Re: Rotational Balance Aircraft models

From: Dan Davis (
Date: Wed May 13 1998 - 14:32:33 EEST

Monica & Glenn Whiteside wrote:
> Anybody out there (any friends at Boeing, Lockheed?) try using
> stereolithography for rotational balance aircraft test models? Our main
> concern with these models is weight - normally they're made out of a
> fiberglas and carved balsa wood laminate structure and their weight is kept
> under or around 12 lbs so the test data is not affected by the mass of the
> test model itself. We want to try QuickCast but we're concerned about it
> holding up during model assembly and test situations (it must have multiple
> detachable sections) and humidity problems. Any suggestions? If the
> QuickCast models don't work out at least we'll be able to easily make our
> mold patterns from the SL models....


QuickCast will not be strong enough because of the outside "skin"
thickness. Also you are correct in assuming that humidity would be a
problem -- my display QC parts built in SL5410 got soft after a couple
of weeks. Other, non-humidity resistant, resins will be worse.

We made similar models of quarter-scale autos on the SLA. We took the
finished STL file and used the "Hollow Part" function in Magics RP.
After trial and error, we found that 1-1.5 mm (about 0.040-0.060") wall
thickness was strong enough for finishing, handling, etc (you have an
'eggshell' effect going on here to add strength). You must then cut a
hole in the part (to eliminate trapped volume) and build the part and
the "plug". Finish, join the plug back into the part and voila.

BTW, to figure the weight of the final model before you build it, after
hollowing the part check the volume. Ciba epoxy SL resin density is
roughly 0.00116 grams per cubic mm, so simply multiply the volume by the
density to get the weight of the final part in grams.

Dan Davis
Head, Rapid Prototyping Division
Shah Alam, Selangor D.E.
+60 3 515-3906 DID
+60 3 515-2380 fax
+60 12 277-5770 hand phone

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