What is TPU Material in 3D printing: material properties, applications, and technologies

3D printing flexible parts can be a challenge. While technologies like injection molding can easily process liquid silicone rubber and various thermosets, 3D printing finds it more difficult to do so. Most 3D printed parts are therefore rigid.

That being said, there are several good options for 3D printing flexible parts. Stereolithography (SLA) 3D printers can print elastomeric resins, while advanced 3D printing technologies like HP’s Multi Jet Fusion can print specialized flexible materials to a very high standard. Silicone 3D printing has also shown promise in recent years — though it is still in its infancy.

But perhaps the best way to 3D print flexible parts is with thermoplastic polyurethane (TPU). Offering rubber-like material properties, TPU can be used with common 3D printing processes like fused deposition modeling (FDM), enabling virtually anyone to “stretch” the rules of 3D printing.

What is TPU material?

Thermoplastic polyurethane is a type of polyurethane plastic belonging to the thermoplastic elastomer (TPE) family. It is elastic like rubber but also strong like plastic.

On a chemical level, TPU is a block copolymer made up of alternating sequences of hard segments (isocyanates) and soft segments (reacted polyol). The manufacturer determines the ratio between them; the more soft segments, the greater the material’s flexibility. [1] Due to this variability, the shore hardness of TPU can range from 60A (similar to mild silicones) to 80D (equivalent to nylons or rigid PVC). [2]

In terms of practical use, TPU offers desirable material properties like flexibility, good tensile strength, and resistance to tears and abrasions. It has emerged as one of the best materials for 3D printing flexible parts, with many applications in consumer products and industrial parts. And unlike other TPE materials, it has good thermal stability and is fairly easy to print using an FDM 3D printer.

TPU is available in different forms to suit different manufacturing technologies. In 3D printing, manufacturers have developed it as both a filament (for FDM) and a powder (for SLS).

TPU material properties

TPU offers some material properties typically found in rubbers and others typically found in plastics. Its principal advantage is its merging of the powerful properties of rubber with the workability of plastics: it is much easier to 3D print than comparable elastomeric materials.

The main material properties of TPU include:

Elasticity

TPU is a flexible elastomer that can behave like rubber. Though it lacks the performance of true rubber, it can bend and compress. These properties also contribute to desirable attributes such as shock resistance and vibration dampening.

The elasticity of TPU can be controlled by modifying its composition. Some TPU materials are hard, while some are much more pliable.

High elongation and tensile strength

In comparison to other thermoplastics in the TPE family, TPU offers high tensile strength, elongation, and tear resistance. This makes it particularly valuable in demanding industrial applications — as a material for gaskets and hoses, for example.

Low-temperature performance

Unlike some flexible materials, TPU retains its elasticity even at low temperatures. This makes it useful for a range of temperature-specific applications, from vehicle components to winter sports products like ski boots.

Abrasion resistance

TPU is resistant to surface damage, hence its suitability for certain consumer goods. Its abrasion resistance can be further improved when it is reinforced with glass fibers or blended with polymers like PVC.

Chemical resistance

In addition to its resistance to abrasions, TPU is a good repeller of chemicals, especially oil and grease. Polyester TPUs are best for resisting oils, while polyether TPUs are typically used in moist environments (where the material’s hygroscopic properties must be minimized).

Pros and cons of TPU

TPU applications

The material properties of TPU — some of which are rarely found in 3D printing materials — make it suitable for a wide range of practical uses.

In the industrial sector, the durability and chemical resistance of TPU contribute to its use in products like gaskets, seals, and tubing (softer TPU), as well as items like caster wheels (harder). 3D printing giant Stratasys recommends its FDM TPU 92A material for products like air ducts and vibration dampeners. [3]

TPU is also a valuable 3D printing material in the automotive industry. Manufacturers use it for printed flexible parts like air intake hoses, side moldings, and seating sections, while its color adaptability allows manufacturers to make cosmetically suitable parts for different car models.

