Coupling flash dry time limit film thickness and sag risk prediction method of curved surface features

Abstract

The application discloses a sagging risk prediction method coupling flash-off time limit film thickness and surface characteristics, and establishes a limit anti-sagging model of automobile real wood interior PUR coating film forming on a three-dimensional curved surface, so that the limit film thickness of the PUR coating film forming can be accurately quantified and characterized, and meanwhile, a substrate correction coefficient is introduced for different wood substrates, so that the limit film thickness of the PUR coating on different wood substrates can be more accurately predicted; and the application constructs a mathematical model based on the rheological critical sagging theory, quantifies and characterizes the mapping relationship between the geometric parameters of the key spraying three-dimensional curved surface and the sagging risk index of the PUR coating film forming process, and realizes the sagging risk prediction and the spraying parameter optimization design.

Description

Technical Field

[0001] This invention belongs to the technical field of automotive wood interior paint processing, specifically involving a method for predicting sagging risk by coupling flash-drying time limit film thickness and surface characteristics. Background Technology

[0002] As a key application area in high-end automotive interiors, genuine wood interior trim pieces offer significant advantages in terms of decorative texture and visual appeal. During the PUR paint application process for automotive genuine wood interior trim pieces, the total coating thickness typically needs to reach 500μm or even greater. Since this thickness is difficult to achieve in a single coat, multiple coats are necessary. To reduce the number of coats and conserve coating resources, operators tend to increase the film thickness of each coat. However, when the shear flow generated by a thick wet film exceeds a certain limit under gravity, it will form strip-like or teardrop-shaped marks on the workpiece surface, resulting in surface defects and rework. Traditional PUR paint application processes rely on empirical parameters and have the following problems: 1. It can only provide a global, most conservative film thickness limit. Based on the film thickness limit, either efficiency is sacrificed to maintain the quality of the painting process, or efficiency is guaranteed, which leads to frequent defects in the local area of ​​the paint film. The entire PUR painting process relies heavily on experience and trial and error. There is a lack of a theoretical model for quantitatively calculating the limit anti-sagging film thickness of PUR paint for automotive wood interiors. The anti-sagging performance cannot be accurately predicted. 2. In real wood decorative parts, when coating drip defects exceed the threshold that can be recognized by the human eye, such as when the coating drip defects are greater than 1mm, the consistency of light and shadow reflection of the wood grain is destroyed, resulting in distortion of the intended decorative effect, causing significant visual defects and a decline in decorative quality. Existing technical solutions have not established a correlation model between wood grain texture and drip defects, and cannot achieve anti-drip control while ensuring the decorative effect, resulting in a rework rate as high as 15-20%. 3. Real wood interior trim pieces, such as dashboards, center consoles, and door panel trims, have complex three-dimensional curved surface structures. They are constrained by multiple curved surface shapes such as vertical surfaces, chamfered surfaces, and R-corner surfaces, and are affected by the combined effects of spraying angle, paint film thickness, and gravity. Traditional spraying solutions only adjust the spraying parameters and do not achieve multi-curved surface linkage control, resulting in a sagging defect improvement rate of less than 30%.

[0003] Therefore, based on the aforementioned problems in the existing technology of PUR paint spraying process for automotive wood interior parts, this invention discloses a method for predicting sagging risk by coupling flash-drying time limit film thickness and surface characteristics. Summary of the Invention

[0004] This invention discloses a method for predicting sagging risk by coupling flash-drying time limit film thickness with surface features. It can couple the effective curvature of the sprayed three-dimensional surface with sagging, and couple the film thickness of the coating film with the effective curvature of sagging. It establishes a prediction model for the limit film thickness on the complex morphology of the sprayed three-dimensional surface, which can more accurately predict the morphology of the sprayed film.

