Vacuum bag hot press molding method of large-curvature PMI foam board

Abstract

The invention discloses a vacuum bag hot-press forming method for a large-curvature PMI (polymethacrylimide) foam board, which comprises the following steps: placing the PMI foam board subjected to drying treatment at a fixed position of a curved surface forming mold, and preheating the curved surface forming mold; a breathable felt is laid on the surface of the PMI foam board, and thermocouples are inserted into at least three area positions selected on the edge of the PMI foam board; a vacuum nozzle is placed, an extraction opening of the vacuum nozzle is located above the foam, and a vacuum negative pressure monitoring opening is located below the foam; sealing the vacuum bag; the curved surface forming mold is heated, vacuumized, depressurized and cooled, and hot pressing of the PMI foam board is completed; and placing the hot-pressed foam in a dry and ventilated environment for at least 2 days to prepare the large-curvature PMI foam board. The technology is high in operability and low in manufacturing cost, it is ensured that the large-curvature foam interlayer formed through hot pressing meets the machining requirement, the forming period is shortened, and the internal quality requirement of a product is met.

Description

Technical Field

[0001] The invention belongs to the technical field of composite material manufacturing, and relates to a vacuum bag hot pressing molding method for a large curvature PMI foam board, and specifically relates to a vacuum bag hot pressing molding method for a large curvature PMI foam board suitable for composite material foam sandwich structural parts. Background Art

[0002] PMI foam (polymethacrylimide foam) has high compressive strength and stiffness, excellent high temperature resistance and moisture and heat resistance, as well as good high temperature creep resistance and dimensional stability. Its outstanding heat resistance and excellent creep resistance at high temperatures enable it to withstand high temperature curing process conditions and achieve one-time co-curing of the foam core and skin. It is widely used in composite foam sandwich structures in the fields of aviation and aerospace.

[0003] With the development of technology, PMI foam can be machined into various complex shapes. For foam interlayers with small curvature, machining can be completed in one time; but for foam interlayers with large curvature, due to the thickness limitation of PMI foam (the maximum thickness of PMI foam board is about 80mm), it needs to be decomposed into multiple pieces for machining and then spliced ​​and glued into shape, resulting in large material waste, long machining cycle, multiple splicing interfaces, and affecting product quality. Summary of the invention

[0004] In view of the deficiencies in the prior art, in order to make PMI foam boards into large-curvature PMI foam boards required for large-curvature foam sandwich structural parts, the present invention provides a vacuum bag hot pressing molding method for large-curvature PMI foam boards, which has high process operability and low manufacturing cost, ensures that the hot-pressed large-curvature foam interlayer meets the machining requirements, shortens the molding cycle, and meets the internal quality requirements of the product.

[0005] The invention discloses a vacuum bag hot pressing molding method for a large-curvature PMI foam board, comprising:

[0006] Step 1: placing the dried PMI foam sheet at a fixed position of a curved surface forming mold, and preheating the curved surface forming mold;

[0007] Step 2: Place a breathable felt on the surface of the PMI foam board, and insert thermocouples in at least three areas on the edge of the PMI foam board; place a vacuum nozzle, with the air extraction port of the vacuum nozzle located above the foam and the vacuum negative pressure monitoring port located below the foam; and seal the vacuum bag;

[0008] Step 3, heating, vacuuming, depressurizing and cooling the curved surface forming mold to complete hot pressing of the PMI foam board;

[0009] Step 4: The hot-pressed foam is placed in a dry and ventilated environment for at least 2 days to obtain a large-curvature PMI foam board.

[0010] As a further improvement of the present invention, in step 1:

[0011] The density of PMI foam board is greater than 50Kg / m 3 , heat deformation temperature is 190~205℃;

[0012] The curvature of the curved surface forming mold is consistent with the curvature of the large-curvature PMI foam interlayer to be prepared;

[0013] The process parameters for drying the PMI foam sheet are: drying at 125±5℃ for at least 2h;

[0014] The curved surface forming mold is preheated to a surface temperature of 50-60°C.

[0015] As a further improvement of the present invention, in step 2:

[0016] For areas where the distance between the foam and the curved surface forming mold is large and the breathable felt is easily sucked under the foam during the hot pressing process, causing air leakage in the vacuum package: use high-temperature tape with a temperature resistance requirement of ≥230℃ to fix the breathable felt in a vertical and horizontal staggered manner, and cut the breathable felt in the middle area in the longitudinal direction of the breathable felt, and overlap it with a length of not less than 100mm to reserve a margin for foam deformation during hot pressing.

