Annealing PLA is a post-processing step that involves heating 3D printed PLA parts to enhance their mechanical strength and thermal resistance. Compared to untreated PLA, annealed parts exhibit improved durability, making them more resistant to heat deformation and mechanical stress over time. 

This improvement in long-term performance makes annealing an appealing post-processing option for projects that demand reliability under challenging conditions. Particularly relevant for makers, hobbyists, and some engineers, this technique addresses common weaknesses in PLA, such as low heat tolerance and brittleness.

This article provides detailed insights into how to execute the annealing PLA process effectively, troubleshoot challenges, and maximize results for a variety of projects.

What is PLA?

PLA is a common thermoplastic material for 3D printing and other processes

Polylactic Acid (PLA) is a thermoplastic polymer derived from renewable resources like corn starch or sugarcane. It is one of the most popular materials used in 3D printing, particularly for Fused Deposition Modeling (FDM) printers. PLA’s widespread use stems from its ease of printing, low cost, and eco-friendly nature.

PLA filament is highly favored by beginners and professionals due to its low printing temperature (190–220°C), minimal warping, and ability to produce detailed, smooth prints. Unlike materials such as ABS or PETG, PLA requires neither a heated bed nor an enclosed printer, making it accessible for low-cost desktop 3D printers.[1] Additionally, it is available in a wide variety of colors and composite forms, such as PLA infused with wood, metal, or carbon fiber for aesthetic or functional enhancements.

One of PLA’s major drawbacks is its low heat resistance. PLA has a glass transition temperature of around 60°C, which means it begins to soften and deform at relatively low temperatures. This limitation arises from its molecular structure, which lacks the crystalline reinforcement seen in materials like ABS or polycarbonate.

The low heat resistance restricts PLA’s use for functional parts exposed to heat, such as car interiors or outdoor applications in hot climates. For higher thermal performance, heat-treated PLA or PLA blends are available, but these solutions often compromise some of PLA's printability advantages.

What Does Annealing Do to PLA?

Annealing 3D printed PLA involves heating the material to a specific temperature (typically 60–100°C) below its melting point and holding it there for a set time. This process alters the molecular structure, increasing crystallinity and improving the material's thermal and mechanical properties.[2]

In its printed state, PLA often has a mostly amorphous structure due to rapid cooling. Annealing allows polymer chains to realign into more orderly crystalline regions. This increased crystallinity raises the material’s heat deflection temperature (HDT), enabling it to withstand higher temperatures. On a simpler level, the heating of the plastic also creates stronger layer adhesion, as layers begin to blend into one another with fewer internal stresses.

Dimensional changes, such as shrinkage or warping, can occur during annealing as the internal structure reorganizes. These changes depend on factors like geometry, print settings, and annealing conditions. Parts with uniform infill and consistent wall thickness tend to anneal more predictably. Furthermore, high-temperature PLA variants (HTPLA) may exhibit less warping during annealing.

By improving heat resistance and strength, annealing makes PLA more suitable for demanding applications like automotive or outdoor use. However, care is needed to prevent deformation during the process.

Recommended reading: A Comprehensive Guide to PLA Melting Point and How it Influences 3D Printing

How to Anneal PLA Prints

Annealing PLA can enhance its thermal and mechanical properties by increasing its crystallinity. Follow these steps to anneal PLA prints effectively while minimizing risks of deformation.

Equipment Required

• 3D printed object: Ensure the print is clean and free of debris.

• Oven: A conventional or toaster oven capable of maintaining precise temperatures between 60°C and 100°C.

• Oven thermometer: To verify and monitor the oven’s temperature, as built-in settings can be inaccurate.

• Heat-resistant surface: A flat metal tray, glass dish, or ceramic plate to place the print on.

• Support material (optional): Sand, uncooked rice, or silicone molds to support complex geometries and prevent deformation.

• Heat-resistant gloves or tongs: For safely handling trays or prints during or after annealing.

• Calipers or ruler: To measure the part before and after annealing to assess dimensional changes.

1. Prepare Your Print

Start with a clean, well-printed object. Uniform wall thickness and consistent infill density help ensure even heat distribution during annealing. If possible, choose PLA formulated for annealing, as some blends are optimized for this process.

2. Preheat the Oven

Set your oven to a temperature between 60°C and 100°C. The ideal temperature depends on the PLA blend and the object itself, so it helps to conduct tests before annealing your final part. On the whole, the ideal temperature will be the highest possible temperature before the material begins to deform to an unacceptable level.

3. Place the Print on a Stable Surface

Put your print on a flat, heat-resistant surface like a metal tray or glass dish. If deformation is a concern, support complex shapes with sand or uncooked rice, which helps maintain the geometry during heating.

4. Anneal the Print

Once the oven reaches the desired temperature, place the tray with your print inside. Leave it in the oven for 15 to 60 minutes, depending on the part's size and thickness. Avoid sudden temperature changes that could cause warping. Alternatives to oven baking the PLA include simply using a heated print bed (and covering the print with a bowl) or using a sous vide immersion heater.

5. Cool Down Gradually

Turn off the oven and allow the print to cool to room temperature inside to prevent thermal shock. Rapid cooling can cause cracks or deformation. Once the oven is cool to the touch, remove the print.