Although TPU is not as medically safe as silicone, the medical sector uses thermoplastic polyurethanes in hospital beds, gel shoe orthotics, and wearable monitor devices. The material can also be used as cable insulation for medical instrument cables.

Many consumer products utilize TPU. The durability and soft texture of printed TPU is particularly attractive for product designers, and the material can therefore be found in items like TPU footwear and raincoats, as well as protective casings for smartphones and other electronic products. Manufacturers can also produce soft-grip handles for products like power tools or toothbrushes using TPU.

The sporting goods industry is another beneficiary of TPU. The low-temperature performance and abrasion resistance of the material makes it ideal for winter sports products such as ski boots and in-line skating equipment.

TPU 3D printing technologies

The main additive manufacturing technologies used to 3D print TPU are fused deposition modeling and selective laser sintering. These technologies differ significantly: FDM is an extrusion process that processes spools of filament; SLS is a powder bed process that sinters particles of powder.

FDM is more widely used than SLS in the fabrication of TPU 3D printed parts (due to its overall market dominance), but both technologies have their respective advantages.

Note, however, that FDM and SLS are not the only 3D printing technologies used to print flexible materials. SLA and PolyJet 3D printers, for example, can print other types of flexible 3D printing material; SLA pioneer Formlabs recommends its proprietary Elastic Resin as a suitable alternative to 50–60A TPU.

Additionally, TPU and other elastomers can be processed with non-additive technologies like injection molding and CNC machining.

Fused deposition modeling

FDM is the most popular 3D printing process, largely thanks to its use in rapid prototyping and the consumer space. To print TPU with an FDM 3D printer, one can purchase TPU filament, popular varieties of which include NinjaTek NinjaFlex, Polymaker PolyFlex, and Stratasys FDM TPU 92A.

One of the major advantages of FDM TPU 3D printing is accessibility. FDM printers are affordable — far more so than SLS machines — which allows a range of non-industrial companies to fabricate TPU components.

A drawback of the technology is its print quality. FDM offers lower dimensional accuracy than SLS, and printed parts can show visible layer lines on the surface. For this reason, FDM is often used for TPU prototypes or non-cosmetic parts rather than end-use products.

Users of FDM technology should note that the printing of TPU (and other flexible filaments) requires special attention. To successfully print the material, the following guidelines should be observed:

• Use direct drive extruders: To prevent TPU coiling up within the extruder, users should print with a direct drive extruder as opposed to a Bowden extruder.
• Mount the filament spool above the extruder: During FDM printing, filament is gradually unwound from the spool as it is pulled by the extruder. But since TPU is elastic, pulling it can cause it to stretch rather than unwind the spool. By mounting the spool above the extruder, gravity reduces the required pulling force.
• Moderate print temperature: TPU filament prints best at 224–250 °C, according to material developers like NinjaTek. [4]
• Slow print speed: TPU does not print well at high speeds, because it can easily compress and cause jams, leading to print failure. A feed rate of no more than 20 mm/s works best.
• Disable or minimize retraction: Retraction is a useful feature of FDM 3D printers designed to prevent stringing and unwanted material blobs. However, TPU is sensitive to fast extruder movements, and retraction — during which the nozzle pulls back excess material — can negatively affect print quality.
• Lower layer heights: Lower layer heights permit slower feed rates, which lead to better TPU prints. Printing software company Simplify3D recommends layer heights in the range of 0.1–0.2 mm. [5]

Selective laser sintering

SLS is a powder bed additive manufacturing process widely used at professional and industrial levels; unlike FDM, there is no consumer market for it. Printing TPU parts with an SLS system requires TPU powders, such as DuraForm TPU Elastomer from 3D Systems or Sinterit’s FlexaSoft.

Although less accessible and affordable than FDM, SLS offers advantages such as improved dimensional accuracy, better mechanical performance, the ability to print complex overhanging sections, powder recycling options, and freedom from support structures (also eliminating the labor-intensive post-processing step of support removal).

