Rail transit vehicle axle surface water-based paint high film thickness one-time forming coating process

Abstract

The application discloses a kind of rail transit vehicle axle surface water-based paint high film thickness one-time forming coating process, belong to the field of coating process.The process includes the following steps: pretreatment is carried out to axle surface, and water-based anticorrosive primer and water-based finish paint system are configured, by optimizing spraying process parameters, including atomization effect, spraying speed and gun path, in combination with baking curing process, realize that paint film reaches high film thickness requirement one-time.The process is by adjusting coating equipment and process flow, VOC emission is significantly reduced, while greatly improving coating efficiency, meets the double requirements of rail transit axle to anticorrosion performance and environmental protection.

Description

Technical Field

[0001] This application belongs to the field of coating technology, specifically relating to a high-film-thickness one-time coating process for water-based paint on the surface of rail transit vehicle axles. Background Technology

[0002] As a crucial component of the national strategic emerging industries, the rail transit equipment manufacturing industry directly impacts the safety of people's lives and property and the stable development of the social economy through the quality of its products and operational safety. In the protective and decorative systems of rail transit vehicle components, coating processes play a vital role, not only enhancing the corrosion resistance and decorative effect of components but also serving as a key link in ensuring the long-term safe operation of vehicles. In recent years, with increasingly stringent environmental regulations globally and the deepening implementation of "dual-carbon" goals, the rail transit equipment manufacturing sector has placed higher demands on the environmental performance of coating processes. Water-based coatings, using water as the primary dispersion medium, have seen their application scope continuously expand in the rail transit vehicle manufacturing industry due to their significant advantages such as low VOC emissions and environmental friendliness, becoming an important direction for the upgrading of coating technology in this field. As the core load-bearing component of the train's running gear, the surface coating quality of rail transit axles directly affects the vehicle's operational safety, fatigue life, and overall reliability, thus imposing extremely stringent technical requirements on axle coating processes.

[0003] Among the key technical challenges currently facing the rail transit vehicle axle field is the one-time high-film-thickness water-based paint coating process. Rail transit vehicle axles typically employ low-roughness surface treatment processes, with surface roughness Ra generally controlled below 3.2 μm. While this characteristic is beneficial for ensuring the fatigue strength and service life of the axle, it also presents a significant challenge to the adhesion of water-based coatings. Furthermore, protective coatings for rail transit vehicle axles usually require a film thickness of over 40 μm to meet stringent corrosion resistance requirements. Traditional multi-stage spraying processes are not only inefficient but also increase energy consumption and VOC emissions. Achieving high-film-thickness one-time spraying while ensuring adhesion, and effectively controlling paint film sagging defects, has become a core technical bottleneck restricting the application of water-based coatings in the rail transit vehicle axle field. In addition, the manufacturing of rail transit equipment needs to adapt to the needs of different regional environments, especially the construction adaptability under high-humidity climate conditions such as southern mountainous areas, which is also an important issue that must be addressed in this technical field.

[0004] In existing technologies, research on water-based coatings mainly focuses on improving and optimizing the formulation. The technical solution with publication number CN116116683A provides an improved water-based paint coating process, achieving a dry-wet process of oil-based clear coat and water-based color coat. However, this process is primarily designed for aluminum alloy wheels and does not fully consider the low roughness characteristics of rail transit axle surfaces, making it difficult to guarantee the adhesion between the paint film and the axle substrate. Furthermore, this process still uses a system combining water-based color coat and oil-based clear coat, failing to achieve true high-film-thickness one-time molding of water-based paint, resulting in relatively high VOC emissions. Additionally, its curing temperature is between 140 and 150°C, leading to significant energy consumption and making it unsuitable for large-scale industrial production of rail transit axles. The technical solution with publication number CN117659835B prepares a thick-film, single-component waterborne paint with high solids content and excellent crack resistance by optimizing the waterborne resin ratio and additive system. However, this solution only involves the improvement of the paint formulation and lacks the design of specific coating process parameters for the surface of rail transit vehicle axles. It cannot solve the technical problem of insufficient adhesion to low-roughness substrates of axles, and it still has limitations in terms of film thickness formed in a single spraying, making it difficult to meet the high film thickness protection requirements of rail transit vehicle axles. At the same time, it does not consider the construction adaptability under humid climate conditions. The technical solution with publication number CN119019920A prepares a high-film-thickness waterborne paint suitable for spraying through batch feeding and process control. However, this solution mainly focuses on the paint preparation process and lacks the design of pretreatment process for low-roughness metal substrates. Although it improves the water resistance, acid and alkali resistance, and salt spray resistance of the paint film, it does not involve the sagging control technology in the one-time coating process and does not consider the characteristics of the on-site construction environment for rail transit vehicle axle coating, and cannot meet the construction requirements under high humidity conditions. In summary, existing water-based paint technologies either focus on improving and optimizing paint formulations or are developed for application scenarios in other industries. Neither has effectively solved the core technical problems faced by high-film-thickness one-time coating of water-based paints on the surface of rail transit vehicle axles. These problems include insufficient adhesion to low-roughness substrates, easy sagging in high-film-thickness one-time coatings, and poor adaptability to construction in humid climates. There is an urgent need to develop a water-based paint high-film-thickness one-time coating process specifically for the surface characteristics of rail transit vehicle axles to meet the urgent needs of the rail transit equipment manufacturing industry for efficient, environmentally friendly, and high-quality coating processes. Summary of the Invention

[0005] The purpose of this invention is to provide a high-film-thickness one-time coating process for water-based paint on the surface of rail transit vehicle axles, which can effectively solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A high-film-thickness, one-step water-based paint coating process for rail transit vehicle axles includes the following steps: Step 1: Substrate pretreatment: The axle substrate surface is degreased and derusted to a cleanliness level of Sa2. The treated axle surface is then naturally dried to ensure it is free of oil, moisture, and dust. Step 2: Water-based paint application parameter control: The water-based paint is a high-solids water-based polyurethane varnish. Before application, the water-based paint is adjusted to the appropriate viscosity, and the volume solids content is controlled to be no less than 50%. Step 3: Spraying application: Air spraying is used under conditions of a spray booth temperature of 20-30℃ and a relative humidity of 50%-70%. During spraying, the distance between the spray gun and the axle surface is maintained at 20-25cm, and the spray gun movement speed is uniform. A staged rotary spraying trajectory control scheme is adopted, and the dry film thickness is not less than 40μm through continuous spraying. Step 4: Leveling and flash drying: After spraying, the axle is placed in the leveling area for flash drying for 5 to 10 minutes to allow the moisture and co-solvent in the paint film to evaporate and eliminate spraying stress. The flash drying endpoint is quantitatively determined by the surface resistivity of the paint film and infrared moisture content monitoring. Step 5: Baking and curing: The flash-dried axle is sent to the baking oven for curing and baking at a temperature of 60 to 85℃ for 30 to 40 minutes. The baking process adopts a step-by-step heating and zoned dehumidification coordinated control strategy to ensure that the paint film is fully cross-linked and cured. Step 6: Cooling and testing: After baking, the axle is allowed to cool naturally to room temperature, and the appearance, thickness and adhesion of the paint film are tested.

