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Does brittle (easily snapped) PLA filament equate to less strong prints?

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I am not a materials expert, but I suppose so. Lets tag @TomHe for some insights. PLA can get brittle when it has been lying next to a radiator for example or is just plain old. Nothing of the 3D printing process (heating it up, cooling down again) would reverse this process, if anything I can imagine it makes it worse. A small print would probably break equally fast, a thick/big print would be stronger because the stress will be divided over many more layers, however, compared to a 'normal' PLA print I think it would break faster.

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In my experience it doesn’t matter as long your feeder doesn’t snap it, or due bowden pressure it snaps inside the bowden. I have printed 2 years old open in the air plas with my bondtech & directdrive without any effect on the prints that I could see. Ofc I dont print + try to break it to see if works better or not.

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I don't know if brittle PLA filament breaks at a lower or higher forces than fresh and more flexible PLA. But if you have circumstances where the model has to flex to function, then brittle will break faster.

For example: a clamp that has to snap around a tube. Fresh PLA filament will flex enough to snap around the tube smoothly. Brittle filament will no longer flex, so you can't snap it around the tube, and you keep pushing, and then it breaks. This applies to all sorts of snap-fit locking systems. Idem for keychains or card holders in your pocket. If you go sitting, the model may get stuck in your pocket and undergo huge forces. If it is very flexible (like nylon), it will bend and survive. If brittle (like old PLA), it will break. Don't ask how I know this... :)

But even "more flexible" materials like PET do break in the latter case. Thus the PET or PETG filament for 3D-printing is not like the PET from cola bottles, which do not break so easily.

So you have to take the practical application in consideration too, not just the technical strenght numbers.

I think, but I am not sure, that flexible filament is better able to distribute applied loads, while hard filament gives higher local stress concentrations. For example, a scratch or an indent (like in chocolate bars) will cause higher local stresses in stiff materials than in flexible materials, and thus it will fail easier on that spot. I guess. If there are material scientists here, feel free to comment and to educate us.

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Indeed @geert_2 could be nice to know if brittle filament makes more brittle prints. I suppose it doesn’t since the water that gives the flexibility should mostly disappear in both cases when going through the hotend. Maybe is other things that make it flexible or brittle, and if so what are they that suffer overtime? Is a pla printed part more brittle after X time?

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Since nzo has commented in another thread that he has some older PLA that is a bit brittle, he may be asking if it is usable. First question would be if it actually goes through the bowden tube ok. If so, then:

I would approach this as you would a print for strength by way of wall thickness and infill:

If it needs to take punishment, use fresh PLA. If it is going to sit on a shelf and look purty, then use the heck out of it.

PLA would pretty much break if dropped anyway, so you are not talking about strong stuff.

If it needs flexibility, then PLA would really not be good anyway. If it needs strength, then PLA is not really usable either.

That is my opinion and I am unanimous in that.

Edited by Guest

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Would coating a new PLA print with a clear fixative or polyurethane spray delay the onset of brittleness? Or baking the print to make it harder and denser?

Maybe what we need is extrudable granite...or maybe not :).

A quality of PLA I like is that it is said to be non-toxic and biodegradable - unlike the Great Pacific Plastic Garbage Gyre that is killing huge numbers of ocean-dwelling species every second. I don't want to contribute to that!

I also don't want to make endless PLA stuff that sits on a shelf...monuments to my ego :). I also don't want to make PLA toys that put children at risk. Usefulness is high on my list of priorities.

Then again, everything disintegrates. Filament, printers, us. The balance seems to be somewhere between usefulness and disintegration?

Edited by Guest

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this thread has more info: https://ultimaker.com/en/community/18137-rejuvenating-filament

tried it and it works to dip it in 70 degrees water.

I cut a few meters of a spool, few seconds in hot water and the filament becomes soft, let it air cool (in the right shape) and it is "good as new"

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If I understood it well, there are two factors that make PLA brittle. But I am no chemist, so my knowledge is limited.

One factor is hydrolysis: water is absorbed into the PLA and destroys its molecules. I think it does so by breaking the PLA down into lactic acid first, and then furter into CO2 and H2O? This damage is irreversible. After all, PLA is bio-degradable. And it is used in surgical wires too, where this chemical breakdown is used to make the wires be absorbed by the human body. However, just to try, I made a PLA sift in the sink of my laboratory, thus sitting in dirty water, but this has survived quite well in a year.

