Environment-friendly integrated treatment process for electrostatic plastic spraying of metal workpieces
By employing wet chemical pretreatment, closed-loop conveying, sequential auxiliary electrodes, and rapid curing technology, the high energy consumption and uneven coating issues in electrostatic powder coating processes have been resolved, achieving environmentally friendly and efficient spraying results.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- DANYANG YONGHAO METAL SURFACE TREATMENT CO LTD
- Filing Date
- 2026-04-27
- Publication Date
- 2026-06-19
Smart Images
Figure CN122230951A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of metal workpiece processing technology, and in particular to an environmentally friendly integrated electrostatic powder coating process for metal workpieces. Background Technology
[0002] Electrostatic powder coating technology is widely used for surface protection and decoration of metal workpieces due to its excellent coating performance and solvent-free evaporation. Traditional electrostatic powder coating processes typically include: degreasing, washing, chemical conversion coating treatment, drying, electrostatic spraying, and hot air curing. However, existing processes have several shortcomings: First, the pre-treated workpieces require high-temperature drying to remove moisture from the film layer. This process not only consumes a lot of energy but may also lead to secondary oxidation or dust contamination on the workpiece surface. Furthermore, the resistivity of the dried film layer is high, which is not conducive to subsequent electrostatic adsorption. Second, electrostatic spraying is usually carried out in ordinary air environments. During the spraying process, the powder is easily oxidized and absorbs moisture. For workpieces with deep grooves, cavities, or complex shapes, due to the Faraday cage effect, the electrostatic field lines have difficulty penetrating the recessed areas, resulting in extremely low powder coverage in these areas. Manual touch-up spraying is often required, which seriously affects production efficiency and coating uniformity. Third, traditional hot air curing methods have slow heating, which can easily lead to over-curing of the surface and insufficient curing of the core in thick-walled workpieces. Moreover, the curing cycle is long and energy consumption is high. In addition, the existing processes have low over-sprayed powder recovery rates and large amounts of wastewater and waste residue discharge, making it difficult to meet increasingly stringent environmental protection requirements.
[0003] To address the aforementioned problems, some existing improvement solutions attempt to employ inert atmosphere spraying or add auxiliary electrodes, but most suffer from complex structures, insufficient control precision, and failure to integrate pretreatment, conveying, spraying, curing, and environmental protection systems. In particular, the technical challenge of directly introducing the wet film layer into the spraying process remains unresolved, and the static arrangement of auxiliary electrodes cannot completely eliminate the Faraday cage effect. Therefore, this invention proposes an environmentally friendly integrated electrostatic powder coating process for metal workpieces to solve the problems existing in the prior art. Summary of the Invention
[0004] To address the aforementioned problems, the present invention aims to propose an environmentally friendly integrated electrostatic powder coating process for metal workpieces, which solves the problems of disconnect between pretreatment and spraying, Faraday cage effect, high energy consumption, poor environmental performance, and uneven curing of thick workpieces in the prior art.
[0005] To achieve the objectives of this invention, the invention is implemented through the following technical solution: an environmentally friendly integrated electrostatic powder coating process for metal workpieces, comprising the following steps:
[0006] Step 1: Wet chemical pretreatment
[0007] The metal workpiece is subjected to degreasing, water washing, and chemical conversion coating treatment in sequence to obtain a wet workpiece with an undried chemical conversion film covering its surface. The chemical conversion film is selected from either a phosphorus-free silane film or a zirconium salt film, and its surface relative humidity is controlled at 30% to 80%, with a film resistivity of 10 Ω·cm. 3 ~10 5 The thickness of the wet film formed by the chemical conversion membrane treatment is 0.5-2.0 μm, and it is directly introduced into step two without drying. By degreasing, washing and phosphorus-free chemical conversion membrane treatment of metal workpieces, and directly entering the next process in a wet state, the energy waste and secondary pollution caused by traditional drying are avoided. At the same time, the low resistivity of the wet film helps subsequent electrostatic adsorption.
[0008] Step 2: Closed-loop anti-oxidation conveying system
[0009] The wet workpiece obtained in step one is transferred to the electrostatic powder spraying chamber through a closed conveyor channel. The closed conveyor channel is filled with inert protective gas, and the relative humidity in the channel is maintained at 50% to 90% to avoid the wet film from drying and cracking and to prevent the workpiece surface from oxidizing. The wet workpiece is conveyed in a closed channel filled with inert gas, while maintaining a high humidity environment, which can effectively prevent the wet film from drying and cracking and the workpiece surface from oxidizing, and ensure that the pretreatment quality is stably transferred to the spraying process.
