The flame-spray technique was recently developed for application ofthermoplatic powder coatings. The thermoplastic powder is fluidizedby compressed air and fed into a flame gun where it is injected througha flame of propane, and the powder melts. The molten coatingparticles are deposited on the workpiece, forming a film onsolidification. Since no direct heating of the workpiece is required, thistechnique is suitable for applying coatings to most substrates. Metal,wood, rubber, and masonry can be successfully coated by thistechnique. This technology is also suitable for coating large orpermanently-fixed objects.
There are particular reasons why flame spraying may be selected over other surface engineering techniques and they are likely to include a combination of the following:
- The part geometry or the working environment requires manual spraying – flame spraying offers the easiest and most tolerable working conditions
- There are cost concerns and the area is large and complex
- The required coating performance is reached with flame spraying
- Dust and fume levels associated with arc spraying are too high
Flame spraying is widely used were lower coating costs are desired and a lower coating quality can be tolerated. Some typical applications include:
- Corrosion protection of structures and components (e.g. bridges, offshore platforms, LPG bottles) with aluminium or zinc coatings. Aluminium is more expensive, but has resistance to acidic gaseous atmospheres (such as those associated with the products of fossil fuel combustion), as well as neutral solutions, such as salt water. Zinc has resistance to alkaline corrosion. Flame spraying is also used to spray corrosion resistant thermoplastic polymer coatings.
- Reclamation of worn shafts, particularly of bearing areas with materials such as stainless steel or bronze alloys. The coatings produced are quite porous and lubricants can be absorbed into the coating, enhancing the performance of the bearing.