1.What are the quality problems in cutting processing?
Coating Scratches/Peeling
Phenomenon: Continuous scratches appear on the cut edge or surface, and in severe cases, the coating may flake off.
Cause: The cutting tool (e.g., saw blade, razor blade) is not sharp enough or directly rubs against the coating surface; or the cutting tool (e.g., saw blade, razor blade) is not lubricated, causing the coating to melt due to high temperatures and adhere to the tool, which in turn scratches the subsequent material.
Impact: The coating is damaged at the scratched area, exposing the substrate and susceptible to rust from the damaged area.
Coating Edge Cracking
Phenomenon: Irregular cracks or chipping occur at the cut edge, especially on thick, pre-coated coils.
Cause: Excessive cutting speed or uneven tool pressure, resulting in the coating experiencing a momentary impact exceeding its toughness limit; or a significant difference in ductility between the substrate and coating (e.g., a hard substrate with a brittle coating).
Inadequate Dimensional Accuracy
Phenomenon: Excessive length/width deviation after cutting, or uneven edges (e.g., wavy edges, beveled cuts).
Cause: Improper positioning of the cutting equipment, or camber (a shaped defect during rolling) of the pre-coated coil itself, causing the reference to shift during cutting.
2.What are the quality problems in bending processing?
Coating Cracking/Pulling at Bends
Phenomenon: Microcracks appear on the coating on the inside or outside of the bend. In severe cases, the coating may flake off in a powdery state (pulling).
Cause: The bend radius is too small (less than the recommended value; for example, conventional polyester coatings typically require a bend radius ≥ 3 times the plate thickness), resulting in tensile/compressive stress on the coating exceeding its elongation at break; the coating is too thick (especially if the topcoat is too thick), resulting in insufficient flexibility.
Impact: Cracks become channels for moisture and corrosive media, accelerating substrate corrosion, and the coating cannot heal itself at cracked areas.
Coating Peeling at Bend Edges
Phenomenon: The coating separates from the substrate at the bend edge, resulting in a "warped edge."
Cause: Uneven stress on the sheet edge during bending, or insufficient adhesion between the coating and the substrate (e.g., poor primer adhesion due to improper pretreatment); or the rough surface of the bending die exacerbates friction between the coating and the substrate.
Zinc Layer Damage (for galvanized substrates)
Phenomenon: The galvanized layer on the substrate at the bend cracks or peels, exposing the cold-rolled base plate. Cause: The galvanized layer of the substrate is too thin or brittle (for example, the pure zinc layer is soft, while the zinc-iron alloy layer is brittle). The zinc layer cannot deform synchronously with the substrate during bending; the bending angle is too large (such as a dead bend close to 180°).
3.What are the quality problems in stamping?
Coating Delamination/Wrinkling in Stretching Areas
Phenomenon: During punching and stretching, the coating in contact with the punch is "pulled off," or irregular wrinkles appear on the surface.
Cause: Insufficient coating adhesion (poor bonding between the primer and the substrate) prevents the coating from withstanding the friction during stretching; excessive stretching speed causes the coating and substrate to deform out of sync, leading to localized wrinkling.
Coating Damage at Punching Edges
Phenomenon: The coating tears or peels off at the edge of the hole after punching, or "burr-like" coating debris remains.
Cause: A blunt punch blade or improper clearance adjustment causes the coating to be squeezed rather than cut; excessive pressure during punching causes the coating to break under shear forces.
Coating Discoloration After Forming
Phenomenon: The coating in the punched area (especially deep-drawn areas) becomes darker or darker, with a color difference from other areas.
Cause: Localized friction during punching generates heat, exceeding the coating's heat resistance limit (e.g., approximately 120°C for ordinary polyester coatings and approximately 150°C for PVDF), resulting in thermal oxidation discoloration of the coating.
4.What are the quality problems in welding processing?
Burned/Carbonized Coating Around the Weld
Phenomenon: The coating near the weld becomes black and charred, losing its luster. In severe cases, it may appear as a carbonized powder.
Cause: The welding current is too high or the dwell time is too long, resulting in excessive heat input, exceeding the coating's high-temperature resistance limit. Failure to implement cooling measures (such as water cooling) allows heat to diffuse into the coating area.
Decreased Adhesion of the Coating in the Heat-Affected Zone
Phenomenon: The coating within 5-10mm of the weld will peel off with a light scratch with a fingernail, or bubbles may appear.
Cause: High temperatures cause the resin components in the coating to decompose, breaking the molecular chains and losing adhesion. The substrate expands upon heating and contracts upon cooling, creating a stress difference with the coating, leading to delamination.
Weld Spatter Damage to the Coating
Phenomenon: Small pits and scratches appear on the coating surface in non-weld areas, or high-temperature slag adheres, causing localized coating breakdown.
Cause: The surrounding area is not shielded during welding, allowing molten droplets (which can reach temperatures exceeding 1000°C) to directly impact the coating, causing physical damage or high-temperature burns.
5.What are the common processing problems?
Coating Scratches/Indentations
Symptoms: Linear scratches, pits, or indentations appear on the sheet surface during processing, or due to stacking and compression.
Causes: Friction with hard objects (such as metal supports or tools) during handling; excessive pressure on the sheets below during stacking; or localized pressure caused by foreign matter (such as grit) on the surface.
Coating Contamination
Symptoms: The coating surface is contaminated with oil, fingerprints, rust, or cutting fluid, making it difficult to clean or leaving marks after cleaning.
Causes: Unclean processing tools (residual oil); operators handling the sheet without gloves (perspiration corrosion); chemical reactions between the cutting fluid and the coating (e.g., acidic cutting fluid corroding polyester coatings).