Extrusion process and associated device

A process for the production of a three-dimensional item printed by a 3D-printing procedure, said process comprising:

• a) providing a 3D printer including an extruder comprising a cylindrical tubular body having a first end closed by a bottom having a nozzle and a second end, and a piston comprising a rod and a piston head that can be removably coupled to one another;
• b) feeding a viscoelastic food dough into said cylindrical tubular body;
• c) providing said piston inside the side wall of said cylindrical tubular body, said piston head being in contact with said side wall of said cylindrical tubular body and being sized to be freely slidable inside said cylindrical tubular body, said viscoelastic food dough being entirely contained between said freely slidable piston head and said bottom, and said freely slidable piston head being in contact with the viscoelastic food material present inside the space comprised between said freely slidable piston head and said bottom of the cylindrical tubular body;
• d) extruding said viscoelastic food dough by progressively advancing said rod and said freely slidable piston head, said freely slidable piston head being coupled with said rod and pushed by said rod thereby contacting and pushing said viscoelastic food dough inside said cylindrical tubular body towards said bottom of the cylindrical tubular body along a sliding path, resulting in the supply at a preset speed of a continuous string of dough from said nozzle;
• e) interrupting the extrusion of said viscoelastic food dough by stopping the progress of said rod at a specific position along said sliding path and by immediately retracting said rod towards said second end to decouple said rod from said freely slidable piston head whereby said freely slidable piston head is pushed by said viscoelastic dough towards said second end of the cylindrical tubular body;
• wherein the progress of said rod inside said cylindrical tubular body towards said bottom and the movement of said rod towards said second end are controlled by means of a control system of the extruder;
• and
• wherein, during said step e) of interrupting extrusion of said viscoelastic food dough, said specific position of said rod along said sliding path corresponds to a preset position previously saved by said control system of the extruder or said specific position of said rod along said sliding path is recorded by said control system of the extruder at the time when said rod is separated from said freely slidable piston head;
• wherein after said step e) the process further comprises an additional step of starting again the extrusion of said viscoelastic food dough, bringing said rod back into said specific position along said sliding path, thus restarting the supply of a continuous string of dough from said nozzle;
• and
• wherein the piston head further comprises a venting valve in fluid communication with the space of the cylindrical tubular body disposed between the piston head and said bottom and with the space of the cylindrical tubular body disposed between said piston head and said second end, said venting valve being convertible between an open mode and a closed mode, and while during said additional step of starting again the extrusion after said step e),
• said venting valve is in said open mode when said rod, coupled with said piston head, advances from a rest position towards an end-stroke position and the piston head is not in contact with the viscoelastic food material contained in the space between the piston head and the bottom of the cylindrical tubular body, and
• said venting valve is in said closed mode when said rod, coupled with the said piston head, advances from a rest position towards an end-stroke position and the piston head is in contact with the viscoelastic food material present inside the space between the piston head and the bottom of the cylindrical tubular body.