A method of forming a large thickness skin foam sandwich composite

Abstract

This invention discloses a method for molding a thick-skinned foam sandwich composite material, comprising: Step 1: raising the temperature of the preform to T1 at a rate of less than 0.5℃ / min, increasing the pressure to P1 during the heating process, and holding for t1; wherein, T1 is not greater than the temperature value corresponding to the minimum resin viscosity, and the pressure P1 does not exceed the compressive strength of the foam core; Step 2: raising the temperature of the preform to T2 at a rate of 0.5℃ / min-1℃ / min, then increasing the pressure to P2, and holding for t2, wherein T2 is between the temperature value corresponding to the minimum resin viscosity and the resin reaction initiation temperature; Step 3: raising the temperature of the preform to the prepreg curing temperature at a rate of greater than 2℃ / min. This invention employs segmented heating and pressurization, wherein the slow heating in the low-temperature stage ensures that the thermal stress of the foam core is fully released; the process window is formed by utilizing the pressure transmission hindrance effect of the viscous resin, protecting the foam core from pressure exceeding the critical value of compressive strength, thereby improving the molding quality of the foam sandwich composite material.

Description

Technical Field

[0001] This invention relates to, but is not limited to, the field of resin-based fiber composite materials, and particularly to a method for molding foam sandwich composite materials. Background Technology

[0002] In the manufacture of high-performance composite materials for aerospace, wind power, and other fields, closed-cell polymethacrylamide (PMI) foam is widely used as the core material for sandwich structures due to its advantages such as high specific strength and low dielectric loss. It is commonly formed using an autoclave bonding process, where the cured lower skin, foam core, and uncured upper skin are integrally molded. However, in engineering practice, the foam core often suffers from pressure damage, becoming a common problem restricting product quality.

[0003] The root cause of this problem lies in the physical mismatch between the foam and the composite material. Firstly, the thermal conductivity of the foam core (<0.06 W / (m·K)) is significantly lower than that of the composite material (>3 W / (m·K)). For thick foam sandwich structures, a drastic temperature difference exists between the inner and outer regions of the core, leading to non-uniform expansion and internal stress concentration in the foam core. Secondly, conventional process schemes neglect the influence of resin rheological behavior on the mechanical response of the foam core. Applying excessive pressure during the low viscosity window of the resin exceeds the compressive strength of the non-uniformly expanding foam core, causing damage. Alternatively, empirically reducing the pressure may mitigate the damage, resulting in excessively low pressure that affects the non-destructive quality of the skin area. In conclusion, a synergistic process control method that considers both the mechanical response of the foam core and the non-destructive quality of the composite material is urgently needed, balancing the molding quality of both the foam core and the skin. Summary of the Invention

[0004] The purpose of this invention is to provide a molding method for thick-skinned foam sandwich composite materials, in order to solve the problem that in existing composite material molding processes, the molding process and the mechanical response of the material are mismatched, resulting in the difficulty in achieving both non-destructive quality in the foam core and skin areas of the foam sandwich composite material.

[0005] The technical solution of this invention is as follows: This invention provides a method for molding a thick-skin foam sandwich composite material. A closed-cell foam core is coated with an adhesive film, and the foam core coated with the adhesive film is placed on a cured lower skin. An upper skin prepreg is laid on the foam core, and the upper skin prepreg includes resin and reinforcing fibers. The curing and molding method for the foam core, adhesive film, and upper skin prepreg includes: Step 1: Increase the temperature of the preform to T1 at a rate of less than 0.5℃ / min, and increase the pressure to P1 during the heating process, and hold for t1 time; wherein, T1 is not greater than the temperature value corresponding to the minimum resin viscosity, and the pressure P1 does not exceed the compressive strength of the foam core; Step 2: Increase the temperature of the preform to T2 at a rate of 0.5℃ / min-1℃ / min, then increase the pressure to P2 and hold for t2. T2 is between the temperature value corresponding to the minimum resin viscosity and the resin reaction initiation temperature. In this step, the heating and pressurization method of first increasing the temperature to T2 and then increasing the pressure to P2 avoids the low viscosity region of the resin during the heating process, that is, avoids the region where the resin pressure transmission efficiency is high, thereby protecting the foam core. Step 3: Increase the temperature of the preform to the curing temperature of the prepreg at a rate greater than 2℃ / min.

