Last-a-Foam OK? Maybe new parts with PVC and Vacuum Infusion?

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wsimpso1

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I was working on my wing skins and getting the spars attached to the fuselage, and in general getting excited about assembling my wings. One of the wing skin layups had obviously parted company with the Last-a-Foam cores... Oh crap. That has serious implications elsewhere.

So I started searching around, and I found on this thread https://www.homebuiltairplanes.com/forums/showthread.php?t=2734 that Glasair had some issues with these foams, and that Orion is recommending that people not use it. And my wing skins are full of the stuff. I have some other issues that I am less than happy with in these wing skins too, so this is just the excuse to make my wife think I am nuts.

So, I am thinking in terms of making new skins on my molds using 3/8" PVC for the cores. I am even thinking of trying out vacuum infusion for these parts too. I took in Corsair82's session at OSH last year, and it looked doable when compared to my current vacuum bagging...

Whadayatink?

Billski
 
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orion

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Ouch!!! I'm sorry you ran into this. One of the first tasks I did for Glasair was an engineering review of selected materials and specifically, foams. This was about a year (give or take a bit) after the company transitioned from Klegecell to the Last-A-Foam material. After about three months of research, my conclusion in the report was that the Polyurethane foam as manufactured by General Plastics is unsuitable for primary structure in any aircraft application. The reason was primarily due to the material's highly friable surface, which was susceptible to damage through impact or vibration, or simply though inadequate preparation prior to lamination. I also saw evidence that the cell and/or chemical structure of the foam at times interfered with the proper curing of the resin, often resulting in inadequate properties in service.

The result of the report was twofold - first, the company modified my text to suit their own needs, then buried it. However they did institute an improved handling and cleaning process (dedicated cutting room and a separate vacuum/clean room) that resulted in a much cleaner surface on the foam prior to it being placed in the mold. Now, it must be pointed out that outside of the damaged firewalls I have not seen nor am aware of any other failures of the sandwich as a result of normal service. But I still hold the potential is there.

BTW, according to my wife anyone heavily interested in airplanes is nuts so you're in good company. Many of us nut-cases out here.

But yes, this might be the perfect excuse to go back and build a wing set you might be happier with. Most PVC core materials will do a proper job and Klegecell, Divinycell, Airex and ATC all make suitable products. There's also one other, a Polyimide foam product (I don't recall the name off hand) that is great for high temperature applications.

I have two favorites out of the standard PVC foams: The first is the classic Airex, which is a non-cross linked PVC foam and is one of the most durable foam cores out there. The second is the extremely tough PVC core made by ATC, called Core-Cell. I've used this so far only for making tools but I like its working properties and the variety of forms it's available in (like bead and cove strips).
 

Othman

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Wow that's really bad news, especially at this stage in the game. How much of your structure has to be redone?

You must have come to terms with the situation because you don't sound too POed... good for you.

- Orion Wrote -
"The result of the report was twofold - first, the company modified my text to suit their own needs, then buried it. However they did institute an improved handling and cleaning process (dedicated cutting room and a separate vacuum/clean room) that resulted in a much cleaner surface on the foam prior to it being placed in the mold. Now, it must be pointed out that outside of the damaged firewalls I have not seen nor am aware of any other failures of the sandwich as a result of normal service. But I still hold the potential is there."
-

SCARY!
 

orion

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Not only that but also surprisingly common. It's amazing how often organizations ask consultants to do a design or to review an analysis, only for them to realize that what the customer wanted was to be told that they were right. Whether they were or not is usually not the issue - they just needed somone to rubber-stamp their decisions.
 

wsimpso1

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There are a couple things that make it less of a problem than you might think:

Only the wing skins are involved;

Building a new wing skin takes only a little more time than fixing the things that I have found need to change;

The molds are sitting right there;

I never have any trouble getting enough folks for these events, and;

Each epoxy party culminates in good beer and excellent pizza with good friends.
It is really a pretty easy decision. Since an ordering mistake resulted in me already having most of the foam and glass I need this won't even result in a whole lot of cost either.

My fuselage tub, bulkheads, even the big flat panels for the wing ribs are all on much sturdier foam. This is OK.

Now the airplanes flying with supposedly sandwich panels that are now hollow, that is scary...

Billski
 

orion

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I think it would be interesting to find some damaged, unflyable Glasairs and to take a few core samples of the wings to see the long term effects of the flight loads on the bonds and/or the foam. However I think most owners might frown on the practice of making their wings look like Swiss cheese.
 

BruceS

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I have two favorites out of the standard PVC foams: The first is the classic Airex, which is a non-cross linked PVC foam and is one of the most durable foam cores out there. The second is the extremely tough PVC core made by ATC, called Core-Cell. I've used this so far only for making tools but I like its working properties and the variety of forms it's available in (like bead and cove strips).
Hey Orion,
I've been trying to find Airex or Core-Cell foam for the last week. I've called from CA to NJ trying my best to purchase foam in small quantities (less than 1-2 sheets) and in a 3 inch thickness. Any leads where I might find some in the Northwest. I'm building a LongEZ and want to use the 3 inch for the fuselage sides so I can carve a rounded shape instead of the flat sides that are called out in the plans. I can get 3 inch Divinycell but I've been told it's really hard to shape, and it can't be hot wired as you know, not at all like the blue foam for the wings. Any leads would be greatly appreciated.

