Adjustable extruders and nozzle geometry effects in additive manufacturing: a review

Extrusion-based additive manufacturing, such as fused filament fabrication and direct ink writing, is valued for its versatility and material compatibility. Nozzle geometry governs material flow, bead morphology, dimensional accuracy, and mechanical performance. Conventional systems rely almost exclusively on fixed-geometry nozzles, whose outlet size and shape are set at manufacture and remain constant for the entire print. Because geometric resolution, deposition rate, and mechanical performance all depend on that same fixed geometry, a nozzle occupies a single point on the trade-off between them, and the geometry best for one region of a part is often suboptimal for another. Dynamic adaptability, by contrast, denotes the capability to change the outlet size or shape during printing, matching the geometry to local process requirements. This review synthesises current knowledge on how outlet size and shape, internal geometry, and deposition conditions affect the printed result, and evaluates extruder designs offering such adaptability. Recent literature and patents are reviewed systematically, identifying adjustable-nozzle concepts based on jaws, sliding plates, iris diaphragms, and compliant structures, compared by response time, mechanical complexity, and flow stability. These approaches show promise against the limitations of fixed-geometry nozzles, but most remain at an early stage of development and require further experimental validation.