[0017] As a further improvement of the present invention, in step 2:

[0018] Thermocouples are arranged at at least three selected areas in the thickness direction, at 1 / 4 and 1 / 2 from the bottom of the foam to the top; the thermocouple at 1 / 2 is used to monitor the temperature at which the foam reaches thermal deformation, with the lower temperature being the standard; the thermocouple at 1 / 4 is used to monitor the temperature during cooling, with the higher temperature being the standard.

[0019] As a further improvement of the present invention, in step 2:

[0020] The vacuum nozzle shall have no less than 2 air extraction ports and 1 vacuum negative pressure monitoring port.

[0021] As a further improvement of the present invention, in step 2:

[0022] At least three sealing strips are used to seal the vacuum bag, and sealing strips with a temperature resistance of more than 230°C and those with a temperature resistance of more than 350°C are used alternately. In order to avoid the pleating position being torn due to insufficient residual volume in the local vacuum bag during vacuuming, the pleats of the vacuum bag must be evenly and reasonably distributed: with consistent height and uniform spacing, and vertical tape is added on both sides of the pleats to reinforce them to reduce the risk of air leakage at the pleated position.

[0023] As a further improvement of the present invention, in step 3:

[0024] During the heating process, the oven heating parameters are set as follows: room temperature → heating to 180°C → keeping warm for 30 to 60 minutes → heating to 220°C → keeping warm until the lowest temperature monitored by the thermocouple at 1 / 2 is ≥192°C.

[0025] As a further improvement of the present invention, in step 3:

[0026] During the vacuuming and pressurizing process, open the vacuum valve at the exhaust port, maintain the vacuum pressure at -0.01±0.001MPa, and control the vacuum negative pressure flow to keep the exhaust rate uniform; after the edge of the foam fits the mold surface, the vacuum pressure is reduced to -0.02±0.002MPa. As the negative pressure in the vacuum bag gradually increases, the foam area with large curvature fits the mold surface; slowly reduce the vacuum pressure, and when the pressure in the vacuum bag is ≤-0.06MPa, fully open the vacuum valve.

[0027] As a further improvement of the present invention, in step 3:

[0028] During the cooling process, keep the oven heating system stopped while maintaining the vacuum negative pressure. When the maximum temperature of the thermocouple at 1 / 4 is ≤120℃, stop the vacuum system. When the maximum temperature of the thermocouple at 1 / 4 is ≤60℃, demoulding is completed.

[0029] As a further improvement of the present invention, it also includes:

[0030] Step 5: Return the prepared large curvature PMI foam board to the mold and monitor the gap between it and the mold, which is required to be ≤3mm; the appearance requires uniform color and no obvious cracks.

[0031] As a further improvement of the present invention, the qualified large curvature PMI foam board is used as the core of the composite foam sandwich structure, and the PMI foam board is co-cured with the skin at one time to obtain the composite foam sandwich structure; wherein the skin includes one of glass fiber cloth and carbon fiber cloth, and the molding process includes one of VARI, autoclave, molding, and vacuum bag pressing.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention selects a high-temperature hot pressing molding process method and adopts improved process parameters. By designing a curved molding mold with the same curvature as the foam interlayer, improving the fixing method of auxiliary materials, reasonably arranging the positions of thermocouples and exhaust ports, controlling the vacuum negative pressure flow at different stages, setting a stress release cycle and other effective measures, not only the quality of foam hot pressing molding is guaranteed, and the gap between the hot-pressed large-curvature foam interlayer and the mold meets the machining requirements, thereby improving the subsequent machining efficiency, but also the number of foam interlayer blocks is reduced to ensure improved material utilization and improve the internal quality of the product. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention is clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] The present invention provides a vacuum bag hot pressing molding method for a large curvature PMI foam board, comprising: a preparation process, a packaging process, a hot pressing process, a static process and an inspection process; wherein:

[0036] Step 1: Preparation process:

[0037] Dry the PMI foam sheet at 125±5℃ for at least 2h, then place the dried PMI foam sheet in a fixed position of a curved surface forming mold, and preheat the curved surface forming mold to a surface temperature of 50-60℃; wherein the density of the PMI foam sheet is greater than 50Kg / m 3 , the heat deformation temperature is 190-205°C; the curvature of the curved surface forming mold is consistent with the curvature of the large curvature PMI foam interlayer to be prepared;

[0038] Step 2: Packaging process:

