What material would you recommend for a prosthetic hand?

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What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-11-Jan

I'm making a prosthetic hand & arm for my father (right arm amputee) based on the theatrical prop inmoov robot hand. I'm experiencing issues with ABS. I am breaking the parts just in the course of trying to assemble the hand. I have a bad feeling about it; this is meant to be a functional prosthetic device and needs to be durable. If he slams the hand in a door, I would prefer it not shatter. I aim to make this hand as strong (think muscles, grip-strength) as a regular human hand and I would prefer the digits not deflect or deform if he squeezes something as hard as he can, so it should be relatively rigid. I would prefer it be abrasion resistant so that the wear and tear of daily use does not demand regular replacement of parts. If he mistakenly grabs something too hot, I would prefer the material not immediately turn to snot - although heat resistance is lowest on the list of importance and if I have to sacrifice, this is where I would prefer to sacrifice.

So far, Nylon 645 looks like the best option, but I'm concerned about it's flexibility. As I saw in this thread it seems superb in strength and impact resistance, but flexes like rubber. Can someone who's printed Nylon 645 weigh in; If I printed the finger digits shown in the inmoov hand link in Nylon 645, are the thin points on the joints going to be so flexible as have a negative impact on the function of the hand?

If so, what other materials should I be looking at? Currently I can only extrude up to 230C, but if the optimal material demands it, I can invest in a higher temp extruder.

Thank you in advance,
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What material would you recommend for a prosthetic hand?

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Re: What material would you recommend for a prosthetic hand?

Postby Nickies08 » 2014-Jan-Thu-12-Jan

I've had some experience with printing small pieces with Taulman 645. The amount of flex seems to depend mostly on the way the component is shaped. I've been printing mounts that have pieces that are roughly, 2cmx3cmx2.5mm and I find they can be pretty rigid. I'm not sure of the rigidity of the other Taulman products though. If you post a couple picture of the small components with an object for size comparison, another member may be able to give you a better idea. I'd be willing to attempt to print one of the components with my Taulman 645 for you and give you an idea of the flex. But I would have to be today because I'm going out of town tomorrow for two weeks.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-13-Jan

Nickies08 wrote:I've had some experience with printing small pieces with Taulman 645. The amount of flex seems to depend mostly on the way the component is shaped. I've been printing mounts that have pieces that are roughly, 2cmx3cmx2.5mm and I find they can be pretty rigid. I'm not sure of the rigidity of the other Taulman products though. If you post a couple picture of the small components with an object for size comparison, another member may be able to give you a better idea. I'd be willing to attempt to print one of the components with my Taulman 645 for you and give you an idea of the flex. But I would have to be today because I'm going out of town tomorrow for two weeks.


Here's a pic:
Image


It's the size of a large man's hand. Here's the part I was trying to assemble last night that was breaking. It's the middle finger. The part that I'm most concerned about is the thin parts (the outsides) of the finger joints. They are maybe 1/8" thick and I do not think any flexing there is appropriate. If you could print that for me and test it out, I would be very appreciative. This specific part also raises a question; how can you glue nylon parts together? Does it take well to super glue, or?
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Re: What material would you recommend for a prosthetic hand?

Postby Nickies08 » 2014-Jan-Thu-14-Jan

One question for you: in what direction are you doing the prints? I find ABS is very poor at holding together between layers. I would print those pieces on their sides so that the "grain" of the print runs from the bottom to top rather than side to side.
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Re: What material would you recommend for a prosthetic hand?

Postby RetireeJay » 2014-Jan-Thu-15-Jan

That "flexing like rubber" is because the walls of that tube were only 0.4mm thick.

In another project, I used thicker cross-sections and have objects that are ALMOST as rigid as ABS would be. viewtopic.php?f=28&t=5209

Printed at the right temperature, nylon 645 has excellent layer-to-layer adhesion (which is what that demo with the single-walled cylinder was intended to demonstrate).

