Forming method of high carbon residue phenolic resin-based composite material

Abstract

The present invention relates to a forming method of a high residual carbon phenolic resin-based composite material, belonging to the technical field of liquid forming of composite materials, and solves the problems of large porosity, low density and insufficient ablation resistance of fiber-reinforced phenolic resin-based composite materials prepared by liquid forming processes in the prior art. The present invention provides a forming method of a high residual carbon phenolic resin-based composite material. A low melting point alloy gasket is arranged in the sealing groove on the outer side of the upper cover plate of the mold; the low melting point alloy gasket is used to realize the dynamic sealing of the mold. By adopting the low melting point alloy for dynamic sealing, the present method realizes the further compression of the space in the mold cavity. With the setting of the one-way breathable film and the regulation of other process parameters, the density and ablation resistance of the composite material are significantly improved, and the porosity is reduced; the porosity of the composite material prepared by the method can be as low as 2.5±0.5%, and the density is increased by 6-10%.

Description

Technical Field

[0001] The present invention relates to the technical field of composite material liquid molding, and particularly to a molding method for a high residual carbon phenolic resin-based composite material. Background Art

[0002] With the rapid development of aerospace technology, the extreme aerodynamic heating and high-temperature environment encountered by aircraft during high-speed flight pose unprecedented challenges to heat-resistant materials. Traditional heat-resistant materials, such as metal and ceramic matrix composites, although having certain high-temperature resistance properties, have inherent defects such as high density, difficult processing, and high cost, and are no longer able to meet the urgent needs of the modern aerospace field for lightweight and high-performance heat-resistant materials.

[0003] In this context, phenolic resin, as an important thermosetting resin, exhibits unique advantages in the field of heat-resistant materials. Phenolic resin is known for its high residual carbon characteristics, and can rapidly form a dense carbonized layer in a high-temperature environment, effectively isolating the intrusion of external heat and protecting the internal structure from heat damage. In addition, phenolic resin-based composites also exhibit excellent thermal stability and mechanical properties, and can withstand thermal stress and mechanical stress under extreme conditions, making them an ideal choice for heat-resistant materials in the aerospace field.

[0004] However, currently, in the process of improving the high residual carbon of phenolic resin, the method mainly relies on introducing macromolecular groups into its molecular structure. Although this approach is effective, it will significantly increase the viscosity of the resin and affect its molding performance. To reduce the viscosity, usually 15%-20% of solvent is added to the phenolic resin. However, during the composite material molding process, if the solvent cannot be completely removed, pores will be formed inside the material, resulting in the density of the composite material being lower than the theoretical value, thereby weakening its structural strength and ablation resistance, and seriously affecting the overall performance of the material.

[0005] Currently, the prior art has been able to relatively fully remove the solvent in the composite material, but the pores left in the composite material after the solvent vaporizes and discharges are difficult to handle, resulting in a relatively high porosity and insufficient density of the composite material, and thus leading to a decline in ablation resistance. Summary of the Invention

[0006] In view of the above analysis, the embodiments of the present invention aim to provide a molding method for a high residual carbon phenolic resin-based composite material to solve at least one of the problems such as large porosity, low density, and insufficient ablation resistance of fiber-reinforced phenolic resin-based composite materials prepared by the liquid molding process in the prior art.

[0007] The present invention provides a molding method for a high residual carbon phenolic resin-based composite material, wherein a low melting point alloy gasket is arranged in a sealing groove on the outer side of the upper cover plate of the mold;

[0008] The low-melting-point alloy gasket is used to achieve dynamic sealing of the mold. During the pressurization and heating stage, the low-melting-point alloy gasket melts and flows out from the overflow port provided on the upper cover plate, thereby further compressing the space inside the mold cavity.

[0009] Specifically, the low-melting-point alloy is one or several of tin-bismuth alloy, bismuth-zinc alloy, and gallium-indium alloy.

[0010] Specifically, the melting point of the low-melting-point alloy is ≤130°C.

[0011] Furthermore, the forming method specifically includes the following steps:

[0012] S1 Gasket casting: Melting the low-melting-point alloy and casting it into a low-melting-point alloy gasket of appropriate specifications;

[0013] S2 Mold assembly: First, place the low-melting-point alloy gasket in the outer sealing groove of the mold upper cover plate, then embed the O-ring rubber seal in the outer sealing groove and fix the low-melting-point alloy gasket in the outer sealing groove;

[0014] Assemble the upper cover plate, positioning frame, and lower bottom plate of the mold in sequence, place the preform in the mold cavity and lock it, and connect the mold to the injection system and vacuum system;

[0015] S3 Injection and degassing: After heating the mold as a whole and evacuating it, perform the injection operation. When resin overflows from the exhaust port, close the vacuum system connected to the exhaust port, and use a stepped pressurization method to degas the resin;

[0016] S4 Exhaust treatment: Continue to slowly heat the mold to 120 - 125°C and evacuate it. When the temperature reaches the target temperature, perform intermittent exhaust operation for exhaust;

[0017] S5 Pressurization treatment: Rapidly heat the mold to 130 - 135°C and keep it warm for 20 - 30 min. After the low-melting-point alloy gasket melts and flows out, lock the mold again and compress the preform;

[0018] S6 Curing and forming: Raise the mold temperature to 160 ± 5°C and keep it warm for at least 2 h, raise the mold temperature to 180 ± 5°C and keep it warm for at least 4 h, and the product cures and forms to obtain a phenolic resin-based composite material.

