Three-dimensional complex curved surface skin structure forming method

Abstract

The invention discloses a three-dimensional complex curved surface skin structure forming method, which relates to the technical field of skin forming, and comprises the following specific steps: selecting a plastic blank, quickly transferring the preheated blank to a forming press machine, performing curved surface downward pressing preliminary forming on a mold cavity and a target curved surface through high-pressure mold pressing, and the semi-finished product is arranged on a working platform to be shaped, the prefabricated insert is subjected to plasma activation, mechanical embedding pressing and hot melt adhesive auxiliary bonding, the special-shaped corner of the skin is subjected to pressure strengthening, nano coating spraying and curing are conducted, and the post-treated product is naturally cooled to the room temperature. By means of the arrangement mode that mechanical embedding and bonding composite structures are matched, the shear strength is improved, a mechanical embedding and chemical bonding composite connection structure is formed, a temperature-controlled hot pressing technology is adopted for pressing and strengthening the special-shaped corner, the cooling rate is controlled, the local shear strength is improved, and the stability of a skin structure forming structure is improved.

Description

Technical Field

[0001] This invention relates to the field of skin forming technology, and in particular to a method for forming a three-dimensional complex curved surface skin structure. Background Technology

[0002] Three-dimensional complex curved surface skin structures are widely used in aerospace, automotive industry, high-end consumer electronics and medical devices. They not only need to meet structural requirements such as lightweight, high strength and high precision, but also need to have excellent appearance quality and environmental durability.

[0003] Traditional skin forming processes mainly rely on thermoforming and vacuum forming technologies. When dealing with highly complex three-dimensional curved surfaces with dramatic curvature changes, such as the complex three-dimensional curved surfaces of automobile door skins and high-speed train front skins, traditional thermoforming or vacuum forming processes have poor control over material flow and pressure distribution uniformity. It is difficult to fit the blank into the mold cavity, which can easily lead to local thinning, wrinkles, or loss of detailed features. The subsequent shaping allowance is large, affecting the final dimensional accuracy and assembly consistency. Moreover, complex curved surfaces, especially stress concentration areas such as irregular corners and flanges, are prone to internal stress concentration, microcracks, or strength weakening during forming and cooling. Existing edge connections are weak, and the edge uses a single adhesive or fastener structure, which makes the edge connections or flanges prone to detachment, affecting the stability of the skin structure. Summary of the Invention

[0004] The purpose of this invention is to provide a method for forming a three-dimensional complex curved surface skin structure to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for forming a three-dimensional complex curved surface skin structure, comprising the following specific steps: Step S1: Select plastic blanks and preheat them evenly using an infrared heating furnace. The heating temperature is adjusted according to the material. Step S2: The preheated blank is quickly transferred to the forming press. The mold cavity and the target curved surface are pressed together under high pressure to achieve the initial forming of the curved surface, and then the blank is initially cooled. Step S3: Set the semi-finished product on the work platform for shaping, perform plasma activation on the prefabricated inserts and mechanical pressing and hot melt adhesive assisted bonding, apply pressure to strengthen the irregular corners of the skin, perform plasma treatment on the side connection surface and outer surface of the skin, and spray and cure the nano-coating. Step S4: Allow the post-processed product to cool naturally to room temperature, and use a measuring instrument to check its dimensions. If there are deviations outside the required range, make secondary corrections until the requirements are met.

[0006] Preferably, in step S2, the fit error between the mold cavity and the target curve is <0.05mm, and a molding pressure of 8-10MPa is applied and held for 10-15s to allow the material to stretch and fit the mold details.

[0007] Preferably, the shaping step of the semi-finished product in step S3 is as follows: S311: The semi-finished product is placed on the work platform by a suction cup robotic arm. The semi-finished product is scanned in full size by a laser scanner to generate a deviation analysis report. The shaping tool is dynamically balanced and calibrated for accuracy. Then, the shaping is carried out in stages. In the rough shaping stage, an 800-grit diamond grinding wheel is used. In the fine shaping stage, a 1200-grit cubic boron nitride grinding wheel is used. The deviation of the side profile is fed back in real time by a laser rangefinder. The controller automatically adjusts the grinding wheel feed based on the deviation data. High-pressure air cooling is used to remove cutting chips during the shaping process. S312: The perpendicularity tolerance of the flanged structure of the semi-finished product is controlled within ±0.03mm after finishing. The burrs of the irregular corner structure are removed by deburring equipment and polished to a roughness Ra≤0.8μm. The structural contour error of the arc edge is ≤0.04mm. S313: The top edge of the semi-finished product is rounded using a CNC chamfering machine, and transition chamfers are added at irregular corners to strengthen the edges.

