A rotary joint for transferring melted plastic from at least one extruder to a plurality of molds of a rotary machine for molding preforms, the joint comprising - a fixed structure provided with a fixed longitudinal element therein, defining a longitudinal axis, wherein a first passage channel is provided within said fixed longitudinal element, - a movable structure (3") adapted to rotate about said longitudinal axis and adapted to be integrally fixed to the rotary machine, comprising in turn: a) a first rotary element, arranged above said fixed longitudinal element and adapted to be integrally fixed to the rotary machine, said first rotary element being provided with a second passage channel coaxial to said first passage channel, said second passage channel being in communication at a first end thereof with said first passage channel, and being in communication at a second end thereof with a plurality of lateral radial channels, said lateral radial channels being provided within said first rotary element, b) a second rotary element arranged underneath the first rotary element and integrally fixed thereto, said second rotary element being provided with a central cylindrical through cavity in which a first end of said fixed longitudinal element is coaxially accommodated, - wherein a gap is provided between the fixed structure and the movable structure (3"), - wherein a spiral seal is arranged in a part of said gap for ensuring the action of sealing the melted plastic between said fixed structure and said movable structure (3"), - wherein said spiral seal comprises a single- or multi-start spiral groove made on an inner surface of said central cylindrical through cavity facing a smooth outer surface of said fixed longitudinal element, said spiral groove being helical and having an inclined helix so as to oppose, with its rotation motion, the natural exiting direction of the melted plastic flow into the gap, characterised in that helix is inclined by an angle not greater than 45o with respect to a plane orthogonal to the longitudinal axis, and in that between crests of the spiral and the outer surface of the fixed longitudinal element there is a distance of at least 2 mm. - a fixed structure provided with a fixed longitudinal element therein, defining a longitudinal axis, wherein a first passage channel is provided within said fixed longitudinal element, - a movable structure (3") adapted to rotate about said longitudinal axis and adapted to be integrally fixed to the rotary machine, comprising in turn: a) a first rotary element, arranged above said fixed longitudinal element and adapted to be integrally fixed to the rotary machine, said first rotary element being provided with a second passage channel coaxial to said first passage channel, said second passage channel being in communication at a first end thereof with said first passage channel, and being in communication at a second end thereof with a plurality of lateral radial channels, said lateral radial channels being provided within said first rotary element, b) a second rotary element arranged underneath the first rotary element and integrally fixed thereto, said second rotary element being provided with a central cylindrical through cavity in which a first end of said fixed longitudinal element is coaxially accommodated, a) a first rotary element, arranged above said fixed longitudinal element and adapted to be integrally fixed to the rotary machine, said first rotary element being provided with a second passage channel coaxial to said first passage channel, said second passage channel being in communication at a first end thereof with said first passage channel, and being in communication at a second end thereof with a plurality of lateral radial channels, said lateral radial channels being provided within said first rotary element, b) a second rotary element arranged underneath the first rotary element and integrally fixed thereto, said second rotary element being provided with a central cylindrical through cavity in which a first end of said fixed longitudinal element is coaxially accommodated, - wherein a gap is provided between the fixed structure and the movable structure (3"), - wherein a spiral seal is arranged in a part of said gap for ensuring the action of sealing the melted plastic between said fixed structure and said movable structure (3"), - wherein said spiral seal comprises a single- or multi-start spiral groove made on an inner surface of said central cylindrical through cavity facing a smooth outer surface of said fixed longitudinal element, said spiral groove being helical and having an inclined helix so as to oppose, with its rotation motion, the natural exiting direction of the melted plastic flow into the gap,