NOVELTY - The joint structure has an intermediate junction element (32) vibratory welded to a pair of thermoplastic blocks (30,31) at respective welding surfaces. A welding surface partially defines preset angle in a plane (42) normal to the junction element, between unwelded regions of the thermoplastic blocks. A flexible rubber seal seals the gap between the thermoplastic blocks.    USE - Welded joint structure with vibratory welded butt corner joint, mitre corner joint, vibratory welded scarf joint, vibratory welded T-joint or mortice-and-tenon joint, for assembly such as truss e.g. roof truss, ladder, balustrades, gate or other substantially flat strut assembly, window, picture frame, solar collector or other panel assembly, door e.g. simulated heritage door, garage panel door, building panel or other multi-panel assembly, cupboard, drawer, bay window or other box panel assembly, table, chair, patio furniture or other combined flat panel and strut assembly, and gazebo, porch or other combined three-dimensional multi-strut assembly (all claimed).    ADVANTAGE - The elastic movement during the vibratory welding process of the thermoplastic blocks is prevented by providing the junction element with sufficient stiffness and rigidity. The visual appearance of the thin joint line formed between the thermoplastic blocks, is improved. The material cost of the welded joint structure is reduced by using thermoplastic blocks made of thermoplastic foam. The visual flash is controlled during the vibratory welding process, as the flash material produced is largely absorbed into the voids within the thermoplastic foam material. The cut foam surfaces offer less friction resistance during the welding process, and so the welding time is reduced. The welding surface is reduced, by welding the V-chamfered protrusions in the junction element with the V-chamfered grooves in the end surfaces of the thermoplastic blocks, and the vibration power required for performing the welding process is reduced.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) method for forming vibratory welded connection between thermoplastic blocks and junction element;    (2) apparatus for forming vibratory welded connection between thermoplastic blocks and junction element; and    (3) system for interconnecting series of elongated frames to form closed frame.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of the welded joint structure.    Thermoplastic blocks (30,31)    Junction element (32)    Welding surfaces of junction element (35,36)    Plane (42)    Joint line (47)