Feeler: DIY / Custom Sandwich Panel Boxes

[DzlToy]

About ten years ago, I built a 12' x 7' x 7' box with a friend of mine and sold it to a customer. That box is currently DIRECTLY mounted on an LMTV and has been from Baja to the Arctic Circle. It is presently being used as a chase/support truck for an outfitter located on the West coast of the U.S.

As with anything in life, the first time you do something, you make mistakes and waste a lot of time. So, knowing what I know now, I would never build that box, that way again. I am considering building a few boxes to order, but with the market correcting itself, I decided to put out this feeler in an effort to gauge genuine interest in the product, not just a passing, "Hey, that is cool." comment. That doesn't pay shop rent.

SIPs (not SIP Panels), just SIPs, are Structural Insulated Panels). They are somewhat compromised in that you are looking for a product that does several things well. The panel must be rigid and 'strong' enough to act as the body of the camper, be fairly light, somewhat affordable and offer some insulation and sound damping. Insulation products such as XPS or EPS performsome, but not all of these tasks well, only moderately. Decades ago, a cottage industry sprung up, manufacturing specialty foams, such as Gurit, Divinycell, Corecell, and Last-a-Foam, mostly for the marine and aerospace industries. These foams are specialised, come in odd sizes and can be expensive and difficult to obtain and work with. Some are made in Europe and must be bought in minimum quantities or via a US distributor.

Sandwich panels can be made of nearly anything that allows a skin to cover some kind of core. Generally 2-3 different materials are used. Insulation and sound damping aren't top priorities, strength and weight, are. Ex: aircraft wings or over head bins, flooring in a train or transit bus, etc.

Vacuum Insulated Panels (VIP) are incredibly good at keeping the cold inside of or outside of your camper, but have virtually no strength on their own and are quite expensive and difficult to work with, in most cases. Their insulation value is phenomenal, however, at R-60 per inch, as measured in the center of the panel.

I could go on and on, but the long and short of it all is that there is no one product that does everything well. So, why try to find or fabricate that product? Build a box that is REALLY robust, then add lightweight, dedicated sound damping and insulation products, if desired.

The product I have developed and am in the process of testing isn't competitive with Chinese plastic or foam panels, gluing blankets to XPS from Home Depot or building a box using plywood and 2x4s from Menards. It is a panel that you can literally beat the crap out of and not hurt it. The technology from the marine transport industry. It takes the first page out of the 'buy once cry once' book. Anyone can glue FRP to plywood or XPS, or nail a few 2x4s together. No one needs me for that.

In a sandwich panel, the skins (think of an Oreo cookie), take tension and compression, while the core take shear loads. So, to make a really strong, robust and relatively lightweight panel, you want thin but strong skins and a thick, lightweight core that can take high shear loads. Thus, skins are 5052 aluminum in 0.0625 or 0.120 and the core is a high-density, structural marine foam with a shear strength between 300 and 650 PSI. For comparison, XPS has a shear strength between 20 PSI and 60 PSI, depending on the product's density, manufacturer and thickness. R-value is estimated at 9 for a 2" panel, but could be higher based on the insulation product chosen.

Completed panels can be as thin as 1" and as thick as 2.25" Weight per square foot ranges from ~3 to more than 8. Insulation adds 0.125 PPSF /inch of thickness. Aromatic and aliphatic polyurea coatings are options, as is epoxy-based paint or automotive-grade 2k paint. Marine aluminum (5083/5086) may be an option, but other than some additional strength and salt-water corrosion resistance, the two offer little advantage over the more readily available 5052 product.

Costs are Estimated at between $15 and $40 per square foot of panel, not of floor space like in residential construction. EX: You want to build the 12' x 7' x 7' box referenced above, fifteen, 4' x 8' sheets will be required to account for waste/drop. 32 SF/sheet x $15 PSF x 15 sheets = $7200. Upgrade to the heaviest-dutiest panel, spray the whole thing in polyurea with Kevlar and top it with automotive paint and you are looking at nearly $40,000.

Please note: these are EMPTY SHELLS, not completed boxes, no bike racks, no wiring, no windows, no solar, no beds, just a box that is built like a brick out house. They are meant to be directly mounted to your truck frame, reducing weight and cost further, by eliminating spring, sub-frame and three/four point mounts. These boxes SHOULD NOT be mounted on Unimogs, which are designed by Daimler to flex drastically. Flex/twist/articulation is NOT suspension, it's just flex.

The panels can be shipped to you 'flat pack', i.e. IKEA furniture or as a completed box. YOU are responsible for mounting, wiring and finishing the box. Use it as a chicken coop or a play house for your kids if you want. I don't care.

The sample box (shell) above is estimated to weigh between 1300 and 3500 pounds, depending on the materials and coatings chosen. I do not think for one second that anyone 'needs' the 8 PPSF panel, but it's just as easy to build as the 3 pound panel. These boxes are meant for MDTs (Medium Duty Trucks) NOT for your Jeep, Tacoma, Ranger, Sprinter or F150. You will double or even triple the weight of the box depending on the interior and gear that you choose to put in it. So, you need an Isuzu NPR, an F350, 450 or 550 or something like a Freightliner M2, an International 4700 or other 'box truck'. Replace the OEM box with the one referenced above. While 3500 pounds may sound heavy, and it is compared to some of the lighter-duty offerings on the market, payload on an Isuzu NRR is in the neighbourhood of 12,000 pounds. An F550 isn't far behind. I am in the camp of, "Do it right, once, the first time. Then go enjoy what you have built instead of worrying about it or repairing it constantly."

