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Re: [TCML] DIY dent-proof Toroids


Don't know about "spray can foam". That stuff (most of it is latex, not polyurethane anyway) requires 
moisture to cure, and will not cure properly in an enclosed space if it cannot be in contact with 
the moisture in the air. That being said, some folks claim success with "pre-moistening" the 
interior space that is to be filled. I have not tried that. Had a very disappointing experience 
several years ago when the "spray can foam" was still available in polyurethane around here. I tried 
to make an ice chest from an old wringer type washing machine shell and a plastic tub. The plastic 
tub went inside the shell, was to be the interior wall of the ice chest. The space between the 
plastic tub and the washing machine shell was filled with several cans of foam. It did not work, only 
part of the foam cured, left a hollow zone in the bottom with a runny mess inside. I have only used 
"spray can foam for its intended purpose since. If it could be in thin layers, maybe. I don't know 
about that either.

"Real" polyurethane foam is available as a 1:1 part A part B mix and pour product. This stuff WILL 
cure in an enclosed space. If I were going to try to fill a toroid I would use this stuff rather than 
the spray can kind. We used something similar to the 2 LB density foam described here:
only from a local supplier to fill the aluminum pontoons on a pontoon boat last year.
If I were going to fill a toroid (I haven't done that, just a mental exercise based on my experience 
with the pontoon boat) I would:

Make a suitably large hole in the inner portion of the torus to pour the foam into. We used 4 inch 
holes as the stuff is pretty thick and goes off fast, so you have to pour it quick. Something like 20 
seconds after it is mixed until it starts to go off :-)

Mount the toroid on a horizontal axle so it can be spun to distribute the foam

Have plenty of suitable funnels and mixing containers available. Measure carefully. Always use the 
same measuring containers for the same parts (part A measured in part A bucket, part B measured in 
part B bucket). You need to have extra foam as some will be wasted on the mixing containers and 
funnels. This stuff sticks well to polyethylene and is quite hard to remove. The more you scratch the 
surface of the bucket or funnel removing cured foam, the better it sticks the next time:-)  We used 
a drywall/paint mixing attachment in an electric drill to mix with. Being steel, a bit easier to 
clean up with a power wire brush :-)

I would pour in layers. Mix and pour in enough to fill to 20% capacity of the torus, then spin slowly 
to distribute the foam about the periphery of the torus until it is set. Repeat adding layers until 
torus is filled. Note that this stuff will develop considerable pressure if it is confined while 
expanding, so probably best to have plenty of vents around the inner periphery of the torus to let 
excess foam escape. The excess which extrudes from the vent/fill holes can be easily removed with a 
saw or knife and filled or sanded to shape.

My 2 cents


On Monday, May 27, 2013 07:15:00 AM David Rieben wrote:
> First of all, wishing a great Memorial Day to all the fellow US based
> coilers. Now to my point ;^) I've been following this "DIY dent-proof
> toroid" thread and I was just wondering.... If you already have the
> typical "dryer duct" type toroid, could you also not carefully drill a few
> holes in it in an inconspicuous location (say along the top and near its
> minor diameter radius) that are just large enough to allow the nozzle of
> one of those urethane foam canisters to be inserted and backfill the
> toroid with the foam in this manner? It seems IIRC that this may have been
> lightly mentioned in a previous post and someone had cautioned the
> possibilty of the foam expanding unevenly and causing unsightly
> "lumpiness".? Reason I'm asking is that I already have a huge 12x56 (inch)
> topload on my large coil and I have already put a good deal of blood sweat
> and tears into its construction. It was based off some industrial quality
> 12" diameter flexible duct that was purchased from McMaster-Carr. I ended
> up cutting strips of aluminum flashing and covering the duct surface with
> the flashing. Adding enough flashing to completely cover the exterior
> surface of the ducting ended up adding significantly to the total weight,
> too ;^o (Estimate 30 - 40 lbs total weight now). This has "helped" in dent
> resistance but it could still be "better". I was wondering if it would be
> safe to the long term outcome of my big topload if I were to backfill it
> with that urethane foam? How would one determine how much foam was
> "enough"? Would like to glean suggestions from the collective advice of
> this list before investing in 2 dozen cans of urethane foam and/or
> possibly ruining my huge work of art. :^/ 
> Thanks,
> David
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