Materials

The following are my rough notes on the properties of different materials we've tested at the Miller Lab as they relate to making flexible joints for prosthetic hands. Testing was mostly done using Steve Wood's Flexy Hand and my own Flexy Beast. So far, the most suitable silicone material is Dragon Skin 30. Filaflex TPU filament as recommended by Steve Wood also has good properties for flexible joints; other flexible filaments may not have the necessary elasticity. For actual quantitative data, see Torture Testing.

Curable resin materials

Dragon Skin 30

• Excellent with the sizes in the “Experimental Dragonskin Joints v1” mold in Flexy Hand
• larger joint size comes out of socket sort of too easily but not as bad as Dragon Skin 10 (will be improved by making the cylindrical part larger). Smaller joint size fits amazingly well.
• still very tear resistant
• may slowly tear after repeated stretching, according to Torture Testing

Dragon Skin 10

• too soft - gravity pulls the fingers closed when palm is turned downward
• probably good for joints when used as a stretchy elastic instead of just flexing
• very resistant to tearing
• very easy to remove intact from one piece PLA mold by pulling from one end with tape/pliers/whatever
• the larger size of joint comes out of the socket too easily - ratio of cylinder diameter to middle thickness is too low

PDMS silicone

• very springy, a lot easier to actuate the finger than with the Flex PLA, hopefully meaning more of the user’s force will be transmitted to the object they’re manipulating instead of being used to bend the fingers
• tears easily when bent too far
• once a tear begins it spreads instantly through the whole piece
• prone to tearing when used in Flexy Hand
• difficult to remove intact from one piece mold

Smooth-On PMC 780 Dry Urethane

• need to get mold release agent and possibly use a 2 part mold - bonds to PLA
• Probably too stiff and brittle

Simpact 60A Urethane

• With a liberal coating of release agent, not extremely difficult to demold from PLA but not as easy as Dragon Skin
• At the smaller size used in the Flexy Hand, the mold does not provide enough space for bubbles to escape without a vacuum chamber or other method, resulting in weak, spongy parts
• At the larger size used in the Flexy Hand, the material is too stiff
• Appears to be similar to Dragon Skin 30 in terms of how easy it is to tear and cut
• Very low friction against PLA – requires side guard walls around joints to keep them from sliding apart
• Not compressible enough to be push-fit like the MCP joint on the Flexy Hand and all joints on the current (as of 7/12/14) Flexy Beast

3D Printable Materials

Recreus Filaflex

• Printed at 20-30mm/s with a 0.5mm Makergear nozzle at 225°C
• extruder idler must be fairly tight to grip the material
• Lots of hysteresis when extruding - needs aggressive retraction
• faster printing results in better quality as long as you don’t go too fast and cause a filament jam
• more elastic and goes back to its original position better than Flex Eco PLA
• seems well suited to repeated motion
• less springy than Dragon Skin silicone so the fingers flop around less when the hand is moved
• not super grippy - could probably benefit from having molded silicone fingertips

Form Futura Flex Eco PLA

• Printed at 8-10mm/s with a 0.5 nozzle
• extruder idler must be loose to avoid crushing the material
• 1 perimeter hollow: too soft to stay in hand when any force is applied
• 2 perimeters hollow: possibly just right; slightly difficult to insert into palm
• 3 perimeters with top and bottom solid layer: too stiff to be inserted
• observation: seems to not go all the way back to its original shape when stretched
  • it bends easily and goes back to its original shape when compression/stretching is minimal, but stays stretched when stretched
  • it slowly (3-8 seconds) goes partway back to its original length but not all the way
  • I expect that this material would wear out over time if used in the Flexy Hand design, which requires the joints to stretch in order for the fingers to fully actuate
• improvement: make sure the circles on the ends print a full loop so the whole joint can’t collapse

Other materials

Sugru

• not really springy so not good for joints
• sticks really well to PLA - not good for molding; probably excellent for making grippy finger pads?
• Apparently too hard for finger pads because they need to conform to the object better