The high cost of SLS hardware means that companies may be more likely to order on-demand SLS TPU parts from a 3D printing service bureau like Shapeways or Sculpteo. These companies can determine the required settings for a print themselves. However, as with flexible FDM filaments, SLS powders require special considerations. Service bureau Shapeways recommends the following design guidelines for parts printed with EOS TPU 1301:

• Minimum wall thickness of 0.7 mm
• Minimum supported wire diameter of 1.0 mm
• Minimum unsupported wire diameter of 2.0 mm
• Minimum 1.0 mm height/width for embossed details
• Minimum 1.5 mm height/width for engraved details
• Use of escape holes to facilitate powder removal: minimum 4.0 mm diameter (single hole) or 3.0 mm (multiple holes)
• Minimum clearance of 1.0 mm between individual parts on the print bed
• Minimum sprue thickness of 1.5 mm; at least two per part [6]

TPE vs TPU

We have looked at the properties, benefits, and applications of TPU, but what is the difference between TPU and TPE, another go-to flexible 3D printing material?

TPE, which stands for thermoplastic elastomer, is a family of flexible materials that includes thermoplastic co-polyester (TCP), thermoplastic polyamide (TPA), and — confusingly — TPU. The technical difference between TPE and TPU is therefore one of specificity: TPE is a group of several materials, while TPU is a specific material within that group.

So if TPU is simply a form of TPE, why is it that many material developers offer a direct choice between TPU and TPE 3D printing material? Unclear though it may be, this is a matter of common usage: in 3D printing, TPE tends to refer to softer, more flexible materials within the TPE spectrum, while TPU is the harder, more durable option.

TPU is a more recent addition to the 3D printing materials market, and it is more commonly used for high-performance parts due to its superior durability. TPU is also easier to print on an FDM 3D printer than soft TPE, which is more prone to clogging and stringing. Perhaps for this reason, Stratasys manufactures TPE in SLS powder form and TPU in FDM filament form. [7]

Conclusion

Thermoplastic polyurethane is a versatile material suitable for a range of flexible 3D printing applications. Although its softness poses some difficulties for 3D printing technology, its benefits — durability, impact resistance, and variable elasticity — more than make up for it.

Users of FDM 3D printers should carry out several test prints to familiarize themselves with the behavior of TPU, after which they will be capable of printing durable, shock-resistant parts with ease. Meanwhile, companies that use service bureaus instead of their own 3D printers can dive in and order TPU parts (SLS or FDM) like any other.

References

[1] What is TPU? [Internet]. Lubrizol. [cited 2021Oct19]. Available from: https://www.lubrizol.com/Engineered-Polymers/About/What-is-TPU 

[2] Thermoplastic polyurethanes bridge the gap between rubber and plastics [Internet]. American Chemistry Council. 2002 [cited 2021Oct19]. Available from: https://prod.americanchemistry.com/industry-groups/center-for-the-polyurethanes-industry-cpi/resources2/library/thermoplastic-polyurethanes-bridge-the-gap-between-rubber-and-plastics 

[3] FDM TPU 92A: Flexible elastomer TPU 3D printing material [Internet]. Stratasys. [cited 2021Oct19]. Available from: https://www.stratasys.com/materials/search/fdm-tpu-92a 

[4] 3D printing with TPU [Internet]. NinjaTek. 2021 [cited 2021Oct19]. Available from: https://ninjatek.com/learn/3d-printing-materials/tpu

[5] Flexible [Internet]. Simplify3D. 2019 [cited 2021Oct19]. Available from: https://www.simplify3d.com/support/materials-guide/flexible

[6] TPU plastic 3D printing material information [Internet]. Shapeways. 2021 [cited 2021Oct19]. Available from: https://www.shapeways.com/materials/tpu 

[7] FDM TPU 92A: Flexible elastomer TPU 3D printing material [Internet]. Stratasys. [cited 2021Oct19]. Available from: https://www.stratasys.com/materials/search/fdm-tpu-92a