1. Field of the Invention
The present invention relates to plasma coating of coating materials on a substrate, and more particularly to equipment and processes for injecting powder into a plasma coating machine.
2. Description of the Prior Art
One of the ongoing problems in the plasma coating industry is "spitting" of prematurely molten powder from the plasmatron or plasma gun. Spitting becomes more of a problem when coating large parts, continuously over a relatively long period. Spitting can occur when applying metal and/or ceramic coatings. When spitting occurs the affected parts have to be stripped and re-coated, and this has a major cost impact.
Several factors are believed to contribute to spitting. One of the major factors is the way powdered material is injected into the plasma stream. Most plasma systems have plasma guns with external powder injection, whereby the powdered coating materials are injected into the plasma stream outside of the anode of the plasma gun through use of inert gas and a tubular injection conduit. Although this outside powder injection reduces the risk of spitting, it also reduces the deposit efficiency as well as the coating quality.
The most efficient way of conventionally injecting powder into a plasma stream is internally, that is, whereby the powdered coating materials are injected into a space between the anode and cathode of the plasma gun through use of inert gas and a tubular injection conduit. This internal powder injection method produces superior quality coatings, but does so at an increased risk of spitting.
Other factors that contribute to spitting are believed to be distance between the arc discharge and the point injection; the temperature of the plasma; and the energy concentration of the plasma. It is known that powder injection close to the arc discharge into a very hot and concentrated plasma stream with high energy density can result in premature melting of the powdered material.