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Printrbot Talk Forum • View topic - What material would you recommend for a prosthetic hand?

What material would you recommend for a prosthetic hand?

Talk about materials used in 3D printing

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 . 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 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,
Strantor
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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

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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

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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

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

Postby strantor » 2014-Jan-Thu-15-Jan

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

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

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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
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Re: What material would you recommend for a prosthetic hand?

Postby strantor » 2014-Jan-Thu-16-Jan

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

Postby thawkins » 2014-Jan-Thu-18-Jan

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

Postby strantor » 2014-Jan-Thu-19-Jan

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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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225mm x 200mm - Heated Bed
RAMPS 1.4 running Marlin 1.1.0
Full Graphic display.
-------------------------------
Zen Toolworks CNC/3d printer
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Dual heated beds.
RAMPS 1.4 running Marlin 0.98
-------------------------------
Flashforge 3d Creator Pro
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220x143x150mm
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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

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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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Printrbot Plus V2.0
Aluminum bearing blocks
Aluminum Build Plate
Robotdigg custom Nema 17 motors with 500mm T8*8 Acme Rods on Z-axis (19" of print height)
Wire extensions to accommodate extra build height
GT2 Belts and Pulleys
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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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Printrbot Plus V2.0
Aluminum bearing blocks
Aluminum Build Plate
Robotdigg custom Nema 17 motors with 500mm T8*8 Acme Rods on Z-axis (19" of print height)
Wire extensions to accommodate extra build height
GT2 Belts and Pulleys
Nickies08
Solid infill...
 
Posts: 237
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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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