[0010] Step 3: Electrostatic spraying under an inert atmosphere
[0011] At least two sets of auxiliary electrodes are arranged around the workpiece in the powder spraying chamber. By sequentially changing the potential state of the auxiliary electrodes, the spatial electrostatic field is periodically deflected toward the concave area of the workpiece to overcome the Faraday cage effect. The electrostatic powder spraying chamber is sealed and filled with inert gas, with an oxygen volume concentration of less than 5%. Powder coating is sprayed onto the surface of the workpiece using an electrostatic spray gun. During the spraying process, the oversprayed powder is recovered and reused through an electret adsorption plate (the electret material adsorbs charged particles under passive conditions), achieving efficient and environmentally friendly reuse. Spraying is carried out in a low-oxygen inert environment. With the sequential displacement of the auxiliary electrodes, the Faraday cage effect can be overcome, and the powder can uniformly cover the deep concave area of the workpiece.
[0012] Step 4: Rapid Induction-Assisted Curing
[0013] The workpiece coated with powder coating is sent into the curing oven. First, it passes through the medium-frequency induction heating zone, which raises the overall temperature of the workpiece's metal substrate to 170-200°C within 5-20 seconds. Then, it enters the hot air insulation zone, where it is kept at 180-200°C for 5-15 minutes to allow the powder coating to completely melt and cross-link, thus completing the curing of the workpiece surface coating. By first rapidly raising the overall temperature of the workpiece substrate to the curing temperature through medium-frequency induction heating before entering the hot air insulation zone, the curing cycle can be significantly shortened, avoiding the problem of over-curing of the surface of thick workpieces while the core is under-cured.
[0014] Step 5: Cooling the unloaded part
[0015] After the surface coating has cured, the workpiece is cooled and then removed from the mold. The electrostatic powder coating treatment of the metal workpiece is completed. After curing, the finished coating is obtained by cooling, which ensures the stability of the coating performance and facilitates subsequent testing and packaging.
[0016] Further improvements are made in the following aspects: In step one, the degreasing treatment uses an environmentally friendly phosphorus-free degreasing agent containing the following components by mass percentage: sodium hydroxide 0.5%–3.0%, sodium carbonate 1.0%–5.0%, sodium gluconate 0.5%–2.0%, fatty alcohol polyoxyethylene ether 0.1%–1.0%, with the balance being water. The treatment temperature is 45–55℃, and the time is 8–12 minutes. The water washing treatment uses 3–5 countercurrent rinsings, with the final rinsing using pure water with a conductivity ≤8μS / cm. The rinsing water is purified by reverse osmosis and then recycled, with a recycling rate ≥90%. The chemical conversion membrane treatment uses a nano-ceramic agent, with a treatment temperature of 30–35℃ and a time of 1–2 minutes. By limiting the degreasing temperature, time, number of countercurrent rinsings, and the conductivity of the pure water, and by using reverse osmosis to recycle the rinsing water, water consumption and wastewater discharge can be significantly reduced, while ensuring the cleanliness of the workpiece surface.
[0017] A further improvement is made in step one, where ultrasonic vibration at a frequency of 28–40 kHz is incorporated during the degreasing process to ensure that the residual oil content is <1 mg / m³. 2 The chemical conversion film treatment uses a retractable spray head and a 360° rotating fixture, combined with vacuum-assisted technology, to allow the treatment liquid to penetrate deep into the inner cavity and blind areas of the workpiece. Ultrasonic vibration is used to improve the degreasing effect. The retractable spray head, rotating fixture, and vacuum-assisted technology ensure that a complete conversion film can be formed even in difficult-to-treat areas such as irregularly shaped parts and complex inner cavities.
[0018] A further improvement is made in the following: In step two, the air pressure inside the sealed conveying channel is 50-200 Pa higher than the atmospheric pressure outside the workshop. An ionization air curtain is installed on the inner wall of the sealed conveying channel to generate an alternating airflow of positive and negative ions. The inert protective gas is carbon dioxide or nitrogen, and its dew point is not higher than -40°C. By limiting the positive pressure inside the channel, the ionization air curtain, and the inert gas dew point, external pollution can be effectively isolated, the wet film state of the workpiece surface can be maintained, and oxidation and dust adhesion can be prevented.
[0019] A further improvement is made in step three, where there are 4 to 8 auxiliary electrodes arranged in a ring around the workpiece. The minimum distance between each auxiliary electrode and the workpiece surface is 100 to 300 mm. The potential state of the auxiliary electrodes is changed sequentially by a programmable logic controller (PLC) with a timing period of 0.5 to 5 seconds. Within each period, the auxiliary electrodes sequentially present at least two states: grounded, +30 to +60 kV positive high voltage, and -30 to -60 kV negative high voltage. The PLC is also connected to an AI vision recognition module, which adjusts the timing period and voltage amplitude in real time according to the shape and size of the workpiece. By limiting the number, ring distribution, distance, timing period, and voltage range of the auxiliary electrodes, and combining this with real-time adjustment using AI vision recognition, the electrostatic field can be precisely and dynamically modulated, completely eliminating the spraying dead angles caused by the Faraday cage effect.
