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

Abstract

The invention relates to a forming method of a high-residual-carbon phenolic resin-based composite material, belongs to the technical field of liquid forming of composite materials, and solves the problems of large porosity, low density and insufficient ablation resistance of a fiber-reinforced phenolic resin-based composite material prepared by a liquid forming process in the prior art. The invention provides a forming method of a high-residual-carbon phenolic resin-based composite material. According to the method, a low-melting-point alloy gasket is arranged in a sealing groove in the outer side of an upper cover plate of a mold; and the low-melting-point alloy gasket is used for realizing dynamic sealing of the mold. According to the method, dynamic sealing is conducted through low-melting-point alloy, further compression of the space in a mold cavity is achieved, the density and the ablation resistance of the composite material are remarkably improved in cooperation with arrangement of a one-way breathable film and regulation and control of other technological parameters, 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 improved by 6-10%.

Description

Technical Field

[0001] The invention relates to the technical field of composite material liquid molding, in particular to a molding method of 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 have posed unprecedented challenges to heat-resistant materials. Although traditional heat-resistant materials, such as metal and ceramic-based composite materials, have certain high-temperature resistance, they are difficult to meet the urgent needs of modern aerospace for lightweight, high-performance heat-resistant materials due to their inherent defects such as high density, difficulty in processing, and high cost.

[0003] In this context, phenolic resin, as an important thermosetting resin, has shown unique advantages in the field of heat-resistant materials. Phenolic resin is known for its high residual carbon property. It can quickly form a dense carbonized layer under high temperature conditions, effectively isolating the invasion of external heat and protecting the internal structure from thermal damage. In addition, phenolic resin-based composite materials also show excellent thermal stability and mechanical properties, and can withstand thermal and mechanical stresses under extreme conditions, making them an ideal choice for heat-resistant materials in the aerospace field.

[0004] However, the current process of improving the high residual carbon of phenolic resin mainly relies on the method of 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. In order to reduce the viscosity, 15%-20% solvent is usually added to the phenolic resin. However, during the molding process of the composite material, if the solvent cannot be completely removed, pores will be formed inside the material, causing the density of the composite material to be lower than the theoretical value, thereby weakening its structural strength and ablation resistance, seriously affecting the overall performance of the material.

[0005] At present, the existing technology can already remove the solvent from the composite material more fully, but the holes left in the composite material after the solvent is vaporized and discharged are difficult to deal with, which leads to a high porosity and insufficient density of the composite material, which in turn leads to a decrease in the ablation resistance. Summary of the invention

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

[0007] The present invention provides a molding method of a high-residual-carbon phenolic resin-based composite material, the method comprising: arranging a low-melting-point alloy gasket in an outer sealing groove of an upper cover plate of a mold; The low melting point alloy gasket is used to achieve dynamic sealing of the mold. During the pressurization and temperature increase stage, the low melting point alloy gasket melts and flows out from the overflow port set on the upper cover plate, thereby achieving further compression of the space in the mold cavity.

[0008] Specifically, the low melting point alloy is one or more of tin-bismuth alloy, bismuth-zinc alloy, and gallium-indium alloy.

[0009] Specifically, the melting point of the low melting point alloy is ≤130°C.

[0010] Furthermore, the molding method specifically comprises the following steps: S1 Gasket casting: melt the low melting point alloy and cast it into low melting point alloy gaskets of appropriate specifications; S2 mold assembly: firstly, place the low melting point alloy gasket in the outer sealing groove of the mold upper cover plate, then embed the O-type rubber sealing ring into the outer sealing groove and fix the low melting point alloy gasket in the outer sealing groove; Assemble the upper cover plate, positioning frame and lower base plate of the mold in sequence, put the preform into the mold cavity and lock it, and connect the mold with the 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 pressure treatment: quickly heat the mold to 130-135°C and keep it warm for 20-30 minutes. After the low-melting-point alloy gasket melts and flows out, lock the mold for the second time and compress the preform. 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.

[0011] Furthermore, the specific operations of step S1 are: A metal block of a low-melting-point alloy is placed in a graphite crucible, and the graphite crucible and the low-melting-point alloy gasket forming mold are placed in a heating device at 130-150°C and heated for 30-60 minutes. When the low-melting-point alloy is melted and has good fluidity, the low-melting-point alloy is cast into the groove of the forming mold through the casting port. The mold is air-cooled, the annular low-melting-point alloy gasket is removed, and then polished and reshaped.