[0008] Preferably, the steps of plasma activation, mechanical embedding, and hot melt adhesive-assisted bonding of the prefabricated insert in step S3 are as follows: S321: The prefabricated insert is selected from plastic inserts or zinc-plated metal inserts that are compatible with the skin material. The insert is placed in a plasma treatment equipment and treated with a mixture of argon and oxygen at a radio frequency power of 50-200W for 30-120s to activate the prefabricated insert. S322: The positioning reference of the pre-set skin bonding structure is identified by the CCD camera, and the insert is pressed into the pre-set skin bonding structure by the servo pressing device, with the fitting depth reaching the pre-set assembly reference surface; S323: EVA modified hot melt adhesive is injected into the pre-set injection channel of the skin structure through a small dispensing machine, heated to 150-200 degrees Celsius, so that it melts and fills the gaps, and after cooling, it forms a composite connection structure of mechanical interlocking and bonding.

[0009] Preferably, the pressure reinforcement at the irregular corner of the skin in step S3 includes scanning the skin surface with a stress detector to identify the stress concentration areas at sharp corners and bending stress concentration areas, selecting a hot press head with a polytetrafluoroethylene coating, the hot press head having a built-in heating element and temperature sensor with a temperature control accuracy of ±2℃, heating the hot press head to 80-100℃, preheating the irregular corner of the skin for 2-3 minutes, applying a holding pressure of 2-3MPa, and ensuring that the shear strength at the skin corner is ≥12MPa after heat preservation and pressure holding.

[0010] Preferably, the steps in step S3, including plasma treatment of the skin edge connection surface and outer surface, followed by nano-coating spraying and curing, are as follows: S331: For the skin side connection surface and outer surface, a plasma processor is used to remove surface oil and oxide layer; S332: Apply a nano-protective coating to the surface of the skin using an electrostatic spraying process, with a coating thickness of 5-10μm. S333: The coated skin is baked in an oven at 120℃ for 20 minutes. The coating adhesion is ≥5B grade, and the wear resistance is improved.

[0011] Preferably, in step S331, the plasma processor uses a mixture of argon and oxygen with a volume ratio of 7:3, the power is controlled within the range of 300-600W, the processing time is 90-120s, the processing speed is 2-3mm / s, the oxygen content is increased by ≥15% after processing, the removal rate of surface oil and oxide layer is ≥99%, the surface roughness Ra is controlled within 0.2-0.4μm, and then the surface is wiped with a lint-free cloth soaked in anhydrous ethanol and dried with hot air.

[0012] Preferably, in step S332, an Al2O3-TiO2 composite nano-coating is selected, with 0.5-1% graphene powder added. The coating slurry is ball-milled to make its particle size ≤50nm. The surface of the skin is coated with a cross-shaped spraying machine using an electrostatic spraying machine. The spraying voltage is 30-50kV, the atomization pressure is 0.3-0.5MPa, the spraying distance is controlled within the range of 20-30cm, the spraying speed is controlled within 5-8cm / s, and the thickness deviation is ≤±0.5μm. During the spraying process, the thickness is detected every 2μm of thickness.

[0013] Preferably, in step S333, the skin is preheated at 80°C for 10 minutes in an oven, then heated to 120°C for 20 minutes. During the curing process, a nitrogen atmosphere is maintained in the oven. After curing, the skin is allowed to cool naturally to room temperature for ≥30 minutes. The coating surface is then lightly polished with 1500-grit sandpaper. The coating adhesion is tested to be ≥5B grade. The wear amount tested by the Taber abrasion tester is ≤0.01g / 1000 cycles. After 48 hours of salt spray testing, there is no peeling or flaking.

