What is the correct maximum volumetric speed for high strength?

I’ve recently been looking at purchasing a P1S (when they finally come back into stock), but I have seen a few tests using the X1C and P1P (since they have the same extruder) that show that the default max volumetric speed (MVS) has measurably worse layer adhesion than slowing down the printer or choosing a smaller MVS value.

For example, this video from CNC Kitchen:

In it, they directly test MVS vs layer adhesion with PLA, and show that around 12mm3/s you achieve the highest strength, whereas at 20mm3/s (which is close to the stock slicer setting of 21mm3/s for Bambu Lab PLA) the strength is only about 92% that of at 12mm3/s.

Similarly, there is this other video:

Which compares the different speed modes vs layer adhesion with Bambu PLA, and shows that normal mode has about 88% the layer adhesion of silent mode. Knowing that silent mode is 50% of the speed on all moves, this would mean that the test was comparing layer adhesion at 10.5mm3/s vs 21mm3/s.

What I also thought was interesting was that in the CNC Kitchen video, they also used the typical MVS test built-in to Orca Slicer which runs a winding shape in vase mode with increasing MVS from bottom to top. And using this test, they show that the max speed achieved before extrusion problems happened was 23mm3/s. However, despite the test showing that the MVS should be set to around 23mm3/s (or that using the default speed of 21mm3/s would also be fine), in reality this value would compromise strength.

Similarly in the same video, they also mentioned that the filament starts to become matte at around 10mm3/s, which further aligns with the previous test about layer adhesion dropping off around 12mm3/s, with both suggesting that 21mm3/s is too fast for PLA.

  1. Therefore, my question is if I want to print a functional part (where I would generally set my slicer settings for maximum strength), what is the highest MVS I could set whilst still maximising strength (such as layer adhesion)?
    Obviously for PLA, I could just use the value of approximately 12mm3/s, but since I will be using other materials such as ASA for my functional parts, and I know that every material has a different MVS, I can’t just use 12mm3/s with other materials.

  2. Alternatively, is there a test I could run to determine the correct value if I don’t have a testing rig (such as is shown in the 2 videos) to destructively test some samples?
    It appears that the standard MVS test such as in Orca Slicer is flawed since as I mentioned above it gives a value (23mm3/s) that is much higher than the value you would want to use for strength or cosmetic purposes (10.5-12mm3/s), so I assume it would need to be a different kind of test?

I suppose it could be possible to just multiply the stock Bambu MVS values by 50-57% (based on the 10.5-12mm3/s range from those 2 videos), but that is assuming that all of the Bambu default MVS values are too high by the same amount, whereas with more expensive materials such as PAHT-CF, Bambu has set the flow rate to 8mm3/s which might suggest that it has already lowered it to less than what a typical MVS test would give in order to maintain high layer adhesion?

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I don’t think that there is a general rule for all materials. To be sure, somebody would have to test the same material at different speeds as Stefan did in the video. I plan to do that for a few PETG brands although with a much simpler test setup. Especially PETG is also known to be very sensitive to cooling. Too much fan speed can severely reduce strength.

On the other hand, you should think about the question how much the absolute strength really matters. I have yet to come across a design where a reduction to 80% decides over failure or no failure. I started first tests and I was amazed how much actual force you need to tear apart a standing test sample with 6mm diameter. I just had printed some hooks to hang the childrens’ bikes on the ceiling. To be on the safe side, I designed it a bit beefier. With the preliminary test in mind, the hooks probably would have no problem when the bikes weighed 100kg :joy:.

So unless you know how to simulate the stress within a part, you probably won’t be able to create a design close to the limits and ±30% won’t make a big difference.

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All the statements you mentioned are true. You must remember that there is a rat race in printing technology. Bambu has swept the market with its printer and now every company wants to match them. But back to what you wrote. If, like me, you print functional things and mechanical durability is the most important thing, you should (unfortunately) perform a set of print tests for each filament - those from Orca Slicer are very good (and durability tests) to select the most optimal settings.
You don’t need any special machine for this. After printing, which shows where the filament changes from shiny to matte, and where it loses adhesion completely, you can already draw some conclusions. Divide such a sample into 5 parts to which you will assign specific MVS values from the program and, on this basis, print, for example, hooks on which you can hang any load. This way you will test the mechanical strength. Of course, this is the simplest test, because I don’t know whether you need an element that is resistant to tension, bending or compression.
I have a certain PLA filament that looks great when printing quickly - it is beautiful matte, but unfortunately it loses about 30% of its strength. What’s worse, slower printing not only makes it shiny, but also makes the ringing visible - such a strange relationship. Over the last 2 years, I have learned that it is worth sticking to proven filaments (I am not saying not to experiment, because more and more cool filaments appear), but there is nothing like always proven, reliable settings. I also know that often cheap PLA (at home, of course) can have much better mechanical properties than PETG, ABS or PA-CF, etc. So, in summary, you need to perform basic tests for each material before you start using it in production.