[0012] Preferably, the height of the gate of the low-melting-point alloy gasket molding mold is 30±5 mm.

[0013] Furthermore, the volume of the low-melting-point alloy gasket is determined according to the actual shape and specifications of the mold. After the low-melting-point alloy gasket melts and flows out, the cavity volume compressed by the secondary locking mold is 10 to 15% of the theoretical volume of the resin in the mold.

[0014] Furthermore, the specific operations of step S3 are: Place the mold in the heating device, wait until the temperature reaches 70-80℃, close the exhaust port valve, open the discharge port valve connected to the vacuum system, evacuate the mold cavity for 3-5 minutes, open the injection port valve, and start injection; When resin overflows from the discharge port, close the vacuum system connected to the discharge port and use a step-by-step pressurization method to remove bubbles from the resin until no bubbles appear in the discharge port pipe. End the injection and close the injection port valve and the discharge port valve.

[0015] Specifically, in step S3, the step pressurization method is four-stage pressurization, and the pressures are 1 bar, 3 bar, 5 bar, and 8 bar respectively.

[0016] Specifically, the intermittent exhaust operation in step S4 is: 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.

[0017] Specifically, the vacuum negative pressure value in step S4 must not be less than 0.95 bar.

[0018] Specifically, in step S5, the mold is locked for a second time and the preform is compressed until the outflow mass of the low-melting-point alloy is ≥ 98% of the total mass of the low-melting-point alloy gasket.

[0019] Furthermore, in step S5, after the mold is locked for the second time, a gap detection is required, and the mold gap shall not be greater than 0.1 mm.

[0020] The invention also discloses a high-residual-carbon phenolic resin-based composite material, and the composite material is prepared by the above-mentioned molding method.

[0021] The present invention also discloses a mold for molding a high-residual-carbon phenolic resin-based composite material, the mold comprising an upper cover plate, a positioning frame, a lower bottom plate and a low-melting-point alloy gasket; The lower surface of the upper cover plate is provided with an inner sealing groove and an outer sealing groove, a low-melting-point alloy gasket is provided in the outer sealing groove, and an overflow port communicating with the outer sealing groove is provided at the corresponding position of the upper surface of the upper cover plate and the outer sealing groove, for discharging molten low-melting-point alloy liquid; The lower surface of the upper cover plate is provided with an insert and a corresponding groove, the insert is provided with an exhaust through hole, and a one-way air permeable membrane is provided between the insert and the groove; The inner sealing groove is arranged at a suitable position of the groove to realize the sealing between the insert and the upper cover plate body; The mold is used in the above-mentioned high-residual-carbon phenolic resin-based composite material molding method.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1. The present invention realizes dynamic sealing of the mold by providing a low-melting-point alloy gasket, which can further compress the cavity space of the mold during the pressurization treatment stage (refer to step S5), thereby eliminating the holes left by solvent vaporization, significantly reducing the porosity of the composite material, and improving the density and ablation resistance of the composite material.

[0023] The low-melting-point alloy gasket can play a good sealing role at a temperature below the melting point, and because the low-melting-point alloy has good thermal conductivity, it can also improve the thermal conductivity of the mold to a certain extent, so that the heating of the entire preform is more uniform; in the pressurized treatment stage, since the heating temperature is higher than the melting point of the gasket, the gasket melts and forms an alloy liquid. At this time, the cavity space of the mold is compressed by locking the mold twice (the common method is mechanical locking). During the compression process, the alloy liquid is squeezed out of the outer sealing groove (discharged from the overflow port), thereby providing a certain secondary compression space for the mold cavity. In this process, the holes left by the vaporization of the solvent are filled with the secondary distributed resin, which reduces the porosity of the composite material and increases the density of the composite material. The reduction in porosity and the increase in density both contribute to the improvement of the anti-ablation ability, and the specific improvement is related to the characteristics of different composite materials themselves.

[0024] It is worth noting that although the technical purpose of eliminating voids in the composite material can be achieved by using a low-melting-point alloy gasket, better results can be achieved by controlling the volume / mass of the low-melting-point alloy gasket.

