A Fiber Bragg Grating Temperature Sensor with Wide Temperature Range Strain Isolation and Its Preparation Method

Abstract

The present invention relates to the field of sensors, and discloses a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation, which includes a substrate. A damping layer is provided on the substrate, and a lower cavity plate is provided on the damping layer. The lower cavity plate is provided with a cavity structure with one side being flat and the other side being arc-shaped. The inside of the cavity is a fiber sensing section. The grating is located in the middle of the fiber sensing section and is encapsulated with a PI tube, and is located at the central position inside the PI tube. The optical fiber leading out of the cavity is an optical fiber transmission section, which is encapsulated and protected with a Teflon tube. An upper cover plate is provided on the lower cavity plate, and an upper skin is provided on the upper cover plate. For the sensor adhered to the surface of the blade, when the blade deforms, the special bending radian of the optical fiber sensing section in the concave cavity of the lower cavity plate of the sensor ensures the free expansion and contraction of the fiber Bragg grating in the environment from low temperature to high temperature (-40°C to 80°C) without generating additional strain.

Description

Technical Field

[0001] The present invention relates to the field of sensors, and specifically to an optical fiber grating temperature sensor with wide-temperature-range strain isolation and a preparation method thereof. Background Art

[0002] Wind turbine blades are the core components of a wind power generation system. However, icing on the wind turbine blades will seriously affect the aerodynamic performance of the blades, thereby reducing the power generation of the generator set, and even causing structural damage to the blades and the generator set, posing a potential safety hazard for safe operation. To solve the problems caused by blade icing to wind turbines, a series of research on anti-icing and de-icing technologies for wind turbine blades has received attention. The main blade de-icing technologies include mechanical de-icing, pneumatic belt de-icing, ultrasonic de-icing, electromagnetic pulse impact, thermal energy de-icing, etc. Except for thermal energy de-icing, the other de-icing methods directly cause the ice layer to break in different ways and break away from the blade under the action of centrifugal force. The de-iced ice amount can reach hundreds of kilograms, and the large amount of ice blocks falling poses a safety hazard to the surrounding area of the blade. How to detect ice accumulation at the early stage of icing and melt the ice by heating the blade, and then remove the ice, the thermal energy de-icing technology has been tried in practice. However, in the blade thermal energy de-icing system, ice-covering detection is a key link in the control system, and accurately measuring the temperature on the blade surface helps to accurately determine the icing conditions. Existing ice-covering detectors fail or affect the correct reading and cause false alarms due to serious icing on their own in a long-term high-humidity and low-temperature environment. Optical fiber grating (FBG) sensors have the advantages of high sensitivity, integrating sensing and transmission, and anti-electromagnetic interference, and are widely used for temperature and strain monitoring of structures. By introducing an optical fiber grating sensor as the temperature monitoring unit of the thermal energy de-icing system, it can respond more quickly and accurately to temperature changes, and then improve the real-time temperature control of the entire de-icing system.

[0003] However, ordinary fiber Bragg grating (FBG) sensors have always suffered from the problem of temperature-strain cross-sensitivity, which has limited their application scenarios to a certain extent. People have to take some measures to solve the troubles brought by temperature-strain cross. In current research, some researchers design the packaging form of fiber Bragg grating (FBG) sensors to achieve the effect of making the fiber Bragg grating sensors free from external force interference. Some scholars also demodulate the signals affected by temperature-strain cross, and extract the temperature signal and strain signal respectively to obtain the required signals. For the demodulation of temperature-strain signals, the design cycle is long and the real-time monitoring is poor. In the research on packaging design, most are based on the tubular packaging form, and further design this form to achieve the effect of isolating external force interference. This packaging form is often effective in scenarios where external pressure is isolated or in small-deformation structures. However, during the service process of wind turbine blades, under the action of wind load, they will bend or twist to a certain extent, resulting in large strains in some areas of the blades, and the strain value can be as high as several thousand microstrains. This design form of tubular packaging is difficult to handle. Therefore, we propose a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation and its preparation method. Summary of the Invention

[0004] (1) Technical problems to be solved

[0005] Aiming at the deficiencies of the prior art, the present invention provides a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation and its preparation method, which solves the above problems.

