The term zero joint defines a machining result of edge banding machines, characterized by an invisible or rather hardly visible joint between panel and edge material. The observer is supposed to not be able to see any joints. It's supposed to look like, as if the edge and the surface are cast from the same mould. How exactly a zero joint is supposed to look like and how it's to be produced isn't exactly defined anywhere. There is, however, a non-official process for proving the existence of a zero-joint, which is called a graphite test. For testing, the graphite powder is to be spread onto the panel and rubbed over the joint. If no joint becomes visible, the test is passed.
The zero-joint is mostly to be reached by activating a reaction layer on the edge banding which affixes the edge banding on the panel by merging.
Temporally, there are four processes which established themselves as means to reach the zero-joint, each one with their own perks and disadvantages.
Hot air technique
Further zero-joint-gluing proceedings