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High Density Foam for Hardpoints

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poormansairforce

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I looked and looked, no joy. For several scratchbuilt composite airplanes, the method of attaching control surfaces is including a high density foam block (20 pcf foam, Wicks stocked it) in the massive cores, glass the surface, then imbed hinges in the clock of high density foam.

Trouble is I can not find 20 pcf foam anywhere and mine is used up. I did find companies that list that density and others near it, but none of their distributors have any of it, and the maker is only interested if you want a semi-trailer full of it. Anybody know what the currently used options are for this sort of construction?

I can make syntactic foam using simple molds and dry micro at about 32-35 pcf (about 0.51-0.55 g/cm^3) but using a previously engineered material is always a better option in my mind...

Billski
As a GC I always thought that pvc trim boards and sheets we use would make for good hard points in composite structures. It's heavier than you are looking for but fairly cheap considering how little is needed. I did some googling and found this:
http://epoxyworks.com/index.php/azek-pvc-deck-boards/

I would test machined cuts since it would expose the cells.
 

pictsidhe

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I weighed a bit of 'royal trimboard' that I have a home 1 1/2x3/4 x 8' was 886g it came from a bigbox. the stuff does have a skin that should plane off pretty easily. Easy to find and cheap enough to experiment with. It would make a lot of sense to redesign around something that can easily obtained, if it doesn't meet the specs of the original.
 

wsimpso1

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One of the things not talked about in all of this discussion is a critical point. The materials we are accustomed to are known to be wetted by epoxy, bond well with epoxy, and to be durable in assemblies with other common composites materials. That is part of the reason for staying within the family of known materials and suppliers. When you start going with trimboards and other materials, we do not have the same knowledge and the materials need some sort of vetting. There is a very real concern for silicone content in plastic foams. ANY silicone in the process greatly reduces bond strength and durability.

Me, I will stick with known materials rather that unknowns. Glass and carbon fiber from standard suppliers, standard epoxies, standard foams, etc. Garolite has become a proven standard, but the trimboards from HomeDepot? Not in my shop. Within this environment, I was looking for options...

Thanks Marc, for the vote of confidence in alodine and primed aluminum for embedded components.

Bill
 

pictsidhe

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Trimboard is foamed PVC. On critical stuff, I test my methods on scrap before using them on the important bits, whether I'm using proven stuff or improvising. Call it self-certification. It's why I'm really, really leery of epoxying aluminium, I've had too many failures. Now I know that alodining is the key, I may try again. Yes, you'd need to evaluate the trimboard, but it could still be quicker than finding that bit of unobtainium you need. Especially the next time round.
 

Marc Zeitlin

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Thanks Marc, for the vote of confidence in alodine and primed aluminum for embedded components.
Probably a minor nit - the alodining is a good prep for the AL surface to protect if from corrosion and to prepare it for bonding - it helps the bond strength. But I wouldn't prime the embedded AL, unless you have knowledge of epoxy bonding to the primer. When I referred to wet install with primer, I was referring to the hardware (screws, nuts, bolts) that are used to hold the VE wing fittings to the composite spar caps. I don't believe that anyone primes the AL wing fittings themselves - they just get alodined, and then the hardware is installed wet to prevent galvanic corrosion and water infiltration.

The VE wing fittings are a particularly terrible design, which is why Burt went away from that technique for the Defiant, Long-EZ and others for the COZY/E-Racer etc. The Jiran wing attach fittings for the VE (very glider like, with extensions into the spar with pins) was very good - I've seen two VE's with that.

Sorry for the digression/diversion.
 

wsimpso1

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Probably a minor nit - the alodining is a good prep for the AL surface to protect if from corrosion and to prepare it for bonding - it helps the bond strength. But I wouldn't prime the embedded AL, unless you have knowledge of epoxy bonding to the primer. When I referred to wet install with primer, I was referring to the hardware (screws, nuts, bolts) that are used to hold the VE wing fittings to the composite spar caps. I don't believe that anyone primes the AL wing fittings themselves - they just get alodined, and then the hardware is installed wet to prevent galvanic corrosion and water infiltration.

The VE wing fittings are a particularly terrible design, which is why Burt went away from that technique for the Defiant, Long-EZ and others for the COZY/E-Racer etc. The Jiran wing attach fittings for the VE (very glider like, with extensions into the spar with pins) was very good - I've seen two VE's with that.

Sorry for the digression/diversion.
These parts are heavily drilled and count on a mechanical interlock of flox/epoxy in those holes. I was preparing to alodine and then prime with PolyFiber epoxy primer. Am I better off not using the epoxy primer?

Billski
 

Marc Zeitlin

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These parts are heavily drilled and count on a mechanical interlock of flox/epoxy in those holes. I was preparing to alodine and then prime with PolyFiber epoxy primer. Am I better off not using the epoxy primer?
I will not claim to be an expert on bonding technology, but I'd guess that a good epoxy primer (as opposed to a zinc chromate or zinc phosphate primer out of a rattle can) is probably going to bond to the alodined AL as well or better than a laminating epoxy, and that if the epoxy primer is either installed when still tacky or roughed up with 220 grit prior to installation, that's probably about as good as it can get (and with the mechanical interlock of flox in holes, I'd certainly think you're fine).

