Re: Stereolith / Rapid Prototyping (trying again)

pullmanboss <tgmadden@...>

Jeff Aley asked:
Is it easier to create a CAD drawing for RP as compared to doing
drawings for injection-molding? I am under the impression that I
could easily do a CAD drawing for a box car, whereas DESIGNING
injection molds requires far more intelligence, skill, and
experience than just re-drawing a paper drawing on the computer.
I would think that the RP method frees one from having to consider
things like material flow, how to eject the part from the mold,
etc. I do, of course, realize that resin casting can only handle
undercuts to a very limited extent.

Are my fantasies even *close* to reality, or am I way off base?
Gee Jeff, I'm not sure I want any insight into your personal fantasies!

CAD for RP is neither easier nor harder than CAD for injection
molding, just different. For starters, your parts can have recesses on
the bottom side, but not details. Remember that honeycomb support
structure I mentioned? All features on the first layer have to bridge
multiple cells of that honeycomb or they will simply float away and
sink to the bottom of the tank. You want to avoid overhangs for the
same reason - everything on a part has to be supported by something as
it is built. Don't put fine details on vertical surfaces - rivets
built sideways don't look very good. Avoid sloped or curved surfaces
if possible. They show stairstepping, which on a 1:1 conventional RP
part is removed by sanding. You can't do that on a miniature part with
rivets. So, the idea is to break down your model into a series of flat
RP parts, then build those flat parts (or first-generation castings of
them) into the actual casting masters. Stereolithography doesn't do
very thin parts well, so if you want thin flat parts, design them on a
thicker support plate and flat cast them later. See the following
photo for an example:

The parts are left & right "wings" for the top of a Pullman blind end
(sloped surfaces on the finished end) and left & right collision posts
for the same end (vertical surfaces on the finished end). I made a
rubber mold from that plate and two others, which I then cut apart so
that the surfaces of the plates became the surfaces of three new,
small molds for flat casting. Easier to show than to describe, but I
just did the first pour on those molds and they're in the pressure
tank curing right now.

My Naperville clinic on this topic will have some "Gee Whiz!" stuff
just for show, but mostly I'll be going over design considerations
like the above.

Tom Madden

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