Fillers 101?

[josepher44]

Milled Fiberblass, Microballoons, Chopped CF, Colloidal Silica, etc. I see these names all over high power build threads. But I don't really know much about their uses.

What thickens and thins epoxy? When would you want extra thick or extra thin epoxy? What adds strength? Do I really need this extra strength to begin with?

Just looking for a crash course on the basics of epoxy fillers. Thanks!

 

[RoyAtl]

Chopped and milled fiberglass or chopped carbon fiber both thicken epoxy and increases strength.

Microballoons also thicken epoxy and increases sandability but reduces its strength after a certain point. (it does not form a matrix). Use with chopped or milled fiberglass for the best of both worlds.

Colloidal Silica and Cab-o-Sil will thicken epoxy and will increase the strength somewhat.

 

[cjl]

Chopped CF is probably the strongest of the addatives you would be likely to use, but in anything greater than the tiniest of quantities, it tends to turn your epoxy into a giant, sticky hairball. It's extraordinarily strong, but I wouldn't use it alone when you need a good finished surface (such as a fin fillet). Colloidal silica is weaker, but is a great choice to thicken epoxy into a smooth, easily shapable paste, and it is still quite strong. It is my choice for external fin fillets most of the time. Milled glass is a bit stronger, but similar for the most part. Microballoons make a wonderful paste that is easy to sand, but they do weaken the epoxy. I wouldn't use them for anything structural.

 

[RoyAtl]

Microballoons, make a wonderful paste that is easy to sand, but they do weaken the epoxy. I wouldn't use them for anything structural.

yes, I made the mistake 20 years ago of making the finishing fillets of my NCR Phantom 4000 with a lot of microballoons. The fins are surface mounted with 60 minute epoxy, tacked on with CA (yes, that was how NCR said to do things back then); i added glue rivets -- 1/16" holes drilled into the tube about 3/4" apart. I then made a structural fillet with regular epoxy, then a nice thick "sculptured" fillet of 15 minute Bob Smith Epoxy with microballoons. Turned out great, but the decorative fillets cracked on the first hard landing. The underlying attachment didn't crack; it flexed a bit. But the cracks always got scrutiny from safety check officers and I always had to somehow prove that the fins weren't going to fly off at launch. Usually they were satisfied if I squirted some CA into the cracks.

After more than fifty flights, one of the fins has torn away a little from the tube, but it is hard to repair as the decorative fillets (and the CA added in attempts to fill the cracks) are in the way and I can't get the fin back into the right position. So I've basically retired that rocket.

I found out a little later about Hobbypoxy Thixotropic epoxy (no longer made as such... you can get it in large quantities as Pettit Thixotropic boat epoxy), which I think is essentially long cure epoxy with colloidal silica already mixed in. I used that on a few other long lasting high power rockets.

 

[troj]

A really good resource for starting out learning about fillers is on the West Systems epoxyweb page.

They don't mention anywhere near everything -- lots of things can be used as a filler, including corn starch. Corn starch thickens the epoxy, and also makes it hard as a rock, so it's not much fun to sand.

Not all fibers are created equal -- fiberglass, carbon and Kevlar are all available, and all give different properties.

Something not really discussed yet is the fact that in addition to strengthening epoxy, sometimes you use a filler to do nothing more than keeping it from running, or reducing the amount it runs.

Boat builders will sometimes add a small amount of colloidal silica or fumed silica, to reduce how much the epoxy runs when doing their layups.

If you look at Crazy Jim's thread about the Wildman LDRS project, you'll see where he makes fillets that hold their shape and cure such that they need little or no work afterwards. That's because he's using thickened epoxy and shaping it while still wet. Because it's about the consistency of peanut butter, in his case, it stays where he puts it.

I use various fillers to provide structure in some cases, and in others so that I can shape the epoxy a bit, and not have it run where I don't want it.

-Kevin

 

[daveyfire]

For large rockets, where filler in fillets is a necessity, I usually put down a structural fillet of epoxy + fiber filler, and then cover that with a cosmetic layer (SuperFil or 5 minute epoxy, depending on my build schedule!).