[0039] 1) Lay the breathable felt on the surface of the PMI foam sheet: For areas where the distance between the foam and the curved surface forming mold is large and the breathable felt is easily drawn under the foam during the hot pressing process, causing vacuum packaging leakage: Use high-temperature tape with a temperature resistance requirement of ≥230℃ to fix the breathable felt in a vertical and horizontal staggered manner, and disconnect the breathable felt in the middle area of ​​the longitudinal direction of the breathable felt, and do an overlap treatment with an overlap length of not less than 100mm to reserve a margin for foam deformation during hot pressing;

[0040] 2) Insert thermocouples at at least three locations on the edge of the PMI foam sheet; thermocouples are arranged at 1 / 4 and 1 / 2 of the thickness direction from the bottom of the foam; the thermocouple at 1 / 2 is used to monitor the temperature at which the foam reaches thermal deformation, with the lower temperature being the standard; the thermocouple at 1 / 4 is used to monitor the temperature during cooling, with the higher temperature being the standard;

[0041] 3) Place the vacuum nozzle, with the air extraction port of the vacuum nozzle located above the foam and the vacuum negative pressure monitoring port located below the foam; the vacuum nozzle shall have no less than 2 air extraction ports and 1 vacuum negative pressure monitoring port;

[0042] 4) Use at least 3 sealing strips to seal the vacuum bag, and use sealing strips with a temperature resistance of more than 230°C and those with a temperature resistance of more than 350°C alternately; In order to avoid the pleating position being torn due to insufficient residual volume of the local vacuum bag during vacuuming, the pleating of the vacuum bag is required to be evenly and reasonably distributed: with consistent height and uniform spacing, and vertical tape is added on both sides of the pleats to reinforce them to reduce the risk of air leakage at the pleating position;

[0043] Step 3: Hot pressing process:

[0044] 1) Heating: set the oven heating parameters as follows: room temperature → heating to 180°C → keeping warm for 30-60 minutes → heating to 220°C → keeping warm until the lowest temperature monitored by the thermocouple at 1 / 2 is ≥192°C; the maximum operating temperature of the oven is not less than 250°C, and the temperature uniformity is required to be ±3°C;

[0045] 2) Vacuum and pressurize, open the vacuum valve at the exhaust port, keep the vacuum pressure at -0.01±0.001MPa, control the vacuum negative pressure flow, and keep the exhaust rate uniform; after the edge of the foam fits the mold surface, the vacuum pressure is reduced to -0.02±0.002MPa, and as the negative pressure in the vacuum bag gradually increases, the foam large curvature area fits the mold surface; slowly reduce the vacuum pressure, and when the pressure in the vacuum bag is ≤-0.06MPa, the vacuum valve is fully opened; the negative pressure in the vacuum bag is not less than -0.1MPa, the maximum range of the vacuum negative pressure gauge is -0.1MPa, and the accuracy is 0.002Mpa;

[0046] 3) Cool down, keep the oven heating system under vacuum negative pressure, and stop the vacuum system when the maximum temperature of the thermocouple at 1 / 4 is ≤120℃; demoulding is completed when the maximum temperature of the thermocouple at 1 / 4 is ≤60℃.

[0047] Step 4: Standing process:

[0048] The foam after hot pressing should be placed in a dry and ventilated environment for at least 2 days to release the stress generated by the high temperature hot pressing process and avoid deformation of the foam after machining;

[0049] Step 5: Inspection process:

[0050] The gap between the large curvature PMI foam board and the mold is monitored and the requirement is ≤3mm. The appearance requires uniform color and no obvious cracks.

[0051] The qualified large curvature PMI foam board is used as the core of the composite foam sandwich structure, and is co-cured with the skin at one time to obtain the composite foam sandwich structure; wherein, the skin can be made of glass fiber cloth, carbon fiber cloth, etc., the resin system can be made of epoxy, phenolic, bismaleimide, etc., and the molding process method can be made of VARI, autoclave, molding, vacuum bag pressing, etc.

[0052] Furthermore, for the foam interlayer with super large curvature, the method of the present invention can be used to form the foam interlayer in blocks by hot pressing, and then machined and glued together to form the foam interlayer.

[0053] Example:

[0054] A vacuum bag hot press molding method for a PMI foam board with a curvature of 1 / 200, comprising:

[0055] S1. Preparation:

[0056] ① Clean the surface of the curved molding mold that is consistent with the curvature of the large curvature foam interlayer, and preheat it in an oven until the surface temperature reaches 50-60℃;

[0057] ② Take the density as 110±10Kg / m 3 1. PMI foam sheet with a thickness of 80mm and 1200*1000mm is placed in an oven for drying: 125±5℃ for 2~2.5h;

[0058] ③ Place the dried PMI foam sheet at the designated position of the curved surface forming mold.

