Eric Petersson:

As Dennis mentions the surface needs sanding to be smooth enough for
our needs.

Well..... The stereolithographic (SLA) parts I showed are certainly
good enough. The louvers on that B&O York AC panel are beautifully
rendered. The problem is with representations of larger sheet metal
surfaces. Such surfaces done by SLA are nice and smooth, but they're
not completely flat at edge or detail boundaries. If surface details
would "float" with the underlying surface, I think we could live with
that. But they don't. In SLA each layer is constructed in absolute
space, not relative to the underlying layer. Each new layer starts out
dead flat - it's a liquid layer, after all - so rivets created on an
undulating underlying layer are like rocks in the ocean, their height
above the water line varying according to wave height. This very small
scale variation in the height of the rivets (~0.001" or so) is more
visually disconcerting than the surface undulations.

PolyJet, on the other hand, renders flat surfaces that really are flat,
but they're not smooth. All the rivets, battens and other surface
details are nicely defined and of the proper height. You can also run
several complete sides at once, whereas with SLA, 80' car sides have to
be made in two pieces, which then have to have the edges milled and
bonded. So Polyjet is a faster, easier and less expensive technology.
The question is, how much surface roughness is acceptible? The
obliquely lit car side was the worst of the five. The surface
grainyness is from rattle can gray primer spray, which I then hit with
a light coat of clear lacquer to accentuate the problem. Such a surface
on a cast resin car should be perfectly fine if it's going to be
weathered. It should be adequate for a flat-finished, unweathered
surface. It's not adequate for a high-gloss surface, especially if it's
brightly lit.

So, is it possible to build the flat surface, remove it from the
machine, sand it and place it back in the machine and then print the
rivets? Has anyone tried this? Would it work?

Not the way the technologies work now. Both SLA and PolyJet build a
reference surface support structure on the machine bed, then create the
parts on that. There's no provision for re-zeroing machines on a
different surface.

Tom Madden