If you are printing a cylinder roughly an inch in diameter and an inch long, with walls 1/8" thick, I think that you will be very pleased with the strength of nylon.

Gluing is a problem. Superglue works best when you have two perfectly mated surfaces - which you will never get with 3D printing because you can always see the traces. There is an epoxy specifically made for plastics that seems to work pretty well, but it's very smelly when you mix it up due to some kind of powerful solvent in it. Available at the big box hardware stores etc.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-15-Jan

Nickies08 wrote:One question for you: in what direction are you doing the prints? I find ABS is very poor at holding together between layers. I would print those pieces on their sides so that the "grain" of the print runs from the bottom to top rather than side to side.

I was printing from the default orientation, all standing up like silos. I know what you mean about not holding together between layers, that is what I was experiencing, and I understand your suggestion, and I appreciate the tip. I may give it a shot, however after my experiences thus far I am inclined to think that even if I print it from a different orientation and the parts manage to stick together long enough for me to get the hand assembled, there will be issues down the road when the hand meets real life. I would much rather upgrade to a more robust material and save myself a lot of worry and heartache.
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Re: What material would you recommend for a prosthetic hand?

Postby Nickies08 » 2014-Jan-Thu-15-Jan

RetireeJay is much more experienced with Taulman 646, I actually turned to him for advice when I went to Nylon printing. When I get home tonight I'll try and split the model into individual pieces so I can print them on their sides and see how they turn out.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-15-Jan

RetireeJay wrote:That "flexing like rubber" is because the walls of that tube were only 0.4mm thick.

In another project, I used thicker cross-sections and have objects that are ALMOST as rigid as ABS would be. viewtopic.php?f=28&t=5209

Printed at the right temperature, nylon 645 has excellent layer-to-layer adhesion (which is what that demo with the single-walled cylinder was intended to demonstrate).

If you are printing a cylinder roughly an inch in diameter and an inch long, with walls 1/8" thick, I think that you will be very pleased with the strength of nylon.

Well, that's very encouraging. Thanks for the info.
RetireeJay wrote:Gluing is a problem. Superglue works best when you have two perfectly mated surfaces - which you will never get with 3D printing because you can always see the traces. There is an epoxy specifically made for plastics that seems to work pretty well, but it's very smelly when you mix it up due to some kind of powerful solvent in it. Available at the big box hardware stores etc.


I don't care much about smell, just strength of the bond. For example in the finger STL file I linked to, each of the 3 finger segments are split in the middle, meant to be glued together with acetone using ABS. If I transition to Nylon, I need a way to make as strong of a bond. How about melting together; have you tried that? I'm thinking, if the layer adhesion is so much better with the nylon during printing, I should be able to take small torch to the two mating surfaces, get them a little melty, and then slap 'em together and call it "welded." Is that all just fantasy? Or does the smelly glue work better?
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-15-Jan

Nickies08 wrote:RetireeJay is much more experienced with Taulman 646, I actually turned to him for advice when I went to Nylon printing. When I get home tonight I'll try and split the model into individual pieces so I can print them on their sides and see how they turn out.


Thank you, I really appreciate it. However, as a proof of concept, I think that printing them upright would be a more direct comparison since that's what I did with ABS. If you can print them (or just one segment) upright with nylon and then insert a small screwdriver in the hole and try to bend the segment outward and see if it breaks or deflects, that would be the most useful data for me.

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Re: What material would you recommend for a prosthetic hand?

Postby RetireeJay » 2014-Jan-Thu-16-Jan

strantor wrote:I don't care much about smell, just strength of the bond. For example in the finger STL file I linked to, each of the 3 finger segments are split in the middle, meant to be glued together with acetone using ABS. If I transition to Nylon, I need a way to make as strong of a bond. How about melting together; have you tried that? I'm thinking, if the layer adhesion is so much better with the nylon during printing, I should be able to take small torch to the two mating surfaces, get them a little melty, and then slap 'em together and call it "welded." Is that all just fantasy? Or does the smelly glue work better?


