A German Maker 3D Prints Hollow Tubes With Compressed Air

What Is This Approach

A maker called Jan, behind the Roetz 4.0 project, has built an extruder that does something most desktop printers cannot do: it deposits a plastic shell around a column of compressed air. The result is a hollow tube, printed in a single pass, with no internal infill pattern and far less material than a solid wall.

The setup is a variation on a dual-material extruder. It feeds two strands of filament into a melt chamber, then pushes the molten plastic out around the tip of a compressed-air line. A high-precision pressure regulator controls the airflow. In principle, the printer builds an airtight tube as it moves, with the air core inflating and setting the geometry of each deposited strand.

The Early Problems

The first tests with PLA showed the concept works in principle but not reliably in practice. The tubes came out with inconsistent diameters. Air pockets formed inside. The low thermal conductivity of the stainless-steel extruder barrel, combined with the cooling effect of the compressed air, caused filament to solidify inside the nozzle before it could be deposited cleanly.

Pressure-based control turned out to be the wrong approach. As the plastic shell gets thinner, it offers less resistance to the incoming air pressure. The tube ballooned outward continuously until the wall burst. Getting stable output from a pressure-regulated system alone proved nearly impossible.

Switching to Volume Control and TPU

Jan solved the consistency problem by switching from pressure control to volumetric air control. Instead of regulating how hard the air pushes, he meters exactly how much air is released per unit of extrusion. He built a peristaltic pump to handle that metering step. The change made a real difference: tube diameters stopped drifting, and walls stayed intact from start to finish.

He also switched from PLA to TPU. The flexible filament bonded to itself across layers far better than a rigid material would, which matters when each layer is being inflated against the previous one. Finished TPU tubes held their shape after inflation, though they did still leak air slowly. That is a solvable sealing problem, not a fundamental one.

Why This Matters

Printing hollow air-filled tubes changes the economics of large-format FDM. A conventional print fills the interior with a zigzag pattern that wastes time and material. A tube-printing approach deposits only the shell. For structural parts where weight matters, or where you need internal passages for airflow or cabling, the technique removes steps from the design and post-processing workflow.

The idea is not entirely new. TU Darmstadt has published research on blow extrusion for additive manufacturing. A similar co-extrusion approach has been used to print pneumatic channels directly into soft robots. What Jan has built is a practical, low-cost desktop implementation that anyone with a dual-extruder setup can experiment with, and he has documented the whole process on YouTube.

What to Watch Next

The leakage issue needs a material solution: a printable filament that seals reliably when inflated, or a post-print coating step that turns an air-filled tube into a rigid pressure vessel. If that gets solved, this approach could show up in large-format construction printing, lightweight drone airframes, and architectural models where visual accuracy matters more than structural strength.