A method for welding between a neck and a plastic vessel tank body adapted to receive a liquid product, with the neck and the body forming a pair of elements to be welded positioned in alignment along a main axis (X'X) and having two opposite contact surfaces, wherein: - the assembling walls (401, 402) of the neck and/or body elements to be welded are provided with one outer and one inner assembling faces, - each assembling wall (401, 402) consists of a first portion having a constant thickness matching a first portion (Sc, Sc') of the assembling wall, - one assembling wall (401, 402) has a thinned thickness relative to the rest of the element (41, 42), with the assembling walls (401, 402) then having a contact surface (Sa, Sa') inclined (α c, α f) relative to the main axis (X'X), - the radial dimensions of the assembling walls (401, 402) are such that the assembling wall (401, 402) of one element (41, 42) is inserted so as to contact the assembling wall (401, 402) of the other element (41, 42), with a force being applied onto one of the elements (41, 42), - the thinned portions of the assembling walls (401, 402) have a deformable structure, - energy input melts the plastic material of the contact faces (Sa, Sa') then joined together, - such melting step is followed by a step of cooling, characterized in that - in addition to the first portion having a constant thickness and the partially linear thinning in thickness, each assembling wall (401, 402) consists of an end portion (Sp, Sp') wherein thinning remains substantially constant, - the inclined contact faces (Sa, Sa') are separated by an angular deviation of at least substantially 3° to 5°, - with the thinned walls of the assembling walls (401, 402) having a radially deformable structure upon the wall offset in thickness, with a force being applied to one of the elements (41, 42) to constrain the contact faces (Sa, Sa'), the inclined geometry of the contact faces (Sa, Sa') induces an elastic deformation of the end of the body element which causes a radial gap (e) between the not thinned outer face of the neck element and the edge of the end (Sp') of the body element towards the end of the offset in thickness, as opposed to the almost inexistent difference at the start of the offset in thickness, - energy input melts the plastic material of the contact faces (Sa, Sa') at local zones (Zc, Zc') provided in such faces to form a volume of molten material, - upon cooling, the edge of the contact face (Sc') of the end of the body elastically returns to contact the neck face and retains the material in excess. - the assembling walls (401, 402) of the neck and/or body elements to be welded are provided with one outer and one inner assembling faces, - each assembling wall (401, 402) consists of a first portion having a constant thickness matching a first portion (Sc, Sc') of the assembling wall, - one assembling wall (401, 402) has a thinned thickness relative to the rest of the element (41, 42), with the assembling walls (401, 402) then having a contact surface (Sa, Sa') inclined (α c, α f) relative to the main axis (X'X), - the radial dimensions of the assembling walls (401, 402) are such that the assembling wall (401, 402) of one element (41, 42) is inserted so as to contact the assembling wall (401, 402) of the other element (41, 42), with a force being applied onto one of the elements (41, 42), - the thinned portions of the assembling walls (401, 402) have a deformable structure, - energy input melts the plastic material of the contact faces (Sa, Sa') then joined together, - such melting step is followed by a step of cooling, - in addition to the first portion having a constant thickness and the partially linear thinning in thickness, each assembling wall (401, 402) consists of an end portion (Sp, Sp') wherein thinning remains substantially constant, - the inclined contact faces (Sa, Sa') are separated by an angular deviation of at least substantially 3° to 5°, - with the thinned walls of the assembling walls (401, 402) having a radially deformable structure upon the wall offset in thickness, with a force being applied to one of the elements (41, 42) to constrain the contact faces (Sa, Sa'), the inclined geometry of the contact faces (Sa, Sa') induces an elastic deformation of the end of the body element which causes a radial gap (e) between the not thinned outer face of the neck element and the edge of the end (Sp') of the body element towards the end of the offset in thickness, as opposed to the almost inexistent difference at the start of the offset in thickness, - energy input melts the plastic material of the contact faces (Sa, Sa') at local zones (Zc, Zc') provided in such faces to form a volume of molten material, - upon cooling, the edge of the contact face (Sc') of the end of the body elastically returns to contact the neck face and retains the material in excess.