The second factor is crystal structure: over time, PLA changes into another crystal structure that is harder and more brittle. But this can be reversed by heating the PLA.

So if the PLA filament became brittle due to chemical breakdown due to water-absorption, it will give weak and brittle prints. If it became brittle due to changes in crystal structure, this should be reversed and should at first give strong prints like fresh PLA.

In real life, both effects will play together, but I have no idea which is strongest. But at least you can minimise the hydrolysis by storing filament dry.

Additionally, if the filament is really moist, melting it in the nozzle may cause water vapour bubbles in the print, which can greatly reduce strength too. Very common in nylon and ABS.

If the PLA filament was stored dry, and it became brittle due to crystal changes, warming it up for a day (at 50°C or so, not sure?) should make it flexible again. Should. At least it should become flexible enough to feed well and to print. However, after one or two years, crystal structure in the printed part will change again, making it harder and brittle again. And hydrolysis will also play a part in the printed model.

If the PLA filament became brittle due to water absorption and chemical breakdown, heating it will not help. That is as far as I understood it.

Anyway, in two years time my printed models did become much harder to the feel (far less flexible), and sometimes a bit dull (I guess this dullness is due to chemical break down?). In models that need flexibility such as clamps or snap-fits, this is a problem. For demo-models it is not. Just my experience.

If you use PLA for RC-model airplanes, or for structures under load, these effects might also cause problems over time.

Stored fresh filament also becomes harder and stiffer, but I haven't had any feeding problems yet. But I do store them dry, in a sealed box with desiccant, so this is probably mostly due to the changes in crystal structure.

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Something strange is going on :)

I've not seen any mention of heating filaments in a microwave.

I've just tried it with several loops of orange and blue PLA in a 1100 watt microwave.

Results:

  • 30 secs=about 35C - the orange relaxed and not so brittle, the blue still snapped easily.
  • 60seconds=both colours heated to about 50C
  • 1m30seconds=both colours now softish and when held by one end relaxed into straight vertical shape. Both colours no longer snapped but bent without breaking.

More experiments on a larger quantity soon. Probably a 'softer' (50%) microwave setting may be beneficial when a whole reel is processed.

Edited by Guest

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How does microwaving work?

From Scientific American

Chad Mueller, assistant professor of chemistry at Birmingham-Southern College in Alabama, replies:

"A microwave oven cooks food because the water molecules inside it absorb the microwave radiation and thereby heat up and heat the surrounding food. Microwave radiation will similarly heat up skin and other body parts. In fact, people stationed at big microwave towers in cold climates used to stand in front of the microwave generators in order to warm themselves. The radiation is harmful mostly to the parts of the body that cannot conduct the heat away very effectively---the eyes especially. I think that heat transfer could explain why one sometimes hears about people (fast-food workers, for instance) getting headaches when exposed to leaking microwave ovens."

David E. Hintenlang, associate professor nuclear and radiological engineering at the University of Florida at Gainesville, adds some further details:

"Microwave ovens cook food by generating intermolecular friction between the molecules of the food. The microwaves cause water molecules to vibrate; the increased friction between the molecules results in heat. Microwaves could affect your tissue in a similar way if they were able to escape from the microwave oven. Modern microwave ovens are designed to allow essentially no leakage of microwaves, however. The only time for concern would be if the door is broken or damaged, in which case the oven should not be used.

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This will be interesting to see. :)I look forward to your results.

Are you just heating raw filament or actual prints?

I know people have annealed PLA in ovens after printing. You can google it and get some results on that.

Matter of fact..I just looked.

Edited by Guest

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Gobble gobble gobble!  The forum just ate my reply to kman :(Aaargh!

Hi @kmanstudios'>kman...

I'm heating the raw filament so far.

When the MW heating cycle ends @ 2mins for a bunch of 6x single PLA loops in different colors, one has only a few seconds to manipulate the filament before it hardens again. The loops straighten easily. No more finger bending!

