Forming process of natural plant fiber-unsaturated polyester composite door sheet

Abstract

The invention belongs to the technical field of composite material forming, and particularly relates to a forming process of a natural plant fiber-unsaturated polyester composite door sheet, which comprises the following steps: opening natural plant fibers, drying, activating with alkali liquor, modifying by acetylation and grafting with a coupling agent to prepare modified plant fibers; performing ultrasonic dispersion on the raw materials, unsaturated polyester resin and the like to prepare a premix; laying a premix and a light core material in the mold in a layered manner, and pre-burying a high-strength insert; a finished product is obtained through dynamic pressing of pressurizing and mold filling, pressure relief and exhaust and high-pressure curing and then gradient cooling and aftertreatment. The invention solves the problems of poor compatibility of plant fiber and resin, easy generation of bubbles during molding and the like, and the product has the advantages of environmental protection, light weight, high strength, stable size and firm installation, and can be widely applied to the fields of building doors and windows, furniture and the like.

Description

Technical Field

[0001] This invention belongs to the field of composite material molding technology, specifically relating to a molding process for a natural plant fiber-unsaturated polyester composite door panel. Background Technology

[0002] Currently, the mainstream door panel reinforcement fibers on the market are mostly artificial synthetic fibers such as glass fiber and basalt fiber. Although these materials can guarantee certain mechanical properties, they have the defect of being non-degradable. Furthermore, during production, processing, and use, synthetic fiber dust can easily irritate human skin and respiratory tract, and long-term exposure may cause health problems.

[0003] Natural plant fibers, with their advantages of being ecological, environmentally friendly, and inexpensive, are gradually becoming an ideal alternative to synthetic fibers. Combining natural plant fibers with unsaturated polyester resin, zinc stearate, and other ingredients, and manufacturing door panels through compression molding, not only aligns with environmental policies but also reduces production costs. Furthermore, the products can be widely used in everyday applications such as building doors and windows, and furniture, indicating a promising market prospect.

[0004] However, in practical applications, natural plant fibers are rich in hydroxyl groups on their surface and have strong polarity, resulting in poor compatibility with non-polar unsaturated polyester resins. This leads to problems such as fiber-resin separation and insufficient mechanical strength in composite materials. Furthermore, existing technologies for modifying plant fibers are mostly single-treatment methods, which cannot simultaneously solve the comprehensive problems of compatibility, hygroscopicity, and mechanical stability. Summary of the Invention

[0005] In view of the above-mentioned shortcomings in the prior art, the present invention provides a molding process for natural plant fiber-unsaturated polyester composite door panels to solve the problems in the background art.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A molding process for a natural plant fiber-unsaturated polyester composite door panel includes the following steps: S1, after opening and drying natural plant fibers, they are successively subjected to alkali activation treatment and coupling agent grafting coating treatment to prepare modified plant fibers with active functional groups on the surface. S2, unsaturated polyester resin, low shrinkage additive, initiator and rheology modifier are mixed and dispersed by high-speed shearing to form a homogeneous resin slurry. Then the modified plant fiber is impregnated in the homogeneous resin slurry to prepare a fiber-reinforced resin premix. S3, in the preheated door panel mold cavity, adopts a layered laying method, sequentially laying the bottom layer fiber-reinforced resin premix, the lightweight structural core material and the surface layer fiber-reinforced resin premix, and pre-embedding high-strength inserts at the lock and hinge positions; S4. After the mold is closed, a dynamic pressing process is performed, which involves pressurizing the rheological filling, depressurizing and venting, and holding the pressure to solidify, so that the material flows to wrap the core material and releases volatiles. S5, after pressure holding and curing, keep the mold closed or transfer to the shaping tooling for staged gradient cooling. After the door panel temperature drops below the resin heat distortion temperature, open the mold and take out the part. After post-processing, the composite door panel is obtained.

[0007] Further, in step S1, the natural plant fiber is a mixture of wood fiber and long fiber, wherein the long fiber is selected from one or more of sisal fiber, jute fiber, and flax fiber; the alkaline activation treatment uses NaOH solution to remove pectin and lignin from the fiber surface; the coupling agent grafting coating treatment uses silane coupling agent or isocyanate coupling agent to chemically bond with the hydroxyl groups on the fiber surface.

[0008] Furthermore, in step S1, after the alkaline activation treatment and before the coupling agent grafting, an acetylation modification step is also included: the alkaline-treated fiber is immersed in a 10%-15% acetic anhydride solution and esterified at 50-60°C for 40-60 minutes.

