1.Why does zinc layer buildup occur?
Material Compression and Flow
When a steel sheet is rolled, the inner and outer metal layers at the bend undergo different deformations: the outer layer is stretched, and the inner layer is compressed.
The zinc layer sandwiched in between (softer than the steel substrate) undergoes plastic flow like butter under immense pressure and compression.
During the final tight pressing of the rolled edge, these compressed zinc layers have nowhere else to go but are "squeezed" out towards the only opening-the edge.
Edge Effect
The edge of the sheet is where the flow resistance of the zinc layer is least. During the pressing of the rolled edge, the zinc squeezed out from the inside of the bend overflows along the edge, accumulating there.
2.What are the specific manifestations of zinc layer deposition?
Appearance: At the seam edge of the rolled edge, noticeable raised deposits of zinc metal, sometimes appearing as nodules or strips, are visible. The color matches the surrounding area, but the surface may be uneven.
Feel: Touching the rolled edge reveals a noticeable bulge; it is not smooth.
3.What potential problems might this bring?
Degraded Appearance Quality: For products with high aesthetic requirements, this buildup is considered unsightly and negatively impacts the product's perceived quality.
Subsequent Coating Issues:
Irregularities in the surface of the accumulated zinc layer can lead to uneven paint film thickness during subsequent spraying, with excessively thin paint films at raised areas.
Severe buildup may result in dripping or orange peel effects after spraying.
Assembly Interference: In certain high-precision assemblies, excessive zinc layer buildup can cause interference, preventing parts from fitting perfectly together.
4.What are the potential quality risks?
The zinc layer in the deposited portion has a relatively loose structure and poor adhesion, making it susceptible to peeling when subjected to external forces (such as scratching or vibration).
Although the deposit itself is dense zinc and does not affect corrosion resistance, if peeling occurs, the exposed substrate edges become the starting point for corrosion.
5.How to control and reduce zinc layer buildup?
Controlling Zinc Coating Thickness (The Most Effective Method)
The thicker the zinc coating, the more severe the buildup. Choose the appropriate zinc coating grade (e.g., Z60 instead of Z120) based on the product's operating environment and processing requirements. While meeting corrosion resistance requirements, choosing a thinner zinc coating is the preferred solution to reduce buildup.
Optimizing Edge Rolling Design
Increasing the edge rolling radius: Appropriately increasing the bending radius during the design phase can reduce the degree of compression on the zinc coating.
Adjusting Edge Rolling Allowance: Accurately calculate and adjust the overlap of the sheet metal used for edge rolling; excessive allowance will exacerbate compression.
Adjusting Process Parameters
Optimize the pressure and feed speed on the edge rolling machine. Excessive pressure will directly lead to more severe compression buildup.
Using Specialized Equipment or Dies
Some high-precision edge rolling equipment or pressing dies with specific groove patterns can better control material flow, thereby reducing buildup.