Foam filled torsion box

[lgroulx200]

Hi,

I posted an idea for using plywood sandwiched with foam for a torsion box. The idea was not well received. They were worried the foam would fail under a light load.

I am renovating my laundry room this spring so last fall I mounted my washing machine on 2 3/4" sheets of MDF in between 1 1/2" foam to make a pedestal. I used PL300 foam adhesive to glue the foam and mdf. I bolted it to the floor.

After 5 months and countless loads of laundry, the foam is fine.

If looking to build an easy torsion box or gantry beam, glue plywood around foam. It will be light weight and very strong.

Larry

[Geof]

If you go ahead with this it may be a good idea to double or triple the plywood in the places that you will be attaching things to your torsion box. Thin plywood on a foam interior will give a very rigid structure provided any loads applied to it are spread out over the surface. However, if you attach something over just a small area only to a thin skin it will flex in this local region.

[lgroulx200]

Originally Posted by Geof If you go ahead with this it may be a good idea to double or triple the plywood in the places that you will be attaching things to your torsion box. Thin plywood on a foam interior will give a very rigid structure provided any loads applied to it are spread out over the surface. However, if you attach something over just a small area only to a thin skin it will flex in this local region.

Hi Geof,

Most DIY routers use single layer plywood or mdf to attach items without a problem.

You are right about spreading the loads out if the load is great. The washing machine "pads" are about 1 1/2" sq inchs. The total weight of the washing machine is transfered to 6" inches (4 pads) into the mdf which is then transfered into the foam. This is well within the load rating of the wood.

On our DIY routers, the weights and loads will be a fraction of the washing machine.

Larry

[Netjams]

I like the idea!

[AndyL]

In theory it would work well - as a reinforcement. Injected polyurethane is used to strengthen (and insulate) in many applications these days. A lot of the construction we see of late uses these newer steel/foam envelopes as the exterior cladding. Basically 24ga steel with injected polyurethane inside.

Same goes for your typical overhead doors, since we've got away from polyethelene sheet insulation - door costs have declined considerably as they're able to use the injected polyurethane to create the strength.

There are some downsides of course, once delamination (Think foam separating from the edges of the wood) starts the strength dissapears. Excessive vibration, heat and thermal expansion / contraction could increase the speed of the breakdown.

[lgroulx200]

Originally Posted by AndyL There are some downsides of course, once delamination (Think foam separating from the edges of the wood) starts the strength dissapears. Excessive vibration, heat and thermal expansion / contraction could increase the speed of the breakdown.

Hi AndyL,

There are plenty of airplanes, KR-2 etc. that use this type of construction. Their engines and props produce hundreds of times more vibration then our routers, they experience -25 to 100 degree weather plus must support hundreds of pounds of weight. Some of these airplanes have been flying for over 15 years.

If you look at some of the designs of DIY tables out there, you will see they are edge glued or screwed with drywall screws or maybe both. These type of joints have very little strengh but provide many trouble free hours of operation to their owners.

Larry

[jjones]

I've used expanding foam in some VERY large bass bins (think PA) speakers with great results. Folded horns playing below 40hz and you can't feel the vibration in the wood.

Not the same application, but I can see that it could be worth a shot!

Jim

[dansutula]

That is what you have made!
They can be surpisingly strong. Used in the home building trade they are aslo known as "structural insulating panels" or SIPs. In order to use them as a torsion box, you will need to box-in the sides and the ends.Might make a decent router bed, in fact I have considered it myself. You would get additional advantage (dimensional and structural) by laminating on a thin layer of metal or fiberglass to the face of the wood skin. I'd be a bit concerned with creep or sag over time (on unsupported spans) and would use care not to keep any sustained load on them which might promote creep.

Oh, and I would not recommend MDF. Building SIPS are typically OSB, but for a router perhaps you could use somthing lighter, stiffer, and moisture resistant, perhaps ADVANTECH (water resistant OSB), marine plywood, or baltic birch plywood.

[lgroulx200]

Hi,

Using foam will simplify construction of some designs. It will reduce the weight, maybe allowing for smaller steppers, provide faster build times and it is just different.

Larry

[AndyL]

I was commenting on the delamination problem more as a cautionary notice... This isn't aircraft construction we're doing here with carefuly regulated materials; there's a huge difference in scale - a torsion box will have only a few sq in of contact space; an aircraft will have numerous square feet of contact area. The adhesion is the same, the distribution of force is radically different. The vibrations from an aircraft engine are not the same as those of a router; the aircraft engine is a highly balanced precice peice of equipment... Router - not so much

Also need to consider construction materials and surface prep to ensure a good adhesion between the polyurethane and your base material. If you're going to do it - do it right because it'll be impossible to go back and rebuild if there is a failure.

[lgroulx200]

I would recommend using styrofoam instead of polyrethane. Easier to glue, cheaper to buy and can get it at any lumber yard.

With this type of construction, the plywood edges are glued and screwed exactly as the other types of construction. It will have the same wood to wood glue area. The foam is used to replace the internal plywood ribs.

Larry