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

A word of warning: this isn't an easy technique. Be prepared for a LOT of wastage until you nail it. I've played around with this quite a bit, but unfortunately it's quite tricky to do on anything but very very simple shapes where the expansion is even and very predictable. Boxes and straight tubes: dead easy. Complex shapes: significantly harder.

There's a bit of maths to do, it isn't quite as simple as just saying your layup should be about 1mm thick so you just make a silicone plug 1mm smaller and presto, perfect results. You need to find the formula for the thermal expansion of silicone and use that to calculate very precisely how much smaller you need to make the plug. The hard bit is that most silicone manufacturers don't provide the CTE of their product in the data sheet, and silicone can have a linear expansion anywhere between 5.9x10-4/C and 7.9x10-4/C. This means that you don't really know how much your silicone is going to expand across a shape of varying size. It also isn't very forgiving. The rate of thermal expansion for silicone might be relatively huge, but if you have a 10" long part you may have a linear expansion along the length anywhere between 7mm and 9.5mm - a rather hefty difference to just be guessing. So on something like an RC helicopter boom, which tapers from relatively large at one end to relatively small at the other, across a relatively long distance; it may well be that you need to make the plug 5mm smaller along the length, but only 0.001mm smaller at the small end and 0.865mm smaller at the large end, and some varying number in between. Those numbers are just random to demonstrate my point.

You will probably find over your first few attempts that you end up with a lot of uneven compaction. One part may be under immense pressure, whilst somewhere else the silicone barely expands at all and you get no compaction. Or it could go the other way, you make your silicone plug too big, and apply such huge forces that it breaks your mould (admittedly very unlikely if you're machining an aluminium mould, but still possible). 


I don't mean to put you off, as I do think this is a very repeatable technique once you get it right. It just isn't as easy as you might be hoping, hence why you can't find a lot of information about it. My approach, given I'm not an engineer and not superb at maths, would be to use the formula to work out a rough idea of the range of expansion you can expect. Then, layup a carbon part seperately in each of your mould halves using vacuum consolidation (pre-preg would be easiest here). Once that is cured and you can measure it's exact thickness rather than just make a rough guess, then you can use sheet wax to add thickness to sections where the silicone will have more expansion. Then bolt the moulds together, and cast your silicone into the shell. Then, rinse and repeat when it doesn't quite work, making changes as needed.