TPU 3D printing: Stringing troubleshooting

Start with a profile that closely matches your desired output. This is particularly useful if you can find a profile that closely matches your filament and the type of extruder you are using. Save this profile before you make any changes. Set nozzle temperature, travel speed, retraction distance, retraction speed and record if the spool has been dried. Confirm nozzle diameter, filament diameter and match the slicer configuration.

Print a small model that simulates stringing and judge both the strings and the quality of the features printed. Use the same settings, model and placement for all tests. Change only one setting at a time.

Look for fine threads, that are still present even with a workable TPU profile. Thick bridges and blobs are a more serious problem, and in cases like these, retraction is not going to help. You will probably run into inconsistent extrusion, popping, rough surfaces, and skipping as well.

Damp TPU can cause bubbling, inconsistent extrusion, stringing, and rough surfaces. To test for damp TPU, listen for occasional popping. However, damp TPU can still string even if you do not hear any popping.

It is better to err on the side of caution and dry the TPU to the given temperatures and times. Different spool materials can withstand different levels of heat. Be especially careful when using a TPU filament dryer to dry a flexible TPU spool, because the filament can melt and the spool can warp.

After drying, perform the baseline test for TPU. It is a good idea to store the TPU in a dried state, either in a dry box, or in a container with desiccant. Desiccant helps dryness, but it is not a substitute for a proper drying cycle.

Make sure your spool unwinds smoothly before adjusting any slicer values. Also, check that the filament isn’t crossed, pinched, or sharp against a guide. TPU is more elastic than rigid filament, therefore inconsistent resistance can result in uneven pressure. Keep the feed path supported and avoid tight bends.

Recommended procedure varies by printer, so check your printer’s docs. Generally, stop motion, isolate power (if needed), and check the extruder. There can be too much idler pressure or too little pressure which can cause filament to slip. Repeated clicking or buckling suggests a feed issue and should be resolved before continuing.

Plastic residue can be left behind on the nozzle, and can cause strings to print across a model. Follow the printers’s instructions to clear nozzle residue, avoiding heater and sensor wiring. If leakage is suspected, stop printing and follow the recommended procedure for repair, rather than adjusting the heater or nozzle.

Test reducing the nozzle temperature by about 5°C, repeating the test after each change, and stopping when filament becomes elastic. Extrusion can become more resistant with less heat, which can also reduce bonding between layers. Oozing can occur with too much heat.

Don't keep lowering the temperature if surfaces get rougher, extrusion becomes intermittent, or layers separate. Choose the lowest temperature that enables consistent flow and produces a good layer bond, and not the temperature that gives the best looking gap between the posts.

Conservative extrusion speeds are a good starting point for tuning TPU. A good starting range for extrusion speed is 15-30mm/s, but this obviously is dependent upon the system. With high-speed TPU, a different profile may be needed. High print speeds require a change in temperature to account for the hotend melting a greater amount of material.

With TPU, some of the retraction motion actually pulls and stretches the material, and can even lead to blockage and jamming.

With direct drive systems, start with no or very little retraction, and with a Bowden system start with a higher amount of retraction. Neither of these are absolute values, and a good starting range with direct drive systems is 0.2-1.0 mm.

Keep retraction speed modest and within recommended parameters. Increase the distance in small steps, approximately 0.2 mm for a direct-drive extrusion test, while keeping the speed constant. Only after the test should slight changes in speed be compared. For the first extrusion after travel, watch for gaps, starting extrusion too slowly, and extruder clicking. Back off the setting if any issues are seen. If increased retraction doesn’t give a clear benefit, revert to the previous value.

View the slicer’s travel preview to see if there are opportunities to route travel within printed regions or to avoid crossing exposed wall regions. These options may string, but detours may extend travel and give more opportunity to ooze. Evaluate the actual route traveled versus the predicted route.

A moderate increase in travel speed can reduce travel time across open space and can be within the limits of the printer’s motion system. Change travel speed independently from print speed. Stop increasing it if movement becomes erratic and positioning becomes unreliable.

If your slicer has wipe, use it and evaluate if there are gaps or missing extrusion. Initially, leave coasting and restart compensation set to their default values. Use Z-hop to clear the nozzle if necessary to prevent collisions. Z-hop increases travel time. Fan at 100% may make strings less visible but also make layers less adherent. Use stringing patterns based on TPU profile.

After the small test is performed and looks acceptable, print a real part that contains similar small features as your usual printed parts. Look at surface finish and extrusion after travel, as well as layer adhesion. Save successful prints with the type of filament used, hardness, filament moisture content, and nozzle size.

If stringing occurs, check spool and moisture content and redo the small test. For the occasional fine hair, let the part cool and remove it using a small cutter. Never use heat or open flames, because TPU can soften and release fumes.