[0059] S2. Packaging:

[0060] ① Place breathable felt on the surface of the PMI foam sheet. For the areas around the PMI foam sheet where the distance between the sheet and the curved surface forming mold is large and the breathable felt is easily drawn under the foam during the hot pressing process, causing air leakage in the vacuum package, use high-temperature tape to fix the breathable felt in a vertical and horizontal staggered manner, and disconnect the breathable felt in the middle area of ​​the vertical and horizontal directions, and overlap it. The overlap length should not be less than 100mm to reserve a margin for foam deformation during hot pressing;

[0061] ② Arrange thermocouples. Insert two thermocouples in the middle of the two long sides and one short side of the edges of the foam. Both thermocouples are inserted in the thickness direction, at 1 / 4 and 1 / 2 from the bottom of the foam. The thermocouple at 1 / 2 monitors the temperature at which the foam reaches thermal deformation, whichever is lower. The thermocouple at 1 / 4 monitors the temperature when the temperature drops, whichever is higher.

[0062] ③ Place the vacuum nozzle, with the air extraction port located above the foam, 3 evenly distributed in a triangle shape, and the vacuum negative pressure monitoring port located below the foam, 1.

[0063] ④ To seal the vacuum bag, use 3 sealing strips, 1 sealing strip with a temperature resistance of more than 230℃ and 2 sealing strips with a temperature resistance of more than 350℃, which are used alternately; in order to avoid the pleating position being torn due to insufficient residual volume of the local vacuum bag during vacuuming, the pleats of the vacuum bag are required to be evenly and reasonably distributed: 6 to 7 pleats on one side, with consistent symmetrical edges, consistent height and uniform spacing, and vertical tape is added on both sides of the pleats to reduce the risk of air leakage at the pleated position.

[0064] S3, hot pressing:

[0065] ① Heating, set the oven heating parameters: room temperature → heating to 180℃, keep warm for 30-60min → heating to 220℃, keep warm for 8-9h, the lowest temperature of the thermocouple at 1 / 2 reaches 195℃;

[0066] ② Vacuum and pressurize, open the vacuum valves at the three exhaust ports, keep the vacuum pressure at -0.01±0.001MPa, control the vacuum negative pressure flow, and keep the exhaust rate uniform; after the edge of the foam fits the mold surface, the vacuum pressure is reduced to -0.02±0.002MPa, and as the negative pressure in the vacuum bag gradually increases, the foam area with large curvature fits the mold surface; slowly reduce the vacuum pressure, and when the pressure in the vacuum bag is ≤-0.06MPa, fully open the vacuum valve; the entire hot pressing takes 15 to 20 minutes;

[0067] ③ Cool down, stop the oven heating system while maintaining the vacuum negative pressure state, and stop the vacuum system when the maximum temperature of the thermocouple at 1 / 4 is ≤120℃; complete demoulding when the maximum temperature of the thermocouple at 1 / 4 is ≤60℃.

[0068] S4, stand still:

[0069] In order to release the stress generated by the high-temperature hot pressing process and avoid deformation of the foam after machining, the hot-pressed foam should be placed in a dry and ventilated environment for 2 days before being transferred to the machining process.

[0070] S5. Inspection:

[0071] Visual inspection shows that there are no crack defects on the outer surface and the color is uniform; after hot pressing, the foam is returned to the mold to monitor the gap between it and the mold, and a feeler gauge is used to evenly measure the gap at points, which is ≤3mm, meeting the technical requirements.

[0072] The advantages of the present invention are:

[0073] The present invention designs a curved surface forming mold, which is consistent with the curvature of the foam interlayer and is used for hot pressing forming foam interlayers with large curvature, reducing the number of blocks or not dividing them, greatly improving the efficiency of foam machining, reducing the waste of foam plates, and reducing labor and material costs; adopting an improved vacuum packaging method to avoid foam bursting caused by inaccurate foam thermal deformation temperature monitoring and unreasonable vacuum port position setting, greatly reducing the risk of air leakage during foam hot pressing, thereby improving the foam hot pressing forming qualification rate; utilizing hot pressing molding at high temperature, heating and cooling based on the temperature at different positions, effectively reflecting the internal temperature of the foam, avoiding foam bursting due to pressurization without heating to the thermal deformation temperature, and foam "rebound" due to pressure relief without cooling to the temperature required by the process, controlling the vacuum negative pressure flow rate through a vacuum valve, and uniform vacuum rate to avoid crack defects on the foam surface, and controlling the vacuum negative pressure in stages according to the bonding state of the foam and the mold surface to ensure the quality of foam hot pressing molding, thereby reducing the machining difficulty and shortening the machining cycle.

