Movement unit for moving a blank comprising ∘ a drum which rotates about a rotation axis (R) along a rotation direction (Ro), o said blank comprising - a central panel , - a lateral panel which is secured at a first end thereof to said central panel by means of a first folding zone and which has a second end which is opposite said first end and which is free, - said lateral panel having an extent in the direction perpendicular to said first folding zone which is at least 10 cm, - said lateral panel being arranged so as to be preceding in said rotation direction (Ro) of said drum with respect to said central panel , o a retention device for retaining said blank the retention device being secured to said drum and comprising - a support , - a first gripping portion which is secured to said support and which comprises first retention elements configured to selectively and fixedly retain said central panel , - a second gripping portion which is secured to said support or to said first gripping portion and which comprises second retention elements which are configured to selectively and fixedly retain said lateral panel of said blank , wherein said retention device comprises ∘ a movement mechanism for moving said second gripping portion with respect to said first gripping portion , which movement mechanism is configured to carry out, during a rotational arc of said drum , a controlled rotation of said second gripping portion with respect to said first gripping portion . ∘ a drum which rotates about a rotation axis (R) along a rotation direction (Ro), o said blank comprising - a central panel , - a lateral panel which is secured at a first end thereof to said central panel by means of a first folding zone and which has a second end which is opposite said first end and which is free, - said lateral panel having an extent in the direction perpendicular to said first folding zone which is at least 10 cm, - said lateral panel being arranged so as to be preceding in said rotation direction (Ro) of said drum with respect to said central panel , - a central panel , - a lateral panel which is secured at a first end thereof to said central panel by means of a first folding zone and which has a second end which is opposite said first end and which is free, - said lateral panel having an extent in the direction perpendicular to said first folding zone which is at least 10 cm, - said lateral panel being arranged so as to be preceding in said rotation direction (Ro) of said drum with respect to said central panel , o a retention device for retaining said blank the retention device being secured to said drum and comprising - a support , - a first gripping portion which is secured to said support and which comprises first retention elements configured to selectively and fixedly retain said central panel , - a second gripping portion which is secured to said support or to said first gripping portion and which comprises second retention elements which are configured to selectively and fixedly retain said lateral panel of said blank , - a support , - a first gripping portion which is secured to said support and which comprises first retention elements configured to selectively and fixedly retain said central panel , - a second gripping portion which is secured to said support or to said first gripping portion and which comprises second retention elements which are configured to selectively and fixedly retain said lateral panel of said blank , ∘ a movement mechanism for moving said second gripping portion with respect to said first gripping portion , which movement mechanism is configured to carry out, during a rotational arc of said drum , a controlled rotation of said second gripping portion with respect to said first gripping portion . <EOS>