Electrostatic spraying process for aluminum profile

Abstract

The invention discloses an aluminum profile electrostatic spraying process which comprises the following steps: (1) extrusion blank pretreatment: grinding an aluminum profile extrusion blank by adopting 120 # abrasive paper or a 800-mesh nylon polishing wheel, and controlling the surface roughness Ra to be less than or equal to 1.6 mu m without scratches with the depth being greater than 0.2 mm; the pretreated aluminum profile is hung on 304 stainless steel hangers, the distance between the hangers is larger than or equal to 50 mm, and the amplitude of the aluminum profile is smaller than or equal to 2 mm; the aluminum profile is immersed into a dilute sulfuric acid-ammonium bifluoride-citric acid composite system, the concentration of dilute sulfuric acid is 30-50 g / L, the concentration of ammonium bifluoride is 5-8 g / L, the concentration of citric acid is 3-5 g / L, the degreasing temperature is controlled to be 20-25 DEG C, and the treatment time is 1-3 minutes. A normal-temperature dilute sulfuric acid composite system is adopted for degreasing, a traditional alkaline or organic solvent degreasing process needing heating is replaced, and energy consumption in the degreasing link is greatly reduced; and meanwhile, the chromium-free passivator is used, so that discharge of hexavalent chromium is thoroughly eliminated, and generation of VOCs is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aluminum profile surface treatment technology, and particularly relates to an aluminum profile electrostatic spraying process. BACKGROUND

[0002] Aluminum profiles are widely used in building curtain walls, automobile parts, photovoltaic supports and rail transit fields due to their lightweight, high strength, easy processing and other characteristics. In order to improve the corrosion resistance, weather resistance and decoration of aluminum profiles, the electrostatic spraying process has become the mainstream surface treatment method, and polyester powder electrostatic spraying accounts for more than 60% of the market share due to no solvent volatilization and strong coating adhesion.

[0003] However, the traditional electrostatic spraying process of aluminum profiles has the following key defects: 1. Defects of degreasing process: The traditional degreasing process uses alkaline degreasing agent (sodium carbonate and sodium silicate compound system) or organic solvent degreasing, which needs to be heated to 40-60℃ and treated for 5-15 minutes, resulting in high energy consumption. The alkaline solution can easily corrode the aluminum substrate, causing surface spots. The waste water contains high concentrations of alkali and surfactants, and the treatment cost accounts for 25%-30% of the total pretreatment cost. The use of organic solvents may cause VOCs emission and fire hazards.

[0004] 2. Water resource waste: The use of tap water directly discharges or simple reverse flow rinsing results in a water resource recycling rate of less than 30%, which does not meet the requirement of "reutilization rate ≥ 80%" in the "Industrial Circulating Water Utilization Guidelines". Moreover, tap water flushing cannot completely remove residual reagents, which may cause coating shrinkage and pinholes.

[0005] 3. Environmental risk of passivation: Long-term reliance on chromate passivation (CrO3 concentration of 2-5g / L) may cause environmental risk. Hexavalent chromium is a strong carcinogen, and the waste water needs to be treated by "reduction-precipitation-membrane filtration" for multiple stages, which costs 40%-50% more than ordinary waste water treatment. Moreover, it does not comply with the EU REACH, RoHS and other regulations, and is limited for export.

[0006] 4. Unstable curing quality: The use of "180-200℃×15-25 minutes" general parameters is not suitable for mainstream pure polyester powder (Tg≈50-70℃), which may cause "over-baking discoloration" or "insufficient curing leading to adhesion reduction". The service life of the coating used outdoors is only 5-8 years.

[0007] Therefore, an aluminum profile electrostatic spraying process is proposed to solve the above problems. SUMMARY

[0008] (I) Invention purpose To solve the technical problems in the background art, the present application provides an aluminum profile electrostatic spraying process, which uses a normal temperature dilute sulfuric acid composite system for degreasing, replacing the traditional degreasing process of heated alkaline or organic solvent, greatly reducing the energy consumption of the degreasing link; at the same time, a chromium-free passivator is used to completely eliminate the emission of hexavalent chromium and reduce the generation of VOCs.