I very much like Kevlar pulp for thickening structural fillets. It's got a lower modulus than carbon, so it's much less likely to crack on a hard landing (vs. carbon or a normal thickening filler), and similar (or better) tensile strength. I really got sold on the use of Kevlar in fillets after seeing Robert DeHate use it on his Balls rocket, both booster and sustainer -- the surface mounted fins with Kevlar-based fillets, two layers of 6 oz. glass and one light layer of Kevlar have held up to several Mach 4+ flights.

The major downside to Kevlar is that it's impossible to sand, hence the dual layer fillet approach. When they're *really* thick (peanut butter or thicker), I like to put peel ply over them and smooth everything out with a gloved finger, and then let it cure. That gives a nice bondable surface without much sanding later on.

For general bonding, I use a lot of the West 406 filler that Kevin pointed to, or flocked cotton from Aircraft Spruce.

 

[dixontj93060]

Something not really discussed yet is the fact that in addition to strengthening epoxy, sometimes you use a filler to do nothing more than keeping it from running, or reducing the amount it runs.

Boat builders will sometimes add a small amount of colloidal silica or fumed silica, to reduce how much the epoxy runs when doing their layups.

Milled fibergalss (<1/32nd shavings) also works well for this application, allowing you to achieve a measure of both thickening and strengthening in one filler.

 

[Sully]

For large rockets, where filler in fillets is a necessity, I usually put down a structural fillet of epoxy + fiber filler, and then cover that with a cosmetic layer (SuperFil or 5 minute epoxy, depending on my build schedule!).

I very much like Kevlar pulp for thickening structural fillets. It's got a lower modulus than carbon, so it's much less likely to crack on a hard landing (vs. carbon or a normal thickening filler), and similar (or better) tensile strength. I really got sold on the use of Kevlar in fillets after seeing Robert DeHate use it on his Balls rocket, both booster and sustainer -- the surface mounted fins with Kevlar-based fillets, two layers of 6 oz. glass and one light layer of Kevlar have held up to several Mach 4+ flights.

The major downside to Kevlar is that it's impossible to sand, hence the dual layer fillet approach. When they're *really* thick (peanut butter or thicker), I like to put peel ply over them and smooth everything out with a gloved finger, and then let it cure. That gives a nice bondable surface without much sanding later on.

For general bonding, I use a lot of the West 406 filler that Kevin pointed to, or flocked cotton from Aircraft Spruce.

Rather than start a new thread, I thought I'd bump an old one.

I am building my first FG rocket (Darkstar Jr) and would like to overbuild it a bit, using Kevlar pulp as filler for the internal fillets. I assume I can just unravel and chop fibers from my tubular Kevlar shock cord. Is this correct? And does anyone have guidance on the ideal length of the pieces? In other places I have seen 1/8" as a guide, which seems about right to me.

I would use chopped CF but don't have a ready source, and it seems that Kevlar, which I have in abundance, will be more than adequate for my needs. This build is more about learning techniques than actually planning for machbusting flights (but I might try a supersonic flight or two down the road, who knows?)

John

 

[daveyfire]

I assume I can just unravel and chop fibers from my tubular Kevlar shock cord.

I think the breakdown in your plan is the "and chop" part -- if you get a good way to chop Kevlar like you chop garlic, I'd love to have it, but the stuff is tougher than nails to shear. (Cryomilling, perhaps.)

You can buy it by the bunch from Fibreglast. Wildman's sells chopped carbon for $7 per jar -- I got a jar two years ago and it's still half full.

Machbusting will be no problem however you choose to do it.

Good luck!

 

[MClark]

The Kevlar used for reinforcements is surface treated so the epoxy sticks better.
If you value your time just buy it chopped. If you must do it, a sharp chisel against a steel plate will do it the fastest.

Kevlar is OK for internal fillets but avoid it for anything that will need to be sanded, it forms a fuzzy surface.

Mark

 

[jd2cylman]

So to bump this along; is there a structural difference in carbon fiber as opposed to milled fiberglass? Are they greatly different in strength? Price?
I have the chopped CF, just wondering about additional options.

Adrian

 

[MClark]

Carbon fiber is lighter and stronger than fiber glass.

However both are way stronger than cardboard and plywood so the strength difference is not very noticeable. It will fail in the substrate. On composite unless you have really good surface prep it fails along the glue line
Non oriented/chopped fibers as filler mostly just prevent cracks from propagating.


Mark