[0006] Optionally, in the molding method for thick-skinned foam sandwich composite materials as described above, the pressure P1 and pressure holding time t1 in step 1 satisfy the following relationship: ; in, The value of α is the compressive strength of the foam core at temperature T1, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. a This refers to the standard pressure for the bonding and curing of the adhesive film. This indicates the actual pressure transmitted to the foam core in step 1. By limiting this pressure to be less than or equal to the compressive strength of the foam core at the same temperature, the product curing quality is ensured by using the highest possible pressure while achieving a safety margin for the compressive strength of the foam core.

[0007] Optionally, in the molding method for thick-skinned foam sandwich composite materials as described above, the pressure P2 and pressure holding time t2 in step 2 satisfy the following relationship: ; in, α is the compressive strength of the foam core at temperature T2, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. r The standard pressure for curing prepreg; This indicates the actual pressure transmitted to the foam core in step 2. By limiting this pressure to be less than or equal to the compressive strength of the foam core at the same temperature, the product curing quality is ensured by using the highest possible pressure while achieving a safety margin for the compressive strength of the foam core.

[0008] Optionally, in the above-described method for molding thick-skin foam sandwich composite materials, after step 2 and before step 3, the method further includes: Step a: Use a pulse pressurization method, that is, increase the pressure to P3, hold for t3, then reduce the pressure to P2, hold for t2, and repeat the above pulse pressurization method multiple times.

[0009] Optionally, in the molding method for thick-skinned foam sandwich composite materials as described above, the pressure to P3 and the pressure holding time t3 in step a satisfy the following relationship: ; in, α is the compressive strength of the foam core at temperature T2, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. r The standard pressure for curing prepreg; In step a, the pressure actually applied to the foam core is controlled by applying pressure P3 to the upper skin prepreg and controlling the pressure application time of P3. The compressive strength of the foam core shall not exceed that of the foam core, thereby protecting the structure of the foam core.

[0010] Optionally, in the above-described method for molding thick-skinned foam sandwich composite materials, the determination method for the pressure transmission loss coefficient in each step includes: S1, Prepare a sample with the same composition as the upper skin prepreg and the adhesive film, with the adhesive film disposed below the upper skin prepreg and a pressure sensor disposed at the bottom of the sample; S2, adjusts the temperature and pressure according to the set program; S3, determine the pressure P monitored by the pressure sensor at different times t. t And α is obtained through a fitting algorithm.

[0011] Optionally, in the above-described method for molding thick-skinned foam sandwich composite materials, the method for adjusting the temperature and pressure according to a set program in step S2 is as follows: According to the curing and molding method of the foam core, film and upper skin prepreg, the product heating and pressurization process involves applying corresponding temperature and pressure at each time period.

[0012] Optionally, in the above-described method for molding thick-skin foam sandwich composite materials, the specific way to obtain α in step S3 is as follows: α is obtained by fitting according to the following relationship; ; Where P is the preset pressure in S2.

[0013] The beneficial effects of this invention are as follows: This invention provides a method for molding a thick-skinned foam sandwich composite material, employing a segmented heating and pressurization approach. In the low-temperature stage, the temperature is raised slowly to ensure sufficient release of thermal stress in the foam core. Furthermore, the heating avoids the low-viscosity plateau of the resin, increasing pressure in the moderate viscosity region. The process window is created by utilizing the pressure-transferring effect of the viscous resin, protecting the foam core from pressure exceeding its critical compressive strength, thereby improving the molding quality of the foam sandwich composite material. The technical solution provided by this invention has the following beneficial effects: First, the molding method for thick-skin foam sandwich composite materials of this invention solves the contradiction between the weak pressure resistance of the foam core and the reliance of the prepreg on high-pressure compaction: A pressure attenuation model for transmission in viscous resin is established. By segmented pressurization and multiple pressure releases to break the pressure transmission chain, pressure transmission is controlled within the skin, ensuring the foam core can withstand safe low pressure and protecting its structural integrity. The spatial pressure distribution problem is transformed into a temporal segmented pressure management problem. A "wave-like" compaction is achieved through cyclic pulses, improving the skin's density and reducing product porosity.