[0009] (II) Technical solutions The present application provides an aluminum profile electrostatic spraying process, comprising the following steps: Step 1), pretreatment of extrusion blank: polishing the aluminum profile extrusion blank with 120# sandpaper or 800 mesh nylon polishing wheel, controlling the surface roughness Ra≤1.6μm, and no deep 0.2mm scratch; Step 2), positioning on the rack: suspending the pretreated aluminum profile on a 304 stainless steel hanger, the hanger spacing is ≥50mm, and the aluminum profile amplitude is ≤2mm; Step 3), dilute sulfuric acid oil removal and degreasing: immersing the aluminum profile in a dilute sulfuric acid-ammonium hydrogen fluoride-citric acid composite system, the dilute sulfuric acid concentration is 30-50g / L, the ammonium hydrogen fluoride concentration is 5-8g / L, and the citric acid concentration is 3-5g / L, controlling the degreasing temperature at 20-25℃, and the treatment time is 1-3 minutes; Step 4), water washing: the degreased aluminum profile enters the middle water spraying area, and is sprayed and cleaned with MBR membrane treated middle water (COD≤50mg / L, electrical conductivity≤500μS / cm), the spraying pressure is 0.2-0.3MPa, and the cleaning time is 1-2 minutes; Step 5), first stage pure water washing: the middle water washed aluminum profile enters the first stage pure water rinsing tank, the pure water is prepared by RO reverse osmosis (electrical conductivity≤50μS / cm), and the rinsing time is 1-1.5 minutes; Step 6), chromium-free passivation: immersing the first stage pure water washed aluminum profile in a passivation tank, the passivation tank contains zirconium titanium salt chromium-free passivator (solid content 10%-15%) or silane composite chromium-free passivator; controlling the passivation temperature at 20-30℃, the treatment time is 1-2 minutes, and the passivation film thickness (70-150mg / m²) is detected by weighing method after passivation; Step 7), second stage pure water washing: the passivated aluminum profile enters the second stage pure water rinsing tank, and is rinsed with 3 sections of RO reverse osmosis pure water (electrical conductivity≤50μS / cm) in countercurrent, the rinsing flow rate is 0.5m / s, and the time is 1.5-2 minutes; the passivator residual amount is detected by spectrophotometry after rinsing, ensuring ≤10mg / L; Step 8), draining and drying: the passivated aluminum profile enters the second stage pure water rinsing tank, and is rinsed with 3 sections of RO reverse osmosis pure water (electrical conductivity≤50μS / cm) in countercurrent, the rinsing flow rate is 0.5m / s, and the time is 1.5-2 minutes; Step 9), polyester powder electrostatic spraying: the dried aluminum profile enters the spraying room, and pure polyester powder is sprayed by using a 360-degree rotating electrostatic spray gun, the spraying voltage is 60-80kV, the atomizing pressure is 0.3-0.5MPa, and the distance between the spray gun and the aluminum profile is 20-30cm; Step 10), curing treatment: the sprayed aluminum profile is sent into an intelligent temperature control curing furnace, the curing temperature is controlled by a PID temperature control system to be 160-180℃, the temperature difference in the furnace is ≤±3℃, the curing time is 20-25 minutes, and the hot air speed in the curing furnace is 1.5m / s; Step 11), finished product inspection: the coating thickness, salt spray resistance, adhesion and impact resistance are detected.

[0010] Further, in the step 3), the degreasing process is detected by titration every 2 hours to control the pH≤1.5, and the degreasing end point is that there is no break in the continuous water film on the surface of the aluminum profile.

[0011] Further, in the step 6), the pH value of the passivator is detected by a precision pH meter every batch, and is controlled to be 5.5-7.0.

[0012] Further, in the step 9), a powder recovery system is matched, the powder recovery rate is ≥95%, and the coating thickness is detected by a film thickness instrument every 5 aluminum profiles.

[0013] Further, in the step 10), the surface temperature of the aluminum profile is monitored in real time by an infrared temperature detector, the data is uploaded to the system, and the glass transition temperature Tg of the coating after curing is about 50-70℃.

[0014] Further, in the step 5), the conductivity is monitored in real time by an online conductivity meter during the rinsing process, and the water is automatically replaced when the conductivity exceeds the standard.

[0015] Further, in the step 6), the pH value of the passivator is detected by a precision pH meter every batch (controlled to be 5.5-7.0).

[0016] Further, in the step 8), the water content of the aluminum profile after drying is detected by the Karl Fischer method, and is ensured to be ≤0.1%.

