Aluminium or Fibre Reinforced Plastic (FRP) as outer skin in a composite panel?

[Mats.P]

I am building a squaredrop/teardrop trailer (not so much for overlanding) and I will use composite panels for my build with XPS foam core but I have a hard time deciding outer skin.

I will use (sort of) an aluminium inner frame and aluminium trims to seal the joints and add some stability. I will bond the panels, the frame and the trims with glue allowing some flexibility for movements between the frame/panels/trims.

As far as I understand FRP(Fibre Reinforced Plastic) has less elongation (lower thermal conductivity) than aluminium, which is a good thing when it comes to cracks in the joints (that could lead to water penetration and so on).
On the other hand, FRP and Aluminium have different temperature elongations. Could this increase the risk for cracks and it would instead be better to use the same material (aluminium) in both frame/trims and skin?

Or is it all theoretical problems since the builds are relatively small and also the temperature caused movements?

Help! Pros and Cons please!! Alu or FRP as outer skin in composite panel?
Thanks and BR
Mats

 

[Peter_n_Margaret]

Absolutely FRP skins (both sides), and no frame at all.
The aluminium skin will be subject to minor damage from the mildest mis-treatment and that damage is virtually un-repairable. FRP skin, on the other hand, is as tough as old boots and in the unlikely event of damage it can be repaired.
I would prefer to use closed cell polyurethane foam in preference to styrene. It has much better thermal insulation properties and probably better crush resistance too.
Cheers,
Peter
OKA196 motorhome

 

[Mats.P]

Absolutely FRP skins (both sides), and no frame at all.
The aluminium skin will be subject to minor damage from the mildest mis-treatment and that damage is virtually un-repairable. FRP skin, on the other hand, is as tough as old boots and in the unlikely event of damage it can be repaired.
I would prefer to use closed cell polyurethane foam in preference to styrene. It has much better thermal insulation properties and probably better crush resistance too.
Cheers,
Peter
OKA196 motorhome

Thanks a lot for your feedback.

I agree the panels normally are rigid enough without frame but since I'm planning to also have a roof rack and possibly a rooftop tent I think I need to at least add some inner structure.

Comparing some of the properties for Extruded polystyrene (XPS) and Closed cell PU it seems like the polyurethane could be even a better solution. I wonder why XPS is popular among offroad camper/trailer producers?? (I have not looked into weight, price, environmental aspects yet...)

Thanks for making me more confused ;-) So FRP skins and closed cell polyurethane core is now the latest bet

 

[ripperj]

I have read that FRP from home stores is not UV stable , so keep that in mind when you buy.


Sent from my iPhone using Tapatalk Pro

 

[madbrainz]

Absolutely FRP skins (both sides), and no frame at all.
The aluminium skin will be subject to minor damage from the mildest mis-treatment and that damage is virtually un-repairable. FRP skin, on the other hand, is as tough as old boots and in the unlikely event of damage it can be repaired.
I would prefer to use closed cell polyurethane foam in preference to styrene. It has much better thermal insulation properties and probably better crush resistance too.
Cheers,
Peter
OKA196 motorhome

Aluminum skin and framework can be repaired without welding depending on the type of damage. There are several aluminum friendly epoxies that will do the job. It also depends on the type of aluminum used and how it is mounted to allow for torsion. Torsion is your biggest enemy on trail rigs. I wouldn't be so concerned with bumps unless you are using leaf springs, without any shock absorbers. I'm a fan of Timbren axleless suspension. Thermal conductivity is also a concern and needs to be accounted for to ensure that you stay warm/cool accordingly.

 

[Alloy]

Thanks a lot for your feedback.

I agree the panels normally are rigid enough without frame but since I'm planning to also have a roof rack and possibly a rooftop tent I think I need to at least add some inner structure.

Comparing some of the properties for Extruded polystyrene (XPS) and Closed cell PU it seems like the polyurethane could be even a better solution. I wonder why XPS is popular among offroad camper/trailer producers?? (I have not looked into weight, price, environmental aspects yet...)

Thanks for making me more confused ;-) So FRP skins and closed cell polyurethane core is now the latest bet

Laminating with FRP is the best unless you go with (heavy) welded aluminum and insulate afterwards. The aluminum skin acts as the support structure.

With FRP an external (rather than internal) support structure could have a secondary purpose as branch/bush bar.



PU (2lb) sprayed under perfect conditions WITH the skin intact is 95% close cell. It will absorb water that takes decades to dry. Homes are sprayed with 1/2lb (R3.5) instead of 2lb because the 1/2lb allows the water to pass through so leaks can be identified before the structure is damaged. In some applicaions the PU is coverd with a liquid vapor barrier to keep moisture out.