[0025] Preferably, the volume of the low-melting alloy gasket is determined in the following principle: according to the actual shape and specifications of the mold, after the low-melting alloy gasket melts and flows out, the cavity volume compressed by the secondary locking 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 the melting during the pressurization treatment stage is too small, and it cannot fully eliminate the holes left by the solvent; if the volume of the gasket is too large, it may cause the preform fibers to be crushed (the primary locking is sufficient, and the preform is relatively fully compressed and exhausted during the exhaust stage, which will cause the final reserved compression margin to be too large, and the preform / composite material may be crushed during the secondary locking).

[0026] Furthermore, in step S5, the mold is locked for the second time and the preform is compressed until the mass of the low-melting-point alloy flowing out is ≥ 98% of the total mass of the low-melting-point alloy gasket. Since the volume of the molten low-melting-point alloy is not easy to measure in 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 margin reserved for the gasket.

[0027] The molding method of a high-residual-carbon phenolic resin-based composite material provided by the present invention is an innovation of the traditional RTM process, can solve the problem of solvent volatilization of solvent-containing resins, and greatly improves the applicable molding process range of solvent-based resins.

[0028] 2. The present invention adopts an intermittent exhaust method to exhaust the phenolic resin at a constant temperature and at a fixed time, which can effectively reduce the volatile matter inside the resin and can more fully remove the solvent in the phenolic resin.

[0029] 3. The present invention uses a one-way breathable film, which can achieve unidirectional gas discharge, while ensuring that the resin in the mold does not leak and the resin content in the composite material is guaranteed.

[0030] 4. The present invention utilizes a combination of simple and inexpensive materials and equipment to achieve the preparation of a high-carbon phenolic resin-based composite material with low porosity, and reduces the molding process cost compared to the traditional autoclave process and molding process.

[0031] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings are only used for the purpose of illustrating specific embodiments and are not to be considered as limiting the present invention. In the entire drawings, the same reference symbols represent the same components; Figure 1 A possible design of the molding die for high carbon residue phenolic resin based composites; Figure 2 A possible schematic diagram of the overall structure of the upper cover; Figure 3 It is a schematic diagram of the assembly of the composite material forming mold and the preform; Figure 4 is a cross-sectional photograph of the composite material in Example 1; Figure 5 This is a cross-sectional photograph of the composite material in Comparative Example 1.

[0033] Reference numerals: 1. Upper cover plate; 2. Positioning frame; 3. Lower base plate; 4. Glue injection port valve; 5. Exhaust port valve; 6. Glue discharge port valve; 7. Overflow port; 8. Locking bolt; 9. Outer sealing ring; 10. Insert; 11. Insert 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 base plate sealing groove; 18. Preform. DETAILED DESCRIPTION

[0034] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0035] The present invention provides a molding method of a high-residual-carbon phenolic resin-based composite material, the method comprising: arranging a low-melting-point alloy gasket in an outer sealing groove of an upper cover plate of a mold; The low melting point alloy gasket is used to achieve dynamic sealing of the mold. During the pressurization and temperature increase stage, the low melting point alloy gasket melts and flows out from the overflow port set on the upper cover plate, thereby achieving further compression of the space in the mold cavity.

[0036] The present invention realizes dynamic sealing of the mold by providing a low-melting-point alloy gasket, and can further compress the cavity space of the mold during the pressurization treatment stage (refer to step S5), thereby eliminating the holes left over due to solvent vaporization, significantly reducing the porosity of the composite material, and improving the density and ablation resistance of the composite material.

[0037] The low-melting-point alloy gasket can perform a good sealing function at a temperature below the melting point, and 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, so that the heating of the entire preform is more uniform (the low-melting-point alloy gasket can assist in heat transfer in the annular direction, so that the heat flows from the high-temperature area of ​​the gasket to the low-temperature area, which helps to improve the uniformity of heating of the mold); in the pressurization treatment stage, since the heating temperature is higher than the melting point of the gasket, the gasket melts and forms an alloy liquid. At this time, the mold is locked for the second time (the common method is mechanical locking) to compress the cavity space of the mold. During the compression process, the alloy liquid is squeezed out of the outer sealing groove (discharged from the overflow port), thereby providing a certain compression space. In this process, the holes left over due to solvent vaporization are eliminated, thereby reducing the porosity of the composite material.