[0005] This invention is achieved through the following technical solution: A method for predicting sagging risk by coupling the flash-drying time-limited film thickness with surface geometry includes the following steps: Step 1: Using the wood veneer layer as the base film, the wood veneer layer is processed onto the substrate layer through an insert injection molding process. Then, the UV-resistant release agent is sprayed onto the surface of the wood veneer layer to form a UV-resistant release agent layer. Step 2: Apply the colored paint to the surface of the UV-resistant release agent layer using a spraying process to form a colored paint layer, and then apply the sealing paint to the surface of the colored paint layer using a spraying process to form a sealing paint layer. Step 3: Establish an ultimate anti-sagging model that correlates the critical sag distance and the ultimate film thickness of PUR coatings, and introduce a substrate correction coefficient for different types of wood into the ultimate anti-sagging model to calculate the ultimate film thickness of PUR coatings based on flash-drying time. Step 4: Extract feature points on the sprayed 3D surface, calculate the effective curvature of the sag at the feature points, and determine the surface characteristics of the sprayed 3D surface at the feature points based on the effective curvature of the sag. If the surface characteristics are planar, proceed to step 5; if the surface characteristics are non-planar, proceed to step 6. Step 5: Use the limit film thickness in Step 3 as the film thickness of PUR material sprayed on the three-dimensional curved surface, without the need for prediction; Step 6: Calculate the effective film thickness of PUR coating on the sprayed three-dimensional curved surface based on the effective curvature of the sag, establish a sag risk index model at the feature point based on the effective film thickness, and predict the film thickness based on the sag risk index model. Step 7: Based on the prediction results, use a spraying process to spray the PUR coating multiple times onto the sealing paint layer to form a PUR coating layer.

[0006] To better realize the present invention, step 4 further includes: Step 4.1: Extract feature points on the sprayed 3D curved surface, establish a tangent plane at the feature points, and select a first principal direction and a second principal direction that are perpendicular to each other within the tangent plane; Step 4.2: Extract the first principal curvature of the sprayed three-dimensional surface in the first principal direction, and extract the second principal curvature of the sprayed three-dimensional surface in the second principal direction; extract the gravity projection direction on the tangent plane, and calculate the angle between the gravity projection direction and the first principal direction; Step 4.3: Calculate the effective curvature of the sprayed three-dimensional surface along the gravity projection direction at the feature point based on the included angle, the first principal curvature, and the second principal curvature; Step 4.4: If the effective curvature of the sag is equal to 0, then the surface characteristic is determined to be planar; if the effective curvature of the sag is not equal to 0, then the surface characteristic is determined to be non-planar.

[0007] To better realize the present invention, the formula for calculating the effective curvature of the sag is further as follows: ; in: Indicates the effective curvature of the sag; Indicates the first principal curvature; Indicates the second principal curvature; This represents the angle between the direction of gravity projection and the first principal direction.

[0008] To better realize the present invention, step 6 further includes: Step 6.1: The thickness of the film formed by spraying PUR coating on the planar characteristic area of ​​the sprayed three-dimensional curved surface is used as the calibration thickness. The effective film thickness is obtained by correcting the calibration thickness through the effective curvature of the sag. Step 6.2: Extract the local normal vector of the tangent plane at the feature point, and calculate the angle between the local normal vector and the direction of reverse gravity; Step 6.3: Establish a sagging risk index model based on the effective film thickness and the angle between the local normal vector and the anti-gravity direction; Step 6.4: Calculate the sag risk index based on the sag risk index model.

[0009] To better realize the present invention, the formula for calculating the effective film thickness is further as follows: ; in: Indicates the effective film thickness; Indicates the initial film thickness; Represents the coupling constant; This indicates the effective curvature of the sag.

[0010] To better realize the present invention, the sag risk index model is further defined as follows: ; in: Indicates the risk index of spillage; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. This represents the angle between the local normal vector and the direction of the opposite gravity. Indicates the effective film thickness.

[0011] To better realize the present invention, step 3 further includes: Step 3.1: Treat the paint film formed by the flow of PUR coating as a Newtonian fluid and calculate the average flow velocity of PUR coating on the flat plate; Step 3.2: Collect the critical sag distance of the PUR coating, and calculate the characteristic sag time of the PUR coating based on the ratio of the critical sag distance to the average flow velocity; Step 3.3: Establish an ultimate anti-sagging model with the constraint that the characteristic sag time of PUR coating is greater than or equal to the flash-drying time of PUR coating process parameter; Step 3.4: After introducing a substrate correction factor into the ultimate anti-sagging model, calculate the ultimate film thickness of the PUR coating based on flash-drying time.

