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I am currently working through various different iterations ofccreating a propeller for a model ICE (nitro engine). So far I have managed to disintegrate each of them eventually. I have been experimenting mostly with PLA and PETG so far. I am currently thinking about using carbon-infused PLA or using PC (I found one that is printable on my printer). However, I am not so sure what properties I exactly have to look for when it comes to a propeller.

I am making the following assumptions:

  • When rotating fast (5-6k RPM) there it probably has to create quite some centripetal force and I assume this would translate to tensile strength
  • When the engine starts or stops it gets accelerated and decelerated almost instantly which probably causes quite some jerk to the propeller. However, I do not know what property that would translate to, also tensile strength.

So far the PLA propeller has held up the longest and I cannot rule out that it accidentally touched something when it broke. That is of course something it does not have to be able to handle, so I am probably going to give PLA another chance.

The PETG propellers all literally broke apart the second the engine started, and it did not even hold up one rotation.

The design of the propeller is actually sacrificing some aerodynamic efficiency in order to make sure there are no "thin" parts. It is typically breaking in the section I have highlighted in the the following picture.

3D rendering of a propeller with a circle around a section

I have also a picture of the "remains" of the broken PLA propeller. It actually held up many runs before it eventually broke. It actually partially delaminated the entire wall.

A damaged print of a propeller

I am looking for two recommendations/tips:

  • What material properties should I look for in my use case?
  • Less important, but some tips regarding print settings are also appreciated. So far I could notice that smaller layer size helps, and thicker walls also helped.
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    $\begingroup$ Are the prints printed solid? And if not why not? Also the geometry of the propeller is going to strongly dictate its strength. A photo of the failed prop will likely improve any answer. $\endgroup$
    – Kezat
    Aug 15 at 12:48
  • $\begingroup$ Oh, I completely forgot to add the pictures I had prepared! $\endgroup$ Aug 15 at 12:49
  • $\begingroup$ Regarding solid yes/no. I did not print them solid assuming some flexibility from the infill and it not being completely rigid might help overcome some "spikes" in force without immediately breaking. $\endgroup$ Aug 15 at 12:56
  • $\begingroup$ Mostly out of curiosity at this point do you have a photo of the failed PETG part.? :) $\endgroup$
    – Kezat
    Aug 15 at 13:24
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    $\begingroup$ With a model aircraft background (building, flying international shows), and seen injuries with propellers, I strongly advise against printing propeller blades, please buy them at your local model aircraft shop or order online. $\endgroup$
    – 0scar
    Aug 15 at 17:38
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As Kezat noted, your part has very low infill, but infill is not what provides most of the part strength anyway. From what I can see of the broken part, it looks like you only have 1 or at most 2 walls, and these are what contribute most to part strength, especially in the orientation you're working in.

You should probably increase the number of walls to the point where the part is entirely solid, and has no infill region in any layers. This gives much better strength than selecting "100% infill", which will normally use alternating diagonal lines to fill the region; even if "concentric" is selected, which "looks like" more walls, there are subtle reasons it's not quite as good.

You may be able to get away with fewer walls, possibly just 3 or 4. If you're trying to keep total mass low, that might be worth trying first. But you definitely need more than the 1 or 2 you have now.

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    $\begingroup$ I used an almost 100% walls (not infill) print for my next iteration. The image looks like its only 1-2 walls, however at least 2 walls had completely delaminated during destruction of the propeller, it was printed with 4 walls. $\endgroup$ Aug 15 at 17:44
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    $\begingroup$ If they delaminated, you should check for underextrusion - either globally, or due to oozing (e.g. from combed travel over the infill region) prior to the wall extrusions. $\endgroup$ Aug 15 at 17:56
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    $\begingroup$ A completely solid propeller would be counterproductive: the material near the tips is providing no strength, but is increasing the load near the hub. If your slicer permits it, increase the wall count just near the point of failure (ideally, tapering off as you go outwards, until the ends of the propeller have just one wall). $\endgroup$
    – Mark
    Aug 15 at 23:16
  • $\begingroup$ @Mark good point, I will play around a bit, I should be able to use various profiles for different parts of the object $\endgroup$ Aug 16 at 7:16
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    $\begingroup$ The almost solid PETG prop went through an entire tank and large parts at full throttle without any issues at all. So that’s already a big improvement! $\endgroup$ Aug 16 at 13:11
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I'm offering a different kind of answer, please do not print your propeller blades. I've seen accidents happen (in my past international model aircraft building and flying hobby) with even the bought injection molded propellers (usually bad handling or aging).