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For those who don’t have a test bench to run destructive tests on, it may be worth turning to more conservative MVS values that indicate good durability and print quality based on observations from other users or your own experience. However, for the most accurate results and optimal setup, running your own tests is the best approach. You can also contact material or slicer manufacturers for recommendations on optimal MVS values for specific materials.

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Yes I agree, unfortunately since every material is different (including the same material but from different manufacturers/product lines), then I would expect the MVS to be different for each of them which would need testing.

For my design in particular, the loads are not precisely defined because it is an ebike battery enclosure, and so the worst case scenario would be something like crashing the bike (and specifically the battery box) into a solid object such as a lamp post, or another vehicle. And so in this case there are a large variety of different speeds of both the battery box and the object it would be crashing into, so it’s hard to specify exactly what kind of impact force the box will need to withstand in terms of location of impact, force of impact, the angle of the impact, and the area of the impact.

Also, as you mentioned, even if I did know the exact conditions of the worst case scenarios, I would also need to be able to either simulate these (including taking into account the exact way in which the box is manufactured such as the slicer settings and toolpath), or I would need to do a realistic real world test with the finished enclosure, both of which are not particularly achievable since I am just an individual without advanced knowledge of automotive impacts or testing equipment, rather than say an automotive company.

Therefore, since the load is unknown, I am trying to design the enclosure conservatively to be as strong as is reasonably possible by using deliberately thick walls (5mm), high impact strength material (ABS), and maximum strength slicer settings, which in this case includes the MVS setting.

Yes, exactly! I was almost going to suggest in the original post that using the glossy to matte transition point would be a good way to estimate the MVS at which the strength is still maximised (without needing any tools other or destructive testing), as obviously if the filament has started to become matte then that implies that the hotend is no longer melting the filament thoroughly and so strength will naturally decrease because of this.

Yes, unfortunately I was expecting that to get an accurate value it would be necessary to somehow run some destructive strength tests, and this in particular seems like a good suggestion since I could probably find some rudimentary solid surface, like the edge of a desk or a folding clothes drying rack, and then hang the hooks and some increasing weight on it.

Alternatively, you could get an electronic hook/luggage scale with a max function, and then pull up or down on the scales attached to the plastic hook on one end and a handle on the other end, in order to test the max load before each sample breaks and then compare the breaking load vs MVS values. Also, it doesn’t require any kind of mechanised test rig with motors, leadscrews, etc. like in the CNC Kitchen video.

I’m not sure exactly what the reason is behind this, but my best guess would be that typically when 3D printing, glossy finishes tend to show off imperfections a lot more compared to matte or carbon filled finishes. This is exactly why ultra shiny filaments like silk PLA tend to be so tricky to print well, because they magnify any existing imperfections in the printing process. So perhaps the ringing was always there, but it was not as noticeable with the matte finish?

I definitely agree, and this is part of the reason why I went with Bambu Lab, because they have pre-tuned settings for the exact combination of their machines (e.g. P1S) with a specific nozzle (e.g. 0.4mm) and their specific filament, which together gets rid of a lot of the time needed to dial in the settings to get optimum quality out of your prints.

Having said that, clearly they don’t have everything tuned optimally, such as having too high of a default MVS if you want to maintain max strength. I suspect that this is exactly because of the rat race that you mentioned, where they can set the MVS artificially high to say 21mm3/s for PLA instead of around 12mm3/s (where it should actually be if you want to maintain max strength and a glossy finish) in order to make their printers look better than they actually are. And they don’t offer any alternative values on their website if you do care about strength either.

It’s somewhat ironic that you can find countless posts on this forum and elsewhere about issues with prints suddenly changing from glossy, when it is all due to Bambu’s default settings meaning that the outer wall volumetric speed can exceed the extruder’s capabilities, which means you get a matte finish when the printer is running full tilt, but then suddenly glossy patches from when the print had to slow down due to minimum layer time kicking in for example.