[0014] Preferably, in step S4, the finished skin product is placed in a constant temperature and humidity workshop for natural cooling to room temperature for 15-20 minutes. The key parameters of complex side dimension accuracy and connection position deviation are detected by installing preset detection points on a coordinate measuring machine. Products are randomly selected for mechanical performance testing and appearance inspection. If the side dimension deviation is >0.04mm or the connection strength is <12MPa, the process returns to step S3 for secondary correction. During correction, the process parameters are adjusted according to the type of deviation until all indicators meet the requirements. Qualified products are then labeled, packaged, and put into storage.

[0015] The technical effects and advantages of this invention are as follows: This invention improves shear strength by combining a composite structure of mechanical interlocking and bonding. It forms a composite connection structure of mechanical interlocking and chemical bonding by combining plasma-activated insert mechanical pressing with EVA hot melt adhesive injection, thereby enhancing the connection strength. It also uses a temperature-controlled hot pressing process to apply pressure to strengthen irregular corners and control the cooling rate, thereby improving the local shear strength and enhancing the stability of the skin structure. This invention employs an infrared heating furnace in conjunction with a thermal imager for real-time monitoring to ensure a uniform temperature field in the billet. Combined with molds and controllable pressure forming, it achieves precise control of material flow and high-fidelity replication of complex curved surface details, enabling efficient preheating and stable forming of the skin. The shaping process of this invention uses laser scanning real-time feedback, servo control and dynamic balancing tools, combined with a setting method that combines roughing and fine finishing, to ensure that the side contours, perpendicularity and surface continuity reach extremely high precision, realize the shaping and stability detection of the skin structure sides, and improve the dimensional consistency of the skin structure. This invention thoroughly cleans the surface and increases its surface energy through plasma treatment with controllable parameters. An Al2O3-TiO2 composite nano-coating is uniformly applied through an electrostatic spraying process with precise and controllable thickness. After optimized curing, the coating exhibits strong adhesion, high wear resistance, and good corrosion resistance, thereby improving the appearance quality and environmental durability of the skin. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention, but do not constitute a limitation thereof. In the drawings: Figure 1 This is a diagram illustrating the method steps of the present invention. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] This invention provides, for example Figure 1 This paper presents a method for forming a three-dimensional complex curved surface skin structure.

