Dear Lisa:
I don't know what's being done in Europe with walnut shells, but here at
Cessna we use crushed walnut shells as a cheap filler material to backfill
stereolithography models we're using as hydroblocks (i.e. formblocks). We
use a stereolithography "shell," say 1/8" thick, to define our part profile
we want to form in sheetmetal (we also model in the material springback)
and then we backfill the inside shell with a high temperature epoxy casting
resin using walnut shells (or you can use aluminum chips) as filler and
then mount this to an aluminum base. One word of caution: as the casting
resin cures it generates heat so it is advisable to model in some
strengthening webs inside the stereolithography shell to keep it from
warping. Oversized holes for part location are also modeled in and
bushings are potted in to accurately locate the part with pins. This has
worked extremely well for us on short-run, experimental parts which we need
FAST. We have successfully formed open-contoured (haven't tried any closed
angles yet!) sheetmetal parts (quantity 7) on a Cincinnati Hydropress at
8,000 psi and also using a rubber-headed drop hammer. We thought the drop
hammer would destroy it for sure but it held up with no visible signs of
damage.
Maybe for larger parts (20' to 30' is EXTREMELY large though) you could
make many thin shell sections (to save resin) using stereolithography, bond
them together, and then backfill or inject them with epoxy or urethane for
strength. Perhaps you could use a mix of CNC machining and
stereolithography, using machining for the easy-to-machine large sections
and stereolithography sections for fine details which would be time
consuming to machine. Hope this helps....
Glenn Whiteside
Cessna Aircraft Company
E-mail: siderwhite@worldnet.att.net
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