Powder flame spraying, a thermal spray process in which the material to be sprayed is in powder form.
Powder flame spraying is probably the simplest of all the spray processes to describe - feed a powder through the center bore of a nozzle where it melts and is carried by the escaping oxy-fuel gases to the work piece. Unfortunately, this approach yields coatings high in oxides and with void contents approaching 20 volume percent (v/o). However, coating quality can be improved by feeding air to the nozzle through a small jet, which reduces the pressure in a chamber behind the nozzle. This chamber is connected to the powder feed hopper. In this way a gentle stream of gas is sucked into the gun and carries powder with it. A typical gun is illustrated below.
This concept was developed by Fritz Schori in the early 1930's. However, the amount of powder that can be supported by a gas stream depends on many factors including powder characteristics. If air is not used then the density of the supporting gas influences the feed rate and, for any particular powder there is an optimum amount that can be carried in a gaseous stream. It depends upon the velocity and volume of the gases used. The usefulness and criticality of flowmeters and pressure gauges are governing factors.
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