Hot-gas welding is a manual plastic welding process for joining thermoplastic materials. A hot-gas torch is used to direct hot air to both the joint surface and weld rod, heating the materials to their softening temperature. Application of pressure on the heated weld rod to the joint surface bonds the materials together to form a completed weld. This technique is not easily automatized and is primarily used for repairs or individual manufacturing needs of small or complex components.
Welding techniques
There are two common forms of welding techniques used in hot gas welding: hand welding and speed welding. Tack welding may be utilized to set the components in position to perform the actual welding process.
Hand welding
Hand welding is a technique in which the weld rod is applied to the joint by the welder directly. This is also referenced as free-hand welding or fan welding. The hot gas torch is maneuvered in one hand to heat both the weld rod and joint surfaces in a pendulum manner in quick succession. Pressure is applied to the welding rod and controlled by hand without the assistance of a nozzle. This technique is suitable for most configurations and can be beneficial for welding tight, constrained areas or complex joint designs since application of the welding rod is only limited to the achievable welding positions.
Speed welding
Speed welding employs a specially designed nozzle which enables the hot gas torch and weld rod to be one cohesive system. The nozzle facilitates application of the weld rod to the joint through a feeder tube. The nozzle evenly heats the weld rod material and allows for a controlled application of pressure. The bottom of the nozzle is designed to heat the joint surface and guide the weld rod into the groove. Nozzles are manufactured for the feeder tubes to accommodate specific welding rod shapes and dimensions and are available for round or triangular rods of common sizes. Use of speed welding is limited to applications of simple joint design and orientation due to the size of the nozzle and maneuverability of the system.
Process parameters
Gas temperature, application pressure, weld travel speed, gas flow rate, and torch orientation all influence the integrity and mechanical properties of the finished weld. Gas temperature and flow rate are controllable parameters based on system inputs. Application pressure, weld travel speed, and torch orientation are all dependent upon the operator performing the weld. These parameters are interrelated and all have a significant impact on the final quality of the weld.
Gas temperature and flow rate
Gas temperature is a controlled input that should be monitored for accuracy prior to initiating the welding process. Hot gas temperatures are selected at values above the material's melting or glass transition temperature. Sufficient temperature is required to overcome a materials activation energy, resulting in a reduction of the viscosity and an increase in flowability to support diffusion across the weld interface. Prolonged exposure to elevated temperatures exceeding material manufacturer's recommendations can result in oxidation, distortion, or molecular deterioration, which can lead to joint failure. Calibration and verification of the output should be performed after the gas temperature has stabilized in the welding gun. Speed tip nozzles focus heat directly on the joint in a specific region, resulting in effective heat transfer to the weld surfaces. If sufficient weld travel speed is not maintained, recommended welding temperatures above the glass or melting temperature of the material in these weld regions can be exceeded and lead to defects.
Thermal expansion from the welding process may result in distortion and development of weld defects if part components are not properly secured. Work surface material should also be considered to avoid heat losses which may result in lack of penetration or lack of fusion due to inadequate heating of the joint surfaces.
Sufficient hot gas flow rate is necessary to maintain adequate, even heating of weld rod and joint surfaces. Flow rate can be controlled through the use of a blower or an air compressor. To avoid weld contamination, supplied hot gas should be free of moisture and should not contain impurities. A properly sized blower or compressor can be utilized for multiple hot gas torches if one is not integrated in the individual welding gun.
Welding energy
The welding energy imparted on the weld surface during hot gas welding can be used to predict the overall strength of the finished joint. Welding energy (Ew) is determined using the gas temperature and flow rate using the following relationship: