A method for producing blow-molded structures where a preform is provided that encloses a cavity and has a preform wall limiting the cavity. The preform is placed into a blow mold and formed to a blow-molded structure, wherein the blow-molded structure has a structural wall. The preform wall or structural wall comprises an inner layer and an outer layer.

A tool presses a joining element against a first object into a first opening of the first object while mechanical vibration energy is coupled into the first object and the joining element. Vibration energy and pressing force make the thermoplastic material of the distal portion of the joining element flowable and interpenetrates structures of the first object to yield, after re-solidification, an anchoring of the distal portion of the joining element in the first object. The second object is placed so that a proximal portion of the joining element is inserted into a second opening, and the first and second objects are pressed against each other while mechanical vibration energy is coupled into one of the first and second objects, until thermoplastic material of the joining element proximal portion is made flowable and interpenetrates structures of the second object to yield, after re-solidification, an anchoring in the second object.