CENTRIFUGAL VS. VACCUM CASTING
Dave Stephens
casting rig and wonder among the
inexpensive equipment available for
making one of a kind small pieces what
is preferred among the pros here,
centrifugal or vacuum casting. I met a
jeweler at a show who swore vacuum
casting had less screw ups and held
better detail. Inquiring minds want to
know!!
Fred Sias
says...
I also will be interested in hearing what the various experts out
there have to say on this subjects. However, I will put my own
prejudices into the discussion.
Both Centrifugal and Vacuum casting are methods of forcing metal
into a mould and expelling the gas that might be trapped in
the flask. One method uses centrifugal force and the other
uses atmospheric pressure. Any trapped gas leaves by way of the
pores in the investment. You can also help matters by adding
extra passageways to reduce the resistance to gas leaving the
mould.
Both methods produce good castings and both have their advocates.
Perhaps a different viewpoint would be to look at the cost of
equipping a shop for either approach:
I find vibrators unreliable for eliminating bubbles in the
investment that result in lumps on the casting. For this
reason alone I would argue that you need a vacuum pump. $500 up
including a table-top casting arrangement.
Next you need a way to melt your metal. If you already have
an oxy-acetylene torch you are all set and you might go to
one of the spring driven centrifugal casting machines.....$300
If you don't have a torch setup...........................$400
then you might consider an electro-melt melting pot. $400
If you buy the melting pot then all you need is a burn-out
oven and you are fixed to do vacuum casting. $400
Main high dollar items for vacuum casting 500+400+400= $1300
Main high dollar items for cent. casting 500+300+400+400=$1600
You might find slightly better prices somewhere and if you
buy used stuff the equation can change considerably.
The electro-melt pot has a graphite melting pot that allows
you to melt your metal in a protective CO2 atmosphere.
Temperature indicator and automatic controls make casting
at a precise metal temperature very easy. Torch casting is
more guesswork although experience can produce good results.
I have heard it argued that centrifugal casting produces
denser castings but I fail to see why if one is careful
to eliminate causes of porosity.
Very expensive production casting equipment is available
for both centrifugal and vacuum casting so there must be
people devoted to both approaches.
Of course, it is possible to cast using home-made
equipment. That also changes the equation.
Finally, it is possible to take ideas from ancient
lost-wax craftsmen and eliminate almost all the
high-price items. Paint on the first layer of investment
to eliminate bubbles, burn out in your charcoal grill, and
melt the metal with charcoal and bellows or a torch.
Let's see what some of the folks with years of experience
out there have to say.
Fred Sias
Andrew Werby
writes:
[If you want to melt in a CO2 atmosphere, you have to get a bottle of
gas and a regulator- add $100,at least]
>Temperature indicator and automatic controls make casting
>at a precise metal temperature very easy. Torch casting is
>more guesswork although experience can produce good results.
>
>I have heard it argued that centrifugal casting produces
>denser castings but I fail to see why if one is careful
>to eliminate causes of porosity.
>
[Straight vacuum (through the bottom of the flask) has an unfortunate
directional quality - downpointing cavities fill well, but horizontal
flow is not so good. More advanced vacuum systems use "perforated
tophat" flasks, which hang into a vacuum chamber and suck air from the
entire periphery of the flask, yielding better results.]
>Very expensive production casting equipment is available
>for both centrifugal and vacuum casting so there must be
>people devoted to both approaches.
>
>Of course, it is possible to cast using home-made
>equipment. That also changes the equation.
>
>Finally, it is possible to take ideas from ancient
>lost-wax craftsmen and eliminate almost all the
>high-price items. Paint on the first layer of investment
>to eliminate bubbles, burn out in your charcoal grill, and
>melt the metal with charcoal and bellows or a torch.
>
>Let's see what some of the folks with years of experience
>out there have to say.
>
>Fred Sias
>
>
[Yeah, well the paint-on method works as long as you use only one batch
of plaster for the painting of the pattern and the filling of the
flask. The melting with charcoal and bellows will tend to blow sparks
everywhere- the metal will melt or it won't. But don't delude yourself
that you can burn out a flask on your barbeque grill, or in your home
oven either; this won't work and it can be dangerous. The wax residue
must be totally eliminated, or not only will your casting be worthless
but the hot metal will spit back in your face. This requires an
internal temperature of at least 950 degrees F for at least an hour,
which a charcoal grill is not going to provide. Perhaps you could
improvise a charcoal-fired kiln, though - even the ancients knew about
kilns, as the development of ceramics technology predates that of
metals-casting by several thousand years]
Andrew Werby - United Artworks
adam fox
its less complecated.
Steve Ramsdell
It was easier to teach with centrifuge than vacuum. The reason
was the students had more trouble with handling and pouring the
liquid metal. The only sticking part with the centrifical was
pulling the arm back, letting the pin fall, then letting go.
Once you get used to either system, you'll be pleased with the
new stuff you can do. Start out simple with store bought molds.
It's rough to build and sprue a model, then lose it in casting.
