What Does Air Wiping Mean?
Air wiping is the process of uniform compressed air jets being used as blades to wipe away excess molten metal. It is used to clean and improve the consistency and quality of surface coatings on pipes, metal sheets and wires.
The uniform wiping off of the molten metal enables the thickness and weight of the coating to be controlled. The air wiping process is applicable either on metals coming out of the furnace or at the final phase of cleaning, coating, polishing or stamping.
Corrosionpedia Explains Air Wiping
Basic air-wiping equipment may consist of an air supply system with:
- Air knives
- Control valves
- Controls for pressure, angle and position
The object to be cleaned or whose surface coating thickness is being controlled is passed through the system. The high-speed air jets are blown parallel to the object's surface and the excess molten metal is wiped off. The excess liquid is blown backward so as not to contaminate the cleaned surface.
Factors that determine the thickness of the coating include:
- Air-jet pressure
- Angle of attack
- Nozzle-to-surface distance
- Line speed, such as the speed of the strip in the hot-dip galvanizing process
Air-wiping equipment varies in design and size depending on the application, the medium and the shape of the object to be cleaned. In corrosion protection processes such as galvanizing, use of compressed air can lead to oxidation. This can produce a rough surface coating due to the formation of residual metal oxides. To avoid oxidation, nitrogen gas, which is an inertia gas, is used as the wiping media.
Air wiping is used in many applications, including:
- Controlling the thickness and uniformity of zinc coating on steel sheets in hot-dip galvanization
- Motor vehicle bodywork cleaning prior to painting
- Removing excess copper dust from wires
- Wiping out liquid and drying thin wires
- Removing excess molten zinc on hot-dip galvanized pipes
- Wiping out water from the surface of insulated wire at the extruder