[0038] It is worth mentioning that in actual implementation, since the gasket is set in the outer sealing groove of the upper cover plate and is tightly fixed by the O-ring, the O-ring and the outer sealing groove are properly matched and the rubber will expand during heating. Therefore, when the gasket melts, no leakage will occur inside the mold, but almost all of it will flow out from the overflow port, with only a small amount remaining in the outer sealing groove.

[0039] Furthermore, the mold is locked for the second time and the preform is compressed until the mass of the low-melting-point alloy flowing out is ≥ 98% of the total mass of the low-melting-point alloy gasket. Since the volume of the molten low-melting-point alloy is not easy to measure in 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 margin reserved for the gasket.

[0040] Specifically, the low melting point alloy is one or more of tin-bismuth alloy, bismuth-zinc alloy, and gallium-indium alloy. These alloys have suitable melting points and good fluidity, and can meet the requirements for forming low melting point alloy gaskets.

[0041] Specifically, the melting point of the low melting point alloy is ≤130°C.

[0042] Specifically, the molding method includes the steps of gasket casting, mold assembly, glue injection and bubble removal, exhaust treatment, pressurization treatment, curing and molding, etc.

[0043] The gasket casting step is to melt the low melting point alloy and cast it into a low melting point alloy gasket of appropriate specifications, and the gasket specifications should correspond to the specifications of the outer sealing groove; During the mold assembly process, the low melting point alloy gasket is placed in the sealing groove on the outer side of the upper cover plate and tightly fixed with an O-type rubber sealing ring to prevent it from leaking into the mold after melting; In the pressure treatment step, the heating temperature is ≥ the melting point of the low-melting-point alloy and the temperature is kept for a period of time to fully melt the low-melting-point alloy, and then the mold is locked for a second time to allow the alloy liquid to overflow from the overflow port.

[0044] Specifically, there is no specific requirement for the size of the overflow port, which can be determined according to specific implementation needs. Generally, the diameter of the overflow port is 1 to 2 mm.

[0045] Furthermore, the molding method specifically comprises the following steps: S1 Gasket casting: melt the low melting point alloy and cast it into a low melting point alloy gasket of appropriate specifications, the gasket specifications should correspond to the specifications of the outer sealing groove; S2 mold assembly: firstly, place the low melting point alloy gasket in the outer sealing groove of the mold upper cover plate, then embed the O-type rubber sealing ring into the outer sealing groove and tightly fix the low melting point alloy gasket in the outer sealing groove; Assemble the upper cover plate, positioning frame and lower base plate of the mold in sequence, put the preform into the mold cavity and lock it, and connect the mold with the injection system and the vacuum system; S3 Glue injection and bubble removal: After heating and vacuuming the mold as a whole for a period of time, the glue injection operation is carried out. When resin overflows from the glue discharge port, the vacuum system connected to the glue discharge port is closed, and the resin bubbles are removed by step-by-step pressurization. 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 pressure treatment: quickly heat the mold to 130-135°C and keep it warm for 20-30 minutes. After the low melting point alloy gasket melts and flows out from the overflow port, lock the mold again and compress the preform. The above-mentioned heat preservation time can ensure that the low alloy gasket is completely melted; S6 curing and molding: raise the mold temperature to 160±5℃ and keep it warm for at least 2h, raise the mold temperature to 180±5℃ and keep it warm for at least 4h, and the product is cured and molded to obtain a dense phenolic resin-based composite material.

[0046] Furthermore, the specific operations of step S1 are: 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 melted and has good fluidity, cast 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. During casting, the molten low-melting-point alloy is slowly cast into the groove of the forming mold through the casting port. Pay attention to controlling the casting speed to avoid bubbles and sputtering. After casting, the mold is quickly cooled by air cooling. After the alloy solidifies, the low-melting-point alloy gasket is removed and finely polished and reshaped using professional polishing tools to ensure that its dimensional accuracy and surface finish meet the requirements of subsequent use.

