Annealing PLA in powdered salt using sous vide

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:

  1. Smoothing – did the texture of the print change?
  2. Strength between layers – was the print any stronger between layers?
  3. Waterproofing – did the print eventually get waterlogged when placed in water?
  4. Temperature resistance – did the print get as flexible when subjected to increased temperatures as a non-annealed print?

Procedure

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%.

A bag of salt powder
A bag of salt powder

I nestled my prints in the salt, then vacuum-sealed the bag and double-sealed the top.

A sealed bag containing prints surrounded by salt powder
Sealed in the bag. The salt prevented sharp edges from poking holes.

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.

Cooking the bag at 80 C
Cooking at 80 C

After that, I turned the sous vide stick off and let the pot come to room temperature.

Observations

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.

Prints partially extracted from salt

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:

PrintOriginal dimension (mm)After annealing (mm)Change (as % of original)
Calibration cube X20.1319.9899.3
Calibration cube Y20.0819.8999.1
Calibration cube Z20.1519.9899.2
Spheroid diameter20.0819.7898.5
Spheroid height (Z)24.8224.7299.6
Bar length (X)30.2330.0699.4
Bar width (Y)5.145.0698.4
Bar height (Z)5.004.9599.0

Results

Broken tabs on the box print
Tabs broke off just as easily after annealing
  1. 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.
  2. 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.
  3. 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.
  4. 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.
A non-annealed bar (bent) and an annealed bar (broken).
The non-annealed part was flexible after heating to 73 C. The annealed part was not.

Conclusion

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.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: