Flexible 3D printer filaments: is PLA flexible?

An FDM 3D printer is a great tool for making all manner of parts and products. The technology is particularly notable for its ability to create hollow or complex geometries on demand. However, the geometry and design of a print is only part of the equation, materials are a major consideration when planning a 3D printing project. The choice of 3D printing filament will influence all sorts of factors, including the cost of a project, the appearance of a print, and, most importantly, the mechanical properties of the final part. For example, if you are looking to make a flexible object, such as a wearable or gasket, it is imperative that you work with a filament that has flexible properties.

Polylactic acid (PLA), while the most commonly used filament today, is not known for its flexibility. In fact, the material is highly rigid and in some cases even brittle. This means that it can fracture when stressed, with little deformation before breaking. In this article, we’ll be diving deeper into PLA’s properties to understand if it can be more flexible. We’ll also be looking at flexible filaments that can be used as an alternative to PLA when flexibility is needed.

PLA filament properties 

Neat PLA filament is a rigid thermoplastic known for its easy printability and high strength. Despite a tensile strength of around 50 MPa (ABS, for comparison has a tensile strength of about 30 MPa), PLA is rigid, offering little flexibility and can actually become more brittle if subjected to stress over time or under extreme conditions. To put that into perspective, PLA has among the lowest elongation at break measurements of the common 3D printing filaments. 

PLA is the most commonly used 3D printing filament, though it is not known for its flexibility.

What is elongation at break? Elongation at break is a material property that refers to the ability of a material to stretch or elongate before it breaks or fractures. It is typically expressed as a percentage of the material's original length and is a measure of ductility or plasticity. In a standard tensile test, a sample of the material is pulled in tension until it breaks. The elongation at break is calculated by measuring the change in length of the sample and dividing it by the original length, then multiplying by 100.

Of course, the specific elongation at break measurement varies depending on the brand of filament used, print settings such as infill and print speed, and geometry of the 3D print.[2] Generally speaking, however, this is how the most common 3D printing filaments compare when it comes to flexibility:

Another key property of PLA is its low melting point and glass transition temperature. This means that the material is easy to process on a desktop FDM 3D printer, since it does not necessarily require a heated print bed or high nozzle temperatures (PLA prints best between 200-220 °C). However, the polymer’s low glass transition temperature of 60 °C also means that printed PLA components cannot be used in applications that require heat resistance. 

Recommended reading: Food-grade 3D printing: Is PLA Food-Safe?

PLA+, Flexible PLA, and Soft PLA

As we’ve seen, neat PLA is not known for its flexibility, however there are some PLA 3D printing products that have been engineered for increased flexibility through the use of special additives. For example, PLA+ (or PLA Plus) is an enhanced version of PLA that integrates plasticizers and toughening agents for better impact resistance and some degree of flexibility. Generally, PLA+ is not highly flexible, but it is somewhat more flexible than standard PLA. It is also designed to be tougher, more impact-resistant, and less prone to breaking under stress. For applications where true flexibility is required, however, PLA+ is still limited. For example, eSun’s PLA+ 3D printing material has an elongation at break of 20%, comparable to ABS.

Flexible PLA is a variation of PLA mixed with flexible materials, such as thermoplastic elastomers (TPE) or thermoplastic polyurethane (TPU). It retains the ease of use associated with PLA, offering low printing temperatures and reduced warping. Flexible PLA filament can stretch and return to its original shape, making it suitable for gaskets, wristbands, and protective covers or phone cases. While it’s easier to print than highly flexible filaments like TPU or TPE, it is still more rigid than those materials.

Soft PLA is another type of filament that typically combines PLA with other additives to improve elasticity and bendability, making it ideal for items like soft toys, flexible parts, or protective covers. However, "soft" in this context doesn't mean highly flexible—the material is still relatively rigid compared to true flexible filaments. It’s worth noting that while Soft PLA is more flexible than neat PLA, the material does not have the same elasticity as elastomers such as TPU.

Recommended reading: TPU vs PLA: Choosing Between Flexible and Rigid Filament for 3D Printing

Flexible alternatives to PLA filament

For applications that require a high degree of elasticity, such as footwear, orthopedic models, tubes, seals, and more, there are elastomer filaments like TPU and TPE to consider.

Flexible filaments are useful for 3D printing wearables, such as footwear.

TPU and TPE Filaments

TPU (thermoplastic polyurethane) is a highly flexible, rubber-like 3D printing filament known for its elasticity, strength, and abrasion resistance. It is a type of thermoplastic elastomer (TPE), meaning it combines the flexibility of rubber with the processing ease of a polymer. TPU can stretch significantly and return to its original shape, making it ideal for parts that require high durability, impact resistance, and flexibility.

Key Features of TPU:

• Flexibility & Elasticity: TPU can stretch in the range of 600% of its original length without breaking, making it perfect for flexible and elastic parts like phone cases, shoe soles, gaskets, and bumpers.

• Durability: TPU is highly abrasion-resistant, tear-resistant, and impact-resistant, which gives it a long lifespan even in harsh conditions.

• Chemical Resistance: It also has good resistance to oils, greases, and various chemicals.