[0006] (2) Technical solutions

[0007] To achieve the above object, the present invention provides the following technical solutions: A fiber Bragg grating temperature sensor with wide-temperature-range strain isolation, including a substrate, a damping layer is provided on the substrate, a lower cavity plate is provided on the damping layer, the lower cavity plate is provided with a cavity structure with one side being flat and the other side being arc-shaped, an optical fiber is provided inside the cavity structure, the middle section of the optical fiber is encapsulated by a PI tube, the part of the optical fiber extending out of the cavity structure is the optical fiber transmission section, the optical fiber transmission section is encapsulated by a Teflon tube, an FBG grating area is at the central position inside the PI tube, an upper cover plate is provided on the lower cavity plate, and an upper skin is provided on the upper cover plate.

[0008] A preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation includes the following steps:

[0009] The first step: Prepare the upper skin;

[0010] The second step: Make the upper cover plate;

[0011] The third step: Make the lower cavity plate;

[0012] The fourth step: Make the damping layer;

[0013] Step 5: Cut the woven fiberglass, take the center of its surface as the origin, cut off a circular area with a diameter of 87 mm, and the remaining material is used as the sensor structure base.

[0014] Step 6: Fix the optical fiber.

[0015] Step 7: Finally, place the upper skin, upper cover plate, lower cavity plate with fiber Bragg grating, damping layer, and base in the lower template of the sensor packaging assembly in sequence, then cover the upper template, and finally put it into a hot press for hot pressing, and then demold to obtain a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation.

[0016] Preferably, the first step includes the following content: Cut the woven fiberglass cloth and lay it on the lower template of the sensor packaging mold, then use a silicone brush to apply water-based polyurethane on the fiberglass cloth so that the fiberglass cloth fits completely with the lower template. After the application, let it stand for 1 hour until the polyurethane cures. The upper skin after demolding is a "cap-shaped" structure with an overall thickness of 0.15 mm. Rounded corners with a radius of 2 mm are selected for the transition areas between the top and side surfaces and between the bottom and side surfaces of the structure.

[0017] Preferably, the second step includes the following content: Cut two pieces of unidirectional carbon fiber prepregs with a diameter of 79 mm and a thickness of 0.15 mm, lay them between two Teflon plates in a layering sequence with the fiber directions orthogonal at 90°, and then put them into a hot press for hot pressing and forming. The pressure is 0.3 Mpa. Adjust the heating temperature of the hot press to rise from room temperature to 120 °C, keep the temperature for 20 min, and finally cool at room temperature. The thickness dimension after hot pressing is 0.2 mm to obtain the upper cover plate.

[0018] Preferably, the third step includes the following content: Cut two pieces of unidirectional carbon fiber prepregs with a diameter of 79 mm and a thickness of 0.15 mm, lay them in the lower cavity plate manufacturing mold in a layering sequence with the fiber directions orthogonal at 90°, and then put them into a hot press for hot pressing and forming. The pressure is 0.5 Mpa. Adjust the heating temperature of the hot press to rise from room temperature to 120 °C, keep the temperature for 20 min, and finally cool at room temperature. The overall thickness of the formed lower cavity plate is 0.2 mm, and it has a cavity structure with a concave upper part and a convex lower part in its central area. The cavity depth is 0.7 mm. One side of the cavity is a common plane, and the other side is composed of a plane and a curved surface. The plane and the curved surface are transitioned by a rounded corner with a radius of 20 mm, and the length of the curved surface is 43 mm.

[0019] Preferably, the fourth step includes the following: Pour 2 g each of food-grade silicone A and B components into a cup, stir evenly, let stand for 2 - 3 minutes, then slowly inject the silicone into the damping layer manufacturing mold. A tool can be used to level the silicone. Wait for 3 hours at room temperature until the silicone cures, and then slowly demold it to obtain the damping layer. The total thickness of the damping layer is 1.5 mm. There is a concave cavity on the upper surface of the damping layer for mating with the lower bottom surface of the lower cavity plate. The size is larger than the cavity of the lower cavity plate, and the side surface is a curved surface for mating with the upper skin.