[0008] Preferably, in step 1, the substrate pretreatment adopts a professional degreasing and derusting process. The degreasing process is used to remove organic contaminants such as cutting oil and lubricating oil from the axle surface, and the derusting process is used to remove oxide scale and rust products from the axle surface. The cleanliness after treatment reaches the Sa2 standard. After treatment, the axle is thoroughly dried by natural drying. Natural drying allows residual moisture on the axle surface to evaporate naturally by controlling the ambient temperature and ventilation conditions, ensuring that the substrate surface is dry and clean, providing a good adhesion base for subsequent coating.

[0009] Preferably, the selection of high-solids waterborne polyurethane varnish in step 2 is a key factor in achieving high film thickness in one-time molding. The volume solids content of this waterborne varnish is not less than 50%. The high solids content ensures that each spray can carry more effective film-forming substances, providing a material basis for achieving a dry film thickness of not less than 40μm. At the same time, the high solids system reduces the amount of water evaporation required per unit area, which is beneficial to shorten flash drying and baking time and reduce overall energy consumption.

[0010] Preferably, controlling the environmental parameters of the spraying process in step 3 is a crucial step in ensuring coating quality. The spray booth temperature is controlled within the range of 20 to 30°C, and the relative humidity is controlled within the range of 50% to 70%. This temperature and humidity range is conducive to the normal evaporation of moisture, avoiding defects such as orange peel or prickly heat caused by excessively high temperature and excessively fast surface drying. At the same time, it avoids affecting the leveling properties by excessively low humidity and excessively fast drying. The distance between the spray gun and the axle surface is maintained at 20 to 25 cm, and the moving speed is uniform to ensure that the paint mist is evenly deposited on the axle surface. A single continuous spray can achieve a dry film thickness of not less than 40 μm.

[0011] Preferably, in step 4, the leveling and flash-drying process allows the paint film sufficient time for natural leveling and initial drying. The flash-drying time is 5 to 10 minutes. During this stage, some of the moisture and co-solvent in the paint film evaporate, the viscosity of the paint film gradually increases and it begins to dry initially. At the same time, the spraying stress is released, effectively preventing cracking or peeling caused by internal stress concentration during the subsequent curing process. The flash-drying area should maintain good ventilation to ensure that solvent gases can be discharged in a timely manner.

[0012] Preferably, the baking and curing temperature in step 5 is set to 60 to 80°C, and the baking time is 30 to 40 minutes. These temperature and time parameters are optimized to ensure that the paint film is fully cross-linked and cured to obtain excellent physicochemical properties, while avoiding yellowing, embrittlement, or energy waste caused by excessive temperature or time. During the curing process, the isocyanate groups and hydroxyl groups in the polyurethane system react fully to form a dense three-dimensional cross-linked network structure, which gives the paint film excellent hardness, wear resistance, and chemical resistance.

[0013] Preferably, the water-based paint enhances the adhesion of the paint film to the metal substrate by adding additives, imparts thixotropic properties to the system by adding thixotropic agents, reduces viscosity at high shear rates for easy atomization, and recovers viscosity at low shear rates to prevent thick film sagging, and enhances the chemical bonding between the coating and the low-roughness metal substrate by adding adhesion promoters. The addition of adhesion promoters can form chemical bonds between the coating and the metal substrate, effectively solving the problem of insufficient adhesion between the low-roughness surface of rail transit vehicle axles and water-based paint.

[0014] Preferably, the introduction of the thixotropic agent imparts shear-thinning properties to the coating system. During the spraying process, when the coating passes through the spray gun orifice, it is subjected to high shear, and the viscosity of the system decreases rapidly, which is beneficial to the atomization and uniform dispersion of the coating. When the coating is deposited on the axle surface, the shear effect disappears, and the viscosity of the system quickly recovers to form a stable structure. This shear-thinning property effectively prevents the flow and sagging of the paint in the thick film state, and provides rheological guarantee for achieving high film thickness in one-time molding.

[0015] Preferably, the application viscosity is controlled at 50 seconds using a Forecast cup (F4). This application viscosity has been verified to ensure that the coating achieves good atomization and deposition efficiency during air spraying. Too low a viscosity will lead to excessive atomization of the coating, resulting in overspray loss and paint mist dispersion. Too high a viscosity will lead to poor atomization, producing large droplets that affect the smoothness of the paint film. The viscosity value of 50 seconds using a Forecast cup (F4) achieves a good balance between spray application workability and paint film quality.

[0016] Preferably, this process is applicable to the coating production of rail transit axles of different specifications. The process parameters can be appropriately adjusted according to the specific size and shape of the axle. For axles with larger diameters or longer lengths, the number of spray coats can be increased or the spray gun movement speed can be adjusted to ensure coating uniformity. This process has simple equipment requirements, does not require special preheating or post-treatment devices, and has good prospects for industrial application.

[0017] Preferably, this process can adapt to the construction requirements under humid climate conditions. The relative humidity of the spray booth is controlled within the range of 50% to 70%. With optimized flash-drying time and baking parameters, the stability of coating quality can be guaranteed even in high humidity environments such as mountainous areas in the south. The synergistic effect of the thixotropic agent system and the low moisture evaporation of the high solids system together ensure the adaptability of construction in humid environments.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. High film thickness in one-time molding: Through the synergistic effect of the high solids system and the thixotropic agent system, this invention achieves a dry film thickness of not less than 40μm in one spraying and a pass rate of not less than 95%, which significantly reduces the number of coating passes, shortens the production cycle, and greatly improves production efficiency compared with traditional multi-pass spraying processes.

[0020] 2. Excellent adhesion: By adding an adhesion promoter, this invention effectively solves the problem of insufficient adhesion of water-based paint on low-roughness axle surfaces, achieving a paint film adhesion level of 1, which meets the stringent requirements of rail transit axles for coating adhesion.

[0021] 3. Excellent anti-sagging performance: By introducing a thixotropic agent, this invention imparts shear-thinning properties to the coating system, effectively preventing paint flow and sagging defects under high film thickness conditions. The paint film has a smooth and even appearance, free from defects such as sagging, bubbling, and pinholes.

[0022] 4. Green and environmentally friendly: VOC emissions are reduced by more than 50% compared with solvent-based coatings. Water is used as the main dispersion medium, which greatly reduces pollution to the atmospheric environment and is in line with the environmental protection development trend of the rail transit equipment manufacturing industry.

