A spiral unit includes a base tube extended along a longitudinal axis and made of a first thermoplastic resin, and a fin disposed on an outer peripheral surface of the base tube along a fin axis spirally extended around the longitudinal axis and made of a second thermoplastic resin. The fin includes a strip portion provided in a state of being bonded to the outer peripheral surface of the base tube and spirally extended along the fin axis. The second resin being mixed with a magnetic material in the strip portion.

A method that includes: butting together end regions of unconnected strip parts, and applying heat and pressure to create a welded joint between the end regions;
at least one end region presents a rear zone and a front zone projecting from the rear zone, the front zone being delimited by two front zone lateral edges each presenting at least one point positioned at a distance from a respective extension of each lateral edge of the rear zone.

The front zone of an end region of a first strip part is situated facing an end region of a second strip part when the end regions are butted together.

A method for producing a rotor blade of a wind turbine includes the following steps: a) providing at least two different components of the rotor blade, b) placing a resistive element between the components, c) placing a thermoplastic or weldable thermoset resin between the components, d) energizing the resistive element so that the resistive element applies heat to the thermoplastic or weldable thermoset resin to melt or to soften it, and e) joining the components together by means of the molten or softened thermoplastic or weldable thermoset resin.

A system and method for fusing at least two thermoplastic composite components together may include placing faying surfaces of the thermoplastic composite components in contact with each other, placing a separation layer over and against one of the thermoplastic composite components, and placing a multi-layer composite pressure pad over and in contact with the separation layer. The method may also include placing an activated inductor against the multi-layer composite pressure pad for induction welding the two thermoplastic composite components to each other. The thermoplastic composite components may comprise conductive fibers while the multi-layer composite pressure pad may comprise a first layer and a second layer with non-conductive fibers. The first layer may at least partially melt during induction welding of the two thermoplastic composite components while the second layer may remain rigid throughout this welding.