[0019] The specific steps include the following: Step S1: Select plastic blanks. The blank size should allow for shaping. The blank size should be 5-8% larger than the target skin. Use an infrared heating furnace to preheat the blanks evenly. The heating temperature should be adjusted according to the material. The control temperature for PC / ABS alloy materials is 190-210℃, the control temperature for glass fiber reinforced PP materials is 180-200℃, and the control temperature for modified PMMA is 200-220℃. The preheating holding time is 8-12 minutes. The surface temperature field of the blanks can be monitored in real time using an infrared thermal imager to ensure that the temperature uniformity error is ≤±5℃, and to avoid local overheating that could lead to material degradation. Step S2: The preheated billet is quickly transferred to the forming press, with a transfer time of ≤15s. The fit error between the mold cavity and the target curve is <0.05mm. A Teflon coating is sprayed onto the surface of the cavity to reduce material adhesion. A forming pressure of 8-10MPa is applied and held for 10-15s. During the holding pressure process, the pressure fluctuation is ≤±0.2MPa to ensure that the material is fully stretched and fits the mold details. The mold cavity and the target curved surface are initially formed by high-pressure molding and then initially cooled. The mold cavity is cooled by water cooling at a temperature of 20-25℃ and a water pressure of 0.3-0.5MPa. The skin temperature is monitored in real time by a temperature sensor. After cooling to 80-100℃, the semi-finished skin is removed to avoid premature hardening of the edges, which would affect the implementation of subsequent processing. After the semi-finished product is removed, it is covered with a dust cover to prevent surface contamination. An infrared heating furnace is used in conjunction with a thermal imager for real-time monitoring to ensure a uniform temperature field of the billet. Combined with the pressure forming mold and controllable pressure forming, the control of material flow and high-fidelity replication of complex curved surface details are achieved. Step S3: Set the semi-finished product on the work platform for shaping, perform plasma activation on the prefabricated inserts and mechanical pressing and hot melt adhesive assisted bonding, apply pressure to strengthen the irregular corners of the skin, perform plasma treatment on the side connection surface and outer surface of the skin, and spray and cure the nano-coating. The following are the steps for reshaping the semi-finished product: S311: The semi-finished product is placed on the work platform using a suction cup robotic arm. The vacuum suction pressure is -0.08 to -0.09 MPa, ensuring the semi-finished product is secure and free from deformation. The deviation between the positioning reference on the work platform and the preset structural coordinates is ≤ ±0.03 mm. A laser scanner performs a full-size scan of the semi-finished product with a scanning accuracy of ±0.01 mm, generating a deviation analysis report to identify areas requiring focused reshaping. Dynamic balancing and precision calibration are performed on the reshaping tools. The grinding wheel is dynamically balanced, with an imbalance ≤0.05 g·cm. The laser rangefinder is calibrated to an accuracy of ±0.02 mm, ensuring the stability and accuracy of the reshaping process. Then, the reshaping is performed in stages. In the rough reshaping stage, an 800-grit diamond grinding wheel is used to initially remove excess material from the edges of the semi-finished product at a reshaping rate of 5-8 mm / s, removing 80% of the excess material. The laser rangefinder provides real-time feedback on the edge contour deviation, measuring every 10 mm to control the process. The tool adjusts the grinding wheel feed based on deviation data to avoid excessive cutting. During the finishing stage, a 1200-mesh cubic boron nitride grinding wheel is used at a finishing speed of 2-3 mm / s. The edges are finely machined using the 1200-mesh cubic boron nitride grinding wheel, focusing on correcting the contour errors generated during the rough finishing stage. This ensures that the surface roughness Ra of the edges is ≤0.6μm. High-pressure air cooling is used to remove cutting debris during the finishing process at a speed of 8-10 m / s to prevent debris adhesion from affecting accuracy. After finishing, the edge contour is scanned again using a laser scanner to verify the edge contour deviation and ensure that the deviations in each area meet the requirements of subsequent processes. The finishing process uses real-time feedback from laser scanning, servo control, and dynamic balancing tools. Combined with precision machining that integrates rough and fine finishing, it ensures that the edge contour, perpendicularity, and surface continuity achieve extremely high precision, such as a contour error ≤0.04mm and a perpendicularity tolerance ±0.03mm. The coordinate measuring machine ensures dimensional consistency throughout the entire process. S312: The perpendicularity tolerance of the flanged structure of the semi-finished product is controlled within ±0.03mm after finishing to ensure the overall perpendicularity consistency. For irregular corner structures, deburring equipment is used to remove burrs and grind to a roughness Ra≤0.8μm. 1500-grit sandpaper is used for grinding. A torque wrench is used to control the grinding force during the grinding process. The grinding force is 5-8N・m to avoid over-grinding and material thinning. The shaping process is monitored in real time by a profile measuring instrument. The structural profile error of the arc edge is ≤0.04mm and the curvature continuity error of the arc edge is ≤0.02mm / m to avoid inflection points or abrupt changes. S313: The top edge of the semi-finished product is rounded using a CNC chamfering machine, and transition chamfers are added at irregular corners to strengthen the edges. The top edge of the flange is rounded with a radius of R0.3-R0.5mm using a CNC chamfering machine, and the surface roughness of the rounded corner is Ra≤0.4μm. Transition chamfers of R0.2-R0.3mm are added at irregular corners, with a chamfer angle of 15-20°, to effectively disperse stress concentration.

