Talk Composites - The Forum for Advanced Composites

Expanding silicone plug

http://www.talkcomposites.com/Topic30734.aspx

By quinn - 10/18/2018 4:18:29 PM

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
By Fasta - 10/30/2018 2:32:51 AM

 
quinn - 10/29/2018 2:26:41 PM
 
Fasta - 10/29/2018 6:24:03 AM
 
quinn - 10/26/2018 3:38:40 PM
 
Fasta - 10/26/2018 4:23:49 AM
These are the two intensifier parts I have made.

The large one has a core piece included so I could reduce the amount of silicone used and also have a bit more flexible plug to aid with releasing. This intensifier was cast inside a fiberglass shell that I pulled from the moulds. I have made 20 of these.

The small one is a tubular foil section, this one was cast within the moulds using sheet wax to simulate the part thickness. I made 6 of these.

No failures except for air in some corners which improved as I tailored the laminates to suit.

The difference I see with the posters project is that my parts here are 1.5mm-2mm thick whereas the heli boom is probably much less so it would have less resin to go around??

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Very cool! Actually not much thinner. Most likely I'll be about 1mm wall, maybe built up thicker in areas at the base. So it sounds like just simulating layup thickness for the plug with wax was all you really had to do. When laying up the foil, is it just left open at the back for the silicone to expand outward? Or do you cap it off to contain the expanding silicone?

The smaller foil intensifier was just sitting in place whereas the larger one had an alloy plate bolted to the end of the mould to hold the silicone in place. It's really simple to make the silicone plugs, I guess the catch is that this method needs to be matched or tailored to some degree whereas air pressure/bladders can essentially conform to any mould shape. But then I would consider the building/design of a bladder system with with fittings etc and containing this in the mould and not leak to be a big challenge too. And time consuming.

My small foil shaped one has had two silicone inserts snap/break when being withdrawn. A soft silicone releases easier as it stretches more/easier while pulling to release. But then with a stronger silicone this requires a  little more pull and so also has the potential to snap. It broke about 100mm from the skinny end which was about 7-8mm thick.

The larger one releases really well literally falling out as long as I used the Mann release on it.
One mistake I made was to make the larger silicone intensifier without degassing the silicone as I ended up with bubbles left in the silicone, the bubbles that are in contact with the moulding surface get epoxy resin forced into them with the cure and that kind of keys the silicone to the inside of the new part making them really hard to get out. The new degassed version was great though.

I think as long as you leave some space at one end of your mould you can use the moulds for either process.


Sounds good. Thanks for the tips. I think I'll give the silicone plug a try first before going to a bladder. 
One more question, how are you preparing the silicone plug before inserting into layup? Are you applying anything for release? I'm using frekote sealer and release on molds. Should I apply that to the plug, or use something else?


https://www.smooth-on.com/product-line/ease-release/