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Platinum Casting Problem


#1

Hello, This morning found me casting platinum . We use a verticle
casting machine , flaskless quicksetting investment [Doc Robinsons]
and a brand new wesgo 10 oz. crucible . This machine was modified
"per" Marc Robinson by addition of a flask cradle and removal of the
spring device that holds crucible to flask . I have used this set up
about 20 times now with a couple of re-do’s [acquiring experience and
technique’s] but mostly with fine results , the equal of some
commercial casters .All in all very pleased , the machine has paid
for its self several times now . Today I used a new crucible ,
melted the metal , moved the crucible/carriage to the flask all looked
good , so I let it “fly” . I had steped away slightly as I hit the
release BANG .I felt a “knock”. I’m sprayed with small droplets of
molten platinum . Taking off my welding helmet I found the flask and
crucible on the Board beside the base of the casting machine the
flask cradle was across the room and little wiff’s of smoke are
everywhere [ my apron , shirt and tie were ruined and I have a
coolset of tiny circular burns , each with a tiny ball of platinum
within ] no big damage but what happened ? There is no obvious
cause . The shield of the machine contained the largest amount of the
spilled platinum [ about 8gr. out of the 30 gr. we were casting] the
shield also shows a significant dent where something [flask/crucible]
hit it , the threaded rod that the counter weight adjusts on is now
bent , the only other clue I see is that the crucible base is broken
along the keyway where it came out of the carriage. Luckly enough of
the metal made it into the flask to get our casting :slight_smile: , so I
believe that we had at least one full revolution of the arm before
disaster.

Has anyone experienced anything similar? Any Ideas ? I’m going to be
setting up a dummy flask and doing some “dry runs” before I do any
more plat casting , but would welcome advice .

Thanks ,
Mark Clodius


#2

I’ve had platinum air-pocket “pops” at the bench-glad
you are OK.


#3

Mark, While I’ve not had that happen, I’m gonna bet the cause was
that your crucible cracked. You say it’s broken at the base, right?
If that happened due to heating shock at first use, or something,
then I’d guess it could come off the machine, and take the flask
with it. While I’ve not had a wesgo crucible come apart and fly off
a machine, i’ve certainly seen them crack. Most of the time when I
retire a crucible it’s because I’m seeing cracks in it so I no longer
trust it NOT to fly apart. usually the cracks are in the main
portion of the thing, but I see no reason why it couldn’t be the
base. I usually “cure” a new crucible off the machine by melting some
platinum in it just with it sitting on a fire brick, to be sure it’s
fully sintered and all that stuff, before actually casting with it.
Some of them seem to not want to survive that first heating…

Peter


#4

Dear Mark, I don’t claim to be a platinum casting expert, but look at
it as a challenging process to master.

I almost had the same thing happen during the last Platinum Seminar
we had here at the Minneapolis Community & Technical College. I
didn’t cast the flask with platinum I used silver. The silver did
spray a bit but not with the fear I would have had with platinum. The
flaskless set up was slightly cracked. We also tried the quickset in
a steel flask because I had seen the cracking before. I switched to a
regular melting crucible and did not use the Wesgo crucible. We did
everything as directed.

My thought? The quickset investment has to be used EXACTLY AS
DIRECTED. Even humidity in the room can be a big problem. Did
anything change in your process or environment? How old was the
investment? Jurgen Maerz THE GREATEST PLATINUM EXPERT ON THE FACE OF
THE EARTH WAS THERE. He said cracking is a problem with that
investment. From my experience all platinum investments are not fool
proof. If I could make any one of them breath more I would. That is
my main complaint, that the air doesn’t exit the flask as well as it
should. A much smoother surface could be obtained with that
happening.

Maybe Jurgen has some comments.

Best Regards,
Todd Hawkinson


#5
    I've had platinum air-pocket "pops" at the bench-glad you are
OK. 

I’ve had these pops quite a few times, but I don’t think that they
have anything to do with “air pockets”.

My experience has seemed to be that they happen when I am pushing
my torch too deep into the work. The inner cone of the flame is
unignited fuel made visible by it’s ignition at the surface of that
cone. My suspicion is that this fuel mixture is disolving into the
metal and later combusting. I’d like to hear a better explanation.

Certainly does provide for a little jolt! 80)


#6
        That is my main complaint, that the air doesn't exit the
flask as well as it should. A much smoother surface could be
obtained with that happening. 

You could try solving this the way sculpture sized castings
sometimes do, with actual vents. Use the finest wax wire you’ve got,
or perhaps even something finer like fine nylon fishing line, for
example, , attached at low points and end points where air will be
trapped, and run these vents back up to the top of the flask, exiting
NEXT to the sprue opening, rather than in it, so metal during casting
doesn’t hit those openings. If it goes as it should, you’ll get the
beginnings of tiny metal wires cast into the form where those vents
attach, which if planned well, will be easy to clip off. that
should fix the back pressure problems you describe…

Peter


#7
If I could make any one of them breath more I would. That is my
main complaint, that the air doesn't exit the flask as well as it
should. 

I have noted that Dpc Robinsons casting elixir tends to crack while
setting up. I have a small operation and my castings are an ounce or
less at a time and I have made breakaway PVC flasks to set up my
waxes in investment. The Doc’s investment tends to crack when it
cannot expand freely while still setting up.

Back to to the investments inability to breath, I sometimes use
sprues that lead not to the buttom but to the upper edge of the flask
adjacent to the button made from 20ga wax wire. Fishing line should
work as well.

Bruce


#8

It sounds like you didn’t quite get the torch all the way out of the
way. With platinum casting you have a very small window when you
pull the torch out of the way and then release the machine. If you
do not pull the torch back quite fast enough the tip of the torch
can sang the edge of the crucible, dislodging it just enough to come
out of the cradle as the velocity increases. You probably won’t feel
this in the heat of battle. If you heat soak the platinum
sufficiently you can spare the split second it takes to get the
torch out of the way.

Spike Cornelius
Portland, Or.
RC ArtMetal