[0019] Furthermore, the specific operation of step S1 is as follows:

[0020] Place the metal block of the low-melting-point alloy in a graphite crucible. Place the graphite crucible and the low-melting-point alloy gasket forming mold in a heating device at 130 - 150 °C and heat for 30 - 60 minutes. When the low-melting-point alloy melts and has good fluidity, pour the low-melting-point alloy into the groove of the forming mold through the casting port. Air-cool the mold to cool it down, take out the annular low-melting-point alloy gasket, and perform grinding and shaping.

[0021] Preferably, the height of the gate of the low-melting-point alloy gasket forming mold is 30 ± 5 mm.

[0022] Furthermore, the principle for determining the volume of the low-melting-point alloy gasket is as follows: According to the actual shape and specifications of the mold, after the low-melting-point alloy gasket melts and flows out, the volume of the cavity compressed by the secondary locking of the mold is 10 - 15% of the theoretical volume of the resin in the mold.

[0023] Furthermore, the specific operation of step S3 is as follows:

[0024] Place the mold in a heating device. When the temperature reaches 70 - 80 °C, close the exhaust port valve, open the degassing port valve connected to the vacuum system, evacuate the mold cavity for 3 - 5 minutes, open the injection port valve, and start injecting resin;

[0025] When resin overflows from the degassing port, close the vacuum system connected to the degassing port, and use a stepwise pressure increase method to degas the resin until no bubbles appear in the degassing port pipeline. End the injection and close the injection port valve and the degassing port valve.

[0026] Specifically, the stepwise pressure increase method in step S3 is four-stage pressure increase, and the pressures are 1 bar, 3 bar, 5 bar, and 8 bar in sequence.

[0027] Specifically, the intermittent exhaust operation in step S4 is as follows: Exhaust by opening / closing the exhaust valve. The exhaust interval is 10 - 15 minutes, the exhaust time is 10 - 15 seconds, and repeat the above process 3 - 5 times.

[0028] Specifically, the vacuum negative pressure value in step S4 shall not be less than 0.95 bar.

[0029] Specifically, in step S5, the mold is secondarily locked and the preform is compressed until the mass of the low-melting-point alloy flowing out ≥ 98% of the total mass of the low-melting-point alloy gasket.

[0030] Furthermore, in step S5, after the mold is secondarily locked, a gap detection is required, and the mold gap shall not be greater than 0.1 mm.

[0031] The present invention also discloses a high residual carbon phenolic resin-based composite material, and the composite material is prepared by the above forming method.

[0032] The present invention also discloses a mold for forming a high carbon residue phenolic resin-based composite material. The mold includes an upper cover plate, a positioning frame, a lower bottom plate, and a low melting point alloy gasket;

[0033] An inner sealing groove and an outer sealing groove are provided on the lower surface of the upper cover plate. A low melting point alloy gasket is arranged in the outer sealing groove. A overflow port communicating with the outer sealing groove is provided at a position corresponding to the upper surface of the upper cover plate and the outer sealing groove for discharging molten low melting point alloy liquid;

[0034] An insert block and a corresponding groove are provided on the lower surface of the upper cover plate. An exhaust through hole is provided on the insert block, and a one-way breathable film is arranged between the insert block and the groove;

[0035] The inner sealing groove is arranged at a suitable position of the groove for realizing the seal between the insert block and the main body of the upper cover plate;

[0036] The mold is used for the above-mentioned forming method of the high carbon residue phenolic resin-based composite material.

[0037] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0038] 1. By arranging the low melting point alloy gasket, the present invention realizes the dynamic seal of the mold, can further compress the cavity space of the mold during the pressure treatment stage (refer to step S5), thereby eliminating the holes left due to solvent gasification, significantly reducing the porosity of the composite material, and improving the density and ablation resistance of the composite material.

[0039] When the temperature of the low melting point alloy gasket is below the melting point, it can play a good sealing role. Moreover, due to the good thermal conductivity of the low melting point alloy, it can also improve the thermal conductivity of the mold to a certain extent, making the entire preform heated more evenly; during the pressure treatment stage, since the heating temperature is higher than the melting point of the gasket, the gasket melts and forms alloy liquid. At this time, the mold is locked again (the common method is mechanical locking) to compress the cavity space of the mold. During the compression process, the alloy liquid is extruded from the outer sealing groove (discharged from the overflow port), thereby providing a certain secondary compression space for the mold cavity. During this process, the holes left due to solvent gasification are filled with the resin redistributed, reducing the porosity of the composite material and increasing the density of the composite material. The reduction of porosity and the increase of density both contribute to the improvement of the ablation resistance ability, and the specific improvement amplitude is related to the characteristics of different composite materials themselves.

[0040] It should be noted that although the technical purpose of removing holes in the composite material can be achieved by using the low melting point alloy gasket, better results can be obtained by controlling the volume / mass of the low melting point alloy gasket.