[0012] To better realize the present invention, the calculation formula for the characteristic flow time is further as follows: ; in: Indicates the characteristic flow time; This represents the average flow velocity of the PUR coating on the flat plate. Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. This indicates the angle of inclination between the flow plate surface and the horizontal plane.

[0013] To better realize the present invention, further, in step 3.4, the ultimate anti-sagging model after introducing the substrate correction coefficient is as follows: ; in: Indicates the maximum film thickness; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. Indicates the angle of inclination between the flow plate surface and the horizontal plane; This indicates the flash drying time, a process parameter. This indicates the substrate correction factor.

[0014] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention establishes an ultimate anti-sagging model for PUR coatings used in automotive wood interiors when sprayed onto a three-dimensional curved surface. This model accurately quantifies the ultimate film thickness of the PUR coating. Furthermore, it introduces a substrate correction coefficient for different wood substrates, enabling more accurate prediction of the ultimate film thickness of the PUR coating on various types of wood. The invention also constructs a mathematical model based on rheological critical sagging theory, quantifying the mapping relationship between the geometric parameters of the key three-dimensional spraying surface and the sagging risk index of the PUR coating film formation process, thus achieving sagging risk prediction and optimized design of spraying parameters. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the method flow of the present invention; Figure 2 This is a schematic diagram of PUR coating flowing on a flow plate to form a paint film. Figure 3 This is a schematic diagram of the tangent plane. Detailed Implementation

[0016] Example 1: This embodiment discloses a method for predicting sagging risk by coupling the flash-drying time-limited film thickness with the surface geometry, such as... Figure 1 As shown, it includes the following steps: Step 1: Using the wood veneer layer as the base film, the wood veneer layer is processed onto the substrate layer through an insert injection molding process. Then, the UV-resistant release agent is sprayed onto the surface of the wood veneer layer to form a UV-resistant release agent layer. The thickness of the UV-resistant release agent layer is controlled between 1μm and 3μm. The UV-resistant release agent layer can provide anti-ultraviolet and anti-aging functions. Step 2: Apply colored paint to the surface of the UV-resistant release agent layer using a spraying process to form a colored paint layer. The thickness of the colored paint layer is controlled between 7μm and 10μm. Then, apply a sealing paint to the surface of the colored paint layer using a spraying process to form a sealing paint layer. The thickness of the sealing paint layer is controlled between 30μm and 40μm. The sealing paint layer seals the wood veneer layer and evens out the physical properties of the wood veneer surface. Step 3: Establish an ultimate anti-sagging model that correlates the critical sag distance and the ultimate film thickness of PUR coatings, and introduce a substrate correction coefficient for different types of wood into the ultimate anti-sagging model to calculate the ultimate film thickness of PUR coatings based on flash-drying time. Step 4: Extract feature points on the sprayed 3D surface, calculate the effective curvature of the sag at the feature points, and determine the surface characteristics of the sprayed 3D surface at the feature points based on the effective curvature of the sag. If the surface characteristics are planar, proceed to step 5; if the surface characteristics are non-planar, proceed to step 6. Step 5: Use the limit film thickness in Step 3 as the film thickness of PUR material sprayed on the three-dimensional curved surface, without the need for prediction; Step 6: Calculate the effective film thickness of PUR coating on the sprayed three-dimensional curved surface based on the effective curvature of the sag, establish a sag risk index model at the feature point based on the effective film thickness, and predict the film thickness based on the sag risk index model. Step 7: Based on the prediction results, the PUR coating is sprayed multiple times onto the sealing paint layer to form a PUR coating layer, which creates a mirror-like high-gloss effect.