[0014] Secondly, the method for molding thick-skin foam sandwich composite materials provided by the present invention is simple to operate, low in cost, and highly compatible: compared with the method of avoiding damage to the foam core by using a secondary bonding process, the present invention only needs to adjust the process parameters, which has significant cost advantages and can be compatible with the molding of low-density foam cores with low compressive strength. Attached Figure Description

[0015] The accompanying drawings are provided to further understand the technical solutions of the present invention and constitute a part of the specification. They are used together with the embodiments of this application to explain the technical solutions of the present invention and do not constitute a limitation on the technical solutions of the present invention.

[0016] Figure 1 A schematic diagram of the curing scheme for the molding method of thick skin foam sandwich composite material provided by the present invention; Figure 2 This is a schematic diagram of a typical resin viscosity-temperature curve and parameter settings. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that, unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

[0018] The present invention provides the following specific embodiments, which can be combined with each other. For the same or similar concepts or processes, they may not be described again in some embodiments.

[0019] The preferred embodiments are described below, and the technical solution of the present invention will be explained more clearly and completely in conjunction with the accompanying drawings.

[0020] Example 1: The sample is a foam-core epoxy resin-based carbon fiber composite material. The matrix resin is epoxy resin (brand name ELM-434). The initial reaction temperature of this resin was determined to be 150℃ and the minimum viscosity temperature to be 130℃ using the DSC characteristic temperature extrapolation method. The sample consists of upper and lower skins and a PMI foam core. The manufacturing process involves first combining the cured lower skin and foam core with the uncured upper skin prepreg, with an adhesive film placed between the foam core and the skin. The sample preform is then vacuum-sealed onto a mold using a vacuum bag and cured in an autoclave. The curing process is as follows: Step 1: Heat the preform to 120°C at a rate of 0.3°C / min, while simultaneously increasing the pressure to 0.3MPa, and maintain this temperature for 20 minutes; Step 2: Heat the preform to 140°C at a rate of 1°C / min, then increase the pressure to 0.5MPa and hold for 10min; Step 3: Increase the temperature of the preform to the prepreg curing temperature of 180°C at a rate of 3°C / min, and keep it at this temperature for 1 hour to complete the curing.

[0021] Comparative Example 1: Comparative Example 1 uses the same sample materials and preparation process as Example 1, and the curing process follows existing methods, as described below: Increase the temperature at a rate of 5℃ / min, pressurize to 0.5MPa between room temperature and 80℃, continue to increase the temperature at a rate of 5℃ / min to 120℃ and hold for 30min, continue to increase the temperature at a rate of 5℃ / min to 180℃ and hold for 1h to complete curing.

[0022] After curing, the sample was subjected to non-destructive testing: Example 1: The foam core showed no damage, and the porosity of the upper skin area was 3%. Comparative Example 1 shows damage to the foam core, with a porosity of 3% in the upper skin area.

[0023] Comparative Example 1 involved uniform heating throughout, with full pressurization applied during the low-temperature phase. The rapid heating rate resulted in poor thermal uniformity of the foam core and significant thermal stress. Because Comparative Example 1 applied full pressure in the low-viscosity resin region, the resin's pressure transmission efficiency was high, leading to substantial pressure on the foam core and, combined with significant internal thermal stress, damage to the foam core. In contrast, Example 1 involved slow heating in the low-temperature phase, resulting in better temperature uniformity and sufficient release of internal thermal stress. Partial pressure was applied during the low-temperature phase, not exceeding the compressive strength of the foam core. A moderate heating rate was used in the mid-temperature phase to ensure sufficient gas release within the product, quickly navigating the low-viscosity resin phase and protecting the foam core structure. Full pressurization was applied after the resin viscosity increased, at which point the resin's pressure transmission efficiency decreased, resulting in relatively lower actual pressure on the foam core and maintaining a good foam core structure. However, both Example 1 and Comparative Example 1 exhibited high porosity in the upper skin.

[0024] It should be noted that the aircraft panel material includes resin and reinforcing fibers. The resin has high pressure transmission efficiency in a low viscosity state. Based on this characteristic, in the embodiment of the present invention, the temperature is first increased in the medium temperature range, and after the temperature reaches the set temperature and the low viscosity stage of the resin is passed, the pressure is applied, which has a better protective effect on the foam core structure.

[0025] Example 2: The sample is the same as in Example 1. The pressure P1 and pressure holding time t1 in curing step 1 satisfy the following relationship: ; in, The compressive strength of the foam core at 120℃ is 0.55MPa, and α is the pressure transmission loss coefficient (68mN). -1 μ is the resin viscosity (0.2 Pas), h r For the prepreg thickness (13.5 mm), P a The standard pressure for bonding and curing the adhesive film is 0.3 MPa.