[0017] Compared with the prior art, the above technical scheme of the present application has the following beneficial technical effects: 1. The aluminum profile electrostatic spraying process completely eliminates the discharge of hexavalent chromium by using a chromium-free passivator, has no heating energy consumption for normal temperature degreasing, reduces the fresh water consumption by 40%-60% by using reclaimed water, can reach the standard by neutralizing the degreasing wastewater to pH 6-9, and has no VOCs emission.

[0018] 2、The aluminum profile electrostatic spraying process, the energy consumption of normal temperature degreasing is lower than that of traditional alkaline degreasing; the middle water recycling reduces the fresh water and waste water treatment cost, the passivation waste water treatment cost is reduced by 30%-50%; the powder recovery rate is greater than or equal to 95%, the labor dependence is reduced, and the cost is reduced comprehensively.

[0019] 3、The aluminum profile electrostatic spraying process does not need to make large-scale transformation to the existing production line, only needs to replace the degreasing agent, the passivation agent and add a middle water recycling system, and can be quickly put into production. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A flow chart of an aluminum profile electrostatic spraying process is provided. DETAILED DESCRIPTION

[0021] To make the purpose, technical scheme and advantages of the present application clearer and more explicit, the present application is further described in detail below with specific embodiments and with reference to the drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present application. In addition, in the following description, the description of known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present application.

[0022] In the description of the application, it should be noted that the orientation or position relationship indicated by the terms "upper", "lower", "inner", "outer", "front end", "rear end", "two ends", "one end" and "the other end" is based on the orientation or position relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as limiting the present application in that the indicated device or element must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as limiting the present application. In addition, the terms "first" and "second" are only for the purpose of description and cannot be understood as indicating or implying relative importance.

[0023] In the description of the application, it should be noted that unless otherwise explicitly specified and limited, the terms "mounting", "provided with", "connection" and the like should be understood broadly, for example, "connection" can be fixed connection, welding, riveting, bonding and the like, or can be detachable connection, threaded connection, key connection, pin connection and the like, or integrally connected; can be mechanical connection, or electrical connection; can be directly connected, or indirectly connected through an intermediate medium; or the internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0024] Example 1 An aluminum profile electrostatic spraying process, comprising the following steps: Step 1), pretreatment of extrusion blank: polishing the aluminum profile extrusion blank with 120# sandpaper, controlling the surface roughness Ra to be 1.2 μm, and no depth > 0.2 mm scratch; Step 2), hanging positioning: the pretreated aluminum profile is hung on a 304 stainless steel hanger with a spacing of 55 mm and an amplitude of 1.5 mm; Step 3), oil removal and degreasing with dilute sulfuric acid: the aluminum profile is immersed in a dilute sulfuric acid-ammonium hydrogen fluoride-citric acid composite system, with a dilute sulfuric acid concentration of 35 g / L, an ammonium hydrogen fluoride concentration of 6 g / L, and a citric acid concentration of 3.5 g / L. The degreasing temperature is controlled at 22°C, and the treatment time is 2 minutes. The degreasing solution concentration is detected every 2 hours by titration method to control the pH at 1.2, and the degreasing endpoint is the continuous uninterrupted water film on the surface of the aluminum profile; Step 4), water washing with reclaimed water: the degreased aluminum profile enters the reclaimed water spraying area, and the reclaimed water (COD 40 mg / L, conductivity 400 μS / cm) treated by MBR membrane is used for spraying cleaning, with a spraying pressure of 0.25 MPa and a cleaning time of 1.5 minutes; Step 5), first-stage pure water washing: the aluminum profile after reclaimed water washing enters the first-stage pure water rinsing tank, and the pure water is prepared by RO reverse osmosis (conductivity 40 μS / cm), with a rinsing time of 1.2 minutes. The conductivity is monitored in real time by an online conductivity meter, and the water is automatically replaced when the conductivity exceeds the standard; Step 6), chromium-free passivation: the aluminum profile after first-stage pure water washing is immersed in a passivation tank containing zirconium-titanium salt chromium-free passivation agent (solid content 12%); the passivation temperature is controlled at 25°C, and the treatment time is 1.5 minutes. The passivation film thickness is 100 mg / m² detected by weighing method after passivation. The pH value of the passivation agent is detected by a precision pH meter for each batch, and is controlled at 6.0; Step 7), second-stage pure water washing: the passivated aluminum profile enters the second-stage pure water rinsing tank, and is subjected to 3-stage countercurrent rinsing with RO reverse osmosis pure water (conductivity 40 μS / cm), with a rinsing flow rate of 0.5 m / s and a time of 1.8 minutes. The passivation agent residual amount is 8 mg / L detected by spectrophotometry after rinsing; Step 8), draining and drying: the passivated aluminum profile enters the drying equipment, and the water content of the aluminum profile is 0.08% detected by Karl Fischer method after drying; Step 9), polyester powder electrostatic spraying: the dried aluminum profile enters the spraying house, and is subjected to pure polyester powder spraying by a 360° rotating electrostatic spray gun, with a spraying voltage of 65 kV, an atomizing pressure of 0.4 MPa, and a distance between the spray gun and the aluminum profile of 25 cm. A powder recovery system is matched, and the powder recovery rate is 96%. The coating thickness is detected by a film thickness meter every 5 aluminum profiles; Step 10), curing treatment: the sprayed aluminum profile is sent into an intelligent temperature control curing furnace, a PID temperature control system is used to control the curing temperature to be 170℃, the temperature difference in the furnace is ±2℃, the curing time is 22 minutes, the hot air speed in the curing furnace is 1.5m / s; the surface temperature of the aluminum profile is monitored in real time through an infrared temperature detector, the data is uploaded to the system, and the glass transition temperature Tg of the coating after curing is 60℃; Step 11), finished product inspection: the coating thickness, salt spray resistance, adhesion and impact resistance are detected, and all indexes meet the standards.