[0023] 5. Strong process adaptability: This process is applicable to the coating production of rail transit axles of different specifications, and can adapt to the construction requirements under humid climate conditions, providing a reliable technical solution for coating operations in high humidity environments such as mountainous areas in the south. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the overall technical solution architecture for a high-film-thickness, one-step coating process for water-based paint on the axle surface of rail transit vehicles according to an embodiment of this application.

[0025] Figure 2 This is a schematic diagram of the core process control principle framework for high film thickness one-time molding of water-based paint according to an embodiment of this application;

[0026] Figure 3 This is a schematic diagram of the main process stages of the axle surface coating process according to an embodiment of this application. Detailed Implementation

[0027] Example 1

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0029] This embodiment takes railway freight car axles as the coating object and systematically describes the overall technical solution and technical implementation details of a water-based high film thickness one-time forming coating process for low roughness axles.

[0030] In step 1, the substrate pretreatment stage, the axle substrate surface undergoes degreasing and rust removal treatment to achieve a cleanliness level of Sa2. The treated axle surface is then naturally dried to ensure it is free of oil, moisture, and dust. Specifically, the substrate pretreatment employs professional degreasing and rust removal processes. Degreasing removes organic contaminants such as cutting oil and lubricating oil from the axle surface, while rust removal removes oxide scale and rust products. The cleanliness level achieved after treatment reaches Sa2. Following treatment, the axle is thoroughly dried naturally. This natural drying process controls ambient temperature and ventilation to allow residual moisture on the axle surface to evaporate naturally, ensuring the substrate surface is dry and clean, providing a good adhesion base for subsequent coating.

[0031] In practice, the pretreatment of axle substrates includes the following detailed procedures: First, the axle surface is immersed in an alkaline cleaning agent, with the concentration controlled between 5% and 10%, the cleaning temperature maintained at 50℃ to 60℃, and the immersion time no less than 15 minutes, to fully break down and remove grease-like organic contaminants from the metal surface; after cleaning, the axle surface is high-pressure rinsed with deionized water to thoroughly remove any residual cleaning agent components; subsequently, rust removal is performed using mechanical abrasive blasting, with steel grit or stainless steel grit selected, a particle size ranging from 0.5mm to 1.2mm, and a blasting pressure controlled at 0.5MPa. The spray pressure is 0.7 MPa, the spray angle is maintained at 70° to 80°, and the treatment time is dynamically adjusted according to the degree of rust on the axle surface to ensure complete removal of oxide scale and rust products. After treatment, the axle is naturally dried. Clean compressed air is used to initially blow-dry the axle surface, and then the axle is placed in a well-ventilated natural drying area for at least 30 minutes. The ambient temperature is controlled between 20°C and 30°C, and the relative humidity is controlled between 50% and 70% to ensure sufficient evaporation of moisture from the axle surface. After drying, the axle body is wiped and inspected with a clean, dry cloth to ensure that the surface is free of oil, moisture, and dust before proceeding to the next process. The surface roughness of the axle substrate is controlled within the range of Ra≤3.2μm after treatment, meeting the technical requirements for low-roughness surface coating.

[0032] In step 2, the waterborne paint application parameter control stage, the waterborne paint is a high-solids waterborne polyurethane varnish. Before application, the waterborne paint is adjusted to the application viscosity, and the volume solids content is controlled to be no less than 50%. The selection of a high-solids waterborne polyurethane varnish is a key factor in achieving high film thickness in a single application. The high solids content of this waterborne paint, with a volume solids content of no less than 50%, ensures that each spray can carry more effective film-forming substances, providing a material basis for achieving a dry film thickness of no less than 40 μm. At the same time, the high-solids system reduces the amount of water evaporation required per unit area, which helps to shorten flash-drying and baking times and reduce overall energy consumption.

[0033] The formulation composition and application parameter control of water-based paint are among the core technologies of this invention. In this embodiment, the water-based paint used is a two-component, high-solids water-based polyurethane varnish. The main agent is a compound of a high-hydroxyl-value water-based acrylic dispersion and a water-based polyurethane dispersion, with the hydroxyl value controlled within the range of 120 mg KOH / g to 150 mg KOH / g. The curing agent is a hydrophilic modified isocyanate curing agent, with the NCO content controlled within the range of 16% to 19%. The main agent and curing agent are mixed at a molar ratio of hydroxyl to isocyanate groups of 1:1.05. The volume solids content of the water-based paint is controlled within the range of 50% to 55%, preferably 52%. This solids content ensures that a single spray... The coating achieves a dry film thickness of over 40μm while maintaining good atomization and leveling properties. The application viscosity is adjusted by adding deionized water, with the viscosity of the Forecast-4 cup controlled within the range of 50 to 55 seconds, preferably 50 seconds. This viscosity value ensures that the coating achieves good atomization and deposition efficiency during air spraying. Too low a viscosity will lead to over-atomization of the coating, resulting in overspray loss and paint mist dispersion, while too high a viscosity will lead to poor atomization, producing large droplets that affect the smoothness of the paint film. The 50-second viscosity value of the Forecast-4 cup achieves a good balance between spray application and paint film quality.

[0034] Additives are added to water-based paints to enhance the adhesion of the paint film to the metal substrate. Thixotropic agents are added to impart thixotropic properties to the system, which reduces viscosity at high shear rates for easy atomization and recovers viscosity at low shear rates to prevent thick film sagging. Adhesion promoters are added to enhance the chemical bonding between the coating and the low-roughness metal substrate. The addition of adhesion promoters can form chemical bonds between the coating and the metal substrate, effectively solving the problem of insufficient adhesion between the low-roughness surface of rail transit vehicle axles and water-based coatings. Specifically, the additive system includes wetting agents, leveling agents, defoamers, and flash rust inhibitors. The wetting agent is a nonionic fluorocarbon surfactant, added at 0.3% to 0.5% of the total formulation amount, used to reduce the surface tension of the coating and enhance its wetting and spreading ability on low-roughness metal substrates. The leveling agent is a polyether siloxane copolymer, added at 0.2% to 0.4% of the total formulation amount, used to improve the leveling performance of the paint film and eliminate surface defects such as orange peel. The defoamer is a modified polysiloxane defoamer, added at 0.1% to 0.3% of the total formulation amount, used to suppress and eliminate bubbles generated during construction. The flash rust inhibitor is an organic flash rust inhibitor, added at 0.5% to 1.0% of the total formulation amount, used to prevent flash rusting of metal substrates in humid environments.

