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What Doesn't Melt You Makes You Stronger

Annealing PLA Prints for Strength – 2 Easy Ways

Picture of Miroslav Sarcevic
by Miroslav Sarcevic
Jul 20, 2019
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When it comes to making your PLA objects stronger and more heat resistant, all you need is some time and heat...

Annealing PLA What Is It and How Does It Work?

Metal annealing in progress.
Metal annealing in progress. Source: Thermal Vac Technology

Background Information

Annealing, when referring to metals, is when you heat up a material to make it less brittle and more pliant. In order for this physical change to occur, you have to heat the material to a sweet spot that’s cooler than its melting point but hotter than the temperature it recrystallizes at. You have to maintain this temperature for a while and then let it naturally cool down.

The question is, why would we want to do that? Well, when metals are cast or formed, tiny crystalline structures, called “grains” are formed within the metal. If the metal is cooled quickly after being cast, these grains tend to be small, which makes the metal hard but brittle. These kinds of metals will crack under stress along the junction lines between individual grains.

Metal annealing involves reheating the metal to a temperature below its melting point and allowing it to cool slowly. This causes larger crystals to form from original small ones – larger crystals make the metal softer and more ductile.

Annealing Plastic

Though this definition refers mostly to metals, a similar process can also be used with plastic. The technology behind plastic annealing is not new. It has been used for decades in the production of injection-molded plastic objects. Annealing is used to reduce internal stress in molded parts and make those parts stronger and less prone to warping.

With 3D printing, plastic is melted so it can be extruded and then cooled relatively quickly to form the desired shape. Since plastic is a poor heat conductor, parts of the extruded plastic are cooled unevenly. This leads to internal stress build-up, the worst of which occur between layers. To combat these stress points, you can anneal your 3D printed object.

As with metal, you should heat the plastic to its glass transition temperature, but below its melting point. This will reorganize the internal crystalline structure and cause bigger grains to form, which will result in much stronger objects that are less prone to layer separation.

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Annealing PLA Reasons to Do It

Broken 3D printed hooks.
Broken 3D printed hooks. Source: Airwolf3D

Regular, untreated 3D printed objects act like wood. They are stronger in one direction than another, and if you apply too much force in the wrong direction, you are going to split the layers of your printed object. This weakness, however, can be remedied by annealing PLA objects.

The biggest reason to anneal PLA is to get greater strength. Tests show that annealed objects can gain up to a 40% increase in mechanical strength. After annealing, the PLA will become less brittle and more ductile. The additional bonus is that the PLA obtains a greater temperature resistance, as well.

The only drawback of the annealing process is, if not done correctly, you will end up with a warped object. During annealing, PLA tends to shrink in the X and Y directions and expand in the Z direction. Yet, this can be avoided if you choose the correct annealing process and parameters.

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Annealing PLA How to Heat PLA in the Oven

Annealing in boiled water - the quick way.
Annealing in boiled water - the quick way. Source: Justin Lam

There are a couple of ways to anneal PLA prints, but the concept and aim are the same: To make the PLA stronger.

The basic concept is to heat PLA above the glass transition temperature of about 60 °C (140 °F), but below the melting point of 170 °C (338 °F) for some time, and then leave it to cool. The following describes how to do this in an oven.

First of all, never use gas ovens for this purpose. The oven thermometer may show a certain temperature, but the flames are typically much hotter. They can melt your object or even start a fire. Microwaves are also a bad choice: Only use an electric oven. Electric ovens with convection heating (the ones with fans) are the best option for annealing, but a regular electric oven will also work.

  1. Heat the oven to 70 °C (158 °F) and leave it hot for about an hour. This will make the temperature as uniform as possible. Do not put your 3D printed object in the oven yet.
  2. Check the oven temperature with an accurate thermometer. Once the oven is at the set temperature, turn off the oven. Place your 3D printed object on the oven-safe surface and put it in the oven. It is important to turn off the oven since the heating elements have a higher temperature than the one you have set. If you leave it on, it can ruin your print.
  3. Leave your prints in the oven until it cools down. A gradual cooling will avoid a reoccurrence of the internal stresses. Slow cooling will also reduce warping.
  4. Once everything has cooled down, you are ready to take your annealed object from the oven.

You may notice that your object shrank along the X- and Y-axes, but grew along the Z-axis. If dimensions are important for you, take this into account when designing your objects. Scale it up on the X- and Y-axes, and scale it down on the Z-axis.

This is the “by the book” method. If you want a quick and dirty anneal, try the next method.

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Annealing PLA How to Heat PLA in Hot Water

Again, the same principles apply.

  1. Heat the water to 70 °C (158 °F).
  2. Put your object in a plastic bag and submerge it in the hot water. Add some coins or screws in the bag to weigh it down.
  3. Allow your part to sit in the water for about 30 minutes at a constant temperature of 70 °C (158 °F).
  4. Leave it to cool down. Do not remove your 3D printed object from the water.
  5. When the water cools down to room temperature, you can remove your object from the water.

This method is faster, but will not get the same results as the previous one. That said, the strength will still be much greater than before annealing.

Feature image source: Airwolf3D

License: The text of "Annealing PLA Prints for Strength – 2 Easy Ways" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.

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