[0020] The steps for plasma activation, mechanical embedding, and hot melt adhesive-assisted bonding of the prefabricated inserts are as follows: S321: Prefabricated inserts are selected from plastic inserts or galvanized metal inserts with good compatibility with the skin material. The dimensional tolerance of the inserts is controlled within ±0.01mm. The inserts are first ultrasonically cleaned with anhydrous ethanol to remove surface oil stains, and then dried with compressed air. The inserts are then placed in a plasma treatment equipment and treated with a mixture of argon and oxygen at a radio frequency power of 50-200W for 30-120s to activate the prefabricated inserts. The power for plastic inserts is 50-100W for 30-60s, and the power for metal inserts is 150-200W for 90-120s. After activation, the contact angle of the insert surface is measured with a contact angle meter to ensure that the surface energy is ≥45mN / m to improve the adhesion performance. The activated inserts are then placed in an oven and preheated at 80-100℃ for 10-15min to reduce the temperature difference between the inserts and the skin and avoid stress during assembly. S322: The positioning reference of the pre-set skin bonding structure is identified by a CCD camera. The position of the pressure head of the servo pressing device is adjusted to ensure that the coaxiality error between the pressure head and the insert is ≤ ±0.02mm. The insert is pressed into the pre-set skin structure by the servo pressing device. The fitting depth is up to the pre-set assembly reference surface. The servo pressing device applies a forming pressure of 3-5MPa and a pressing rate of 1-2mm / s. During the pressing process, a force sensor is used to monitor the pressure change in real time. When the pressure change exceeds 10%, the machine will stop automatically to check for jamming problems. The fitting depth is strictly controlled to the pre-set assembly reference surface with a depth error of ≤ ±0.03mm to ensure connection stability. After pressing, the insert is initially inspected. The initial inspection uses a microscope to observe the fit between the insert and the skin. No gaps or tilting are acceptable. Unacceptable products are reworked in time. S323: EVA modified hot melt adhesive is injected into the pre-set injection channel of the skin structure using a small dispensing machine. The adhesive is preheated in a hot melt adhesive machine to 150-200 degrees Celsius and held for 30 minutes to remove air bubbles and ensure that the adhesive is uniform and free of impurities, allowing it to melt and fill the gaps. The hot melt adhesive is then injected into the pre-set injection channel of the skin structure using a small dispensing machine at a pressure of 0.8-1.2 MPa and a speed of 0.5-1 ml / s. The adhesive layer thickness is strictly controlled at 0.3-0.5 mm. During the injection process, a CCD camera is used to monitor the flow of the adhesive in real time to avoid leakage and overflow. After the injection is completed, the air in the cavity is discharged through the pre-set vent holes with a diameter of 0.5-0.8 mm to prevent air bubble formation. Then, it is kept at 80-100℃ for 3-5 minutes for curing. During the curing process, a temperature monitoring instrument is used to monitor the curing temperature to ensure that the adhesive layer is fully cured. After cooling, a composite connection structure of mechanical interlocking and bonding is formed.

[0021] By combining mechanical interlocking and adhesive composite structures with a proprietary bonding design, the shear strength reaches 12-18MPa, which is more than 40% higher than that of traditional single connections. After 500 cycles of high and low temperatures from -40℃ to 80℃ and vibration testing, there is no loosening or cracking, meeting the requirements of harsh working conditions. The combination of plasma-activated insert mechanical pressing and EVA hot melt adhesive injection forms a composite connection structure of mechanical interlocking and chemical bonding, which improves the connection strength and enhances the shear strength of the structure.

[0022] Pressure reinforcement is applied to irregular corners of the skin, including sharp corners and areas of stress concentration during bending. A stress detector scans the skin surface to identify stress concentration areas. A hot press head with a PTFE coating is selected. The PTFE coating on the hot press head prevents adhesion to the skin. The hot press head has a built-in heating element and temperature sensor with a temperature control accuracy of ±2℃. The hot press head is heated to 80-100℃ and preheated for 2-3 minutes at the irregular corner of the skin to ensure a uniform local temperature increase and reduce thermal stress. A holding pressure of 2-3MPa is applied, with a pressure uniformity error ≤±0.1MPa. The holding time is 5-8 minutes, adjusted according to the corner size. The smaller the diameter, the longer the pressure holding time. After heat and pressure holding, ensure that the shear strength at the skin corner is ≥12MPa. After keeping the pressure constant, use air cooling to cool the corner area to room temperature at a rate of 5-8℃ / min to avoid secondary stress caused by rapid cooling. Use a tensile testing machine to test the shear strength at the skin corner to ensure it is ≥12MPa. At the same time, use strain gauges to measure the residual strain after strengthening. The residual strain is ≤500με, which meets the structural requirements. For stress concentration areas such as irregular corners, use temperature-controlled hot pressing process for pressure strengthening and control the cooling rate to effectively improve the local shear strength and reduce the residual strain, ensuring the integrity of the structure under extreme temperature cycles and vibrations.

