Products Description
Precision Sheet Metal Powder Coating Line
The precision sheet metal powder coating line is designed for high-precision workpieces such as electronic equipment enclosures, medical device sheet metal parts, and automotive precision components (with a dimensional tolerance of ±0.1-0.5mm and a thickness of 0.5-3mm). Its core goal is to form a uniform, defect-free powder coating while ensuring the dimensional stability of the sheet metal parts. The process flow centers on three core requirements: "deformation prevention, high-precision thickness control, and no impurity contamination", and is mainly divided into five stages, each equipped with specialized equipment and process parameters:
I. Workpiece Loading and Positioning (powder coating line)
Operation Points
Operators use "soft-support hangers" (the contact parts of the hangers are covered with silica gel or rubber) to fix the precision sheet metal parts, preventing workpiece deformation caused by hanger pressure. For thin and light sheet metal parts (such as electronic enclosures with a thickness < 1mm), multi-point support positioning is adopted to ensure no wobbling or displacement of the workpieces during transportation.
Equipment Configuration
A precision conveyor chain driven by a servo motor is equipped, and the conveying speed can be accurately adjusted (usually 0.5-1m/min) to avoid uneven processing in subsequent processes due to fluctuations in conveying speed. The distance between hangers is customized according to the workpiece size to ensure no collision or blocking between adjacent workpieces.
Core Control
The workpiece positioning deviation must be controlled within ±0.2mm, and the load-bearing capacity of the hangers should not exceed 1.5 times the weight of the workpiece to prevent deformation caused by excessive pulling.
II. Pretreatment Stage (Low-Temperature, Low-Impact Process)
Pretreatment is crucial for ensuring coating adhesion and must avoid the impact of high temperature and high pressure on the dimensions of precision sheet metal parts. The process is divided into 4 steps:
Low-Temperature Degreasing
Workpieces enter a low-temperature degreasing tank (temperature: 40-50℃, lower than the 55-65℃ of general lines). A weakly alkaline degreaser (concentration: 2%-3%) is used, and surface oil stains are removed by spraying (instead of soaking). The spraying pressure is controlled at 0.2-0.3MPa to prevent workpiece deformation caused by high-pressure impact. The degreasing time is 8-10 minutes to ensure complete removal of oil stains without damaging the sheet metal surface.
Precision Water Washing
Three stages of countercurrent spray water washing are set up. The first stage removes residual degreaser, the second stage uses deionized water for cleaning, and the third stage uses ultrapure water for rinsing. The water temperature is controlled at 35-40℃ to avoid thermal deformation of the workpiece due to excessive temperature difference. After water washing, the water film on the workpiece surface must be uniform and continuous without residual water marks (to prevent pinholes in the subsequent coating).
Micro-Film Phosphating
Workpieces enter a micro-film phosphating tank (temperature: 35-45℃) to form a dense phosphating film with a thickness of 5-8μm (the phosphating film thickness of general lines is 10-15μm). The phosphating time is 5-6 minutes, which not only ensures coating adhesion but also avoids excessive film thickness affecting subsequent assembly accuracy. After phosphating, hot air drying (temperature: 60-70℃) is used instead of high-temperature drying to prevent thermal expansion and contraction of the workpiece.
Precision Drying
Workpieces enter a low-temperature drying oven (temperature: 70-80℃, drying time: 15-20 minutes). A hot air circulation + exhaust system is adopted to ensure complete evaporation of moisture on the workpiece surface (moisture content < 0.1%). At the same time, the temperature fluctuation in the oven is controlled within ±2℃ to avoid local overheating causing workpiece deformation.
III. Powder Spraying Stage (High-Precision Thickness Control)
In response to the high requirement for coating thickness uniformity of precision sheet metal parts, the spraying stage adopts a "precision atomization + real-time monitoring" process:
Sealed Spraying Booth
The spraying booth adopts a fully sealed design and is equipped with a negative-pressure dust removal system (negative pressure value: -50 to -80Pa) to prevent dust contamination of the workpieces. The booth is equipped with 2-4 high-precision electrostatic spray guns (atomized particle size: 50-80μm, compared with 80-120μm for general spray guns). The distance between the spray guns and the workpieces is controlled at 200-250mm, and the angle can be fine-tuned by a servo motor (accuracy: ±1°).
