1.What are the core principles of bending pre-coated steel coils?
All considerations in bending processes revolve around one core principle: minimizing damage to the organic coating. Once the coating cracks, moisture and oxygen can seep in through the cracks, corroding the underlying metal and causing rust to spread from the inside.
2.What should be noted regarding the direction of bending?
Prioritize lateral bending: Bending should be performed as close as possible to the rolling direction (i.e., the length direction) of the pre-coated coil. In other words, the bending line should be perpendicular to the rolling direction.
Avoid longitudinal bending: Avoid bending along the width direction of the pre-coated coil (i.e., perpendicular to the rolling direction) as much as possible.
Reason: During the rolling process, the grains of the pre-coated substrate (usually galvanized sheet) have a certain orientation. Lateral bending provides better ductility and reduces the likelihood of coating cracking.
3.How to choose the bending radius?
Basic Principle: The larger the bending radius, the better. A larger radius reduces tensile stress on the coating surface, effectively preventing microcracks in the coating.
Minimum Bending Radius: The specific minimum bending radius depends on the substrate material and thickness, as well as the type and thickness of the coating.
Substrate Influence: Hot-dip galvanized substrates have better ductility than aluminized zinc substrates, allowing for smaller bending radii. The thicker the substrate, the larger the required minimum bending radius.
Coating Influence: PVC plastisol coatings (thicker, commonly used in home appliances) are more flexible and have stronger resistance to bending cracking than polyester coatings (thinner, commonly used in construction).
Operating Guidelines: Always consult the technical data provided by the color-coated coil manufacturer, which will clearly specify the minimum allowable bending radius for the product (usually expressed as a multiple of the sheet thickness, such as 1.5t, 2t, etc.).
4.What should be considered when selecting molds and adjusting equipment?
Die Surface Finish: A smooth, clean, rust-free, and burr-free bending die must be used. Any defects in the die will be imprinted on the coating surface, causing damage.
Avoid Sharp Angles: The tip of the upper die should have an appropriate rounded corner; avoid using overly sharp dies.
Pressure Adjustment: Use the lowest possible bending pressure while maintaining the correct bending angle. Excessive pressure will severely compress the coating.
Speed Control: Use a uniform, slow bending speed. Rapid pressing will impact the coating, increasing the risk of cracking.
5.What are the common problems and solutions?
Problem: Fine cracks or an "orange peel" effect appear in the coating at bends.
Cause: The bending radius is too small, exceeding the coating's flexibility limit.
Solution: Increase the bending radius; choose a coating with better flexibility (such as high-durability polyester, PVC plastic paint, etc.).
Problem: Coating peels off at bends.
Cause: Rough or damaged mold; excessive bending pressure; incorrect embossing plate orientation.
Solution: Inspect and polish the mold; reduce pressure; ensure the embossed surface faces outwards when bending.
Problem: Whitening or change in gloss at bends.
Cause: The coating has been overstretched, causing microscopic deformation.
Solution: This is also a sign of an insufficient bending radius; the radius also needs to be increased.