1.What effect does the stress during bending have on the coating?
The coating is not ductile enough to follow the deformation of the substrate
The color coating is mainly composed of resin, pigment and additives, and its ductility is much lower than that of the metal substrate.
When the bending angle is small, the substrate will undergo plastic deformation, and if the coating is not ductile enough, it will be "forced to stretch", causing cracks inside. If the cracks penetrate the coating, paint will fall off and peel off. ◦ When the coating thickness is too thick, the ductility will further decrease (thick coating is more "brittle"), and it is more likely to crack due to deformation mismatch during bending.
Stress concentration at the bending part exceeds the adhesion limit of the coating
The bending part is the area where the substrate deformation is most severe, and the interface between the coating and the substrate will be subjected to huge shear stress. If the adhesion (i.e. bonding strength) between the coating and the substrate is insufficient, the stress will directly destroy the bonding between the two, causing the coating to peel off from the surface of the substrate.
2.What are the performance defects of the coating itself?
The coating is not tough enough and is too brittle
The toughness of the coating is related to the resin type and the degree of curing: ◦ Resin type: For example, epoxy coatings have low toughness and are prone to brittle cracking when bent; while polyester and fluorocarbon coatings have better toughness and better bending resistance. ◦ Over-curing: If the temperature is too high or the time is too long during baking and curing, the coating will be too cross-linked, hard and brittle, lack elastic buffering when bending, and easily fall off.
Uneven coating thickness or pinholes
If there are thin spots, pinholes or bubbles in the coating itself at the bend, these defects will become "weak points" where stress is concentrated. When bending, the stress is released first from the defect, causing the coating to crack and fall off from the defect.
3.What is the impact of substrate and pretreatment?
Poor surface condition of substrate
When there are burrs or pits on the surface of substrate (such as steel plate), bending will cause the coating in these parts to be "broken" or overstretched, causing paint to fall off.
If the zinc layer of galvanized sheet has poor adhesion (such as loose zinc layer), the zinc layer may fall off first when bending, and the coating will be peeled off together.
Pretreatment process defects
Passivation is a key step to enhance the adhesion between coating and substrate: If the passivation film is too thin or uneven, it cannot effectively connect the substrate and coating, and the interface is easy to separate when bending.
If the water washing after passivation is not thorough, the residual chemicals (such as acid and alkali) will weaken the bonding between the coating and the substrate and reduce the anti-bending performance.
4.What kind of impact will unreasonable bending process parameters bring?
The bending radius is too small
The smaller the bending radius (i.e. the size of the fillet at the bend), the greater the stretching degree of the coating. If the radius is smaller than the minimum bending radius that the coating can withstand (such as some coatings require a bending radius ≥ 3 times the thickness of the plate), the coating will inevitably break due to excessive deformation.
Bending speed is too fast or force is uneven
Fast bending will cause the coating to be subjected to instantaneous impact force, and there will be no time to buffer the deformation; while uneven force (such as uneven contact between the bending mold and the plate) will cause excessive local stress, and paint peeling will occur first.
5.What are the key measures to prevent bending and paint peeling?
Optimize coating performance: select highly ductile resins (such as flexible polyester), control coating thickness (usually 50~100μm dry film thickness is appropriate), and avoid over-curing.
Strengthen pretreatment: ensure that the substrate is free of burrs and oil stains, the passivation film is uniform and dense, and the coating adhesion is improved.
Reasonably set bending parameters: select the appropriate bending radius according to the coating type (such as fluorocarbon coating can adapt to a smaller radius), control the bending speed and force uniformity.