With that little piece of magic, we developed a process to create custom molds for thin shell casting using various granular media. We worked with two primary variables. The granular media and the shape of the bladder. The thin shell casts take on the form of the shaped bladder as well as the granular media, crystallizing the intricate texture created by the relatively coarse grain. Choice of granular media affects the stiffness of the bladder, the stiffness of the cast, and the texture of the cast. Glass marbles casts have a stiff structure with a hexagonal pattern of craters while rice creates a fragile structure with an irregular speckled texture.
Unlike traditional hard molds, casting into a jammed mold can have undercuts since the bladder returns to a soft state when air is supplied back into the bladder. With this small advantage, jamming bladders can be freely shaped with folds and undercuts that are difficult to achieve with hard molds.
The following are the steps to our process:
Build a custom latex bladder
Fill the bladder with coarse granular media
Pull a vacuum at 15” Hg
Shape the bladder by hand
Increasing the vacuum to 20-28” Hg
Cast Hydrostone or Hydrocal FGR-95 to coat the surface of the hardened form
Rotate the mold to evenly coat the surface until the material hardens
Wait for the cast material to harden
Let the air back into the bladder
Remove the cast
Another thing we tried was using the jamming molds for laying up composites to build stool. In this case we used burlap and Hydrostone. The outcome was something unexpectedly stiff with more than enough strength to hold a person's weight
Because the molds are open and irregular, each mold needs to be rotated carefully, but unevenly while the material cures. To help automate the thin shell casting process, we built a simple programmable 1.5 axis rotation machine (one axis is manual, the other is automatic). The device is programmed by rotating the piece manually while holding down the program button. When the program button is released, the motor activates and loops through the recorded motions (speed and direction).