In industry, glass fibre reinforced and high temperature plastics are mainly joined by infrared (IR) welding, since the generation of fine particulates or fluff during vibration welding is no longer required or permissible in manufacturing. However, since IR welding is not without problems in practice either, especially due to the very complex construction and set-up of the IR heating elements, another non-contact heating process, heating with hot gas, is becoming increasingly important. The challenge in hot gas welding is the concept and design of the tool. The prediction of the actual heating and plasticizing behaviour on the special component is directly related to the used nozzle system, the existing seam geometry and the process parameters. Also challenging is the material behaviour during heating with hot gas. Polyamides in particular are prone to oxidation if no inert gas is used. This paper discusses different nozzle geometries. The various heating conditions of tube and slot nozzles are considered and their advantages and disadvantages are shown. Furthermore, the influences of the process parameters on the melt profile are shown and their effects on the mechanical properties of the seam are analysed.