Method for forming a shell type body of self-reinforced composite thermoplastic polymer material from a plate-like element (10, 60), comprising a central region and a peripheral region which surrounds said central region, said method comprising: - providing a press including a first and a second die portion (3, 4) which can be moved between a forming configuration, in which they are pressed against each other, and a release configuration, in which they are moved apart from each other; - at least one of said first and/or said second die portion (3, 4) being provided with ribs on an operative face (3A, 4A) thereof intended to abut said plate-like element; - providing said plate-like element on said press so that said central region is in abutment on said operating face of said second die portion, - retaining said plate-like element at said peripheral region, - moving said first and/or said second die portion towards said forming configuration until said first die portion abuts said plate-like element continuing to retain said plate-like element at said peripheral region, - a forming step in which said first and said second die portion are retained in said forming configuration for a predefined time, and said plate-like element is retained between said first and said second die portion at a temperature for forming said self-reinforced composite thermoplastic polymer material so as to generate the at least partial fusion of a matrix thereof without causing the substantial fusion of the fibers - so as to form by means of said ribs on said central region a plurality of recesses defining on said central region a plurality of lateral panels which are mutually connected to a central panel by means of at least one corresponding recess, - moving said first and/or said second die portion in said release configuration and extracting said plate-like element, - successively folding said lateral panels with respect to said central panel at said recesses (20A, 20B) so as to form said shell type body. - providing a press including a first and a second die portion (3, 4) which can be moved between a forming configuration, in which they are pressed against each other, and a release configuration, in which they are moved apart from each other; - at least one of said first and/or said second die portion (3, 4) being provided with ribs on an operative face (3A, 4A) thereof intended to abut said plate-like element; - providing said plate-like element on said press so that said central region is in abutment on said operating face of said second die portion, - retaining said plate-like element at said peripheral region, - moving said first and/or said second die portion towards said forming configuration until said first die portion abuts said plate-like element continuing to retain said plate-like element at said peripheral region, - a forming step in which said first and said second die portion are retained in said forming configuration for a predefined time, and said plate-like element is retained between said first and said second die portion at a temperature for forming said self-reinforced composite thermoplastic polymer material so as to generate the at least partial fusion of a matrix thereof without causing the substantial fusion of the fibers - so as to form by means of said ribs on said central region a plurality of recesses defining on said central region a plurality of lateral panels which are mutually connected to a central panel by means of at least one corresponding recess, - moving said first and/or said second die portion in said release configuration and extracting said plate-like element, - successively folding said lateral panels with respect to said central panel at said recesses (20A, 20B) so as to form said shell type body.