This method of welding was the first to be developed for plastics and is a modification of gas welding of metals. Basically the process involved melting the weld area by means of stream of hot gas supplied from a hand held torch. A welding or filter rod made from the same material as the parts to be joined is also used.

During the welding the weld bed and the welding rod are heated simultaneously by gently moving the hot gas stream back and forward between the two. When softening occurs, slight pressure is applied to the rod to feed it into the melt pool whilst steadily progressing along the joint.

The welding torches are usually electrically heated and capable of providing a gas temperature of between 200° C and 300° C at the nozzle orifice. The gas should be dry and clear with a flow rate of 15 - 60 litres per minute at high pressure. Tests have shown that the type of gas affects the bond strength. Strongest bonds are achieved with oxygen where as carbon dioxide tends to produce weak bonds. In many cases the most convenient gas to use is compressed air since it is inexpensive and produce satisfactory welds. However several plastics, such as polythlylene and acetal, oxidize very easily and are best welded using nitrogen.

Rigid PVC is the most suitable material for hot gas welding but other plastics can also be joined very satisfactorily by this method. The major advantage of this welding technique is that large structural shapes can be fabricated relatively easily, but it has the drawback of being a slow process, relying on operator skill for good welds.