[0047] In addition, the volume of the low-melting-point alloy needs to be calculated in advance. The volume determination principle 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 cavity volume compressed by the secondary locking mold is 10-15% of the theoretical volume of the resin in the mold, so as to ensure the best compression and sealing effect in the subsequent steps. For example, when the area of ​​the upper cover plate / lower base plate of the mold is large, a small height change (the locking height change caused by the gasket melting before and after melting) will lead to a large change in the volume of the cavity. ; When the area of ​​the mold upper cover plate / lower base plate is small, a large height change (the locking height change before and after the gasket melts) will not necessarily cause a large volume change in the cavity. Therefore, the above principles need to be flexibly grasped according to the actual implementation situation.

[0048] Preferably, the height of the gate of the low-melting-point alloy gasket molding mold is 30±5 mm.

[0049] It is worth noting that although the technical purpose of eliminating holes in composite materials can be achieved by using low-melting alloy gaskets, better results can be obtained by controlling the volume / mass of low-melting alloy gaskets. Preferably, the volume determination principle of the low-melting alloy gasket is: according to the actual shape and specifications of the mold, after the low-melting alloy gasket melts and flows out, the cavity volume compressed by the secondary locking 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 the melting during the pressurization treatment stage is too small, and it cannot fully eliminate the holes left by the solvent; if the volume of the gasket is too large, it may cause the preform fibers to be crushed (the first locking is sufficient, and the preform is relatively fully compressed and exhausted during the exhaust stage, which will cause the final reserved compression margin to be too large, and the preform / composite material may be crushed during the secondary locking).

[0050] Specifically, mold assembly is one of the key steps to ensure the quality of composite material molding. Figure 1 Taking the molding die shown in the figure as an example, it can be assembled according to the following process: First, place the polished and trimmed low-melting-point alloy gasket 13 in the outer sealing groove 12 of the mold upper cover plate to ensure that the gasket fits tightly with the sealing groove without any gap. Next, install O-type rubber sealing rings in the corresponding upper cover plate outer sealing groove 12, upper cover plate inner sealing groove 16, and lower bottom plate sealing groove 17 of the mold upper cover plate 1 and the lower bottom plate 3 to enhance the sealing performance of the mold and prevent resin leakage.

[0051] Then, two layers of one-way breathable membrane 14 are laid in the center of the groove of the upper cover plate of the mold. The selection of the one-way breathable membrane should ensure that it has good air permeability and resistance to resin erosion. The breathable membrane must be laid flat and wrinkle-free to ensure that the gas can be discharged smoothly and the resin will not penetrate. Subsequently, the upper cover plate insert 10 is accurately placed in the groove and fastened with the insert bolt 11 to ensure that the insert is stable and does not shake. During the tightening process, it is necessary to pay attention to controlling the tightening torque of the bolts to avoid deformation of the mold due to over-tightening or decreased sealing performance due to over-loosening. Next, the positioning frame 2 is accurately positioned and installed on the upper cover plate 1 of the mold to form a tight rectangular cavity. The preform 18 prepared in advance is placed in the cavity to ensure that the position is accurate and the preform fits tightly against the cavity wall.

[0052] Finally, position the mold bottom plate 3 and use the locking bolts 8 to lock all parts of the entire mold to ensure that all parts are tightly combined without looseness, and check the mold gap. The mold gap should not be greater than 0.1mm. Then install the injection port valve 4, the discharge port valve 6, and the exhaust port valve 5 to control the injection and discharge of resin and gas in subsequent operations. Connect the assembled mold to the injection system and vacuum system through pipelines to prepare for the subsequent resin infiltration process.

[0053] Furthermore, the specific operations of step S3 are: Place the mold in the heating equipment, wait until the temperature reaches 70-80℃, and start the resin infiltration process after the temperature inside the mold is stable and evenly distributed; close the exhaust port valve, open the glue discharge port valve connected to the vacuum system, and evacuate the mold cavity for 3-5 minutes to remove the air in the cavity, then open the glue injection port valve and start glue injection; during the glue injection process, the glue discharge port situation must be closely observed. Once resin overflows, immediately close the vacuum system connected to the glue discharge port to prevent resin loss.

[0054] When resin overflows from the discharge port, close the vacuum system connected to the discharge port and use a step-by-step pressurization method to remove bubbles from the resin until no bubbles appear in the discharge port pipe. End the injection and close the injection port valve and the discharge port valve.