[0009] Furthermore, in step S2, the rheology modifier comprises zinc stearate or calcium stearate, and an ultrasonic-assisted dispersion process is introduced when preparing the fiber-reinforced resin premix, with an ultrasonic frequency of 20-40kHz, a power of 300-500W, and a dispersion time of 15-25min.

[0010] Furthermore, in step S2, the rheology modifier comprises zinc stearate or calcium stearate, and an ultrasonic-assisted dispersion process is introduced when preparing the fiber-reinforced resin premix, with an ultrasonic frequency of 20-40kHz, a power of 300-500W, and a dispersion time of 15-25min.

[0011] Furthermore, during layered installation, the projected area of ​​the lightweight structural core material is smaller than the projected area of ​​the mold cavity to ensure that the upper and lower premixed materials meet and seal at the edges during molding.

[0012] Furthermore, in step S4, the specific process of the dynamic pressing process is as follows: Pressurized rheological filling: Apply an initial pressure of 2-5 MPa and hold for 10-30 seconds to allow the premix to soften and flow, filling the gap between the core material and the mold. Pressure relief and exhaust: Quickly reduce the pressure to 0-0.5MPa, and control the upper mold to open slightly by 1-3mm for 2-5 seconds to exhaust the water vapor and small molecule volatiles released by the plant fibers when heated; High-pressure curing: Close the mold again and apply a pressure of 8-15 MPa, and maintain at 120-145℃ for 3-10 minutes until the resin crosslinks and cures.

[0013] Further, in step S5, the staged gradient cooling refers to cooling the door panel at a curing temperature of 120-145℃ at a cooling rate of 2-5℃ / min. The door panel temperature is reduced to below the resin heat distortion temperature by means of the temperature-controlled oil channel in the mold or the air-cooling system of the shaping fixture, and then the pressure is released.

[0014] Furthermore, post-processing includes: The surface of the door panel is sanded and leveled, and then decorated with a spraying or heat transfer process. The edges of the door panel are coated with a color paste of the same material as the base resin for a second sealing process.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. A multi-stage fiber modification + fiber hybridization approach significantly improves the overall performance of composite materials. Through a three-stage treatment process involving alkali activation, acetylation modification, and coupling agent grafting, combined with a hybrid system of wood fibers and sisal / jute long fibers, the interfacial compatibility between natural plant fibers and unsaturated polyester resin is effectively improved, fiber hygroscopicity is reduced, and the mechanical strength and dimensional stability of the material are enhanced. This solves the problems of easy deformation and insufficient strength in traditional plant fiber composite materials.

[0016] 2. Innovative dynamic pressing process ensures stable door panel molding quality. A complete process of "pressurized rheological filling → pressure release and venting → high-pressure holding and curing" is designed to specifically remove water vapor and small volatile molecules generated during the molding process, avoiding defects such as bubbles and pores inside the door panel. At the same time, it ensures that the material fully wraps the lightweight core material, resulting in a smooth and flat product surface, high density, and strong consistency in molding quality.

[0017] 3. The design employs a "layered installation + embedded inserts + secondary edge sealing" approach to achieve multi-functional balance. The layered installation structure, combined with a lightweight honeycomb core, ensures structural strength while reducing the weight of the door panel. The embedded high-strength inserts with countersunk teeth enhance the robustness of the lock and hinge connections. The secondary edge sealing process improves the waterproof sealing of the edges, giving the door panel the characteristics of being lightweight, high-strength, reliable in installation, and durable, adapting to various usage scenarios. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the molding process of a natural plant fiber-unsaturated polyester composite door panel according to the present invention. Detailed Implementation

[0019] To enable those skilled in the art to better understand the present invention, the technical solution of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual images. They should not be construed as limiting the scope of this application. To better illustrate the embodiments of the present invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0021] It should be noted that the raw materials used in this embodiment are all commercially available products, and the equipment used is conventional equipment in the field of composite material molding, such as opening machines, drying ovens, high-speed shear mixers, compression molding machines with temperature control and pressure regulation functions, shaping tooling, etc.