[0074] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A vacuum bag hot pressing method for forming a large curvature PMI foam board, characterized in that: include: Step 1: placing the dried PMI foam sheet at a fixed position of a curved surface forming mold, and preheating the curved surface forming mold; Step 2: Place a breathable felt on the surface of the PMI foam board, and insert thermocouples in at least three areas on the edge of the PMI foam board; place a vacuum nozzle, with the air extraction port of the vacuum nozzle located above the foam and the vacuum negative pressure monitoring port located below the foam; and seal the vacuum bag; Step 3, heating, vacuuming, depressurizing and cooling the curved surface forming mold to complete hot pressing of the PMI foam board; Step 4: The hot-pressed foam is placed in a dry and ventilated environment for at least 2 days to obtain a large-curvature PMI foam board.

2. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 1, characterized in that: In step 1: The density of PMI foam board is greater than 50Kg / m 3 , heat deformation temperature is 190~205℃; The curvature of the curved surface forming mold is consistent with the curvature of the large-curvature PMI foam interlayer to be prepared; The process parameters for drying the PMI foam sheet are: drying at 125±5℃ for at least 2h; The curved surface forming mold is preheated to a surface temperature of 50-60°C.

3. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 1, characterized in that: In step 2: For areas where the distance between the foam and the curved surface forming mold is large and the breathable felt is easily sucked under the foam during the hot pressing process, causing vacuum packaging leakage: use high-temperature tape with a temperature resistance requirement of ≥230℃ to fix the breathable felt in a vertical and horizontal staggered manner, and cut the breathable felt in the middle area in the longitudinal direction of the breathable felt, and overlap it with an overlap length of no less than 100mm.

4. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 1, characterized in that: In step 2: Thermocouples are arranged at at least three selected regions in the thickness direction, at 1 / 4 and 1 / 2 from the bottom of the foam to the top; the thermocouple at 1 / 2 is used to monitor the temperature at which the foam reaches thermal deformation, and the thermocouple at 1 / 4 is used to monitor the temperature during cooling.

5. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 1, characterized in that: In step 2: At least three sealing strips are used to seal the vacuum bag, and sealing strips with a temperature resistance of more than 230°C and more than 350°C are used alternately; the pleats of the vacuum bag must be evenly and reasonably distributed: consistent height, uniform spacing, and vertical tape reinforcement is added on both sides of the pleats.

6. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 4, characterized in that: In step 3: During the heating process, the oven heating parameters are set as follows: room temperature → heating to 180°C → keeping warm for 30 to 60 minutes → heating to 220°C → keeping warm until the lowest temperature monitored by the thermocouple at 1 / 2 is ≥192°C.

7. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 4, characterized in that: In step 3: During the vacuuming and pressurizing process, open the vacuum valve at the exhaust port, maintain the vacuum pressure at -0.01±0.001MPa, and control the vacuum negative pressure flow to keep the exhaust rate uniform; after the edge of the foam fits the mold surface, the vacuum pressure is reduced to -0.02±0.002MPa. As the negative pressure in the vacuum bag gradually increases, the foam area with large curvature fits the mold surface; slowly reduce the vacuum pressure, and when the pressure in the vacuum bag is ≤-0.06MPa, fully open the vacuum valve.

8. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 4, characterized in that: In step 3: During the cooling process, keep the oven heating system stopped while maintaining the vacuum negative pressure. When the maximum temperature of the thermocouple at 1 / 4 is ≤120℃, stop the vacuum system. When the maximum temperature of the thermocouple at 1 / 4 is ≤60℃, demoulding is completed.

9. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 1, characterized in that: Also includes: Step 5: Return the prepared large curvature PMI foam board to the mold and monitor the gap between it and the mold, which is required to be ≤3mm; The appearance requires uniform color and no obvious cracks.

10. The vacuum bag hot pressing forming method of the large curvature PMI foam board according to claim 1 or 9, characterized in that: The qualified large curvature PMI foam board is used as the core of the composite foam sandwich structure, and is co-cured with the skin at one time to obtain the composite foam sandwich structure; wherein the skin includes one of glass fiber cloth and carbon fiber cloth, and the molding process includes one of VARI, autoclave, molding, and vacuum bag pressing.