[0020] Preferably, the sixth step includes the following: Insert the optical fiber with the FBG grating region into a 55-mm-long PI tube, and make the FBG grating region located at the center of the PI tube. Use a syringe to inject epoxy resin solution into the PI tube, then fix the optical fiber at both ends of the PI tube with glue and slowly bend the PI tube naturally. Place the transmission optical fibers at both ends in a Teflon tube with epoxy resin glue, and then fix the Teflon tube, the bare optical fiber, and both ends of the PI tube on the carbon fiber plate with epoxy resin glue. The fixed lengths are 3 mm, 4 mm, and 4 mm respectively, and the depth is the depth of the groove. After the glue dries on the surface at room temperature, put it into a drying oven and heat it at 45°C for 1 hour to accelerate the curing of the glue. After 1 hour, connect this structure to the fiber Bragg grating demodulator, and then pull the Teflon tubes at both ends while observing the change in the FBG wavelength signal. If the change is not significant, it means the glue has been completely fixed and can be used for overall molding and encapsulation.

[0021] Preferably, the parameters in the hot press in the seventh step are as follows: The pressure is 0.5 MPa. After the heating temperature rises from room temperature to 120°C, keep it at a constant temperature for 20 minutes, and then demold it after natural cooling at room temperature.

[0022] (III) Beneficial effects

[0023] Compared with the prior art, the present invention provides a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation and a preparation method, having the following beneficial effects:

[0024] 1. For the sensor pasted on the surface of the blade, when the blade deforms, the special bending radian of the sensing optical fiber segment in the concave cavity of the lower cavity plate of the sensor ensures the free expansion and contraction of the fiber Bragg grating in the environment from low temperature to high temperature (-40°C to 80°C), without generating additional strain, and solves the influence of the thermal effect of the fiber Bragg grating superimposed on the external strain.

[0025] 2. For the fiber Bragg grating temperature sensor with wide-temperature-range strain isolation and the preparation method, the ultra-thin structure design (2 mm) of the upper skin can achieve conformal installation on the surface of the wind turbine blade, avoiding affecting the aerodynamic performance of the wind turbine blade. Its "cap-shaped" shape design facilitates the more uniform coating of the blade coating on the sensor.

[0026] 3. The fiber Bragg grating temperature sensor with wide-temperature-range strain isolation and its preparation method can isolate the large-strain interference caused by the deformation of the object to be measured and improve the temperature measurement accuracy of the fiber Bragg grating sensor by adding a damping layer, the bending design of the lower cavity plate and the upper skin on the basis of the substrate packaging of the fiber Bragg grating sensor.

[0027] 4. The fiber Bragg grating temperature sensor with wide-temperature-range strain isolation and its preparation method are more suitable for outdoor work than ordinary electrical signal temperature sensors, have good weather resistance, do not require waterproof and lightning protection treatments, have high sensitivity, and have fewer wiring connections. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the present invention;

[0029] Figure 2 It is a split schematic diagram of the present invention;

[0030] Figure 3 They are schematic diagrams of the upper template and the lower template;

[0031] Figure 4 They are schematic diagrams of the upper template and the lower template;

[0032] Figure 5 It is a schematic diagram of the damping layer;

[0033] Figure 6 It is a top view schematic diagram of the lower cavity plate;

[0034] Figure 7 It is a schematic diagram of the upper skin;

[0035] Figure 8 It is a schematic diagram of the upper cover plate;

[0036] Figure 9 It is a schematic diagram of the substrate.

