Silicone casting exercise

For my final project for soft robotics class I am interested in making a soft robot with an exoskeleton.  The idea is that the exoskeleton will protect the soft materials from punctures and cuts. The robot would still be based on soft actuation mechanisms. To test out the idea I thought of making a small 3 prong gripper with an exoskeleton. The plan was to 3D print the grippers exoskeleton in a flexible material and fuse that with the inner silicone gripper in the silicone casting process.

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The above picture shows a 3D model of the 3 pronged gripper. The red part being the tough but flexible exoskeleton and the inner light grey part being the silicone casting, this is the part that is actuated. The exoskeleton being flexible, conforms to the silicon inflation.

I took these files to 3D print. But it turned out that the 3D printers were not capable of printing in flexible material. So as a workaround I printed the upper cylindrical part and the 3 flaps separately and thought of joining them with black electrical tape.

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Pictured above is a CAD model of the mould for casting silicone, the sloping feature to the right is the spout from where the silicone will be poured.

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the silicone casting mould is attached to one of the 3 flaps on the exoskeleton as pictured above and the silicone is poured from top (indicated by the black arrow)

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3D printed parts ready for casting!

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I poured in the silicone and set the part aside for curing. I only applied mould release to the inner part as I wanted the silicone to adhere to the gripper flaps. I have to admit, the silicone pouring process was extremely difficult as the size of my nozzle was very small. The silicone wasn’t flowing freely into the cast, instead it was just sitting at the nozzle opening after a point which made me think that the silicone was fully poured in.

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As you can see, it was a massive failure! But it left me with some valuable learning.

  1. Silicone wont naturally bind to any surface. I should create a physical/mechanical bond to secure the two together. One possible way of doing that is to create holes on the one surface (usually the harder surface) and let the silicone flow through it over to the other side. On the other side the silicone needs to be fixed in place with a slight flange. 
  2. The nozzle for pouring silicone should have a good enough opening, at least 10mm will make it easier to pour in.
  3. Incorporate small runners throughout the mould on the opposite ends of the nozzle to let the air out as you pour the silicone. This minimizes bubbles from forming in the silicone and at the same time helps the silicone flow into the mould. Sometimes if the nozzle is too small to clog the silicone and the mould doesnt have any means of releasing the air the silione will not pour in.
  4. An alternative way of casting this would be to first pour the silicone in its mould and then secure it to the flap, that way i could have easily poured all the silicone into it’s mould.

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