A spray process for electrostatically spraying a zinc steel guard rail

Abstract

The application belongs to the technical field of guardrail processing, and particularly relates to a spraying process for electrostatic spraying of zinc steel guardrails, and the specific steps of the spraying process are as follows: base material pretreatment, low-temperature preheating, segmented stable voltage electrostatic spraying, gradient curing, constant-temperature ripening air cooling; a multi-process composite pretreatment process is adopted, through alkaline degreasing, surface conditioning, phosphorization and multi-stage water washing, zinc steel base material surface oil stains, zinc ash and oxide layers are completely removed, a clean and uniform phosphorization transition film is formed on the base material surface, the coating and base body bonding performance is effectively improved, and the problem of poor coating adhesion is solved. The application additionally adds a low-temperature preheating process, dries profile gap residual moisture, eliminates the internal and external temperature difference of the workpiece, avoids base material condensation and moisture return, and guarantees stable powder adhesion.

Description

Technical Field

[0001] This invention relates to the field of guardrail processing technology, specifically to a spraying process for electrostatic spraying zinc steel guardrails. Background Technology

[0002] Zinc steel guardrails are widely used in municipal roads, residential walls, garden landscapes, factory protection and other scenarios. Zinc steel guardrails have the advantages of high structural strength, convenient processing and assembly, and outstanding cost performance. In order to increase the service life of the guardrails, the surface is often coated with a spraying process during the processing to avoid rust during later use. However, the existing spraying process still has the following technical problems when in use: Existing electrostatic spraying processes for zinc-steel guardrails generally suffer from problems such as simple pretreatment, limited electrostatic spraying parameters, and crude curing methods.

[0003] The surface of zinc-coated steel substrates naturally contains zinc ash and an oxide layer, which cannot be thoroughly cleaned by ordinary degreasing methods, resulting in poor adhesion of the powder coating. Furthermore, traditional constant-voltage spraying methods cause concentrated electric fields at the edges and corners of the profiles, making them prone to substrate exposure, while the electric field on the surface is weak, leading to powder accumulation and sagging. One-time high-temperature baking easily causes orange peel texture, cracking, and uneven color in the coating.

[0004] During outdoor use, products are prone to fading, powdering, rusting, peeling, and flaking, resulting in a short product lifespan and high rework costs.

[0005] Therefore, an electrostatic spraying process for zinc-coated steel guardrails is proposed to solve the aforementioned problems. Summary of the Invention

[0006] The purpose of this invention is to provide a spraying process for electrostatic spraying zinc steel guardrails to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a spraying process for electrostatic spraying zinc steel guardrails, the specific steps of which are as follows: Step 1: Pretreatment of substrate: The zinc-steel guardrail substrate after hot-dip galvanization is pretreated in the following order: alkaline degreasing, first water wash, surface conditioning, phosphating conversion treatment, second water wash and pure water wash. After the pretreatment is completed, a continuous conversion film without ash residue, watermarks and zinc ash residue is formed on the surface of the workpiece. Step 2, Low-temperature preheating: The zinc-steel guardrail workpieces that have completed the pretreatment and drained the water are sent into the preheating oven for low-temperature preheating; the preheating temperature is 75-95℃, and the temperature is maintained for 6-10 minutes. Step 3, Segmented voltage-stabilized electrostatic spraying: The preheated workpiece is transported to the electrostatic powder spraying room and segmented voltage-stabilized electrostatic spraying is adopted. The spraying voltage is differentiated for the flat area and the corner and crevices of the profile. Outdoor weather-resistant polyester powder is selected for spraying. Step 4, Gradient Curing: After spraying, the workpiece is transported into the curing oven and cured by segmented gradient heating to avoid one-time high-temperature baking; Step 5, Constant Temperature Curing and Air Cooling: After curing, the workpiece is removed from the oven and placed in the curing and air cooling station. First, constant temperature curing is performed, with the ambient temperature of the workpiece maintained at 120-140℃ for 4-7 minutes. After curing, clean and dry compressed air is used for gradient air cooling, with the air cooling speed gradually decreasing from 3-6 m / s. Cooling is stopped when the surface temperature of the workpiece is ≤45℃, and the spraying process is completed.