[0022] like Figure 1 As shown, this invention discloses a molding process for a natural plant fiber-unsaturated polyester composite door panel, comprising the following steps: S1, after opening and drying natural plant fibers, they are successively subjected to alkali activation treatment and coupling agent grafting coating treatment to prepare modified plant fibers with active functional groups on the surface. S2, unsaturated polyester resin, low shrinkage additive, initiator and rheology modifier are mixed and dispersed by high-speed shearing to form a homogeneous resin slurry. Then the modified plant fiber is impregnated in the homogeneous resin slurry to prepare a fiber-reinforced resin premix. S3, in the preheated door panel mold cavity, adopts a layered laying method, sequentially laying the bottom layer fiber-reinforced resin premix, the lightweight structural core material and the surface layer fiber-reinforced resin premix, and pre-embedding high-strength inserts at the lock and hinge positions; S4. After the mold is closed, a dynamic pressing process is performed, which involves pressurizing the rheological filling, depressurizing and venting, and holding the pressure to solidify, so that the material flows to wrap the core material and releases volatiles. S5, after pressure holding and curing, keep the mold closed or transfer to the shaping tooling for staged gradient cooling. After the door panel temperature drops below the resin heat distortion temperature, open the mold and take out the part. After post-processing, the composite door panel is obtained.

[0023] Specifically, in this embodiment, step S1 first requires selecting natural plant fiber raw materials. To address the issue of unstable performance of single plant fibers, a hybrid system of wood fiber and long fiber is adopted. The wood fiber used is poplar fiber, and the long fiber is a mixture of sisal and jute fibers. The mass ratio of wood fiber to long fiber is 7:3, and the mass ratio of sisal fiber to jute fiber is 1:1. The above-mentioned mixed fibers are fed into an opening machine for opening treatment, dispersing the fiber clumps into loose single fibers or fiber bundles. After opening, the fibers are sent to a drying oven for drying treatment. The drying temperature is set at 105℃, and the drying time is 2 hours to ensure that the fiber moisture content is reduced, avoiding defects such as bubbles caused by moisture evaporation during subsequent molding processes.

[0024] The dried fibers are first subjected to alkaline activation treatment using a 5% sodium hydroxide solution. The fibers are completely immersed in the solution at a bath ratio of 1:20 for 60 minutes at room temperature. The alkalinity of the sodium hydroxide solution removes pectin, lignin, and some impurities adhering to the fiber surface. Simultaneously, it breaks down the cellulose molecular chains on the fiber surface, increasing the number of hydroxyl groups and improving surface roughness. This provides more active sites for subsequent modification treatments and resolves the compatibility issue between plant fibers and non-polar unsaturated polyester resin.

[0025] After alkaline activation treatment, the fibers are removed and repeatedly rinsed with deionized water until the washing solution is neutral, followed by acetylation modification. A 12% (w / w) acetic anhydride solution is prepared as the acetylation treatment solution. The washed fibers are completely immersed in the treatment solution at a bath ratio of 1:15. The system is then placed in a constant temperature water bath at 55°C for 50 minutes. Through the acetylation reaction, the hydroxyl groups on the fiber surface undergo esterification with acetic anhydride, introducing hydrophobic acetyl groups. This effectively reduces the hygroscopicity of the fibers, improves the dimensional stability of the subsequently prepared composite material, and reduces the risk of deformation during use.

[0026] After the acetylation reaction, the fibers were removed and dried in a 60°C oven to constant weight, followed by coupling agent grafting treatment. In this embodiment, silane coupling agent KH550 was selected. The coupling agent was diluted with anhydrous ethanol to prepare a 3% (w / w) treatment solution with a bath ratio of 1:18. The dried fibers were immersed in the treatment solution and stirred at room temperature for 30 minutes. One end of the silane coupling agent can chemically bond with the hydroxyl groups on the fiber surface, while the other end can crosslink with the double bonds in the subsequent unsaturated polyester resin. This "molecular bridge" effect significantly improves the interfacial bonding strength between the natural plant fiber and the unsaturated polyester resin, preventing fiber-resin delamination under stress and thus improving the overall mechanical properties of the composite material.

[0027] After the coupling agent treatment is completed, the fiber is taken out and dried at 80°C for 30 minutes to obtain modified plant fiber with active functional groups on the surface, which is then ready for use.

