at a constant speed of 10-15 rpm around its main axis (the axis of symmetry of the parabola). The rotation continues for 15 minutes until the foam is completely gelled and solidified. Stop rotating and place the mold into an 80℃ oven to mature for 2 hours; After the mold cools down to below 50°C, it is demolded to obtain a parabolic polyurethane foam core material with a diameter of 1.2 meters.

[0055] Three-dimensional scanning and density sampling tests were performed on the above parabolic core material. The results showed that the density at various points of the entire curved core material ranged from 78 to 82 kg / m³. 3Within the specified range, the density difference is less than 5%, meeting the requirements for high-precision antenna use.

[0056] Example 3 This embodiment relates to the fabrication of a sandwich structure parabolic antenna, specifically including the following steps: Parabolic polyurethane foam core material with a density difference of less than 5% was prepared according to the method in Example 2; Two layers of carbon fiber prepreg are first laid on the mold surface as the front skin, with the first layer at 45° and the second layer at 0°; then the foam core material is placed; then two layers of carbon fiber prepreg are laid on the surface of the foam core material as the rear skin, with the third layer at 0° and the fourth layer at 45°; to enhance the interfacial bonding force, an epoxy resin-based structural adhesive film (thickness 0.1-0.3mm) is used to reinforce the bonding between the foam core material and the carbon fiber prepreg. Encapsulation and curing: Sequentially lay a peelable cloth, a non-porous release film, a breathable felt, and a vacuum bag on the product surface. Use sealing tape to seal the entire product on the mold. Vacuum compact the layers, ensuring the vacuum level is not lower than -0.06MPa during compaction, for 5 to 15 minutes. Then, place the product in an autoclave and cure it at 0.4MPa and 120℃ for 90 minutes. Once the mold has cooled to below 40°C, the bag is removed from the mold and sent to the fitter for post-processing, ultimately yielding a qualified antenna curved sandwich structure with a diameter of 1.2 meters.

[0057] Industrial applicability The method for preparing uniformly dense curved polyurethane foam core material by rotational foaming, the antenna curved sandwich structure and its manufacturing method provided by this invention can be widely used in satellite communication antennas, radar antennas, radio telescopes and aerospace sandwich structures, etc., and have significant industrial applicability.

[0058] In summary, the method for preparing uniformly dense curved polyurethane foam core materials using rotational foaming provided by this invention, by driving the mold to rotate continuously and uniformly around the central axis of the antenna surface during the key stage of the foaming reaction, ensures that the polyurethane mixture is in a microgravity state where centrifugal force and gravity are balanced during the foaming process. This effectively eliminates the density gradient problem caused by bubble floating in traditional static foaming processes, producing curved polyurethane foam core materials with an overall density difference controlled within 5%. When this core material is composited with a fiber-reinforced resin composite skin through co-curing or secondary bonding processes, the resulting antenna curved sandwich structure exhibits consistent thermal conductivity and coefficient of thermal expansion under varying ambient temperatures, with uniform thermal deformation in each region. This significantly improves the surface accuracy stability and electrical performance consistency of the antenna reflector.

[0059] This invention achieves near-net-shape molding of complex curved foam core materials through a mold cavity conformal design with the antenna reflector surface, requiring little or no subsequent machining, significantly improving material utilization and reducing manufacturing costs. By placing an adhesive film reinforcement layer between the skin and the core material, the interfacial bonding strength is further enhanced, while simultaneously buffering thermal stress between the skin and the core material, effectively preventing interfacial debonding failure and improving the long-term reliability and durability of the sandwich structure.

[0060] The process and equipment of this invention are simple and easy to operate. It can be realized by simply adding a rotation drive device to the existing foaming mold. It is suitable for the manufacturing needs of antenna reflectors with different curvatures and sizes. It is especially suitable for the preparation of large, high-precision hyperboloid or parabolic antennas. It has the advantages of simple process, low cost and wide applicability. It can be widely used in satellite communication antennas, radar antennas, radio telescopes and aerospace sandwich structures.

[0061] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a uniformly dense curved polyurethane foam core material using rotational foaming, characterized in that, Includes the following steps: a. Prepare a mold with a curved cavity that matches the reflective surface of the target antenna; and mount the mold on a drive device that can rotate about the central axis of the antenna surface; b. Pour the polyurethane mixture into the cavity of the mold through the feed port; c. During the period from the start of the foaming reaction of the polyurethane mixture to the basic gelation and solidification of the foam, the driving device is controlled to drive the mold to rotate continuously and uniformly around the central axis of the antenna surface, so that the polyurethane mixture is in a microgravity state in which centrifugal force and gravity are balanced during the foaming process. d. After the foam has cured and solidified, demold it to obtain curved polyurethane foam core material.

2. The method for preparing uniformly dense curved polyurethane foam core material by rotational foaming according to claim 1, characterized in that, In step c, the rotational speed of the continuous uniform rotational motion is 10 to 15 rpm.

3. The method for preparing uniformly dense curved polyurethane foam core material by rotational foaming according to claim 1, characterized in that, In step c, when the driving device drives the mold to rotate at a constant speed around the central axis of the antenna surface, the rotation axis passes through or approaches the geometric center of the mold cavity.

4. The method for preparing uniformly dense curved polyurethane foam core material by rotational foaming according to claim 1, characterized in that, The density range of the curved polyurethane foam core material is ≤5%.

5. A method for manufacturing a curved sandwich structure for an antenna, characterized in that, Includes the following steps: a. A curved polyurethane foam core material is prepared using the method according to any one of claims 1-4; b. Reinforcing fiber prepreg or dry fiber fabric is laid on the upper and lower surfaces of the curved polyurethane foam core material, respectively; c. The upper skin, lower skin and the curved polyurethane foam core material are compositely molded into an integral curved sandwich structure through a co-curing process or a secondary bonding process. d. The edges of the formed curved sandwich structure are trimmed and post-processed to obtain a high-precision antenna reflector.

6. The method for manufacturing the antenna curved sandwich structure according to claim 5, characterized in that, In step b, the reinforcing fiber prepreg is one or more of carbon fiber prepreg, glass fiber prepreg, or aramid fiber prepreg, and the number of layers is at least two, with the fiber directions of adjacent layers being orthogonal or cross-laid.

7. The method for manufacturing the antenna curved sandwich structure according to claim 5, characterized in that, In step c, the co-curing process uses an autoclave for curing, with a curing pressure of 0.3–0.5 MPa, a curing temperature of 100–130°C, and a curing time of 60–120 minutes.

8. An antenna curved sandwich structure, characterized in that, include: Curved polyurethane foam core material prepared by the method according to any one of claims 1-4; The upper skin is composited on the upper surface of the curved polyurethane foam core material; The lower skin is composited on the lower surface of the curved polyurethane foam core material; Both the upper and lower skins are fiber-reinforced resin composite laminates.

9. The antenna curved sandwich structure according to claim 8, characterized in that, The curved polyurethane foam core material has a parabolic or hyperboloid shape that conforms to the reflective surface of the target antenna.

10. The antenna curved sandwich structure according to claim 8, characterized in that, An adhesive film reinforcement layer is also provided between the upper and lower skins and the curved polyurethane foam core material.

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