6. Inspect the Results

After annealing, check for any dimensional changes, such as shrinkage or warping. Measure critical dimensions to ensure they meet your requirements. Keep in mind that some slight deformation is normal, especially for complex geometries.

Tips for Success

• Use test prints to fine-tune your annealing process.

• Avoid convection ovens, as airflow can lead to uneven heating.

• Monitor the process closely to prevent overheating, which could melt or deform the part.

Annealing Time

PLA annealing times typically range from 15 to 60 minutes, depending on the part’s size and thickness. Smaller or thinner parts require less time, while larger or bulkier pieces need more. Monitoring is important to ensure even heat treatment and to prevent overheating, which can cause warping or deformation.

Annealing Temperature

Annealing temperatures for PLA typically range between 60°C and 100°C, depending on the filament's composition and other factors. It helps to use an accurate oven thermometer to maintain a consistent temperature. Too low won’t effectively increase crystallinity, while too high risks melting or deforming the print. Manufacturer guidelines can help determine the optimal range.

Shrinkage, Warping & Other Considerations

Always measure the annealed prints to check for dimensional changes

Shrinkage and warping are common concerns when annealing PLA prints, arising from changes in the material's internal structure. As PLA heats during annealing, its polymer chains reorganize into a more crystalline arrangement. This densification process can cause the material to contract, leading to dimensional changes or shrinkage. Uneven heating or cooling exacerbates the issue, causing some areas to shrink more than others, which results in warping or distortion.

The degree of shrinkage varies based on PLA composition, print geometry, and annealing conditions. Standard PLA typically shrinks by 1–2% with the aforementioned annealing times and temperatures, but high-crystallinity or annealing-specific PLAs may exhibit less shrinkage. Parts with uneven wall thickness or complex geometries are more prone to warping due to differential stresses during heat treatment.

To offset these effects, careful preparation is crucial. Supporting the print with sand or uncooked rice during annealing may help stabilize its shape, particularly for intricate designs, though there is debate over the effectiveness of this method.

Recommended reading: The Ultimate Guide to 3D Printing Post-Processing Techniques

How Do Other Filaments React to Annealing?

Many 3D printing filaments, such as ABS, PETG, and nylon, are less suited for annealing because their required temperatures are higher than PLA’s, increasing the risk of warping or deformation. ABS and nylon, for example, require annealing temperatures near or above their heat deflection points, which can destabilize the print.

While PETG has a less crystalline structure than PLA, annealing can still provide some benefits, such as improving impact strength.[3] However, the process often leads to higher shrinkage rates and thermal stresses, making it challenging to achieve consistent results. These factors limit annealing's practicality for most non-PLA materials.

Conclusion

Annealing PLA is an effective method to enhance the thermal and mechanical properties of 3D printed PLA parts. By carefully heat treating the material, its crystalline structure improves and layer lines are reduced, resulting in increased temperature resistance and strength. 

While shrinkage and warping are potential challenges, these can be mitigated through proper preparation, controlled heating, and gradual cooling. Compared to other filaments, PLA’s relatively low annealing temperature makes the process accessible and practical for most users.

With the right approach, annealing transforms PLA prints into durable components suitable for a broader range of applications, extending the versatility of this eco-friendly and widely-used 3D printing material.

Frequently Asked Questions

What is the purpose of annealing PLA prints?

Annealing PLA prints increases their crystallinity, enhancing their heat resistance, strength, and overall durability. This process makes PLA more suitable for applications requiring higher thermal stability, such as automotive or outdoor components, while maintaining its eco-friendly properties.

How do you prevent warping when annealing PLA?

To prevent warping, ensure uniform wall thickness and infill during printing. Support complex shapes with materials like sand or uncooked rice during annealing. Additionally, gradually cool the print by leaving it in the oven after turning it off, preventing thermal shock.

Can all types of PLA be annealed?

Most PLA filaments can be annealed, but some, particularly blends designed for specific properties, may have limited heat resistance or low crystallization potential. Always check the manufacturer’s guidelines for recommended annealing temperatures and times for the best results.

Why isn’t annealing suitable for other 3D printing filaments like ABS or PETG?

Other filaments like ABS and PETG require much higher annealing temperatures, which can lead to warping or deformation. ABS, for example, has a high heat deflection point that risks destabilizing prints, while PETG’s less crystalline structure means annealing offers little benefit.

References

[1] Steenhuis HJ, Pretorius L. Consumer additive manufacturing or 3D printing adoption: an exploratory study. Journal of Manufacturing Technology Management. 2016 Sep 5;27(7):990-1012.

[2] Wach RA, Wolszczak P, Adamus‐Wlodarczyk A. Enhancement of mechanical properties of FDM‐PLA parts via thermal annealing. Macromolecular Materials and Engineering. 2018 Sep;303(9):1800169.

[3] Kumar KS, Soundararajan R, Shanthosh G, Saravanakumar P, Ratteesh M. Augmenting effect of infill density and annealing on mechanical properties of PETG and CFPETG composites fabricated by FDM. Materials Today: Proceedings. 2021 Jan 1;45:2186-91.