[0028] In step S2, raw materials are selected according to a preset ratio, including 100 parts of unsaturated polyester resin, 8 parts of low-shrinkage additive, 2 parts of initiator, and 3 parts of rheology modifier. In this embodiment, the unsaturated polyester resin is type 196 weather-resistant unsaturated polyester resin; the low-shrinkage additive can be a polystyrene-based low-shrinkage agent, which can effectively inhibit volume shrinkage during resin curing and prevent dents and cracks on the surface of the product; the initiator can be a 50% concentration of methyl ethyl ketone peroxide solution, used in combination with cobalt isooctanoate accelerator at a mass ratio of 3:1. First, cobalt isooctanoate is mixed evenly with the resin slurry, and then methyl ethyl ketone peroxide is added in portions. It can quickly decompose at the molding temperature to generate free radicals, initiating the cross-linking and curing reaction of the unsaturated polyester resin; the rheology modifier can be zinc stearate. The above-mentioned unsaturated polyester resin, low-shrinkage additive, initiator, and zinc stearate are added sequentially to a high-speed shear mixer, with the rotation speed set to 2500 rpm and the shearing dispersion time set to 15 minutes, so that the components are fully mixed to form a homogeneous resin slurry. The addition of low-shrinkage additives can effectively reduce the shrinkage rate during the resin curing process, avoiding defects such as surface depressions and cracks after the composite door panels are formed; zinc stearate, as a rheology modifier, can improve the flow properties of the resin slurry, facilitating subsequent wetting and mixing with fibers and material flow during the molding process.

[0029] The modified plant fibers prepared in step one were added to the homogeneous resin slurry described above, with a mass ratio of modified plant fibers to resin slurry of 1:1.2. The mixture was then placed in an ultrasonic dispersion device for ultrasonic-assisted dispersion treatment, with the ultrasonic frequency set to 30 kHz, power to 400 W, and dispersion time to 20 minutes. The ultrasonic vibration effectively broke up agglomerates in the resin slurry and promoted the resin slurry to fully penetrate the surface and internal pores of the modified plant fibers, ensuring uniform mixing of fibers and resin and avoiding localized fiber agglomeration or uneven resin distribution. After ultrasonic-assisted dispersion, a uniform fiber-reinforced resin premix was obtained for later use.

[0030] In step S3, the door panel mold is preheated to 80°C and maintained at a constant temperature. Preheating facilitates the softening and flow of the fiber-reinforced resin premix, improving molding efficiency. Subsequently, a layered application method is used, sequentially laying the bottom layer of fiber-reinforced resin premix, the lightweight structural core material, and the top layer of fiber-reinforced resin premix within the mold cavity. The bottom and top layers of fiber-reinforced resin premix have identical compositions, both being the fiber-reinforced resin premix prepared in step two, with a thickness of 5 mm. This design ensures uniform mechanical properties on both the upper and lower surfaces of the door panel and simplifies the production process. The lightweight structural core material is a plant fiber-resin composite honeycomb core with a pore size of 8 mm, a wall thickness of 0.5 mm, and a compressive strength ≥15 MPa, sufficient to withstand holding pressure and ensure no internal collapse after molding. This core material is lightweight and high-strength, significantly reducing the overall weight of the door panel while maintaining its mechanical properties, achieving a lightweight design.

[0031] When laying the lightweight structural core material, its projected area is controlled to be smaller than the projected area of ​​the mold cavity, with a difference of 5 cm x 5 cm. This design ensures that the fiber-reinforced resin premixes of the upper and lower layers can fully converge at the edge of the mold cavity during subsequent molding, achieving automatic edge sealing, improving the density and waterproof performance of the door panel edges, and preventing gaps or warping at the edges. Simultaneously, according to the design dimensions of the door panel and the installation positions of the locks and hinges, high-strength inserts are pre-embedded in the corresponding areas. These inserts are made of stainless steel, with a thickness matching the thickness of the door panel core layer, and the surface of the insert has a reverse tooth structure. The pre-embedded fixing method adopts "mold positioning pin + local pre-compression fixing": positioning pins are preset in the mold cavity corresponding to the position of the insert. The gap between the positioning pin and the reserved installation hole of the insert is ≤0.2mm. After the insert is placed, it is accurately positioned by the positioning pin. Then, a 2-3mm thick fiber-reinforced resin premix is ​​laid around the insert. It is then lightly pressed with a special pressure block until the premix is ​​dense. During the molding process, the premix fills the gap between the teeth on the surface of the insert, forming a mechanical interlock with the insert. At the same time, the resin and the oxide layer on the surface of the insert form an interface bond, which can offset the difference in thermal expansion coefficient between plant fiber and metal, and prevent the insert from falling off under high and low temperature cycles. By pre-embedding the insert, the connection strength of the lock and hinge after installation can be significantly improved, ensuring the reliability of the door panel.