[0020] A further improvement is made in the following aspects: In step three, the electrostatic spray gun applies a voltage of 40–70 kV, the powder supply pressure is 0.3–0.5 MPa, the distance between the electrostatic spray gun and the workpiece is 150–300 mm, and the powder coating is a low-temperature curing environmentally friendly powder, composed of the following components by weight:
[0021] Epoxy resin: 55-70 parts;
[0022] Dicyandiamide curing agent: 3-8 parts;
[0023] Pigments and fillers: 15-30 parts, wherein the pigments and fillers are at least one of titanium dioxide, barium sulfate, talc, and mica powder;
[0024] Acrylic leveling agent: 0.5–1.5 parts;
[0025] Benzoin degassing agent: 0.3–1.0 parts;
[0026] Nano-reinforcing agent: 2-5 parts, wherein the nano-reinforcing agent is a mixture of nano-silica and nano-alumina in a mass ratio of 1:1 to 3:1;
[0027] Phase change microcapsules: accounting for 2% to 10% of the total mass of powder, wherein the core material of the phase change microcapsules is paraffin or fatty acid ester, the phase change temperature is 160 to 180℃, the latent heat is 120 to 200 J / g, the shell is melamine-formaldehyde resin or polyurea resin, and the microcapsule particle size is 1 to 10 μm.
[0028] The powder coating is free of VOCs and heavy metals, with a particle size D50 of 32-42 μm. By limiting the spray gun voltage, pressure, distance, and the particle size of the low-temperature curing powder and the amount of phase change microcapsules added, the uniformity of spraying can be guaranteed and low-temperature rapid curing can be achieved, thus reducing energy consumption.
[0029] A further improvement is made in step three, where the electret adsorption plate is a porous polytetrafluoroethylene plate or a porous polypropylene plate that has undergone corona charging treatment. Its surface potential is -5 to -10 kV, and its porosity is 40% to 70%. It does not have an external power supply. After adsorption saturation, the powder is detached by reverse corona discharge. The reverse corona discharge uses a pulse voltage of +8 to +12 kV, a frequency of 1 to 5 Hz, and a discharge duration of 0.1 to 0.5 seconds per discharge. The recovered powder is 100% reused after fine screening and impurity removal. By limiting the material, potential, porosity, and reverse corona discharge parameters of the electret adsorption plate, the oversprayed powder recovery rate reaches over 95% and can be 100% reused, avoiding powder waste and environmental pollution.
[0030] A further improvement is made in step four, where the operating frequency of the intermediate frequency induction heating zone is 5–20 kHz and the power density is 10–50 kW / m². 2 A transition section with a length of 1 to 2 meters is provided between the medium-frequency induction heating zone and the hot air insulation zone. The waste heat from the medium-frequency induction coil and the hot air circulation are activated simultaneously in the transition section to ensure that the temperature difference between the surface and core of the workpiece does not exceed 10°C. The hot air insulation zone uses indirect gas heating or electric heating, with an air circulation speed of 1 to 3 m / s and a temperature deviation of ≤ ±3°C. The curing oven is equipped with a waste heat recovery system with a waste heat recovery efficiency of ≥85%. The recovered waste heat is used for heating the pretreatment process and for workshop heating. By limiting the medium-frequency induction frequency, power density, temperature difference control of the transition section, and hot air insulation circulation, combined with the waste heat recovery system, energy consumption can be saved by more than 30%, while ensuring uniform cross-linking of the coating.
[0031] Further improvements are made in that: the wastewater generated by the wet chemical pretreatment is treated by a combination of neutralization, precipitation, reverse osmosis, and ion exchange processes to achieve zero wastewater discharge; the exhaust gas from the curing furnace is purified before being discharged, and the inert gas in the electrostatic powder spraying chamber is circulated and purified. After removing micro-powder and moisture through a cyclone separator, a high-efficiency filter, and a condenser dryer, the gas is sent back to the electrostatic powder spraying chamber, forming a closed-loop circulation system. Through zero wastewater discharge, exhaust gas purification, and inert gas circulation purification, the entire process achieves environmentally friendly closed-loop operation, meeting the requirements of clean production.
