How to Anneal PLA Part for Strength
PLA may be one of the most widely used and popular filament materials used in 3D printing. However, it’s not exactly known for being particularly strong or heat-stable. While most people use PLA because of how easy it is to handle, what if there was a way to enhance the properties of your PLA print? The trick is annealing, Annealing could improve the strength and reduce inner force.
In this guide, we will explain the definition of annealing PLA and how to anneal PLA part for high strength.
What is annealing?
In annealing, the material is exposed to temperatures between its glass transition temperature and its melting temperature. To reach that temperature, heat must be applied very gradually to avoid the buildup of thermal stress. Once the target temperature has been reached, it is maintained for a period based on the prescribed annealing program.
In this high-temperature state, the material undergoes alteration at the molecular level. The molecular structure of the material reorganizes, achieving a more “crystalline” state. This merely means that its atoms and molecules become more orderly and take on repeating patterns across all three dimensions. This shift from being amorphous or non-ordered to crystalline enhances the rigidity and strength of the material.
Once the prescribed holding time at the annealing temperature has been completed, it is equally important for the temperature to be reduced gradually. This is the phase of crystal growth, and higher crystal grain sizes can be achieved with slow cooling. Larger crystal grains are associated with better mechanical properties. A slow cooling rate also prevents the buildup of additional thermal stress in the solid.
Not all materials are capable of attaining a more crystalline state. For truly amorphous solids, annealing is still beneficial as it eases the thermal stress resulting from the rapid heating and cooling of 3D printing. In such cases, the benefits of annealing are not as pronounced as those in solids that achieve a more crystalline state.
Why PLA needs annealing
As a 3D printing filament, the main strengths of PLA are that it prints at low temperature, is not prone to warping, and is produced from sustainable sources. However, there’s also a reason why 3D printed PLA products are typically only used for display items – they are not particularly strong or durable.
Compared to other 3D printing materials like ABS, PLA has very low tensile strength. It also has a glass transition temperature of only about 65 °C. For this reason, it is not recommended to use PLA for any outdoor applications. Left inside a car on a hot day, an object made of PLA would almost certainly become deformed.
Annealing is a way to address these limitations. A fairly standard manufacturing technique, annealing a material improves its strength, rigidity, toughness, and heat resistance. Luckily for you, you don’t need any fancy equipment to anneal PLA.
Advantages and disadvantages of annealing
1. Creates stronger parts
Whether it’s PLA or other 3D printing materials, annealing is a process that can make your finished part stronger. This is ideal if you want your 3D printed parts to be able to withstand greater amounts of mechanical stress. Specifically for PLA, annealing addresses the lack of strength and rigidity that the material is well-known for. It also does not alter the 3D printing process, which means that PLA is just as easy to handle as always.
2. Improves heat stability
Another noteworthy benefit of reconfiguring PLA (or any material) at a molecular level through annealing is that it improves the material’s heat stability. This pertains to the materials’ ability to maintain their shape when exposed to higher temperatures. Again, enhanced heat stability addresses one of the common flaws of PLA as a manufacturing material.
Heat stability enhancements typically become better at higher annealing temperatures. However, as we shall see later, keeping a thermoplastic at elevated temperatures for a long period also has undesirable consequences.
1. Lengthens the manufacturing process
No matter the settings for your annealing procedure, it will always be an extra step to an existing manufacturing process. If you’re doing 3D printing for any commercial purpose, annealing will significantly lengthen the turnaround time for any completed part. It also requires a bit of work. If you have limited resources, adding an extra annealing step becomes a question of higher throughput versus better product quality.
2. Consumes additional energy
Heat is a central element to the annealing process. Whether you’re delivering heat via gas or electricity, this means consuming more energy for post-processing a finished 3D print. This likely isn’t a big deal for one-off products but can get quite expensive for commercial-scale manufacturing. If you’re planning to sell annealed PLA products or parts, then the extra cost of heating is something you cannot ignore.
3. Dimensional accuracy may suffer
In annealing, the target temperature can lie anywhere between a material’s glass transition temperature and its melting temperature. This is a very wide range. The benefits of annealing are typically magnified at higher temperatures, producing stronger and more heat-resistant materials.
The problem with heating any material well past its glass transition temperature is that it also starts to deform. This balance is critical to a successful annealing process for PLA. When annealed at very high temperatures, PLA starts to warp. If you’re working with a model with lots of intricate details, dimensional accuracy may suffer because of annealing.