[0025] Example two An aluminum profile electrostatic spraying process, comprising the following steps: Step 1), pretreatment of extruded blank: the extruded blank of aluminum profile is polished by using a 800 mesh nylon polishing wheel, the surface roughness Ra is controlled to be 1.0μm, and there is no scratch with a depth of more than 0.2mm; Step 2), positioning on the rack: the pretreated aluminum profile is hung on a 304 stainless steel hanger, the distance between the hangers is 60mm, and the amplitude of the aluminum profile is 1.0mm; Step 3), oil removal and degreasing by dilute sulfuric acid: the aluminum profile is immersed in a dilute sulfuric acid-ammonium hydrogen fluoride-citric acid composite system, the concentration of dilute sulfuric acid is 40g / L, the concentration of ammonium hydrogen fluoride is 7g / L, and the concentration of citric acid is 4g / L, the degreasing temperature is controlled to be 23℃, and the treatment time is 1.5 minutes; the concentration of the degreasing solution is detected by titration every 2 hours during the degreasing process, the pH is controlled to be 1.0, and the end point of degreasing is that there is no break in the continuous water film on the surface of the aluminum profile; Step 4), water washing with intermediate water: the degreased aluminum profile enters the intermediate water spraying area, and is sprayed and cleaned by using the intermediate water (COD 35mg / L, conductivity 350μS / cm) treated by MBR membrane, the spraying pressure is 0.28MPa, and the cleaning time is 1.2 minutes; Step 5), first-stage pure water washing: the aluminum profile washed with intermediate water enters the first-stage pure water rinsing tank, the pure water is prepared by RO reverse osmosis (conductivity 35μS / cm), and the rinsing time is 1.0 minute; the conductivity is monitored in real time by an online conductivity meter during the rinsing process, and the water is automatically replaced when the conductivity exceeds the standard; Step 6), passivation by chromium-free agent: the aluminum profile washed with the first-stage pure water is immersed in a passivation tank, and the passivation tank contains silane composite chromium-free passivation agent; the passivation temperature is controlled to be 28℃, the treatment time is 1.2 minutes, the passivation film thickness is 120mg / m² after passivation is detected by weighing method, and the pH value of the passivation agent is detected by a precision pH meter for each batch, and is controlled to be 6.5; Step 7), second-stage pure water washing: the passivated aluminum profile enters the second-stage pure water rinsing tank, and is rinsed by 3-stage countercurrent rinsing by using RO reverse osmosis pure water (conductivity 35μS / cm), the rinsing flow rate is 0.5m / s, and the time is 1.6 minutes; the residual amount of the passivation agent after rinsing is 7mg / L detected by spectrophotometry; Step 8), draining and drying: After passivation, the aluminum profile enters the drying equipment, and after drying, the water content of the aluminum profile is detected by Karl Fischer method, which is 0.07%; Step 9), polyester powder electrostatic spraying: After drying, the aluminum profile enters the spraying room, and pure polyester powder is sprayed by using a 360° rotating electrostatic spray gun, with a spraying voltage of 70 kV, an atomizing pressure of 0.35 MPa, and a distance between the spray gun and the aluminum profile of 22 cm. A powder recovery system is used, and the powder recovery rate is 97%. Every 5 aluminum profiles are detected by a thickness gauge to detect the coating thickness. Step 10), curing treatment: The sprayed aluminum profile is sent into an intelligent temperature control curing furnace, and a PID temperature control system is used to control the curing temperature to be 165℃, the temperature difference in the furnace is ±1.5℃, the curing time is 23 minutes, the hot air speed in the curing furnace is 1.5 m / s. The surface temperature of the aluminum profile is monitored in real time by an infrared temperature detector, and the data is uploaded to the system. The glass transition temperature Tg of the coating after curing is 55℃. Step 11), finished product inspection: The coating thickness, salt spray resistance, adhesion and impact resistance are detected, and all indicators meet the standards.

