The difference between rigid and flexible parts is significant. Imagine if the caster wheels on your office chair were rigid and brittle, or if your wrench was made of rubber: you’d have scratched floors, and you’d never be able to loosen a nut from its bolt. The objects simply wouldn’t perform as they’re supposed to.

Fortunately, FDM 3D printing accommodates the printing of both rigid and flexible parts. For rigid parts, the most common entry-level material is PLA, a low-cost and versatile thermoplastic with good strength. And there are plenty of other options too: POM and PVC are both great for rigid 3D printed parts, while ABS is solid but with a touch more flexibility than PLA.

When printing flexible parts using an FDM 3D printer, the options are more limited, but you can still achieve excellent results. One of the best flexible filaments is TPU, material from the TPE family with good durability, excellent abrasion resistance, and a high level of flexibility. TPU is harder to print than PLA but easier than other flexible materials.

In some cases, the choice of PLA vs TPU is clear and obvious: if flexibility is what you need, TPU is the way to go. In other situations, however, it can be a more difficult decision. This article looks at the respective material characteristics of PLA and TPU 3D printer filament, giving tips on which to choose in certain situations.

What is PLA filament?

Polylactic acid, abbreviated as PLA, is the most widely used 3D printing filament for the fused deposition modeling (FDM/FFF) printing process. Cheap, widespread, and biodegradable (it can be made from natural resources like corn starch or sugar cane), PLA nonetheless offers a good level of strength that is suitable for most prototyping projects.[1]

PLA’s popularity in the world of extrusion 3D printing is also helped greatly by its low printing temperature, layer adhesion, and overall printability. Compatible with extruder temperatures as low as 180 °C and not requiring a heated bed, a spool of PLA is very easy to print, even at fast printing speeds. To improve bed adhesion, the print bed surface can be lined with painter’s tape, PEI, or glue stick.[2] In general, even beginner-level hardware can process PLA without issue — something that can’t be said of engineering materials, high-performance materials, or even other common 3D printing materials like TPU.

The flip side of PLA’s printability is its low level of temperature resistance. A low melting point makes for easy printing, but it means that printed parts are prone to deformation and warping when subjected to high temperatures. Because of this, PLA is useless for end-use parts that go near heat (parts located near automotive engines, for instance). The material has other limitations too: while its rigidity can be a benefit, its brittleness makes it unsuitable for load-bearing applications.

PLA filament is widely used to make 3D printed prototypes, consumer products, biodegradable medical devices, electronics housings, and plant pots.

3D printed plant pots

PLA filament properties

• Printing temperature: 180–230 °C

• Print bed temperature: Unheated or 60–80 °C

• Medium strength

• Low flexibility

• Medium durability

What is TPU filament?

Thermoplastic polyurethane, known as TPU, is a flexible material that is part of the thermoplastic elastomer (TPE) family. Less elastic than other TPEs, TPU is highly durable, strong, and easier to extrude than other flexible filaments.

TPU behaves partly like plastic, partly like rubber, and there’s a good reason for this: the material is a block copolymer with alternating sequences of isocyanates (hard) and reacted polyol (soft). The ratio between these hard and soft segments can be altered by the material manufacturer, and this ratio ultimately determines the flexibility of the TPU.[3] Some applications demand a softer TPU material, while others are better with a firmer one.

Although TPU is not the only flexible filament for FDM 3D printing, it is part of a small group. It also has very good abrasion resistance, chemical resistance (oil and grease), and fairly high shear strength. It is quite difficult to print (low print speeds are required) but still easier than other TPEs and exhibits a low level of shrinkage. It requires PEI or painter’s tape applied to the build surface.

Note that in other manufacturing processes (injection molding and stereolithography, for example) flexible parts are often made using thermosets: polymers that are liquid at room temperature and cured to become solid, after which they cannot be returned to their liquid state. There are plenty of flexible thermosetting materials, but these are incompatible with extrusion 3D printers since they cannot be melted and reformed. TPU is special because, although it is a thermoplastic, it offers material properties comparable to elastic thermosets.

TPU filament is often used to make parts like caster wheels, phone cases, vibration dampeners, wearables, gel shoe orthotics, and soft-grip handles. Highly elastic TPU formulations can be used for items like gaskets and tubing.[4]

3D printed shoe

TPU filament properties

• Printing temperature: 225–235 °C

• Print bed temperature: Unheated

• Low strength

• High flexibility

• Medium durability

TPU vs PLA: Which one to choose?

When faced with the choice of PLA vs TPU, 3D printer users are confronted with one obvious fact: TPU is a flexible filament, while PLA is a very rigid one. But there are other factors to consider too.

Flexibility and rigidity are the main differentiating factors between TPU and PLA, so we’ll start our comparison there. Having read the previous section, it should be evident that TPU is a great material for making flexible, bendy, elastic, rubbery, or squishy 3D printed parts, while PLA is not. So if you need a 3D printer filament for an item like a shock absorber, a gasket, or a protective phone case, TPU may be an excellent choice. Conversely, if you need a stiff plastic component, PLA is the better option.

Of course, you might be making parts that do not require a particularly high level or flexibility or stiffness. In that case, you might consider other important factors such as heat resistance: TPU requires a higher extruder temperature, but TPU parts consequently maintain their shape and function even at high temperatures. PLA parts, meanwhile, will begin to soften at around 60 °C so are only useful at low temperatures.

Another difference between TPU and PLA is its tensile strength, where PLA comes out on top. Although TPU offers better durability, polylactic acid is the stronger of the two 3D printing filaments. However, TPU prints have the upper hand when it comes to abrasion resistance.

Finally, the choice of PLA vs TPU may ultimately come down to simple factors like cost and printability. PLA wins out in both cases: TPU can be around three times more expensive than PLA on platforms like Amazon and is also more difficult to print — especially for beginners.

Conclusion

PLA and TPU are two very different materials, so they are not usually considered direct alternatives to one another. However, many 3D printer users will keep a stock of rigid 3D printing filament like PLA and a stock of flexible filament like TPU, putting a range of material properties at their disposal. To conclude, here is a simple table showing when to choose PLA and when to choose TPU:

References

[1] Humphreys R. From Corn to Poly Lactic Acid (PLA): Fermentation in Action [Internet]. Polymer Innovation Blog. Innocentrix, LLC; 2012 [cited 2022Jan3]. Available from: https://polymerinnovationblog.com/from-corn-to-poly-lactic-acid-pla-fermentation-in-action/

[2] 3D printing with PLA [Internet]. NinjaTek. 2021 [cited 2022Jan4]. Available from: https://ninjatek.com/learn/3d-printing-materials/pla/

[3] What is TPU? [Internet]. Lubrizol. [cited 2022Jan3]. Available from: https://www.lubrizol.com/Engineered-Polymers/About/What-is-TPU 

[4] Thermoplastic polyurethanes bridge the gap between rubber and plastics [Internet]. American Chemistry Council. 2002 [cited 2022Jan4]. 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