[0026] To ensure thorough compaction of the prepreg and good wetting of the bonding surfaces of the adhesive film, foam core, and lower skin, engineering experience suggests that higher pressure is more beneficial, with a compaction time of no less than 600 seconds. Therefore, the pressure holding time t1 is set to 600 seconds, and P1 is set to 0.45 MPa. The actual pressure transmitted to the foam core at this time... The pressure is 0.45 MPa. Relative to the compressive strength of the foam core at this temperature, the pressure should be increased as much as possible while maintaining a certain safety margin to ensure product quality.

[0027] In this embodiment 2, the pressure P2 and the pressure holding time t2 in curing step 2 satisfy the following relationship: ; in, P represents the compressive strength of the foam core at 140℃ (0.5 MPa). r The standard pressure for prepreg curing is 0.45 MPa. α is the pressure transmission loss coefficient (5.6 × 10⁻⁶). 2 mN -1 μ is the resin viscosity (28 Pas), h r The thickness of the prepreg is 13.5 mm.

[0028] To ensure thorough compaction of the prepreg and good wetting of the adhesive film and the bonding surface, based on engineering experience, higher pressure is more beneficial, and the compaction time should not be less than 600 seconds. Therefore, the pressure holding time t2 is set to 600 seconds, P2 is set to 0.6 MPa, and the actual pressure transmitted to the foam core is [not specified]. The pressure is 0.42 MPa. Relative to the compressive strength of the foam core at this temperature, the pressure should be increased as much as possible while maintaining a certain safety margin to ensure product quality.

[0029] In summary, the curing process of the sample is as follows: Step 1: Increase the temperature to 120℃ at a rate of 0.3℃ / min, while simultaneously increasing the pressure to 0.45MPa, and maintain this temperature for 20min; Step 2: Increase the temperature to 140℃ at a rate of 0.3℃ / min, then increase the pressure to 0.6MPa and hold for 600s; Step 3: Increase the temperature to the prepreg curing temperature of 180°C at a rate of 3°C / min, and hold at this temperature for 1 hour to complete the curing process; After curing, the sample was subjected to non-destructive testing: Example 2: The foam core showed no damage, and the porosity of the upper skin area was 1.5%. While ensuring the quality of the foam core, Example 2 showed a lower porosity compared to Example 1. The reason for this is that the pressure applied to the prepreg in Example 2 was higher than that in Example 1, which was more conducive to compacting the prepreg.

[0030] Example 3: Example 3 uses the same samples as Examples 1 and 2, except that the curing process in Example 2 adds step a between steps 2 and 3: A pulsed pressurization method was used, where the pressure was increased to P3 (0.8 MPa), held for t3 (400 s), then decreased to 0.6 MPa and held for 600 s. This process was repeated three times. The actual pressure acting on the foam core during the pressurization process is... The pressure is 0.47 MPa, and the pressure P3 and the pressure holding time t3 satisfy the following relationship: ; in, α is the compressive strength of the foam core at temperature T2, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. r The standard pressure for curing prepreg.

[0031] After curing, the sample was subjected to non-destructive testing: Example 3: The foam core showed no damage, and the porosity of the upper skin area was 1%. Example 3, while maintaining the quality of the foam core, further reduced the porosity compared to Example 2. The reason for this is the pulsed pressurization method. By controlling the pressurization time when applying significant pressure to the upper skin prepreg, the actual pressure applied to the foam core can be kept below its compressive strength, thus protecting the foam core structure. Repeating the pressurization cycle multiple times further improves the compaction of the prepreg and eliminates residual gas.

[0032] Example 4: In Example 4, based on Examples 2 and 3, the pressure transmission loss coefficient is determined as follows: S21: Prepare three sets of samples with the same composition as the upper skin prepreg and the adhesive film, with the adhesive film placed below the prepreg and a pressure sensor placed at the bottom of the sample; S22: Increase the temperature of sample 1 to 120℃ and pressurize it to 0.45MPa; increase the temperature of sample 2 to 140℃ and pressurize it to 0.6MPa; increase the temperature of sample 3 to 140℃ and pressurize it to 0.8MPa. S23: After pressurization is completed, measure the pressure P monitored by the pressure sensor at different times t for the above samples. t α is obtained by fitting the following relationship; ; Where P is the preset pressure of each sample in S22.