[0032] The beneficial effects of this invention are as follows: By directly conveying a wet workpiece with an undried chemical conversion film on its surface into an inert atmosphere for spraying, this invention eliminates the drying process, saves energy, and avoids secondary oxidation. The low resistance of the wet film facilitates electrostatic adsorption, and the use of a time-displaced auxiliary electrode effectively overcomes the Faraday cage effect, ensuring uniform powder application in deep recessed areas. At the same time, the electret adsorption plate enables efficient recovery and 100% reuse of oversprayed powder, reducing waste. In addition, the combination of medium-frequency induction rapid heating and hot air insulation results in a small internal and external temperature difference and uniform curing of thick workpieces, which can significantly reduce energy consumption. This significantly improves the coating quality and production efficiency of metal workpieces, demonstrating outstanding energy-saving and environmental protection advantages and industrial application value. Attached Figure Description
[0033] Figure 1 This is a schematic diagram of the process for the integrated electrostatic powder coating of environmentally friendly metal workpieces according to the present invention. Detailed Implementation
[0034] 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.
[0035] It should be noted that the technical means not described in detail in the following embodiments are all conventional means in the art, are not the key points of the invention, and will not be elaborated upon.
[0036] Example 1
[0037] See Figure 1 This embodiment provides an environmentally friendly integrated electrostatic powder coating process for metal workpieces. The metal workpiece is made of Q235 carbon steel, with external dimensions of 300mm × 200mm × 5mm. The workpiece surface has a rectangular groove with a depth of 30mm and an opening width of 40mm (used to verify the effectiveness of overcoming the Faraday cage effect). The method includes the following steps:
[0038] Step 1: Wet chemical pretreatment
[0039] First, degreasing was performed. An environmentally friendly, phosphorus-free degreasing agent was used, composed of the following components by weight percentage: sodium hydroxide 1.5%, sodium carbonate 3.0%, sodium gluconate 1.0%, fatty alcohol polyoxyethylene ether 0.5%, and the balance being water. The degreasing agent was heated to 50°C, and the workpiece was immersed in the degreasing tank for 10 minutes. Ultrasonic oscillation was simultaneously activated during the degreasing process at a frequency of 35kHz. After degreasing, the residual oil content on the workpiece surface was measured to be 0.4 mg / m³. 2 Less than 1 mg / m2 Requirements.
[0040] Then, a water washing process is performed. Four counter-current rinsing stages are used, with the water flow direction opposite to the workpiece conveying direction. The final rinse uses pure water with a conductivity of 5 μS / cm, meeting the requirement of ≤8 μS / cm. The rinse water is purified by a reverse osmosis system and then recycled, with a calculated recycling rate of 92%, meeting the requirement of ≥90%.
[0041] Finally, a chemical conversion coating treatment is performed. A nano-ceramic agent (mainly composed of fluorozirconic acid and organosilane, commercially available model T-100) is used at a treatment temperature of 32℃ for 1.5 minutes. During the treatment, a retractable spray head and a 360° rotating fixture are employed, along with vacuum-assisted technology (vacuum degree -0.08MPa) to allow the treatment solution to penetrate deep into the grooves and corners of the workpiece. After treatment, a zirconium salt chemical conversion film is formed on the workpiece surface, with a wet film thickness of 1.0μm, a surface relative humidity of 65%, and a film resistivity of 5×10⁻⁶. 3 Ω·cm. The wet workpiece proceeds directly to step two without any drying process.
[0042] Step 2: Closed-loop anti-oxidation conveying system
[0043] The wet workpiece obtained in step one is suspended on a conveyor chain and enters a closed conveyor channel. The channel is filled with nitrogen as an inert protective gas, with a dew point of -45℃ (meeting the requirement of not exceeding -40℃). The relative humidity inside the channel is controlled at 70%, and the air pressure is 100Pa higher than the atmospheric pressure of the external workshop. An ionized air curtain is installed on the inner wall of the channel. This air curtain generates an alternating airflow of positive and negative ions, causing any suspended particles in the air inside the channel to become electrostatically charged and adsorbed onto the dust collection plates on the channel wall. The workpiece is conveyed in the channel for approximately 30 seconds. At the exit, the wet film on the workpiece surface is checked and no drying cracks are observed, and no oxidation marks are found on the workpiece surface.
[0044] Step 3: Electrostatic spraying under an inert atmosphere
[0045] The electrostatic powder coating chamber is completely sealed. First, the indoor air is replaced with nitrogen to reduce the oxygen concentration to 3% (below the 5% requirement). After the workpiece enters the chamber, six auxiliary electrodes are arranged in a ring around it, with each electrode at least 200mm away from the workpiece surface. The auxiliary electrodes are controlled by a programmable logic controller (PLC) to sequentially change their voltage state. The timing cycle is set to 2 seconds. Within each cycle, the auxiliary electrodes sequentially present three states: grounded for 0.5 seconds, +45kV positive high voltage for 0.5 seconds, -45kV negative high voltage for 0.5 seconds, and then back to grounded for 0.5 seconds. Simultaneously, the PLC is connected to an AI vision recognition module. This module uses a camera to collect real-time information about the workpiece's shape and size, automatically identifying difficult-to-coat areas such as grooves and inner corners. Based on the recognition results, it adjusts the timing cycle and voltage amplitude of the auxiliary electrodes in real time (for example, when a groove is directly opposite an auxiliary electrode, the positive high voltage duration of that electrode is extended to 0.8 seconds).