[0035] The introduction of a thixotropic agent imparts shear-thinning properties to the coating system. During spraying, the coating is subjected to high shear as it passes through the spray gun orifice, causing a rapid decrease in system viscosity, which is beneficial for atomization and uniform dispersion. Once the coating is deposited on the axle surface, the shear effect disappears, and the system viscosity quickly recovers, forming a stable structure. This shear-thinning characteristic effectively prevents paint flow and sagging in thick film conditions, providing rheological assurance for achieving high film thickness in a single application. The thixotropic agent is a compound system of organically modified bentonite and fumed silica. The amount of organically modified bentonite added is 0.8% to 1.2% of the total formulation, and the amount of fumed silica added is 0.5% to 0.8% of the total formulation. The two work synergistically to form a three-dimensional network structure, giving the coating system excellent thixotropic properties; at high shear rates (10000 s⁻¹), the coating system exhibits excellent thixotropic performance. -1 Under the above conditions, the system viscosity can be reduced to the range of 0.5 Pa·s to 1.0 Pa·s, which is beneficial for the atomization and uniform dispersion of the coating; at a low shear rate (0.1 s⁻¹), the viscosity can be reduced to the range of 0.5 Pa·s to 1.0 Pa·s. -1 Under the following conditions, the system viscosity recovers to above 100 Pa·s, effectively preventing paint flow and sagging in thick film conditions. The introduction of the thixotropic agent system enables the coating to maintain good anti-sagging properties even under high film thickness application conditions, which is a key technical guarantee for achieving a dry film thickness of over 40 μm in a single spray.

[0036] The addition of adhesion promoters is a key measure to solve the problem of insufficient adhesion to low-roughness metal substrates. The adhesion promoter selected is a compound system of silane coupling agents and phosphate ester compounds. Its mechanism of action is as follows: The silane coupling agent (such as a 1:1 mass ratio compound of γ-aminopropyltriethoxysilane KH-550 and γ-glycidoxypropyltrimethoxysilane KH-560) contains both hydrolyzable silane groups (-Si(OR)3) and organic functional groups (such as amino or epoxy groups). During application, the silane groups first hydrolyze to generate silanol groups (-Si-OH), which can undergo a dehydration condensation reaction with the hydroxyl groups (-Me-OH) on the surface of the metal substrate to form a strong covalent bond (-Si-O-Me-), thereby chemically anchoring the coupling agent molecule to the substrate surface. Simultaneously, its organic functional groups can chemically react or strongly hydrogen-bond with the carboxyl or hydroxyl groups in the waterborne polyurethane resin, achieving a chemical bridge connection between the coating and the metal substrate. The phosphate group (-PO4) in phosphate ester compounds (preferably phosphate ester acrylate oligomers, added at 1.5% to 2.5% of the total formulation) 3-It possesses strong coordination ability, enabling it to form a dense phosphate passivation layer on the metal surface and chemically adsorb onto the substrate surface through P-O-Me bonds. Its organic segments are intertwined and cross-linked with the coating resin. The synergistic effect of the chemical bonding of the silane coupling agent and the coordination adsorption of the phosphate ester compound significantly enhances the bonding strength between the coating and the low-roughness metal substrate, resulting in a stable paint film adhesion level of Grade 1 or higher.

[0037] The synergistic working mechanism of the thixotropic agent and adhesion promoter is as follows: the thixotropic agent prevents thick film sagging by controlling the rheological behavior of the coating film, providing a uniform thickness basis for the coating film; the adhesion promoter establishes a chemical bond at the interface between the coating film and the substrate. The two work synergistically from the two levels of "macro-sagging control" and "micro-interfacial bonding" to jointly ensure the integrity and bonding strength of the high-thickness one-time forming coating.

[0038] In step 3, the spraying process, air spraying is used under conditions of a spray booth temperature of 20-30℃ and a relative humidity of 50%-70%. During spraying, the distance between the spray gun and the axle surface is maintained at 20-25cm, and the spray gun moves at a uniform speed. A staged rotating spray trajectory control scheme is employed, achieving a dry film thickness of at least 40μm through a single continuous spray. Controlling the environmental parameters during spraying is crucial for ensuring coating quality. Maintaining the spray booth temperature within the range of 20-30℃ and the relative humidity within the range of 50%-70% facilitates normal moisture evaporation, preventing defects such as orange peel or blistering caused by excessively high temperatures and rapid surface drying, while avoiding excessively low humidity that could affect leveling. Maintaining a distance of 20-25cm between the spray gun and the axle surface, with a uniform movement speed, ensures that the paint mist is evenly deposited on the axle surface, achieving a dry film thickness of at least 40μm through a single continuous spray.

[0039] The specific operating parameters, equipment configuration, and trajectory control scheme for the spraying application are as follows: An air spray gun is used, with a nozzle diameter of 1.8mm to 2.0mm, preferably 2.0mm. The spraying pressure is controlled within the range of 0.35MPa to 0.45MPa, preferably 0.40MPa. The distance between the spray gun and the axle surface is maintained within the range of 20cm to 25cm, preferably 22cm. To achieve uniform coverage and resolve the logical conflict between rotary spraying and cross-spraying, this process adopts a staged rotary spraying trajectory control scheme: the spraying process is divided into an axial main spraying stage and a circumferential correction spraying stage. In the axial main spraying stage, the axle rotates continuously at a constant speed of 15r / min to 25r / min, and the spray gun moves uniformly along the axial direction of the axle at a speed of 15cm / s to 25cm / s, spraying 2 to 3 times along the axial direction to ensure full coverage deposition of paint mist in the circumferential direction of the axle. This stage mainly completes the main thickness construction of the paint film. After the axial main spraying stage is completed, the axle stops rotating and remains stationary, entering the circumferential correction spraying stage: the spray gun performs segmented arc spraying along the circumferential direction of the axle (i.e., the cross-sectional direction perpendicular to the axis), with the spray gun's movement trajectory perpendicular to the axle axis. Each 90° rotation constitutes a segment, and the spray gun moves along the arc direction during each segment's spraying. The overlap width between segments is controlled within the range of 3cm to 5cm to eliminate any circumferential thickness unevenness that may occur during the axial main spraying stage and to fill any missed areas at the joints of the spiral trajectory. The time interval between the two spraying stages is controlled within 30 seconds to avoid any joint marks. This staged scheme decouples the rotational motion and cross-spraying in time, utilizing the rotational motion to ensure the circumferential uniformity of the axial spraying while achieving true orthogonal grid superposition through vertical supplementary spraying in a stationary state, fundamentally resolving the logical contradiction that cross-spraying cannot be achieved during dynamic rotation. The target dry film thickness for a single continuous spray is 40μm to 50μm, preferably 45μm.

[0040] Environmental control during spray painting is crucial for ensuring coating quality. The spray booth temperature should be controlled between 20℃ and 30℃, preferably 25℃. Too low a temperature will slow down the paint drying process, prolonging flash-off and curing time; too high a temperature will cause excessively rapid surface drying, affecting leveling. Relative humidity should be controlled between 50% and 70%, preferably 60%. Too low humidity will cause excessively rapid evaporation of moisture from the paint, resulting in dry spraying; too high humidity will prolong drying time and may lead to flash rust. The airflow velocity within the spray booth should be controlled between 0.2m / s and 0.5m / s, and the airflow direction should be coordinated with the spraying direction to avoid interfering with the normal deposition path of the paint mist. The air cleanliness within the spray booth should reach ISO Class 7 (Class 10,000) or higher, with the concentration of airborne dust particles controlled below 35,000 particles / m³ to prevent dust particles from falling onto the uncured paint film surface and causing surface defects. Before application, the water-based paint should be thoroughly stirred and filtered for at least 10 minutes to ensure that all components in the paint are mixed evenly. Use a 200-300 mesh filter to remove agglomerated particles and mechanical impurities from the paint to avoid clogging the spray gun or affecting the appearance quality of the paint film.

