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

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Hi kman. How are your bronze/copper filament experiments going? Have you been able to solve the problems you were having?

Nope, had to get on a production model for a client and that has taken the last 4 days to print out parts and get them ready to assemble. And, I designed a lamp for them as well.

Will be hoping on that soon. I have to solve an issue with my hardcore nozzle. I think a piece of copper is stuck in it preventing proper flow. But, as soon as I get past this basic model for the client I am back onto the metal fills.

I gotta get onto the Christmas gifts for kids and grandkids......

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Something about straightening filament and holding it in that position for 10 hours makes it very brittle.

This is most likely caused by microcracks slowly growing in the material, due to the stress. See these pictures: first one taken with macro-function in daylight, second one taken through a 10x microscope under warm white LED-light. This is colorFabb "natural": due to its translucency the cracks are very visible, but I also see them in other materials. This is fairly old filament, about 1.5 years, although it has been stored dry in a sealed box with desiccant. These pictures were taken after manually straightening this tightly wound filament near the end of the spool, and then relaxing it again (so the cracks stop growing), to make it feed better in my UM2, without underextruding.

But I guess it is the same effect that plays when the filament sits "artificially straightened" in a bowden tube (or wherever else) for a prolonged time. I have been wondering too about the reason why it cracks? Is it spooled while still warm, and then cools? Or spooled cold, but the crystal structure takes some time to change from amorphe (more flexible) to crystaline (harder and more brittle), so it is wound while still in the more flexible phase, and then it hardens while already wound, and gets fixed in that bent shape?

DSCN5649.thumb.JPG.6bbdd1ca1552266ce1b66269de7edea5.JPG

DSCN5654.thumb.JPG.8d711ff4f62ff3d1d91eaee3b0ffa713.JPG

These bright lines are cracks: you can see them beginning and growing while straightening tightly wound filament.

Edited by Guest

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Is it spooled while still warm, and then cools? Or spooled cold, but the crystal structure takes some time to change from amorphe (more flexible) to crystaline (harder and more brittle), so it is wound while still in the more flexible phase, and then it hardens while already wound, and gets fixed in that bent shape?

This is shape memory at work. The same function of the filament being more difficult to push through the bowden tube as you get to the much more tightly coiled end of a spool rather than the larger diameter of the outer parts of the roll when new.

If you take a fresh spool, and print the heck out of it, you will still get some resistance at the end of the spool. Age of the filament only worsens the effect.

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Won't this type of cracking be eliminated in the "furnace" of the hotend?

 

That is a good question, but it may be that the material itself does not actually regain its original composition/qualities once it has aged.

I do not know enough about material sciences to answer that. It would be nice if one of the Team Ultimaker guys could give us some feedback on this.

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Most of my data comes from this amazing research here:

https://ultimaker.com/en/community/17102-why-did-the-filament-break#reply-120629

But after reading that my experience completely agrees - usually the problem only happens on older filaments and only the portion of the filament that was straightened for a few hours. Sometimes it cracks a few times in the bowden but most often it cracks right where it goes into the feeder. In my experience. All locations where the filament has straightened.

Having it crack in the bowden is bad because it usually gets stuck when the crack area gets into the print head.

My solution is to not allow old filament to stay inside the bowden overnight.

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Won't this type of cracking be eliminated in the "furnace" of the hotend?

 

Yes.  Exactly what I suspect as well.  And parts made from old filament seem fine but I haven't done any mechanical testing on old filament versus new.  I've done lots of mechanical testing but not on old filament.

 

I haven't been printing long enough to have old filament. I burn through it like wildfire with experiments and such.

So, I depend on much more experienced people to advise on things like this since mostly any input I have is conjecture.

Although I did experiment with annealing nylon prints and it really made the material stiff and hard. Nothing flimsy or thin though.

I have not tried to anneal any other materials as of yet.

Edited by Guest

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Won't this type of cracking be eliminated in the "furnace" of the hotend?

 

That is a good question, but it may be that the material itself does not actually regain its original composition/qualities once it has aged.

