3D printing stringing: those thin threads between parts, causes and fixes

3D printing stringing describes the thin threads, sometimes barely visible, that connect two walls of the same part or two neighbouring parts on the build plate. They form during travel moves, when the nozzle crosses empty space without extruding. Molten plastic keeps flowing, stretches, then settles as a web.

In our Lille workshop, stringing is one of the most common FDM defects, alongside warping. The good news: unlike other issues, it has few causes and almost always responds to settings. The key is testing them in the right order, one at a time.

What stringing tells you about the material

A viscosity issue, not a mechanical one

A filament that is too fluid flows out under gravity and residual pressure inside the melt zone, even when the nozzle stops moving. That pressure is what creates the threads. The hotter the material, the more fluid it is, the more it strings.

So stringing is almost never a printer fault. A perfectly calibrated machine will string if the filament is wet or the temperature too high. Conversely, an entry-level printer that is properly dialled in produces clean parts.

The role of water vapour

Technical polymers absorb moisture from the air. Inside the nozzle, that water flashes into steam and pushes material outwards. The result is twofold: threads, but also audible crackling and a rough, matte surface.

A spool of PETG left open for three weeks in a humid workshop can be enough to trigger stringing on a part that printed cleanly the month before. That is the first thing to check when a proven profile suddenly stops working.

The four main causes

Extrusion temperature too high. This is cause number one. Every spool has a usable window, often 20 to 30 °C wide. Working at the top of that window improves layer adhesion but degrades travel cleanliness.

Insufficient filament retraction. Retraction pulls the filament back before a travel move to release pressure. If the distance or speed is too low, pressure remains and material keeps oozing.

Wet filament. As described above. The most sensitive materials are nylon, PETG, TPU and polycarbonate.

Poorly optimised travel paths. A part with many small islands — a comb, a grille, a set of teeth — multiplies the number of crossings over open space. The same profile that yields a clean cube will produce threads on that geometry.

Materials that string the most

Not every material behaves the same way. Here is what we observe in production:

• PLA: low sensitivity, wide window, forgiving settings. The easiest material to control.
• PETG: clearly more prone to stringing. Its viscosity and moisture uptake make it the usual suspect. We routinely drop the temperature 5 to 10 °C below factory profiles and increase retraction.
• ABS and ASA: moderate sensitivity, though a heated chamber keeps the material fluid for longer.
• TPU: a special case. Flexible filament compresses instead of retracting, which makes retraction largely ineffective. We compensate with lower flow and lower temperature.
• Nylon (PA) and polycarbonate: highly hygroscopic. Without prior drying, stringing is close to guaranteed.
• Carbon-filled composites (PA-CF, PETG-CF): the filler stiffens the melt and slightly reduces oozing, but the base polymer remains hygroscopic.

Our materials pages detail the mechanical properties of each family, which helps with selection before settings even come up.

Settings to test, in order

We always work from the most likely cause to the finest. One parameter at a time.

1. Dry the filament

Before touching any setting, remove the doubt. Four to eight hours of drying usually suffices for PETG and PLA, around 45 to 60 °C depending on the material. Nylon and polycarbonate need longer and hotter. Properly dried filament, then stored in a sealed box with desiccant, stays stable for weeks.

2. Lower the temperature in 5 °C steps

Print a test tower, or two small pillars spaced 30 mm apart. Drop 5 °C, observe. Keep going until the threads disappear or layer bonding starts to suffer. The right value sits just above that threshold.

3. Adjust retraction

On direct-drive extruders such as those on Bambu Lab machines, useful distances stay short, typically under 1 mm. Increase in 0.1 mm steps, then raise retraction speed. Be careful: excessive retraction causes under-extrusion on restart, visible as small gaps on the walls.

4. Speed up travel moves

The faster the nozzle crosses the gap, the less time the material has to stretch. High travel speeds are one of the most effective levers, with no downside for surface quality.

5. Enable wipe and travel-avoidance options

Wiping during retraction deposits the excess onto an existing wall. Options that avoid crossing outer perimeters keep the threads inside the part, where they will not be seen. These settings do not remove the cause, but they make the defect invisible.

6. Reduce nozzle size or layer height

Less flow means less residual pressure. This is a last-resort lever, since it extends print time.

When post-processing is the right answer

On some geometries — fine threads, grilles, several parts printed side by side — a few threads remain despite a clean profile. On an internal technical part, that has no consequence. On a visible part, we remove them with a heat gun or a blade, then sand if required.

We prefer that approach to an extreme setting that would weaken the part. A part with two threads to trim beats an under-extruded part. Our portfolio shows the finish obtained after that kind of rework.

If your geometry contains many small islands, it is worth discussing before we start. We can rearrange the plate, change orientation, or suggest resin when fine detail matters most. To price a project, use the quote page; for recurring professional needs, the pro quote is better suited.

Do not confuse stringing with other defects

A few useful distinctions:

• Threads between walls during travel moves: stringing.
• Corners lifting off the plate: see our article on warping.
• A small blob at the start of each layer: a seam issue, not stringing.
• Grainy surface with crackling sounds: moisture, often alongside stringing.

Identifying the defect correctly avoids changing the wrong parameters. File preparation matters too: geometry designed for printing mechanically reduces the number of open-space crossings.

Frequently asked questions

Does stringing weaken the part?

Not directly. The threads are material deposited outside the useful volume; they take nothing away from the structure. That said, heavy stringing often signals unstable extrusion, and that instability can degrade layer bonding.

Should I lower the temperature or increase retraction first?

Start with temperature: it is the most common cause and the easiest variable to evaluate. Filament retraction comes next, in small increments. Changing both at once makes the results impossible to interpret.

How do I know if my filament is wet?

Three signs: audible crackling during extrusion, a matte and grainy part surface, and stringing that appeared without any change in settings. A test drying cycle settles the question within a few hours.

Does PETG always string a little?

In practice, yes, on complex geometries. Very clean PETG parts are achievable on simple shapes, but on a grille or a comb some residual threading is common. We handle it in finishing rather than pushing settings to extremes.

Does stringing occur in resin printing?

No. MSLA cures liquid resin layer by layer, with no nozzle and no travel moves. Resin has its own defect list: badly placed supports, delamination, poorly washed surfaces.