[0041] Preferably, the principle for determining the volume of the low-melting-point alloy gasket is as follows: According to the actual shape and specifications of the mold, after the low-melting-point alloy gasket melts and flows out, the volume of the cavity compressed by the secondary locking of the mold is 10-15% of the theoretical volume of the resin in the mold. If the volume of the gasket is too small, the additional compression space / compression margin provided by melting during the pressure treatment stage is too small to effectively remove the solvent remaining holes; if the volume of the gasket is too large, it may cause damage to the preform fibers (when the first locking is sufficient and the preform is compressed and exhausted sufficiently during the exhaust stage, this will result in too large a remaining compression margin finally, and the preform / composite material may be damaged during the secondary locking).

[0042] Furthermore, in step S5, the mold is locked secondly and the preform is compressed until the mass of the low-melting-point alloy flowing out ≥ 98% of the total mass of the low-melting-point alloy gasket. Since in the specific implementation process, the volume of the molten low-melting-point alloy is not easy to measure, the mass is used for measurement. When the above conditions are met, it is considered that the locking operation has fully utilized the compression margin reserved by the gasket.

[0043] The forming method of the high-residual-carbon phenolic resin-based composite material provided by the present invention is an innovation to the traditional RTM process, which can solve the problem of solvent volatilization of solvent-containing resins and greatly expand the applicable forming process range of solvent-based resins.

[0044] 2. The present invention adopts an intermittent exhaust method, enabling the phenolic resin to exhaust at a constant temperature and for a fixed time, which can effectively reduce the volatile components inside the resin and more fully remove the solvent in the phenolic resin.

[0045] 3. The present invention selects a unidirectional breathable membrane, which can achieve unidirectional gas exhaust while ensuring that the resin in the mold does not flow out and ensuring the resin content in the composite material.

[0046] 4. The present invention realizes the preparation of a high-carbon phenolic resin-based composite material with a low porosity by combining simple and inexpensive materials and equipment, and reduces the forming process cost compared with the traditional autoclave process and compression molding process.

[0047] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the subsequent specification, and some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained from the content specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings are only for the purpose of showing specific embodiments and are not considered as a limitation to the present invention. Throughout the drawings, the same reference signs represent the same components;

[0049] Figure 1 A possible design for a molding die for high residual carbon phenolic resin-based composites;

[0050] Figure 2 A schematic diagram of the overall structure of a possible upper cover plate;

[0051] Figure 3 A schematic assembly diagram of a composite material molding die and a preform;

[0052] Figure 4 A cross-sectional photo of the composite material in Example 1;

[0053] Figure 5 A cross-sectional photo of the composite material in Comparative Example 1.

[0054] Reference numerals:

[0055] 1. Upper cover plate; 2. Positioning frame; 3. Lower bottom plate; 4. Glue injection port valve; 5. Exhaust port valve; 6. Exhaust glue port valve; 7. Overflow port; 8. Locking bolt; 9. Outer sealing ring; 10. Insert block; 11. Insert block bolt; 12. Outer sealing groove; 13. Low melting point alloy gasket; 14. One-way breathable membrane; 15. Inner sealing ring; 16. Inner sealing groove; 17. Lower bottom plate sealing groove; 18. Preform. Detailed implementation manners

[0056] The following will specifically describe the preferred embodiments of the present invention in conjunction with the accompanying drawings, where the accompanying drawings form a part of this application and are used together with the embodiments of the present invention to explain the principle of the present invention, and are not used to limit the scope of the present invention.

[0057] The present invention provides a molding method for high residual carbon phenolic resin-based composites, and the method includes arranging a low melting point alloy gasket in the outer sealing groove of the upper cover plate of the mold;

[0058] The low melting point alloy gasket is used to achieve dynamic sealing of the mold. During the pressurization and heating stage, the low melting point alloy gasket melts and flows out from the overflow port provided on the upper cover plate, thereby further compressing the space inside the mold cavity.

[0059] The present invention realizes dynamic sealing of the mold by arranging a low melting point alloy gasket, can further compress the space inside the mold cavity during the pressurization treatment stage (refer to step S5), thereby eliminating the holes left due to solvent gasification, significantly reducing the porosity of the composite material, and improving the density and ablation resistance of the composite material.

[0060] When the low-melting-point alloy gasket is at a temperature below its melting point, it can play a good sealing role. Moreover, due to the good thermal conductivity of the low-melting-point alloy, it can also improve the thermal conductivity of the mold to a certain extent, making the heating of the entire preform more uniform (the low-melting-point alloy gasket can assist in heat transfer in the circumferential direction, enabling heat to flow from the high-temperature area to the low-temperature area of the gasket, which helps to improve the uniformity of mold heating); during the pressure treatment stage, since the heating temperature is higher than the melting point of the gasket, the gasket melts and forms alloy liquid. At this time, the mold is locked again (the common method is mechanical locking) to compress the cavity space of the mold. During the compression process, the alloy liquid is extruded from the outer sealing groove (discharged from the overflow port), thereby providing a certain compression space. During this process, the holes left due to solvent gasification are excluded, reducing the porosity of the composite material.

[0061] It should be noted that in actual implementation, since the gasket is arranged in the outer sealing groove of the upper cover plate and is tightly fixed by an O-ring rubber seal, the O-ring rubber seal has an excessive fit with the outer sealing groove and the rubber will expand during the heating process. Therefore, when the gasket melts, there will be no liquid leakage inside the mold, but almost all of it will flow out from the overflow port, and only a small amount remains in the outer sealing groove.

