Plastic spraying process of high mast light poles
Among the various surface treatment processes, electrostatic spraying (powder coating) has become the mainstream choice for anti-corrosion, decoration, and long-term protection of high-mast light poles due to its excellent comprehensive performance.
I. Core process of plastic spraying: precise and orderly protection
Pretreatment: the basis of success or failure, every detail is important
Purpose: Thoroughly remove oil, rust, scale, welding slag, dust, and all impurities that affect the adhesion of the coating on the surface of the high mast light poles to form a clean, active, and uniform substrate surface.
Key steps:
Degreasing (degreasing): Use alkaline or neutral chemical cleaning agents to spray or soak to remove rolling oil, cutting fluid strongly, and grease dirt contaminated by handling. The cleaning effect must achieve a "continuous water film" state.
Water washing: Multi-stage countercurrent rinsing to thoroughly rinse the residual cleaning agent and dissolved dirt to prevent secondary pollution and carry-in of subsequent processes.
Surface adjustment (optional): For phosphating pretreatment, fine-tune the metal surface charge state to promote the subsequent phosphating film crystallization to be uniform and dense.
Phosphating/ceramic/silane treatment: core conversion film process.
Zinc phosphating: mainstream choice. A porous, microcrystalline zinc phosphate film (phosphating film) is formed on the surface of clean steel. This film greatly enhances the mechanical bite and chemical bonding between the coating and the substrate, providing an excellent anti-corrosion barrier effect. The control requirements for process parameters (temperature, concentration, time, accelerator) are extremely high.
Ceramic/silane treatment (phosphorus-free conversion film): It is increasingly widely used under the trend of environmental protection. The formation of a nano-scale organic-inorganic hybrid film can also significantly improve the adhesion and corrosion resistance of the coating. It does not contain heavy metal phosphorus, and wastewater treatment is simpler.
Water washing: Wash thoroughly again to remove the residual treatment liquid.
Pure water washing (deionized water washing): Key step! Remove calcium, magnesium, and other ions that may be brought in by ordinary water washing, ensure that the resistivity of the final washing water meets the standard, and prevent the salt "white frost" left after the water is dried to affect the appearance and adhesion of the coating.
Drying/water removal: Ensure that the workpiece is completely dry and free of moisture before entering the powder spraying room. Hot air circulation drying or strong wind drying is commonly used.
Electrostatic spraying: uniform coverage, electrostatic adsorption
Environment: carried out in a dedicated, cleanliness-controlled powder spraying room. Good ventilation and powder recovery systems are essential.
Powder coating: Outdoor durable polyester resin powder is preferred. Its characteristics include:
Excellent weather resistance: strong resistance to ultraviolet (UV) degradation, excellent long-term gloss, and color retention (not easy to powder and discolor).
Good mechanical properties: hardness, toughness, and wear resistance meet outdoor use requirements.
Chemical corrosion resistance: can resist acid rain, salt spray, and other erosion.
Rich color and gloss selection: meet the needs of urban beautification.
Electrostatic spray gun: core equipment. The powder is given a high-voltage negative charge (usually 60-100kV) in the spray gun. The negatively charged powder particles are pushed by compressed air to form an atomized airflow and shoot toward the grounded (zero potential) high mast light poles.
Adsorption principle: electrostatic attraction causes the powder particles to be evenly adsorbed on the surface of the high mast light poles. The Faraday cage effect (weak electric field in depressions and dead corners) is a challenge for spraying large and complex workpieces, which needs to be overcome by adjusting the spray gun angle, voltage, air pressure, and powder output, and using friction guns to assist.
Film thickness control: high mast light poles require thick coatings (usually total dry film thickness is above 120μm, or even 150-200μm) to provide more durable physical barrier protection. It needs to be precisely controlled through multiple reciprocating sprays ("multiple guns and multiple passes") to ensure uniform coverage and avoid sagging or being too thin.
High-temperature curing: plasticizing and molding, performance sublimation
Purpose: Heat the powder coating adsorbed on the surface of the high mast light poles to a specific temperature and keep it for a sufficient time to melt, level, cross-link, and cure the resin to form a continuous, dense, tough coating with ultimate performance.
Equipment: A large curing oven (oven) must meet the requirements for the passage of super-long dimensions of high mast light poles. The heating method is usually natural gas or electric heating and forced hot air circulation.
Process core:
Temperature curve: This is the soul of curing. The curing curve provided by the powder coating supplier must be strictly followed. Typical polyester powder curing conditions are usually between 180°C - and 200°C, and the holding time is 10-20 minutes (referring to the duration after the metal body of the workpiece reaches this temperature, not the oven-set temperature).
Temperature uniformity: The temperature uniformity of different areas in a large drying tunnel and different parts of the high mast light poles (especially the flange and lamp arm with large thickness differences) is crucial (usually the temperature difference is required to be ≤±5°C). The hot air circulation system needs to be reasonably designed, and multi-point temperature measurement monitoring should be used when necessary.
