We used centrifugal casting to fabricate the ellipsoid expansion bladders used in the combustion and buoyancy experiments. A basic description of our manufacturing process is as follows: First spray the 3D printed ellipsoid mold with Universal® Mold Release, then mix equal parts of Ecoflex 10 part A and B and pour the mixture into our ellipsoid mold (making sure to coat the walls). Next, we place the filled mold into the rotational caster, and let it run for approximately 2 hours. After the 2 hours have passed, the mold is placed into an oven at 65 degrees Celsius for 30 minutes to ensure that the silicone inside fully cures. This general process creates a completely sealed ellipsoid expansion bladder, more detail is given in the subsequent pages.
The rotational rate of the slower rotational caster axis is 0.77 rotations per second. With an approximate 2:1 spin rate, the faster axis turns at 1.54 rotations per second. This difference in the spin rates results in an even Ecoflex coating on the interior of the ellipsoid mold, as long as the mold’s centroid is located about the center of rotation. If the mold is not centered about the center of rotation, the bladder will be misshapen. An uneven coating can also occur if the inside of both halves of the mold are not lightly brushed with Ecoflex prior to placing it in the caster. An optional additional step is turning the filled mold upside down in the rotational caster, and letting it sit for 30 seconds before starting the rotations. This may be necessary because a significant amount of time generally has elapsed between when the silicone is poured and when it is mounted in the rotational caster. The problem with that is letting the mold sit too long on one side causes the silicone to pool, which will result in a bladder with uneven walls, even after being turned by the rotational caster. There has to be a relatively even silicone distribution for the casting to work.