[0041] In step 4, the leveling and flash-drying stage, after spraying, the axle is placed in the leveling area for flash-drying for 5 to 10 minutes. This allows some of the moisture and solvent in the paint film to evaporate, eliminating spraying stress. The leveling and flash-drying stage provides sufficient time for the paint film to level naturally and dry initially. During this stage, some of the moisture and solvent in the paint film evaporate, the viscosity of the paint film gradually increases, and it begins to dry initially. At the same time, the spraying stress is released, effectively preventing cracking or peeling caused by internal stress concentration during the subsequent curing process. The flash-drying area should maintain good ventilation to ensure that solvent gases can be discharged in a timely manner.

[0042] The flash-drying endpoint is determined using quantitative control standards, completely replacing subjective "touch-drying" experience-based judgment. This process establishes a dual-parameter joint judgment system of "resistivity-moisture content": a non-contact surface resistivity monitoring probe and an infrared moisture detector are installed in the flash-drying zone. The surface resistivity monitoring probe uses a ring electrode array to continuously measure the surface resistivity of the paint film online at a distance of 5mm to 10mm from the paint film surface; the infrared moisture detector scans the axle shaft at specific wavelengths (1.4μm and 1.9μm dual-band absorption peaks) to calculate the internal moisture content of the paint film in real time. The quantitative criterion for the flash-drying endpoint is: the surface resistivity of the paint film stabilizes at 10... 8 Ω·cm to 10 10Flash drying is considered complete when both conditions are met: resistivity falls within the Ω·cm range and the moisture content of the paint film decreases to between 8% and 12%. This criterion has a clear correlation with the actual surface dryness of the paint film—resistivity falling within this range indicates that a continuous polymer-rich shell has formed on the surface of the paint film, and the moisture content meeting the standard ensures that the internal free moisture is insufficient to cause bubble defects during subsequent heating. Environmental parameters in the flash drying zone are synchronously controlled: the relative humidity in the flash drying zone is set between 45% and 55%, and the temperature is controlled between 23°C and 28°C, maintaining a slightly negative pressure environment through an independent exhaust system. If the above dual-parameter criterion is not met within the preset flash drying time (10 minutes), an alarm is triggered, and the control system continues to extend the flash drying time until the standard is met before automatically releasing the process to the next step. This quantitative criterion mechanism eliminates subjective differences from operators and ensures stable process reproducibility.

[0043] In step 5, the baking and curing stage, the flash-dried axle is placed in an oven for curing and baking at a temperature of 60 to 85°C for 30 to 40 minutes to ensure full cross-linking and curing of the paint film. The baking and curing temperature of 60 to 80°C and the baking time of 30 to 40 minutes are optimized to ensure full cross-linking and curing of the paint film to achieve excellent physicochemical properties, while avoiding yellowing, embrittlement, or energy waste caused by excessively high temperatures or long baking times. During the curing process, the isocyanate groups and hydroxyl groups in the polyurethane system react fully to form a dense three-dimensional cross-linked network structure, giving the paint film excellent hardness, abrasion resistance, and chemical resistance.

[0044] The baking process employs a step-by-step heating and zoned dehumidification coordinated control strategy, fundamentally solving the core problem of dehumidification and blister prevention during baking of high-film-thickness water-based paints. The baking and curing process employs a three-stage stepped heating procedure: The first stage is a low-temperature preheating and dehumidification stage, where the temperature is increased from room temperature to 45°C to 50°C at a rate of 1.5°C / min to 2.0°C / min, and held constant for 8 to 10 minutes. This stage allows residual moisture inside the paint film to slowly and evenly escape under a low temperature gradient, while preventing rapid cross-linking on the surface to form a closed shell. The second stage is a medium-temperature transition stage, where the temperature is increased from 45°C to 50°C to 60°C to 65°C at a rate of 1.0°C / min to 1.5°C / min, and held constant for 5 to 8 minutes. During this stage, the polyurethane cross-linking reaction begins, and the viscosity of the paint film gradually increases, but the cross-linking density is still at a low level, and internal moisture still has channels to escape. The third stage is a high-temperature curing stage, where the temperature is increased from 60°C to 65°C to the final curing temperature of 75°C to 80°C at a rate of 0.5°C / min to 1.0°C / min, and held constant for 15 to 22 minutes, allowing the cross-linking reaction to proceed fully to a conversion rate of over 95%. The total baking time should be controlled between 30 and 40 minutes.

[0045] To complement the stepped heating system, the oven is equipped with three independent dehumidification ducts (shaft end section, shaft section, and transition section) arranged along the axle axis. Each duct is equipped with an independent airflow regulating valve and humidity sensor. In the low-temperature preheating dehumidification section, the dehumidification ducts are fully open, and the exhaust airflow in each section is uniformly set to 800 m³ / h. 3 / h to 1200m 3 / h, forcibly expel moisture and fluxes escaping from the paint film, and control the relative humidity inside the furnace to below 40%; in the medium-temperature transition section, the exhaust volume of the shaft section is reduced to 400m³ / h. 3 / h to 600m 3 / h (this area has the thickest paint film; maintain moderate dehumidification to avoid excessively rapid surface drying), and maintain an exhaust volume of 600m³ / h for the shaft end section and transition section. 3 / h to 800m 3 / h; In the high-temperature curing section, the exhaust volume of each section is uniformly reduced to 300m³ / h. 3 / h to 500m 3 / h, to maintain temperature uniformity within the furnace, at which point the paint film has essentially completed cross-linking, and the residual moisture content is below 2%, eliminating the risk of blistering. Real-time monitoring of exhaust humidity and airflow adjustment in each section form a closed-loop control: when a humidity sensor detects a relative humidity exceeding 45% in a certain section, the exhaust volume in that section is automatically increased by 20% to 30% until the humidity returns to the set range. This synergistic strategy of segmented, stepped heating and zoned dehumidification ensures that moisture inside the paint film escapes smoothly along the temperature gradient, and that the surface and interior cross-link simultaneously, completely avoiding the fatal defect of "surface sealing and internal blistering." Batch verification shows that the blistering rate is controlled below 0.2%.