[0037] In the figure: 1. Upper skin; 2. Upper cover plate; 3. Lower cavity plate; 4. Damping layer; 5. Substrate; 6. Optical fiber; 7. Teflon tube; 8. PI tube; 9. FBG grating area; 10. Upper template; 11. Lower template. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] Please refer to Figures 1-9, A fiber Bragg grating temperature sensor with wide-temperature-range strain isolation, comprising a substrate 5, a damping layer 4 is provided on the substrate 5, a lower cavity plate 3 is provided on the damping layer 4, the lower cavity plate 3 is provided with a cavity structure with one side being flat and the other side being arc-shaped, an optical fiber 6 is arranged inside the cavity structure, the middle section of the optical fiber 6 is encapsulated by a PI tube 8, the part of the optical fiber 6 extending out of the cavity structure is an optical fiber transmission section, the optical fiber transmission section is encapsulated by a Teflon tube 7, there is an FBG grating region 9 at the central position inside the PI tube 8, an upper cover plate 2 is provided on the lower cavity plate 3, and an upper skin 1 is provided on the upper cover plate 2.

[0040] A preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation, comprising the following steps:

[0041] Prepare the upper skin 1, cut a woven glass fiber cloth with a size of 106mm * 106mm and lay it on the lower template 11 of the sensor packaging mold, then use a silicone brush to apply waterborne polyurethane on the fiber cloth to make the fiber cloth fully fit with the lower template 11. After the application is completed, let it stand for 1 hour until the polyurethane cures. The demolded upper skin 1 is a "cap-shaped" structure with an overall thickness of 0.15mm. A fillet with a radius of 2mm is selected for the transition region between the top surface and the side surface and between the bottom surface and the side surface of the structure. One of the purposes is to make the woven glass fiber cloth fit better with the mold and avoid wrinkles when it is laid in the mold. Another purpose is to reduce its impact on the aerodynamic performance of the blade when it is attached to the blade surface.

[0042] Below the upper skin 1 is the upper cover plate 2 and the lower cavity plate 3 structure for fixing the optical fiber 6.

[0043] Fabrication of the upper cover plate 2, cut two pieces of unidirectional carbon fiber prepreg with a diameter of 79mm and a thickness of 0.15mm, lay them between two Teflon plates in a ply stacking sequence with the fiber directions orthogonal at 90°, and then put them into a hot press for hot pressing and forming. The pressure is 0.3Mpa. After adjusting the heating temperature of the hot press to rise from room temperature to 120°C, keep the temperature for 20min, and finally cool at room temperature. The thickness dimension after hot pressing is 0.2mm, and the structure is as Figure 8 shown.

[0044] Fabrication of the lower cavity plate 3: Cut two pieces of unidirectional carbon fiber prepreg with a diameter of 79 mm and a thickness of 0.15 mm, lay them in the fabrication mold of the lower cavity plate 3 according to the lay-up sequence with the fiber directions orthogonal at 90°, and then put them into a hot press for hot pressing and forming. The pressure is 0.5 Mpa. After adjusting the heating temperature of the hot press to rise from room temperature to 120 °C, keep the temperature for 20 min, and finally cool it at room temperature. The overall thickness of the formed lower cavity plate 3 is 0.2 mm, and it has a cavity structure with an upper concave and lower convex shape in its central area. The cavity depth is 0.7 mm. One side of the cavity is a common plane, and the other side is composed of a plane and a curved surface. The plane and the curved surface are transitioned by a fillet with a radius of 20 mm. The length of the curved surface is 43 mm. The design of the cavity is used to ensure the free expansion and contraction of the FBG grating area 9 at ultra-low temperature and high temperature without generating additional strain, and to solve the influence of the thermal effect superposition strain of the fiber grating.

[0045] Fabrication of the damping layer 4: Take a low-modulus flexible medium. In this case, select 2 g of each of the food-grade silicone A and B components, pour them into a cup and stir evenly. After standing for 2 - 3 minutes, slowly inject the silicone into the fabrication mold of the damping layer 4. You can use tools to level the silicone. Wait for 3 hours at room temperature until the silicone cures, and then slowly demold it as the damping layer 4. The total thickness of the damping layer 4 is 1.5 mm. There is a concave cavity on the upper surface of the damping layer 4 for mating with the lower bottom surface of the lower cavity plate 3, and its size is slightly larger than the cavity of the lower cavity plate 3. The side surface is a curved surface for mating with the upper skin 1. The structure is as Figure 5 shown.

