We are currently trying to manufacture a carbon fiber spar for a UAV.

We want to make it a square box section 16mmx16mm and a length of 2 meters, using hand-layup and then vacuum bagging.

We are planning to wrap the fiber around a square box section (wood, steel or aluminum hollow section), and try to pull it out after curing.

We tried foam, but because the dimensions are small, the foam couldn't handle the vacuum pressure. With aluminum sections we failed to extract the part, so, we tried wrapping the aluminum section with vacuum bagging film and adding release agent. That barely worked, but we are yet to try it on a 2m long part.

What's your experience manufacturing box sections? we have no access to specific machines to help us and its not possible to import ready-made ones so we are stuck with hand-layup and vacuum bagging.

We are ok with the resultant surface finish produced by the vacuum bag, we are more concerned about the success of the process.

Try to rollwrap the aluminium with cash register paper. Only fix the ends with tape. Then rollwrap the paper covered tube with packing tape. The resin will not stick to the tape, and the paper will make everything slippy enough to get the mandrel out. Afterwards you could easily extract paper and tape by pushing it out with a rod.