Flexible Filaments 101

tpu flexible filament header
Courtesy: Felfil

In this article, we will be exploring Flexible material. Flexible filament is an interesting elastic filament in a way that it is the only type that offers an alternative to rubber and can be used for such applications.

It has an important place in the entire material library of FDM 3D printing. We will explore the flexible material through this article.

What is flexible Filament?

Flexible filaments are elastomers and often found as Thermoplastic Elastomers (TPE) which is a blend of soft rubber and hard plastic. It generally refers to the complete range of flexible filaments and is a non-specific name. Thermoplastic Polyurethane (TPU), however, is a rigid-flexible filament and the most common type of TPE filament. It is more rigid than TPE. TPU is a newer formulated material while TPE is in use for a long time.

TPU is a block copolymer consisting of alternating sequences of hard and soft segments. It has high flexibility, good durability offering good resistance to abrasion and impact. It also offers great rigidity which means that it can bend upon application of force and easily return to its original shape with no deformation.

The most common applications of TPU is in automotive (gear knobs, instrument panels, or console parts), Agriculture (animal identification tags used for livestock), Seals & Gaskets, conveyor belts, inflatable items, or military equipment, wires, and cables, breathable films for leisure, sports, and grips for instruments and equipment, etc.

Understanding TPU 3D Printing Filament

TPU is the most commonly used flexible filament in 3D printing. It is a flexible material offering elastic properties to the 3D printed products. The TPU filament exhibits good impact resistance, durability, and flexibility.

 It also has an excellent vibration dampening property making it suitable for applications requiring the use of a dampening agent for heavy machinery. Besides, it has a long shelf life.

TPU exhibits a higher shore hardness – a measure of a material’s resistance to deformation, than TPE. It lies anywhere between 60A and 55D.

While TPU offers great advantages, it also brings some amount of difficulty in 3D printing. It is quite difficult to print and imposes multiple printing restrictions.

 It exhibits poor bridging characteristics, TPU printed parts show stringing and blobs, it cannot be printed with Bowden extruders and if any printer has such arrangement then the same has to be detached before TPU can be printed.

Applications of TPU 3D printing filament include vibration dampening products, grips, phone cases, car tires for toys and similar projects, stamps, etc.

General Printing Settings for TPU Filament

  • Extruder Temperature: 220oC to 230oC
  • Printing Speed: ~ 30 mm/sec
  • Heated Bed: 50oC (Not necessary but can be used)
  • Enclosure: Not Necessary

Filament Printing Tips

PETG requires very specific print settings and conditions and thus users need to carry out considerable trial and error to determine the perfect settings for their printer-material combination. 

We will look at how to take the necessary precautions to experience a successful PETG print every time.

1. Direct Drive Extruders

extruder types graphic

Direct Drive vs Bowden Extruder/Courtesy: Recreus

In direct drive, the filament is directly fed into the nozzle. The action of the extruder gears (whether it pulls the filament inside the nozzle or out of it as in during retraction) is immediately conveyed to the filament and there is no lag. This especially helps in the case of TPU filament.

The lag can cause the material to create a blockage in the Bowden tube in case of the Bowden extruder system leading to a failed print. The shorter path to the nozzle ensures a successful print.

So, TPU should always be used with printers having direct-drive extruders.

2. Printing Speed

PETG can give out the best printing results without the use of a fan. Higher the PETG filament temperature, better will be the layer adhesion. But for prints with finer details, a cooling fan is necessary. So, the best recommendation is that users turn off the fan for the initial few layers and then use a fan.

3. Retraction Settings

For best results, retraction should be kept zero while printing with TPU filaments. Retraction can cause stringing and, sometimes, even nozzle clogging and so it should be avoided.

4. Avoid Bed Adhesion Tools

TPU filament shows excellent bed adhesion, better than most materials, and so bed adhesion tools like raft and brim should be avoided. It should also be avoided as the raft and brim tools require higher extrusion rates and this can cause a problem for TPU printing.

Benefits of TPU Filament

  • Elasticity: TPU is a great replacement for rubber materials in 3D printing. It can easily be bent, twisted, and pressed making it suitable for specific applications that cannot be fulfilled through other FDM filaments.
  • High Rigidity: TPU filament offers great rigidity which means it can regain its original shape even if it is twisted.
  • Minimal-to-No Warping: TPU does not exhibit warping issues that are generally seen in other FDM filaments like ABS, Nylon, etc.
  • Wide-Ranging Applications: TPU serves a very specific set of applications like grips, knobs, dampening applications, products requiring high elastic properties, etc.

Drawbacks of TPU Filaments

  • Specific Requirements: TPU filament printing requires some very specific settings and arrangements like only using direct drive extrusion system, modifying the retraction settings, calibrating the distance between the nozzle and the bed, etc. This can be tedious work for users if they ever want to print with flexible material.
  • Specific Applications: TPU is not a general-purpose material and has very defined and limited applications. This can limit the use of the material.
  • Stringing: As mentioned earlier, TPU is susceptible to a high amount of stringing, and is proper settings are not used then the stringing can create problems.
  • Difficult for New Users: New users may find the material difficult to print with as it is not an off-the-shelf printing material.
  • Hygroscopic: Like a lot of thermoplastic materials, TPU is also hygroscopic, that means, it absorbs moisture.
  • Dimensional Accuracy: TPU has a problem when it comes to printing dimensionally accurate parts. Moreover, it cannot be easily post-processed and thus the layer lines cannot be smoothened through sanding or other post-processing techniques.

TPU Filament Price

TPU filament is sold by numerous manufacturers and the price tends to fall anywhere around $25 to $50 per spool.

Verdict

TPU has very specific applications that cannot be sufficed by any other FDM filament and this makes it an important material in the entire library. It offers good elasticity and rigidity that makes the 3D printed products quite durable.

However, this material is generally used by beginners to create products like phone cases, stamps, grips and knob covers, etc. It has recently found important applications in the automotive, footwear, medical sector, and specific products like wearable devices, seals, gaskets, etc. The material will only gain in popularity as newer applications are continually explored.

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