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Ideal vacuum casting temperature


#1

Hello,

I’m just about to try my fist casting on my new equipment. What is
the best recommended casting temperature for the flask for sterling
and gold ? I was looking for this over the weekend and could see
that the burn out part was similar in everything I was finding but
the casting temp was between 480 C and 970 C. Does anyone what to
share their experience on various size flask and regular to fine
detail pieces? I will be using Americast Investment by Ransom &
Randolph with has the following burn out recommendation:

6 HR. Cycle 2 122" x 2 122" Flask

2 HR. @ 149 C
1 HR. @ 371 C
2 HR. @ 732 C
1 Hr. Heat Soak At Mold Casting Temperature

8 HR. Cycle 3 1/2" x 4" Flasks

2 HR. @ 149 C
2 HR. @ 371 C
3 HR. @ 732 C
1 Hr. Heat Soak At Mold Casting Temperature

12 HR. Cycle 4" x 8" Flasks

2 HR. @ 149 C
2 HR. @ 315 C
2 HR. @ 482 C
4 HR. @ 732 C
2 Hr. Heat Soak At Mold Casting Temperature

Also if you have some input for fine silver and brass it would be
appreciated.

Thanks
CHristian


#2

Hi,

I tend to use small flasks for my casting so use a slighly different
burn out cycle and soak my flasks at 590 deg C. The reason for this
temp is hot enough not to chill the metal when casting but is a few
degrees below the cristobalite squeeze temperature. This means that
I can cool the flasks back down to room temp without worrying about
my investment degrading. Above this temp and you change the
investment properties by making “dead burnt” CaSO4 and the silica is
altered which changes the volume of the investment slightly. Not a
problem if you are going to cast right away but can be problemmatical
if you are using large volume flasks as the heating and cooling and
heating again can cause cracking if you dont use them at the time. AS
a rule of thumb a flask temp of 200 deg C cooler than the metal MP
and a superheat of the metal of 50-100 deg c above melting point (if
using a temp controller) works with about any metal.

Nick Royall


#3

Hi Christian,

I bring my flasks down to 450C for casting sterling silver. When I
have cast gold in the past we cast at around 500 but that was with
40mm flasks in a centrifugal caster. For vacuum I use 85mm perforated
flasks so they have a reasonable thermal mass to keep sufficiently
hot while I do the pour. They certainly take ages to cool down s I
can quench them after the pour. I actually expect that the core
temperature of the first flask I pour might be somewhat higher than
the kiln temperature because of this thermal mass and by the time the
kiln has got down to 450 I tend to be in a bit of a hurry and fire up
the crucible furnace. Mind you, my propane fired crucible furnace is
a bit slow on the first melt so maybe things balance out.

I would be a bit cautious about your soak temperature of 732.
Although some investment specifications suggest this as the maximum
soak temperature (mine specifies 735) I think it is a little too
close for comfort to the breakdown point of the Gypsum particularly
if the calibration of thermocouple or controller is a little out or
the kiln is a little hotter where the flasks are. I now usually keep
my maximum soak to 650 and maybe 700 if I’m casting from a pattern
made from something other than all wax (I work with a lot of natural
organics). 650 seems to work perfectly fine to give a clean burnout
with wax and has the added advantage of substantially reducing the
energy consumption of the kiln and thus reducing its contribution to
GHG emissions and your power bill.

All the best
Jenny


#4

Final casting temp can not be over 900F… The specification for the
rubber gasket is 900f… So it must be keep at about 850F or you burn
rings in your gasket…

Jim.


#5
Final casting temp can not be over 900F... The specification for
the rubber gasket is 900f.. So it must be keep at about 850F or you
burn rings in your gasket.... 

So? We cast at temps from 850 to 1200, sometimes even higher,
depending on the metal and the nature of the models. Yes, it burns
rings into the rubber, but it’s not that bad in any single use, and
this doesn’t seem to detract from the rubber’s ability to cast more
flasks, especially if you set the next flask in roughly the same
position. And you’ve got two good sides of the gasket to use up. We
replace the rubber gaskets when they get too trashed, but each one
generally lasts us perhaps six months. That’s a lot of flasks, and
the gaskets we’re now using (red rubber) were found at a closeout
sale at Contenti.com, for under 3 dollars each. Given the current
cost of metal, the gasket fades to non-importance so long as it
works. So choose your casting temperatures according to the needs of
the metal and casting, to maximize casting quality. If that shortens
the life of the gasket, well, that’s simply not as important

Peter


#6

I don’t remember where I learned this (it might well have been on
this site 2 years ago), but I place a wet paper towel between the
rubber gasket and my casting flask. I make sure to tear a hole in it
to match up the air hole. It protects the rubber like a charm.


#7

Hi Peter,

A trick that I used to use on a plate vac caster (not a pit vac) was
to put a pad made up of 5-6 sheets of newspaper, soaked in water, on
top of the silicone pad. Tear out a bit in the middle over the
vacuum port, and away you went. Would give a nice seal, even on
not-entirely- perfect vac flasks, and it saved a lot of wear & tear
on the silicone pad. Never caught fire either. (Got pretty crispy on
occasion, but no fires.)

Probably wouldn’t be that hard to do something similar with a pit
vac, just more of a pain to get the hole right. Probably would want
to rig up some sort of disk punch for that.

For whatever that’s worth,
Brian