[0017] To better realize the present invention, step 3 further includes: Step 3.1: Treat the paint film formed by the flow of PUR coating as a Newtonian fluid and calculate the average flow velocity of PUR coating on the flat plate; Step 3.2: Collect the critical sag distance of the PUR coating, and calculate the characteristic sag time of the PUR coating based on the ratio of the critical sag distance to the average flow velocity; Step 3.3: Establish an ultimate anti-sagging model with the constraint that the characteristic sag time of PUR coating is greater than or equal to the flash-drying time of PUR coating process parameter; Step 3.4: After introducing a substrate correction factor into the ultimate anti-sagging model, calculate the ultimate film thickness of the PUR coating based on flash-drying time.

[0018] To better realize the present invention, the calculation formula for the characteristic flow time is further as follows: ; in: Indicates the characteristic flow time; This represents the average flow velocity of the PUR coating on the flat plate. Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. This indicates the angle of inclination between the flow plate surface and the horizontal plane.

[0019] To better realize the present invention, further, in step 3.4, the ultimate anti-sagging model after introducing the substrate correction coefficient is as follows: ; in: Indicates the maximum film thickness; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. Indicates the angle of inclination between the flow plate surface and the horizontal plane; This indicates the flash drying time, a process parameter. This indicates the substrate correction factor.

[0020] To better realize the present invention, step 4 further includes: Step 4.1: Extract feature points on the sprayed 3D curved surface, establish a tangent plane at the feature points, and select a first principal direction and a second principal direction that are perpendicular to each other within the tangent plane; Step 4.2: Extract the first principal curvature of the sprayed three-dimensional surface in the first principal direction, and extract the second principal curvature of the sprayed three-dimensional surface in the second principal direction; extract the gravity projection direction on the tangent plane, and calculate the angle between the gravity projection direction and the first principal direction; Step 4.3: Calculate the effective curvature of the sprayed three-dimensional surface along the gravity projection direction at the feature point based on the included angle, the first principal curvature, and the second principal curvature; Step 4.4: If the effective curvature of the sag is equal to 0, then the surface characteristic is determined to be planar; if the effective curvature of the sag is not equal to 0, then the surface characteristic is determined to be non-planar.

[0021] To better realize the present invention, the formula for calculating the effective curvature of the sag is further as follows: ; in: Indicates the effective curvature of the sag; Indicates the first principal curvature; Indicates the second principal curvature; This represents the angle between the direction of gravity projection and the first principal direction.

[0022] To better realize the present invention, step 6 further includes: Step 6.1: The thickness of the film formed by spraying PUR coating on the planar characteristic area of ​​the sprayed three-dimensional curved surface is used as the calibration thickness. The effective film thickness is obtained by correcting the calibration thickness through the effective curvature of the sag. Step 6.2: Extract the local normal vector of the tangent plane at the feature point, and calculate the angle between the local normal vector and the direction of reverse gravity; Step 6.3: Establish a sagging risk index model based on the effective film thickness and the angle between the local normal vector and the anti-gravity direction; Step 6.4: Calculate the sag risk index based on the sag risk index model.

[0023] To better realize the present invention, the formula for calculating the effective film thickness is further as follows: ; in: Indicates the effective film thickness; Indicates the initial film thickness; Represents the coupling constant; This indicates the effective curvature of the sag.

[0024] To better realize the present invention, the sag risk index model is further defined as follows: ; in: Indicates the risk index of spillage; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. This represents the angle between the local normal vector and the direction of the opposite gravity. Indicates the effective film thickness.

[0025] Example 2: This embodiment discloses a method for predicting sagging risk by coupling the flash-drying time-limited film thickness with the surface geometry, which is an optimization based on Embodiment 1, specifically as follows: The sagging problem in coatings is a competition between the sagging process and the fixation process on a time scale. The sagging process is driven by gravity, during which the paint film flows a specific distance within a given time period, and its speed is characterized by the sagging time. To characterize it. The immobilization process is characterized by a rapid increase in viscosity driven by solvent evaporation and chemical reactions, and the time required for the surface coating viscosity to increase sufficiently to effectively freeze the flow. Given process parameters, flash drying time... This refers to the upper limit of the time for this fixation process.

