Extrusion

Extrusion in concrete printing is the process of forcing fresh mortar through a shaped nozzle under pressure so that the material leaves as a continuous filament of defined cross-section. The material is delivered either by pumping it through a hose to the print head, or by feeding it into a screw extruder mounted on the head, which meters the flow and gives the operator direct control over the deposition rate. Twin-screw designs such as WASP's LDM extruder are common on crane and gantry machines.

In practice

Flow rate has to be matched to travel speed: too much material gives a bulging, over-wide bead, too little gives a thin or broken one. The mortar must stay pumpable long enough to reach the nozzle yet stiffen quickly once deposited, which is the central conflict in printable mix design. Screw extruders help by adding shear at the head, temporarily reducing viscosity in a thixotropic mix just before deposition. Interruptions matter more than in other processes, since a stopped extruder leaves material standing in the hose, where it can stiffen and block the line.

Related terms

• Extrudability — the ability of a mix to pass through the nozzle without blocking or segregating.
• Nozzle — the shaped outlet that sets the width and profile of the filament.
• Printable mortar — a mix formulated to be pumped, extruded and stacked.