Polylactic acid (PLA) is one of the most popular materials used in 3D printing today. Its low cost, ease of printing, and biodegradable nature make it an accessible option for hobbyists and professionals alike. However, to achieve optimal print results with PLA, it is important to understand the behavior of the material at various temperatures – specifically, the glass transition temperature.
What is the Glass Transition Temperature?
The glass transition temperature (Tg) is the point at which an amorphous polymer like PLA transitions from a hard, glassy state to a rubbery, flexible state as it is heated. Below the Tg, polymer chains are frozen in place, lending rigidity to the material. Above the Tg, polymer chains become mobile, allowing the material to soften and take on new shapes.
For PLA, the glass transition temperature is typically between 60-65°C. This means that below 60°C, PLA remains rigid and glass-like. Once the material is heated above 60-65°C, it will begin to soften as polymer chains gain mobility.
Why Understanding Tg Matters for 3D Printing
When 3D printing with PLA, the glass transition temperature determines the upper temperature limit for the material. If PLA becomes too hot and exceeds its Tg during printing, the material will lose its rigid structure and begin to deform under its own weight. This can lead to warping, shrinking, and layer adhesion issues.
Conversely, if the temperature is too low, the PLA will remain glassy and brittle, resulting in cracks and splits during printing. Staying within the optimal temperature window – above the minimum extrusion temperature but below the Tg – is key to achieving clean, strong prints with good layer adhesion.
The Tg also determines the maximum use temperature for finished PLA prints. Once an object is printed, heating it above the Tg will cause the PLA to soften and lose its shape. Understanding this thermal behavior allows makers to select PLA for appropriate applications and operating conditions.
Factors that Influence PLA’s Glass Transition Temperature
While 60-65°C is the commonly cited glass transition temperature range for PLA, several factors can influence the actual Tg:
1. Molecular Weight
Higher molecular weight PLA polymers have a higher Tg, increasing rigidity. Lower molecular weights are more flexible.
2. Isomer Ratio
PLA contains both crystalline (L-lactide) and amorphous (D-lactide) regions. A higher crystalline ratio raises Tg.
3. Additives
Colorants, fillers, and other additives can either raise or lower Tg depending on type and concentration.
4. Processing
Orientation of polymer chains during extrusion and layer adhesion in printing impacts mobility and Tg.
5. Ambient Conditions
Exposure to humidity can plasticize PLA, lowering its Tg. Annealing can increase crystallinity and rigidity.
Signs of Exceeding the Glass Transition Temperature
When printing PLA, several visible issues can emerge if the Tg is exceeded:
- Warping – Corners of the print lift and detach from the print bed as soft PLA deforms.
- Shrinking – Printed objects become smaller than designed as warm PLA layers contract.
- Layer Separation – Layers detach and split apart rather than fusing together cleanly.
- Stringing – Molten filament oozes excessively and creates messy thin strings.
- Jamming – Soft, rubbery PLA can jam extruders if temperatures are too high.
Monitoring for these defects and adjusting temperatures accordingly can help dial in optimal settings.
Techniques to Avoid Exceeding the Glass Transition Temperature
Here are some best practices when printing with PLA to maintain temperatures below the Tg:
- Print in a Cool Area – Prevent ambient temperatures from softening PLA.
- Optimize Bed Temperature – Heat bed enough to aid adhesion, but not so high as to soften layers. 50-60°C is generally recommended.
- Adjust Nozzle Temperatures – Start low at 190-195°C for finer prints, 200-210°C for faster prints.
- Enable Cooling Fans – Active cooling keeps deposited material below Tg before next layer.
- Slow Print Speeds – Slower printing allows more cooling time and avoids overheating.
- Dry Filament – Wet PLA has lower Tg, increasing risk of softening.
Annealing PLA Above the Glass Transition Temperature
While exceeding Tg during printing should be avoided, annealing printed PLA parts above Tg can actually improve properties once the print is finished.
Annealing involves reheating a completed print to just below the melting point, which allows polymer chains to relax and orient themselves. For PLA, a temperature of 80-100°C is typically used.
Benefits of annealing PLA include:
- Increased Strength – Better polymer alignment improves mechanical properties.
- Reduced Internal Stresses – Annealing relaxes stresses built up during printing.
- Smoother Surfaces – Partial melting creates a glossy, smooth finish.
- Higher Heat Resistance – Enhanced crystallinity increases Tg for better high-temp performance.
With some experimentation, PLA parts can be annealed to achieve optimum mechanical or aesthetic properties.
Signs of Printing Below Glass Transition Temperature
Printing PLA at temperatures too far below the glass transition point can also create issues:
- Cracking – Brittle, glassy PLA can crack during printing or when removed from the bed.
- Poor Layer Adhesion – Layers separate easily with minimal contact area when deposited.
- Weak Infill – Infill fails to adhere well to perimeter with low heat and pressure.
- Clogging – Cool temperatures impede flow and increase viscosity, blocking nozzles.
- Rough Texture – Without warmth, layers cannot reflow together into a smooth surface finish.
While staying below Tg is important to avoid deformation, sufficient heat is necessary to produce strong, high-quality PLA prints.
Optimizing Print Temperatures Based on Environmental Factors
The ideal nozzle and bed temperatures for printing PLA depend on ambient conditions in the print environment:
- In Cool Settings – Use higher temps up to 220°C for the nozzle and 70°C for the bed.
- In Warm Environments – Reduce temperatures down to 190°C nozzle, 50°C bed to prevent softening.
- With Enclosed Printers – More insulation retains heat, allowing lower temperatures.
- With Heated Chambers – Significantly higher ambient heat requires adjusting down extruder temperatures.
- With Air Conditioning – Colder surrounding air necessitates hotter print temperatures.
- At Higher Altitudes – Thinner air makes cooling less efficient, so reduce temperatures.
- With Humidity Control – Dry conditions allow higher temps without softening PLA.
Fine-tuning temperatures for specific conditions is key to dialing in the optimal print quality.
Selecting the Right PLA Filament for Your Application
With all PLA filaments having a Tg in the 60-65°C range, material choice comes down more to factors like quality control, durability, appearance, and value. Nonetheless, slight Tg variations can help guide selection:
- For High Heat Applications – Choose PLAs with higher molecular weights or L-lactide ratios to increase Tg.
- For Flexibility – Amorphous-rich PLAs with lower Tg allow more flex without fracturing.
- For Annealing – Opt for higher purity PLAs to achieve the best heat-stabilized properties.
- For Outdoors – Weather-resistant PLAs with UV inhibitors prevent environmental degradation over time.
- For Food Safety – Medical-grade, FDA-compliant PLAs enable direct food contact.
Within the PLA family, different formulations exist to suit distinct use cases where thermal behavior is a priority.
Is PLA’s Glass Transition Temperature a Major Limitation?
While PLA’s relatively low Tg compared to other thermoplastics introduces some printing challenges, the material remains extremely popular due to its ease of use and safety. For applications where temperature resistance and durability are priorities, materials like ABS, PETG, and nylon may be better suited.
However, with the right technique PLA’s thermal properties are manageable for a wide range of applications. By understanding and respecting the glass transition temperature during printing and use, beautiful and functional PLA objects can be created successfully. With continued material innovation, glass transition limitations may also become less constraining in the future as well.
So for makers seeking an entry into 3D printing, PLA’s qualities make it a great place to start learning despite its temperature sensitivity. Just be mindful of ambient conditions, fine-tune print settings, and avoid exposing finished prints to excessive heat. With prudence and practice, PLA can become a staple material for desktop fabrication.
