A Maker Built a 3D Printer That Extrudes Hollow Air-Filled Tubes

Most 3D printers melt plastic and push it through a nozzle in one continuous strand. The result is a solid part. A maker called Jan, who runs the Roetz 4.0 project in Germany, asked a different question: what if the strand had nothing inside it at all?

The Setup

His modified extruder takes two strands of filament, melts them, and extrudes both as a shell around the outlet of a compressed-air line. A high-precision pressure regulator controls the airflow during the print. In theory, the nozzle deposits a tube of plastic with air running through the middle, not solid rod.

Initial tests used PLA. The nozzle produced what looked like airtight tubes, but the results were inconsistent. Walls ballooned unpredictably, and diameters jumped from layer to layer. The stainless-steel extruder also cooled the filament too aggressively thanks to the compressed air running through it, causing jams mid-extrusion.

Why Pressure Control Failed

The core problem is that thin plastic walls cannot sustain stable pressure. As the molten shell gets thinner, it offers almost no resistance to the air pushing outward. Once ballooning starts, it keeps accelerating until the tube bursts. Purely pressure-based control made reliable prints almost impossible.

Jan switched strategies. Instead of regulating pressure, he started regulating the volume of air flowing through the system. He built a small peristaltic pump that meters air by the milliliter, giving the firmware a stable number to work with. That change produced the first consistently formed tubes.

TPU Changed Everything

PLA did not want to cooperate as a thin-walled pressure vessel. Jan switched to TPU flexible filament, which has far better layer adhesion and can stretch slightly without tearing. The TPU tubes held their shape through inflation, though they still leaked air at the seams.

Finished TPU structures actually expand slightly when inflated, which could be useful if you want your prints to change shape after printing. That is a genuinely interesting property for functional parts like seals, damping elements, or lightweight structural ribs.

Where This Could Go

Academic researchers at TU Darmstadt have already explored blow-extrusion style additive manufacturing, so this is not purely a bench experiment. The same co-extrusion principle has also been used to print pneumatic channels inside soft-robot parts, where hollow passages built directly into the structure let the robot move without any separate air lines.

For desktop makers, hollow printed tubes have a few obvious uses. You can route wires or tubing through prints without drilling. You can make lightweight enclosures that are rigid but contain nothing inside. You can print inflatable structures for props or robotics.

The practical barrier is consistency. Getting walls thick enough to hold pressure without bursting, while thin enough to be useful as a tube, is a narrow window. Jan's volumetric pump approach is a real step forward, but it is not a turnkey solution yet.

The Takeaway

This is an early-stage technique, not a product. What stands out is that the breakthrough came from changing what you measure. Moving from pressure to volume sounds simple, but it completely changed the behavior of the system. That is the kind of lateral thinking that moves homemade 3D printing experiments forward.