[0008] Preferably, the specific details of the pretreatment process in step one are as follows: Alkaline degreasing: Use a weak alkaline degreasing agent, treat at a temperature of 45-55℃, and soak for 8-12 minutes; First rinse: overflow rinse at room temperature, rinse time 2-3 minutes; Surface conditioning treatment: Use zinc-based surface conditioning agent and soak at room temperature for 1.5-3 minutes; Phosphating conversion treatment: Further treatment with zinc-based phosphating solution at a temperature of 38-45℃ for 5-8 minutes to form a dense and uniform phosphating conversion film on the zinc-steel substrate surface, with a film weight of 1.2-2.5 g / m². Second rinse: room temperature spray rinse for 1-2 minutes; Pure water wash: Use deionized pure water with conductivity ≤50μS / cm to spray and wash for 1-2 minutes.

[0009] Preferably, in the three-stage voltage-stabilized electrostatic spraying step, the spraying distance is controlled at 180-240mm; the three-stage voltage-stabilized electrostatic spraying includes a first stage and a second stage. The first stage targets the flat parts of the profile, with the electrostatic output voltage controlled at 60-70kV and the powder output controlled at 120-160g / min; the second stage targets the edges, grooves, and gaps of the profile, with the electrostatic output voltage reduced to 40-50kV and the powder output at 80-110g / min; a reciprocating spray gun is used, and the spraying is repeated 2-3 times; the final overall dry film thickness is controlled at 60-90μm.

[0010] Preferably, in step four, the gradient heating in the curing oven includes a first stage and a second stage. In the first stage, the temperature is raised to 110-130℃ and held for 8-12 minutes, during which the powder melts and flows smoothly, and the residual gas inside the coating is slowly released. In the second stage, the temperature is raised to 175-185℃ and held for 14-18 minutes to complete the full cross-linking and curing of the polyester powder. The gradient heating rate is controlled at 4-6℃ / min.

[0011] Preferably, in the alkaline degreasing step and the phosphating conversion treatment step, the workpiece is suspended and immersed, and the draining time after the workpiece is removed from the tank is 1-3 minutes.

[0012] Preferably, in the segmented voltage-stabilized electrostatic spraying process, the internal temperature of the powder spraying booth is 18-28℃, the relative humidity is 40-65%, the zinc steel guardrail workpiece is grounded through a conductive hanger with a grounding resistance ≤1MΩ, and the reciprocating speed of the spray gun is 0.25-0.45m / s.

[0013] Preferably, in the constant temperature curing and air cooling step, the compressed air used for air cooling needs to be treated with dust removal and filtration.

[0014] Compared with the prior art, the beneficial effects of the present invention are: 1) This invention employs a multi-stage substrate pretreatment process, sequentially performing alkaline degreasing, a first water wash, surface conditioning, phosphating conversion treatment, a second water wash, and a pure water wash. Alkaline degreasing removes oil stains from the workpiece surface; surface conditioning activates the zinc-steel substrate surface, breaking down the surface oxide layer; phosphating conversion treatment generates a phosphating conversion film on the substrate surface; combined with multi-stage water washing and pure water rinsing, zinc ash and chemical residues are removed from the workpiece surface, resulting in a continuous, ash-free, watermark-free, and zinc ash-free conversion film. Compared to existing simple pretreatment and ordinary degreasing processes, this solution can effectively remove zinc ash and the oxide layer from the zinc-steel substrate surface. The phosphating conversion film serves as an intermediate transition layer, improving the interfacial bonding conditions between the powder coating and the galvanized substrate, thus solving the problem of poor coating adhesion. 2) This application sets up a low-temperature preheating process, where the workpiece is kept at 75-95℃ for 6-10 minutes to dry the residual moisture inside the gap of the profile, eliminate the temperature difference between the inside and outside of the workpiece substrate, avoid condensation and moisture return on the substrate surface during the spraying process, prevent damage to the spraying interface due to condensation on the substrate surface, and provide basic conditions for the stable adhesion of subsequent powder. 3) This application abandons the traditional single constant voltage spraying mode and adopts segmented voltage-stabilized electrostatic spraying, using differentiated spraying voltages for the flat areas and corner and crevices of the profile. Corresponding electrical parameters are matched to different structural parts to alleviate the electric field concentration phenomenon at corners, improve the Faraday cage effect in grooves and crevices, avoid the problem of exposed substrate at corners, and improve the powder coating on flat areas, suppressing powder accumulation and sagging defects, resulting in a more uniform powder coating on different structural locations. 4) Abandoning one-time high-temperature baking, a segmented gradient heating curing mode is adopted instead of the rough direct high-temperature baking. The gradient heating mode allows the powder to first complete the melting and leveling, and then complete the cross-linking and curing. The coating is heated gradually, avoiding orange peel, cracking and uneven color difference caused by sudden high temperature heating, thus ensuring the quality of coating film formation. Attached Figure Description

