1.How to ensure the uniformity of substrate surface condition?
Substrate selection and quality screening
Cold-rolled substrates are preferred, with substrate thickness deviation ≤0.05mm and plate flatness ≤5I.
Galvanized substrates need to control the uniformity of zinc layer thickness and the size of zinc flowers should be consistent (large zinc flowers are prone to excessive local roughness and need to be polished to Ra=0.8-1.2μm).
Substrate pretreatment: Eliminate surface defects and pollution
Degreasing and rust removal: Use a combination of spraying + immersion to ensure that oil and scale on the substrate surface are thoroughly removed to avoid uneven coating adhesion caused by local residual pollutants.
Chemical conversion treatment: Control the uniformity of phosphating bath concentration and temperature to ensure that the thickness and crystal density of the phosphating film are consistent - if the film is too thick, it will be rough, and if it is too thin, the substrate will be partially exposed, which will cause subsequent coating roughness fluctuations.
2.How to ensure the consistency of "pressure, speed, and roughness"?
Selection and maintenance of coating rollers:
A combination of chrome-plated steel rollers and rubber rollers is used. The surface roughness of the steel rollers needs to be pre-processed to the target value, and the Ra deviation of the entire roller surface is ≤0.2μm.
The hardness of the rubber rollers needs to be controlled to be uniform to avoid local wear. The surface of the rubber rollers needs to be checked for scratches and bulges every 500 tons of production.
Coating roller pressure and parallelism:
A servo hydraulic control system is used to ensure that the pressure difference at both ends of the coating roller is ≤0.5bar (too much pressure will squeeze the coating, causing the local coating to become thinner and the roughness to decrease; too little pressure will cause the coating to be thicker and the roughness to be too high).
Adjust the parallelism of the coating roller to avoid the striped roughness difference of "one side thick, one side thin" in the coating due to "tilting".
3.How to match coating properties with coating parameters?
Coating viscosity stability: Real-time monitoring through online viscometer, excessive viscosity fluctuations will lead to differences in coating fluidity. Stabilize viscosity through constant temperature system when necessary.
Coating speed and roller speed ratio: Set matching line speed according to coating type, and the speed ratio of coating roller to substrate must be constant - speed ratio fluctuations will lead to uneven coating transfer and periodic roughness fluctuations.
4.How to avoid roughness deviation caused by uneven drying/cooling?
Oven temperature field uniformity control
Multi-stage temperature control + hot air circulation system is used to ensure that the lateral temperature difference in the oven is ≤±3℃, avoiding local high temperature causing premature curing of the coating or low temperature causing excessive leveling.
Calibrate the thermocouple regularly and clean the dust or coating residue in the oven.
Matching leveling and curing time
Low-viscosity coatings need to extend the leveling time to avoid surface unevenness due to rapid curing; high-viscosity coatings (such as fluorocarbon coatings) need to shorten the leveling time (1-2 seconds) to prevent excessive sagging and local smoothness.
The curing time needs to match the line speed (such as curing time ≥30 seconds at 230℃) to ensure that the curing degree of the full-width coating is consistent (areas that are not fully cured are prone to form fine wrinkles due to subsequent cooling and shrinkage, increasing roughness).
Cooling uniformity
Roller cooling (rather than local air cooling) is used to ensure that the surface temperature of the cooling roller is uniform (±2℃) and is closely attached to the full width of the substrate - uneven cooling will lead to differences in stress distribution of the coating, causing local "warping" or "microcracks", which indirectly increase the roughness.
5.How to ensure the stability of raw materials?
Coating batch consistency: The same batch of coatings is required for the same order, and the pigment dispersion (particle size deviation ≤ 2μm) and solid content (deviation ≤ 1%) of the coating are stable - uneven pigment distribution will lead to local "graininess" differences in the coating and increase roughness fluctuations.
Diluent ratio: Add diluent strictly according to the formula (such as the dilution ratio of solvent-based coatings ± 0.5%) to avoid arbitrary adjustments that lead to unstable coating viscosity and affect coating uniformity.