1.What are some common causes of substrate defects?
Substrate Surface Contamination
Grease or Dust: If rolling oil, rust-preventive oil, or adsorbed dust are not thoroughly removed from the surface of cold-rolled or galvanized steel sheets during the degreasing stage of the color coating production line, these contaminants will act as a "barrier film," severely affecting coating adhesion.
Residual Passivation Solution: Galvanized substrates typically require chemical conversion treatment (such as passivation) to form a protective film. If the passivation solution is not thoroughly rinsed after passivation, or if it remains, the residue will vaporize or cause poor adhesion during coating baking.
Fingerprints and Water Stains: During production or handling, direct contact between human hands and clean substrates leaves sweat and grease, which can also become sources of contamination.
Substrate Defects
Zinc Plating Issues: If the zinc plating is of poor quality, such as containing excessive impurities or having a loose plating structure (e.g., a sponge-like zinc plating), the adhesion will be weak. During subsequent processing or use, the adhesion problem between the zinc layer and the iron substrate will be transmitted to the coating, manifesting as blistering.
Original Board Corrosion: If the substrate already has slight surface rust, the rust products (iron oxides) will expand in volume, and their adhesion to the substrate is already poor, preventing the coating from adhering firmly.
2.What are the reasons related to the production process?
Incomplete Pretreatment
Poor Degreasing: As mentioned above, insufficient temperature, concentration, and spray pressure of the degreasing bath result in inadequate cleaning power.
Poor Chemical Conversion Film: The conversion film (such as chromium-plated or chromium-free conversion film) is too thin, uneven, or incompletely formed. This film acts as a "bridge" connecting the metal substrate and the organic coating; a weak bridge leads to poor adhesion.
Inadequate Rinsing: Residual chemicals on the surface were not thoroughly rinsed off with pure water after pretreatment.
Coating and Curing Process
Excessive Coating Thickness and Solvent Residue: During coating (roller coating), if a single coat is too thick, or if the solvent evaporation rate of the coating itself is inappropriate, the surface layer cures too quickly in the oven, forming a dense film, preventing the internal solvent from completely evaporating. When the product is exposed to sunlight, the residual solvent expands due to heat, causing blistering.
Insufficient Curing: Insufficient oven temperature or time prevents the coating from fully cross-linking and curing, resulting in poor cohesion and adhesion.
Baking temperature too high: Excessive temperature may cause the crystal water or certain contaminants on the substrate surface to vaporize rapidly, thereby lifting the coating.
3.Can mechanical damage cause bulges?
Insufficient bending radius: During processing, if the bending radius is less than the material's minimum allowable bending radius, the coating may develop microcracks due to excessive stretching, or even peel off from the substrate, forming visible bulges or peeling.
Local impact: During transportation, hoisting, or installation, the color-coated sheet may be subjected to localized, severe impacts, causing the coating to detach from the substrate in certain areas.
4.What impact does environmental erosion have on the blistering of pre-coated steel coils?
Water penetration: This is the most common cause during use. If the cuts, drilled holes, or scratches on the color-coated steel sheets are not properly protected after installation, moisture can seep in through these areas. When sunlight raises the temperature of the sheets, the moisture vaporizes, creating steam and generating enormous pressure, which can push up the coating and cause blistering. This "breathing" effect is particularly accelerated in areas with large temperature differences between day and night.
Condensation: Inside buildings with large temperature differences between indoors and outdoors (such as steel structure factories), if insulation is inadequate, the hot and humid indoor air will condense into water droplets on the cold inside of the color-coated steel sheets. Long-term immersion in this water can also lead to decreased coating adhesion and even blistering.
5.When a bulge is discovered, how can the cause be initially determined?
Observe the location and pattern:
Random, irregular bulges are often related to substrate contamination or poor pretreatment.
If concentrated at the board edge or cut, it's often related to moisture penetration.
If appearing along processing bends, it's often related to over-processing.
Assess the timing:
Discovered before leaving the factory or upon opening: This is a production quality issue (substrate or manufacturing process).
Appearing after processing or after a period of use: This is a processing or usage environment issue.