[0023] The steps for plasma treatment of the skin side connection surface and outer surface, followed by nano-coating spraying and curing are as follows: S331: For the skin side connection surface and outer surface, a plasma processor is used to remove surface oil and oxide layer; The plasma processor uses a 7:3 volume ratio of argon and oxygen mixed gas, with power controlled within the range of 300-600W, processing time of 90-120s, and processing speed of 2-3mm / s. It scans uniformly along the skin surface. After processing, the surface elemental composition is analyzed using X-ray photoelectron spectroscopy. The oxygen content is increased by ≥15%, the removal rate of surface oil and oxide layer is ≥99%, and the surface roughness Ra is controlled within 0.2-0.4μm, improving coating adhesion. The surface is then wiped with a lint-free cloth soaked in anhydrous ethanol to remove trace deposits generated by plasma treatment. It is then dried with hot air at a temperature of 50-60℃ to avoid residual moisture affecting coating quality. Through efficient surface activation and functional coating settings, and using plasma treatment with controllable parameters, the surface is thoroughly cleaned and its surface capabilities are improved. Contact angle detection ensures that the surface energy is ≥45mN / m, laying the foundation for subsequent coating adhesion. S332: Apply a nano-protective coating to the surface of the skin using an electrostatic spraying process, with a coating thickness of 5-10μm. The coating uses an Al2O3-TiO2 composite nano-coating, with an Al2O3 content of 60-70% and a TiO2 content of 30-40%, and 0.5-1% graphene powder added. The coating slurry is ball-milled to ensure a particle size ≤50nm, guaranteeing uniform dispersion. The surface of the skin is then coated using an electrostatic spraying machine in a cross-hatching pattern. The spraying voltage is 30-50kV, the atomization pressure is 0.3-0.5MPa, the spraying distance is controlled within 20-30cm, the spraying speed is controlled within 5-8cm / s, and the thickness deviation is ≤±0.5μm. During the spraying process... Thickness testing is performed after spraying a 2μm layer to ensure coating uniformity. The coating thickness is strictly controlled between 5-10μm. During the spraying process, a coating thickness gauge is used to check the thickness every 2μm of spraying, and the spraying parameters are adjusted in time to avoid local coatings being too thick or too thin. The Al2O3-TiO2 composite nano-coating is uniformly applied through electrostatic spraying, and the thickness is precisely controllable. After optimization and curing, the coating has strong adhesion, high wear resistance, and good corrosion resistance, achieving an adhesion grade ≥5B, a wear resistance of ≤0.01g / 1000 cycles, and no abnormalities in the 48h salt spray test, thus improving the appearance quality and environmental durability of the skin. S333: The coated skin is baked in an oven at 120℃ for 20 minutes. The coating adhesion is ≥5B grade, and the wear resistance is improved. The skin is preheated in an oven at 80℃ for 10 minutes, then heated to 120℃ for 20 minutes. During the curing process, a nitrogen atmosphere is maintained in the oven to prevent coating oxidation. After curing, the skin is allowed to cool naturally to room temperature for ≥30 minutes to avoid rapid cooling that could cause coating cracking. The coating surface is then lightly polished with 1500-grit sandpaper to remove tiny particles generated during spraying, resulting in a surface roughness Ra≤0.2μm and improved appearance quality. The coating adhesion is tested to be ≥5B grade, and the wear amount tested by the Taber abrasion tester is ≤0.01g / 1000 cycles. After a 48-hour salt spray test, there is no peeling or flaking. From preheating, rapid transfer, molding, cooling to post-processing shaping, insert assembly, and coating spraying, parameters are controlled at each stage, and key steps are equipped with real-time monitoring sensors to enable immediate warning and adjustment of process anomalies, ensuring seamless process connection and full-process monitoring.

[0024] Step S4: Allow the post-processed product to cool naturally to room temperature, and use a measuring instrument to check its dimensions. If there are deviations outside the required range, make secondary corrections until the requirements are met.