[0062] Furthermore, the mold is locked again and the preform is compressed until the mass of the low-melting-point alloy flowing out ≥ 98% of the total mass of the low-melting-point alloy gasket. Since in the specific implementation process, it is not easy to measure the volume of the molten low-melting-point alloy, mass is used for measurement. When the above conditions are met, it is considered that the locking operation has fully utilized the compression margin reserved by the gasket.

[0063] Specifically, the low-melting-point alloy is one or several of tin-bismuth alloy, bismuth-zinc alloy, and gallium-indium alloy. These alloys have appropriate melting points and good fluidity, and can meet the requirements for the formation of the low-melting-point alloy gasket.

[0064] Specifically, the melting point of the low-melting-point alloy ≤ 130 °C.

[0065] Specifically, the forming method includes steps such as gasket casting, mold assembly, glue injection and bubble removal, exhaust treatment, pressure treatment, and curing forming.

[0066] Among them, in the gasket casting step, the low-melting-point alloy is melted and cast into a low-melting-point alloy gasket of appropriate specifications, and the gasket specifications should correspond to the specifications of the outer sealing groove;

[0067] During the mold assembly process, the low-melting-point alloy gasket is arranged in the outer sealing groove of the upper cover plate and tightly fixed with an O-ring rubber seal to prevent liquid leakage into the mold after it melts;

[0068] In the pressure treatment step, the heating temperature is ≥ the melting point of the low-melting alloy and is kept for a period of time to fully melt the low-melting alloy, and then the mold is locked again, so that the alloy liquid overflows from the overflow port.

[0069] Specifically, the size of the overflow port is not specifically required and can be determined according to specific implementation needs. Generally, the diameter of the overflow port is 1 - 2 mm.

[0070] Furthermore, the forming method specifically includes the following steps:

[0071] S1 Spacer casting: Melt the low-melting alloy and cast it into a low-melting alloy spacer of appropriate specifications, and the spacer specifications should correspond to the specifications of the outer sealing groove;

[0072] S2 Mold assembly: First, place the low-melting alloy spacer in the outer sealing groove of the upper mold cover, then embed the O-ring rubber seal in the outer sealing groove and tightly fix the low-melting alloy spacer in the outer sealing groove;

[0073] Assemble the upper mold cover, positioning frame, and lower bottom plate of the mold in sequence, place the preform in the cavity of the mold and lock it, and connect the mold with the injection system and the vacuum system;

[0074] S3 Injection and degassing: After heating the whole mold and evacuating for a period of time, perform the injection operation. When resin overflows from the exhaust port, close the vacuum system connected to the exhaust port, and use a stepped pressure method to degas the resin;

[0075] S4 Exhaust treatment: Continue to slowly heat the mold to 120 - 125 °C and evacuate. When the temperature reaches the target temperature, perform intermittent exhaust operation for exhaust;

[0076] S5 Pressure treatment: Rapidly heat the mold to 130 - 135 °C and keep it warm for 20 - 30 min. After the low-melting alloy spacer melts and flows out from the overflow port, lock the mold again and compress the preform. The above-mentioned holding time can ensure that the low-alloy spacer is completely melted;

[0077] S6 Curing and forming: Raise the mold temperature to 160 ± 5 °C and keep it warm for at least 2 h, raise the mold temperature to 180 ± 5 °C and keep it warm for at least 4 h, and the product is cured and formed to obtain a dense phenolic resin-based composite material.

[0078] Furthermore, the specific operation of step S1 is:

[0079] Place the metal block of the low-melting-point alloy in a graphite crucible. Place the graphite crucible and the low-melting-point alloy gasket forming mold in a heating device at 130 - 150 °C and heat for 30 - 60 minutes. When the low-melting-point alloy is molten and has good fluidity, pour the low-melting-point alloy into the groove of the forming mold through the casting port. The mold is cooled by air, and the annular low-melting-point alloy gasket is removed and polished and shaped. During casting, slowly pour the molten low-melting-point alloy into the groove of the forming mold through the casting port, and pay attention to controlling the casting speed to avoid generating bubbles and splashing. After casting, use the air-cooling method to quickly cool the mold. After the alloy solidifies, remove the low-melting-point alloy gasket and use professional polishing tools to finely polish and shape it to ensure that its dimensional accuracy and surface finish meet the requirements for subsequent use.

[0080] In addition, the volume of the low-melting-point alloy needs to be calculated in advance. The principle for determining the volume of the low-melting-point alloy gasket is as follows: According to the actual shape and specifications of the mold, after the low-melting-point alloy gasket melts and flows out, the volume of the cavity compressed by the secondary locking of the mold is 10 - 15% of the theoretical volume of the resin in the mold, so as to ensure the best compression and sealing effects in the subsequent steps. Exemplarily, when the area of the upper cover plate / lower bottom plate of the mold is large, a small height change (the locking height change generated before and after the gasket melts) will cause a large change in the volume of the cavity. ; when the area of the upper cover plate / lower bottom plate of the mold is small, a large height change (the locking height change generated before and after the gasket melts) may not necessarily cause a large change in the volume of the cavity. Therefore, the above principle needs to be flexibly grasped according to the actual implementation situation.

[0081] Preferably, the height of the casting port of the low-melting-point alloy gasket forming mold is 30 ± 5 mm.

