Mold Making

Published February 26, 2018

This week, I made a two-part mold for an inflatable silicone actuator. My hope for this design was that the three corners of the triangular actuator would bend inwards to act as a simple gripper. To that end, I designed the mold with the following features:

  • fabric was embedded into the inside half of the gripper to restrict stretching on that side. When inflated, I hoped this differential stretching between the two halves of the gripper would encourage it to bend inward,

  • the inner air chamber was designed to be largest toward the corners of the triangle, which I again hoped would encourage those areas to experience the most stretching, and the most bending inwards toward the restricted fabric-embedded side. This air chamber was to be made by spraying mold release through a stencil. The two halves should remain unconnected where the mold release was sprayed, leaving an inner chamber which can be filled with air.

Test Cast

The two halves of the mold were poured separately at first. I then used the stencil to pattern mold release on one finished half of the actuator, and ‘glued’ the two halves together using a fresh batch of silicone:

Lessons Learned

After the two halves of the casting finished setting, and with a few extra hours to boot (about 48 hours total), I mixed a second batch of silicone to act as a glue between the two. While this method seemed to work at the time, I later heard that silicone that has already set is less likely to form a strong bond with wet silicone. A better approach would have been…

  • make a single mold with is deep enough for the whole casting,
  • pour a first batch of silicone up to one half of the depth of the mold,
  • wait just long enough for the silicone to retain its shape (as little time as possible) during the next steps,
  • apply the mold release pattern to the now-set silicone in the mold,
  • finally, pour a second batch of silicone directly on top of the first.

This approach (specifically not waiting two days between pours) would have ensured a strong bond between the two layers of silicone. Finally, if the second layer of silicone (the ‘bending’ layer without the embedded fabric) had been relatively thinner than the first layer, I believe it would have ensured preferential actuation toward the fabric-bound layer.