Propeller blades can be bought online or at your local model aircraft shop against reasonable prices. You don't want to lose your eye or somebody else's.

Even if used on a car or a hovercraft, please use a bought injection molded propeller and provide proper shielding.

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    $\begingroup$ I wouldn't go so far as to make this an absolute, but yes, please practice proper safety with whatever you use. Personally, for me that would mean shielding for any propeller operating with sufficient energy to cause injury. $\endgroup$ Aug 15 at 17:59
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    $\begingroup$ I cannot disagree much with your safety recommendations, however at the moment it’s calculated risk. The only person possibly getting harmed is myself, there are hundreds of yards of open terrain with no one on it except me, so as long as I am following safety precautions for myself which is easy enough it should be fine. $\endgroup$ Aug 16 at 17:20
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You need WAY more infill or make it solid. Normal props like this are 100% solid for a reason.

To add to that, in my experience with 3D printing the the infill percent is only part of the story for part strength. In many cases adding perimeter loops and not infill is a better solution for part strength.

With a solid part I suspect PETG will come out ahead over PLA as PLA can be more brittle and just snap under shock or vibration.

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  • $\begingroup$ I am going to give a solid version with both PLA and PETG a try, the PLA one was very early and I used my default settings. Recently I tried a PETG that was more material, but it did not help much: i.imgur.com/dFK54y0.png . $\endgroup$ Aug 15 at 13:19
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    $\begingroup$ Don't use "100% infill" setting though. Increase the number of walls so that there is no infill region. This will give much greater strength. You might even be able to make it sufficiently strong without eliminating the infill region, just with 3 or 4 walls. Right now it looks like you only have 1 or 2, which is ridiculously low for a functional part under significant forces. $\endgroup$ Aug 15 at 14:38
  • $\begingroup$ If the issue is tensile strength, PLA beats PETG. PETG is better for handling shocks and other varying loads, but for raw strength, PLA wins (at least until material creep starts deforming it). $\endgroup$
    – Mark
    Aug 15 at 23:19
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    $\begingroup$ Well the commercial ones are solid because they are injection molded. $\endgroup$
    – FarO
    Aug 16 at 8:30
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A nitro engine rotates much faster than an electric one, 30k rpm vs 10k rpm top.

Also, it has much more vibrations (the engine gives torque in sharp pulses, not a continuous power generation). There's a reason if the propellers for electric are not compatible with nitro engines. You will need to step up the game with fiber-infused plastics, and shock resistant plastics, so check CF-ABS, CF-ASA, CF-PC, nylon-CF, nylon-GF.

Forget PETG or PLA...

And use an infill that is equally resistant in every direction, so cubic or gyroid.

And do a simple calculation, concerning the infill: use the weight and the spinning speed to calculate the strength you need for the area where the blade attaches to the hub. I think you need an almost solid infill to hold (solid infill = as many perimeters you need to fill it), and then you can progressively decrease the perimeters closer you get to the tip, replacing them with infill.

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  • $\begingroup$ I did not mention this in my question, but I am using it on an aircraft engine, it has a practical range of 2-10k RPM. But I am definitely going to try different materials as well. $\endgroup$ Aug 16 at 8:41
  • $\begingroup$ The vibrations are still there, even if the RPM are lower. You need fiber reinforced materials. Normal ones are not because injection molded plastic is stronger than FDM parts not only due to lack of layers, also due to the pressure involved, but you need the reinforcement. $\endgroup$
    – FarO
    Aug 16 at 12:19
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For safety on a project like this, it's best to have a self-contained chamber to run a life test until the propeller falls apart. Once a material and design has a reasonable lifetime, then you could use the print.

Reputable propeller manufacturers will already do this. They may even run a shorter burn-in test on product being sold to eliminate defects.

I would suspect PLA as being too brittle. Nylon carbon fiber is probably a better material, but not easy to use. Unless you're wanting to experiment with propeller design, printing propellers will cost you more then buying them.

As Brydon mentioned life testing usually does acceleration such as twice the rate. Temperature is also important. Plastic is more brittle cold and softer hot. You may even want to off balance the axis slightly to accelerate the testing.

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    $\begingroup$ To add to this - I wouldn't feel confident using a propeller unless it handles twice the rated max RPM it would be used at. This is coming from someone who carves their own wooden propellers $\endgroup$ Aug 17 at 16:23

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