Bruce
 

orion

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I use two sources - one is Fiberlay in Seattle and the other is Composites One in Arlington. I don't however know how they are for small quantities.

One outfit that was great for cores was Tacoma Fiberglass but I'm not sure if they're still around.
 

BruceS

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I use two sources - one is Fiberlay in Seattle and the other is Composites One in Arlington. I don't however know how they are for small quantities.

One outfit that was great for cores was Tacoma Fiberglass but I'm not sure if they're still around.

Thanks for the leads, I'll give them a call on Monday.

Bruce
 

BruceS

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Hey Orion,

I've called both places and it looks like I'll be able to get a small quantity from Fiberlay, thanks again.

Just a small technical question, where they say the density is 2.5 lbs or 3.0 lbs, etc, they are saying that one cubic foot weighs 2.5 or 3.0 lbs and so on, correct?

Bruce
 

orion

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To Bruce: Yes, the densities are quoted as pounds per cubic foot. Skinned on both sides, the light stuff should be fine for your fuselage sides. For use as core material in glass wing skins for instance, I prefer to see maybe a half inch in a higher density, possibly in the 6-8 pcf range. For graphite application you can go to a thinner core but usually about a 10 pcf makes a nice compatible application.

To JMillar: Personally, yes I would see Last-A-Foam as a problem in a primary structural application. Many kit companies and/or plan sets have gone to the Polyurethane mainly because it is significantly cheaper than the PVC products. But the thing to remember in this process is simply that even in a whole airplane you actually don't use all that much material so overall, the cost tends to be at least tolerable if not all that inexpensive, especially considering your life may constitute the final cost.
 

JMillar

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Yeah, I take your point, Orion. I certainly like the idea of using the best materials available (up to a point, I don't talk all carbon fiber and titanium for my seat and dash :) ).But what I'm wondering is, if they use a material that is unsuitable, while you can replace the material, should that be a red light saying, there may be more design issues ahead?
 

orion

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Hard to say without being the proverbial fly on the wall. While small airplanes are certainly not rocket science, I have seen enough questionable to downright scary practices to make me somewhat skeptical in approaching any new design. But that does not mean that any one particular design is bad - it's just that if you don't have a track record of doing this, or haven't received proper experience and/or training in light plane development, yes, I'd probably have a somewhat skeptical approach to my evaluation of the project.

In general, if someone does pick a less than ideal material in the process, yes, it might be an indication that not all ducks were in place before the decision was made and that there could be other less than ideal or compromised areas ahead. But whether one particular design has a problem or not I cannot say since I haven't had direct involvement with the product.

The Vision process is clever and from what I've seen, has areas that were certainly well thought out. But beyond that I don't know without doing a more in-depth investigation.
 

Hunt Johnsen

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Re: Last-a-Foam OK? Too bad Clarke isn't making theirs anymore

I've used the lastafoam urethane sheet foam as a core in one-off canoes and prototype/plug applications, but I design my stuff so that the laminate will hold up even if there is some degradation of the foam. When I started building the first of my foam sandwich boats back in the eighties there was a white urethane sheet foam available from Clarke Foam - the surfboard blank manufacturers - it was beautiful stuff and had very good bonding and mechanical qualities and shaped like a dream. It was described as a urethane-polyester foam. I sure wish there was something like it available now.
 

orion

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Last-A-Foam will probably work satisfactorily in many applications where the loads are lower and where your life may not be at significant risk. But airframe structures are a bit more critical and tend to be subject to different loading configurations and/or magnitudes than what you might encounter within a typical white-water canoe.

The General Plastics foam is a poly-urethane product that has a sightly different formulation from the white foam you mention, which is just a basic urethane. The basic urethane foam is fine for tooling but is unsuitable to flight structure use. The very properties that make it easy to shape disqualify it from any application where the structure is subject to repeated loading and/or subject to physical or acoustic vibration.
 

Hunt Johnsen

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I totally agree with regard to the unsuitability of a straight, brittle. urethane.
The Clarke foam I was nostalgic about had much better mechanical qualities than the Lastafoam that I've seen, and bonded to fiberglass really well - failure usually occured deep in the core when it did happen.
Incidentally, the white water environment is not quite the same as a big wave ocean surf situation. Urethane cores and polyester laminates have been the standard for 50 years - but we break boards all the time too.
 

Hunt Johnsen

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Orion, I just went to the General Plastics foam page and their current line looks much better than the plain urethane I've seen and may indeed be better than the old Clarke foam product. The pure urethane I use these days is brittle and suitable only for fin cores and lightweight flotation. We won't even talk about free-blown do it yourself foamkits.
 

orion

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Unfortunately the GP foam is not a whole lot better since it too has a friable structure and will debond when subject to shock or vibration.
 
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