1.How to reduce the impact of "innate defects" on coating uniformity?
Surface flatness and roughness
If the substrate has wavy bends, local depressions or protrusions, it will cause uneven contact pressure between the coating roller and the substrate, forming "thick and thin stripes". The flatness of the substrate needs to be controlled at ≤31 by a flattening machine to ensure that the substrate and the coating roller are fully fitted during coating.
The surface roughness of the substrate needs to match the type of coating: the primer usually requires Ra=1.0-2.0μm. If the roughness is too high, local pits will accumulate too much paint; if it is too low, the coating may sag due to insufficient adhesion. It needs to be accurately controlled through processes such as pickling and sandblasting.
Surface cleanliness
Oil, scale, and dust on the surface of the substrate will cause local coatings to fail to adhere or accumulate. It needs to be processed through three steps: alkaline washing → electrolytic cleaning → passivation to ensure that the surface oil residue is ≤5mg/m² and there are no visible impurities.
2.How to accurately control coating amount and pressure distribution?
The diameter deviation of the coating roller and the backing roller must be ≤0.01mm/m, the cylindrical error must be ≤0.005mm, and the surface roughness Ra must be ≤0.8μm. If there is a 0.1mm protrusion on the roller surface, the local coating thickness may increase by 5-10μm.
Pressure and gap control between rollers: The pressure between the coating roller and the backing roller must be "zoned controlled" through the servo hydraulic system to ensure that the lateral pressure deviation is ≤±2%. When the pressure is too high, the coating is squeezed and thinned, and when the pressure is too low, the coating accumulates and becomes thicker.
At the same time, the linear speed ratio of the coating roller and the strip must be stable at 1.05-1.10: a speed ratio that is too high can easily lead to an overly thick coating, and a speed ratio that is too low can easily lead to an overly thin coating and "missing coating".
Cleaning and maintenance of the coating roller: The surface of the coating roller needs to be cleaned with a special solvent every 8 hours to avoid "scaling" caused by residual coating; the wear of the coating roller should be checked weekly, and when the surface coating wear is greater than 5μm, it needs to be re-grinded.
3.How to ensure "continuous, stable and uniform" delivery of paint?
Viscosity and temperature control:
Paint viscosity is the core parameter that affects the coating amount. The temperature of the paint needs to be controlled at ±1℃ through a constant temperature heating system, and the online viscometer is used for real-time monitoring, and the viscosity is automatically adjusted by adding solvents. If the viscosity fluctuates too much, it will cause "periodic fluctuations" in the coating thickness.
Paint stirring and filtration: During the storage and transportation of the paint, the agitator must be used to prevent the pigment from settling, and avoid "granular protrusions" in the coating due to excessive local pigment concentration. At the same time, the paint needs to be filtered through a 100-200 mesh filter to remove impurity particles.
4.How to avoid thickness deviation caused by "uneven drying"?
Curing furnace temperature distribution
The lateral temperature deviation in the furnace must be ≤±3℃. If the temperature difference is too large, the coating in the high temperature area will dry quickly and shrink more, forming a "lateral thickness gradient". It needs to be controlled in real time through a multi-point thermocouple + hot air circulation system.
Curing speed matches substrate speed
The substrate running speed must match the curing time. If the speed is too fast, the coating enters the cooling section before it is completely dry, which is prone to local accumulation due to "sagging"; if the speed is too slow, the coating may become brittle due to excessive drying, which indirectly affects subsequent coating.
5.How to reduce "human and environmental interference"?
Equipment calibration regularly
The coating roller needs to be re-grinded every 500 tons to ensure surface accuracy;
The thickness gauge is calibrated with a standard sample every week, and the error must be ≤1μm;
The pressure sensor is calibrated monthly to ensure pressure control accuracy.
Environmental control
The temperature and humidity of the workshop must be stable: too high humidity will cause the viscosity of the coating to increase and the coating amount to increase; too low humidity will cause the coating to dry easily and block the gap of the coating roller, which will cause thickness fluctuations.