[0015] Figure 1 This is a flowchart of the process steps of the present invention; Figure 2 Flowchart of substrate pretreatment steps; Figure 3 A flowchart illustrating the steps of segmented voltage-stabilized electrostatic spraying; Figure 4 This is a flowchart of the gradient curing process. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example:

[0018] Please see Figure 1-4 The present invention provides a technical solution: An electrostatic spraying process for zinc-coated steel guardrails, the specific steps of which are as follows: Step 1: Substrate Pretreatment: The hot-dip galvanized zinc-steel guardrail substrate undergoes pretreatment, with the following steps in sequence: alkaline degreasing, first water wash, surface conditioning, phosphating conversion treatment, second water wash, and pure water wash. After pretreatment, a continuous conversion film free of residue, watermarks, and zinc ash is formed on the workpiece surface. For hot-dip galvanized zinc-steel guardrail substrates, a complete set of processes—alkaline degreasing, water washing, surface conditioning, phosphating, and multi-stage water washing—is used to replace the industry's simple degreasing process. This removes oil, zinc ash, and zinc oxide layer from the substrate surface, generating a phosphating conversion base free of residue, watermarks, and zinc ash on the workpiece surface. This eliminates the interference of impurities on the coating bonding interface, improving the adhesion between the powder coating and the galvanized substrate from the source and reducing the risk of peeling and flaking later. Step 2, Low-temperature preheating: The zinc-steel guardrail workpieces that have completed pretreatment and drained water are sent into a preheating oven for low-temperature preheating; the preheating temperature is 75-95℃, and the temperature is maintained for 6-10 minutes; the purpose is to dry the residual moisture in the gaps of the workpiece and the cavity of the profile, increase the overall temperature of the workpiece substrate, eliminate the temperature difference between the inside and outside of the profile, avoid the substrate from condensing and becoming damp during spraying, and provide a temperature basis for subsequent powder adhesion. Step 3: Segmented Voltage Stabilized Electrostatic Spraying: The preheated workpiece is transported to the electrostatic powder coating booth. A segmented voltage stabilized electrostatic spraying method is adopted, differentiating the spraying voltage for the flat areas and corner / seam areas of the profile. Outdoor weather-resistant polyester powder is selected for spraying. This method abandons the traditional single constant voltage spraying mode, differentiating the spraying voltage for the flat areas and corner / seam areas. This ensures efficient powder application in flat areas, while reducing the electric field concentration effect in corner / seam areas, improving the Faraday cage effect, and alleviating defects such as powder accumulation and sagging at corners and localized exposure of the substrate in seams. The use of outdoor weather-resistant polyester powder ensures the basic outdoor weather resistance performance of the finished product. Step 4, Gradient Curing: After spraying, the workpiece is transported into the curing oven and cured using a segmented gradient temperature increase to avoid one-time high-temperature baking; abandoning one-time direct high-temperature baking, a two-stage gradient temperature increase is adopted; first, a low-temperature stage allows the powder to melt and flow, slowly releasing the gas inside the coating, and then the temperature is increased to the curing temperature to complete the cross-linking and curing of the polyester powder; the heating rate is controlled to avoid the coating being suddenly heated to a high temperature, which would cause orange peel, cracking, uneven color difference, and ensure that the powder fully forms a film; Step 5, Constant Temperature Curing and Air Cooling: After curing, the workpiece is removed from the oven and placed in the curing and air cooling station. First, constant temperature curing is performed, maintaining the workpiece ambient temperature at 120-140℃ for 4-7 minutes to further stabilize and cross-link the molecular chains within the coating, releasing internal stress. After curing, clean, dry compressed air is used for gradient air cooling, with the air speed gradually decreasing from 3-6 m / s. Cooling is stopped when the workpiece surface temperature is ≤45℃, completing the spraying process. A constant temperature curing stage is set after curing to further stabilize the coating. Then, clean compressed air is used for gradient cooling with decreasing air speed to slow the coating's cooling rate, release internal thermal stress, and suppress later cracking and powdering defects caused by residual internal stress. The final temperature is controlled to prevent the high-temperature workpiece from directly contacting the air and causing rapid oxidation and discoloration.