PolyIso will absorb water that take decades to dry. Over time it looses R value and performance drops as the temp drops. Both of these factors are due to the blowing agent used in the foam. Some Polyiso has alum foil that slows off gassing and stops UV from the sun. Mfg use different blowing agent which affects loss of R value so each brand needs to be compared. For example PolyIso that starts out at R8 per inch might drop to R5 (same as XPS) over time.

EPS will absorb water, is cheap, can be made in large sheets and it is easy to grind/recycle the scraps. EPS will dry faster than PU.

XPS does not absorb water but is limited in sheet size.

In my experience people are of 2 minds when choosing insulation:
1.) it will not leak...while I own it.
2.) it will leak at some point

**Condensation is water so a panel/wall that is air permeable will get water in it.

 

[rruff]

I wonder why XPS is popular among offroad camper/trailer producers?? (I have not looked into weight, price, environmental aspects yet...)

It's cheap at the hardware store...

Even the 25psi stuff will probably work, and they may be using stronger grades (up to 100 psi)... which are sometimes available to consumers depending on where you live. Note that some people have had issues with outgassing and delamination with XPS core if it gets hot. Definitely use a white roof if you go that route. Sand the foam, texture it if possible, use an appropriate adhesive (PL Premium?) etc. It's hard to get anything to stick to it very well.

If you use PU I'd recommend sheets that are 4+ lbs/cu ft.

I'm using PVC foam from Carbon Core on my camper; 4 lb/cu ft. It's >2x the weight of 25psi XPS and ~6x the price... but also 6x stronger. Plus the cells are much larger and epoxy adheres to it like crazy (unlike XPS).

Can't find the thread, but there is a couple who built a truck camper recently using home built panels with XPS, plywood interior, and aluminum exterior, then vinyl wrapped. That had an aluminum exoskeleton.

I have read that FRP from home stores is not UV stable , so keep that in mind when you buy.

Definitely buy sheets made for camper exteriors. Usually has gelcoat.

 

[Peter_n_Margaret]

I agree the panels normally are rigid enough without frame but since I'm planning to also have a roof rack and possibly a rooftop tent I think I need to at least add some inner structure.

Not needed.
We have a pop top over the bed area. It is a single sheet of FRP sandwich. It is pretty strong



Cheers,
Peter
OKA196 motorhome

 

[high-and-dry]

Yes polyurethane foam can be stronger, but as with every foam the stronger it is the heavier it is. EPS is cheap, its easy to find in 25 psi ( not home depot ), and its easy to work with. Believe it or not epoxy does glue to it very well, but the foam structure it self is weaker for peel strength. You can do a peel test and the epoxy to foam is not what fails, it will pull a thin layer of foam.

Pu foam would be the idea, just expensive, I did look it up and i think it runs 6 to 10 times more ( iirc )

Both sawtooth ultimate and I did 25 psi eps foam with fiber glass skins. I have steel tube in mine for bracing the pop top roof opening. I still would not want a RTT on mine.

If your going to go that do a steel frame with ACM bonded to the outside with vhb tape, fill with foam and vhb the inside skin on.

 

[Mats.P]

Thanks for all the good comments!! Just to clarify, I will not manufacture the panels myself but buy them pre made from a professional sandwich supplier. What Im mainly looking for here is to increase my knowledge so I can have good start-point for a discussion regarding the specification with the supplier.

 

[Mats.P]

Not needed.
We have a pop top over the bed area. It is a single sheet of FRP sandwich. It is pretty strong

I agree! But what I´m concerned about is more the shear forces. You have a design that is good for shear forces on your camper (the bed area on top of the cab adds strengths). Compare with an empty box, put 100kg on top of it and then turn in high speed. Here I think it could be good with also some inner structure

 

[Peter_n_Margaret]

Here I think it could be good with also some inner structure

That inner structure can be the furniture, cupboards, bench top etc all of which become a structural part of the whole box.
Cheers,
Peter
OKA196 motorhome

 

[high-and-dry]

That inner structure can be the furniture, cupboards, bench top etc all of which become a structural part of the whole box.
Cheers,
Peter
OKA196 motorhome

Yup in my build the front half of the bed also helps hold the side walls and front of the trailer. The kitchen slide out support, helps hold the bed.

BTW Peter its all your fault, I love the OKA truck you used for the base of your build. I spent an hour checking them out yesterday, but there are none is the US for sale.

 

[plh]

I've heard its difficult to source aluminum skin at the moment, kinda like the beer can shortage I guess.

 

[rruff]

Thanks for all the good comments!! Just to clarify, I will not manufacture the panels myself but buy them pre made from a professional sandwich supplier.

Who?