How to Prevent TPU (Flexible) Filament From Stringing?

Stringing is, without a doubt, one of the most widespread issues in the world of 3D printing, with every 3D printing enthusiast most likely coming across the problem at least once in their 3D printing journey, where the stringy plastic ends up invading the whole surface of the 3D printed model and completely ruins its aesthetics.

Considering that it’s practically impossible to clean up and fix a model once the damage is done by stringing, preventing the issue from happening at all is the only solution to the problem at hand, making it vital to take the necessary measures before starting another print to ensure that time and material isn’t wasted on yet another failed print.

In today’s article, our topic will be preventing stringing during the 3D printing process, specifically in the case of using TPU filament, as stringing is an issue that you are even more likely to face when printing with TPU compared to other filament types, especially if it’s your first time printing with it.

So, how can you prevent TPU filament from stringing during the print?

Below, you can find the solutions that we recommend applying to prevent TPU from stringing:

  • Increase the retraction distance
  • Increase the retraction speed
  • Decrease the retraction minimum travel value
  • Lower the print temperature
  • Increase the travel speed
  • Activate Coasting in Cura
  • Activate Combing in Cura
  • Activate Wiping in Cura
  • Clean the nozzle of your 3D printer
  • Dry the filament spool

Next up, we will further examine the measures you can take to prevent TPU filament from stringing during the printing process in greater detail, go through the reasons that will cause TPU filament to become stringy, and finally, discuss whether it’s possible to print TPU with a Bowden extruder successfully.

How to Prevent TPU (Flexible) Filament From Stringing?

Even though stringing is an issue you can face with any filament type, the problem is amplified in the case of TPU due to the flexible nature of the filament that makes it more likely for it to become stringy, requiring some specific measures to be taken for a successful and stringing-free print with TPU.

Here are the procedures we recommend following to prevent stringing from occurring when printing with TPU filament, along with detailed explanations that clarify how they can help resolve the problem.

Increase the Retraction Distance

Increasing the retraction distance will cause the 3D printer to pull the TPU further away from the nozzle, reducing the chance of plastic coming out of it inadvertently as the printhead moves to its next destination and creating the issue of stringing as a result.

When tuning the retraction distance for printing with TPU filament, we recommend starting with a value of 6 millimeters for Bowden extruders and 2 millimeters for direct drive extruders, which should provide a solid foundation.

For further fine-tuning, our recommendation would be to increase or decrease the value by 1 to 2 mm at each step, where an increase is suitable in cases where you’re still experiencing stringing, and a decrease should be preferred if under-extrusion is taking place.

Increase the Retraction Speed

Increasing the retraction speed will allow the 3D printer to pull the TPU back as quickly as possible once a retraction is triggered, reducing the likelihood of it unintentionally dropping a blob of plastic on the position of the nozzle while the retraction is taking place.

As the blob of plastic can end up coming with the nozzle once the printhead starts moving and create the issue of stringing in the process, increasing the retraction speed value can be helpful to eliminate stringing from occurring.

Since retracting TPU way too quickly can lead to the jamming of the nozzle, our recommendation would be to start with a low retraction speed value of 20 mm/s, in this case, to avoid creating another problem while trying to solve one.

When fine-tuning, you can increase or decrease the value by 5 millimeters at each step, with the goal of finding the highest retraction speed where the issue of filament grinding does not take place.

Decrease the Retraction Minimum Travel Value

Decreasing the retraction minimum travel value will increase the frequency of retractions by causing the 3D printer to trigger them over shorter distances, which can be helpful in preventing TPU from stringing in cases where stringing occurs due to a lack of retractions.

Utilizing a standard retraction minimum travel value of 1.5 mm should do the job when printing TPU and create an environment where the 3D printer is able to perform the correct amount of retractions.

If you would like to fine-tune this value further, our recommendation would be to find the highest value possible where stringing does not occur, which will bring print time optimizations to the table.

Lower the Print Temperature

Lowering the print temperature value will slow down the rate at which TPU melts and liquefies, preventing unexpected amounts of plastic from flowing out of the nozzle in an uncontrollable manner, both during extrusion and during travel, and stopping stringing from occurring as a result.

When tuning the print temperature, our primary recommendation would be to start with the value at the bottom end of the temperature range specified by the manufacturer.

If this information is not present, you can start with a print temperature value of 220 degrees Celsius, which we can consider to be a relatively small temperature value for printing with TPU filament.

From this point, you can increase the print temperature value in increments of 5 degrees and find the figure that works best through trial and error.

Increase the Travel Speed

Increasing the travel speed value will allow the printhead to reach its destination as quickly as possible once it’s done extruding, without spending too much time in the air, which will reduce the likelihood of TPU flowing out of the nozzle and causing stringing while the travel move is still taking place.

As the optimal travel speed value is primarily based on the hardware of your 3D printer, our recommendation would be to use the highest travel speed value possible that your 3D printer can handle.

To find this value for your 3D printer, you can either refer to the specifications sheet or run test prints by incrementally increasing the travel speed value.

Enable Coasting in Cura

Enabling the Coasting feature causes Cura to instruct the 3D printer to stop extruding plastic for the last bits of the extrusion, causing any TPU that would otherwise end up oozing from the nozzle during the upcoming travel move to end up on the model itself instead, which prevents stringing from occurring.