[0082] It should be noted that although using a low-melting-point alloy gasket can achieve the technical purpose of removing holes in the composite material, better results can be obtained by controlling the volume / mass of the low-melting-point alloy gasket. Preferably, the principle for determining the volume of the low-melting-point alloy gasket is as follows: According to the actual shape and specifications of the mold, after the low-melting-point alloy gasket melts and flows out, the volume of the cavity compressed by the secondary locking of the mold is 10 - 15% of the theoretical volume of the resin in the mold; if the volume of the gasket is too small, the additional compression space / compression margin provided by melting during the pressurization process is too small to play a sufficient role in removing the holes left by the solvent; if the volume of the gasket is too large, it may cause damage to the preform fibers (the first locking is sufficient, and the preform is compressed and exhausted sufficiently during the exhaust stage, which may result in too large a remaining compression margin in the end, and the preform / composite material may be damaged during the second locking).

[0083] Specifically, mold assembly is one of the key steps to ensure the forming quality of the composite material. Exemplarily,Figure 1 Taking the shown molding die as an example, the assembly can be carried out according to the following process:

[0084] First, place the polished and shaped low-melting alloy gasket 13 in the outer sealing groove 12 of the upper die plate to ensure that the gasket fits tightly with the sealing groove without gaps. Then, install O-ring rubber seals in the corresponding outer sealing groove 12 of the upper die plate, inner sealing groove 16 of the upper die plate, and lower base plate sealing groove 17 between the upper die plate 1 and the lower base plate 3 to enhance the sealing performance of the die and prevent resin leakage.

[0085] Then, lay two layers of unidirectional breathable membranes 14 at the center position of the groove on the upper die plate. The selection of the unidirectional breathable membrane should ensure its good air permeability and resistance to resin erosion. The laying of the breathable membrane should be flat and without wrinkles to ensure that gas can be discharged smoothly and resin will not penetrate. Subsequently, accurately place the upper die plate insert 10 in the groove and fasten it with insert bolts 11 to ensure that the insert is stable and does not shake. During the fastening process, attention should be paid to controlling the tightening torque of the bolts to avoid deformation of the die caused by over-tightening or a decrease in sealing performance caused by under-tightening. Then, accurately position and install the positioning frame 2 on the upper die plate 1 of the die to form a tight rectangular cavity. Place the pre-prepared preform 18 in the cavity to ensure accurate positioning and tight fit between the preform and the cavity wall.

[0086] Finally, position the lower base plate 3 of the die and use the locking bolts 8 to lock all parts of the die to ensure that all parts are tightly combined without looseness, and check the die clamping gap. The die clamping gap shall not be greater than 0.1 mm. Subsequently, install the injection port valve 4, exhaust port valve 6, and vent port valve 5 for subsequent operation to control the injection and discharge of resin and gas. Connect the assembled die to the injection system and vacuum system through pipelines to prepare for the subsequent resin infiltration process.

[0087] Further, the specific operation of step S3 is as follows:

[0088] Place the die in a heating device. When the temperature reaches 70 - 80 °C and the internal temperature of the die is stable and evenly distributed, start the resin infiltration process; close the vent port valve, open the exhaust port valve connected to the vacuum system, evacuate the die cavity for 3 - 5 minutes to remove the air in the cavity, and then open the injection port valve to start injection; during the injection process, closely observe the situation of the exhaust port. Once resin overflows, immediately close the vacuum system connected to the exhaust port to prevent resin loss.

[0089] When resin overflows from the exhaust port, close the vacuum system connected to the exhaust port, and use a stepped pressure method to defoam the resin until no bubbles appear in the exhaust port pipeline, then end the injection and close the injection port valve and exhaust port valve.

[0090] Specifically, the stepwise pressure increase method in step S3 is a four-stage pressure increase, and the final pressure increase must not be less than 8 bar. Otherwise, all the resin bubbles in the resin cannot be removed. Each stage of pressure is maintained for 5 - 10 minutes until no bubbles appear in the exhaust port pipeline, indicating that the resin has fully infiltrated the fibers and the internal bubbles have been effectively removed. During this process, the pressure value needs to be strictly controlled. Preferably, the four-stage pressure gradients are 1 - 2 bar, 3 - 4 bar, 5 - 6 bar, and 8 - 10 bar respectively.

[0091] Specifically, the specific operation of step S4 is as follows: Set the heating program of the mold to heat the mold at a heating rate of 1 - 2 °C (slow heating) to 90 ± 5 °C, open the exhaust port valve connected to the vacuum system, and use the vacuum negative pressure to promote the further volatilization of the solvent in the resin. When the mold temperature reaches 120 °C, close the exhaust port valve, keep it for 20 - 30 minutes to allow the resin to fully react, and then perform intermittent exhaust operation.

[0092] Specifically, the intermittent exhaust operation in step S4 is as follows: Exhaust by opening / closing the exhaust valve. The exhaust interval is 10 - 15 minutes, and the exhaust time is 10 - 15 seconds to discharge the solvent gas and the small molecule gas generated by the reaction. This process needs to be repeated 3 - 5 times.

[0093] Specifically, the vacuum negative pressure value in step S4 must not be less than 0.95 bar to ensure the exhaust effect.

[0094] Specifically, in step S5, the mold is locked again and the preform is compressed until the mass of the molten low-melting alloy flowing out ≥ 98% of the total mass of the low-melting alloy gasket. Since the volume of the molten low-melting alloy is not easy to measure during the specific implementation process, the mass is used for measurement. When the above conditions are met, it is considered that the locking operation has fully utilized the compression allowance reserved by the gasket.