[0026] Example Three An aluminum profile electrostatic spraying process, comprising the following steps: Step 1), pretreatment of extrusion blank: The aluminum profile extrusion blank is polished with 120# sandpaper, and the surface roughness Ra is controlled to be 1.5 μm without deep (>0.2 mm) scratches; Step 2), positioning on the rack: The pretreated aluminum profile is hung on a 304 stainless steel hanger with a spacing of 50 mm, and the aluminum profile has an amplitude of 2.0 mm; Step 3), oil removal and degreasing with dilute sulfuric acid: The aluminum profile is immersed in a dilute sulfuric acid-ammonium hydrogen fluoride-citric acid composite system, with a dilute sulfuric acid concentration of 45 g / L, an ammonium hydrogen fluoride concentration of 5 g / L, and a citric acid concentration of 5 g / L. The degreasing temperature is controlled at 25℃, and the treatment time is 3 minutes. During the degreasing process, the concentration of the degreasing solution is detected every 2 hours by titration, and the pH is controlled at 1.5. The end point of degreasing is that there is no break in the continuous water film on the surface of the aluminum profile. Step 4), water washing with reclaimed water: The degreased aluminum profile enters the reclaimed water spraying area, and is sprayed and cleaned with MBR membrane treated reclaimed water (COD 45 mg / L, conductivity 450 μS / cm) at a spraying pressure of 0.22 MPa for 2 minutes; Step 5), first-stage pure water washing: The aluminum profile after reclaimed water washing enters the first-stage pure water rinsing tank, and the pure water is prepared by RO reverse osmosis (conductivity 45 μS / cm), and the rinsing time is 1.5 minutes. During the rinsing process, the online conductivity meter is used for real-time monitoring, and the water is automatically replaced when the conductivity exceeds the standard. Step 6), passivation with chromium-free agent: the first-stage water-washed aluminum profile is immersed in a passivation tank containing a zirconium-titanium salt chromium-free passivation agent (solid content 14%); the passivation temperature is controlled at 22°C and the treatment time is 2 minutes; the passivation film thickness is 80 mg / m2 after passivation, as detected by the weighing method; the pH value of the passivation agent is detected by a precision pH meter for each batch, and is controlled at 5.8; Step 7), second-stage water rinsing: the passivated aluminum profile enters a second-stage water rinsing tank, and is rinsed with RO reverse osmosis pure water (electrical conductivity 45 μS / cm) for 3 stages of countercurrent rinsing, at a rinsing flow rate of 0.5 m / s and for 2 minutes; the passivation agent residual amount after rinsing is 9 mg / L, as detected by the spectrophotometric method; Step 8), draining and drying: the passivated aluminum profile enters a drying device, and the water content of the aluminum profile after drying is 0.09%, as detected by the Karl Fischer method; Step 9), polyester powder electrostatic spraying: the dried aluminum profile enters a spraying house, and is sprayed with pure polyester powder by using a 360° rotary electrostatic spray gun, at a spraying voltage of 75 kV, an atomizing pressure of 0.45 MPa, and a distance between the spray gun and the aluminum profile of 28 cm; a powder recovery system is used, and the powder recovery rate is 96%; the coating thickness is detected by a film thickness meter for every 5 aluminum profiles; Step 10), curing treatment: the sprayed aluminum profile is sent into an intelligent temperature control curing furnace, and the curing temperature is controlled at 175°C by using a PID temperature control system, the temperature difference in the furnace is ±2.5°C, the curing time is 24 minutes, the hot air speed in the curing furnace is 1.5 m / s; the surface temperature of the aluminum profile is monitored in real time by an infrared temperature detector, and the data is uploaded to the system; and the coating glass transition temperature Tg after curing is 65°C; Step 11), finished product inspection: the coating thickness, salt spray resistance, adhesion and impact resistance are detected, and all the indexes meet the standards.

