Vacuum ( negative pressure) is used 2 ways in casting.
One after the mixing of the silicone mould material. Air bubbles are vacuumed out of the mixture prior to pouring and after pouring as well. If a pump is not available , gentle agitation will help air bubbles to rise up and out of the silicone.
So now your mould is ready for resin. Mix the resin and vacuum that mixture if possible. Pour the resin and vacuum that as well.
Here are some pics of a chamber made from a pressure cooker:
This is a little fancy with the digital pressure guage but it works great. You can see the lexan viewing window bolted and epoxied on , and a DeVilbiss petcock on the top. The petcock introduces vacuum at a controllable rate, and after vacuum can be used to re-pressurize the mould with the introduction of atmosphere back into the chamber , forcing the resin into the details of the mould.
Qualifying pics of metal props , rims and hubs cast in metal and resin successfully:
There is a LOT to the work. Mould box's must be sturdy and rigid , there must be at least 1/2" of silicone around the master to be cast , dont cheap out! This will really apply to a long thin wing casting attempt.
Mix all ingredients thouroughly!!! As per instructions. Use water to guage the amount of resin needed , fill the mould with it and add 30% to be sure.
Purchase the longest curing resin to start and work from that.
You WILL fail and fail miserably if you do not apply rigid standards to your attempt. Cutting corners is simply not possible.
I researched until I was satisfied that I could complete the procedure tooling up followed along with some practise casting.
I have a pile of useless moulds , and crappy castings , so get ready for some material waste.
The amount of pre-planning you do will help to allieviate this.
Its impossible to detail the entire procedure here, youtube and ither modellers can help.
Good luck!