In summary, annealing is not a perfect method. It may provide benefits in improving the mechanical strength of a material but seriously compromises its dimensional accuracy. With this in mind, annealing may not be a suitable post-processing step in all situations.
How annealing PLA works
PLA loses the benefits of its semi-crystalline structure when it goes through the 3D printing process. This is because of how quickly the material is heated and then cooled, thereby preventing the growth of crystals in the structure. At this state, PLA takes on a more amorphous form. This form of PLA is more chemically reactive, less heat-resistant, and less resilient against mechanical stress.
When PLA is heated beyond its glass transition temperature, it undergoes a restructuring at the molecular level. Internal stresses are relieved spontaneously via the laws of thermodynamics. This is achieved by the removal of “dislocations” or linear defects in the material’s molecular structure.
After the period of annealing, PLA is cooled down very gradually. During this phase, recrystallization happens. Polymer chains start orienting in semi-regular patterns. Again, this happens spontaneously towards the material’s natural equilibrium state.
How to anneal PLA
It’s fairly easy to try annealing your PLA print with some basic equipment. You will need three things – a stopwatch, a digital thermometer, and an electric oven with temperature controls. Once you have finished 3D printing your project with PLA, these are the steps to take in coming up with your very own annealed PLA:
1. Pre-heat the oven to the desired annealing temperature.
For PLA, this is typically between 65 to 70 °C. The specific value is up to you – benefits are more pronounced at higher temperatures, but there is also a higher chance of warping.
2. Wait for the oven to reach a stable target temperature.
This is important as it common for electric ovens to overshoot the target temperature before settling down on a stable reading.
3. Place the PLA print inside the oven along with the digital thermometer.
Make sure that the thermometer is as close to the print as possible. It is best to put the object on top of a non-stick surface such as a silicone tray. This will save you the hassle of having to pry it off after.
4. Leave the PLA print inside the oven for the prescribed annealing time, all the while making sure that the target temperature is being maintained. The proper annealing time will vary depending on the dimensions of the print. A good rule of thumb to start at is 30 minutes for every 1/4-inch thickness.
5. Turn down the oven without opening its door. This will slow down the cooling process sufficiently to allow for recrystallization of the PLA structural grains. Let the print cool down until it naturally reaches room temperature.
Take note that you may not get the best results on your first shot at annealing. You may have to try different combinations of annealing temperature and time depending on the dimensions and level of detail of your 3D printed object. As long as you hit the prescribed annealing temperature, there should some profound improvements in the properties of the PLA material of your project.
Other tips you need to know
1. Use an electric oven
When using an oven for annealing, choosing an electric oven over a gas-fired one is advantageous for two reasons.
Most electric ovens use convection as the primary heat delivery system. This provides much more consistent, which should help ensure that no parts of your piece are significantly heated more than the others.
An electric oven also provides better temperature control. This is highly critical when doing annealing of PLA. It’s not enough that the oven is set to about 70 °C – it has to be at exactly that temperature. PLA can be easily ruined with improper temperature settings.
2. Scale up your piece
Warping is virtually unavoidable when annealing a PLA piece, even at the lowest temperatures. The effect of warping is more pronounced if you’re working with small pieces or those that are heavily detailed. An easy way to make warped features less glaring is to simply scale up your model. Of course, this isn’t always an option. However, if there is some leeway to the size of your final piece, you should always try and make it larger.
Printing at 100% infill may seem counter-intuitive for most users of 3D printers, but these settings deliver the best annealing performance. The added PLA material on the piece makes it much stronger after annealing.
More importantly, printing at 100% infill goes a long way towards preserving dimensional accuracy. This is simply because having additional material inside the piece helps resist the natural tendency of PLA to shrink during annealing.
3. Keep your eye on the piece
If it’s your first time to try annealing PLA, there will be a healthy dose of trial and error before you can get it right. Given this, it’s extremely important to not walk away from your oven once you have placed the PLA piece in. Look at it every now and then and watch out for signs of warping.
What you’re trying to avoid is the worst-case scenario where you end up with a puddle of molten PLA on the bottom of your oven. More than just the wasted hours spent on 3D printing, this would be a nightmare to clean up.
Aside from polishing and painting, there aren’t many post-processing options for PLA. If you want your PLA prints to become stronger or more resilient than standard PLA, then we suggest trying your hand at annealing. It’s not terribly difficult once you get the hang of it.
Annealing PLA is an excellent way to improve your PLA prints, especially if you’re selling them or using them as tools or replacement parts around the house.