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Surface Finishing Tutorial

#185 Powder Painting (Thermal Spraying Method)

Category : Painting
May23, 2014
The thermal spray method of powder painting evolved from thermal spraying of metal powder, and the method is often used with thermoplastic resin paints such as polyethylene and nylon. For thermosetting resin paints such as epoxy and etc., the films need to be reheated to re-melt after thermally spraying.

For the thermal spraying systems, there are Gas Spraying system, Plasma Jet Spraying system, and etc.

The Gas Spraying system on [Fig.1], device structure on [Fig.2], a Plasma Jet Gun structure is shown on [Fig.3].
With the gas spraying system, high temperature flames of 2,000〜3,000°C by combustion of oxygen and acetylene gas (or propane gas) is generated. Compressed air is sprayed enveloping the flame to adjust for the length of the flame while preventing oxidation of the powder paint. The powder paint is injected in the center of the flame to melt, or semi-melt to be sprayed onto the workpiece.
Normally, the workpieces are preheated to 100〜200°C in order to improve paint adhesion, as well as the surface is seared to smooth the paint film.
With this method, paint baking furnace is not needed, enabling painting of large structures, outdoor objects, thick coating on interior walls of large chemical equipment.

With Plasma Jet Spraying, as shown on [Fig.3], an arc is generated by applying a DC current between a cathode and an anode in the gun. When inert gas is introduced into this arc, the gas is ionized and the plasma is generated by the heat of the arc.
When the ionized gas is cooled by the gas around the arc and the coolant, the electrons, proton ions and atoms reversibly bond and a type of thermal reaction generates heat of 20,000°C or more. There is an application example, DC50〜75v/130〜150A, nitrogen gas of 50~70L/minute, coolant 15~20L/Minute, epoxy powder paint of 30~60g/Minute.

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