I cross-sectioned the red PLA with a scalpel after heating and cooling and what I saw reminded me of a tree-ring structure. The inner core appeared glassy (like those red gelatin-filled chocolates) and was surrounded by a ring of normal 'crystallized' PLA. Maybe the center was still crystallizing. I tried to photograph this but it really needs a microscope. Mine is in pieces at the moment. I'll repeat this when it's resurrected.

Edited by Guest
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@kmanstudios'>kmanstudios...I printed this tiny forklift by agepbiz a few days ago, using the UM 0.25mm nozzle and yellow PLA. I decided to sacrifice the print in the microwave @ 2mins on the high setting to see any resizing effects.

loader.jpg.9fbd1f8863de74c51c02766efdb3aed5.jpg

Top to bottom of chassis before MW=17.28mm

Top to bottom of chassis after MW=17.22mm

I need to test this more for consistency.

Edited by Guest
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@SandervG'>SandervG...I'll start testing filament loops again soon. Microwaves heat the water molecules in humidified filament. I have no ABS or Nylon but my sense is that it could work. In my first PLA tests loops were exposed to a maximum of 1100 watts. They got hot quickly but not evenly and cooled very quickly at room temp. I feel it needs a slower process.

Next I would place the filament on a flat surface and run the microwave at a 50% on/off cycle, to give the plastic time to soak. Microwave ovens tend to have low thermal mass and uneven heat, and when the door is opened cooling is rapid.

Added thermal mass, such as a dense piece of firebrick or unglazed tile placed in the MW for a pre-heat phase would even out the temperature and hold the heat for longer. There's probably a sweet spot where the heat hovers around 60 to 70C where the annealing is happening.

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A microwave heats molecules by applying a fast changing electric field. Thus the molecules are violently shaken left and right, which causes friction and warms them up, like rubbing. But this only works for molecules which have a polarity, like water, H2O: both H-atoms (hydrogen, positive) are sitting on one side , and the O (oxygen, negative) is on the other side. So, an H2O molecule has a clear polarity, and can be shaken by an alternating electric field. Food also contains a lot of water, so that is why it works. But I don't know if PLA has some polarity? If yes, it might work. If not, it could only work as far as there is moisture trapped in the PLA. Is there anyone who knows more about the chemical composition of PLA? Anyway, heating in a microwave does not work at all for dry non-polar molecules, like glass and most oil-based plastics. You can take them out of the oven with your bare hands.

If you want to be sure, a conventional oven (well controlled, like an incubator), or the radiator of your living room, or an incandescent spot, might be a better heat source, I think. Or heating "au bain marie": in a closed dry box or pot, with desiccant, which is swimming in warm water. But not directly in the water, since that would increase hydrolysis.

Anyway, it are interesting experiments. I would really like to know the results.

Edited by Guest
corrected typo

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Thanks for that info @geert_2'> geert. Moist PLA might have enough H20 molecules to work. The last batch of PLA I tested in the MW certainly responded by softening. No moisture = no softening @ 70C.

We had this BS marketing hype last Black Friday in NZ and I kept an eye on  deals for bench top ovens marked down to around $50. Today they were up to $145 -$180 again. I'll wait until mid December and look again. An oven like that could be useful for cooking filament, backing bread, heating pizza and roasting a chicken :) I'll stick a pottery pyrometer in through the oven shell to more accurately monitor internal temp.

I'll keep you updated.

Edited by Guest

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short answer - printed part won't be brittle. I've heard some people say it has to do with "micro fractures". Also when the filament gets brittle it's only the 1 meter or so that has been straightened. Something about straightening filament and holding it in that position for 10 hours makes it very brittle.

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short answer - printed part won't be brittle.  I've heard some people say it has to do with "micro fractures".  Also when the filament gets brittle it's only the 1 meter or so that has been straightened.  Something about straightening filament and holding it in that position for 10 hours makes it very brittle.

Maybe doing that without heat causes the plastic to 'violate' its shape memory?

It is probably spooled warm. And when it cools, that makes one side longer than the other by virtue of the cylindrical shape and that is the condition the polymers remember once cooled. Pulling it straight and keeping it that way (or not the same coiled shape would be more accurate) causes it to pull on one side and stretch on the other without being heated and made malleable.

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