3D printing strong objects can prove challenging. Tougher filament types cost more, require hardened nozzles, and benefit from things you might not have, like a chamber heater. So what if you could get improved strength and heat resistance from basic filaments like PLA and PETG?
It turns out you can, and the process is straightforward, but there are a few things you should know beforehand.
“Annealing” your 3D prints in an oven
Annealing is a heat treatment that is most commonly associated with metallurgy (and also used for glass). The process involves heating an item to temperature, holding it there, and then allowing it to cool at a slow pace to improve its overall strength.
The term “annealing” is typically not associated with plastics, but it has entered the vocabulary of many 3D printing enthusiasts. By heating 3D printed objects to a point where the molecular structure is able to change, plastics can be “annealed” in a similar way to metal or glass by reforming in a stable manner with less internal tension.
This involves bringing the filament close to its glass transition temperature (where the object becomes malleable) but not so high that it will melt. One of the best studies on this behavior came from Prusa Research in 2019, where it was discovered that both PLA and PETG can benefit greatly from heat treatment. The same set of experiments suggested that ASA and ABS are poorly suited.
There are some things you should know before you try this for real. For starters, objects that are annealed will often exhibit shrinkage along the X and Y axes, and may even grow along the Z axis. This can be a problem for prints that need to be dimensionally accurate, though with some careful experimentation, you could account for this by changing your model’s size accordingly.
Furthermore, not all filaments are built equal. Different filaments have different glass transition and melting temperatures. You might need to perform your own tests to find the sweet spot that balances dimensional changes with the desired outcome of improved strength and heat resistance.
In Prusa’s research, the best results yielded around a 20% improvement in weight bearing for PLA, and around 70% improvement in heat deflection. In terms of impact resistance, PETG measured an improvement of around 200%.
Here’s how you can try for yourself
To ensure even application of heat to the entire object and in order to help mitigate dimensional changes, annealing in sand or salt has emerged as the go-to method. There are other methods, including annealing in plaster, as covered by YouTube channel CNC Kitchen, but it’s messy and incredibly time-consuming.
The hardest part of this process is choosing a temperature to use. Some recommend starting low at around 60ºC, below the glass transition temperature (which is typically 65ºC for PLA). Prusa’s research revealed that temperatures of around 90ºC started to yield significant improvements for PLA, and around 130ºC for PETG.
The lower the temperature you pick, the less dimensional change you should encounter. For best results, I’d recommend creating some test prints and experimenting for yourself. Ideally, keep one print as a control to observe how the process has affected the print.
With your oven heating, place enough sand in a heat-proof container so that you can fully submerge your object. Bury the object with a gap of sand at the bottom, since if the object touches the container, it will melt and deform. Now place the object into the oven for around 30 to 60 minutes. Prusa’s numbers are based on 30-minute annealing times, but the researcher notes that “annealing time depends on the size and shape of the object.”
Once your time is up, turn off the oven, but do not remove the object. Allow it to cool down slowly and naturally, until it has completely cooled before performing any tests. You can now measure your item (and compare it with your control print) to see how much it has changed in the oven.
Don’t use your kitchen oven for this
It’s probably a good idea to avoid using your kitchen oven for this sort of thing. While PLA is widely considered safe and non-toxic, you still don’t want to breathe in too many of the fumes generated by hot plastic. If you’re going to try annealing other filaments, health outcomes could be much worse.
Consider buying a cheap toaster oven for these sorts of experiments, and only use it for project-related tasks. For example, you can turn your waste filament into new items by casting printer poop and supports in molds. Ideally, you’ll want an oven that has a low starting temperature of around 50ºC (120ºF), and it won’t hurt to get a proper oven thermometer to account for any calibration errors.
Just make sure you’re heating and melting your plastic in a well-ventilated space, away from anyone who doesn’t want a headache. A shed, workshop, garage, or even outside on a dry day is perfect.
The other way you can improve the strength of your 3D-printed objects is by making sure that you orient them properly on the print bed.
Stephan is the sports journalist for the Maple Grove Report.