[0046] The temperature uniformity inside the baking oven should be controlled within ±3℃ to ensure that all parts of the axle can achieve a uniform curing effect. During the baking and curing process, the isocyanate groups (-NCO) and hydroxyl groups (-OH) in the polyurethane system undergo an addition polymerization reaction to generate urethane bonds (-NHCOO-). As the reaction proceeds, the linear molecular chains gradually form a three-dimensional cross-linked network structure, giving the paint film excellent hardness, wear resistance, chemical resistance and adhesion. After baking, the paint film should reach a completely dry state, that is, the hardness of the paint film reaches H level or above, and there is no adhesion on the paint film surface.

[0047] In step 6, the cooling and testing stage, after baking, the axle is allowed to cool naturally to room temperature, and the appearance, thickness and adhesion of the paint film are tested. The specific testing items and judgment criteria are as follows: For paint film appearance inspection, visual inspection is used. Observation is conducted inside a standard light source box with a color temperature of 6500K and an illumination intensity of 1000Lux to 1500Lux. The observation distance is 50cm. The paint film surface should be smooth and free from surface defects such as runs, bubbles, pinholes, orange peel, and shrinkage craters. For paint film thickness inspection, a magnetic thickness gauge is used for non-destructive measurement. Measurement points are selected at the middle, end, and transition areas of the axle body. A measurement point is taken every 120°, and each area is measured three times, with the average value taken. The dry film thickness should not be less than 40μm, preferably 45μm to 55μm, and the thickness uniformity deviation should be controlled within ±5μm. For paint film adhesion inspection, a cross-cut test is used, performed according to GB / T 9286 standard. A 1mm × 1mm grid is drawn on the paint film surface using a single-edged cutting tool, with the scratches penetrating to the metal substrate. After removing debris with a soft brush, a 3M... Apply 600 type tape to the grid area and peel it off quickly. An adhesion level of 0 or 1 is acceptable. Level 0 indicates that the cut edges are completely smooth and there is no peeling. Level 1 indicates that there is a small amount of paint film peeling off at the intersection of the cuts, but the peeling area does not exceed 5%. In addition, the hardness, abrasion resistance, chemical resistance and other indicators of the paint film can be tested as needed to comprehensively evaluate the protective performance and service life of the paint film.

[0048] The test results of this embodiment show that after construction according to the above process parameters, the paint film has a smooth and even appearance, without defects such as sagging, blistering, or pinholes; the dry film thickness is 48μm to 52μm, which meets the technical requirement of not less than 40μm; the adhesion is grade 0 according to the cross-cut test, which is excellent; the VOC emission is reduced by about 55% compared with solvent-based coatings, which has good environmental performance.

[0049] The process flow of this embodiment can be further summarized into the following core technical points: Substrate pretreatment ensures the axle surface cleanliness reaches Sa2 level, and the surface roughness is controlled within the range of Ra≤3.2μm, providing a good adhesion foundation for subsequent coating; the volume solids content of the high-solids water-soluble polyurethane clear varnish is not less than 50%, achieving a dry film thickness of over 40μm with a single spray; the introduction of the thixotropic agent system imparts shear-thinning properties to the coating, effectively preventing sagging under high film thickness conditions; the addition of adhesion promoters enhances the chemical bonding between the coating and the low-roughness metal substrate, achieving an adhesion level of 1 or higher; the spraying environment temperature is controlled between 20℃ and 30℃, and the relative humidity is controlled between 50% and 70%, with a staged rotating spraying trajectory scheme ensuring uniform coverage; the leveling and flash-drying time is controlled between 5 and 10 minutes, and the flash-drying endpoint is quantitatively determined by resistivity and moisture content; baking and curing employ a stepped heating and zoned dehumidification coordinated control strategy to ensure full cross-linking and curing of the paint film without blistering defects.

[0050] Example 2

[0051] To meet the coating requirements of different application scenarios, this embodiment provides a modified coating process for subway vehicle axles. While retaining the core technical solution of this invention, this solution adaptively adjusts some process parameters to meet the higher coating quality requirements of subway vehicle axles.

[0052] In step 1, the substrate pretreatment stage, the same pretreatment process as in Example 1 is used to degrease and remove rust from the surface of the subway vehicle axle, achieving a cleanliness level of Sa2. The subway vehicle axle is made of EA4T alloy steel, with a surface roughness typically Ra≤1.6μm, which is an ultra-smooth surface with even lower roughness. Therefore, the rust removal process in the pretreatment step needs to be appropriately extended, and the mechanical abrasive blasting treatment time needs to be increased by 20% to 30% to ensure a uniform micro-roughening effect on the substrate surface, providing sufficient mechanical bonding for subsequent coating.

[0053] In step 2, the waterborne paint application parameter control stage, the same high-solids waterborne polyurethane clear varnish system as in Example 1 was used. However, considering the ultra-smooth surface characteristics of subway vehicle axles, the proportion of adhesion promoters was optimized and adjusted. The total addition amount of silane coupling agent was increased to 1.2% to 1.5% of the total formulation amount, and the addition amount of phosphate ester compounds was increased to 2.5% to 3.0% of the total formulation amount to enhance the chemical bonding effect on low-roughness substrates. Simultaneously, considering that subway vehicle axles typically operate underground, with high humidity and potential condensation, the addition amount of anti-flash rust agent was increased to 1.2% to 1.5% of the total formulation amount to ensure that flash rust does not occur in high-humidity environments.

[0054] In step 3, the spraying process, the environmental parameters remain consistent with those in Example 1: the spray booth temperature is controlled between 20°C and 30°C, and the relative humidity is controlled between 50% and 70%. The phased rotating spraying trajectory control scheme is also applicable. However, considering the dimensional characteristics of subway vehicle axles, the spray gun movement speed in the axial main spraying stage is appropriately reduced to between 15cm / s and 20cm / s, and the overlap width of the segmented arc spraying in the circumferential correction spraying stage is increased to between 4cm and 6cm to ensure uniform paint mist deposition on the ultra-smooth surface. Simultaneously, given that subway vehicle axles typically have precision components such as bearing seats and journals, these components require special protection before spraying. High-temperature masking tape and protective caps are used to cover and mask these components to prevent paint mist deposition from affecting subsequent assembly.

[0055] In step 4, the leveling and flash-drying stage, the leveling and flash-drying time is appropriately extended to the range of 8 to 12 minutes. The determination of the flash-drying endpoint still adopts the resistivity-moisture content dual-parameter joint system, and the determination threshold is adjusted to resistivity 10. 9Ω·cm to 10 11 Ω·cm and moisture content of 6% to 10% are used to adapt to the ultra-smooth surface characteristics of subway vehicle axles, ensuring that the paint film reaches a suitable surface dry state before entering the baking and curing process.

