1.What are the main reasons for the "orange peel texture" on the surface of pre-coated steel coils?
Insufficient Coating Leveling
Excessively high coating viscosity or inappropriate thinner leads to poor flowability.
Poor resin-solvent matching in the coating formulation results in excessively rapid evaporation.
Coating Process Issues
Inappropriate Roller Coating Parameters: Mismatched roller speed ratios and inter-roller pressures lead to uneven coating thickness or insufficient transfer.
Inadequate Film Thickness Control: Excessively thick or thin wet films affect leveling.
Curing Temperature/Speed Mismatch: Excessively rapid surface curing during baking causes solvent evaporation from the underlying layer, resulting in unevenness.
Substrate or Primer Coating Condition
Substrate temperature that is too high or too low affects coating wettability.
Large roughness or slight unevenness in the primer coating amplifies defects in the topcoat.
2.What are some solutions?
Adjusting Coating Parameters:
Optimizing Thinner: Select a slow-drying thinner to extend the leveling time.
Adjusting Viscosity: Control the coating viscosity within the appropriate range according to the roller coating process requirements (usually monitored using a Ford cup or rotational viscometer).
Collaborating with Suppliers: Improve the coating formulation and increase the proportion of leveling aids (such as siloxanes or acrylic leveling agents).
Controlling the substrate and coating condition:
Ensure the substrate is clean and at a stable temperature (it is recommended to control the substrate temperature with cooling water in summer).
Check the surface roughness of the primer coating (Ra≤0.8μm is preferred); polish or adjust the primer coating process if necessary.
Environmental and Equipment Management
Control the temperature and humidity of the coating room (recommended temperature 20-30℃, humidity ≤70%).
Regularly maintain the roller coater to ensure the roller surface is clean and undamaged.
Check the airflow speed of the oven nozzles to avoid direct airflow onto the coating surface.
3.How to optimize the coating process?
Roller Coating Process Adjustment:
Check the speed ratio of the coating roller and the feed roller to ensure uniform paint transfer.
Adjust the roller pressure to avoid excessive compression that could cause texture.
Curing Profile Optimization:
Appropriately reduce the oven inlet temperature and extend the leveling zone (flash zone) time.
Ensure uniform oven temperature and avoid localized overheating (temperature difference should ideally be controlled within ±5℃).
4.What are the steps for rapid on-site investigation?
Observe the regularity of defects:
If the orange peel texture appears periodically, check for damage to the coating roller or back roller.
If the distribution is irregular, focus on investigating coating or environmental factors.
Small-batch testing verification:
Adjust a single variable (such as thinner type, roller speed ratio) and compare the effects.
Use a leveling test board to simulate production conditions.
Instrument-aided analysis:
Use a gloss meter to check the uniformity of surface gloss (orange peel texture is usually accompanied by fluctuations in gloss value at 60°).
Quantify surface roughness (Sa value) using a 3D profilometer.
5.What are the preventative measures?
Establish process windows: Record and solidify the allowable ranges for key parameters (viscosity, roller speed, temperature, etc.).
Regularly monitor coating performance: Test leveling and curing speed for each batch of coating.
Train operators: Improve their sensitivity to coating conditions and equipment parameters.