PLA is one of the most user-friendly filaments to use, but it’s no secret that PLA(polylactic acid) can’t withstand much heat at all, so you have to consider where you’ll use your PLA parts and make sure they won’t be exposed to too much heat.
So what temperature can PLA withstand?
To understand the temperature PLA can withstand, we need to see what the glass transition temperature is – the temperature at which a polymer like PLA transitions from a solid state into an amorphous (softened) state.
The glass transition temperature of PLA is around 60 to 65 degrees C, or 140 to 150 degrees F. At these temperatures, PLA will begin to soften.
What is the melting point of PLA?
The melting point for PLA is much higher, at around 180 to 190 degrees C, which is the typical printing temperature for PLA parts.
What is the heat resistance of PLA plus?
PLA+ is advertised as a more advanced form of PLA, which happens to overcome some of the weaknesses of PLA such as brittleness and finish.
While PLA+ is indeed a little stronger than PLA, and does look better in the finished product, tests done on the heat resistance of PLA+ found that it began to soften at pretty much the same temperature as PLA.
As for heat resistance, whether you’re using PLA or PLA+, you’ll need to stick to lower temperature applications.
So where can you use PLA?
If your PLA parts are used in settings that won’t go above room temperature, then they should be ok to use.
However, if the temperature will go above 60 degrees, then the PLA will begin to soften.
Typically, the inside of a car will get incredibly hot in the summer, so if you print a visor or a clip that comes directly under the windshield, the part will not last long.
However, other parts of the car may be cooler, such as below the seat, or under the hood at a place not directly in contact with metal.
As soon as the temperature goes up, the PLA will begin to experience warping, shrinking, and will bend under even the least amount of stress.
Annealed PLA heat resistance
One solution to solve the issue of PLA’s low heat resistance is to make it undergo a process called annealing, where the parts are gradually exposed to a certain level of heat to rearrange the molecular structure and make it stronger.
The heat resistance of annealed PLA depends on how much you heated it during the annealing process.
According to tests conducted by Prusa Research, annealing at temperatures of around 90 degrees C and above tended to produce the strongest PLA heat resistance, with the annealed parts now withstanding nearly 90 degrees C temperatures.
What is annealing and how does it work
Annealing is a rather ancient process. It was originally used for metals, and with the advent of polymers, it is used for polymers as well.
When metals or polymers are heated and rapidly cooled, the molecules are not arranged evenly. Due to this uneven arrangement, there are many fracture points and stress points in the material.
Annealing means slowly heating up the material to a certain temperature: typically beyond the glass transition temperature but well below the melting temperature.
At these temperatures, the molecular structure relaxes and the molecules are free to move about. Now, when the part cools, the molecules will settle into a much more stable structure, resulting in an overall stronger part that also has higher thermal resistance.
So how is this cooling process different from the cooling that occurs when your printer is printing?
When an FDM printer puts down layer after layer, the bottom layers will cool nearly instantly, and the new, fresh layer will be very hot. As soon as that layer is complete, it will also cool, but it won’t be as cool as the layers beneath it.
By this time, another hot layer of plastic will have been laid on top!
You get the gist!
Add to this the fact that plastic is a poor conductor of heat, which makes it even more prone to uneven cooling.
How to anneal PLA
To anneal PLA, you’ll need an electric oven.
It’s important that you don’t use a gas fired oven, as the flames are much hotter than the rest of the oven.
This may result in the bottom of your parts melting, or worse, your parts catching fire!
Use an electrical oven, and for best results, use a convection oven as it circulates hot air over the entire oven very evenly.
Set your oven temperature to 70 degrees C, and use a thermometer to make sure the inside temperature is indeed 70 degrees and no higher.
Note: if you wish to anneal at higher temperatures, use the temperature you need, whether it’s 80 or 90 or even 100.
Give it a few minutes to let the heat distribute evenly, and place your parts on a tray in the oven.
Now turn the oven off, and leave the parts in the oven for an hour or two until they have cooled. This will allow enough time for the parts to gradually heat up, cool down, and for the molecules to have rearranged themselves in a stronger configuration.
When you anneal your parts, they’ll slightly change in size and shape as they soften and harden again. The temperature that you use will determine the change in size of your annealed part.
Typically, the print will shrink along the X and Y axis and slightly get taller on the Z axis.
Before annealing your final part, make sure to test the process out on a random scrap print and measure before and after to see the percentage of change that your oven and particular brand of PLA produces, as it will slightly vary from plastic to plastic.
One final tip is to orient your print in a way that it is very sturdy on the surface you place it on. For example, a large overhang will likely fall over as the plastic softens, so make sure to lay it down in a way that the overhang is supported!
PLA is not the most heat-resistant material, but there are methods like annealing which increase the strength and temperature resistance.
ABS, PETG, and nylon are far stronger and more heat-resistant, so if you need high-stress parts, consider using those filaments instead of PLA.