[0056] In step 5, the baking and curing stage, the baking temperature is increased to the range of 75°C to 85°C, preferably 80°C, and the baking time is extended to the range of 35 minutes to 45 minutes, preferably 40 minutes, to meet the higher requirements of subway vehicle axles for coating protection performance. The stepped heating and zoned dehumidification coordinated control strategy remains unchanged, but the temperature nodes of each stage are increased by 5°C accordingly: the low-temperature preheating and dehumidification stage is increased to 50 to 55°C, the medium-temperature transition stage is increased to 65 to 70°C, and the high-temperature curing stage is increased to 80 to 85°C; the dehumidification air volume setting remains unchanged, and the humidity control threshold is adjusted to 50%. Higher baking temperatures and longer baking times can promote more complete cross-linking and curing of the polyurethane system, forming a denser three-dimensional network structure, giving the coating film higher hardness and better chemical resistance.

[0057] In step 6, the cooling and testing stage, the appearance, thickness, and adhesion of the paint film are tested using the same testing method as in Example 1. The coating quality requirements for subway vehicle axles are even higher; the dry film thickness should be no less than 45 μm, preferably 50 μm to 60 μm; the adhesion grade should reach level 0; in addition, salt spray resistance testing must be conducted according to GB / T 1771 standard, with a testing cycle of 240 hours, and the paint film should show no blistering, peeling, or rust.

[0058] The test results of this embodiment show that after construction according to the adjusted process parameters, the paint film has a smooth and even appearance, without defects such as sagging, blistering, or pinholes; the dry film thickness is 52μm to 58μm, which meets the technical requirement of not less than 45μm; the adhesion is grade 0 according to the cross-cut test; and the salt spray resistance reaches 240 hours without abnormalities, meeting the stringent requirements of subway vehicle axles for coating protection performance.

[0059] Example 3

[0060] To verify the applicability of the process of this invention under different climatic conditions, this embodiment provides a coating process solution for high humidity environments in mountainous areas of southern China. The climate characteristics of mountainous areas in southern China are high annual average relative humidity, typically between 70% and 85%, with summer relative humidity reaching over 90%, and frequent rainfall, making environmental control for coating operations quite challenging. This embodiment, based on the core technical solution of Embodiment 1, adapts the process parameters to the characteristics of high humidity environments.

[0061] In step 1, the substrate pretreatment stage, forced drying is used instead of natural drying after pretreatment to ensure the substrate surface is thoroughly dried in a high-humidity environment. Forced drying uses hot air drying equipment, with the drying temperature controlled between 50°C and 60°C, the drying time no less than 20 minutes, and the hot air velocity controlled between 2m / s and 3m / s. After forced drying, an infrared moisture detector is used to detect the moisture content on the axle surface, ensuring the surface moisture content is below 0.1% before proceeding to the next process. Furthermore, an independent temperature and humidity control unit is set up in the pretreatment area to control the relative humidity of the treatment area below 60%, preventing condensation and the formation of a water film on the substrate surface due to high humidity.

[0062] In step 2, the water-based paint application parameter control stage, the following adjustments are made to the water-based paint formulation to address the characteristics of high humidity environments: First, the amount of thixotropic agent added is increased, with the amount of organically modified bentonite increased to 1.0% to 1.5% of the total formulation amount, and the amount of fumed silica increased to 0.8% to 1.0% of the total formulation amount, to enhance the thixotropic properties of the coating system and improve its anti-sagging ability; Second, the application viscosity is reduced, with the viscosity controlled within the range of 45 to 50 seconds using the Forecast-4 cup. Lower viscosity facilitates the evaporation of moisture in the coating and accelerates film drying; Third, the amount of anti-flash rust agent added is increased to 1.0% to 1.5% of the total formulation amount to prevent flash rust in high humidity environments; Fourth, an appropriate amount of film-forming aid is added to the formulation, with propylene glycol butyl ether selected as the film-forming aid, added at 2% to 3% of the total formulation amount. The addition of the film-forming aid can lower the minimum film-forming temperature of the coating and promote the formation of a continuous and complete film even in high humidity environments.

[0063] In step 3, the spraying process, environmental control is crucial. An independent dehumidifier is installed in the spray booth to maintain relative humidity between 50% and 65%, preferably 55% to 60%. Although the external humidity is high, the continuous operation of the dehumidifier ensures that the relative humidity inside the spray booth remains within the required range. During spraying, the local exhaust system in the spray booth is activated to promptly remove overspray paint mist, while maintaining a balance between exhaust and supply air volumes to ensure stable airflow and prevent uneven paint mist dispersion or deposition. In the phased rotary spraying trajectory control scheme, the spray gun movement speed is appropriately reduced to between 12cm / s and 18cm / s during the axial main spraying stage to ensure sufficient time for partial drying of the paint mist before reaching the substrate surface, reducing the risk of paint dripping. The overlap width during the circumferential correction spraying stage is increased to 4cm to 6cm.

[0064] In step 4, the leveling and flash-drying stage, the relative humidity in the flash-drying zone needs to be controlled more strictly, set to 40% to 50%, and the threshold for determining the flash-drying endpoint is adjusted to a resistivity of 10. 7 Ω·cm to 109 The resistivity is set at Ω·cm, and the moisture content is 10% to 14% to accommodate the slow evaporation of moisture in high humidity environments. A lower lower resistivity limit and a higher upper moisture content limit allow the paint film to reach a surface-dry state in an engineering sense in high humidity environments, avoiding excessively prolonged flash-drying time that could lead to moisture absorption and re-wetting. Simultaneously, a forced ventilation system in the flash-drying zone maintains directional airflow, ensuring the paint film surface remains in a low local water vapor pressure environment, promoting continuous moisture escape. An auxiliary dehumidification device is installed in the flash-drying zone to control the relative humidity within the range of 40% to 50%, while maintaining good ventilation to promote the evaporation and removal of moisture and co-solvents from the paint film.

[0065] In step 5, the baking and curing stage, the baking temperature is increased to the range of 75°C to 85°C, preferably 80°C, and the baking time is maintained at 30 to 40 minutes. In the step-by-step heating and zoned dehumidification coordinated control strategy, the low-temperature preheating and dehumidification section is extended to 12 to 15 minutes (the heating rate is reduced to 1.0°C / min to 1.5°C / min) to provide a more sufficient time window for the moisture inside the paint film to escape in a high-humidity environment; the dehumidification air volume in each section is uniformly increased by 20% to 30%, the forced dehumidification rate is enhanced, and the target relative humidity control in the oven is tightened to below 35%. The higher baking temperature and enhanced dehumidification capacity can accelerate the evaporation of moisture in the paint film and the cross-linking and curing of the polyurethane system, reducing the impact of the high-humidity environment on the final paint film performance. During the baking process, the dehumidification function of the baking oven is turned on to promptly remove the hot and humid gas in the oven and prevent moisture from circulating in the oven and affecting the curing effect.

[0066] In step 6, the cooling and inspection stage, a more rigorous visual inspection is required after cooling, paying particular attention to any signs of flash rust, water spots, or other defects that may occur under high humidity conditions. The testing methods for performance indicators such as paint film thickness and adhesion are consistent with those in Example 1.

