Re: [ATM] "Render Unto Ceasar. . . . "

[artbianconi@blast.net]

Back in the sixties there was a popular foam core material whose 
chemical origins I do not recall but which was referred to as Clark 
Foam. It was said to be made in Europe but I could not verify 
that. It was available in an assortment of densities from 3 to 12 
lbs per cubic inch and in various sheet thicknesses starting at 
1/4 inch.

Clark foam, at least as a descriptive name, has disappeared from 
the lexicon of today's composite enthusiasts but there are still 
many, many alternative materials that can coexist with a wide 
variety of resin systems without desolving.

You can get a free catalog from Aircraft Spruce by going to their 
web site but, when it comes to tech specs, scope of materials, 
tech support I prefer one group for composite materials: 
Fiberglast. They too have a web site. 

Core material can be just about any low density substance. The 
best, of course continues to be the oldest of the bunch: Dupont's 
honeycomb but the cost is prohibitive and it resists bending 
around very tight radii.

The most friendly, in my opinion, and most suited to OTA's is a 
low density foam core that is cut into small squares and held 
together with a fabric scrim. It's much the same method used for 
holding small  bathroom tiles together (like you use for making 
mirror tools).

It's perfect for wrapping around a mandrel when making a tube.. 
Depending on size and thickness, it will impart a slight prismatic 
effect to the glass unless you sand it lightly beforehand. It does 
however, create an exceptionally stiff part and does offer a 
modicum of compression resistance to threaded fasteners, 
something low density foam cores lack.

I personally prefer end grain balsa as a core material, especially 
in areas where stress concentrations are likely such as the 
primary cell attachments, the attach points for the EQ mount and 
perhaps the focuser.

The most fun, however, is to be had when you uncover 
something that's common but not normally considered for this 
application. 3M, for example makes a red insulation material that 
comes in a roll about 30 feet long. It's about 6 inches wide and 
perhaps 3/8 inches thick. It's in the building department at 
Loews and Home Depot. 

My first attempt to use it was a failure but only because I did not 
use a microballoon mixture between it and the glass layup. I 
know better but got lazy and the result was poor peel strength. 

These foams do not stand up to resin systems that employ an 
MEKP hardener. That's the hardener of choice for most 
Vinylester based mixes. Unfortunately, that the most likely to be 
found in auto parts stores and Home Depot!

MEKP is dangerous! The fumes are highly toxic and the P in 
MEKP is for the Peroxide chemistry which will blind you should it 
splatter into your eye. 

Safer polyester resin systems like West are not nearly as risky 
and will not desolve any of the foam cores. Even junk styrofoam 
can withstand it.

Good luck

Art Bianconi


On 25 May 2004 at 23:15, Daniel Fundo wrote:

<< I am finding numerous core materials that are equal to or 
better
than structural foam and less expensive. One thing is clear: there 
is
absolutely no excuse for the continued use of sono tubes for an
optical system.  >>

Art,  I agree......fiberboard tubes are too heavy.  I got hold of 
some
foam packing material that is phenominal  Its almost as light as 
high
density styro, but it's hard......takes a bit of pressure to put a
fingernail into it, but its still flexible.  I cut (with some
difficulty) some thin strips and laminated them with epoxy and 2 
oz
'glas.  The strips were rigid as hell and only weighed a few 
ounces.
Better than aluminum at the least.  Wish I could find out what 
the
foam board is called.  Just for kicks, I'd like to make a Dr.
Seuss-looking telescope......sort of an organic, free form, 
functional
sculpture.

*****-----
Dan F.


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