[0026] Therefore, the limiting conditions for preventing paint sagging and failure are: ≥ ; Since sagging mainly occurs in the initial stage when the viscosity is lowest, this embodiment has been simplified as follows to derive a simple, clear formula that is easy to apply in engineering: (1) At low shear rates, the paint film is considered as a Newtonian fluid without yield stress; the shear rate refers to a shear rate between 10 and 10. -9 s -1 -10s -1 between; (2) During the flash-drying time, the viscosity of the coating remains at the initial viscosity. The density of the coating remains at its initial density. ; (3) The coating film thickness is maintained at the initial film thickness; For the angle of inclination is A Newtonian fluid film in steady state, incompressible, and fully developed laminar flow on a flat plate, with a flow velocity v y Represented as: ; Where: h represents the coating thickness; y represents the vertical distance of the fluid from the stationary wall to any position within the flow layer, 0≤y≤h.

[0027] Calculate the average flow velocity of PUR coating on a flat plate: ; in: Indicates the characteristic flow time; This represents the average flow velocity of the PUR coating on the flat plate. Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. Indicates the angle of inclination between the flow plate surface and the horizontal plane; This indicates the initial film thickness.

[0028] The characteristic flow hanging time is calculated as follows: ≥ ; Further deduction: ; It should be noted that, as Figure 2 As shown, This represents the angle between the paint film flow plate surface and the horizontal plane. In reality, the paint film flow plate surface and the horizontal plane cannot be perfectly parallel, hence the angle... It can only approach 0° infinitely, but it can never equal 0°, that is... It can only approach 0 infinitely, but cannot equal 0. When the included angle... When it approaches 0°, at this point Approaching infinity.

[0029] Based on the above formula The maximum value of the limit film thickness of the two-component PUR coating based on flash-drying time is expressed as: ; Considering the differences in paint permeability among different wood veneers, a wood substrate correction coefficient is introduced to distinguish different wood types, resulting in the ultimate anti-sagging model after incorporating the substrate correction coefficient. ; in: Indicates the maximum film thickness; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. Indicates the angle of inclination between the flow plate surface and the horizontal plane; This indicates the flash drying time, a process parameter. This indicates the substrate correction factor, 0.60≤ ≤0.95, The specific value is determined based on the type of wood used as the base material.

[0030] For complex three-dimensional curved surfaces, both the gravitational tangential direction and film thickness distribution are related to local geometry. The effective curvature of the sag is used to correct the film thickness, and a location-dependent sag risk index model is constructed to express the surface location characteristics. The sag risk index model is as follows: ; in: Indicates the risk index of spillage; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. This represents the angle between the local normal vector and the direction of the opposite gravity. Indicates the effective film thickness.

[0031] The steps to calculate the effective curvature of the sag are as follows: like Figure 3 As shown, feature points are extracted on the sprayed 3D curved surface. A tangent plane is established at the feature points, and mutually perpendicular first principal directions d1 and second principal directions d2 are selected within the tangent plane. The first principal curvature k1 of the sprayed 3D curved surface on the first principal direction d1 is extracted, the second principal curvature k2 of the sprayed 3D curved surface on the second principal direction d2 is extracted, and the gravity projection direction t on the tangent plane is extracted. g And calculate the gravitational projection direction t g The angle between the first principal direction d1 and the first principal direction d1 The first principal direction d1 and the second principal direction d2 are preferably the directions with the largest absolute value of curvature and the directions with the smallest absolute value of curvature, respectively.

[0032] Based on the included angle The first principal curvature k1 and the second principal curvature k2 are used to calculate the three-dimensional surface to be sprayed along the gravitational projection direction t at the feature point. g The effective curvature of the sag is as follows: ; in: Indicates the effective curvature of the sag; Indicates the first principal curvature; Indicates the second principal curvature; This represents the angle between the direction of gravity projection and the first principal direction.

