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3D-print molded Protosil RTV 245 (durometer shore 40A) silicone covers for Toughware Equilux [proof of concept, demo of "bionic" grip]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - 3D-print molded Protosil RTV 245 (durometer shore 40A) silicone covers for Toughware Equilux [proof of concept, demo of "bionic" grip]; published February 4, 2018, 11:50; URL: https://www.swisswuff.ch/tech/?p=8248.

BibTeX: @MISC{schweitzer_wolf_1571570036, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - 3D-print molded Protosil RTV 245 (durometer shore 40A) silicone covers for Toughware Equilux [proof of concept, demo of "bionic" grip]}}, month = {February},year = {2018}, url = {https://www.swisswuff.ch/tech/?p=8248}}


It is sometimes more fun to present the technical results before or even instead of explaining exactly why.  So in short, I 3d-designed and then printed molds to make grip covers for really serious grip performance of a Toughware Equilux device.

3D print with normal / average draft print parameters

So the 3D-print was performed with sufficient resolution - but I did not overdo it there seeing as if that was just a prototype. Once this becomes established I may start putting the cast angles at 2 degrees ; ) But as is, a silicone mold for 40A durometer probably is cool with just a bit of infill and not too much tweaking. At any rate I just printed these with fast forward.

Manually processed silicone

At this point I will point out that the prosthetic industries in 2018 has reached such a state whereas the arm amputee himself, single-handedly, will not only design and print molds using 3D design and printing, but manually process silicone, to achieve mad grip skills with a new prosthetic device. Not that achieving mad grip skills by tweaking my stuff is entirely new to me (TRS Prehensor, V2P Prehensor, .. ). Sometimes copied (whodathunkit), but never properly referenced, and definitely never reached. Before that, it had to become the year 2018, wherein a body powered hook or similar gripper device is still by far the best for real work. And don't tell me I am responsible for that - even though I do know what works and what does not. Just saying.

I used Protosil RTV 245 by Altropol (Germany) with a declared Durometer Shore 40A hardness and 12 hours curing time. To test this using these new molds, I just mixed a small amount (80 grams of component A and about 8 grams of component B).

This is the mixed stuff. It is pretty sticky to be honest.

Filling the molds

Here are the molds filled up with still liquid silicone. I wrapped them in plastic bags to protect the molds and of course the rest of the kitchen.

Finished gripper pads

Here are some pictures from taking the silicone bits out of the mold. I did design the molds so these would be relatively easy to take out, using the blunt end of a pencil. If you cannot believe that, get these printed yourself (3D files below), fill them up, let them cure then see for yourself. So if you ever design 3D printed molds for an environment where an arm amputee processes the results using very available materials all by himself / herself, such considerations may play a role.

Below is a picture of the finished silicone grip bit already with screws inserted to mount on Toughware Equilux gripper device.

The rectangular grooves are to hold a steak knife to cut meat. Or to properly eat pizza with a knife and with this gripper device. Or such. Really, I established the great use of that particualr shape all already last summer with a bit of PLA prototype testing. So, yes the groove needs to be that deep, yes the direction really makes sense, it has to be at that angle.

Tools used

This shows most of the tools and parts used.

The molds (left side) designed of course to be filled up easily and to also allow for relatively easy removal of the cured grippers.

I wear variable focus glasses via Aliexpress (102 USD) for perfect vision during such work, which easily would cost me over 10x as much if purchased locally.

Sharp knife to remove the mount bits off the cured and removed silicone parts.

Screw driver to mount the bits onto the Toughware Equilux gripper (right side in image).

Application of the grip pads

As the deformable grip pad to achieve grip form closure usually now has the correct hardness characteristics for grip shape optimization, it lacks tensile strength so it routinely wants to be protected by a suitable fabric. In other words, your perfect grip surface necessarily wants to be multilayered. I use cut-up work gloves for that and fix the fingers to the Toughware Equilux device using electrical tape ("Isolierband").

Finished device. Grip performance bordering on the truly insane now. With this type of material, I would call the approximation of true finger tip deformability "bionic".

Pulling a magnet off a metallic surface

Update Feb 8th 2018.

The Toughware Equilux (TE) here is in its VO mode (voluntary opening) so it is held close by virtue of that red rubber band. If you followed the previous posts closely you may have realized that the TE is not the strongest VO device ever - which totally does not matter as you can switch it to VC (voluntary closing) operation by simply flicking the cable lever over. But this here is in weaklish easy relaxing VO mode, and just for show and giggles.

Us amputees have it hard enough as is. Why make things extra hard for us - and with regard to grip surfaces, this can be taken literally. Gripper surfaces must be so soft the material falls apart just by looking at it and then covered by deformable stuff with high tensile strength, as here. Otherwise the soft material will tear up upon first contact with, say, a simple car wash, and so how will you make it through the day after that happened. But with soft stuff inside and deformable but protective textile covering it, I can use low grip forces, achieve perfect form closure grip wise, and then exert maximal lift forces. Learn this by heart if you must, otherwise maybe come back here.

I have spent the last few days filled with awe with the absolutely mad grip skillz that I have somehow managed to build into this thing. Also cutting a steak with a standard cafeteria knife now is rock solid and safe - the first time I had used the TE with its standard issue pads, the knives would fall to the floor a few times, no way even with VC and full force this would work out ever. The hard grip pads were not able to stabilize a knife. It is a shape thing, really, as wearing any Hosmer 5 model will allow knife placement without problems. But now? The Toughware Equilux grip and cable forces are almost impercetibly low and I can literally bathe in lift force. To be beside oneself.

Just so you can see this for yourself. A moderately strong magnet with a weak grip force and insane mad grip pads. Test piloting new stuff requires step-wise approximations. Cheers.

Maybe I will find more time to explain, here, why this is so cool grip-mechanics wise. Maybe you have to come back a few weeks later to check whether I edited this post to explain all that.

3D-models - downloading files, ordering ready-made models

Files are here:

You do not need a 3D printer, you can get these 3D-printed as high quality models for molds and soft part creation directly here:


 

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