Stefan of CNC Kitchen recently posted about annealing (and later re-melting) PLA prints to improve their strength and temperature resistance. I decided to do some experiments of my own.
My method was to print out some PLA objects, pack them in a salt powder that gets vacuum-sealed, and then toss that in the sous vide for 18 hours. The sous vide doesn’t get hot enough to re-melt PLA, so it doesn’t have exactly the same characteristics, but it does gain some benefit. The advantage to the sous vide method is that it’s more available than a constant-temperature oven.
I did informal tests on the annealed prints for a few characteristics:
- Smoothing – did the texture of the print change?
- Strength between layers – was the print any stronger between layers?
- Waterproofing – did the print eventually get waterlogged when placed in water?
- Temperature resistance – did the print get as flexible when subjected to increased temperatures as a non-annealed print?
I used four objects: a hollow spheroid, a 30x5x5 mm bar, a calibration cube, and a box with tabs that were oriented perpendicular to the layers. I ground up plain (uniodized) salt in a spice mill, and put that into a FoodSaver bag that was double-sealed at the bottom. Relative humidity when I sealed the bag was around 33%.
I nestled my prints in the salt, then vacuum-sealed the bag and double-sealed the top.
Next, I tossed the bag in a pot of water along with an Anova Nano sous vide stick. I set the temperature for 80 C (176 F) and left it that way for 18 hours.
After that, I turned the sous vide stick off and let the pot come to room temperature.
When I removed the salt from the bag, it had formed a solid mass that could be broken apart by hand. I carefully extracted the printed objects. The box lid and letters on the calibration cube letters were packed with salt, but that could be washed out easily.
I measured most of the objects, and found that they had indeed changed size – all were a few percent smaller than before annealing. So I knew annealing did something:
|Original dimension (mm)||After annealing (mm)||Change (as % of original)|
|Calibration cube X||20.13||19.98||99.3|
|Calibration cube Y||20.08||19.89||99.1|
|Calibration cube Z||20.15||19.98||99.2|
|Spheroid height (Z)||24.82||24.72||99.6|
|Bar length (X)||30.23||30.06||99.4|
|Bar width (Y)||5.14||5.06||98.4|
|Bar height (Z)||5.00||4.95||99.0|
- Smoothing: unlike Stefan’s experiments with PLA re-melting, the annealed objects did not change texture. The plastic still showed the same layer lines and other 3D printer artifacts that non-annealed prints did. I attribute that to the fact that the plastic never flowed – the annealing temperature was always below the melting point.
- Strength: when I assembled the box, the tabs broke off exactly as they had when I printed non-annealed PLA using the “wrong” orientation. I couldn’t measure it, but I don’t think annealed PLA was any stronger between layers.
- Waterproofing: after immersing the spheroid in water for a week, it seemed to sink a little lower, but then stop. I don’t think I’d want to claim that annealing changed the waterproofing of the prints.
- Temperature resistance: this did appear to change. I put both the annealed and non-annealed bar prints on the printer bed at 73 C (163 F). After a few minutes, the non-annealed bar was flexible enough that it could be bent between my fingers. In contrast, the annealed print remained solid, and in fact snapped when I applied more pressure.
Annealing sous vide is useful for imparting temperature resistance to PLA, and may be a valuable technique for PLA objects that need to live in a hot environment (such as a car in the summertime). Sous vide annealing is easier and more reliable than trying to anneal in a kitchen oven. Annealing alone does not change texture, strength between layers or waterproofing characteristics.