[0055] Specifically, in step S3, the step pressurization method is four-stage pressurization, and the final pressurization pressure shall not be less than 8 bar, otherwise the resin bubbles in the resin cannot be completely eliminated. Each level of pressure is maintained for 5 to 10 minutes until no bubbles appear in the discharge pipe, indicating that the resin has fully infiltrated the fiber and the internal bubbles are effectively eliminated. In this process, the pressure value needs to be strictly controlled. Preferably, the four-stage pressure gradient is 1 to 2 bar, 3 to 4 bar, 5 to 6 bar, and 8 to 10 bar, respectively.

[0056] Specifically, the specific operation of step S4 is: set the temperature rise program of the mold, so that the mold is heated up (slowly heated up) to 90±5°C at a heating rate of 1-2°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.

[0057] Specifically, the intermittent exhaust operation in step S4 is: exhaust by opening / closing the exhaust valve, the exhaust interval is 10 to 15 minutes, the exhaust time is 10 to 15 seconds, so as to exhaust the solvent gas and the small molecule gas generated by the reaction, and this process needs to be repeated 3 to 5 times.

[0058] Specifically, the vacuum negative pressure value in step S4 must not be less than 0.95 bar to ensure the exhaust effect.

[0059] Specifically, in step S5, the mold is locked for the second time and the preform is compressed until the mass of the low-melting-point alloy flowing out is ≥ 98% of the total mass of the low-melting-point alloy gasket. Since the volume of the molten low-melting-point alloy is not easy to measure in 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 margin reserved for the gasket.

[0060] Furthermore, after the mold is locked for the second time in step S5, a gap detection is required, and the mold gap shall not be greater than 0.1 mm and all parts of the mold are tightly combined without looseness.

[0061] Specifically, the temperature and time parameters need to be strictly controlled during the curing process of step S6 to avoid the degradation of the composite material performance caused by excessively high or low temperatures. At the same time, the temperature inside the mold needs to be uniformly distributed to avoid thermal stress that causes cracking or deformation of the composite material. After the curing is completed, the product can be taken out after the mold is cooled to room temperature to obtain a dense and high-performance high-carbon phenolic resin-based composite material.

[0062] The present invention also discloses a high carbon residue phenolic resin-based composite material, which is prepared by the above molding method. The porosity of the composite material prepared by the preparation / molding 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 conventional technology.

[0063] The present invention also discloses a mold for molding a high-residual-carbon phenolic resin-based composite material, wherein the mold comprises, from top to bottom, an upper cover plate, a positioning frame, a lower bottom plate, and a low-melting-point alloy gasket; The lower surface of the upper cover plate is provided with an inner sealing groove and an outer sealing groove, a low-melting-point alloy gasket is provided in the outer sealing groove, and an overflow port communicating with the outer sealing groove is provided at the corresponding position of the upper surface of the upper cover plate and the outer sealing groove, for discharging molten low-melting-point alloy liquid; An insert and a corresponding groove are arranged at the center of the lower surface of the upper cover plate, an exhaust through hole is arranged on the insert, and a one-way breathable membrane is arranged between the insert and the groove; The inner sealing groove is arranged at a suitable position of the groove to realize the sealing between the insert and the upper cover plate body; An exhaust port is provided in the center of the upper cover plate; The lower bottom plate is provided with a lower bottom plate sealing groove; The upper cover plate, the positioning frame and the lower base plate are provided with bolt holes at corresponding positions for setting locking bolts; The mold is provided with a glue injection port and a glue discharge port; The mold is used in the above-mentioned high-residual-carbon phenolic resin-based composite material molding method.

[0064] Specifically, the one-way breathable membrane is two or more layers, and the one-way breathable membrane should be selected to ensure that it has good air permeability and resin erosion resistance. The breathable membrane should be laid flat and wrinkle-free to ensure that the gas can be discharged smoothly and the resin will not penetrate. Exemplarily, the one-way breathable membrane can be Dahltexx SP-2 brand one-way breathable membrane of Airtech.

[0065] Specifically, the number of exhaust holes on the insert is ≥9, and the diameter of the exhaust holes is 1±0.02 mm.