[0033] The principle of this invention is as follows: Traditional foam sandwich structure manufacturing faces the challenge of simultaneously achieving high quality in both the skin and the foam core molding process. High pressure leads to foam core damage, while low pressure results in insufficient prepreg compaction and high porosity. The reasons for these problems can be summarized as follows: 1. Compared to prepreg, foam core has lower thermal conductivity. The process parameters of using prepreg result in "cold inside and hot outside" foam core, with greater thermal stress, which causes the foam core to be damaged under pressure below its compressive strength.

[0034] 2. When applying high pressure within a low temperature range, the resin viscosity-temperature curve is approximately "U"-shaped, with a low viscosity plateau. At this point, the resin viscosity is very low, and the pressure transmission efficiency is very high. It is easy to press in along weak areas such as pores and seams in the foam core, resulting in stress concentration and exacerbating the damage to the foam core.

[0035] To address the aforementioned problems, the present invention provides a method for molding a thick-skin foam sandwich composite material. This method involves curing and molding the overall structure of the foam core, adhesive film, and upper skin prepreg. The upper skin prepreg in this structure includes resin and reinforcing fibers, and the foam core is located below the upper skin prepreg. The molding scheme for this structure adopts the following technical approach: 1. Segmented heating and pressurization, with slow heating at low temperatures to ensure full release of thermal stress in the foam core.

[0036] 2. The heating process in the medium-temperature stage avoids the low viscosity region of the resin. That is, the temperature is rapidly increased in the medium-temperature stage to quickly pass through the low viscosity stage of the resin. After the medium-temperature stage is over, the pressure is increased in the medium viscosity region of the resin. The time difference is created by using the viscous resin to hinder the pressure transmission. That is, the pressure gradually decreases as it is transmitted downward along the surface. When the pressure on the foam core reaches the critical value, the temperature is increased to cure and protect the structure of the foam core.

[0037] The viscosity-temperature curve of epoxy resin can be approximated as a "U" shape. In the initial low temperature stage, the viscosity is relatively high. In the middle stage of temperature rise, the resin segment transport intensifies, and the viscosity drops significantly, reaching the lowest point (taking the resin selected in Examples 1-4 as an example, the temperature at this time is 120°C). As the temperature continues to rise, the resin begins to undergo a slight cross-linking and curing reaction, and the viscosity begins to rise. When the temperature reaches the initial reaction temperature (taking the resin selected in Examples 1-4 as an example, the temperature at this time is 140°C), the curing reaction intensifies, and the viscosity rises sharply.

[0038] During the molding process, pressure acts on the surface of the preform and is transferred to the foam and lower skin through the prepreg. This pressure decays, and this process is significantly related to the resin state. When the resin is not cured, pressure decay is dominated by viscous dissipation. Resin segments rub against each other under high shear forces, converting pressure into heat energy. Furthermore, according to Darcy's law, resin flow rate is negatively correlated with viscosity; increased viscosity hinders resin flow, impeding pressure transfer via convection, resulting in significant resin hindrance to pressure transmission. During the low viscosity plateau phase of the resin, viscous dissipation is insignificant, and pressure transmission efficiency is high. When the resin cures, the segments cannot move, viscous dissipation disappears, and pressure transmission efficiency remains high.

[0039] Considering that the process time window for pressure transmission retardation by resin is significantly affected by resin thickness, this invention mainly focuses on foam sandwich structures with thick skins. To describe the attenuation behavior of pressure transmission in the prepreg, pressure transmission experiments were conducted, and the actual pressure under the prepreg at different times was monitored. The experimental patterns were summarized, and a pressure transmission attenuation model was established.

[0040] P foam The actual pressure transmitted to the foam core is the attenuation behavior of which is related to the resin viscosity, prepreg thickness, and pressure transmission time. The attenuation coefficient α can be obtained by fitting, which is used to determine the relationship between the applied pressure and the actual applied pressure.