Steve Ramsdell
Donald Dietz
casting as well. Vaccum give most professional results if used
correctly. It is also the most expensive and, unless you are going to
do many copies of very intricate models, is not worth the expense in
a small shop (between $1200 to $1800 new). Centrifugal will give good
dense castings if used correctly and is decidedly less complex.(Cost
between $300 to $500 new). Check the Rio Grande tools catalogue or
Bourget Bros. If you are only going to do a piece now and then, check
out publications on steam casting. You can set up for less than $100
and the results can be excellent. The Lapidary Journal has had a
number of outstanding articles on steam casting over the years,
probably the best by John Planey of the Patuxent Lapidary Guild in
Maryland. Don't forget the old cuttle fish method either - it has
been used for a very long time with considerable success. Before
settling on one method, check them all out.
Charlie O Fredregill
with a grate set in bottom. Put about 1 - 2 inches of water in bottom,
place flasks open end down, above water level, cover, & bring water to
boil. takes about 30 minutes to eliminate about 90 -95 % of wax.
That reduces wax burn-out
fumes - smoke - residue to a very low level. Flasks can be steamed out
within about 20 - to 30 minutes of investing with no problems. Don't use
pot for food after doing this.
Charlie
Edresden
the vacuum method works extremely well as long as the models don't have
very thin sections to them such as are found in filagree pieces. These do
better with the greater forces applied to the molten metal that a
centrifugal machine provides.
With both methods, one of the most important factors is being sure that
you do an adequate job of sprueing the model so that the metal has easy
access to all parts of it, especially the thinner parts. Inadequate
sprueing is one of the greatest causes of incomplete castings and nothing
is more agravating than spending many hours creating a fine wax model and
having the casting come out incomplete.
Hope this helps a bit Dave.
Earl
edre...@aol.com
Bruce Holmgrain
>[Straight vacuum (through the bottom of the flask) has an unfortunate
>directional quality - downpointing cavities fill well, but horizontal
>flow is not so good. More advanced vacuum systems use "perforated
>tophat" flasks, which hang into a vacuum chamber and suck air from the
>entire periphery of the flask, yielding better results.]
>
I'd like to suggest that wax wires can be employed to ventilate the
side of the "straight vacuum" flasks. This will eliminate virtually all
horizontal flow problems. There are companies selling "wax webbing" to further
allow the ventilation of straight flasks.
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JJerie
especialy under an inert enviorment.Most vacume casting machines can also
but modified to produce grain which provides cleaner grain and more
acurate melting temps but for small difficult to fill parts ie.filigree
the centrifugal casting machine can sometimes prove more
efficient.Centrifugal casting machines are also available in vacume
models.
good luck
N.Y.C. CASTER
Goldsmith1
do use my vacuume caster for debubling my molds.
I have found that vacuum chamber casting works better than vacuum assist.
Allthough I have had good luck with all, vacuum assist has a tendancy with
fine detailed patterns to be more likely incomplete. To me centrifical is
much less hassel as far as flask temp, you don't have to worry about
gaskets making a good vacuum. But with what ever method you decide on it
would be nice to have three hands "grin".
I don't think it would be a good idea to burnout on a charcoal grill????
unless maybe if you are casting lead!
Mike Cirelli
Cirelli's Fine Jewelry Inc
Mfg of Fine Jewelry
Chunor
enough.If you need more heat than that look for "chemolene"it's also clean
burning and burns at 5900 with O2(oxy acetelene burns at 5950).Centrifigal
is the easiest to learn on and works in a broader range of casting
situations.I'm going to let you in on a little known secret."Read the
manufacturers reccommendations on the casting equipment and the
investment."
PeterWRowe
<you...@eng.clemson.edu> writes:
>
>Have you ever tried lost-wax burnout with charcoal?
>
>I have been trying to develop methods to teach lost-wax casting
>that require a minimum of equipment. The burn-out oven has
>always been a stumbling block. However, the lost-wax method has
>been around since 4-6000 B.C. All ancient metal workers used
>charcoal.
You might also be interested in a brief account of Ashante techniques.
The nigerian/ivory coast regions of Africa have long and wonderful
traditions of goldsmithing and lost wax casting. As a graduate student at
Tyler (Philadelphia) in 1987, the studio was honored by a visit from the
esteemed swiss teacher and metalsmith, Max Froelich, who was continuing on
to (I think) Penland to teach a class in these techniques, which he'd
spent some years researching in Africa. The techniques, as he
demonstrated them, are not only about as low tech and primative as you can
get, but also seemed to be capable of extraordiary detail.
Models are made primarily from pure beeswax, which can be locally obtained
at low cost (a few bee stings...). Many involve very fine wax wires,
coiled or wound into intricate designs, beads, spools, and the like.
Solid forms are usually made of thin sheet wax formed over a core of the
investment material, and then usually embellished with more wire and
grains.
