Plastics joining

Hot Gas Welding technique

The process

Hot gas welding of thermoplastics is a manual process. A stream of hot gas (typically air but can be nitrogen) is directed towards a prepared joint between the two thermoplastic parts to be joined, where it softens or melts the polymer. A filler rod is also heated in the stream of hot gas and this is fed into the joint between the two parts. A weld is formed by the fusing together of the thermoplastic parts and the filler rod.

The hot gas welding tool (torch) consists of a heater unit to heat the gas, and a nozzle to direct the gas onto the workpieces. If air is used, this is provided by either a remote air compressor or an integral blower. If nitrogen is used, suitable connection to a gas supply is made.

The temperature of the hot gas stream is governed by the electrical supply to the heater and is typically in the range 200-400°C depending on the material to be welded. A range of nozzle shapes is available and selection is based on the type of weld preparation. Thermoplastic consumable filler rods are generally circular in section although rods with triangular section are also available in most thermoplastic materials. It is important that the filler rod is made from the same base material as the two parts to be joined.

The variable welding parameters are gas type, flow rate and temperature, weld speed and pressure, and the angle of the filler rod to the parts being welded (typically around 90°). Typical gas flow rates are in the range 16-60 litre/minute.

The main advantage of hot gas welding is that the equipment is easily portable. The main disadvantages of the process are that it is slow and weld quality is dependent on the skill of the operator.

Types of welding

There are two main types of hot gas welding:

Hand, or Pendulum, Welding

This is used where the length of the welded joint is short, around a small radius or difficult to access. The welding gun is moved in a pendulum fashion along the joint axis heating both the rod and joint groove.

Speed Welding

This is used where the length of the welded joint is long, the radii are large and access is easy. The welding rod is fed through a speed welding nozzle into a prepared groove. The nozzle outlet is designed to give the correct proportion of hot gas to the rod and joint area.

Typical applications

Applications for the process are based around the fabrication of sheet and tube into vessels and pipework. Industrial sectors that have exploited the technology in these applications include industrial plant, agriculture and building.

Further information

Hot gas welding of plastics: Part 1 - the basics