[0019] The specific details of the pretreatment process in step one are as follows: Alkaline degreasing: Use a weak alkaline degreasing agent, treat at a temperature of 45-55℃, and soak for 8-12 minutes to remove oil, floating zinc, and zinc ash from the surface of the workpiece. First rinse: overflow rinse at room temperature for 2-3 minutes to remove residual degreasing agent from the workpiece surface; Surface conditioning treatment: Use zinc-based surface conditioning agent, immerse at room temperature for 1.5-3 minutes to activate the surface of the zinc-plated substrate, remove the oxide layer, and provide a uniform film-forming substrate for phosphating; Phosphating conversion treatment: Further treatment with zinc-based phosphating solution at a temperature of 38-45℃ for 5-8 minutes to form a dense and uniform phosphating conversion film on the zinc-steel substrate surface, with a film weight of 1.2-2.5 g / m². Second rinse: room temperature spray rinse for 1-2 minutes; Pure water wash: Use deionized pure water with conductivity ≤50μS / cm to spray and clean for 1-2 minutes to reduce the amount of chemical residue on the workpiece surface.

[0020] The weakly alkaline degreasing temperature and time are suitable for hot-dip galvanized substrates, effectively removing oil and zinc ash while avoiding excessive corrosion of the galvanized layer by strong chemicals. Surface conditioning parameters ensure full activation of the substrate, providing numerous nucleation sites for phosphating, resulting in a fine and uniform phosphating film grain. The phosphating film weight is limited to 1.2-2.5 g / m²; too low a weight results in insufficient adhesion, while too high a weight leads to brittle phosphating films that are prone to peeling. Pure water washing ensures a conductivity ≤50 μS / cm, minimizing salt and chemical residues on the workpiece surface and preventing residual impurities from causing coating blistering and premature corrosion. In the three-stage voltage-stabilized electrostatic spraying process, the spraying distance is controlled at 180-240mm. The three-stage voltage-stabilized electrostatic spraying includes a first stage and a second stage. The first stage targets the flat parts of the profile, with the electrostatic output voltage controlled at 60-70kV and the powder output controlled at 120-160g / min. The second stage targets the edges, grooves, and gaps of the profile, with the electrostatic output voltage reduced to 40-50kV and the powder output at 80-110g / min. A reciprocating spray gun is used, and the spraying is repeated 2-3 times. The final overall dry film thickness is controlled at 60-90μm.

[0021] Principle: Reducing the voltage at the corners weakens the electric field concentration effect, suppresses powder accumulation at the corners, and prevents sagging; maintaining a higher voltage on the plane ensures powder application efficiency, improves the Faraday cage effect in the gaps, and reduces local exposure of the base material.