[0027] Comparative Example One The traditional alkaline degreasing agent (sodium carbonate and sodium silicate complex system) is used for degreasing, heating to 50°C and treating for 10 minutes, and the other steps are the same as in Example One. It is detected that the energy consumption of this comparative example is 30% higher than that of Example One, and a small amount of spots appear on the surface of the aluminum profile, and the wastewater treatment cost increases by 28%.

[0028] Comparative Example Two The water washing process uses tap water direct discharge, without water reuse and multi-stage pure water washing, and the other steps are the same as in Example Two. It is detected that the water resource reuse rate of this comparative example is only 25%, which does not meet the requirements of the Industrial Circulating Water Utilization Guide, and the coating appears shrinkage and pinhole phenomenon.

[0029] Comparative Example Three Chromate passivation (CrO3 concentration 3g / L) was used, and other steps were the same as example three. It was detected that the wastewater treatment cost of the comparative example was 45% higher than that of example three, and there was hexavalent chromium emission, which did not meet the requirements of relevant regulations of the European Union.

[0030] Experimental example

[0031] In summary The energy consumption reduction rate of degreasing: the energy consumption of examples one to three is reduced by 28%, 32% and 30% respectively compared with comparative example one (traditional alkaline degreasing), which intuitively reflects the energy-saving advantage of normal temperature degreasing.

[0032] Water resource recycling rate: the recycling rate of examples one to three is 82%-85%, which is much higher than that of comparative example two (25%), and meets the requirement of “≥80%” in the “Guidelines for Industrial Circulating Water Utilization”.

[0033] Wastewater treatment cost: the cost of examples one to three is reduced by 35%, 38% and 36% respectively compared with comparative example three (chromate passivation), which highlights the environmental protection and economy of chromium-free passivation.

[0034] Outdoor service life: the service life of the coating of examples is estimated to be 12-15 years, which is nearly 1 times higher than that of the traditional process (comparative examples one to three, 5-8 years), which verifies the effect of the optimization of the curing parameters.

[0035] The electrostatic spraying process of the aluminum profile proposed in the application has obvious advantages in reducing energy consumption, saving water resources, reducing wastewater treatment cost, improving coating quality and environmental performance.

[0036] Although the embodiments of the application have been shown and described, it can be understood by those of ordinary skill in the art that various changes, modifications, replacements and variations can be made to the embodiments without departing from the principles and spirits of the application, and the scope of the application is defined by the appended claims and their equivalents.