[0095] Furthermore, after the mold is locked again in step S5, a gap detection needs to be carried out. The mold gap must not be greater than 0.1 mm and all parts of the mold are tightly combined without looseness.

[0096] Specifically, during the curing process of step S6, the temperature and time parameters need to be strictly controlled to avoid the decline of the composite material performance caused by too high or too low temperature. At the same time, the temperature inside the mold needs to be kept evenly distributed to avoid the generation of thermal stress leading to cracking or deformation of the composite material. After curing is completed, wait for the mold to cool to room temperature and then take out the product to obtain a dense and excellent-performance high-carbon phenolic resin-based composite material.

[0097] The present invention also discloses a high carbon residue phenolic resin-based composite material, which is prepared by the above-mentioned forming method. The porosity of the composite material prepared by the preparation / formig method provided by the present invention can be as low as 2.5±0.5%, and the density is increased by 6-10% compared with the composite material prepared by the conventional technology.

[0098] The present invention also discloses a mold for forming a high carbon residue phenolic resin-based composite material. The mold sequentially includes an upper cover plate, a positioning frame, a lower bottom plate and a low melting point alloy gasket from top to bottom;

[0099] An inner sealing groove and an outer sealing groove are provided on the lower surface of the upper cover plate. A low melting point alloy gasket is provided in the outer sealing groove. An overflow port communicating with the outer sealing groove is provided at a corresponding position on the upper surface of the upper cover plate for discharging molten low melting point alloy liquid;

[0100] An insert block and a corresponding groove are provided at the center of the lower surface of the upper cover plate. Exhaust through holes are provided on the insert block, and a one-way breathable film is provided between the insert block and the groove;

[0101] The inner sealing groove is arranged at a suitable position of the groove for realizing the seal between the insert block and the main body of the upper cover plate;

[0102] An exhaust port is provided at the center of the upper cover plate;

[0103] The lower bottom plate is provided with a lower bottom plate sealing groove;

[0104] Bolt holes are reserved at corresponding positions of the upper cover plate, the positioning frame and the lower bottom plate for arranging locking bolts;

[0105] The mold is provided with a glue injection port and a glue discharge port;

[0106] The mold is used for the above-mentioned forming method of the high carbon residue phenolic resin-based composite material.

[0107] Specifically, the one-way breathable film is two layers or more. The selection of the one-way breathable film should ensure that it has good air permeability and resin erosion resistance. The laying of the breathable film needs to be flat and without wrinkles to ensure that gas can be discharged smoothly and resin will not penetrate. Exemplarily, the one-way breathable film can be the Dahltexx SP-2 brand one-way breathable film of Ertac.

[0108] Specifically, the number of exhaust through holes on the insert block is ≥9, and the diameter of the exhaust holes is 1±0.02 mm.

[0109] Exemplarily, taking Figure 1 the shown forming mold as an example, the assembly can be carried out according to the following process:

[0110] First, place the polished and shaped low-melting-point alloy gasket 13 in the outer sealing groove 12 of the upper die plate, ensuring that the gasket fits tightly with the sealing groove without gaps. Then, install O-ring rubber seals in the corresponding outer sealing groove 12 of the upper die plate, inner sealing groove 16 of the upper die plate, and sealing groove 17 of the lower base plate between the upper die plate 1 and the lower base plate 3 to enhance the sealing performance of the mold and prevent resin leakage.

[0111] Next, lay two layers of unidirectional breathable membranes 14 at the center position of the groove on the upper die plate. The selection of the unidirectional breathable membrane should ensure good air permeability and resistance to resin erosion. The laying of the breathable membrane should be flat and without wrinkles to ensure that gas can be discharged smoothly and resin will not penetrate. Subsequently, accurately place the upper die plate insert 10 in the groove and fasten it with insert bolts 11 to ensure that the insert is stable and does not shake. During the fastening process, pay attention to controlling the tightening torque of the bolts to avoid deformation of the mold caused by over-tightening or a decrease in sealing performance caused by over-loosening. Then, accurately position and install the positioning frame 2 on the upper die plate 1 of the mold to form a tight rectangular cavity. Place the pre-prepared preform 18 in the cavity, ensuring that the position is accurate and the preform fits tightly with the cavity wall.

[0112] Finally, position the lower base plate 3 of the mold and use the locking bolts 8 to lock all parts of the entire mold, ensuring that all parts are tightly combined without looseness, and check the mold closing gap. The mold closing gap shall not be greater than 0.1 mm. Subsequently, install the injection port valve 4, exhaust port valve 6, and vent valve 5 for subsequent operation to control the injection and discharge of resin and gas. Connect the assembled mold to the injection system and vacuum system through pipelines to prepare for the subsequent resin infiltration process.

[0113] Example 1

[0114] Use the mold as Figure 1 shown to prepare the composite material. The specific process and parameters are as follows:

[0115] (1) Casting of the low-melting-point alloy gasket: Place the metal block of the low-melting-point alloy (tin-bismuth alloy) in a graphite crucible, place the graphite crucible and the low-melting-point alloy forming mold in a heating device at 130 - 150 °C and heat for 30 - 60 min. When the low-melting-point alloy is molten and has good fluidity, pour the low-melting-point alloy into the recess of the forming mold through the casting port. The mold is cooled by air and the annular low-melting-point alloy is removed, and then polished and shaped to obtain the low-melting-point alloy gasket; the secondary compression allowance (secondary compression volume in the cavity) provided after the low-melting-point alloy gasket melts is about 12% of the theoretical volume of the resin in the mold.