[0032] In step S4, after the paving is completed, the mold is closed, and the compression molding machine is started to perform a dynamic pressing process of pressurized rheological filling, depressurization and venting, and high-pressure holding and curing. First, the pressurized rheological filling stage is carried out, and an initial pressure of 3 MPa is applied to the mold and maintained for 20 seconds. During this process, the fiber-reinforced resin premix inside the mold softens due to heat and flows slowly under pressure, fully filling the gap between the lightweight structural core material and the mold, ensuring that the core material is completely wrapped by the material.

[0033] The pressure relief and venting phase then begins, rapidly reducing the mold pressure to 0.3 MPa and controlling the upper mold to open slightly by 2 mm, maintaining this state for 3 seconds. Since natural plant fibers release a small amount of water vapor during heating, and the resin system also produces some small-molecule volatiles during the reaction, the pressure relief and slight mold opening allow these water vapors and volatiles to be quickly expelled from the mold, preventing them from remaining inside the composite material and forming defects such as bubbles and pores, thus improving the density and mechanical properties of the composite door panel.

[0034] After the pressure is released and the air is vented, the high-pressure holding and curing stage begins. The mold is closed again and a pressure of 12 MPa is applied to the mold. At the same time, the mold temperature is raised to 135°C and maintained at this pressure and temperature for 6 minutes to allow the unsaturated polyester resin to undergo a cross-linking and curing reaction, forming a stable three-dimensional network structure that firmly combines the natural plant fiber and the lightweight structural core material into a whole.

[0035] In step S5, after high-pressure holding and curing, the mold is kept closed, and the temperature is gradually reduced in stages through the temperature-controlled oil channels within the mold. Specifically, the mold temperature is first reduced from 135°C to 90°C at a cooling rate of 3°C per minute, then reduced to 60°C at a cooling rate of 4°C per minute, and finally the door panel temperature is reduced to below the heat distortion temperature of the 196 type unsaturated polyester resin used in this embodiment (measured according to GB / T 1634 standard, the heat distortion temperature of this resin is 85°C). This gradient cooling method effectively reduces internal temperature stress in the door panel, avoiding deformation and cracking caused by rapid cooling, and ensuring the dimensional accuracy and shape stability of the door panel.

[0036] Once the door panel reaches the required temperature, the mold pressure is released, the mold is opened, and the panel is removed for post-processing. The post-processing begins with sanding the surface of the door panel. Coarse and fine sandpaper are used sequentially to remove burrs, bumps, and other defects, resulting in a smooth and even surface. Subsequently, a heat transfer process is applied to decorate the surface. The heat transfer temperature is set to 160℃ for 20 seconds, ensuring the patterns on the transfer film adhere firmly to the door panel surface, enhancing its aesthetics and decorative effect.

[0037] Finally, a second edge-sealing coating is applied to the door edges. A colorant (60% solids concentration) of the same material as the unsaturated polyester resin base is used. The colorant is evenly applied to the door edges using a brush, with a coating thickness of 0.5 mm. After coating, it is left to dry at room temperature for 30 minutes. This second edge-sealing treatment further enhances the waterproof and sealing performance of the door edges, while also ensuring the edge color matches the surface, improving the overall aesthetics.

[0038] After all the above steps, the final product is a natural plant fiber-unsaturated polyester composite door panel. This product is lightweight and high-strength, with high dimensional accuracy, a smooth and beautiful surface, securely installed locks and hinges, excellent waterproof performance and durability, and can be widely used in building doors and windows, furniture and other fields.

[0039] Example 1 This embodiment represents a preferred solution for the molding process of natural plant fiber-unsaturated polyester composite door panels. The combination of its process parameters can achieve an optimal balance between product performance and production efficiency. The specific steps are as follows: Step S1: Select natural plant fiber raw materials: A hybrid system of poplar wood fiber and sisal-jute long fiber is adopted, wherein the mass ratio of wood fiber to long fiber is 7:3, and the mass ratio of sisal fiber to jute fiber is 1:1. The mixed fibers are fed into an opening machine to open into loose fiber bundles, and then sent to a drying oven to dry at 105℃ for 2 hours to reduce the fiber moisture content to 5%-8%.

[0040] The dried fibers are then subjected to alkaline activation treatment: a 5% NaOH solution with a bath ratio of 1:20 is used to soak the fibers at room temperature for 60 minutes to remove pectin and lignin from the fiber surface, followed by rinsing with deionized water until neutral.

