Welding is a process of joining two flaps of film, usually plastic, through the use of tools.
New frontiers are looking at cold sealing which reduces the amount of waste due to the gentleness of the system on the contained product, such as chocolate bars, even during machine downtime. In addition, the use of problematic cold sealing adhesives can be avoided. For this technology, the ultrasonic generator allows monitoring of the quality of the pouch seam: hermetic sealing is guaranteed even if there is product in the seam area that is normally contained in the package (defined as ‘contamination’).

Source:

Herrmannultraschall, “Ultrasonic sealing technology“, updated 08-07-2020, taken from: https://static.herrmannultraschall.com/fileadmin/user_upload/Donwloads/FOOD_ITA.pdf