Re: Big Toroids, collective conscious brain storm

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 12:55 PM 9/12/2006, Tesla list wrote:
> Original poster: "Nick Field - HVFX Ltd." <nick.field@xxxxxxxxxx>
>
> Finn,
>
> This is actually quite close to a process called 'rotational 
> moulding' which is commonly used for making large plastic parts.



> Only major difference is that the tool is normally steel or 
> aluminium, and the process relies on a bulk plastic being heated 
> inside the tool.

or even, wood.. although the longevity isn't all that great.

The trick is that you need to find a rotomolder with a large enough 
oven to hold your molds.

Call around, some molder may have one already.

The finish won't be glassy smooth though.  Most rotomolded stuff has 
some texture to it.




> I think to avoid distortion a stiffer tool than a tractor inner tube 
> would be needed.  Perhaps take a fibreglass moulding from one to 
> provide a female mould?

Easier to make a plaster and burlap positive to take the fiberglass 
mold from.  This is the traditional approach.. you start with wood 
and chicken wire to get the approximate shape. You build a template 
with the desired cross section that can mount onto a pipe in the 
center as a pivot (the template will look like a semicircular cutout 
in the bottom of the template). Then you lay burlap over that that's 
been impregnated with a bit of plaster and paris.  Check with the 
template to make sure that it's not too tall anywhere, but it can 
still be rough.  Wait for the plaster to cure.

Mix up plaster, smear it on everywhere, spin the template, wait for 
plaster to cure.  Sand smooth, goop on mold release, layup gel coat, 
then fiberglass, then resin, etc.  Pull mold off positive.

repeat.

Any fiberglass place can do this for you.  They'll use a chopper gun 
for both making the female mold and for doing the positive 
inside.  It's up to you to do the joins.



For another approach, you could use hydroforming to bend sheet 
aluminum over a sturdy form to make toroid halves, then get someone 
to TIG weld the halves.  Hydroforming: mold in bottom of swimming 
pool, lay aluminum over top, hang explosives in water over aluminum, 
detonate...