Electrostatic Spraying Parameters
The electrostatic voltage is controlled at 60-80kV, the electrostatic current at 50-80μA, and the powder output at 50-80g/min (adjusted according to the required coating thickness). For sheet metal parts with complex structures (such as electronic enclosures with small holes and grooves), a "multi-angle spray gun + touch-up spraying station" is used to ensure no missing spraying in grooves, corners, and other parts (coating coverage rate: 100%).
Real-Time Thickness Measurement and Adjustment
An online coating thickness gauge (accuracy: ±1μm) is installed at the exit of the spraying booth to real-time detect the coating thickness on the workpiece surface (target thickness: 40-80μm). If the thickness deviation exceeds ±3μm, the system automatically adjusts the spray gun parameters (such as output volume, spraying distance) to ensure uniform coating thickness for each workpiece.
Powder Recovery
A two-stage recovery system of "cyclone separation + filter element filtration" is adopted, with a powder recovery rate of over 98% (higher than the 95% of general lines). The recovered powder must be sieved (sieve mesh size: 300 meshes) before reuse to avoid impurities mixing in and affecting coating quality.
IV. Low-Temperature Curing Stage (Slow Heating, Precision Temperature Control)
The curing stage must avoid workpiece deformation caused by high-temperature rapid curing while ensuring complete curing of the coating:
Stepwise Heating Curing
Workpieces enter a stepwise curing oven, which is divided into a preheating section (temperature: 120-140℃, time: 5-8 minutes), a curing section (temperature: 160-180℃, time: 25-30 minutes), and a cooling section (temperature: 100-120℃, time: 5-8 minutes). The heating rate is controlled at 5-8℃/min, and the cooling rate at 3-5℃/min to avoid thermal stress deformation of the workpiece due to excessive temperature difference. The temperature uniformity in the oven is controlled within ±3℃ to ensure uniform coating curing.
Curing Quality Control
After curing, the coating must meet the following requirements: pencil hardness ≥ 2H, adhesion cross-cut test Grade 0, salt spray resistance test ≥ 480 hours (≥ 720 hours for medical device sheet metal parts). At the same time, the coating surface must be smooth and flat without defects such as sagging, pinholes, or orange peel (defects with an area ≤ 0.1mm² are considered qualified).
V. Cooling and Unloading Stage (Damage Prevention, Precision Inspection)
Gradient Cooling
After being sent out of the curing oven, the workpieces first enter a gradient cooling chamber (temperature: 100-80℃, time: 5-10 minutes), and then enter a room-temperature cooling area (temperature: 25-30℃, time: 10-15 minutes) to avoid uneven dimensional shrinkage of the workpiece due to rapid cooling. A soft conveyor belt (surface covered with polyurethane) is used during the cooling process to prevent scratches on the workpiece surface.
Precision Inspection
Before unloading, the workpieces must undergo "dual inspection": ① Appearance inspection (using strong light irradiation + magnifying glass to check for coating defects); ② Dimensional inspection (using a coordinate measuring machine to detect key dimensions of the workpiece, with deviation ≤ ±0.1mm); ③ Coating thickness inspection (using an offline thickness gauge, sampling ratio ≥ 10%, thickness deviation ≤ ±3μm).
Qualified Unloading
Qualified workpieces after inspection are stored in specialized pallets (with soft cushioning materials laid inside the pallets) to avoid stacking pressure. Unqualified workpieces (such as those with coating defects or dimensional deviations) must be marked and sent to the rework area. A low-temperature paint stripper (temperature: 50-60℃) is used for paint stripping before they re-enter the production line, to avoid workpiece damage caused by high-temperature paint stripping.
VI. Core Advantages of the Process Flow (Compared with General Coating Lines)
Deformation Prevention Design: A low-temperature process is adopted throughout the entire flow (degreasing, drying, and curing temperatures are all 10-15℃ lower than those of general lines), and operations that easily cause deformation such as high-pressure spraying and soaking are avoided, ensuring the dimensional accuracy of precision sheet metal parts remains unchanged.
High-Precision Thickness Control: From spray gun atomization and real-time thickness measurement to curing parameter control, the coating thickness deviation is guaranteed to be ≤ ±3μm in all links, meeting the high coating precision requirements in fields such as electronics and medical care.
No Impurity Contamination: Ultrapure water rinsing, sealed spraying booths, and high-precision powder recovery are adopted to ensure the coating is free of impurities and pinholes, meeting the appearance and performance standards of precision products.
