1.What is the fundamental physical principle?
When in a liquid state, coatings possess surface tension and tend to shrink to their minimum surface area.
When a contaminant (such as oil) with low surface tension exists on the substrate surface, that spot cannot be properly wetted.
The coating around the contaminant has higher surface tension and will shrink away from the contaminant as if being "stretched."
Ultimately, this results in a permanent depression or a hole exposing the substrate at the location of the contaminant.
2.What are the effects of substrate surface contamination?
Rolling oil/lubricating oil residue: Grease from upstream cold rolling or galvanizing processes was not completely removed by the cleaning line.
Rust-preventive oil contamination: Uneven or excessive application of rust-preventive oil on galvanized or aluminized zinc substrates.
Silicone contamination: Silicone release agents, sealants, or lubricants used in upstream processes; even trace amounts of silicone can cause severe shrinkage cavities.
Workshop environmental contaminants: Dust, water stains, fingerprints, hydraulic oil droplets, oil-water mixtures in compressed air, etc., acquired during transportation, uncoiling, or pretreatment.
Pretreatment residues or contamination:
Chemical conversion solution contaminated with grease.
Passivation film (e.g., chromium-free passivation) unevenly dried or adhered to environmental contaminants.
3.What are the consequences if there are problems with the paint itself?
Paint contamination: Foreign matter has entered the paint during manufacturing, transportation, or the coating line circulation system.
Contamination from different paint systems: For example, incomplete cleaning during paint type switching on the production line, leaving residual old paint or solvents that are incompatible with the new paint.
Foreign matter contamination: Such as silicone, oil, water, or other surfactants from the air, equipment, or packaging.
4.What impact will coating processes have on the environment?
Coating roller issues: The coating roller (roller coater) is damaged, not properly cleaned, or contains contaminants.
Coating room environment issues:
Airborne contaminants: Oil mist, silicone spray, sanding dust, etc., from the workshop.
Improper temperature and humidity control: Excessive humidity may cause moisture condensation and contamination of the wet film.
Curing oven contamination: Grease or decomposition products carried by the oven's hot air circulation system drip onto the uncured wet film.
5.What are the corrective and preventative measures?
Emergency Handling: Filter or discard contaminated paint; thoroughly clean all rollers on the production line that come into contact with the substrate.
Source Control: Strengthen the cleanliness testing of incoming substrates; strictly implement cleaning procedures when switching between different paints; isolate processes that may generate contamination (such as silicone).
Process Optimization: Optimize the leveling and wetting properties of the paint (fine-tune additives in the formulation); ensure that the coating room meets cleanliness requirements.