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Old Oct 30, 2006, 02:48 PM
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What type of silicone is used for the silicone plugs used in moulding fuselages?

A Brand name would be very helpful or a type would suffice.

Thanks for any info.


T.D.
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Old Oct 30, 2006, 03:42 PM
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You should check Smooth-On.com

There are literally hundreds of different silicons. You should pick the one that will work for your specific application.

Sean
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Old Oct 30, 2006, 04:34 PM
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"You should pick the one that will work for your specific application."

Thats just it Sean, I am asking for specific information on which one will work for a specific application, namely the silicone inserts that are used in moulds during layup.

I had a look at Smooth On but no luck because I'm not sure what to look for.

So has anyone here purchased silicone for making these inserts and if so would you please post the name of the product?

Thanks.


T.D.
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Old Oct 30, 2006, 05:41 PM
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It depends on your alititude above sea level. But from Smooth-On you'd probably be O.K. with Mold Max 30 or Smooth-Sil 950.

Sean
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Old Oct 30, 2006, 06:21 PM
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Folks use Oomoo 30 or Mold Max 40 as well. In these cases the number refer to the hardness of the silicone once it's cured. I haven't used Mold Max, but it is reported to make a more tear resistant mold.

"Molding fuselages" is a bit generic, which is probably why Sean wasn't specific. Do you mean glider, electric, gas, jet, helicopter, etc.? All are likely to have different needs.

PS-my suggestions are based on discus launch glider molds as I believe Sean's are too.
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Old Oct 30, 2006, 08:00 PM
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Thanks guys for replying and yes I should have been more specific.

I've seen photos' of the silicone inserts that are placed in the mould (handlaunch pods) on top of the layup and these are then vacuum bagged or perhaps just squished in to get a better bond between layers.

What I want to do is use the silicone in a mould I have built for making double taper carbon fibre spar caps.

Since the carbon tows taper (in thickness .080 to .020) from the centre out to the tips I want to get an even amount of pressure all the way accross the layup so the spar caps are evenly bonded accross the span.

It's basically a channel (female part) 1.5 metres long by 12.5mm wide (ID) that I will lay up carbon tows into and then place the silicone insert on top, then insert a piece of steel barstock (male part) on top of the silicone and either bag it or use clamps.

So far I've been making spar caps by bagging the carbon between two pieces of hardwood and they're just not up to snuff over the long haul.

Just about each wing I build uses a custom spar so buying the caps is out and I think this silicone moulding technique is the way to go.

So any info. is greatly appreciated.


Tony Dempsey
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Old Oct 30, 2006, 08:23 PM
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Is the steel bar the same width as the channel?

If you have piece of steel bar, why bother with the silicone? Put down some plastic wrap and clamp that sucker down. Or put it in the vac bag and vac the bar down. The bar will act as a hard mold.
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Old Oct 30, 2006, 08:43 PM
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Found this from a recent post elsewhere, scroll down and you'll see a sparcap press illustration. It looks like they are using steel plate then silicone then another clamping plate.

It says they can get 30 bar of pressure which is about 363 PSI.

A good vac pump will generate 25inches of mercury which is about 12 PSI.

Wow, I'm glad you asked this, it helps answer a question I had in another thread

http://www.airplane-model.com/tech-built-up-d-box.html
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Old Oct 30, 2006, 09:41 PM
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I'd stay away from silicon for spar caps unless you build a special box like Will's link shows. Even there, the silicon expands with heat, and it's confined within the box, so all the force goes down to the epoxy layup. Like Will said, just use steel plates and "C" clamp them with as many clamps as you can get on it and crank them down. Just one 9" C-clamp cranked pretty tight, but not gorilla cranked, is over 800psi. Now, with thin metal plates, this load is not carried across the entire bar. This is why numerous clamps is better.

Sean
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Old Oct 30, 2006, 10:28 PM
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"If you have piece of steel bar, why bother with the silicone? Put down some plastic wrap and clamp that sucker down. Or put it in the vac bag and vac the bar down. The bar will act as a hard mold."

I've tried that already, the problem is that no matter how hard it is clamped the steel bar rides high on the transition areas as the carbon tapers accross the spar cap and the bond is only partial between the layers of carbon which is where the silicone comes in as it conforms to the transition areas and ensures that all the carbon is under equal pressure and you get a nice linear spar cap.

"I'd stay away from silicon for spar caps unless you build a special box like Will's link shows. Even there, the silicon expands with heat, and it's confined within the box, so all the force goes down to the epoxy layup."

I have built a special mould (box) very much like the photo at Vlads' site, what I was asking for was the type (name brand) of silicone that I should use.

I'll be using a type of spring clamp that gives constant pressure as the epoxy flows out of the layup and into a double layer of Bounty paper towels that are in the bottom of the mould. There is a layer of Peel Ply between the carbon and paper towels.

Maybe I'll post a photo when I have it all set up.


Tony D.
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Old Oct 30, 2006, 10:42 PM
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I don't know what type of silicon to use for that application...something really hard. Maybe you should use a semi-flexible Urethane like ReoFlex.

What if you place C-clamps at the transitions, using the steel? How thick is the steel you're using?

Photos would be nice.

Sounds like a fun challenge though. I'm game!

Sean
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Old Nov 05, 2006, 12:00 AM
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I've assembled my spar cap compressing device and here's a photo, it's a bit Spanish Inquisition but should get the job done.

I got the bolts and .25" thick steel plates at the junk yard for $5. and they are an almost perfect fit width wise.

Instead of using silicone I'm going to use a 25 durometer rubberized plastic which is available cheap from a local flooring shop. It has good conformity and is immune to epoxy bonding.

First cap should go in next week.


T.D.
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Old Nov 19, 2006, 12:03 PM
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Quote:
Originally Posted by Tony D.
Discussion - What type of silicone is used for the silicone plugs used in moulding fuselages?
For mold inserts, like my SuperGee II pod molds all the way to 48-inch long wingeron slope sled fuse molds and spar cap molds, I use RTV silicone.

I purchase RTV silicone where ever I can find it the cheapest, typically I purchase 1 gallon or greater "kits" on eBay. I look for the stuff that has a hardness of 25 to 40 Shore A. The kits I've puchased are for either blue or purple silicone and has brand names on it like AeroMarine, Tap Plastics, etc.

I would highly encourage you to try making silicone inserts for your spar cap mold and to vacuum bagging the material into your mold with silicone inserts.

Jay Decker
Kennewick, WA
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Old Nov 19, 2006, 01:35 PM
working to the closest cm
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Tony - Wondering in what way have your spars not been acceptable? I have already seen reference to the discontinuities due to the steel clamping plate.

If you use a vacum technique (admittedly much less pressure than clamping) the discontinuites would not be possible as the bag would account for the steps in layers.

What are your goals :- maximum removal of epoxy?

I favour the simple vacum approach:- there are a lot of molded gliders out there winning major contests that have had the spar layers simply vacumed wet into a mold.

Usually the spar is designed for stiffness and is way over strong - would any minor loss due to just the vacum technique compromise the job?

I have seen spars fail due to insufficient epoxy.

cheers jeff
Keeping it simple.
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Old Nov 19, 2006, 03:04 PM
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Thanks Jay, that's what I was looking for.

jeff, I've had success bagging the spar caps flat on a piece of mylar but I'm looking for a perfectly pre-dimentioned carbon cap that requires no trimming after it is laid up and the mould with silicone and metal insert will (I think) achieve that.

I'm going to go with Sleds suggestion using the silicone with steel insert in the mould and vacuum bag it.

I'll post a photo of the finished cap.


T.D.
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