Coating cracking after bending of stainless steel color plates is a common quality defect in sheet metal processing. Its causes involve multiple factors, including material properties, process parameters, equipment condition, and operating procedures. Coating cracking not only affects the product's appearance but also reduces its corrosion resistance and shortens its service life. Therefore, it is necessary to analyze the root causes and develop systematic preventative measures.
The quality defects of the material itself are the primary factor leading to coating cracking. If recycled materials or substandard chemical compositions (such as excessively high carbon content) are used in the stainless steel substrate, the material's plasticity and toughness will be significantly reduced, making the substrate prone to micro-cracks during bending, which in turn leads to coating peeling. Furthermore, defects such as scratches, holes, or oxide scale on the substrate surface will become stress concentration points, accelerating coating cracking during bending. For example, one company used stainless steel substrate with excessive impurities, resulting in radial cracks in the coating of a batch of products after bending. The problem was ultimately resolved by changing the supplier and strengthening incoming inspection.
The rationality of bending process parameters directly affects the integrity of the coating. Insufficient bending radius is a common cause. When the radius is below the material's limit (e.g., forcibly bending a 1.5mm thick sheet to R=0.5mm), both the substrate and coating will be subjected to excessive stretching, causing the coating to crack from the edge inwards. Furthermore, the relationship between the bending direction and the rolling direction is crucial: bending perpendicular to the rolling direction reduces the material's tensile strength, easily leading to cracks; bending along the rolling direction reduces the risk of cracking. A car parts manufacturer once experienced cyclical cracks in a batch of stainless steel color plate door frames after bending due to a lack of consideration for the rolling direction; this was later resolved by adjusting the blanking layout.
The equipment condition and die matching significantly affect coating quality. Fixed lower dies, with their sharp groove edges and high coefficient of friction, easily cause excessive stretching of the sheet metal during bending, leading to coating tearing; while CNC bending machines equipped with adjustable roller dies can reduce friction through rolling compensation, lowering the risk of cracking. Mold wear is also a significant concern. After long-term use, worn corners of the mold can cause uneven pressure on the sheet metal during bending, leading to localized stress concentration. One appliance manufacturer experienced a batch of stainless steel color plates suffering from dense pinhole cracks after bending due to inadequate mold maintenance; the problem was immediately resolved after replacing the molds.
The coating's inherent properties and application process are also crucial factors. If the coating is too thick, the inner layer is compressed and the outer layer stretched during bending, making it prone to cracking due to uneven stress distribution. Conversely, a coating that is too thin cannot provide sufficient adhesion, leading to peeling. Furthermore, excessively high baking temperatures or prolonged baking times can cause the coating to become brittle, reducing its resistance to deformation. In one architectural decoration project, improper coating baking parameters caused the stainless steel color plates to peel off in a cracked manner after bending; the coating performance was restored by adjusting the curing temperature and time.
To address these issues, multi-dimensional preventative measures are necessary. First, strictly control the quality of raw materials, ensuring the chemical composition of the substrate meets standards through spectral analysis and adding bending tests to verify plasticity. Second, optimize the bending process, rationally setting the bending radius based on material thickness and performance, and standardizing the rolling direction. Third, strengthen equipment maintenance, regularly check mold wear, and replace or repair them promptly. Finally, standardize coating application, control coating thickness uniformity, and strictly follow the coating instructions to set baking parameters.
For products with existing coating cracks, targeted repair measures must be taken according to the degree of cracking. If the cracking is minor and adhesion is still acceptable, the cracked area can be smoothed by sanding, loose coating fragments removed, and then primer and topcoat applied. If the cracking is severe or adhesion is poor, the old coating must be completely removed down to the substrate surface, and the coating process re-applied, with strict control over construction quality. Through systematic prevention and timely repair, the yield and product quality of stainless steel color plate bending can be significantly improved.