[0046] Fabrication of the substrate 5: Cut a woven glass fiber with a size of 106 mm * 106 mm, take the center of its surface as the origin, and cut off a circular area with a diameter of 87 mm. The remaining material is used as the sensor structure substrate 5.

[0047] Fixing of the optical fiber 6: As Figure 6 shown, put the optical fiber 6 with the FBG grating area 9 into a 55-mm-long PI tube 8, and make the FBG grating area 9 located at the center of the PI tube 8. Use a syringe to inject epoxy resin solution into the PI tube 8, and then fix the optical fiber at both ends of the PI tube 8 with Loctite 480 glue and slowly bend the PI tube 8 naturally. The purpose of bending is to prevent the blade from deforming and causing tensile or compressive forces on the overall structure, thereby resulting in axial forces on the grating area. Next, use epoxy resin glue to place the two ends of the transmission optical fiber 6 in the Teflon tube 7 for protection, and then use epoxy resin glue to fix both ends of the Teflon tube 7, the bare optical fiber 6, and the PI tube 8 on the carbon fiber board. The fixing lengths are 3 mm, 4 mm, and 4 mm respectively, and the depth is the depth of the groove. After the glue dries on the surface at room temperature, put it into a drying oven and heat it at 45 °C for 1 hour to accelerate the curing of the glue. After 1 hour, connect this structure to the fiber grating demodulator, and then pull the two ends of the Teflon tube 7 while observing the change of the FBG wavelength signal. If the change is not significant, it means that the glue has been completely fixed and can be used for overall forming and encapsulation.

[0048] Finally, place them in the following order into the lower template 11 of the sensor packaging assembly: the upper skin 1, the upper cover plate 2, the lower cavity plate 3 with fiber Bragg gratings, the damping layer 4, and the substrate 5. Then cover it with the upper template 10. Finally, put it into a hot press for hot pressing. The pressure is 0.5 MPa. After the heating temperature rises from room temperature to 120 °C, keep it at a constant temperature for 20 min, and then demold after natural cooling at room temperature. Thus, the sensor is fabricated.

[0049] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A fiber Bragg grating temperature sensor with wide-temperature-range strain isolation, characterized in that, It includes a base (5), a damping layer (4) is provided on the base (5), a lower cavity plate (3) is provided on the damping layer (4), the lower cavity plate (3) is provided with a cavity structure with one side being a plane and the other side being arc-shaped, an optical fiber (6) is arranged inside the cavity structure, the middle section of the optical fiber (6) is encapsulated by a PI tube (8), the part of the optical fiber (6) extending out of the cavity structure is an optical fiber transmission section, the optical fiber transmission section is encapsulated by a Teflon tube (7), there is an FBG grating area (9) at the central position inside the PI tube (8), an upper cover plate (2) is provided on the lower cavity plate (3), and an upper skin (1) is provided on the upper cover plate (2).

2. A preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation, characterized in that, It includes the following steps: The first step: Prepare the upper skin (1); The second step: Make the upper cover plate (2); The third step: Make the lower cavity plate (3); The fourth step: Make the damping layer (4); The fifth step: Cut a woven glass fiber, take its surface center as the origin, cut off a circular area with a diameter of 87 mm, and the remaining material is used as the sensor structure base (5); The sixth step: Fix the optical fiber (6); The seventh step: Finally, place the upper skin (1), the upper cover plate (2), the lower cavity plate (3) with an optical fiber grating, the damping layer (4), and the base (5) in sequence in the lower template (11) of the sensor packaging assembly, then cover the upper template (10), finally put it into a hot press for hot pressing, and then demold to obtain a fiber optic grating temperature sensor with wide-temperature-range strain isolation.