The floor is open.

 

[Victorian]

Love this topic!
Couple of questions/observations:
You mention no subframe needed between truck rails and box. How do you compensate for the torsional frame twist of the truck?
Without subframe and internal floor framework, how are you supporting the floor including the load from the side walls/roof etc?

 

[rruff]

...skins are 5052 aluminum in 0.0625 or 0.120 and the core is a high-density, structural marine foam with a shear strength between 300 and 650 PSI. For comparison, XPS has a shear strength between 20 PSI and 60 PSI, depending on the product's density, manufacturer and thickness.

Completed panels can be as thin as 1" and as thick as 2.25" Weight per square foot ranges from ~3 to more than 8. Insulation adds 0.125 PPSF /inch of thickness.

I'm using the lightest PVC foam @ 4 PPCF, which works out to 0.333 PPSF/inch. Check the math.

Your skins make these very heavy, with adequately robust panels typically ~1.5 PPSF total, rather than 3+. Have you experimented with thinner aluminum?

 

[DzlToy]

Yes, thin aluminum dents and dings easily and is difficult to transport and work with during assembly. Using 0.060" or 0.120" does add weight, but that is, after all, the design brief, a bulletproof panel.

Of course I could make a lighter panel... I could buy Nomex honeycomb and vacuum infuse thin carbon skins onto it. That panel would be very stiff/rigid, but not very resistant to impact, scrapes or scratches and would offer virtually nothing in the way of sound damping or insulation. Labour and material costs are incredibly high with custom/small scale composites, not to mention all of the brushes, rollers, cups, vacuum bags, hoses, peel ply, breather cloth and stir sticks you will toss in the garbage along the way.

The ~3 PPSF panels are pretty dang tough and would only be insufficient in applications such as building very large boxes or something that would be highly loaded, such as a cargo or vehicle trailer.

 

[Spencer for Hire]

Are these like Total Composite or close?

 

[DzlToy]

All sandwich panels share the same basic design. There is a thin skin, usually aluminum, fiberglass or carbon fiber on each side of a lightweight core material such as Nomex, Divinycell or even wood, though the latter isn't advisable.

This foam core isn't very strong on it's own, after all, it's just foam. Products such as Divinycell, Last-a-Foam and Corecell are engineered structural foams, for these applications. They are quite different than buying pink/blue foam from your local hardware store. Those products are insulation and have very little structural strength. EPS, sometimes called Styrofoam, which is a brand name, is even weaker than XPS is most cases. While you can physically use these products to make a sandwich panel, that isn't advisable.

Using something like H200 or even an aerospace grade Nomex will provide a massive gain in strength over an EPS or XPS-based panel, with a massive price tag to go along with it. Not everyone needs this strength.

In a sandwich panel, any sandwich panel, the skins take compression and tensile loads and the core material, whatever it may be, takes in-plane shearing loads.

Think of trying to pull a thin piece of string tied to a fix object until it breaks. Now, clamp a sheet of aluminum to a work bench, grab it and try to 'stretch it'. The latter is MUCH more difficult; impossible for a human to do, actually. That isn't a scientific test by any means, but that is the concept of tensile strength. Compressive strength, as the name suggests, is the opposite. Get a sheet of plywood, set one end down against a fixed object and push the other (free) end with your hands, legs or even with your friends'. It is very unlikely that you will compress that sheet of plywood a micron. You may bend it, but it will not compress.

Now, if that piece of plywood, or a sheet of aluminum or a sheet of FRP (Vetroresina), were to be glued to a core material or even another sheet of plywood, it would be that much more difficult to bend, right? This is the basis of all sandwich panels.

The thick, but lightweight core material, e.g. foam, resists bending to a much greater degree once the skins are bonded to it. The skins resist bending for the same reason, they are bonded to the thick foam core.

The disadvantage of this design is that the individual 'ingredients' are still quite weak. Drag your camper box along an embankment trying to climb out of washed out muddy ditch on the side of a road or slide off the trail into a tree and do the same thing, drag the camper box along the tree in an effort to escape and you will absolutely destroy that panel. After all, it's just thin fiberglass and foam. Swinging a hatchet, heavy hammer or an ax will produce the same results.

Now, put a sheet of steel or aluminum plate down on the floor of your shop and whack it with a hammer or an ax as hard as you can, drive over it with a truck and get all of your friends over to jump up and down on it for hours, if you want. There isn't much you are going to do that will significantly or severely damage a steel or aluminum plate.

This is the concept behind a VERY robust sandwich panel. It's more rigid, and stronger in tensile, compression and shear than anything else on the market. Do you need this? Most people don't need it any more than they 'need' 100k Jeep Gladiator or a 500k luxo-barge camper-ma-jiggy. There was no shortage of either at the last Expo show that I attended.

 

[simple]

For me, I'd rather lean toward the side of ultralight and avoid damage. Bashing a habitat box through a narrow road sounds like stuffing a square peg in a round hole.

For really rugged construction, at some point it seems like a foam core sandwich panel starts to lose it's advantage when compared with tube and skin fabrication or molded fiberglass or glass reinforced plywood like a U-haul box with a layer on insulation on the inside.