Expanding silicone plug

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Expanding silicone plug

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quinn	q quinn posted 7 Years Ago HOT Topic Details
q Group: Forum Members Posts: 155, Visits: 992	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
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quinn	q quinn posted 7 Years Ago HOT Post Details
q Group: Forum Members Posts: 155, Visits: 992	+x oekmont - 10/20/2018 9:24:27 AM You are not dealing with ordinary forces here. Heat expansion is an atomic force. Once the plug filled the cavity completely, there is nothing except neutron star like gravity, wich will stop the plug from bursting your mold like it was nothing. However, in your case the plug will expand through the ends. But because the boom is tapered, this will mainly happen into the wider direction. This, however will likely lead to high pressures on the tighter side (the plug can't really expand in this direction) and low pressure on the wider side (the plug is pushed in this direction, and therefore ever mould "section" gets a thinner plug section than planned, resulting in lower pressure). Because of that I would advice you to close both mould ends, and make a silicone bladder. Either with rotation casting, or with silicone sheets. with a pressure inlet on one side of course (bladder and mould). That way you can perfectly control the pressure, and get a evenly pressure along the part. Not quite neutron star gravity forces lol. Solids are compressible and bulk modulus of rubber is lower than most solids. But yeah, you're probably right, still too much force for my mold. So the bladder was actually my original idea when first thinking about this boom, but couldn't find much info on that either. What do you use for rotation casting? I imagine you want pretty runny stuff to evenly coat the walls as you spin it around. One other thought to make the silicon plug possibly more controllable. Let's say before pouring the silicone, you have a small plastic thin wall container than can hold let's say 40psi. This little container is suspended in the middle of the mold while casting the silicone. So what you end up with is a silicone plug that has a small 40psi cavity in it. When baking the layup, silicone will expand until pressing against all walls with 40psi of pressure, then any expansion left over will compress the 40psi capsule in the middle. Obviously pressure goes up a bit as volume of capsule goes down, but can be accounted for. I realise the silicone is going to try and expand differently in different directions, but it's elastic, I honestly think it would just displace itself to give pretty consistent pressure against all walls and excess pressure compresses cavity in the middle.
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