[0046] Powder coating is applied to the workpiece surface using an electrostatic spray gun. The spray gun is powered by a voltage of 55kV and a powder supply pressure of 0.4MPa. The distance between the spray gun and the workpiece is maintained at 200mm. The spray gun's movement speed is adaptively adjusted according to the workpiece surface curvature: 400mm / s in flat areas and automatically reduced to 200mm / s at the edges of grooves.
[0047] The AI visual recognition module in this embodiment employs a deep learning-based object detection algorithm, specifically YOLOv8 (You Only Look Once version 8) or Mask R-CNN (Region Convolutional Neural Network) models. This model is pre-trained on an image dataset containing various metal workpieces (such as areas difficult to coat, including grooves, cavities, deep holes, and irregular curved surfaces). The training dataset includes pixel-level annotations of feature regions such as workpiece edges, groove openings, and cavity boundaries. After training, the model is deployed in an industrial control computer. Workpiece images are acquired in real-time using an industrial camera (such as a 5-megapixel CMOS camera with an LED ring light source) installed in the powder coating chamber. The system outputs category labels and bounding boxes for each region on the workpiece surface at a rate of at least 30 frames per second, with a focus on identifying low-electric-field-accessible areas such as grooves, cavities, and bending dead angles.
[0048] The programmable logic controller (PLC) dynamically calculates the potential timing period and voltage amplitude of each auxiliary electrode based on the regional location information output by the AI model and the spatial distribution coordinates of the current auxiliary electrodes. For example, when a groove is detected to be directly opposite the third auxiliary electrode, the duration of the positive high voltage of that electrode is extended from the default 0.5 seconds to 0.8 seconds, and the voltage amplitude is increased from +45kV to +60kV. At the same time, the negative high voltage amplitude of the adjacent electrodes is reduced to guide the electrostatic field lines to concentrate into the groove area. The AI vision recognition module communicates with the PLC via Ethernet or RS485, and the overall recognition-control delay does not exceed 50 milliseconds, ensuring real-time response to moving workpieces on the conveyor chain.
[0049] The powder coating used in this embodiment is a low-temperature curing environmentally friendly powder, composed of the following components by weight: 65 parts epoxy resin; 4.5 parts dicyandiamide curing agent; 25 parts pigments and fillers, including 15 parts titanium dioxide and 10 parts precipitated barium sulfate; 1.0 part acrylate leveling agent; 0.6 parts benzoin degassing agent; and 2.5 parts nano-reinforcing agent, which is a mixture of nano-silica and nano-alumina in a mass ratio of 1.5:1. In addition, phase change microcapsules accounting for 5% of the total powder mass are added. The core material of these phase change microcapsules is paraffin wax, with a phase change temperature of 170℃ and a latent heat of 150J / g; the outer shell is melamine-formaldehyde resin, and the microcapsule particle size is 5μm. The above components are subjected to high-speed mixing, melt extrusion (extruder heating zone temperature 110℃), tableting and cooling, ACM milling, and grading through a 200-mesh sieve to obtain a powder coating with a particle size D50 = 38μm. Tests have shown that the powder coating does not contain VOCs or heavy metals such as lead, cadmium, mercury, and hexavalent chromium.
[0050] During the spraying process, oversprayed powder is recovered by an electret adsorption plate. The electret adsorption plate is a porous polytetrafluoroethylene plate treated with corona charging, with a surface potential of -8kV and a porosity of 60%, and requires no external power source. This adsorption plate relies on its own permanent electrostatic field to adsorb the dispersed powder. When adsorption saturation occurs (determined by a weight sensor or accumulated time), a reverse corona discharge is initiated: a pulse voltage of +10kV, a frequency of 3Hz, and a discharge duration of 0.3 seconds per discharge, causing the adsorbed powder to detach and fall into the powder hopper below. The collected powder undergoes fine screening (200-mesh sieve) and impurity removal (magnetic separation to remove metal impurities) before being 100% returned to the powder supply center for secondary reuse. Calculations show that the oversprayed powder recovery rate in this embodiment reaches 96%.