[0116] (2) Mold assembly: Place the low-melting alloy in the sealing groove outside the upper mold cover. Place an O-ring seal in the sealing grooves of the upper mold cover and the lower base plate. Place the Eltex Dahltexx SP-2 brand one-way breathable film (2 layers) in the groove position of the upper mold cover. Place the upper cover insert into the groove of the upper mold cover and lock it with bolts. Position and install the mold frame on the upper mold cover to form a rectangular cavity. Place the preform in the cavity. Position the lower base plate and lock the entire mold with bolts. Install the ball valve assembly and connect the mold to the injection system and the vacuum system.

[0117] (3) Resin infiltration of fibers: Place the mold in a heating device. When the temperature reaches 70 - 80 °C, close the exhaust port valve, open the degassing port valve connected to the vacuum system, evacuate the mold cavity for 3 - 5 minutes, open the injection port ball valve, and start injecting resin. When resin overflows from the degassing port, close the vacuum system connected to the degassing port, and use a stepwise pressure increase method to remove air bubbles from the resin. The stepwise pressure increase values are 1 bar, 3 bar, 5 bar, and 8 bar in sequence, and each pressure level is maintained for 5 - 10 minutes until no air bubbles appear in the degassing port pipeline. End the injection and close the injection port valve and the degassing port valve.

[0118] (4) Exhaust control: Set the mold heating program to heat the mold at a heating rate of 1 - 2 °C (slow heating) to 90 ± 5 °C. Open the exhaust port valve connected to the vacuum system to allow further evaporation of the solvent in the resin. When the mold temperature reaches 120 °C, close the valve. At regular intervals, open the exhaust port valve for exhaust to discharge the solvent gas and the water vapor generated by the reaction, and then close the exhaust port valve. The vacuum negative pressure value is not less than 0.95 bar, the exhaust interval is 10 - 15 minutes, and the exhaust time is 10 - 15 seconds to discharge the solvent gas and the small molecule gas generated by the reaction. This process needs to be repeated 3 - 5 times.

[0119] (5) Pressure treatment: Rapidly heat the mold temperature to 130 °C and keep it warm for 20 - 30 minutes. Wait for the low-melting alloy to melt and flow out (the mass of the low-melting alloy flowing out ≥ 98% of the total mass of the low-melting alloy gasket), and use a mechanical locking method to compress the preform (secondary locking) to allow the resin to better infiltrate the fiber preform. After secondary locking, a gap detection needs to be carried out. The mold gap shall not be greater than 0.1 mm and all parts of the mold shall be tightly combined without looseness.

[0120] (6) Curing and forming: Raise the mold temperature to 160 ± 5 °C and keep it warm for 2 hours, then raise the mold temperature to 180 ± 5 °C and keep it warm for 4 hours. The product is cured and formed to obtain a dense phenolic resin-based composite material.

[0121] After testing, the density of the phenolic resin-based composite material is 1.50 g / cm 3, the porosity is 2-3%.

[0122] Example 2

[0123] Do not set the one-way breathable membrane, and other processes and parameters are the same as those in Example 1.

[0124] Without adding the one-way breathable membrane, during the exhaust process of the resin, some resin is carried out, and the overall resin quality in the mold decreases. Although the space in the mold is compressed later, due to the small amount of resin in the mold, the overall product is loose. Although there are no large pores, there are relatively more small pores.

[0125] After testing, the density of the phenolic resin-based composite material is 1.45 g / cm 3 , the porosity is 5-6%, and the relative density is 3-4% lower than that of the composite material in Example 1.

[0126] Comparative Example 1

[0127] Do not pour and configure the low-melting-point alloy gasket, and other processes and parameters are the same as those in Example 1.

[0128] Without adding the low-melting-point alloy gasket, the product compression process is missing. Although the vaporized solvent is completely discharged during the preparation of the product and the gas is discharged from the resin, there is no excess resin in the mold to fill the pores. Eventually, large pores appear inside the product, and the density of the fiber preform is low.

[0129] After testing, the density of the phenolic resin-based composite material is 1.40 g / cm 3 , the porosity is 8-10%, and the relative density is 6-8% lower than that of the composite material in Example 1.

[0130] A preferred implementation mode of the present invention is to give a certain pressure to the resin in the mold by reducing the mold cavity on the premise of ensuring the resin quality in the mold cavity, so that the solvent-free resin in the mold is redistributed, and the resin is fully filled into the pores generated due to the volatilization of the solvent in the resin, reducing the porosity of the prepared composite material and increasing the density.

[0131] Therefore, for improving the performance of the composite material, it is crucial that the resin retention effect in the mold is good and the resin can replace the space generated due to the volatilization of the solvent in the mold (referred to as "space replacement"). Further, for the present invention, the influence of realizing "space replacement" through secondary space compression is more significant, which is also one of the most core invention points of the present invention. Comparative Example 1 did not achieve space replacement, and the resin retention effect in the mold of Example 2 was not good, so the composite material performance (mainly referring to porosity and density) was lower than that of Example 1; however, the density of the composite material in Example 2 was still significantly higher than that of Comparative Example 1, and the porosity was also significantly lower.