[0033] The effective film thickness of PUR coatings sprayed onto a three-dimensional curved surface is calculated based on the effective curvature of the sag. ; in: Indicates the effective film thickness; Indicates the initial film thickness; Represents the coupling constant; This indicates the effective curvature of the sag.

[0034] like If the value is 0, the surface characteristic is judged to be planar, indicating that the sprayed three-dimensional surface does not bend along the first principal direction and the second principal direction, and can be regarded as a plane. In this case, the ultimate film thickness is directly calculated using the ultimate anti-sagging model. The thickness of the film formed by spraying PUR material onto a three-dimensional curved surface.

[0035] like >0, at this time, > If the surface characteristics are determined to be concave and non-planar, it is characterized by gravity driving the PUR coating to converge towards the bottom of the concave surface, causing the paint film formed at the bottom of the concave surface to be compressed and accumulated, making it more prone to sagging.

[0036] like <0, at this time, If the surface characteristics are determined to be non-planar characteristics of a convex surface, it is characterized by gravity driving the PUR coating to spread from the top of the convex surface to the surrounding areas, which stretches the paint film, reduces the amount of paint per unit area, and makes it less prone to sagging.

[0037] When the sprayed three-dimensional curved surface exhibits non-planar characteristics, let With the limit of film thickness Replace the initial film thickness Then we have: ; Based on the above sag prediction, a real wood interior trim component of a car with typical high-risk features such as deep concave surfaces was selected as the prediction object, and a two-component thermosetting polyurethane topcoat was used for spraying. The control group used a uniform film thickness, and the traditional empirical process of setting the vertical surface single-gun spraying thickness to no more than 40μm was used. The experimental group used the above-mentioned differentiated film thickness control based on the sag risk field of curved surfaces as a reference for spraying. A total of 10 sprays were applied, with a total spray thickness of 500μm. 300 sets of spraying were completed for each group. The comparison results are shown in Table 1 below: Table 1. Comparison of Spraying Effects

[0038]

[0039] The rest of this embodiment is the same as that of Embodiment 1, so it will not be described again.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for predicting sagging risk by coupling flash-drying time-limited film thickness and surface characteristics, characterized in that, Includes the following steps: Step 1: Using the wood veneer layer as the base film, the wood veneer layer is processed onto the substrate layer through an insert injection molding process. Then, the UV-resistant release agent is sprayed onto the surface of the wood veneer layer to form a UV-resistant release agent layer. Step 2: Apply the colored paint to the surface of the UV-resistant release agent layer using a spraying process to form a colored paint layer, and then apply the sealing paint to the surface of the colored paint layer using a spraying process to form a sealing paint layer. Step 3: Establish an ultimate anti-sagging model that correlates the critical sag distance and the ultimate film thickness of PUR coatings, and introduce a substrate correction coefficient for different types of wood into the ultimate anti-sagging model to calculate the ultimate film thickness of PUR coatings based on flash-drying time. Step 4: Extract feature points on the sprayed 3D surface, calculate the effective curvature of the sag at the feature points, and determine the surface characteristics of the sprayed 3D surface at the feature points based on the effective curvature of the sag. If the surface characteristics are planar, proceed to step 5; if the surface characteristics are non-planar, proceed to step 6. Step 5: Use the limit film thickness in Step 3 as the film thickness of PUR material sprayed on the three-dimensional curved surface, without the need for prediction; Step 6: Calculate the effective film thickness of PUR coating on the sprayed three-dimensional curved surface based on the effective curvature of the sag, establish a sag risk index model at the feature point based on the effective film thickness, and predict the film thickness based on the sag risk index model. Step 7: Based on the prediction results, use a spraying process to spray the PUR coating multiple times onto the sealing paint layer to form a PUR coating layer.