[0041] Next, acetylation modification was carried out: the fiber was immersed in a 12% acetic anhydride solution (bath ratio 1:15) and reacted in a constant temperature water bath at 55°C for 50 minutes to introduce hydrophobic acetyl groups, and then dried at 60°C to constant weight.

[0042] Finally, coupling agent grafting was performed: silane coupling agent KH550 was selected and prepared into an anhydrous ethanol solution with a mass concentration of 3% (bath ratio 1:18). The solution was stirred at room temperature for 30 minutes and then dried at 80℃ for 30 minutes to obtain modified plant fibers with active functional groups on the surface.

[0043] Step S2: Take the following raw materials according to the specified ratio: 100 parts of type 196 weather-resistant unsaturated polyester resin, 8 parts of polystyrene-based low-shrinkage additive, 2 parts of 50% concentration methyl ethyl ketone peroxide initiator (combined with cobalt isooctanoate accelerator, mass ratio 3:1), and 3 parts of zinc stearate rheology modifier. Add the above raw materials to a high-speed shear mixer and shear at 2500 r / min for 15 minutes to form a homogeneous resin slurry.

[0044] Modified plant fiber (fiber to slurry mass ratio 1:1.2) was added to the slurry and placed in an ultrasonic dispersion device. The mixture was dispersed at a frequency of 30kHz and a power of 400W for 20 minutes to obtain a uniform fiber-reinforced resin premix.

[0045] Step S3: Preheat the door panel mold to 80℃ and keep it at a constant temperature. Use layered installation: sequentially lay a 5mm thick bottom layer premix, a plant fiber-resin composite honeycomb core (8mm pore size, 0.5mm wall thickness, compressive strength ≥15MPa), and a 5mm thick top layer premix. The projected area of ​​the honeycomb core is 5cm×5cm smaller than the mold cavity to ensure edge sealing.

[0046] Pre-embed 304 stainless steel inserts (with 1.5mm high back teeth and 3mm tooth spacing) at the lock / hinge position: After positioning with mold positioning pins (gap ≤0.2mm), spread 2-3mm of premixed material around the insert and lightly press it to achieve temporary fixation.

[0047] Step S4, after closing the mold, perform dynamic pressing: pressurized rheological filling: apply an initial pressure of 3MPa and hold for 20 seconds to allow the premix to flow and wrap around the core material; depressurize and vent: reduce the pressure to 0.3MPa, slightly open the upper mold with a 2mm gap, and continue for 3 seconds to release volatiles; high pressure holding and curing: close the mold, apply a pressure of 12MPa, and hold at 135℃ for 6 minutes to allow the resin to crosslink and cure.

[0048] Step S5: Keep the mold closed and cool the mold through the temperature control oil channel: reduce the temperature from 135℃ to 90℃ at 3℃ / min, then reduce it to 60℃ at 4℃ / min, and finally reduce the door panel temperature to below the heat distortion temperature of 196 type resin (85℃ as measured by GB / T 1634).

[0049] After mold opening and part removal, post-processing is carried out: the surface is sanded and leveled, and then decorated by heat transfer printing at 160℃ for 20 seconds; the edges are coated with a color paste of the same material containing 60% solids (thickness 0.5mm), and cured at 80℃ for 20 minutes to complete the edge sealing.

[0050] Example 2 This embodiment uses limited lower limit process parameters to adapt to the production scenario of low-pressure molding equipment. The specific steps are as follows: Steps S1-S3 are completely consistent with Example 1 (fiber ratio, modification process, premix preparation, mold laying and insert fixing are all the same as in Example 1).

[0051] Step S4, after closing the mold, perform dynamic pressing: pressurized rheological filling: apply an initial pressure of 2MPa and hold for 10 seconds to allow the premix to flow slowly and wrap around the core material; depressurize and vent: reduce the pressure to 0MPa, slightly open the upper mold with a 1mm gap, and continue for 2 seconds to release volatiles; high pressure holding and curing: close the mold, apply a pressure of 8MPa, and hold at 120℃ for 10 minutes to complete the resin crosslinking and curing.

[0052] Step S5 is the same as in Example 1 (gradient cooling process and post-processing are the same as in Example 1).

[0053] Example 3 This embodiment uses the upper limit of process parameters, adapted to high-efficiency curing mass production lines. The specific steps are as follows: Steps S1-S3 are completely consistent with Example 1 (fiber ratio, modification process, premix preparation, mold laying and insert fixing are all the same as in Example 1).