[0132] In summary, the porosity of the composite material prepared by the preparation method provided by the present invention can be as low as 2.5 ± 0.5%, and the density is increased by 6-10% compared with the composite material prepared by the conventional technology. The reduction of porosity and the increase of density both contribute to the improvement of the ablation resistance ability, and the specific improvement amplitude is related to the characteristics of different composite materials.

[0133] Exemplarily, taking Example 1 and Comparative Example 1 as examples, the relative density is 1.5 / 1.4≈1.07. In Comparative Example 1, relatively excellent process parameters were actually adopted and a unidirectional breathable film was set. Therefore, the density of the composite material is increased compared with the conventional technology. That is to say, if the composite material corresponding to Example 1 is compared with the composite material prepared by the conventional technology, the density increase is greater (the relative density is higher).

[0134] As described above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A molding method for a high-residual-carbon phenolic resin-based composite material, characterized in that: The method comprises: arranging a low melting point alloy gasket (13) in an outer sealing groove (12) of an upper cover plate (1) of a mold; The low melting point alloy gasket (13) is used to achieve dynamic sealing of the mold. During the pressurization and temperature increase stage, the low melting point alloy gasket (13) melts and flows out from the overflow port (7) provided on the upper cover plate (1), thereby achieving further compression of the space in the mold cavity; the melting point of the low melting point alloy is ≤130°C.

2. The molding method according to claim 1, characterized in that: The low melting point alloy is one or more of a tin-bismuth alloy, a bismuth-zinc alloy, and a gallium-indium alloy.

3. The molding method according to any one of claims 1 to 2, characterized in that: The specific steps include: S1 gasket casting: melting the low melting point alloy and casting it into a low melting point alloy gasket (13) of appropriate specifications; S2 mold assembly: firstly, a low melting point alloy gasket (13) is placed in the outer sealing groove (12) of the mold upper cover plate (1), then an O-type rubber sealing ring is embedded in the outer sealing groove (12) and the low melting point alloy gasket (13) is fixed in the outer sealing groove (12); The upper cover plate (1), the positioning frame (2), and the lower base plate (3) of the mold are assembled in sequence, the preform (18) is placed in the mold cavity of the mold and locked, and the mold is connected to the glue injection system and the vacuum system; S3 Glue injection and bubble removal: After heating and vacuuming the mold as a whole, perform glue injection. When resin overflows from the glue discharge port, close the vacuum system connected to the glue discharge port and use a step-by-step pressurization method to remove the resin bubbles. S4 exhaust treatment: continue to slowly heat the mold to 120-125°C and evacuate the mold. When the temperature reaches the target temperature, use intermittent exhaust operation to exhaust the mold. S5: Pressurization treatment: Rapidly heat the mold to 130-135° C. and keep the temperature for 20-30 minutes. After the low melting point alloy gasket (13) melts and flows out, lock the mold for a second time and compress the preform (18). S6 curing and molding: the mold temperature is raised to 160±5℃ and kept warm for at least 2h, the mold temperature is raised to 180±5℃ and kept warm for at least 4h, the product is cured and molded to obtain a phenolic resin-based composite material.

4. The molding method according to claim 3, characterized in that: The step S1 comprises: The metal block of the low melting point alloy is heated to be molten, and then cast into the groove of the forming mold. The mold is cooled by air to remove the annular low melting point alloy gasket (13).

5. The molding method according to claim 4, characterized in that: The height of the gate of the molding die for the low melting point alloy gasket (13) is 30±5 mm.

6. The molding method according to claim 3, characterized in that: The volume of the low melting point alloy gasket (13) is determined in the following principle: according to the actual shape and specifications of the mold, after the low melting point alloy gasket (13) melts and flows out, the volume of the cavity compressed by the secondary locking mold is 10-15% of the theoretical volume of the resin in the mold; In step S5, the mold is locked for a second time and the preform (18) is compressed until the outflow mass of the low-melting-point alloy is greater than or equal to 98% of the total mass of the low-melting-point alloy gasket (13).

7. The molding method according to claim 3, characterized in that: The intermittent exhaust operation in step S4 includes: exhausting by opening / closing the exhaust valve, the exhaust interval is 10 to 15 minutes, the exhaust time is 10 to 15 seconds, and the above process is repeated 3 to 5 times.

8. A high residual carbon phenolic resin-based composite material, characterized in that: The composite material is obtained by the molding method according to any one of claims 1 to 7.

9. A mold for molding a high carbon residue phenolic resin-based composite material, characterized in that: The mold comprises an upper cover plate (1), a positioning frame (2), a lower base plate (3) and a low melting point alloy gasket (13); The lower surface of the upper cover plate (1) is provided with an inner sealing groove (16) and an outer sealing groove (12), a low-melting-point alloy gasket (13) is provided in the outer sealing groove (12), and an overflow port (7) in communication with the outer sealing groove (12) is provided at a position corresponding to the upper surface of the upper cover plate (1) and the outer sealing groove (12) for discharging molten low-melting-point alloy liquid; The lower surface of the upper cover plate (1) is provided with an insert (10) and a corresponding groove, the insert (10) is provided with an exhaust through hole, and a one-way air permeable membrane (14) is provided between the insert (10) and the groove; The mold is used to implement the molding method according to any one of claims 1 to 7.

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