2. The method for predicting sagging risk based on coupled flash-drying time limit film thickness and surface characteristics according to claim 1, characterized in that, Step 4 specifically includes: Step 4.1: Extract feature points on the sprayed 3D curved surface, establish a tangent plane at the feature points, and select a first principal direction and a second principal direction that are perpendicular to each other within the tangent plane; Step 4.2: Extract the first principal curvature of the sprayed three-dimensional surface in the first principal direction, and extract the second principal curvature of the sprayed three-dimensional surface in the second principal direction; extract the gravity projection direction on the tangent plane, and calculate the angle between the gravity projection direction and the first principal direction; Step 4.3: Calculate the effective curvature of the sprayed three-dimensional surface along the gravity projection direction at the feature point based on the included angle, the first principal curvature, and the second principal curvature; Step 4.4: If the effective curvature of the sag is equal to 0, then the surface characteristic is determined to be planar; if the effective curvature of the sag is not equal to 0, then the surface characteristic is determined to be non-planar.

3. The method for predicting sagging risk based on coupled flash-drying time limit film thickness and surface characteristics according to claim 2, characterized in that, The formula for calculating the effective curvature of the sag is: ； in: Indicates the effective curvature of the sag; Indicates the first principal curvature; Indicates the second principal curvature; This represents the angle between the direction of gravity projection and the first principal direction.

4. The method for predicting sagging risk by coupling flash-drying time-limited film thickness and surface characteristics according to claim 1, characterized in that, Step 6 specifically includes: Step 6.1: The thickness of the film formed by spraying PUR coating on the planar characteristic area of ​​the sprayed three-dimensional curved surface is used as the calibration thickness. The effective film thickness is obtained by correcting the calibration thickness through the effective curvature of the sag. Step 6.2: Extract the local normal vector of the tangent plane at the feature point, and calculate the angle between the local normal vector and the direction of reverse gravity; Step 6.3: Establish a sagging risk index model based on the effective film thickness and the angle between the local normal vector and the anti-gravity direction; Step 6.4: Calculate the sag risk index based on the sag risk index model.

5. The method for predicting sagging risk based on coupled flash-drying time limit film thickness and surface characteristics according to claim 4, characterized in that, The formula for calculating the effective film thickness is: ； in: Indicates the effective film thickness; Indicates the initial film thickness; Represents the coupling constant; This indicates the effective curvature of the sag.

6. The method for predicting sagging risk based on coupled flash-drying time limit film thickness and surface characteristics according to claim 5, characterized in that, The sag risk index model is as follows: ； in: Indicates the risk index of spillage; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. This represents the angle between the local normal vector and the direction of the opposite gravity. Indicates the effective film thickness.

7. The method for predicting sag risk based on the coupled flash-drying time limit film thickness and surface characteristics according to any one of claims 1-6, characterized in that, Step 3 specifically includes: Step 3.1: Treat the paint film formed by the flow of PUR coating as a Newtonian fluid and calculate the average flow velocity of PUR coating on the flat plate; Step 3.2: Collect the critical sag distance of the PUR coating, and calculate the characteristic sag time of the PUR coating based on the ratio of the critical sag distance to the average flow velocity; Step 3.3: Establish an ultimate anti-sagging model with the constraint that the characteristic sag time of PUR coating is greater than or equal to the flash-drying time of PUR coating process parameter; Step 3.4: After introducing a substrate correction factor into the ultimate anti-sagging model, calculate the ultimate film thickness of the PUR coating based on flash-drying time.

8. The method for predicting sagging risk based on coupled flash-drying time-limited film thickness and surface characteristics according to claim 7, characterized in that, The formula for calculating the characteristic inflow time is as follows: ； in: Indicates the characteristic flow time; This represents the average flow velocity of the PUR coating on the flat plate. Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. Indicates the angle of inclination between the flow plate surface and the horizontal plane; This indicates the initial film thickness.

9. The method for predicting sagging risk based on coupled flash-drying time-limited film thickness and surface characteristics according to claim 8, characterized in that, In step 3.4, the ultimate anti-sagging model after introducing the substrate correction factor is: ； in: Indicates the maximum film thickness; Indicates the critical sag distance; Indicates the initial viscosity of the PUR coating; The initial density of the PUR coating is represented by ; g represents the acceleration due to gravity. Indicates the angle of inclination between the flow plate surface and the horizontal plane; This indicates the flash drying time, a process parameter. This represents the substrate correction factor.

Images