1.What is the core reason for the color difference of color coating?
Raw material fluctuations: color variations between paint batches, substrate surface conditions (such as roughness and phosphate coating uniformity), and variations in diluent purity or ratio.
Uneven pretreatment: Incomplete substrate degreasing and inconsistent phosphate coating thickness and crystal structure (such as missing or excessive phosphate coating in certain areas) lead to variations in paint adhesion and leveling.
Fluctuations in coating process parameters: Uneven coating weight (film thickness variations), abnormal coating roller condition (such as wear and contamination), and insufficient paint stirring (pigment settling).
Unstable drying and curing conditions: Uneven temperature distribution within the drying oven (local overheating or underheating) and inconsistent curing times lead to variations in the chemical cure of the paint.
Environmental and testing deviations: Fluctuations in temperature and humidity within the paint shop, inconsistent light sources during testing (such as natural light versus standard light sources), and colorimeter calibration failures.
2.What impact does batch control of paint have on color difference?
Prioritize paint from the same batch, and specify "same order, same batch" when placing an order. If a batch change is necessary, conduct a batch matching test in advance. The new and old batches of paint should be applied to the same substrate using the same process. The color difference (ΔE value, typically ΔE ≤ 1.5) should be measured before batch use.
Before storage, the paint should be tested for color stability: use a colorimeter to measure the color of the paint solution (compared to a factory standard sample). Also, check for stratification and sedimentation. If so, thoroughly stir until uniform. The stirring time varies depending on the paint type; for example, solvent-based paint requires 30-60 minutes.
3.How to control the surface condition of the substrate?
Substrates (such as cold-rolled and galvanized steel) must maintain a consistent surface roughness (typically Ra = 1.0-2.0μm). Upon storage, sample substrate surfaces should be inspected to avoid localized roughness abnormalities caused by rolling defects (such as roller marks and oxidation spots).
If phosphate treatment is required, the phosphate film thickness (typically 5-10μm) and uniformity must be controlled. This is achieved by ensuring the stability of the phosphate bath concentration (e.g., total acidity, free acidity), temperature (40-50°C), and treatment time (1-3 minutes). Regular testing should be conducted to ensure full coverage of the phosphate film (e.g., a copper sulfate drop test indicates no red spots within 30 seconds).
4.How to avoid "uneven discoloration in later stages" during storage and transportation?
Even if the color difference during production is acceptable, improper storage and transportation can still lead to "later color difference" (e.g., local aging or contamination):
Store the panels upright (avoid horizontal stacking, which can cause the coating to adhere under pressure). Use spacers (such as kraft paper) between panels to prevent friction damage to the coating.
Store in a dry, well-ventilated environment (relative humidity ≤ 60%), away from direct sunlight (UV rays can cause discoloration of the coating, especially light-colored coatings such as white and beige), and away from corrosive gases (such as acidic and alkaline gases to prevent chemical discoloration).
During transportation, cover with a tarpaulin (to avoid exposure to rain), maintain a steady speed (to reduce vibration and friction), and avoid collisions during loading and unloading (to prevent localized damage to the coating, which can lead to water absorption and discoloration).
5.What is the "core logic" of color difference control?
The root cause of color variation in pre-painted products is a lack of consistency throughout the entire process. The key to controlling color variation lies in eliminating variables at every stage, from raw materials to finished product. This is achieved through standardized raw materials, stable process parameters (temperature, pressure, speed), precise equipment control (coating rollers, furnace temperature), and real-time testing and feedback (online film thickness gauges, colorimeter). This creates a closed loop of "prevention-monitoring-adjustment." Furthermore, personnel operating procedures must be strengthened (such as regular training on equipment adjustment and testing skills) to ensure consistent execution at every stage to fundamentally avoid color variation.