I don't know. I have used a heat gun (the kind they sell in craft stores) to try to melt away unwanted threads, and it seems that the critical temperature for melting nylon without charring it requires a pretty tight tolerance. I'm afraid that if you got the surface to a melting point without overheating it, you would only have milliseconds to bond your parts before that surface cooled too low for bonding.

I have not really stressed the epoxy to see how well it does.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-16-Jan

RetireeJay wrote:
strantor wrote:I don't care much about smell, just strength of the bond. For example in the finger STL file I linked to, each of the 3 finger segments are split in the middle, meant to be glued together with acetone using ABS. If I transition to Nylon, I need a way to make as strong of a bond. How about melting together; have you tried that? I'm thinking, if the layer adhesion is so much better with the nylon during printing, I should be able to take small torch to the two mating surfaces, get them a little melty, and then slap 'em together and call it "welded." Is that all just fantasy? Or does the smelly glue work better?


I don't know. I have used a heat gun (the kind they sell in craft stores) to try to melt away unwanted threads, and it seems that the critical temperature for melting nylon without charring it requires a pretty tight tolerance. I'm afraid that if you got the surface to a melting point without overheating it, you would only have milliseconds to bond your parts before that surface cooled too low for bonding.

I have not really stressed the epoxy to see how well it does.

Ok then, it sounds like some experimentation is in order. I'm wait til this evening to hear back from Nickies. If his results are promising then I will most likely order some and perform some different methods of bonding and post the results.
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Re: What material would you recommend for a prosthetic hand?

Postby thawkins » 2014-Jan-Thu-18-Jan

strantor wrote:
Nickies08 wrote:One question for you: in what direction are you doing the prints? I find ABS is very poor at holding together between layers. I would print those pieces on their sides so that the "grain" of the print runs from the bottom to top rather than side to side.

I was printing from the default orientation, all standing up like silos. I know what you mean about not holding together between layers, that is what I was experiencing, and I understand your suggestion, and I appreciate the tip. I may give it a shot, however after my experiences thus far I am inclined to think that even if I print it from a different orientation and the parts manage to stick together long enough for me to get the hand assembled, there will be issues down the road when the hand meets real life. I would much rather upgrade to a more robust material and save myself a lot of worry and heartache.


Thats is going to put the weakest direction perpendicular to where most of the stress is coming from, plus the force from putting in the fixings is going to push the layers apart. You really need to print this lying down. so that the shear planes are along the length of the part.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-19-Jan

thawkins wrote:
strantor wrote:
Nickies08 wrote:One question for you: in what direction are you doing the prints? I find ABS is very poor at holding together between layers. I would print those pieces on their sides so that the "grain" of the print runs from the bottom to top rather than side to side.

I was printing from the default orientation, all standing up like silos. I know what you mean about not holding together between layers, that is what I was experiencing, and I understand your suggestion, and I appreciate the tip. I may give it a shot, however after my experiences thus far I am inclined to think that even if I print it from a different orientation and the parts manage to stick together long enough for me to get the hand assembled, there will be issues down the road when the hand meets real life. I would much rather upgrade to a more robust material and save myself a lot of worry and heartache.


Thats is going to put the weakest direction perpendicular to where most of the stress is coming from, plus the force from putting in the fixings is going to push the layers apart. You really need to print this lying down. so that the shear planes are along the length of the part.


Point taken, but my thinking is that, with the complexity of the hand, or some of the parts of it, there are going to be some scenarios where printing everything in the ideal orientation/grain direction just isn't possible, and in the everyday use of the hand it will be put through an infinite myriad of different direcrional forces that may not be agreeable to the print from the most ideal grain direction, much less the least ideal grain direction. So im hoping to use a material that doesn't complain about which orientation I print the part in. This could be a pipe dream, let me know if it is. I'm a noob who doesn't know any better. I hope I'm not dreaming, because if I am, then it means this whole project is a bust unless I decide to whittle the whole thing out of UHMWPE by hand with a dremel.
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Re: What material would you recommend for a prosthetic hand?