3. The preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation according to claim 2, wherein: The first step includes the following content: Cut a woven glass fiber cloth and lay it on the lower template (11) of the sensor packaging mold, then use a silicone brush to apply waterborne polyurethane on the fiber cloth so that the fiber cloth is completely attached to the lower template (11). After the application is completed, let it stand for 1 hour. After the polyurethane cures, the demolded upper skin (1) is a "cap-shaped" structure with an overall thickness of 0.15 mm. A fillet with a radius of 2 mm is selected for the transition area between the top surface and the side surface and between the bottom surface and the side surface of the structure.

4. The preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation according to claim 2, characterized in that: The second step includes the following content: Cut two pieces of unidirectional carbon fiber prepregs with a diameter of 79 mm and a thickness of 0.15 mm, lay them between two Teflon plates in a lay-up sequence with the fiber directions orthogonal at 90°, and then put them into a hot press for hot pressing and forming. The pressure is 0.3 Mpa. After adjusting the heating temperature of the hot press to rise from room temperature to 120 °C, keep the temperature for 20 min, and finally cool at room temperature. The thickness dimension after hot pressing is 0.2 mm to obtain the upper cover plate (2).

5. The preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation according to claim 2, wherein: The third step includes the following content: Cut two pieces of unidirectional carbon fiber prepregs with a diameter of 79 mm and a thickness of 0.15 mm, lay them in the lower cavity plate (3) manufacturing mold in a lay-up sequence with the fiber directions orthogonal at 90°, and then put them into a hot press for hot pressing and forming. The pressure is 0.5 Mpa. After adjusting the heating temperature of the hot press to rise from room temperature to 120 °C, keep the temperature for 20 min, and finally cool at room temperature. The overall thickness of the formed lower cavity plate 3 is 0.2 mm, and it has a cavity structure with an upper concave and lower convex shape in its central area, the cavity depth is 0.7 mm, one side of the cavity is a common plane, and the other side is composed of a plane and a curved surface. The plane and the curved surface are transitioned by a fillet with a radius of 20 mm, and the length of the curved surface is 43 mm.

6. The preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation according to claim 2, characterized in that: The fourth step includes the following: Pour 2 g of each of the food-grade silicone A and B components into a cup, stir well, let stand for 2 - 3 minutes, then slowly inject the silicone into the mold for making the damping layer (4). A tool can be used to level the silicone. Wait for 3 hours at room temperature until the silicone cures, and then slowly demold it to obtain the damping layer (4). The total thickness of the damping layer (4) is 1.5 mm. There is a concave cavity on the upper surface of the damping layer (4) for mating with the lower bottom surface of the lower cavity plate (3), and its size is larger than the cavity of the lower cavity plate (3). The side surface is a curved surface for mating with the upper skin (1).

7. The preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation according to claim 2, characterized in that: The sixth step includes the following: Insert the optical fiber (6) with the FBG grating region (9) into a 55-mm long PI tube (8), and make the FBG grating region (9) located at the center of the PI tube (8). Use a syringe to inject epoxy resin solution into the PI tube (8), then fix the optical fiber (6) at both ends of the PI tube (8) with glue and slowly bend the PI tube (8) naturally. Place the transmission optical fibers (6) at both ends in a Teflon tube (7) with epoxy resin glue, and then fix the Teflon tube (7), the optical fiber (6), and both ends of the PI tube (8) on a carbon fiber board with epoxy resin glue. The fixing lengths are 3 mm, 4 mm, and 4 mm respectively, and the depth is the depth of the groove. After the glue dries on the surface at room temperature, put it into a drying oven and heat it at 45°C for 1 hour to accelerate the curing of the glue. After 1 hour, connect this structure to an optical fiber grating demodulator, and then pull the Teflon tubes (7) at both ends while observing the change in the FBG wavelength signal. If the change is not significant, it means that the glue has been completely fixed and can be used for overall molding and encapsulation.

8. The preparation method of a fiber Bragg grating temperature sensor with wide-temperature-range strain isolation according to claim 2, wherein: The parameters in the hot press in the seventh step are as follows: The pressure is 0.5 MPa. After the heating temperature rises from room temperature to 120°C, keep it at a constant temperature for 20 min, and then demold it after natural cooling at room temperature.

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