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Material Choice for Leg Braces


JayWatt
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Posted (edited) · Material Choice for Leg Braces

I am designing 3d-printable AFOs (leg braces) and am hoping that someone with materials experience can point me to an appropriate filament type/brand  to use.  I am currently operating a Creality CR-10 V2 equipped with an E3D Hemera hotend/direct-drive extruder.  I have made the braces using PETG, and they crack after a day of use.  I have to use a heat gun to get them soft so that they can be custom-fit to my legs, and that additional heat seems to exacerbate the brittleness problem.  I just tried TPU for the first time and am excited about its potential, but the stuff I used flexes too much.  (Duramic Filament TPU, Shore hardness 95A)  I'd like something that will stay more or less solid on thick 100% infilled parts, but manufacturers tend to prioritize softness over strength on the brands that I have seen.  I am considering Nylon, but I've never printed with it.  I'll have to modify an old food dehydrator to pay filament out to the printer if I go that route.  An improvised enclosure is also on the menu.  I have heard that actual custom AFOs are usually made from either Polypropylene or HDPE.  Those filaments are apparently available, but I'd heard that they can be very challenging to use.  I'd just start buying and trying stuff if I could, but it'd all get too expensive very quickly.  'd also really like to try Polycarbonate, but the extremely high bed and nozzle temperatures involved are somewhat daunting.  It can be done if that's the only option, but I'd consider 40 hour print sessions with a 300+ degree nozzle and 150+ degree bed to be something of a last resort.  Does anyone have any experience with a material that might be well suited for what I need?  If I can get something that works, I'm going to upload it with instructions.  Maybe someone with more talent will wind up outdoing me.  That's the hope anyway.

Edited by JayWatt
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    Posted · Material Choice for Leg Braces

    Definitely try nylon. Taulman Alloy 910 is great if you are good with basic black or natural/transparent. I like Matterhackers PRO nylon if you want color. The Matterhackers stuff is a bit more flexible, I think. I love nylon. It can take a while to get adhesion dialed in. Magigoo PA works well on glass. You might need a brim depending on which type of nylon you are using. 

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    Posted (edited) · Material Choice for Leg Braces

    Color doesn't matter so much so long as it isn't overly vibrant.  I was looking through Taulman's catalog and they have some neat stuff.  They've also got fiberglass filled Nylon.  But then, there are several types of carbon fiber filled Nylon too from other brands.  That's really exciting.  Their PCTPE is also very intriguing.  Combining the flex of TPE with the rigidity of Nylon could yield some interesting possibilities.  Thank you for bringing them up.  They're working on some cool formulas.  I'm pretty blown away with that Hemera too.  I just installed it a couple of weeks ago.  I couldn't print TPU at all on the stock Bowden setup, but that Hemera will extrude it at over 70 mm/s with no issues.  The printer bed can't keep up with anything faster than that, so I'll probably never know how fast I could go.  I'm very impressed with it.  Just need a thermistor upgrade so that I can go over 270 degrees.  Anyway, thanks again.

    Edited by JayWatt
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    Posted · Material Choice for Leg Braces

    Matterhackers NylonX (carbon-fiber + nylon) s good stuff. You will need to use a brim, and I wouldn't recommend a flexible build plate for large parts. Small parts are ok. Remember that you will need a hardened nozzle to print CF- and fiberglass-reinforced materials. 

     

    Ultimaker clear nylon, followed by the black, makes the best looking nylon parts I've ever seen. 

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    Posted · Material Choice for Leg Braces
    10 minutes ago, dcschooley said:

    Matterhackers NylonX (carbon-fiber + nylon) s good stuff.

     

    I've wondered if the carbon fiber really does much. The strength of carbon fiber is in the fibers crossing over each other, glued by the carrier. With filament 3D printing, no fibers cross layers. I can see it increasing wear resistance, but since the biggest potential weakness in filament 3D printing is between layers, does the carbon fiber really help the strength by a significant amount?

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    Posted · Material Choice for Leg Braces
    1 hour ago, alienrelics said:

    does the carbon fiber really help the strength by a significant amount?

    That I can't say for sure.  I imagine that it would be helpful if layer adhesion is good.  That PETG that I was previously using was not cracking along layer lines.  Those cracks were perpendicular to them.  One claimed benefit that I have read is that the CF helps prevent the part from warping while its printing.  So much so that I have heard from several pages that it makes it significantly less challenging to print than regular Nylon.  I don't know that from experience, but that's what they're saying anyway.  I'm going to get some hardened nozzles and try a few things.  Should be interesting.  Getting a new pair of professionally made braces in a week, so I don't have to be in a major hurry to get something printed.  Custom-made plastic ones can be as much as $10,000 per pair.  It bothers me that there are people that can't afford what they need.  If not for insurance then that would have been my situation.  