Postby thawkins » 2014-Jan-Thu-19-Jan

One technique i used a lot when desiging parts is to put holes perpendicular to the shear plane, and put small bolts with washers on each end through them. They serve two purposes, a) they compress the layers together and prevent delamination. B) they prevent movement parallel to the shear plane which also stops the layers from seperating. If you put small insets at each end, both the head and the bolt can be kept below the surface so they dont protrude. I use various lengths of 1/8 stove bolts which are very very cheap. M3 bolts are almost the same size.

I also use it a lot on columns, by having a central hole to take a bolt. They can in that case just be self tapping and dont need a nut on the other end. I have a load of parts that have an 8mm column to take a 608zz bearing, and before doing this the columns would often snap off.
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Re: What material would you recommend for a prosthetic hand?

Postby Nickies08 » 2014-Jan-Thu-19-Jan

Most of your needed support material comes off very easily, you would not need an extensive amount of effort to remove it. I can usually pull it off my parts with tweezers. The main reason for printing with the grain is for strength where you are connecting it. When I mentioned printing the fingers on their side, that would allow the finger to take force from any direction. If the layers go from bottom to top, pressure against any side would be pushing along the weak part, the join between layers. I honestly feel that you can print all the parts for high strength without giving you too much of a headache! For the effort you are putting in and the willingness to research materials, taking the time to print for maximum strength is wort it. You do not have to print every part this way, but as many as possible would be good. We can look at any model you want and give advice. I just won't be able to print any other test pieces for you over my two week vacation.

On another note, your pieces are printing as you requested, bottom to top. I'll test them once done and let you know the results.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-21-Jan

Nickies08 wrote:Most of your needed support material comes off very easily, you would not need an extensive amount of effort to remove it. I can usually pull it off my parts with tweezers. The main reason for printing with the grain is for strength where you are connecting it. When I mentioned printing the fingers on their side, that would allow the finger to take force from any direction. If the layers go from bottom to top, pressure against any side would be pushing along the weak part, the join between layers. I honestly feel that you can print all the parts for high strength without giving you too much of a headache! For the effort you are putting in and the willingness to research materials, taking the time to print for maximum strength is wort it. You do not have to print every part this way, but as many as possible would be good. We can look at any model you want and give advice. I just won't be able to print any other test pieces for you over my two week vacation.

On another note, your pieces are printing as you requested, bottom to top. I'll test them once done and let you know the results.


Alright, I'm sold. I will print them sideways. I will try it with ABS and see if I have better luck. But I still am very interested in the nylon. Waiting anxiously for the results. Thank you again for taking thr time to print that for me, I really appreciate it.
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Fri-17-Jan

I guess Nickies got tied up. Have a good vacation sir!

Well, I decided to just go ahead and order some nylon and do my own testing. I'm going to set up some force measurement stuff in my garage and try to collect some data.
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Re: What material would you recommend for a prosthetic hand?

Postby Nickies08 » 2014-Jan-Fri-23-Jan

I printed them while I slept and tossed them in my luggage this morning so I could test them. Just finished a 15 hour drive to Ottawa so I have to dig them out and do some testing =)
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Re: What material would you recommend for a prosthetic hand?

Postby Nickies08 » 2014-Jan-Fri-23-Jan

Alright: as expected the parts are nice and solid! They don't have any more flex than you'd find in an injection moulded piece of ABS. I forgot my new hotend runs 15C hotter than my old one and I used my existing Taulman profile to print the parts. As a result, there are spiderwebs everywhere =( Not pretty, but still testable. Layer adhesion held up to prying but still best to print for long term life. Best of luck on your project!
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Sat-09-Jan

Awesome! this great news! Thanks again Nickies, that was going way above & beyond to carry it with you into vacation.
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