[0054] Step S4, after closing the mold, perform dynamic pressing: pressurized rheological filling: apply an initial pressure of 5MPa and hold for 30 seconds to allow the premix to quickly fill the gaps in the core material; depressurization and venting: reduce the pressure to 0.5MPa, slightly open the upper mold with a 3mm gap, and continue for 5 seconds to release volatiles; high-pressure holding and curing: close the mold, apply a pressure of 15MPa, and hold at 145℃ for 3 minutes to complete the resin crosslinking and curing.

[0055] Step S5 is the same as in Example 1 (gradient cooling process and post-processing are the same as in Example 1).

[0056] Proportional Design To further verify the necessity of each core process of the present invention, the following comparative examples are provided: Comparative Example 1 The "depressurization and venting" operation in step S4 was not performed; the remaining process parameters were the same as in Example 1.

[0057] Comparative Example 2 The "acetylation modification" operation in step S1 was not performed; the remaining process parameters were the same as in Example 1.

[0058] Comparative Example 3 The "coupling agent grafting and coating" operation in step S1 was not performed, and the remaining process parameters were the same as in Example 1.

[0059] Performance testing and analysis The core performance indicators of the products obtained in the above embodiments and comparative examples were tested based on the following criteria: Examples 1-3 involve a full set of tests on the product, including: Core mechanical and waterproof performance (verifying the process's improvement on interface / molding quality): 24h water absorption rate: Performed according to GB / T 1034-2008 (sample 50mm×50mm×door panel thickness, soaked at 23±2℃, 3 parallel samples per group); Bending strength: Performed according to GB / T 1449-2022 (three-point bending, span = 16 × door panel thickness, 3 parallel samples per group).

[0060] Environmental durability (verification of long-term stability): Bending strength retention rate after damp heat aging: After aging at 70℃ and 95%RH for 168h, the samples were removed and subjected to re-humidification at 23±2℃ and 50%±5%RH for 24h. The bending strength was retested according to GB / T 1449-2022 (3 parallel samples were tested in each group). Retention rate = (average bending strength after aging / average initial bending strength) × 100%.

[0061] Core product applicability (matching the actual usage scenarios of the door): Surface flatness: Comply with GB / T 29498-2013 (measure 3 points on the front of the door panel with a 2m straightedge and feeler gauge); Lock insert pull-out resistance: Vertical pull-out test using a universal testing machine (loading speed 5mm / min, record the maximum failure force); Repeated opening and closing durability: Performed according to the combination of GB / T14154-2023 and GB / T14155-2023 (test ≥ 10,000 times).

[0062] Mandatory safety and environmental protection requirements (compliance): Combustion performance rating: Compliant with GB 8624-2021, the result shall not be lower than Class B; Formaldehyde emission: Compliance with GB18580-2017 (climate chamber method).

[0063] Comparative Example 1 core tests: 24h water absorption rate, flexural strength, surface smoothness, and repeated opening and closing durability (test requirements are the same as in the example); Comparative Example 2 and Comparative Example 3 core tests: 24h water absorption rate, bending strength, bending strength retention rate after damp heat aging, and pull-out force of lock inserts (test requirements are the same as in the examples).

[0064] The performance test results of each sample are shown in Table 1: Table 1 Performance test results of each sample As shown in Table 1, all indicators of Examples 1-3 meet the requirements for door panel use: 24h water absorption rate ≤4%, bending strength ≥35MPa, damp heat aging retention rate ≥89%, surface flatness ≤0.3mm / m, repeated opening and closing ≥10,000 times, proving that the process of the present invention can stably prepare composite door panels with qualified performance.

[0065] Comparative Example 1: Due to defects in the molding bubbles, the water absorption rate increased by 50%+ in 24 hours, the bending strength decreased by 35%, the surface flatness exceeded the standard, and it failed after only 4200 repeated opening and closing cycles—proving that "pressure relief and air release" is the key to ensuring the tightness, waterproofness and structural stability of the door panel. Comparative Examples 2 and 3: Poor fiber-resin interface bonding resulted in higher 24-hour water absorption, reduced flexural strength by 8%-26%, decreased wet heat aging retention rate to 77%-80%, and slightly reduced pull-out resistance of lock inserts—confirming that "multi-level fiber modification" is the core means to improve interface compatibility and optimize waterproof and mechanical properties.