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

TPE (thermoplastic elastomer) is a flexible 3D printing filament that combines the elasticity and rubber-like properties of traditional elastomers with the processability of thermoplastics. Like TPU (a similar material), TPE is designed to provide high flexibility, stretchability, and reliability in applications requiring soft, bendable parts.

Key Features of TPE:

• Flexibility and Softness: TPE can stretch significantly, offering rubber-like elasticity. It is commonly used to print parts like soft toys, wearables, gaskets, flexible hinges, and grips.

• Durability: TPE is abrasion-resistant, impact-resistant, and has excellent fatigue resistance, making it suitable for items that endure frequent bending or compression.

• Chemical Resistance: TPE is resistant to oils, greases, and various chemicals, offering good durability in environments where such substances are present.

The main drawbacks of using elastic 3D printing filaments is that they are a) more expensive than PLA spools and b) they are more challenging to 3D print. Specifically, TPU and TPE’s elasticity can cause issues like stringing, clogs, or jamming, especially with Bowden extruders, which struggle to push flexible filament through the tube. Direct-drive extruders are therefore recommended for smoother printing. Flexible filaments also require slower print speeds (typically 20-30 mm/s) and lower retraction settings to avoid extrusion problems. Additionally, TPU and TPE print at higher temperatures compared to PLA. TPE and TPU also come with more limited post-processing options that more rigid filaments.

Ultimately, while PLA is user-friendly and forgiving, TPU demands more careful settings and attention during printing for optimal results. Below are some 3D printing tips for flexible filaments:

3D Printing with TPU:

• Print Settings: TPU requires slower print speeds (typically around 20-30 mm/s) to prevent issues like stringing or clogging. It also requires lower retraction settings due to its flexibility.

• Print Temperature: TPU filament prints best at 224–250 °C.

• Extruder Compatibility: Direct-drive extruders are generally recommended for TPU, as Bowden setups may struggle with the filament’s flexibility.[1]

TPE and TPU can be tricky to 3D print and makers should be aware of problems like stringing and nozzle clogs.

Polypropylene (PP)

Polypropylene (PP) filament is flexible, but it has a distinct set of characteristics compared to other flexible filaments like TPU or TPE. Polypropylene is a lightweight, durable, and flexible thermoplastic known for its high impact resistance, chemical resistance, and fatigue resistance. It can bend and stretch without cracking, making it ideal for applications like hinges, living hinges, containers, and flexible parts that need to withstand repeated stress.

In tests, PP filament has demonstrated an elongation at break in the range of >800% and an elongation at yield in the range of 35%.[3] It also tends to be more challenging to print due to its low surface adhesion and tendency to warp. PP generally requires a heated bed (around 100-110°C) and sometimes a heated chamber for optimal results. 

Conclusion

In the end, while PLA is a popular and easy-to-use filament, it is not particularly flexible and can become brittle under stress. For applications requiring flexibility, alternatives like PLA+, Flexible PLA, and Soft PLA offer some improved flexibility, though they are still not as elastic as elastomeric materials. TPU and TPE are a suitable option for applications that require flexibility, such as phone cases, gaskets, or wearables. However, they come with their own set of challenges, including difficult printing conditions, higher temperatures, and more intricate print settings. Materials like Polypropylene (PP) offer flexibility with high impact and chemical resistance but can also be tricky to print. Ultimately, the choice of filament depends on the specific requirements of the project, including flexibility, durability, and ease of printing. For maximum flexibility and durability, TPU and TPE are recommended, but users should be prepared for a steeper learning curve compared to PLA.

Frequently Asked Questions (FAQs)

Q: Is PLA flexible?

A: No, PLA is a rigid filament and is not known for its flexibility. It is brittle and can break under stress with little deformation. It has a low elongation at break (1-6%), meaning it doesn’t stretch much before snapping.

Q: What is PLA+ and how flexible is it?

A: PLA+ (or PLA Plus) is an enhanced version of PLA that incorporates toughening agents and plasticizers for better impact resistance and some flexibility. PLA+ has an elongation at break of around 20%, making it tougher and more resilient than standard PLA but still not highly flexible.

Q: What is Flexible PLA?

A: Flexible PLA is a blend of PLA mixed with elastomers like TPU or TPE, making it more flexible than regular PLA. It is suitable for items like gaskets and phone cases. However, it is still more rigid than true flexible filaments and lacks the stretchability of materials like TPU or TPE.

Q: What are the challenges of 3D printing with TPU and TPE?

A: TPU and TPE filaments are more challenging to print compared to PLA due to their flexibility, which can cause issues like stringing, clogs, and jamming, especially with Bowden extruders. These filaments require slower print speeds (around 20-30 mm/s) and lower retraction settings to avoid extrusion problems. Additionally, TPU and TPE require higher print temperatures and are less forgiving during the post-processing stage compared to rigid filaments like PLA.

References

[1] Flexible Materials Guide [Internet]. Simplify3D, 2024. Available from: https://www.simplify3d.com/resources/materials-guide/flexible/ 

[2] Leite M, Fernandes J, Deus AM, Reis L, Vaz MF. Study of the influence of 3D printing parameters on the mechanical properties of PLA.

[3] Ultimaker PP Technical data sheet [Internet]. UltiMaker, April 20, 2022. Available from: https://um-support-files.ultimaker.com/materials/2.85mm/tds/PP/Ultimaker-PP-TDS-v3.00.pdf