[0066] For example, Figure 1 Taking the molding die shown in the figure as an example, it can be assembled according to the following process: First, place the polished and trimmed low-melting-point alloy gasket 13 in the outer sealing groove 12 of the mold upper cover plate to ensure that the gasket fits tightly with the sealing groove without any gap. Next, install O-type rubber sealing rings in the corresponding upper cover plate outer sealing groove 12, upper cover plate inner sealing groove 16, and lower bottom plate sealing groove 17 of the mold upper cover plate 1 and the lower bottom plate 3 to enhance the sealing performance of the mold and prevent resin leakage.

[0067] Then, two layers of one-way breathable membrane 14 are laid in the center of the groove of the upper cover plate of the mold. The selection of the one-way breathable membrane should ensure that it has good air permeability and resistance to resin erosion. The breathable membrane must be laid flat and wrinkle-free to ensure that the gas can be discharged smoothly and the resin will not penetrate. Subsequently, the upper cover plate insert 10 is accurately placed in the groove and fastened with the insert bolt 11 to ensure that the insert is stable and does not shake. During the tightening process, it is necessary to pay attention to controlling the tightening torque of the bolts to avoid deformation of the mold due to over-tightening or decreased sealing performance due to over-loosening. Next, the positioning frame 2 is accurately positioned and installed on the upper cover plate 1 of the mold to form a tight rectangular cavity. The preform 18 prepared in advance is placed in the cavity to ensure that the position is accurate and the preform fits tightly against the cavity wall.

[0068] Finally, position the mold bottom plate 3 and use the locking bolts 8 to lock all parts of the entire mold to ensure that all parts are tightly combined without looseness, and check the mold gap. The mold gap should not be greater than 0.1mm. Then install the injection port valve 4, the discharge port valve 6, and the exhaust port valve 5 to control the injection and discharge of resin and gas in subsequent operations. Connect the assembled mold to the injection system and vacuum system through pipelines to prepare for the subsequent resin infiltration process.

[0069] Example 1 Use Figure 1 The mold shown is used to prepare the composite material. The specific process and parameters are as follows: (1) Casting of low melting point alloy gasket: a metal block of a low melting point alloy (tin-bismuth alloy) is placed in a graphite crucible, and the graphite crucible and the low melting point alloy forming mold are placed in a heating device at 130-150°C and heated for 30-60 minutes. When the low melting point alloy is melted and has good fluidity, the low melting point alloy is cast into the concave of the forming mold through the casting port. The mold is cooled by air, the annular low melting point alloy is removed, and the low melting point alloy is polished and repaired to obtain a low melting point alloy gasket; the secondary compression margin (secondary compression volume in the mold cavity) provided by the low melting point alloy gasket after melting is approximately 12% of the theoretical volume of the resin in the mold.

[0070] (2) Mold assembly: Place the low-melting-point alloy in the sealing groove on the outer side of the mold upper cover plate, place the O-ring in the sealing groove between the mold upper cover plate and the lower base plate, place the Dahltexx SP-2 brand one-way breathable membrane (2 layers) of Airtech in the groove position of the mold upper cover plate, place the upper cover plate insert into the groove of the mold upper cover plate and lock it with bolts, position the mold frame and install it on the mold upper cover plate 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 vacuum system.

[0071] (3) Resin impregnation of fiber: Place the mold in a heating device, wait until the temperature reaches 70-80°C, close the exhaust valve, open the discharge port valve connected to the vacuum system, evacuate the mold cavity for 3-5 minutes, open the injection port ball valve, and start resin injection. When resin overflows from the discharge port, close the vacuum system connected to the discharge port, and use a step-by-step pressurization method to remove bubbles from the resin. The step-by-step pressurization values ​​are 1 bar, 3 bar, 5 bar, and 8 bar, respectively. Each level of pressure is maintained for 5-10 minutes until no bubbles appear in the discharge port pipe. End the injection, and close the injection port valve and the discharge port valve.