[0066] It should be noted that the above embodiments of the present invention are merely examples. In practical applications, the process parameters can be reasonably adjusted according to the specific performance requirements of the door panel, the characteristics of the raw materials, and the production conditions. For example, long fibers in natural plant fibers can be selected as flax fibers alone, isocyanate coupling agents can be selected as coupling agents, aluminum honeycomb cores or flame-retardant rigid foam boards can be selected as lightweight structural core materials, calcium stearate can be selected as rheology modifiers, and the pressure, temperature, and time parameters in the dynamic pressing process can be finely adjusted within the corresponding range. Gradient cooling can be achieved through the air cooling system of the shaping fixture, and surface decoration treatment can be achieved through spraying processes, etc. These adjustments all fall within the protection scope of the present invention.

Claims

1. A molding process for a natural plant fiber-unsaturated polyester composite door panel, characterized in that... This includes the following steps: S1, after opening and drying natural plant fibers, they are successively subjected to alkali activation treatment and coupling agent grafting coating treatment to prepare modified plant fibers with active functional groups on the surface. S2, unsaturated polyester resin, low shrinkage additive, initiator and rheology modifier are mixed and dispersed by high-speed shearing to form a homogeneous resin slurry. Then the modified plant fiber is impregnated in the homogeneous resin slurry to prepare a fiber-reinforced resin premix. S3, in the preheated door panel mold cavity, adopts a layered laying method, sequentially laying the bottom layer fiber-reinforced resin premix, the lightweight structural core material and the surface layer fiber-reinforced resin premix, and pre-embedding high-strength inserts at the lock and hinge positions; S4. After the mold is closed, a dynamic pressing process is performed, which involves pressurizing the rheological filling, depressurizing and venting, and holding the pressure to solidify, so that the material flows to wrap the core material and releases volatiles. S5, after pressure holding and curing, keep the mold closed or transfer to the shaping tooling for staged gradient cooling. After the door panel temperature drops below the resin heat distortion temperature, open the mold and take out the part. After post-processing, the composite door panel is obtained.

2. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: In step S1, the natural plant fiber is a mixture of wood fiber and long fiber, wherein the long fiber is selected from one or more of sisal fiber, jute fiber, and flax fiber; the alkaline activation treatment uses NaOH solution to remove pectin and lignin from the fiber surface; the coupling agent grafting coating treatment uses silane coupling agent or isocyanate coupling agent to chemically bond with the hydroxyl groups on the fiber surface.

3. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: In step S1, after the alkaline activation treatment and before the coupling agent grafting, an acetylation modification step is also included: the alkaline-treated fiber is immersed in a 10%-15% acetic anhydride solution and esterified at 50-60°C for 40-60 minutes.

4. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: In step S2, the rheology modifier includes zinc stearate or calcium stearate, and an ultrasonic-assisted dispersion process is introduced when preparing the fiber-reinforced resin premix, with an ultrasonic frequency of 20-40kHz, a power of 300-500W, and a dispersion time of 15-25min.

5. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: In step S3, the lightweight structural core material is one of plant fiber-resin composite honeycomb core, aluminum honeycomb core, or flame-retardant rigid foam board.

6. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 5, characterized in that: During the layered paving process, the projected area of ​​the lightweight structural core material is smaller than the projected area of ​​the mold cavity to ensure that the upper and lower premixed materials meet and seal at the edges during molding.

7. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: In step S4, the specific process of the dynamic pressing process is as follows: Pressurized rheological filling: Apply an initial pressure of 2-5 MPa and hold for 10-30 seconds to allow the premix to soften and flow, filling the gap between the core material and the mold. Pressure relief and exhaust: Quickly reduce the pressure to 0-0.5MPa, and control the upper mold to open slightly by 1-3mm for 2-5 seconds to exhaust the water vapor and small molecule volatiles released by the plant fibers when heated; High-pressure curing: Close the mold again and apply a pressure of 8-15 MPa, and maintain at 120-145℃ for 3-10 minutes until the resin crosslinks and cures.

8. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: In step S5, the phased gradient cooling refers to cooling the door panel at a curing temperature of 120-145℃ at a cooling rate of 2-5℃ / min. The door panel temperature is reduced to below the resin heat distortion temperature by means of the temperature-controlled oil channel in the mold or the air cooling system of the shaping fixture, and then the pressure is released.

9. The molding process of a natural plant fiber-unsaturated polyester composite door panel as described in claim 1, characterized in that: The post-processing includes: The surface of the door panel is sanded and leveled, and then decorated with a spraying or heat transfer process. The edges of the door panel are coated with a color paste of the same material as the base resin for a second sealing process.

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