Claims

1. An electrostatic spray process for aluminum profiles, characterized in that, The method comprises the following steps: Step 1), pretreatment of extrusion blank: polishing the aluminum profile extrusion blank with 120# sandpaper or 800 mesh nylon polishing wheel, controlling the surface roughness Ra≤1.6 μm, and no deep scratches greater than 0.2 mm; Step 2), positioning: hanging the pretreated aluminum profile on a 304 stainless steel hanger, the distance between the hangers is greater than or equal to 50 mm, and the amplitude of the aluminum profile is less than or equal to 2 mm; Step 3), oil removal and degreasing with dilute sulfuric acid: immersing the aluminum profile in a dilute sulfuric acid-ammonium hydrogen fluoride-citric acid composite system, the concentration of dilute sulfuric acid is 30-50 g / L, the concentration of ammonium hydrogen fluoride is 5-8 g / L, and the concentration of citric acid is 3-5 g / L, the degreasing temperature is controlled at 20-25°C, and the treatment time is 1-3 minutes; Step 4), water washing: after degreasing, the aluminum profile enters the water spraying area, and is sprayed and cleaned with MBR membrane treated water (COD≤50 mg / L, electrical conductivity≤500 μS / cm), the spraying pressure is 0.2-0.3 MPa, and the cleaning time is 1-2 minutes; Step 5), first-stage pure water washing: after water washing, the aluminum profile enters the first-stage pure water rinsing tank, and the pure water is prepared by RO reverse osmosis (electrical conductivity≤50 μS / cm), and the rinsing time is 1-1.5 minutes; Step 6), passivation with chromium-free agent: immersing the aluminum profile after the first-stage pure water washing in a passivation tank, the passivation tank contains zirconium-titanium salt chromium-free passivation agent (solid content 10%-15%) or silane composite chromium-free passivation agent; the passivation temperature is controlled at 20-30°C, the treatment time is 1-2 minutes, and the passivation film thickness is detected by weighing method (70-150 mg / m²) after passivation; Step 7), second-stage pure water washing: after passivation, the aluminum profile enters the second-stage pure water rinsing tank, and is rinsed with RO reverse osmosis pure water (electrical conductivity≤50 μS / cm) for 3 stages of countercurrent rinsing, the rinsing flow rate is 0.5 m / s, and the time is 1.5-2 minutes; after rinsing, the passivation agent residual amount is detected by spectrophotometry, and is ensured to be less than or equal to 10 mg / L; Step 8), draining and drying: after passivation, the aluminum profile enters the second-stage pure water rinsing tank, and is rinsed with RO reverse osmosis pure water (electrical conductivity≤50 μS / cm) for 3 stages of countercurrent rinsing, the rinsing flow rate is 0.5 m / s, and the time is 1.5-2 minutes; Step 9), polyester powder electrostatic spraying: after drying, the aluminum profile enters the spraying room, and is sprayed with pure polyester powder by using a 360° rotating electrostatic spray gun, the spraying voltage is 60-80 kV, the atomization pressure is 0.3-0.5 MPa, and the distance between the spray gun and the aluminum profile is 20-30 cm; Step 10), curing treatment: sending the sprayed aluminum profile into an intelligent temperature control curing furnace, controlling the curing temperature by using a PID temperature control system, the curing temperature is 160-180°C, the temperature difference in the furnace is less than or equal to ±3°C, the curing time is 20-25 minutes, and the hot air speed in the curing furnace is 1.5 m / s; Step 11), finished product inspection: detecting the coating thickness, salt spray resistance, adhesion and impact resistance.

2. The electrostatic spray process for aluminum shapes as claimed in claim 1 wherein, In step 3), the concentration of the degreasing solution is detected by titration every 2 hours during the degreasing process, the pH is controlled to be less than or equal to 1.5, and the degreasing end point is that there is no break in the continuous water film on the surface of the aluminum profile.

3. The electrostatic spray process for aluminum shapes as claimed in claim 1 wherein, In step 6), the pH value of the passivator in each batch is detected by a precision pH meter, and the pH value is controlled to be 5.5-7.

0.

4. The electrostatic spray process for aluminum shapes as claimed in claim 1 wherein, In step 9), a powder recovery system is matched, the powder recovery rate is ≥95%, and the coating thickness is detected by a film thickness instrument every 5 aluminum profiles.

5. The electrostatically sprayed process for aluminum shapes as claimed in claim 1 wherein, In step 10), the surface temperature of the aluminum profile is monitored in real time by an infrared temperature detector, and the data is uploaded to the system, and the glass transition temperature Tg of the coating after curing is about 50-70℃.

6. The electrostatically sprayed process for aluminum shapes as claimed in claim 1 wherein, In step 5), the online conductivity meter is used to monitor the rinsing process in real time, and the water is automatically replaced when the conductivity exceeds the standard.

7. The electrostatically spray painting process of aluminum profiles according to claim 1, characterized in that, In step 6), the pH value of the passivator in each batch is detected by a precision pH meter (controlled to be 5.5-7.0).

8. The electrostatically spray painting process of aluminum profiles according to claim 1, characterized in that, In step 8), the water content of the aluminum profile is detected by the Karl Fischer method after drying to ensure that it is ≤0.1%.

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