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    Posted · Material Choice for Leg Braces
    1 hour ago, alienrelics said:

     

    I've wondered if the carbon fiber really does much. The strength of carbon fiber is in the fibers crossing over each other, glued by the carrier. 

     

    I wish I knew the answer to this. A lot depends on how you define "strength." The carbon fiber definitely does something. My overall impression is that NylonX is stiffer and harder than unfilled nylon. It might also be less dense. NylonG (glass filled) is similar but not entirely the same. I don't know how much of this is due to the composition of the fill vs. the composition of the nylon. My formal training in materials science consists of a single undergraduate course decades ago. From what I do remember, adding particles will have some sort of impact because the polymer molecules in the nylon will have to rearrange themselves differently due to the presence of the little bits of carbon (or glass.) The problem with much of the testing you see on the internet is you don't know the composition of the nylon used in the composite. You can also make the argument that the mere presence of some sort of added particles might be more important than the composition, i.e., carbon fiber vs. glass. 

     

    From my experience, in terms of stiffness, which is just one measure of strength:

    NylonX/NylonG > Ultimaker Nylon/910 > Matterhackers PRO nylon > Taulman 645/Bridge

     

    Ultimaker Nylon and Taulman 910 appear to be very similar and are nearly as stiff as the composites, but that is based on my part designs. Someone else might get different results. 

     

    I like to print the same parts in different materials just to see how they do. I really need to do some more formal testing. 

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    Posted · Material Choice for Leg Braces
    12 minutes ago, JayWatt said:

    That PETG that I was previously using was not cracking along layer lines.  Those cracks were perpendicular to them. 

     

    PETG might be the most overrated material on the planet. It is much more brittle than it is given credit for being. PETG is nice because the prints look good, the temperature resistance is better than PLA, and it's easier to print than ABS. The parts also break when you drop them onto a hard floor. I've seen them break when dropped onto a hard floor covered in carpet. Most of what I do is not temperature sensitive enough to require PETG over PLA/Tough PLA, so I plan to use up my PETG on test parts and then only buy PLA, Tough PLA, or ABS when I need fancy colors. 

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    Posted (edited) · Material Choice for Leg Braces

    PETG definitely has its quirks.  One thing I LOVE about the stuff though is easy support removal.  When I need supports, I'll use a ~5mm support interface at ~30%+ density.  Then I turn the "Support Interface Flow" down to about 70%.  Supports generally pop right off with little hassle.  That technique works somewhat with PLA-based filaments, but PETG is amazing in that regard.  It's unfortunate that its other properties make it a non-starter for a lot of projects.

    Edited by JayWatt
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    Posted · Material Choice for Leg Braces
    18 hours ago, dcschooley said:

     

    What drew me to PETG at first was its ability to handle UV: I make a lot of parts that get used outdoors.   You're right about the shattering when dropped on a hard floor.   Some material manufacturers actually use the word "glass" in their product title for the stuff.   However if the CAD design for a widget gets dawn up in a way that expects this, then the part won't break very easily.  Solid PETG parts break, but if there are stress-absorbing structures, then the impact gets spread out and handled better. I liked printing the filament enough that it pushed me to learn better design processes so I wouldn't need to change over to some other UV-friendly filament, which are few and far between.  

     

    Another positive point for PETG:  Cold pulls are something I never need to bother with.   PETG can sit in a nozzle or caked on a nozzle for hours, get gunked up as heck, but will extrude right out when a little heat is added.  To clean the gunk off the nozzle, I just turn the hot end up to 240C, use a wire brush on the underside of the head and it's mostly clean again in short order.

     

    I haven't had such rosy experiences with PLA or ABS.   Could be that I just need to practice more with those.  Or I'm just lazy 🙂

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    Posted · Material Choice for Leg Braces
    6 hours ago, chrisw said:

    However if the CAD design for a widget gets dawn up in a way that expects this, then the part won't break very easily.  Solid PETG parts break, but if there are stress-absorbing structures, then the impact gets spread out and handled better. 🙂

    You are totally correct about designing to distribute the stress, but you can't always do that. Several years ago I was making interlocking cube-puzzles for coworkers. I was making them in either school colors or flag colors for those from overseas using high-quality PETG. Because of the way the pieces needed to lock together, there wasn't a lot of room to adjust the design, and at the time I trusted the material.  The parts didn't weigh very much, but they could still break if you dropped them. 

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