[0025] After processing, the skin products are placed in a temperature- and humidity-controlled workshop to cool naturally to room temperature. The temperature in the workshop is controlled at 23±2℃ and the humidity at 50±5%. The cooling time is 15-20 minutes to avoid secondary stress caused by rapid cooling. During the cooling process, a special support is used to prevent product deformation. A coordinate measuring machine is used to install preset detection points to detect key parameters such as the accuracy of complex side dimensions and the deviation of connection positions. Randomly selected products are then subjected to mechanical performance testing and appearance inspection. For mechanical performance testing, three products are randomly selected. A tensile testing machine is used to test the shear strength of the composite connection, and an impact testing machine is used to conduct a cantilever beam impact test to ensure that there is no breakage or cracking. For appearance inspection, a visual inspection system is used in combination with manual visual inspection to check for defects such as scratches, bubbles, and coating peeling on the skin surface. The appearance quality meets industry standards. If the side dimension deviation is >0.04mm or the connection strength is <12MPa, the process returns to step S3 for secondary correction. During correction, the process parameters are adjusted according to the type of deviation until all indicators meet the requirements. Qualified products are then labeled, packaged, and put into storage.

[0026] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for forming a three-dimensional complex curved surface skin structure, characterized in that, The specific steps include the following: Step S1: Select plastic blanks and preheat them evenly using an infrared heating furnace. The heating temperature is adjusted according to the material. Step S2: The preheated blank is quickly transferred to the forming press. The mold cavity and the target curved surface are pressed together under high pressure to achieve the initial forming of the curved surface, and then the blank is initially cooled. Step S3: Set the semi-finished product on the work platform for shaping, perform plasma activation on the prefabricated inserts and mechanical pressing and hot melt adhesive assisted bonding, apply pressure to strengthen the irregular corners of the skin, perform plasma treatment on the side connection surface and outer surface of the skin, and spray and cure the nano-coating. Step S4: Allow the post-processed product to cool naturally to room temperature, and use a measuring instrument to check its dimensions. If there are deviations outside the required range, make secondary corrections until the requirements are met.

2. The method for forming a three-dimensional complex curved surface skin structure according to claim 1, characterized in that, In step S2, the fit error between the mold cavity and the target curve is <0.05mm. An 8-10MPa molding pressure is applied and held for 10-15s to allow the material to stretch and fit the mold details.

3. The method for forming a three-dimensional complex curved surface skin structure according to claim 1, characterized in that, The shaping step of the semi-finished product in step S3 is as follows: S311: The semi-finished product is placed on the work platform by a suction cup robotic arm. The semi-finished product is scanned in full size by a laser scanner to generate a deviation analysis report. The shaping tool is dynamically balanced and calibrated for accuracy. Then, the shaping is carried out in stages. In the rough shaping stage, an 800-grit diamond grinding wheel is used. In the fine shaping stage, a 1200-grit cubic boron nitride grinding wheel is used. The deviation of the side profile is fed back in real time by a laser rangefinder. The controller automatically adjusts the grinding wheel feed based on the deviation data. High-pressure air cooling is used to remove cutting chips during the shaping process. S312: The perpendicularity tolerance of the flanged structure of the semi-finished product is controlled within ±0.03mm after finishing. The burrs of the irregular corner structure are removed by deburring equipment and polished to a roughness Ra≤0.8μm. The structural contour error of the arc edge is ≤0.04mm. S313: The top edge of the semi-finished product is rounded using a CNC chamfering machine, and transition chamfers are added at irregular corners to strengthen the edges.

4. The method for forming a three-dimensional complex curved surface skin structure according to claim 1, characterized in that, The steps in step S3, including plasma activation of the prefabricated insert followed by mechanical embedding and hot melt adhesive-assisted bonding, are as follows: S321: The prefabricated insert is selected from plastic inserts or zinc-plated metal inserts that are compatible with the skin material. The insert is placed in a plasma treatment equipment and treated with a mixture of argon and oxygen at a radio frequency power of 50-200W for 30-120s to activate the prefabricated insert. S322: The positioning reference of the pre-set skin bonding structure is identified by the CCD camera, and the insert is pressed into the pre-set skin bonding structure by the servo pressing device, with the fitting depth reaching the pre-set assembly reference surface; S323: EVA modified hot melt adhesive is injected into the pre-set injection channel of the skin structure through a small dispensing machine, heated to 150-200 degrees Celsius, so that it melts and fills the gaps, and after cooling, it forms a composite connection structure of mechanical interlocking and bonding.

