1.How do substrate properties affect processability?
Yield strength and elongation:
The substrate with low yield strength and high elongation has better plasticity, is not easy to break when bending or stamping, and is suitable for complex forming.
The high-strength steel substrate with high yield strength has good rigidity but low plasticity, which is suitable for simple forming. Excessive bending is prone to "rebound" or substrate cracking.
Coating characteristics: The hot-dip galvanized layer is softer and has good ductility. It is not easy to crack during forming and is suitable for processing with a small bending radius.
The aluminum-zinc coating contains aluminum, zinc, and silicon alloys, has higher hardness and slightly poorer ductility. Excessive bending may cause the coating to crack, and the bending radius needs to be controlled.
2.How do coating properties affect processability?
Flexibility: The coating needs to have a certain elasticity and be able to stretch with the deformation of the substrate. The "T-bend test" is usually used for evaluation: After the color-coated coil is bent, the smaller the minimum bending radius at which the coating does not crack, the better the flexibility.
Differences in flexibility of different coatings: Polyester (PE) > Silicon-modified polyester (SMP) > Fluorocarbon (PVDF), so PE coating is more suitable for complex molding (such as home appliance stamping parts), and PVDF coating is more suitable for simple bending (such as building exterior wall panels).
Adhesion: The bonding force between the coating and the substrate must be strong enough to avoid coating peeling due to substrate stretching during processing. The commonly used "grid test" is evaluated: Use a blade to scratch a cross grid and then paste the tape to tear it off. The coating peeling area is less than 5% to be qualified. The less it peels off, the better the adhesion.
Hardness: If the hardness of the coating is too high, it is easy to become brittle and crack during processing; if it is too low, it is easy to scratch. It is necessary to balance hardness and flexibility. Usually, the hardness of color-coated coils for home appliances is controlled at H-2H, and that for construction is controlled at 2H-3H.
3.How does machining technology affect machining performance?
Bending radius (R): The smaller the bending radius, the greater the tensile deformation of the substrate and the coating, and the more likely the coating will crack; conversely, a larger bending radius can reduce deformation stress and reduce defects.
Forming speed: During high-speed rolling or stamping, if the mold is not lubricated enough, the coating is prone to scratches and peeling due to friction overheating or local stress concentration, and a lubricant suitable for the speed must be matched.
Mold design: If the mold radius is too small, it will aggravate the wear of the coating. A mold with a radius of ≥2mm must be used to reduce rigid friction with the coating.
4.What are the differences in formability requirements for different processing scenarios?
Simple forming
Processing method: rolling into simple cross-sections such as corrugation and trapezoid, deformation is mainly linear bending, and the coating flexibility requirements are medium.
Requirements: substrate elongation ≥20%, coating T-bend test ≥2T without cracks, adhesion ≥4B.
Medium forming
Processing method: 90° folding, shallow stamping, large local deformation.
Requirements: substrate elongation ≥25%, coating T-bend test ≥1T without cracks, cup depth ≥6mm, ensure that the coating does not fall off at the folding edge.
Complex forming
Processing method: deep stamping, tensile deformation, extremely high requirements for substrate plasticity and coating flexibility.
Requirements: substrate elongation ≥30%, coating T-bend test 0T without cracks, cup depth ≥8mm, and the coating must have "self-healing" ability.
5.What are the measures to improve processing formability?
Substrate selection: Prioritize substrates with low yield strength and high elongation, such as DC51D+Z (hot-dip galvanized) or SPCC (cold-rolled sheet).
Coating adjustment: Choose a coating system with better flexibility, such as polyester coating (PE) with added plasticizer, or adopt a "primer + topcoat" double-layer structure (primer improves adhesion, topcoat ensures flexibility).
Process optimization:
Increase the bending radius (such as from 1T to 2T) to reduce local deformation stress;
Use special lubricants (such as water-based lubricants) during processing to reduce friction between the coating and the mold;
Reduce the forming speed (such as rolling speed from 30m/min to 20m/min) to avoid stress concentration.