Key Takeaways
The air knife wipes surplus molten zinc from the strip with a high-velocity gas jet, setting coating weight and uniformity.
Streaks are usually caused by partial nozzle blockage, pressure instability during speed changes, or poor strip shape/tension in the knife zone.
Immediate fixes: open the knife at welds, clean blocked nozzles with the proper equipment, and re-check knife gap and tension-roll settings.
Prevention: control incoming strip shape, manage line-speed changes smoothly, and run a scheduled knife maintenance programme.
1. What the Air Knife Does
In hot-dip galvanizing, the strip leaves the zinc pot with excess liquid zinc on its surface. The air knife - a narrow slot nozzle blowing high-velocity gas across the strip width - wipes this surplus away, controlling the final coating weight and its uniformity. Because the knife sets the coating profile, anything that disturbs the gas jet produces visible defects. The most common is the streak: a longitudinal band where the coating is thicker or thinner than the target.
2. Main Causes of Streaks
- 1.Partial nozzle blockage:
- zinc splash, zinc ash or dust partially blocks the nozzle slot; the disturbed jet produces a streak at the corresponding strip position.
- 2.Pressure instability:
- a large change in line speed suddenly changes knife pressure; pressure surges can splash zinc and aggravate blockage, or momentarily alter coating weight.
- 3.Geometry and strip stability:
- too small a knife-to-strip gap, poor strip shape (buckles, waves), uneven strip heating or unstable tension can bring the strip too close to the knife or cause flutter, disturbing the jet.
- 4.Incoming material:
- poor incoming strip shape and coating carry-over are upstream contributors that show up at the knife.
3. Immediate Corrective Actions
- Open the air knife briefly at strip welds so weld-area coating does not disturb the jet, and clean the knife zone with the proper cleaning equipment.
- Clear blocked nozzles promptly using the mill's standard nozzle-cleaning procedure; do not run with a partially blocked knife.
- Re-check the knife-to-strip gap and the tension/stable-roll settings so the strip runs flat and centred in the middle of the knife jet.
- Control the amplitude of line-speed changes so pressure follows smoothly instead of surging.
4. Prevention and Process Control
- Monitor incoming strip shape and reject or condition material with poor shape before the line.
- Keep strip tension and heating uniform across the width to avoid local jet disturbance.
- Run scheduled air-knife maintenance: nozzle inspection, cleaning and gap verification on a fixed cycle.
- Log streak occurrences against line parameters (speed, knife pressure, gap) to identify the process window that produces defect-free coating.
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Frequently Asked Questions
Q1. Why do streaks appear at the same strip position every coil?
A fixed-position streak usually points to a permanently blocked or damaged nozzle at that position, or a local strip-shape problem; inspect the nozzle first.
Q2. Can line speed cause streaks without nozzle blockage?
Yes. Rapid speed changes cause knife-pressure surges that momentarily change coating weight across the strip.
Q3. How often should the air knife be cleaned?
Cleaning frequency depends on line conditions and zinc-pot operation; a scheduled maintenance cycle with nozzle inspection is recommended.