[0051] Step 4: Rapid Induction-Assisted Curing
[0052] After the coating is completed, the workpiece enters the curing oven. It first passes through a medium-frequency induction heating zone, which operates at a frequency of 10kHz and a power density of 30kW / m². 2The workpiece passes continuously through the induction coil, and the metal substrate heats up to 185°C within 12 seconds. It then enters a 1.5-meter-long transition section where the waste heat from the medium-frequency induction coil and hot air circulation are simultaneously activated to ensure the surface and core temperature difference of the workpiece does not exceed 8°C. Next, it enters a hot air insulation zone, which uses indirect gas heating with an air circulation speed of 2 m / s and an internal temperature set at 190°C, with temperature deviation controlled within ±2°C. The workpiece remains in the insulation zone for 10 minutes to allow the powder coating to completely melt and cross-link. The curing oven is equipped with a waste heat recovery system, including waste heat collection pipes, heat exchangers, and insulation and storage modules, with a measured waste heat recovery efficiency of 86%. The recovered waste heat is used through a heat exchanger to heat the degreasing tank in the pretreatment process and for winter heating in the workshop, resulting in an overall energy consumption reduction of approximately 32% compared to traditional processes.
[0053] Step 5: Cooling the unloaded part
[0054] After curing, the workpiece is naturally cooled to below 45℃ in the cooling section before being unloaded. A full range of tests are performed on the finished coating: the coating thickness is measured using an eddy current thickness gauge, resulting in a thickness of 65–75 μm with a deviation within ±12%; the adhesion is tested using a cross-cut adhesion tester, achieving grade 0 (no peeling); a neutral salt spray test is conducted in a salt spray chamber, and after 1000 hours, no red rust appears on the workpiece surface; visual inspection reveals no defects such as runs, pinholes, or color differences. After passing all tests, the workpiece is put into storage.
[0055] Environmental closed-loop treatment
[0056] This embodiment also incorporates environmentally friendly closed-loop treatment for the entire process. Wastewater generated during pretreatment is collected and treated through a combination of neutralization (pH adjusted to 7.5), sedimentation, reverse osmosis, and ion exchange processes. The treated water has a conductivity of less than 50 μS / cm and is entirely reused in the rinsing process, achieving zero wastewater discharge. Exhaust gas from the curing oven is purified by activated carbon adsorption and catalytic combustion before being discharged in compliance with standards. The inert gas (nitrogen) in the powder spraying chamber undergoes a circulating purification process: an exhaust vent at the top of the powder spraying chamber extracts nitrogen containing micropowder and sends it to a cyclone separator to remove powder particles larger than 5 μm. It then passes through a high-efficiency filter (0.5 μm filtration accuracy), is cooled to 10°C by a condenser dryer to remove moisture, and is finally reheated to 25°C with a small amount of fresh nitrogen added (approximately 2% of the circulating volume) before being returned to the air distribution plate at the bottom of the powder spraying chamber, forming a closed-loop circulation system.
[0057] Example 2
[0058] See Figure 1This embodiment provides an environmentally friendly integrated electrostatic powder coating process for metal workpieces. The metal workpiece is made of aluminum alloy (6061) with external dimensions of 500mm × 400mm × 3mm. The workpiece has a complex irregularly shaped internal cavity with a minimum opening size of 20mm and a depth of 80mm. The method includes the following steps:
[0059] Step 1: Wet chemical pretreatment
[0060] The degreasing treatment used the same environmentally friendly, phosphorus-free degreasing agent as in Example 1, but the ratio was adjusted to: sodium hydroxide 1.0%, sodium carbonate 2.5%, sodium gluconate 1.2%, fatty alcohol polyoxyethylene ether 0.8%, with the remainder being water. The treatment temperature was 48°C, the treatment time was 9 minutes, and ultrasonic oscillation was simultaneously activated at a frequency of 40 kHz. The residual oil content after degreasing was 0.3 mg / m³. 2 .
[0061] The water washing process employs five countercurrent rinsing stages, with the final stage using pure water with a conductivity of 4 μS / cm. The reverse osmosis recycling rate of the rinsing water is 93%.
[0062] The chemical conversion membrane treatment uses a phosphorus-free silane membrane. The treatment solution consists of the following components: γ-glycidyl etheroxypropyltrimethoxysilane (KH-560) 2.0%, fluorozirconic acid (calculated as ZrO2) 0.5%, fluorotitanic acid (calculated as TiF6) 0.5%. 2- The treatment solution consisted of 0.2% (by weight) deionized water, with the balance being deionized water. The pH was adjusted to 4.8 with ammonia. The treatment temperature was 30℃, and the treatment time was 2 minutes. A retractable spray head, 360° rotating fixture, and vacuum-assisted technology (vacuum degree -0.09MPa) were used to ensure that the treatment solution entered every corner of the irregularly shaped cavity. The resulting wet film was a phosphorus-free silane film with a thickness of 0.8μm, a surface relative humidity of 70%, and a film resistivity of 8×10⁻⁶. 3 Ω·cm. The wet workpiece is not dried and proceeds directly to step two.
