Joining thermoplastics and thermoplastic composites involves heating the polymer to a viscous state and physically causing polymer chains to interdiffuse, usually by application of pressure . In hot gas/air and extrusion welding, the weld joint is filled with a partially- or fully-molten polymer, respectively. The weld joint can be either a groove cut between two sheets, as in a butt weld, or in the corner made by two pieces of polymer positioned at right angles. Lap and other types of joints can be welded by these techniques.
In hot gas/air welding, heated gas or air is blown onto the surfaces of the joint area and, simultaneously onto a filler rod, to melt both surfaces. These surfaces are then pressed together using either manual pressure or a metal tab (pressure foot) that is built into the tip of the welding tool
In extrusion welding, the heating method is sometimes similar (heated tools or infrared heaters can also be used). The difference is that a fully molten polymer is injected into the weld joint. The molten polymer is generated inside the welding tool and pumped into the weld joint as the tool is moved along the weld .
These processes are often used to weld long weld joints. Typical applications include welding of seams in environmental pond liners, assembly of large thermoplastic air ducts, manufacture of polymeric tanks, and attachment or repair of polymeric pipelines.