Plastic welding is the process of creating a molecular bond between two compatible thermoplastics. Welding offers superior strength, and reduces cycle times. There are three main steps to any weld: pressing, heating, and cooling. Plastic welding processes are primarily differentiated by their heating methods. Hot gas welding, also known as hot air welding, is a plastic welding technique using heat. ... Two sheets of plastic are heated via a hot gas (or a heating element) and then rolled together.