One of the first things I fabricated was a single arm that can be expanded to curl around an object. I 3D printed the mold and cast the arms in EcoFlex 00-30 silicone. To inflate the cavity inside the arm, I used a mini DC motor pump. Observing the arm and how it behaves was quite interesting and prompted me to see what other improvements and designs I could make.
This was my first attempt at making a 4 arm soft gripper. While I was making this particular prototype, I added a layer of fabric on the bottom of the arms. The fabric embedded into the silicone prevents that entire face from expanding too much. It essentially aids in making the arms curl. Depending on the thickness of the silicone walls, the arm will expand less if the wall are thicker. The same effect can be achieved if the face with fabric is much thicker than the rest of the walls. This prototype however did not turn out too well for reasons including the design of the mold and grip mount, as well as the fabrication of the arms themselves. Any slight inconsistencies in the thickness of any face on the arms will result in the thinner walls expanding further outward due to the pressure in the volume of the cavity.
The following video shows an example of an improperly fabricated gripper where different parts of the face have varied wall thicknesses. The bottom face can be seen expanding at different rates for different arms. For me, this was an undesirable effect, but it was interesting to observe how much the EcoFlex 00-30 silicone could expand without failing from the hoop stress experienced.
The following shows an eight arm gripper, like an octopus, working as intended with all arms expanding at similar rates. The photos below show that various stages of the fabrication process for these octopus arms and a little head.
In order to cut down on the fabrication time, I explored laser cutting acrylic pieces to quickly prototype molds. An issue with 3D printing molds is the overall time to print. One would want a high resolution print depending on the printer so that there are no gaps between the layers. When casting the silicone into the mold, there is a chance that silicone can seep into the print which will make removing the part difficult, even with mold release agent applied. For prototyping mold purposes, if the same function can be achieved with laser cut material and a little bit of glue, the overall fabrication time can be greatly reduced.
Unfortunately with the particular design I put together, certain walls inside the cavity of the casted actuator were ripping away and the final result just looks like balloon inflating. But by altering the mold design so that the inner cavity walls have more surface area for the walls to glue together, this laser cut method of creating molds can be very fast and effective.
This entire study with soft robots and how to design and fabricate them was a great introduction into molding and casting, as well as a eye opener to the growing technology of soft robotic actuators.
- 3D Modelling
- 3D Printing
- Laser Cutting
- Molding and Casting