[0022] Limit the spraying distance, two-stage spraying voltage, powder output, number of spray gun passes, and overall dry film thickness of 60-90μm. Limiting the spraying distance ensures a stable electrostatic field; high voltage and large powder output on the surface ensure efficient powder application; reducing voltage and powder output at corners and crevices suppresses charge accumulation and reduces powder buildup and sagging; controlling the dry film thickness range is crucial—too thin a film results in insufficient corrosion protection, while too thick a film increases powder loss and sagging; 2-3 passes of reciprocating spraying ensure complete powder coverage of all parts of the profile, further reducing the risk of exposed substrate. In step four, the gradient heating in the curing oven includes a first stage and a second stage. In the first stage, the temperature is raised to 110-130℃ and held for 8-12 minutes, during which the powder melts and flows smoothly, slowly releasing residual gas inside the coating. In the second stage, the temperature is raised to 175-185℃ and held for 14-18 minutes to complete the full cross-linking and curing of the polyester powder. The gradient heating rate is controlled at 4-6℃ / min to prevent the coating from being heated too quickly, resulting in orange peel, cracking, and color difference.

[0023] The first stage temperature is specifically used for powder melting and leveling, and slow release of internal gas to prevent gas from being trapped inside the coating and forming pinholes and bubbles. The second stage temperature and holding time ensure the full cross-linking of outdoor polyester powder; the heating rate is 4-6℃ / min. If the heating is too fast, the temperature difference between the inside and outside of the workpiece will be large, and the coating will easily show orange peel and color difference; if the heating is too slow, the production efficiency will be greatly reduced. This parameter range takes into account both coating quality and production cycle. In the alkaline degreasing and phosphating conversion steps, the workpieces are suspended and immersed in the solution, with a draining time of 1-3 minutes after removal from the tank. Suspension immersion ensures full contact between the inside and outside of the guardrail profiles, as well as the seams, with the solution, addressing the issue of inadequate treatment of the seams in the guardrail components. Setting a draining time reduces the amount of solution carried by the workpieces into the next washing tank, preventing cross-contamination, stabilizing the concentration of chemicals in each treatment tank, and ensuring process stability between batches. In the segmented voltage-stabilized electrostatic spraying process, the internal temperature of the powder spraying booth is 18-28℃, the relative humidity is 40-65%, the zinc-steel guardrail workpiece is grounded via a conductive hanger with a grounding resistance ≤1MΩ, and the reciprocating speed of the spray gun is 0.25-0.45m / s. Excessive temperature and humidity can cause the powder to absorb moisture and clump, resulting in poor electrostatic adsorption; excessive humidity can easily cause static electricity to spark and discharge. The grounding resistance ensures reliable grounding of the workpiece, allowing the electrostatic powder to adhere properly to the workpiece surface. Poor grounding directly leads to poor powder application and exposed substrate. Controlling the spray gun's travel speed prevents excessively fast spraying (thin coating) and excessively slow spraying (localized powder accumulation and dripping), further stabilizing the spraying quality. In the constant temperature curing and air cooling step, the compressed air used for air cooling needs to be treated with dust removal and filtration. This prevents dust and impurities carried by the compressed air from being blown onto the hot, wet, soft coating surface, avoiding dust adhesion and particle pitting defects on the coating surface, and ensuring the appearance quality of the finished coating.