[0063] Step 2: Closed-loop anti-oxidation conveying system
[0064] The sealed conveying channel is filled with carbon dioxide as an inert protective gas (carbon dioxide is cheaper than nitrogen and more friendly to some aluminum alloy surfaces), with a dew point of -50°C. The relative humidity inside the channel is maintained at 80%, and the air pressure is 150 Pa higher than the atmospheric pressure in the workshop. The parameters of the ionized air curtain machine are the same as in Example 1. The workpiece is conveyed in the channel for approximately 25 seconds, and the wet film remains intact when it exits the channel.
[0065] Step 3: Electrostatic spraying under an inert atmosphere
[0066] The oxygen concentration in the powder coating chamber is controlled at 2%. Eight auxiliary electrodes are arranged in a ring around the workpiece, with a minimum distance of 150mm from the workpiece. The PLC timing cycle is set to 1 second, and within each cycle, the auxiliary electrodes sequentially display: ground for 0.3 seconds, +60kV positive high voltage for 0.3 seconds, -60kV negative high voltage for 0.3 seconds, and ground for 0.1 seconds. The AI vision recognition module adjusts the voltage amplitude in real time for irregularly shaped cavities. When an opening in the cavity is detected, the positive high voltage is automatically increased to +60kV and the duration is extended to 0.5 seconds.
[0067] The electrostatic spray gun is operated at a voltage of 60kV and a powder supply pressure of 0.35MPa, with the distance between the spray gun and the workpiece maintained at 180mm. The spray gun's moving speed is adaptively adjusted according to the workpiece's surface curvature, decreasing to 150mm / s at the entrance of the irregularly shaped inner cavity.
[0068] The powder coating formulation is as follows: 60 parts epoxy resin; 5 parts dicyandiamide curing agent; 28 parts pigments and fillers, including 12 parts titanium dioxide, 8 parts talc, and 8 parts precipitated barium sulfate; 1.2 parts acrylic leveling agent; 0.5 parts benzoin degassing agent; 3.5 parts nano-reinforcing agent, including 2.0 parts nano-silica and 1.5 parts nano-alumina (mass ratio approximately 1.33:1); phase change microcapsules account for 8% of the total powder mass. The core material of these microcapsules is a fatty acid ester (phase change temperature 165℃, latent heat 180J / g), and the outer shell is polyurea resin. The microcapsule particle size is 3μm. The powder coating preparation method is the same as in Example 1, with a particle size D50 = 35μm.
[0069] The electret adsorption plate is made of porous polypropylene with a surface potential of -10kV and a porosity of 70%. Reverse corona discharge parameters: pulse voltage +12kV, frequency 5Hz, 0.2 seconds per discharge. Recovery rate: 97%.
[0070] Step 4: Rapid Induction-Assisted Curing
[0071] The medium-frequency induction heating zone operates at a frequency of 15kHz and has a power density of 25kW / m². 2 The workpiece is heated to 180℃ within 10 seconds. The transition section is 1.2 meters long, and the temperature difference between the surface and the core is controlled within 6℃. The hot air insulation zone is set to 185℃ and maintained for 12 minutes, with an air circulation speed of 2.5 m / s and a temperature deviation of ±1.5℃. The waste heat recovery efficiency is 87%, and the recovered heat is used for pretreatment heating.
[0072] Step 5: Cooling the unloaded part
[0073] Forced air cooling to 40℃ was applied to the parts. Test results: Coating thickness 55-65μm, adhesion grade 0, no corrosion after 800 hours of salt spray testing on aluminum alloy (according to GB / T 10125-2012), and the coating was intact and without any missed areas in irregularly shaped cavities and dead corners after cutting and inspection. No defects were found in the appearance.
[0074] Environmental closed-loop treatment: It is basically the same as in Example 1, except that the inert gas circulation system uses a special desiccant for carbon dioxide (molecular sieve) instead of a condenser dryer to adapt to the physical properties of carbon dioxide.
[0075] Comparative example (traditional process)
[0076] To verify the effectiveness of this invention, a comparative example was set up. A Q235 carbon steel workpiece of the same material (same as Example 1) was used and treated according to traditional processes: degreasing (ordinary alkaline degreasing agent, no ultrasound, temperature 60℃, time 15min) → water washing (2 overflow rinsings, no pure water) → zinc phosphating (temperature 45℃, time 10min) → water washing → drying (120℃, 15min) → electrostatic spraying in an air environment (no auxiliary electrode, no inert atmosphere, spray gun voltage 70kV, powder supply pressure 0.5MPa) → hot air curing (200℃, 25min). Test results: the powder application rate at the grooves was only 40%, requiring manual re-spraying; the coating adhesion was grade 1 (a small amount of peeling occurred in the cross-cut test); red rust appeared after 720 hours of salt spray testing; energy consumption was 45% higher than in Example 1; the wastewater contained pollutants such as nickel and phosphorus, resulting in high treatment costs.
