Inflatable Silicone Pressure Bag Fabrication
- Most tooling or mandrels for bag fabrication are made from aluminum.
- Apply a TFE aerosol release or use a solution of Joy detergent and water. Do not use a silicone base release.
- If a metal stem is to be used for inflation of the bag, it should be installed now. Use the following procedure for aluminum or carbon steel. Sand blast or abrade the metal surface. Clean with a solvent.
- Prime the metal with one uniform coating of Thixon 305 primer. Allow the primer to dry at ambient conditions for at least 30 minutes.
- Wrap the primed metal with uncured silicone sheet. Use doubled nylon fabric to remove air and skive or feather the edges next to the threaded portion of the stem.
- Place the valve stem into the desired location on the tool. The silicone will be tacky enough to ensure that the stem stays in position. If desired, a cap of uncured silicone can be laid over the stem and extended 1 to 2 inches back on the mandrel. This cap should be feathered or skived to make a smooth transition to the tube.
- Next, lay the uncured silicone sheet into position to form the body of the tube. Feather or skive the edges and allow about 1/4 to 1/2 inch overlap. Use doubled nylon fabric to work all of the air out from underneath. Start at one end and work around and forward to push air to the open end of the tube. Apply extra pressure to the overlap seam area and if desired the excess can be carefully removed. Skive or feather the edge of the sheet.
- This section of the tube is ready to be vulcanized. An inert gas autoclave is normally recommended to cure silicone if it cannot be vacuum bagged. Air will cause a slight reversion of the unprotected silicone surface during its vulcanization. This situation does not occur if nitrogen is used or if a saturated steam autoclave is used. Cure the tube for at least 30 minutes at 300°F and 60 psi. Start timing the cure cycle after the tool reaches 300°F. Allow the part to cool to room temperature before removing it from the mandrel.
- Removing the bag is sometimes difficult, especially if the tube has undercuts or different thicknesses in sections. The easiest way is to apply very slight air pressure to the stem end to slightly expand the bag. By holding the opposite end by hand, the air can be
maintained until the bag slides off. - A more difficult procedure involves wrapping the tube with nylon fabric and peeling it off inside out as you would remove a sock. The tube should then be turned back to the right side out position.
- After the tube is fabricated, the end can be sealed up by several methods. A molded cap can be spliced into place using uncured silicone to bond the cured bag to the cured cap. Uncured silicone can be packed into the tube to affect a seal which is then vulcanized in the autoclave.
- After the end is closed, post cure the bag for 3 hours at 400°F. It is then ready to use.
