A while back I posted about a helicopter tail boom, about 800mm long, tapered from 40mm to 30mm. Put that project on hold for a while while i did the canopy, coming back to it now. Decided i wamy yo go with 2 peice female aluminum mold and prepreg. Vacuum bagging would work but one of you guys mentioned a positive silicone plug for squeezing the prepreg to the wall. Really like the idea of it and I might be doing a run of 100 or so. The silicone plug sounds like it would really cut down on lay up time rather than bagging each one. I've done some searching and can't find much detailed info on this method. Here's some questions I have:
1- what type of silicone do I purchase for this?
2- how do you cure the silicone? I'm picturing the stuff in tubes that obviously doesn't cure inside the tube,  so I assume it wouldn't be any different sealed inside a mold. Is heat used? Is this some kind of different silicone than what I'm picturing? 
3-  what size offset does the plug need? Prepreg layup will be about 1mm. Is it absolutely necessary to make a separate smaller mold for the plug, or is there another way around that? Was thinking maybe if silicone could be cured with heat inside the actual mold for prepreg, it would possibly shrink down enough when cooling to allow layup thickness. Any other way other than separate mold? Maybe just layup one prepreg part with vacuum bag and then pour silicone inside that? 
4- does the silicone plug need to be 100% contained to be effective? If the ends of mold are left open, is the silicone just gonna expand out the ends and not put enough pressure on walls? If that's the case, I can make bolt on caps for ends of mold to contain it. 
That's all that comes to mind for now. Any advice is very much appreciated. Thanks

Stuff looks interesting, would love to try it. Waiting on a reply from Vac Innovation. Do they have a MOQ Rob, do you know? 


Quinn, I wasnt saying do a small scale part to save on pre-preg costs, I know pre-preg is cheap in this instance. But at around AU$85 a Kg, those silicone plugs are going to cost you a packet if you need to make them several times in trial and error. Your labour time in laying up each piece is also worth something, and it would be quicker to layup a smaller scale piece. 

Anyway, there is a better way than 20 trial and error processes - do the maths, work out the actual expansion to expect for your silicone across its specific form, put that into CAD and then CNC machine two moulds. One that you use as your actual pre-preg mould, and one that you use as the mould for casting your silicone plug. If you've done your maths right, that will be accurate enough to work reliably. But to get the maths right, you'll need to get some silicone and test it for its expansion rate.