When it comes to making your PLA objects stronger and more heat resistant, all you need is some time and heat...
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.
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.
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.
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.
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.
Again, the same principles apply.
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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