In my opinion, this could be easily fixed by just lowering the outer wall speed in all the default profiles such that the outer wall MVS is low enough that it remains fully glossy even at full speed. Effectively what you are doing is limiting the MVS to say 12mm3/s, but only for the outer walls rather than the whole print, since a cosmetic part can be matte (and weaker) on the inside without any issues.

On the other hand, if you look at it from a time efficiency standpoint rather than just having the biggest number for marketing, you’re only gaining about 10% strength but you lose about 40% of your flowrate, which means a total time per print increase of more than 10%. So if you just want to churn out a batch of parts as quickly as possible (either cosmetic or functional), then it might make more sense to just use the defaults if you can tolerate the 10% hit to strength.

But overall, this default high MVS value is continuing a worrying trend I have been noticing with Bambu slicer settings, which is that they seem to prioritise cosmetic parts over actual functional printing. For example, Bambu Studio doesn’t address a critical problem with 3D printing that is needed for functional parts, and that is the shrinkage of the plastic caused by the plastic cooling after printing, which means that all parts come out undersized by anywhere from about 0.3% to 1% depending on the material. In other words, an edge that is supposed to be 150mm long would come out as 149.55mm just due to thermal shrinkage of the material when printing in PLA or PETG, and for TPU or even higher shrinkage materials like ABS, it would be even more undersized. Given that most good printers are supposed to have a dimensional accuracy of ±0.2mm, 0.45mm undersized (or more) is just unacceptable for printing a functional part. And not only does Bambu not include shrinkage factors for all their filaments pre-tuned in the slicer profile (to make it “just work”), they don’t even have a shrinkage factor field in the slicer at all!

This was one of the key reasons I decided to switch to using OrcaSlicer after pretty much the 1st day of using a Bambu printer, because having to manually scale up all your objects in the slicer in only X and Y (since the shrinkage doesn’t affect Z due to the way FDM works) would just be a nightmare to do every single time I want to print a functional part. So at least with OrcaSlicer you can save the shrinkage factor into the filament profile.

Having said that, it’s yet another calibration step I have to do manually for each material since like I said, Bambu doesn’t even include this information for their own filaments. This combined with the fact that X-Y hole compensation and X-Y contour compensation are both set to 0 by default means that it’s almost impossible to print functional parts with good dimensional accuracy using Bambu defaults, and now there’s the MVS problem to add to the pile of things that don’t “just work” and that I need to tune myself.

Yes exactly, I think the glossy to shiny transition point would be a good estimate as I mentioned previously, but in the end the only accurate test is going to be some kind of destructive strength test.

As for contacting filament manufacturers, I have seen that they usually provide a general speed range in mm/s on the filament product page, but firstly that is just the linear speed not the volumetric speed (you would need to know the exact line width and layer height that is used with those speeds), and secondly it’s usually a fairly wide range so I’m not sure how you could figure out a specific MVS value given those two issues?

I’m in the middle of testing different PETG brands and print settings for the influence on strength.
For that I built a simple jig with some wooden levers, hinges and a force meter with peak hold function. I also designed a sample that combines some axial and some bending load and fits the measurement range of the meter. So it is kind of a poor man’s tensile testing machine with manual actuation and only peak strength.

So far I have tested 6 brands at 120 and 300 mm/s and 4 samples each. The results were very mixed. Most brands are weaker at 300 than at 120, but not all. And variation between brands is really impressive. I think, the transition from glossy to matte can be an indicator but it is not reliable. One filament had exactly the same strength at both speeds. So i think there is really no point in trying to generalize findings.l

Regarding compensation of shrinkage, in defense of Bambulab, it is just based on Prusa Slicer which does not offer it either, and many other slicers do not too.

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Interesting that there are some brands of PETG that are just as strong at 300mm/s than at 120mm/s - perhaps they have some kind of modifier in the formula that makes it print better, like you see with those “high speed” PLAs?

Regarding the shrinkage compensation, I agree that PrusaSlicer and many other slicers don’t have a field for it, but just because Bambu Studio is a fork of PS doesn’t give it an excuse in my opinion, since there are other PS forks such as SuperSlicer (which I used before switching to OrcaSlicer) that do have a field for it.

And in my opinion if Bambu wants to be taken seriously as a manufacturer that caters towards professionals or people who focus on functional printing (including winning over people who use more expensive machines like Ultimakers because they “just work”) then having a field for shrinkage in the slicer is going to be pretty essential, not to mention having it pre-calibrated for all their filaments, as well as having X-Y hole compensation pre-tuned in their default profiles.