composite walls

[hossesdad]

Tnttt has broken me down and I find myself thinking about a 3.5 m/11’6” three person caravan. I have wheels, torsion suspension halves (lightly used), hydraulic coupling, rotor hubs and callipers. Yesterday I bought the steel, which is one way of getting started. (The other way is to draw up a detailed plan, which I haven't done yet).

I am no craftsman, I could probably get a job as an bodger but only if there was a leading hand bodger to show me how to do the tricky bits.

I fancy vacuum bagging the walls as a composite...1mm aluminium, 50mm (2 inch) foamular and 6mm or 9mm (1/4 or 3/8ths) treated plywood epoxied together. I am off to buy the foam tomorrow. My neighbour brought me his heavy duty vacuum pump. It ran a dairy once. I could bolt down my compressor and run the pump off the compressor motor which is about 3hp.

Does anyone have any experience of vacuum bagging walls? I am doing it because I believe the pressure (one might manage one thousand pounds per square foot) improves the epoxy bond but is this true? Of two composites manufacturers who I checked on, one did it this way the other other had a huge press so they believe in pressure. But often pressure is used to force epoxy through glassfibre laminate and I am not doing that, just using epoxy as glue. Views welcome, experiences particularly welcome. I should not be surprised if vacuum bagging is a stupid idea, I have had stupid ideas before now.
Hope “foamies” is the right forum for this question.

Next week I shall start a build journal and I will submit photos

[OP827]

Just sharing my experience: I feel that 3mm plywood is more than enough for internal skin in a wall foam sandwich. More will just add weight. I also feel that my 1.5in foamular foam is more than enough of wall thickness. My next build will probably be just 1in foam. 1mm aluminum skin seems to be also too thick to me, but all it is of course my personal preference.

I have not tried epoxy glue-up vacuum press on large surfaces yet, I used bricks to glue panels and found that epoxy gives me a very good working time comparing to TB2(dries too fast). Lately I started using PL Premium glue instead of epoxy for gluing plywood to foam and I like the outcome so far, I used a notched spreader for PL and bricks for clamping on large panels at home. PL is more economical than epoxy and less toxic. I would probably use epoxy for gluing aluminum to XPS foam though, as PL requires porous surface, while epoxy does not. I think that I would still use PL if I was gluing EPS(white beaded, not an xps foam) to aluminum because EPS is porous.

[swoody126]

when building boats w/ epoxy we have found that extreme pressure is not necessary or even desired for a really strong bond

most folks are building using the BRICK method for face to face bonding and making some really strong boats

DUCKWORKS now offers an epoxy that is way less obnuckinfockshus to the nose & skin though the old safety precautions are still recommended

the stuff is DWX

http://www.duckworksbbs.com/supplies/ep ... /index.htm

me tinks VACUUM BAGGING is contraindicated

sw

[hossesdad]

OP827 and SWOODY126, thank you for your support.

OP827: (I admire the quality of finish inside your poptop, Oleg). On your answer to my question so far as it concerns plywood thickness, a local builder of RV walls told me that he uses 2mm fibreglass bonded both sides of the foam. RV builders use light construction, in my understanding, so I am trying (without being an engineer) to match his build in aluminium, foam and plywood. I will look up the relative stiffness and strength and post again. You and swoody126 are both right on the vacuum bagging being overkill, see below.

swoody126: thank you, you and Oleg caused me to research this morning and may have saved me a lot of time. It turns out that vacuum bagging with too much vacuum could be disasterous. Epoxy as glue only needs two or three psi of vacuum. The following are relevant.

http://www.westsysteminternational.com/ ... stem-epoxy (quote):
“Use only enough clamping pressure to squeeze a small amount of the thickened mix from the joint. This indicates that the epoxy is making good contact with both of the so-called ‘mating surfaces’. Do not squeeze all the thickened mix from the joint by using too much clamping pressure.”

https://support.systemthree.com/hc/en-u ... lue-joint- (quote):
“Epoxy adhesives, being gap-filling glues, need only enough clamping to close the joint. Unlike other glues, epoxy requires low pressure and a proper film thickness of resin to make a proper glue joint.”

MY COMMENT As my sides are 12’x 6’, I will need a lot of bricks. I may do a low pressure vacuum bag just to avoid moving all those bricks six times (why six? I am thinking of two panels for the roof, so four walls plus 2 roof panels). I have time to think about it as I am going to build the chassis first. Oleg, I will use 2” foam on a belief (that may be wrong) that the strength of composites is improved the further apart the tension members are and (secondary considerations) hot summers here, the need for good cold insulation in a bigger caravan (well, bigger than a teardrop) with minimal heating, a desire for sound insulation and the fact that the dollar difference to the build is minimal. Forgive me, I seem to like to have a few years to regret not taking good advice.

[KennethW]

Instead of block. How about a sheet of plastic and a bed of sand.?

