OT: (but could be related) Jay Leno's 3D Printer Makes Old Car Parts - NextEngine 3D scanner and Dimension 3D printer - P

[Ethan Dicks]

On Wed, Jul 15, 2009 at 2:11 PM, Vincent
Slyngstad<v.slyngstad at verizon.net> wrote:
>> > http://www.cosam.org/images/pdp11-35/filler_bracket.jpg
>
> I have cast those with pretty good success.  The way that seemed to work
> best was to cast the RTV mold with the thing laying as pictured.  Then when
> the RTV sets up, flip it over, and remove the original, exposing the whole
> whole back.  Then  fill with the 2-part plastic resin, being careful not to
> leave air bubbles in the recesses that will form the bulbs.

My dad used to do vacuum casting in his home jewelry shop.  I doubt he
kept his rig when he moved from Ohio to Maine to California, but it's
not impossible to make one large enough to hold molds the size of
coffee mugs.  Finding a bell jar is probably the hardest part.  You
just need enough vacuum to get the air bubbles to express themselves
before the medium sets up, so a pump shouldn't be hard to find.

>  I also like to
> fill it slightly over-full, so that the back is bulgy, then sand the face
> that bolts to the rack nice and flat.

Good tip.

> The bulbs will fatigue the mold as parts are removed, so each mold is
> probably only good for 6 or 8 good castings.

I can believe that (and it's probably simpler to use a semi-disposable
RTV mold than make a proper two-part mold).

-ethan