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I recall seeing that it's possible to "bake" or "anneal" or even "remelt" PLA prints to strengthen them. As I understand it, the main effect is improving layer adhesion, which is the main source of weakness in tension in the original Z-axis direction.

Yes, I can print with a hotter nozzle (up to a point) to increase layer adhesion, print with a larger nozzle orifice, print with increased layer height, print slower -- but at a minimum those require reslicing, and affect print appearance (hotter nozzle causes stringing, thicker layers are more visible, etc.).

Baking a print does none of that, as long as it's supported (often done by bedding it in salt).

Question is, how hot? I presume I don't want to go all the way to printing temperature of 180-220 °C, but the plastic transition level of roundly 50 °C isn't hot enough for this.

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  • $\begingroup$ source for improved layer adhesion? I remember annealing as a way to improve temperature resistance, mostly. $\endgroup$
    – FarO
    Commented Jul 16, 2021 at 18:57
  • $\begingroup$ @FarO At least one of the articles linked in the answer below talks about layer adhesion, and that was the subject of the video (of which I've only seen the thumbnail) that I was recalling when I wrote the question. Makes sense that if the material softens enough to recrystallize, the layers will sinter together if they weren't already well joined. $\endgroup$
    – Zeiss Ikon
    Commented Jul 16, 2021 at 19:02

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I've only annealed PEEK personally, but a quick search returns varying recommendations:

  • All3DP recommends 1 hour at 70 °C.

  • X3D recommends 1 hour at 110 °C.

  • Matterhackers recommends 10 minutes at 105 °C for Tough PLA or HTPLA.

  • A post on Reddit recommends "a few hours" at 110 °C.

I'd do a test suite with pieces at 70 °, 90 °, and 110 °C for an hour each and test/compare.

Be mindful of any chemicals that might be released.

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CNC Kitchen has a series of videos covering research on this topic, ending up with

using salt. The oven temperature used for PLA was 200˚C reaching 190˚C at the core of the salt. Lower temperatures (especially without support) are of much more questionable value.

From my experience, I believe that similar results might be achievable just by improving the printing process, without any annealing. This is because a lot of the weakness along layer lines seems to be caused by inconsistent extrusion (largely due to oozing in the model interior) and marring of the layer by combing travel over it before the next layer can be laid. With these issues solved, my prints got a lot stronger. So if your goal is just to fix a practical part strength problem, it may be worthwhile to see if you can improve things here before trying an elaborate annealing process (which necessarily requires 100% infill as well as a lot of messy postprocessing work).

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