5. The method for forming a three-dimensional complex curved surface skin structure according to claim 1, characterized in that, In step S3, applying pressure to strengthen the irregular corner of the skin includes scanning the skin surface with a stress detector to identify stress concentration areas at sharp corners and bending stress concentration areas. A hot press head with a polytetrafluoroethylene coating is selected. The hot press head has a built-in heating element and temperature sensor with a temperature control accuracy of ±2℃. The hot press head is heated to 80-100℃ and preheated for 2-3 minutes at the irregular corner of the skin. A holding pressure of 2-3MPa is applied. After heat preservation and pressure holding, the shear strength at the skin corner is ensured to be ≥12MPa.

6. The method for forming a three-dimensional complex curved surface skin structure according to claim 1, characterized in that, The steps in step S3, including plasma treatment of the skin edge connection surface and outer surface, followed by nano-coating spraying and curing, are as follows: S331: For the skin side connection surface and outer surface, a plasma processor is used to remove surface oil and oxide layer; S332: Apply a nano-protective coating to the surface of the skin using an electrostatic spraying process, with a coating thickness of 5-10μm. S333: The coated skin is baked in an oven at 120℃ for 20 minutes. The coating adhesion is ≥5B grade, and the wear resistance is improved.

7. The method for forming a three-dimensional complex curved surface skin structure according to claim 6, characterized in that, In step S331, the plasma processor uses a mixture of argon and oxygen with a volume ratio of 7:3, with power controlled within the range of 300-600W, processing time of 90-120s, and processing speed of 2-3mm / s. After processing, the oxygen content is increased by ≥15%, the removal rate of surface oil and oxide layer is ≥99%, and the surface roughness Ra is controlled within 0.2-0.4μm. Then, the surface is wiped with a lint-free cloth soaked in anhydrous ethanol and dried with hot air.

8. The method for forming a three-dimensional complex curved surface skin structure according to claim 6, characterized in that, In step S332, an Al2O3-TiO2 composite nano-coating is selected, and 0.5-1% graphene powder is added. The coating slurry is ball-milled to make its particle size ≤50nm. The surface of the skin is coated with a cross-shaped spraying machine using an electrostatic spraying machine. The spraying voltage is 30-50kV, the atomization pressure is 0.3-0.5MPa, the spraying distance is controlled within the range of 20-30cm, the spraying speed is controlled within 5-8cm / s, and the thickness deviation is ≤±0.5μm. During the spraying process, the thickness is detected every 2μm of thickness.

9. The method for forming a three-dimensional complex curved surface skin structure according to claim 6, characterized in that, In step S333, the skin is preheated at 80°C for 10 minutes in an oven, then heated to 120°C for 20 minutes. During the curing process, a nitrogen atmosphere is maintained in the oven. After curing, the skin is allowed to cool naturally to room temperature for ≥30 minutes. The coating surface is then lightly polished with 1500-grit sandpaper. The coating adhesion is tested to be ≥5B grade. The wear amount tested by the Taber abrasion tester is ≤0.01g / 1000 cycles. After 48 hours of salt spray testing, there is no peeling or flaking.

10. The method for forming a three-dimensional complex curved surface skin structure according to claim 1, characterized in that, In step S4, the finished skin product is placed in a constant temperature and humidity workshop for natural cooling to room temperature for 15-20 minutes. The key parameters of complex side dimension accuracy and connection position deviation are detected by installing preset detection points on a coordinate measuring machine. Products are randomly selected for mechanical performance testing and appearance inspection. If the side dimension deviation is >0.04mm or the connection strength is <12MPa, the process returns to step S3 for secondary correction. During correction, the process parameters are adjusted according to the type of deviation until all indicators meet the requirements. Qualified products are then labeled, packaged, and put into storage.

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