Re: RP of Die Cast Parts

From: Rob Connelly (Motorola)
To: Douglas vanPutte (Eastman Kodak  Company)
Cc: RP-ML
Date: Tuesday, September 27, 1994
Subject: Re: RP of Die Cast Parts

> Can anyone tell me any good ways to rp a metal die cast part?

You may have already thought of this, but a process I used for RP'ing some
die cast parts is called RPM (Rubber Plaster Molding) and a vendor who does
a GREAT job at it is Metalcast Engineering in Oakland CA.  Call John Collins
or Randy Erickson at (510) 534-2320 and tell 'em I sent you.

The process involves taking your RP positive, laying it up to establish a
parting surface, then making urethane negative mold halves and inserts (yes, 
they can do undercuts and shutoffs.)  Next, they make RTV molds of the urethane
halves and insert pieces which are quite flexible.  Finally, they squirt an
aerated plaster slurry into each of the resulting boxes to make a plaster mold
for each part to be made.  The plaster molds are assembled together with
their insert pieces and metal (aluminum, zinc, zamak, magnesium at the time) 
is poured in, to be liberated by smashing open the mold.  I visited their
facility to watch the whole process, and they are quite good at it.

You're probably thinking, as I did when it was first described to me, that the
tolerances couldn't be very good with all those processes going on, but they do
an amazing job.  When I was doing parts with them, they were quoting +/- 10 
mils anywhere on the part with an extra 5 mils across any parting line.  I was
making small parts, on the order of 4" max. dimension.  Shrink was very 
uniform and consistent from part to part, showing slight directionality.  On
my parts they met their quoted tolerances right on the money, and I seem to 
remember a turnaround time of about one week for the first parts.  We had them
make 200 total.

Hope this helps you in your search.

Rob Connelly
Motorola
Paging Products Group
Boynton Beach FL