1.What are the effects of coating and raw material factors?
Low paint viscosity: This is the most direct cause. Excessive addition of thinner during paint mixing, or the use of an incompatible fast-drying thinner, results in paint with excessively low viscosity, making it difficult to adhere to the surface like water.
Poor paint thixotropy: High-quality paints should have good thixotropy-high viscosity at rest to prevent sedimentation; viscosity decreases during application shearing (e.g., via a coating roller) for easy leveling; and a rapid return to high viscosity after shearing to prevent sagging. Paints with poor thixotropy cannot quickly recover viscosity after application, making them prone to dripping.
Slow solvent evaporation: If the proportion of slow-drying solvents in the formula is too high, the paint will remain in the oven for too long, resulting in slow solvent evaporation and prolonged fluidity of the coating, leading to sagging.
High paint temperature: Excessively high temperatures during paint storage or circulation will decrease its viscosity, increasing the risk of sagging.
2.What impact do production processes and parameters have?
Excessive wet film thickness: This is a major cause on the production line.
Improper roller gap setting: The roller gap of a precision roller coater (such as a two- or three-roller coater) is crucial for controlling the coating amount. Too large a gap results in excessive coating being transferred to the board surface.
Mismatched roller speed ratio: An improper speed ratio between the feed roller and the coating roller (or strip) leads to excessive coating transfer.
Improper coating machine operation:
Uneven roller pressure adjustment.
Paint level in the tray is too high, causing the coating roller to penetrate too deeply.
Inappropriate curing oven temperature profile:
Preheating zone (or first zone) temperature is too low: The coating fails to reach the "gel" state in time and is heated while still having good fluidity, exacerbating sagging.
Overall oven temperature is too low, resulting in a slow curing reaction.
Slow production line speed: With the same coating amount, the slower the line speed, the longer the wet coating is subjected to gravity before entering the oven, leading to more pronounced sagging.
3.What impact does the equipment status have?
Roller wear or damage: Pitted, scratched, or unevenly worn roller surfaces can lead to abnormal localized paint transfer, resulting in streaks and runs.
Roller system vibration or bearing damage: This causes uneven coating.
Unstable tension control: Fluctuations in the tension of the sheet during coating cause changes in the contact pressure with the rollers, affecting coating uniformity.
Uneven hot air circulation in the oven: This results in localized low temperatures on the sheet surface, leading to slow curing.
4.What are the effects of substrates on the environment?
Low substrate temperature: A cold substrate can cause a sudden increase in the viscosity of the coating in contact with it. However, temperature differences can also cause solvent condensation, affecting leveling and indirectly causing problems.
High ambient temperature and humidity: Especially in high humidity environments, solvent evaporation can be affected, and the coating may absorb moisture, altering its rheological properties.
Oil or contamination on the substrate surface: This affects coating wetting and adhesion, potentially leading to localized, irregular leveling defects.
5.What are some possible solutions?
First, check the following process parameters:
Immediately adjust the coating amount: Reduce the gap between the coating rollers; this is the fastest and most effective correction method.
Check and adjust the line speed and oven temperature: While ensuring curing, appropriately increase the preheating zone temperature or the overall line speed.
Confirm the coating viscosity: Use a viscosity cup to measure on-site and ensure it is within the range specified in the process.
Inspect the coating itself:
Check if the thinner is excessive or of the wrong type.
Confirm batch consistency with the coating supplier, especially when switching to a new batch.
Check equipment condition:
Inspect the surface condition of the coating rollers and ensure the bearings operate smoothly.
Confirm the pressure and parallelism of each roller on the coating head.
Consider the environment and substrate:
Ensure the substrate is clean and dry before entering the coating chamber.
Control the temperature and humidity within the coating chamber.