[OP827]

Thanks for your kind compliments of my build. I was thinking that if the panel is too big and bricks are too many, maybe a simple vapour barrier heavy mil polyethylene and shop vac will do the glue up job as some of people here have done. The shop vac would have to be reliable though, as it needs to continuously run for several hours at least as epoxy sets. I am thinking of doing that in my future builds, it's just the shop vac is noisy and may not be as friendly with electric bill as bricks
I wish you success with your approach and build. I personally like the idea of pre-made glued-up panels as they can be made flat, including their finish. That is how many travel trailers are made nowadays.

[OP827]

Oleg, I will use 2” foam on a belief (that may be wrong) that the strength of composites is improved the further apart the tension members are and (secondary considerations) hot summers here, the need for good cold insulation in a bigger caravan (well, bigger than a teardrop) with minimal heating, a desire for sound insulation and the fact that the dollar difference to the build is minimal. Forgive me, I seem to like to have a few years to regret not taking good advice.

I agree that 2" foam is going to be very good to insulate from cold and heat and provide a very rigid wall structure if delamination from external skin is prevented. Also, think about the temperatures that the foam will be exposed to. XPS will soften quite well if the roof is hot, a thin plywood and thin fiberglass cloth with epoxy may be a good protection in that case. Aluminium, if it is not white, may get very hot on the sun and then foam may separate from it by the glue line.
I will paint my build in white to reduce the risk of heating the foam core while being exposed to extreme sun heat.

[QueticoBill]

I would be concerned that the aluminum on one side and wood on the other would make the wall bow like a bimetal strip in a thermostat. Might only be a little but could show up in doors and hatches.

[GPW]

You need to make some samples and test them to suit Your needs ... the only way to know for sure ...

“ I will paint my build in white to reduce the risk of heating the foam core while being exposed to extreme sun heat.” No worries , even black painted foam never got beyond 140F in our SOUTHERN Summer Sun ... where it gets real HOT for many months ... acctually it works to advantage as the foam puffs itself ( the air inside expands) getting rid of any dents that may have occurred ...

[CharlieL]

According to DuraSip brochure the smallest radii with the thinnest solid panel would be 5 inches. probably more th sable in a teardrop front and rear roof line. see DuraSip.com for a peek at the chart. comes in 3 thicknesses and smallest radii runs from 5 in to 19 inch radius. Have fun with it, thinking about durasip and 2 1 inch roof bends for a teardrop. one sided (outside) and pmf for the other
what do you think? don't have a clue what it costs though.
Charlie L

[rruff]

1mm aluminium, 50mm (2 inch) foamular and 6mm or 9mm (1/4 or 3/8ths) treated plywood epoxied together.

I'll 2nd the comment about about the thick plywood being overkill. If you can get 2.7mm luan where you live, that should be more than adequate for the interior surface.

Are you after the look of aluminum? It will get hot in the sun on a calm day due to low emissivity (does not radiate heat). If you are comfortable with epoxy and glass you could do the exterior in that and eliminate seams. If you want a nice smooth surface though fiberglass will be a lot of work.

1 inch of XPS is a lot of insulation for a small camper. You are going to need a decent amount of ventilation and that will be most of your heat loss. It's very different from a house where the volume of air exchange needed is relatively less compared to the interior volume and surface area.

1inch will be plenty stiff with those skins, at least for the walls. Try some test panels. Greater thickness will get you greater bending strength and stiffness, but it will be overkill I think. Regarding durability, denting of the aluminum and failure of the foam core are greater concerns.

I've had very good luck using PL Premium for panel bonding (plywood to foam), using a notched trowel to spread it over the surface. Some advantages over epoxy are that it's quite a bit cheaper (buy the big tubes and just rip them open), no mixing, and it's ok to use it in cold temperatures. You don't need much pressure at all.

The cheap XPS foam is weak and doesn't bond very well. It helps a lot to texture the surface with a carpet seam roller. That goes for epoxy too, or any adhesive I know of. Best to make samples of whatever sounds good to ensure you are happy with the results before you dive in. Good luck on your project!

[hossesdad]

Thank you for your thoughts. Every scrap is valuable right now. I am still aiming for an aluminium/foam/ply sandwich though I am pricing fibreglass sheet.

Here is an update:

1. I have bought the 2” foam, below is a pic of it on my trailer.

2. I drove 150 miles to get the foam (our local shops sell teat cream, calving chains and culvert pipe but they don’t sell this) and took Ebi to the zoo while I was there. We camped on mattresses in the back of my suv overnight. Good living.

3. PLYWOOD THICKNESS. I am persuaded to use no larger than ¼” plywood for the internal lining. I haven’t the courage to go to 1/8th on a structure this size (12’x’6’x7.2’). The RV manufacturers are comfortable with 2mm fibreglass on each side of the foam. The internet suggests that a ply or wood strip hull can be half the weight of fibreglass. It turns out that the modulus, etc, of each is adequate but the ply and wood strip are more serviceable because of stiffness. On that view, light ply might suffice. I guess (could be wrong) that flexing is the enemy in a caravan as it will stress the joints; a ballooning side wall would pull the roof joint down and the floor joint up, and not gently. Using both thumbs and all my fingers to count, driving at 50 mph in a 20mph side wind when an semi-trailer passes could generate temporary winds of 80mph putting roughly 1500 to 1800lbs of pressure on the sidewall. (https://www.physicsforums.com/threads/w ... si.364785/, but I am not joining arguments on this one, though you are welcome to have one).