But that opinion is worth what you paid for it :).
 

BJC

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This has been an interesting thread. May I ask a related question?

What factors make flox a preferred filler over milled fibers, or vice versa?

Thank you.


BJC
 

wsimpso1

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I will not claim to be an expert on bonding technology, but I'd guess that a good epoxy primer (as opposed to a zinc chromate or zinc phosphate primer out of a rattle can) is probably going to bond to the alodined AL as well or better than a laminating epoxy, and that if the epoxy primer is either installed when still tacky or roughed up with 220 grit prior to installation, that's probably about as good as it can get (and with the mechanical interlock of flox in holes, I'd certainly think you're fine).

But that opinion is worth what you paid for it :).
Thanks mark, I have a similar commentary about the potential value of my opinions... Appreciate the candor.

Billski
 

wsimpso1

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What factors make flox a preferred filler over milled fibers, or vice versa?
Byron, as long as we are drifting the thread, might as well make the answer comprehensive. Several reasons to add fillers to resins:

To make it thixotropic - keep it from running off of a layup or out of a bond joint that is vertical - use fumed silica or flox to thicken;

To fill broken cells in the surface of foams before layups - 50-50 mix of epoxy and micro balloons is lighter and less costly than straight epoxy;

To fill and fair shapes at min weight - 4:1 micro balloons to epoxy - this is light and sands easily. Not terribly strong but works great for fairing and for fillets under glass cloth in corners and such;

To fill in bonding jobs where gaps are too large or rough for straight epoxy - wet flox does the job nicely;

Building an outside corner that would otherwise be thin composites backed by foam - it would be fragile, easily broken or peeled - so glass one side and let cure, fill the corner with flox, then glass the other side.

Hmm, no where did I mention milled glass. In most construction, it is not needed. Gougeon Brothers tells us no joint is stronger with milled glass than with flox. We know that the milled glass fibers are stronger than the milled cotton fibers in flox, how can they be equivalent? The usual answer is that it takes 1000 diameters of a fiber to fully get load on a fiber and another 1000 diameters to get it back out, and you start on the next one up some distance from the ends. In typical joints and other uses, you do not have enough room to get loads up and into the fibers enough to use the strength of milled glass. And since milled glass is tougher to work with and embeds in our skin, we usually do not bother.

Are there places where it makes sense? A few. Repairing high density things like centerboards and rudder blades for sailboats is one place where some folks seem to find value in it. Not airplanes, I know. The cores are typically high density foam or wood with glass skins, and when they get damaged, you gotta build up the cores again with something strong and stiff. As the fleet captain for a university sailing club, I never mandated use of milled glass, and suspect that a repair every bit as good was effected with flox. Nonetheless, I kept milled glass on hand because some of our members preferred to use it saying it was stronger. Hey, you accommodate volunteers, particularly when they are teaching, putting in a lot of time, and helping out a bunch.

I suppose if one were repairing a part built up with a chopper gun (usually polyester resin), you should repair using chopped glass of about the same character. If you find such a thing in an airplane, you might do well to redesign it with a somewhat thicker composite fabric shell and a lighter core. It will be both sturdier and lighter at the same time... My brother built centerboards and a rudder assembly for his own design boat of hotwired foam and glass skins. After he crushed a centerboard, he included a 1/2" wide oak spar in the core, and you could jump on the thing with impunity during capsize recovery. These parts would blow away in a strong wind, were strong as all Hell, and no milled glass.

I think that I bought one bag of milled glass in the mid 1990's, and still have it.

Other people may disagree with me. I would love to know where it is needed and the best material...

Billski
 

BJC

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Thanks Bill.

The Glasair uses milled fibers and cabosil in resin to make an adhesive, but the Sportsman uses no milled fibers and practically no qcell. Instead the Sportsman just uses resin with cabosil.


BJC
 

BoKu

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...I think that I bought one bag of milled glass in the mid 1990's, and still have it...
I think I have that exact same bag at my shop. It's like the one Christmas fruitcake that travels forward and backward through time to occupy all possible instances where a fruitcake appears to exist.

I don't have actual data, but what I've heard is that Cabocil makes joints harder, but also more brittle. And that's the sense I get from handling the cured mixture. So in my shop we use a mix of around 3 parts flox to 1 part cabocil to do critical bondlines for wings and fuselages. The cab is just in there for thixotropicity; a very necessary property since most of the fuselage bonding joggles stand vertical while we are spreading the bonding paste.

For captured joints where we want hardness but don't need additional tensile strength we use up to a 1:1 mix of cab and flox. That's what we use in the bonding paste injection operation where we pot in the bushings for the wing main pins in our spar stubs.

--Bob K.
 
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