[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. An environmentally friendly integrated electrostatic powder coating process for metal workpieces, characterized in that, Includes the following steps: Step 1: The metal workpiece is sequentially degreased, washed with water, and treated with a chemical conversion film to obtain a wet workpiece with an undried chemical conversion film on its surface. The chemical conversion film is selected from either a phosphorus-free silane film or a zirconium salt film. Step 2: The wet workpiece is transferred to the electrostatic powder spraying chamber through a sealed conveyor channel filled with inert protective gas; Step 3: Electrostatic spraying under an inert atmosphere At least two sets of auxiliary electrodes are arranged around the workpiece in the powder spraying chamber. The Faraday cage effect is overcome by changing the potential state of the auxiliary electrodes in a sequential manner. The electrostatic powder spraying chamber is sealed and filled with inert gas. Powder coating is sprayed onto the surface of the workpiece using an electrostatic spray gun. Oversprayed powder is recovered and reused through electret adsorption plates during the spraying process. Step 4: The workpiece coated with powder coating is sent into the curing oven. First, it passes through the medium frequency induction heating zone, and then enters the hot air insulation zone, so that the powder coating can be completely melted and cross-linked, and the surface coating of the workpiece is cured. Step 5: Cooling the unloaded part After the surface coating has cured, the workpiece is cooled and then removed from the mold, completing the electrostatic powder coating treatment of the metal workpiece.
2. The environmentally friendly integrated electrostatic powder coating process for metal workpieces according to claim 1, characterized in that: In step one, the degreasing treatment uses an environmentally friendly, phosphorus-free degreasing agent, with a treatment temperature of 45–55°C and a time of 8–12 minutes. The environmentally friendly, phosphorus-free degreasing agent contains the following components by mass percentage: sodium hydroxide 0.5%–3.0%, sodium carbonate 1.0%–5.0%, sodium gluconate 0.5%–2.0%, fatty alcohol polyoxyethylene ether 0.1%–1.0%, and the balance being water. The water washing treatment uses 3–5 countercurrent rinsings, with the final water wash using pure water with a conductivity ≤8 μS / cm. The chemical conversion membrane treatment uses a nano-ceramic agent, with a treatment temperature of 30–35°C and a time of 1–2 minutes.
3. The environmentally friendly integrated electrostatic powder coating process for metal workpieces according to claim 1, characterized in that: In step one, the degreasing process is combined with ultrasonic oscillation at a frequency of 28–40 kHz to ensure that the residual oil content is <1 mg / m³. 2 .
4. The environmentally friendly integrated electrostatic powder coating process for metal workpieces according to claim 1, characterized in that: In step two, the air pressure inside the sealed conveying channel is 50-200 Pa higher than the atmospheric pressure outside the workshop, and the inert protective gas is carbon dioxide or nitrogen with a dew point not higher than -40°C.
5. The environmentally friendly integrated electrostatic powder coating process for metal workpieces according to claim 1, characterized in that: In step three, there are 4 to 8 auxiliary electrodes, which are arranged in a ring around the workpiece. The minimum distance between each auxiliary electrode and the workpiece surface is 100 to 300 mm. The potential state of the auxiliary electrodes is changed sequentially by a programmable logic controller.
6. The environmentally friendly integrated electrostatic powder coating process for metal workpieces according to claim 1, characterized in that: In step three, the electrostatic spray gun applies a voltage of 40-70kV and a powder supply pressure of 0.3-0.5MPa. The distance between the electrostatic spray gun and the workpiece is 150-300mm. The powder coating is a low-temperature curing environmentally friendly powder with a particle size D50 of 32-42μm. It contains 2%-10% by mass of phase change microcapsules. The core material of the phase change microcapsules is paraffin or fatty acid ester, and the outer shell is melamine-formaldehyde resin or polyurea resin.
7. The environmentally friendly electrostatic powder coating integrated treatment process for metal workpieces according to claim 1, characterized in that: In step three, the electret adsorption plate is a polytetrafluoroethylene porous plate or a polypropylene porous plate that has been corona-charged, with a surface potential of -5 to -10 kV and a porosity of 40% to 70%. After adsorption saturation, the powder is detached by reverse corona discharge.
8. The environmentally friendly electrostatic powder coating integrated treatment process for metal workpieces according to claim 1, characterized in that: In step four, the operating frequency of the intermediate frequency induction heating zone is 5–20 kHz, and the power density is 10–50 kW / m². 2 A transition section with a length of 1 to 2 meters is provided between the medium-frequency induction heating zone and the hot air insulation zone. The residual heat of the medium-frequency induction coil and the hot air circulation are activated simultaneously in the transition section to ensure that the temperature difference between the surface and core of the workpiece does not exceed 10°C. The hot air insulation zone adopts indirect gas heating or electric heating, with an air circulation speed of 1 to 3 m / s and a temperature deviation of ≤ ±3°C.