MBe International - Combustion wire description

This process is the oldest of the Thermal Spray processes. It dates back to C.1909 in Switzerland. The ballistics engineer Max U. Schoop developed the first known Thermal Spray gun, surprisingly enough called, the “Schoop Gun”.

Schematic diagram of the Combustion Wire process

The story goes that Dr. Schoop stumbled across the idea whilst he was setting the sights on a long barrelled gun. Using a brick wall as a target, he was unhappy with the resulting damage to the wall after each test firing. He decided to secure a metal plate against the wall to protect it from further damage. The “shot” from the gun was said to “stick” to the metal plate on impact and so the concept of Thermal Spraying was born.

Whether this is a true account of those events is irrelevant because the story holds up and we can see from the account that the process relies upon two fundamental energies; Thermal and Kinetic (Heat and Speed).

We do know for certain that the first Thermal Spray patent was filed by Dr. Schoop on March 30th 1915 and that his original Schoop gun bares a remarkable resemblance to the modern day Combustion Wire guns.

We also know that in 1917 Dr. Schoop was presented with the “John Scott Award” for “The Schoop Metal Spraying Process”. John Scott was an Edinburgh druggist who wanted to create a method of recognising ingenious or unique inventions. He chose the City of Philadelphia in the USA to administer the annual awards.

In 1922 Metallisation Ltd started trading in the UK after buying the rights to the Schoop patent.

In 1933 Metco Inc started trading in the USA, again after buying the rights to the Schoop patent.

Principle of operation

The combustion wire process involves driving a wire (usually metallic) through the gun by means of a compressed air-driven turbine or motor. An oxy-fuel gas flame then melts the wire as it passes through the nozzle at the front of the gun. As the wire is melting, compressed air then atomises the molten wire and accelerates it onto a prepared surface.

It can been seen therefore that the thermal energy (heat) is supplied by the oxy-fuel gas flame and that the kinetic energy is supplied by the compressed air as it atomises the particles from the wire.

For hard wires such as steels, bronzes, molybdenum etc. acetylene (C2H2) is the usual fuel gas of choice as it can generate a heat source temperature of up to 3,100°C. These higher melting point materials are usually used for machine element reclamation (worn shafts, journal areas, seal areas etc.).

These types of gun and their ancillary equipment are widely used within the anti-corrosion industry. By preparing the steel or iron surface (by grit-blasting) and applying a coating of between 0.100mm – 0.300mm (100µm - 300µm) of zinc or aluminium, corrosion protection can be offered for up to 30 years in some cases.

Zinc and Aluminium have relatively low melting temperatures compared to the harder materials used in machine element work. For this reason, the favoured gas for the anti-corrosion industry is propane (C3H8) which gives a heat source temperature of up to around 2,600°C. Propane is generally cheaper than acetylene so combining this with the low melting points of zinc and aluminium, propane becomes very attractive in this very completive market.

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