Time control: Insufficient holding time leads to incomplete curing (poor coating performance), while too long holding time may cause the coating to be over-baked (discoloration, brittleness).
Effect: The cured coating presents the final color and gloss, and achieves the required adhesion, hardness, flexibility, impact resistance, chemical resistance, and weather resistance.
Cooling and inspection: final quality control
Cooling: After curing, it needs to be cooled naturally or forced to a temperature that can be safely handled. Avoid excessive stress on the coating due to rapid cooling.
Comprehensive inspection:
Appearance: Uniform color, smooth and flat, without defects such as particles, shrinkage holes, pinholes, orange peel, sagging, spray leakage, scratches, etc. As a city landscape, high mast light poles have extremely high appearance requirements.
Film thickness: Use magnetic or eddy current thickness gauges for multi-point measurement to ensure that the overall and key parts (welds, corners, flanges) The film thickness meets the design requirements and is uniform.
Adhesion: Cross-cut test (GB/T 9286), requiring the highest level (usually level 0, that is, the cutting edge is completely smooth and no grid falls off). This is the basis for the coating to serve for a long time without peeling.
Hardness: Pencil hardness test (usually requires ≥H).
Sampling performance test: Impact resistance, cupping test, salt spray resistance test (such as GB/T 10125, requiring no obvious corrosion for more than 1000 hours), artificially accelerated aging test (such as QUV), etc., are carried out according to relevant standards or contract requirements.
II. The core value of plastic spraying technology for high mast light poles
Excellent corrosion resistance: The formed coating is dense and pinhole-free, which can effectively isolate corrosive media such as water vapor, oxygen, and salt from contact with the steel substrate. Combined with high-quality pre-treatment (such as phosphating), the anti-corrosion life far exceeds that of ordinary paints, usually up to 15-25 years or more, greatly reducing maintenance costs. This is particularly economical for high mast light poles that require large lifting equipment for later maintenance.
Superior weather resistance and color and light retention: Special outdoor powders (especially pure polyester or HAA systems) have excellent UV resistance, can resist sunlight exposure for a long time, are not easy to powder, discolor, and lose light, and keep the appearance of the high mast light poles beautiful for a long time, maintaining the image of the city.
Excellent mechanical properties: The cured coating has high hardness, good toughness, wear resistance, and impact resistance, and can effectively resist bumps and scratches during transportation and installation, as well as wind and sand wear in outdoor environments.
Good decorative properties: Rich and diverse colors, optional gloss (high gloss, flat gloss, matte), good coating leveling, and smooth and delicate surface, can meet the high requirements of urban landscaping.
Thick coating advantages and environmental protection: A thicker coating (more than 100μm is easy to achieve) can be obtained in one spraying, with high efficiency. No solvent volatilization (VOC emissions are almost zero), the production process is more environmentally friendly and safe, the powder can be recycled (the recycling rate can reach more than 95%), and the material utilization rate is high.
High overall cost-effectiveness: Although the initial equipment investment and process requirements are high, its ultra-long service life, extremely low maintenance frequency and cost, and excellent protection performance significantly reduce the overall cost of high mast light poles throughout their life cycle.
III. Key elements and challenges of process control
Thoroughness and consistency of pre-treatment: Cleaning of large workpieces, uniformity of phosphating, and quality stability are difficult points, and it is necessary to ensure precise control of tank liquid parameters and proper equipment maintenance.
Uniform spraying of large workpieces: Overcoming the Faraday cage effect and ensuring uniform coating thickness of complex structures (lamp arms, flanges, internal welds) and ultra-long high mast light poles (30 meters +) is the core challenge of spraying technology. Experienced operators and optimized tooling design are required.
Accurate control of curing temperature: Temperature uniformity control of large drying ovens is the key to ensuring complete and uniform curing of the coating of the entire high mast light poles. Temperature monitoring and drying oven design are essential.
Environment and cleanliness control: The cleanliness of the powder spraying room and drying oven directly affects the appearance quality of the coating (particles, impurities). Ambient temperature and humidity also affect spraying and curing.
Powder quality and batch stability: The selection of high-quality and stable outdoor powder coating suppliers is a prerequisite.
Conclusion
The spraying process of high mast light poles is far from being as simple as "spraying a layer of plastic". It is a system engineering that integrates physical chemistry, material science, electrical automation, and precision control. From the meticulous pre-treatment to the uniform coverage of electrostatic spraying, and then to the precise sublimation of high-temperature curing, each link determines the final quality of this "steel armor". A set of strictly implemented and precisely controlled spraying processes gives the high mast light poles the tenacity to resist the erosion of time, ensuring that they shine for a long time on the city's skyline, silently guarding the safety and prosperity of the night. This is not only a reflection of technical craftsmanship, but also a solemn commitment to the long-term quality of urban infrastructure.