[0041] Based on the comparative effects of various embodiments and comparative examples, the present invention provides a preferred curing scheme, the execution steps of which are as follows: Step 1: Increase the temperature of the preform to T1 at a rate of less than 0.5℃ / min, while simultaneously increasing the pressure to P1, and hold for t1. T1 is not greater than the minimum viscosity temperature of the resin. The pressure P1 and the pressure holding time t1 satisfy the following relationship: ; The intermediate term is the actual pressure transmitted to the foam core. This pressure is limited to be less than the compressive strength of the foam core to protect the structure of the foam core. Since the foam core is a rigid body, the attenuation of pressure transmitted downward from the foam core is negligible. Limiting the actual pressure to be greater than the standard adhesive curing pressure of the adhesive film helps to ensure the bonding quality between the foam core and the lower skin.

[0042] Step 2: Increase the temperature of the preform to T2 at a rate of less than 0.5℃ / min, then increase the pressure to P2 and hold for t2. T2 is between the minimum viscosity temperature of the resin and the initial reaction temperature of the resin. The pressure P2 and the pressure holding time t2 satisfy the following relationship: ; By appropriately increasing the temperature to improve the resin viscosity, the resin has a more significant effect on impeding pressure transmission at higher viscosity. Increasing the pressure promotes the compaction of the upper skin prepreg. At this time, the actual pressure borne by the foam core is relatively small. As the compaction time is extended, the actual pressure borne by the foam core increases. By controlling the time and pressure, the actual pressure borne by the foam core is kept below the critical value of the foam compressive strength, thus protecting the foam core structure. The actual pressure is also limited to be greater than the standard pressure for prepreg curing, thereby improving the quality of skin molding.

[0043] Step 3: Use a pulsed pressurization method, that is, increase the pressure to P3 (0.8MPa), hold for t3 (400s), then reduce the pressure to 0.6MPa and hold for 600s. Repeat the above method 3 times. During the pressurization process, the actual pressure acting on the foam core... The pressure is 0.47 MPa, and the pressure P3 and the pressure holding time t3 satisfy the following relationship: ; When applying significant pressure to the upper skin prepreg, controlling the pressurization time ensures that the actual pressure applied to the foam core does not exceed its compressive strength, thus protecting the foam core structure. Repeating the pressurization cycle multiple times further enhances the prepreg's compaction and eliminates residual gas.

[0044] Step 4: Increase the temperature to the prepreg curing temperature at a rate greater than 2°C / min.

[0045] Rapid heating and curing transforms the upper skin into a robust, integral structure, acting as a pressure equalizer that distributes pressure evenly across the foam core, preventing stress concentration and protecting the foam structure.

[0046] This invention proposes a method to further promote the compaction of prepregs and reduce porosity: A pulsed pressurization method is used, where the pressure is increased to P3, held for t3, then reduced to P2 and held for t2, and this process is repeated several times. The pressure is further increased, and the holding time is appropriately shortened, controlling the actual pressure applied to the foam core within the compressive strength range. Repeated high-pressure applications promote further compaction of the prepreg.

[0047] The method for molding thick-skinned foam sandwich composite materials provided in this invention employs a segmented heating and pressurization approach. In the low-temperature stage, the temperature is increased slowly to ensure sufficient release of thermal stress in the foam core. Furthermore, the heating avoids the low-viscosity plateau of the resin, increasing pressure in the moderate viscosity region. The process window is created by utilizing the pressure-transferring effect of the viscous resin, protecting the foam core from pressure exceeding its critical compressive strength, thereby improving the molding quality of the foam sandwich composite material. The technical solution provided by this invention has the following beneficial effects: First, the molding method for thick-skin foam sandwich composite materials of this invention solves the contradiction between the weak pressure resistance of the foam core and the reliance of the prepreg on high-pressure compaction: A pressure attenuation model for transmission in viscous resin is established. By segmented pressurization and multiple pressure releases to break the pressure transmission chain, pressure transmission is controlled within the skin, ensuring the foam core can withstand safe low pressure and protecting its structural integrity. The spatial pressure distribution problem is transformed into a temporal segmented pressure management problem. A "wave-like" compaction is achieved through cyclic pulses, improving the skin's density and reducing product porosity.

[0048] Secondly, the method for molding thick-skin foam sandwich composite materials provided by the present invention is simple to operate, low in cost, and highly compatible: compared with the method of avoiding damage to the foam core by using a secondary bonding process, the present invention only needs to adjust the process parameters, which has significant cost advantages and can be compatible with the molding of low-density foam cores with low compressive strength.