You can enable Coasting by checking the Enable Coasting checkbox in the Experimental section of Print Settings.

Enable Combing in Cura

Enabling the Combing feature in Cura causes the travel paths of the printhead to be re-mapped in a way that only allows it to go over the printed areas and prevents it from going over the empty spaces, causing any TPU that would ooze out of the nozzle during a travel move to end up on the model.

You can enable Combing by choosing any option that isn’t “Off” from the Combing Mode dropdown located in the Travel section of Print Settings. Our recommendation for general usage would be the “Within Infill” option.

Enable Wiping in Cura

Enabling the Wiping feature in Cura prompts the 3D printer to wipe the excess filament on a brush after the completion of each layer, which ensures that no oozing can take place as the printhead is traveling from one location to the other.

You can enable Wiping by checking the Wipe Nozzle Between Layers checkbox in the Experimental section of Print Settings.

Clean the Nozzle

Cleaning the nozzle will give the plastic a clear path to its destination, allowing it to adhere smoothly to either the build plate or the previous layer without any chance of it clumping up on the nozzle.

For cleaning the nozzle and getting rid of a partial clog as effortlessly as possible, our recommendation would be to utilize the cold pulling method, which should offer a quick solution that does not require the disassembly of the nozzle.

Once you’re through with the cleaning process, our recommendation would be to run a test print to ensure that the problem is indeed resolved.

Dry the Filament

Drying the filament will ensure that there is no moisture in the filament, allowing a smooth and steady flow of the plastic without any inconsistencies in nozzle pressure.

As there are many different methods that you can employ to dry your spool of filament, ranging from using an electric oven to purchasing a filament dryer manufactured explicitly for the task, feel free to use the one you find to be the most convenient.

Even after drying the filament, we recommend keeping a close look at the printing process, as it won’t be possible to tell whether the drying process was successful or not without exposing the filament to the high temperatures of the nozzle.

What Can Cause TPU (Flexible) Filament to String?

Understanding the factors that can cause TPU to string and become a problem for the quality of your 3D printed models is an essential part of dealing with the problem, as it will allow you to spot the scenarios where TPU is likely to string before the issue takes place and act accordingly.

Here are the potential culprits that can create the issue of TPU stringing and explanations that describe how they can contribute to the occurrence of the problem:

  1. The retraction distance value is too low. A low retraction distance can cause the TPU to stay way too close to the nozzle as the printhead travels, potentially creating a situation where the filament leaks out.
  2. The retraction speed value is too low. A low retraction speed value can cause the TPU to leak out as retraction is taking place, creating a blob that the nozzle may pick up and drag across the space as it travels.
  3. The retraction minimum travel value is too high. A high retraction minimum travel value can reduce the frequency of the retractions to a point where the TPU is not being retracted before long travel move sequences where filament can easily ooze.
  4. The print temperature value is too high. A high print temperature value will cause the TPU to melt and liquefy way too rapidly, increasing the chance that it leaks out of the nozzle uncontrollably.
  5. The travel speed value is too low. A low travel speed value causes the printhead to move way too slowly as it travels from one point to the other, which can end up with the TPU eventually oozing out of the nozzle due to the long time spent in the air.
  6. The nozzle is partially blocked. A partial blockage in the nozzle can cause the melted plastic to stick to it, causing the plastic to travel with the nozzle instead of sticking to the intended spot.
  7. The filament has absorbed moisture. Filament that has absorbed moisture can cause the nozzle pressure to become higher due to steam being generated as a result of the water being exposed to high temperatures, forcing filament out of the nozzle inadvertently.

Is It Possible to Successfully Print TPU (Flexible) Filament With a Bowden Extruder?

It’s no secret that TPU filament and Bowden extruders don’t play well due to the flexible nature of TPU filament, which makes it difficult for the filament to move smoothly within the PTFE tube, potentially creating issues such as extruder jamming in the process.

That being said, even though we can’t deny that it will be a challenging process, it is indeed possible to successfully print TPU filament with a Bowden extruder while avoiding any of the issues that stem from the incompatibility between TPU filament and the PTFE tube by utilizing the correct setup and configuration.

For best results, our recommendations would be to minimize the distance that TPU will need to travel in the PTFE tube as much as possible (which can be achieved by bringing the extruder closer to the hotend), to ensure that the PTFE tube follows a straight path between the extruder and the hotend, and to use TPU filament with higher shore hardness whenever possible.

Wrapping Up

Stringing is, without a doubt, an annoying issue to face due to its capability to completely ruin the aesthetic quality of your 3D prints, and the fact that TPU filament is more likely to string than its alternatives does not help things either.

To quickly recap, optimizing the main retraction parameters, decreasing print speed, increasing travel speed, printing with a clean nozzle, utilizing a dry spool of TPU, and utilizing the anti-stringing features of Cura, such as coasting, combing, and wiping, can all contribute to preventing TPU from stringing.

As there will be a considerable amount of trial and error involved in configuring each of the related parameters as optimally as possible, we highly recommend running a test after each modification and carefully observing the results to find out how it affected the severity of the stringing problem.

Happy printing!