1.What impact do substrate and pretreatment issues have?
Substrate surface contamination: Grease, rust-preventive oil, dust, salt, etc., are not thoroughly cleaned. This is the most common cause of bubbles; contaminants vaporize during baking, breaking through the coating and forming bubbles.
Pretreatment conversion film issues:
Poor passivation/formation treatment: The conversion film is loose, uneven, or excessively thick; its inherent water of crystallization or impurities release gas at high temperatures.
Incomplete rinsing: Inadequate rinsing after pretreatment allows residual chemicals to be carried into the coating.
Substrate defects: Minor rust spots or uneven zinc plating on cold-rolled or galvanized steel sheets can affect adhesion and trap moisture.
2.What impact will problems with the coating process have?
Coating Too Thick: If the wet film is too thick, a skin forms rapidly on the surface, preventing the internal solvent from evaporating and eventually causing it to "break through" and form bubbles.
Improper Baking Profile:
Rapid Heating: The surface solvent evaporates rapidly to form a film, while the internal solvent and air rapidly vaporize and become trapped.
Excessively High Baking Temperature: This also leads to excessively rapid surface curing.
Insufficient Baking: The solvent does not completely evaporate and slowly escapes later or during use, forming bubbles.
Roller Coating Process Parameters: Mismatched roller gap, pressure, and speed can cause air to be trapped during coating.
3.What impact will problems with the paint itself have?
Improper solvent matching: Mismatched evaporation rates. If low-boiling-point solvents evaporate too quickly, leaving excessive residue of high-boiling-point solvents, bubbles are easily generated during later baking.
Air or moisture mixed into the coating: Introduced during coating transportation, mixing, and pumping.
Coating formulation problems: Poor compatibility between the resin and additives (such as leveling agents and defoamers), or inappropriate selection or ineffective defoamer.
Expired or deteriorated coating: Some components react to produce gas.
4.What impact will environmental and storage issues have?
Excessive ambient humidity: Especially during the rainy season in southern China, high humidity air is drawn into the coating or condenses on the substrate.
Improper storage of color-coated rolls: Storing finished rolls in humid environments with large temperature differences may also cause micro-bubbles to slowly form later.
5.What suggestions do you have?
First, check the pretreatment: This is the lowest-cost and highest-priority step. Confirm that the concentration, temperature, and spray pressure of each bath solution in the cleaning section are normal, and check the quality and uniformity of the passivation film.
Optimize the baking process: Appropriately reduce the temperature of the preheating or initial curing section to allow the solvent to evaporate slowly in a gradient, avoiding "boiling over". Ensure the oven temperature profile matches the characteristics of the coating used.
Check the coating and coating parameters:
Measure and adjust the coating viscosity (solid content).
Check the wear of the coating rollers and the set parameters.
If the coating is suspected, take a sample and simulate coating it on a clean test piece in the laboratory using a scraper to observe whether bubbles still exist, thus isolating production line factors.
Control the environment: Ensure that the temperature and humidity in the coating room are controllable, especially avoiding the effects of high humidity weather.
Record and compare: Record all process parameters (speed, temperature, viscosity, etc.) when bubbles appear, and compare them with the parameters when there are no problems to find differences.