I do not know enough about material sciences to answer that. It would be nice if one of the Team Ultimaker guys could give us some feedback on this.

 

When this material is molten, the cracks disappear and it flows smoothly out of the nozzle, like always. So, in my prints there are absolutely no cracks indeed.

But as kman says, I also wonder how much strength or flexibility old filament recovers, compared to brand new material?

I guess if the material became harder and brittle due to reversible changes in crystal structure only, it might recover fully. But if the material became harder due to hydrolysis (break down of molecules), or by plasticizers that evaporate or degenerate, to me it seems unlikely that it would fully recover. But this is only a guess. I would welcome the views of a materials engineer specialised in PLA polymers.

Anyway, printed parts that get old, do get much harder and brittle, in my experience. This is very clear in my clamps. For example the "snake clamp":

snake_clamp1.thumb.jpg.aebd165b44691e50a1c55abe0f07e3f8.jpg

When freshly printed, this easily flexes and smoothly slides over the bowden tube. But after 1.5 years now, they are so stiff that they are very difficult to get around the tube. And sometimes they just break. I have seen this too in other designs like keychains, which I made specially to test real life behaviour. I have no scientific data and no numbers, but the subjective feel is obvious. So this is something I keep in mind when printing parts that need to keep their functional flexibility over a longer time.

I will see if I can find a few old unused clamps, and warm them up in the oven (=incubator, electronically controlled) for some time at different temps, and see what that does.

snake_clamp1.thumb.jpg.aebd165b44691e50a1c55abe0f07e3f8.jpg

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@Tomhe - any comments about the above post and the post further up with close up of filament showing cracks?  In particular:

What makes filament brittle

why when one straightens the filament for 8 hours or so does that make it suddenly brittle

Why is older filament more brittle (is it water related, age, outgassing, micro fractures?)

An experiment on UM filament that is very interesting (watch the 40 second video):

https://ultimaker.com/en/community/17102-why-did-the-filament-break#reply-120629

Edited by Guest

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Thanks for the excellent article link gr5! This "bad batch" problem seems to be widespread. About 7 out of 10 Wanhao spools I've used are 'snappy' brittle, although i've never had filament break in the bowden tube.

May be time for Ultimaker to purchase - or hire time on - a scanning electron microscope for their materials expert...no? :)

See what filament looks like at a deeper crystalline level.

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@Tomhe - any comments about the above post and the post further up with close up of filament showing cracks?  In particular:

What makes filament brittle

why when one straightens the filament for 8 hours or so does that make it suddenly brittle

Why is older filament more brittle (is it water related, age, outgassing, micro fractures?)

An experiment on UM filament that is very interesting (watch the 40 second video):

https://ultimaker.com/en/community/17102-why-did-the-filament-break#reply-120629

 

Sorry for the very late answer, I've been a bit busy lately.

First of all: please watch out with placing non-food objects in your microwave. Your microwave works by heating water, but if there is not enough water in the microwave, it can overheat. It is often recommended to place a glass of water in the corner of the microwave to absorb energy. I know this because I've spend some time microwaving stuff for my graduation research (making Graphene) and just for fun :).

I believe PLA can become brittle because of multiple reasons.

- Polymers have relaxed into a certain state, and alignment (crystallinity) makes it stay in that state. Changing the shape of the filament will break these alignments, reducing the stability. It will take a lot of time for PLA to relax again, as this is a very slow crystallizing material. A higher temperature leads to faster relaxation, which is why annealing (hot water, oven, microwave, etc) changes how the material behaves.

- Degradation leads to brittleness. I don't think a measurable amount of hydrolysis will happen at normal storing conditions, but UV damage can make a polymer brittle. Degradation is irreversible damage. This damage is mostly happening on the surface of the material, exactly where you will have the most stresses.

- Changing of composition will also lead to changes in the behaviour. But for PLA, I believe the only change will be that some water is absorbed. Absorbing water could make the material slightly softer at first, but it could also be a 'seed' for crystallization, increasing brittleness...

Edited by Guest
forgot a part.
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