[0049] While the embodiments disclosed in this invention are as described above, they are merely illustrative of the embodiments to facilitate understanding of the invention and are not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and variations in the form and details of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection for this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for molding a thick-skinned foam sandwich composite material, characterized in that, The closed-cell foam core is covered with an adhesive film, and the foam core covered with the adhesive film is placed on the cured lower skin. An upper skin prepreg is laid on the foam core, and the upper skin prepreg includes resin and reinforcing fibers. Curing and molding methods for prepregs of foam core, adhesive film, and upper skin include: Step 1: Increase the temperature of the preform to T1 at a rate of less than 0.5℃ / min, and increase the pressure to P1 during the heating process, and hold for t1 time; wherein, T1 is not greater than the temperature value corresponding to the minimum resin viscosity, and the pressure P1 does not exceed the compressive strength of the foam core; Step 2: Increase the temperature of the preform to T2 at a rate of 0.5℃ / min-1℃ / min, then increase the pressure to P2 and hold for t2. T2 is between the temperature value corresponding to the minimum resin viscosity and the resin reaction initiation temperature. In this step, the heating and pressurization method of first increasing the temperature to T2 and then increasing the pressure to P2 avoids the low viscosity region of the resin during the heating process, that is, avoids the region where the resin pressure transmission efficiency is high, thereby protecting the foam core. Step 3: Increase the temperature of the preform to the curing temperature of the prepreg at a rate greater than 2℃ / min.

2. The method for molding a thick-skinned foam sandwich composite material according to claim 1, characterized in that, The pressure P1 and the pressure holding time t1 in step 1 satisfy the following relationship: ； in, The value of α is the compressive strength of the foam core at temperature T1, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. a This refers to the standard pressure for the bonding and curing of the adhesive film. This indicates the actual pressure transmitted to the foam core in step 1. By limiting this pressure to be less than or equal to the compressive strength of the foam core at the same temperature, the product curing quality is ensured by using the highest possible pressure while achieving a safety margin for the compressive strength of the foam core.

3. The method for molding a thick-skinned foam sandwich composite material according to claim 1, characterized in that, The pressure P2 and the pressure holding time t2 in step 2 satisfy the following relationship: ； in, α is the compressive strength of the foam core at temperature T2, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. r The standard pressure for curing prepreg; This indicates the actual pressure transmitted to the foam core in step 2. By limiting this pressure to be less than or equal to the compressive strength of the foam core at the same temperature, the product curing quality is ensured by using the highest possible pressure while achieving a safety margin for the compressive strength of the foam core.

4. The method for molding a thick-skinned foam sandwich composite material according to any one of claims 1 to 3, characterized in that, The process after step 2 and before step 3 also includes: Step a: Use a pulse pressurization method, that is, increase the pressure to P3, hold for t3, then reduce the pressure to P2, hold for t2, and repeat the above pulse pressurization method multiple times.

5. The method for molding a thick-skinned foam sandwich composite material according to claim 4, characterized in that, The pressure to P3 and the pressure holding time t3 in step a satisfy the following relationship: ； in, α is the compressive strength of the foam core at temperature T2, α is the pressure transmission loss coefficient, μ is the resin viscosity, and h is the viscosity of the resin. r P represents the thickness of the prepreg. r The standard pressure for curing prepreg; In step a, the pressure actually applied to the foam core is controlled by applying pressure P3 to the upper skin prepreg and controlling the application time of pressure P3. The compressive strength of the foam core shall not exceed that of the foam core, thereby protecting the structure of the foam core.

6. The method for molding a thick-skinned foam sandwich composite material according to claim 5, characterized in that, The determination method for the pressure transmission loss coefficient in each step includes: S1, Prepare a sample with the same composition as the upper skin prepreg and the adhesive film, with the adhesive film disposed below the upper skin prepreg and a pressure sensor disposed at the bottom of the sample; S2, adjusts the temperature and pressure according to the set program; S3, determine the pressure P monitored by the pressure sensor at different times t. t And α is obtained through a fitting algorithm.

7. The method for molding a thick-skinned foam sandwich composite material according to claim 6, characterized in that, The method for adjusting temperature and pressure according to the set program in S2 is as follows: According to the curing and molding method of the foam core, film and upper skin prepreg, the product heating and pressurization process involves applying corresponding temperature and pressure at each time period.

8. The method for molding a thick-skinned foam sandwich composite material according to claim 6, characterized in that, The specific method for fitting α in S3 is as follows: α is obtained by fitting according to the following relationship; ； Where P is the preset pressure in S2.

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