Fabrication Overview
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.
Bill of Materials
This page offers a detailed Bill of Materials for replicating the silicone combustion chamber.
Molds
The combustion chamber was fabricated through rotational casting, using a two part, Objet printed mold that has a sealing interface.
Molds
The Solidworks and .STL files can be downloaded from the Downloads Section.
Lab Tools
Latex Gloves Mixing Cups Tongue Depressors
Equipment
Laboratory Scale Rotational Caster
Vacuum Chamber Laboratory Oven
Materials
ECOFLEX 10
Manufacturing Process
1. Into a clean cup, pour 90 g of Ecoflex 10 part A and 90 g Ecoflex 10 part B. The final mixture should therefore weigh 180 g.
2. Using a rigid stirrer (e.g., a tongue depressor), mix the two halves together slowly, to avoid creating air bubbles. Air bubbles, when the silicone has cured, can result in rupturing at lower stresses than you would need the silicone bladder to be able to withstand during inflation.
3. To eliminate any air bubbles, put the mixing cup in a vacuum chamber. Removal of the air just above the surface of the liquid silicone in the cup will draw out any pockets of air under the surface.
4. Coat the interior of the mold halves with a mold release, then let them dry for the appropriate amount of time (generally listed on the side of the mold release can).
5. Pour half of the Ecoflex 10 mixture in each mold half, carefully coating the walls. Ecoflex 10 has a relatively low viscosity, but you should still be able to get some of it all over the interior of the mold. This will help the silicone spread once we put the filled mold on the rotational caster.
6. Carefully fit the mold halves together. While not necessary, it is generally advised that you use some kind of external binding to make sure the halves stay together.
7. Fix the mold on the rotational caster, centering it about the axis of rotation. Depending on your rotational caster, you may have to affix the mold using zip ties, bolts, etc. Your rotational caster should have an approximate 2:1 spin rate. We used 0.77 rotations per second and 1.54 rotations per second. Place the mold vertically, upside down (the bottom part is the cylindrical nozzle) in the caster for 30 seconds. This will spread the silicone.
8. Turn the mold with the nozzle facing down, and put it on the caster using the same settings.
9. Let cure for 2 hours.
10. Take the mold off the rotational caster and place it in the oven at 65 Celsius for 30 minutes.
11. Remove the mold and let it cool down.
12. Open the mold and carefully demold the silicone chamber.
Downloads
 | sldprt.zip | 457 KB |
| stl.zip | 305 KB |