[0072] (4) Exhaust control: Set the mold heating program to heat the mold at a rate of 1 to 2°C (slowly) to 90±5°C, open the exhaust valve connected to the vacuum system to allow the solvent in the resin to evaporate further, and when the mold temperature reaches 120°C, close the valve. Open the exhaust valve at intervals to exhaust the solvent gas and water vapor generated by the reaction, and then close the exhaust valve. The vacuum negative pressure value is not less than 0.95 bar, the exhaust interval is 10 to 15 minutes, and the exhaust time is 10 to 15 seconds to exhaust the solvent gas and small molecular gas generated by the reaction. This process needs to be repeated 3 to 5 times.

[0073] (5) Pressurization treatment: Rapidly raise the mold temperature to 130°C and keep it warm for 20 to 30 minutes. When the low-melting-point alloy melts and flows out (the mass of the low-melting-point alloy flowing out ≥ 98% of the total mass of the low-melting-point alloy gasket), compress the preform by mechanical locking (secondary locking) to allow the resin to better infiltrate the fiber preform. After the secondary locking, the gap detection is required. The mold gap shall not be greater than 0.1mm and the various parts of the mold shall be tightly combined without looseness.

[0074] (6) Curing and molding: Raise the mold temperature to 160±5℃ and keep it for 2 hours, then raise the mold temperature to 180±5℃ and keep it for 4 hours. The product is cured and molded to obtain a dense phenolic resin-based composite material.

[0075] After testing, the density of the phenolic resin-based composite material is 1.50g / cm 3 , the porosity is 2~3%.

[0076] Example 2 No one-way breathable membrane is provided, and other processes and parameters are the same as those in Example 1.

[0077] Without adding a one-way breathable film, part of the resin will be taken out during the exhaust process, and the overall resin mass in the mold will be reduced. Although the space inside the mold is compressed later, the product is loose as a whole due to the small amount of resin in the mold. Although there are no large pores, there are relatively many small pores.

[0078] After testing, the density of the phenolic resin-based composite material is 1.45g / cm 3 The porosity is 5-6%, and the relative density is 3-4% lower than that of the composite material in Example 1.

[0079] Comparative Example 1 No low melting point alloy gasket is cast or configured, and other processes and parameters are the same as those in Example 1.

[0080] Without adding the low-melting-point alloy gasket, the product compression process is omitted. Although the vaporized solvent is exhausted during the preparation process and the gas is removed from the resin, there is no excess resin in the mold to supplement the pores. Eventually, large pores appear on the inside of the product and the density of the fiber preform is low.

[0081] After testing, the density of the phenolic resin-based composite material is 1.40g / cm 3 The porosity is 8-10%, and the relative density is 6-8% lower than that of the composite material in Example 1.

[0082] A preferred implementation of the present invention is to ensure the quality of the resin in the mold cavity by reducing the mold cavity to apply a certain pressure to the resin in the mold, so that the solvent-free resin in the mold is redistributed, and the resin is fully filled into the pores generated by the volatilization of the solvent in the resin, thereby reducing the porosity of the prepared composite material and increasing the density.

[0083] Therefore, for improving the performance of composite materials, the resin retention effect in the mold is good and the resin can replace the space generated by the solvent volatilization in the mold (referred to as "space replacement"), both conditions are critical. Further, for the present invention, the effect of "space replacement" achieved through secondary space compression is more significant, which is also one of the 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; but the density of the composite material of Example 2 was still significantly higher than that of Comparative Example 1, and the porosity was also significantly lower.

[0084] 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 conventional technology. The reduction of porosity and the increase of density are both helpful to improve the ablation resistance, and the specific improvement is related to the characteristics of different composite materials themselves; For example, taking Example 1 and Comparative Example 1 as examples, the relative density is 1.5 / 1.4≈1.07, wherein Comparative Example 1 actually adopts more optimal process parameters and sets a one-way breathable membrane, so the density of the composite material is improved relative to conventional technology, that is to say, if the composite material corresponding to Example 1 is compared with the composite material prepared by conventional technology, the density is more improved (relative density is higher).

[0085] The above description is only a preferred specific implementation manner 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 any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within 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.

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 claim 1, characterized in that: The melting point of the low melting point alloy is ≤130°C.

4. The molding method according to any one of claims 1 to 3, 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.

5. The molding method according to claim 4, 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).

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

7. The molding method according to claim 4, 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).

8. The molding method according to claim 4, 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.

9. 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 8.

10. A mold for molding a high-residual-carbon 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 8.

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