4. AUMINIUM THICKNESS. The consensus out there seems to be that for stiffness, to equal half inch ply would take quarter inch aluminium ( eg, http://www.millerwelds.com/resources/co ... s-aluminum). On the other hand the strength-to-weight of aluminium is probably (cannot find conclusive figures just now) the same as that of ply. Aluminium is two to three times denser than ply. I cannot imagine someone skinning outside foam with less than 3mm ply so I see no reason to go below 1mm aluminium (.040” to you) and should probably go above it, to be logical. This aluminum is structural, not decorative.

5. ATTACHING ALUMINIUM. The threat here seems to be heat expansion. the problem is more than “oil canning’ in that there is a danger of breaking the glue line and weakening the structure. The difficulty can clearly be reduced by lowering the temperature (by coating the aluminium in a light colour) and would be helped by using a flexible glue. But I have tested the substrate and the surface flexibility of the foam is plenty to allow the glue to move with the aluminium. On bending and stretching the foam, the compression and tension sides move more an 3/16ths” per 4” relative to the core. I would consider it highly flexible. Quantifying the problem, the long axis here is 3.5m (sorry, I will have to do this in metric), the coefficient of expansion of aluminium is about .000023 m/degreeC . On 3.5m between -30 and +50C there would be approximately 4mm of expansion. (That's over an 80 degrees C range). If I fixed the aluminium to the foam at 20degrees C, going to 60degrees C would expand it by 2mm or about 3/32nds” Thats enough to wrinkle it. If I use 4’x8’ sheets tall side vertical, three will do the width of a wall. A 1.5mm gap at each join should allow for any reasonable expansion across the whole. I can cover the slots with a 3mm thick, 10 mm high “top hat” or “bowler hat” extrusion fixed with VB tape or epoxy. Hopefully that would absorb the movement at its corners and, at the same time, restore some strength to the panel.

The alternative would be a single corrugation in the middle of each sheet. leave the corrugation unglued and movement in it would absorb the expansion. I might look into that. Can anyone think of a good way to put one corrugation into an aluminium sheet at home? Without a sheet metal brake, that is.

Another alternative might be to use galvanised and painted steel in a very light guage. The coefficient of expansion is half that of aluminium, the price is right and steel has a greater tensile strength than aluminium, weight for weight. (Or does it? I must look that up)

6. I am going to do some test panels.

Thanks, everyone.

[GPW]

RE: Aluminum .... All the “oil canning “ can be pretty much avoided by attaching the Aluminum when the weather is the HOTTEST .... metal fully expanded , so when it cools it will only shrink and tighten up further ... We arrived at that deduction by checking with several Cargo trailer owners in the arrea , The most wrinkly ones in the summer were the ones made in the winter ... Summer made were all drum tight ... Just sayin’ ...

Ps. our little all Steel Cargo trailer is a little heavy , but very TOUGH ...

[KCStudly]

A few thoughts,
Don't get hung up on the individual material traits. You are building a composite structure (even traditional builds are composite structures!). It is the unity of the materials and the geometry of how they are put together that really adds to the strength. Airplanes are both light and strong. Armor plated tanks are just plain heavy.

A composite wall might not end up being any lighter than a plywood wall, but it will be just as strong (or stronger) and it will have better insulating properties. (To me the thickness also makes for better ergonomic fits and sealing at side doors and galley hatch.)

If you break your outer skins down into sections with expansion joints, you lose some of that unity. But even if you decide to go that way, why bother with the complexity of a hat channel or complex breaks? Just leave a gap at the butts and fill with flexible sealer.

But, to be honest, I think you are over thinking this. Look at all of the successful builds that float their aluminum, use .040 max thk for side walls and .030 for roof (for enough flexibility to follow profiles). By the time you add hard points for doors and other various attachments, your composite structure will be about the same as a traditional build (depending on the complexity of bunks/cabinets and other features you may want to add). (My build started out as a simple foamie with thin ply inner skins, but after modelling all of the hard points, even using individual blocking points in lieu of skeleton/sandwich ply construction, when I stripped away the outer skin and foam it looked like one of those dinosaur kits.)


I'm all for innovation and have done some things differently on my build than most people do. I can appreciate the desire to be creative and to experiment a bit, but I can also vouch that sometimes things aren't as easy as they look at first, and don't always turn out like you planned. Most importantly, breaking new ground takes a lot longer to sort through when you have to develop your own techniques, methods and material applications. If I build another it will be much more traditional. $0.02

[GPW]

“Studlysaurus “ ... Very Cool !!