[0024] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description. Therefore, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0025] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A spraying process for electrostatic spraying zinc steel guardrails, characterized in that, The specific steps of this spraying process are as follows: Step 1: Pretreatment of substrate: The zinc-steel guardrail substrate after hot-dip galvanization is pretreated in the following order: alkaline degreasing, first water wash, surface conditioning, phosphating conversion treatment, second water wash and pure water wash. After the pretreatment is completed, a continuous conversion film is formed on the surface of the workpiece, free of ash, watermarks, and zinc ash residue. Step 2, Low-temperature preheating: The zinc-steel guardrail workpieces that have completed the pretreatment and drained the water are sent into the preheating oven for low-temperature preheating; the preheating temperature is 75-95℃, and the temperature is maintained for 6-10 minutes. Step 3, Segmented voltage-stabilized electrostatic spraying: The preheated workpiece is transported to the electrostatic powder spraying room and segmented voltage-stabilized electrostatic spraying is adopted. The spraying voltage is differentiated for the flat area and the corner and crevices of the profile. Outdoor weather-resistant polyester powder is selected for spraying. Step 4, Gradient Curing: After spraying, the workpiece is transported into the curing oven and cured by segmented gradient heating to avoid one-time high-temperature baking; Step 5, Constant Temperature Curing and Air Cooling: After curing, the workpiece is removed from the oven and placed in the curing and air cooling station. First, constant temperature curing is performed, with the ambient temperature of the workpiece maintained at 120-140℃ for 4-7 minutes. After curing, clean and dry compressed air is used for gradient air cooling, with the air cooling speed gradually decreasing from 3-6 m / s. Cooling is stopped when the surface temperature of the workpiece is ≤45℃, and the spraying process is completed.

2. The electrostatic spraying process for zinc-coated steel guardrails according to claim 1, characterized in that: The specific details of the pretreatment process in step one are as follows: Alkaline degreasing: Use a weak alkaline degreasing agent, treat at a temperature of 45-55℃, and soak for 8-12 minutes; First rinse: overflow rinse at room temperature, rinse time 2-3 minutes; Surface conditioning treatment: Use zinc-based surface conditioning agent and soak at room temperature for 1.5-3 minutes; Phosphating conversion treatment: Further treatment with zinc-based phosphating solution at a temperature of 38-45℃ for 5-8 minutes to form a dense and uniform phosphating conversion film on the zinc-steel substrate surface, with a film weight of 1.2-2.5 g / m². Second rinse: room temperature spray rinse for 1-2 minutes; Pure water wash: Use deionized pure water with conductivity ≤50μS / cm to spray and wash for 1-2 minutes.

3. The electrostatic spraying process for zinc-coated steel guardrails according to claim 1, characterized in that: In the three-stage voltage-stabilized electrostatic spraying process, the spraying distance is controlled at 180-240mm. The three-stage voltage-stabilized electrostatic spraying includes a first stage and a second stage. The first stage targets the flat parts of the profile, with the electrostatic output voltage controlled at 60-70kV and the powder output controlled at 120-160g / min. The second stage targets the edges, grooves, and gaps of the profile, with the electrostatic output voltage reduced to 40-50kV and the powder output at 80-110g / min. A reciprocating spray gun is used, and the spraying is repeated 2-3 times. The final overall dry film thickness is controlled at 60-90μm.

4. The electrostatic spraying process for zinc-coated steel guardrails according to claim 1, characterized in that: In step four, the gradient heating in the curing oven includes a first stage and a second stage. In the first stage, the temperature is raised to 110-130℃ and held for 8-12 minutes, during which the powder melts and flows smoothly, and the residual gas inside the coating is slowly released. In the second stage, the temperature is raised to 175-185℃ and held for 14-18 minutes to complete the full cross-linking and curing of the polyester powder. The gradient heating rate is controlled at 4-6℃ / min.

5. The electrostatic spraying process for zinc-coated steel guardrails according to claim 2, characterized in that: In the alkaline degreasing and phosphating conversion steps, the workpiece is suspended and immersed, and the draining time after the workpiece is removed from the tank is 1-3 minutes.

6. The electrostatic spraying process for zinc-coated steel guardrails according to claim 3, characterized in that: In the segmented voltage-stabilized electrostatic spraying process, the internal temperature of the powder spraying booth is 18-28℃, the relative humidity is 40-65%, the zinc steel guardrail workpiece is grounded through a conductive hanger with a grounding resistance ≤1MΩ, and the reciprocating speed of the spray gun is 0.25-0.45m / s.

7. The electrostatic spraying process for zinc-coated steel guardrails according to claim 1, characterized in that: In the constant temperature curing and air cooling step, the compressed air used for air cooling needs to be treated with dust removal and filtration.

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