[0067] The test results of this embodiment show that, under the high humidity conditions of the southern mountainous area, after construction according to the adjusted process parameters, the paint film has a smooth and even appearance, without defects such as sagging, blistering, or pinholes, and no signs of flash rust; the dry film thickness is 45μm to 52μm, meeting the technical requirement of not less than 40μm; the adhesion, tested by the cross-cut test, is grade 0 to 1; and the VOC emission is reduced by approximately 52% compared to solvent-based coatings. These results demonstrate that the coating process of this invention can adapt to the construction requirements under high humidity conditions and has good process adaptability.

[0068] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-film-thickness, one-step coating process for water-based paint on the surface of rail transit vehicle axles, characterized in that, Includes the following steps: Step 1, substrate pretreatment: The surface of the axle substrate is degreased and derusted to remove organic contaminants, oxide scale and rust products. After treatment, it is dried naturally to ensure that the surface is free of oil, moisture and dust. Step 2, control of water-based paint application parameters: Select high-solids water-based polyurethane varnish as the coating material. Before application, adjust the water-based paint to the application viscosity, with a volume solids content of not less than 50%. Step 3, spraying construction, using air spraying, spraying is carried out under the conditions of spray booth temperature of 20-30℃ and relative humidity of 50%-70%, using a staged rotating spraying trajectory control scheme, and the dry film thickness reaches the process requirement value through one continuous spraying. Step 4, leveling and flash-drying: After spraying, place the axle in the leveling area for flash-drying to allow some of the moisture and solvent in the paint film to evaporate, eliminating spraying stress. The flash-drying endpoint is quantitatively determined by a combination of surface resistivity and infrared moisture content detection. Step 5, baking and curing: The flash-dried axle is sent into the baking oven for curing and baking. The baking process adopts a three-stage stepped heating and multi-stage independent dehumidification coordinated control strategy to ensure that the paint film is fully cross-linked and cured. Step 6, Cooling and Inspection: After baking, allow the axle to cool naturally to room temperature and inspect the appearance, thickness, and adhesion of the paint film.

2. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, The phased rotational spraying trajectory control scheme in step 3 includes an axial main spraying stage and a circumferential correction spraying stage: In the axial main spraying stage, the axle rotates continuously at a constant speed of 15-25 r / min, and the spray gun moves at a constant speed along the axial direction of the axle, spraying back and forth 2-3 times along the axial direction; after the axial main spraying stage is completed, the axle stops rotating and remains stationary, and enters the circumferential correction spraying stage, where the spray gun performs segmented arc spraying along the circumferential direction of the axle, with a specified overlap width between each segment.

3. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, The quantitative judgment criteria for the flash-drying endpoint in step 4 are as follows: the surface resistivity of the paint film is measured online using a non-contact ring electrode array, and the internal moisture content of the paint film is calculated in real time using a dual-band infrared absorption peak scanning method. When the surface resistivity reaches the first threshold range and the moisture content of the paint film drops to the second threshold range, the flash-drying is judged to be completed. When the flash-drying time reaches the preset upper limit value but the two conditions are still not met, the control system automatically extends the flash-drying time until the two conditions are met simultaneously before releasing the vehicle.

4. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, The three-stage stepped heating program in step 5 is as follows: the first stage is a low-temperature preheating and dehumidification stage, in which the temperature is increased from room temperature to 45-50℃ at a rate of 1.5-2.0℃ / min and held constant for 8-10 minutes; the second stage is a medium-temperature transition stage, in which the temperature is increased to 60-65℃ at a rate of 1.0-1.5℃ / min and held constant for 5-8 minutes; the third stage is a high-temperature curing stage, in which the temperature is increased to 75-80℃ at a rate of 0.5-1.0℃ / min and held constant for 15-22 minutes.

5. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, The multi-segment independent dehumidification coordinated control strategy in step 5 is as follows: the baking oven is equipped with three independent dehumidification ducts arranged along the axle axis, and each duct is equipped with an independent air volume regulating valve and humidity sensor; in the low-temperature preheating dehumidification section, the exhaust air volume of each section is uniformly set to 800-1200 m³ / h. 3 / h; In the intermediate temperature transition section, the exhaust volume of the shaft section is reduced to 400-600 m³ / h, while the exhaust volume of the shaft end section and the transition section is maintained at 600-800 m³ / h. 3 / h; In the high-temperature curing section, the exhaust volume of each section is uniformly reduced to 300-500m³ / h; When the humidity sensor of any section detects that the relative humidity exceeds the set threshold, the exhaust volume of that section is automatically increased until the humidity drops back to the set range.

6. The high-film-thickness one-step coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, In step 2, the water-based paint is a two-component high-solids water-based polyurethane varnish. The main agent is a compound of water-based acrylic dispersion and water-based polyurethane dispersion. The curing agent is a hydrophilic modified isocyanate curing agent. The main agent and curing agent are mixed according to the molar ratio of hydroxyl to isocyanate groups. The application viscosity is adjusted by adding deionized water, and the viscosity of the Forecast cup 4 is controlled within the range of 50-55s.

7. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, In step 2, a thixotropic agent system is added to the water-based paint formulation to impart shear-thinning properties to the coating. The viscosity decreases at high shear rates to facilitate atomization, and the viscosity recovers at low shear rates to prevent thick film sagging. The thixotropic agent is a compound system of organic modified bentonite and fumed silica. An adhesion promoter is also added to the water-based paint formulation to enhance the chemical bonding between the coating and the metal substrate. The adhesion promoter is a compound system of silane coupling agent and phosphate ester compound.

8. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, The substrate pretreatment in step 1 includes: degreasing treatment by soaking and cleaning with an alkaline cleaning agent, followed by high-pressure rinsing with deionized water; rust removal treatment by mechanical abrasive spraying, using steel grit or stainless steel grit, with the spraying pressure controlled at 0.5-0.7 MPa and the spraying angle maintained at 70°-80°; and the surface roughness of the treated axle is controlled within the range of Ra≤3.2μm.

9. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, In step 3, the dry film thickness of a single continuous spraying is not less than 40μm. During spraying, the distance between the spray gun and the axle surface is maintained at 20-25cm, and the air cleanliness in the spray booth reaches ISO 7 or above. In step 6, the paint film adhesion reaches level 1 or above according to the cross-cut test.

10. The high-film-thickness one-time coating process for water-based paint on the axle surface of rail transit vehicles according to claim 1, characterized in that, This process is suitable for construction in humid climates. The relative humidity of the spray booth is controlled within the range of 50%-70%, and the relative humidity of the flash drying zone is controlled within the range of 45%-55%. The flash drying zone maintains the specified humidity environment through an independent exhaust system and an auxiliary dehumidification device. In step 5, the baking and curing temperature is 60-80℃, and the baking time is 30-40 minutes.

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