Why Your PLA Filament is Becoming Brittle (and How to Prevent It)

As a 3D printing enthusiast, you've probably encountered the frustrating issue of your PLA filament becoming brittle over time, or sometimes straight out of a new spool. You go to load a fresh roll, and snap! The filament breaks before it even reaches the extruder. This can be a real headache, leading to failed prints, clogs, and wasted material.

But what actually causes this brittleness, and more importantly, how can you prevent it? In this guide, we'll explore the science behind filament degradation, common mistakes that accelerate the problem, and practical solutions to keep your PLA in optimal condition.

What Causes PLA Filament to Become Brittle?

The main culprits behind brittle PLA filament are usually moisture absorption, UV exposure, and extreme temperature fluctuations. These environmental factors can break down the molecular structure of the PLA, making it more rigid and prone to cracking.

1. Moisture: The #1 Enemy of PLA

PLA is hygroscopic, meaning it actively absorbs moisture from the air. When PLA absorbs water, the water molecules infiltrate the polymer chains and cause hydrolysis, a chemical reaction that breaks down the material's molecular structure. This results in:

• Brittleness: The filament becomes stiff and snaps easily

• Print defects: Steam bubbles cause popping sounds, inconsistent extrusion, and poor surface finish

• Reduced layer adhesion: Prints become weaker and more prone to de-lamination

Manufacturers typically extrude filament directly onto spools, then dry and vacuum-seal them to ensure they remain in good condition. However, once the filament is opened and exposed to ambient air, it can start degrading quickly. Even unopened spools aren't immune as clear vacuum bags can slowly allow moisture ingress over extended periods (months or years), which is why you might occasionally receive a brittle spool straight from the factory.

2. UV Exposure

Sunlight (specifically UV radiation) breaks down the chemical bonds in PLA. Even indirect sunlight through a window can gradually degrade filament stored on a shelf. UV exposure causes:

• Photo-degradation: Polymer chains break down, making filament brittle

• Discoloration: Colours fade or yellow over time

• Surface degradation: The outer layer becomes chalky or rough

3. Temperature Fluctuations

PLA has a glass transition temperature of around 60-65°C. While your storage area probably isn't that hot, repeated temperature cycling can still cause issues:

• Thermal expansion and contraction: Repeated cycles create microscopic stress fractures

• Storage near heat sources: Avoid placing filament near printers, heaters, or in cars on hot days

4. Material Additives and Formulations

One common misconception is that brittleness always indicates poor quality filament. In reality, some of the highest-performing PLA filaments are actually more prone to brittleness due to their increased stiffness and rigidity. This is a trade-off for superior mechanical properties or specific visual finishes.

Filaments containing additives are more likely to be brittle:

5. Physical Stress and Rewinding

A common mistake that accelerates brittleness is rewinding filament onto a different spool. Here's why this matters:

When filament is manufactured, it's extruded in a semi-fluid state and wound directly onto the spool. In this state, it conforms to the spool's radius and "sets" in that curved shape. When you later unwind and rewind it onto a different spool, especially one with a different diameter you introduce mechanical strain.

This strain causes creep failure over time. The filament wants to maintain its original curvature, and the stress of being forced into a new shape creates micro fractures. The stiffer the filament (due to additives or moisture degradation), the more likely it is to snap during rewinding or develop brittleness later.

The golden rule: Never respool filament unless absolutely necessary. If you must, use a spool with the same or larger diameter than the original, and do it slowly.

How to Identify Brittle Filament

Not sure if your filament has gone bad? Look for these signs:

How to Prevent PLA Brittleness

1. Proper Storage: The First Line of Defense

Short-term storage (days to weeks):

• Keep filament in a cool, dry place away from direct sunlight

• If your printer is in a sunny room, cover your spools when not in use

• If you room have high humidity, its recommenced you store in seal box/bag/dry box. 

Medium to long-term storage (weeks to months):

• Use airtight containers with desiccant (silica gel packets)

• Re-sealable bags (many filaments come with them) are adequate if you squeeze out excess air or using silver mylar bags that sealable

• Budget solution: Large food-grade cereal containers with desiccant work surprisingly well

• Consider vacuum-sealing clear/mylar bags for very long-term storage

Pro tip: Monitor your storage humidity. Ideally, keep filament below 15-20% relative humidity.

2. Drying Wet Filament

If your filament has already absorbed moisture, all is not lost. You can dry it and often restore mechanical properties.

Recommended methods:

• Filament dryer: Purpose-built devices like the Creality Pi, Sunlu S4, or Polydryer are safest and most effective

• Food dehydrator: Use a dedicated dehydrator never used for food (dedicated for filament only)

• Oven: NOT RECOMMENDED as temperature fluctuations and hot spots can melt filament and can create fire hazards

Critical safety warning: Never use an oven or food dehydrator that you also use for cooking. Filament fumes and residues can contaminate your kitchen equipment and food.

Drying times and temperatures:
PLA typically dries at 45-50°C for 4-6 hours. For specific recommendations, check our detailed guide:

Filament Drying Time Recommendations

3. Avoid Rewinding

As discussed earlier, rewinding introduces mechanical stress. If you absolutely must re spool:

• Choose a spool with the same or larger diameter than the original

• Do it slowly and gently

• Expect that the filament may be more brittle afterward

• Consider printing a respooling jig to maintain consistent tension

Source: 3D Printing Solutions - Filament Rewinding is a Bad Idea

4. Choose the Right Filament for Your Needs

When selecting PLA filament, consider the trade-offs:

If you're experiencing brittleness with standard PLA, try switching brands or formulations. Some manufacturers offer "tough" or "durable" PLAs that incorporate flexibility additives to reduce brittleness while maintaining printability.

5. Rotate Your Stock

Like any material, filament has a shelf life. Practice FIFO (First In, First Out):

• Use older spools before opening new ones

• If you have multiple spools of the same color, mark purchase dates

• Consider buying smaller spools if you print infrequently

Can You Save Already-Brittle Filament?

If your filament is snapping but hasn't fully degraded, you may be able to salvage it:

1. Dry thoroughly: Run it in a filament dryer for 6-8 hours at 45-50°C some may require longer up to 24 hours or if you're dryer cannot reach those drying temperatures. 

2. Test a section: After drying, bend a sample, if it bends without snapping or less brittle, it's usable

3. Print a small test: If printing goes smoothly without popping or under extrusion, you're good to go

Brittle PLA Troubleshooting Quick Reference

Final Thoughts

Brittle PLA is frustrating, but understanding its causes helps you prevent it. The key takeaways:

• Moisture is enemy #1, store filament properly and dry when needed

• UV light degrades PLA, keep spools out of direct sunlight

• Some brittleness is by design, additives for strength or appearance increase stiffness

• Never respool unless absolutely necessary

• Choose filament based on your needs, standard PLA for flexibility, engineered PLAs for performance

By following these guidelines, you can help ensure your PLA filament stays in top condition and continues producing high-quality 3D prints for a long time to come.

Happy printing!