The investment material used is of two types. First a mix of a fine local
clay, along with charcoal powder. Max used instead commercial clay mixes
from ceramic supply houses. Sorry, but I don't have the exact recipe
handy. This is the fine grained mix used for cores, and as the first
layer applied around the models. Additional layers are a similar
material, but with the addition of dried straw or grass, which serves to
make the mix rather more porous as it burns out.
Models are completely covered, except for a sprue which leads to a large
funnel shaped area corresponding to what our techniques would call a
button, after casting. After drying, these molds are burned out over
relatively low heat, over a charcoal fire. After being allowed to cool,
the required metal for casting is placed in the aformentioned bowl, and
additional clay mix is build up around it to completely enclose the
melting chamber, with the metal inside. The result is rather like a
dumbbell, with the metal inclosed in the crucible on one end and the mold
on the other. This is now placed crucible down in a charcoal furnace or
fire, designed to be hot enough to melt the metal in that enclosed space.
The carbon content of the clay, the reducing atmosphere of the fire
itself, and the enclosed melting chamber all contribute to clean oxide
free melts. With brass casting, the metal can be judged as being the
right molten temperature for casting by emitted fumes coloring the flames
in the furnace. To cast, one simply invertes the mold/crucible
arangement, allowing the metal to gravity cast into the mold. As the mold
is only a little cooler than the crucible, this allows plenty of time for
the gold or brass to flow, and it does, with surprisingly high fidelity.
Its broken out after cooling.
The furnace Max used was a large steel pail, lined with a dried/fired
layer of clay or firebrick, with a simple blower made from a hair dryer
type blower, substituting for hours with a bellows... It was fired with
simple charcoal, and was quite sufficient to melt gold or bronze, not to
mention burning out the wax.
The only real equipment required was the furnace, which was homebuilt, and
even with purchased parts, cost only about $25 or so. A good scrounger
could do it for just about free. And the clay components of the mold
material are a lot cheaper than investment powder...
(See my next post for another thought on cheap casting)
Peter Rowe
PeterWRowe
>
>Have you ever tried lost-wax burnout with charcoal?
>
>I have been trying to develop methods to teach lost-wax casting
>that require a minimum of equipment. The burn-out oven has
>always been a stumbling block. However, the lost-wax method has
>been around since 4-6000 B.C. All ancient metal workers used
>charcoal.
At one time I had a small booklet, which I've seen around, so it's still
in print, which talked about homemade lost wax casting equipment. One
item in there is plans for a simple burnout kiln. Mostly it's simple
sheet metal work, some firebrick, heating elements (can be obtained from
ceramics supply houses), and some sort of controller. Back in 1974 I
built one of those for under $25. Now I suspect it might cost rather
more, but even so, it shouldn't run much over $50. I think the authors
name was Ed. Soukoup or something (Don't shoot me if I'm wrong...).
But anyway, I also burned out a number of flasks in those beginning days,
just over my gas stove's burner. I used a clay flower pot (one of those
red terra cotta ones- they're real cheap) just slightly larger than the
flask, lined with several layers of furnace tape (It was asbestos then,
but the current products will work just as well), and then a double layer
of aluminum foil (which had to be replaced each time). Fortunatly, the
stone was equipped with an exhaust hood, or I'd have had the apartment
house wondering what I was burning... While a little slower than a
commercial kiln, and I'm not sure I'd want to try a large flask, my little
2.5" x 3" flasks worked just fine this way. I'd set them up at night with
the burner on high, go to bed, and get up a hour earlier than usual, so as
to allow me time to turn off the burner, start breakfast, cast the flask,
shower, quench the flask, get dressed and ready for work, and be able to
have a cleaned flask and casting ready for cleanup by the time it was time
to go on to work at my "day" job...
I had tried steam casting, and rejected it, as the fancy hoops you have to
go through to use tiny multiple sprues so as to be able to melt the metal
over the holes without if flowing before you wanted it to, just seemed
like too much work. (Just like that last sentence was way too long...
(grinn)).
So I'd built a homemade broken arm type centrifuge, driven by an old
surplus motor. I won't repeat the story of that machine's maiden
voyage... But after some redesign, it worked quite well for some time...
And cost me very little to build.
Peter Rowe
Charles Hofmeister
>At one time I had a small booklet, which I've seen around, so it's
still
>in print, which talked about homemade lost wax casting equipment.
I have this somewhere.
>One item in there is plans for a simple burnout kiln. Mostly it's
simple
>sheet metal work, some firebrick, heating elements (can be obtained
from
>ceramics supply houses), and some sort of controller.
I built one but never got it to work. I guess I used the wrong
kind of firebrick. I wanted the light, porous stuff that I use as a
solder block, but all I found was hard, heavy fireplace firebrick.
PLEASE, PLEASE, PLEASE help me find the right stuff. I tried several
catalogs, but no luck. Please, email me any info. I think I need
about 20 bricks.
Do I sound desperate enough. ;)
Fred Sias
says...
>
> I built one but never got it to work. I guess I used the wrong
>kind of firebrick. I wanted the light, porous stuff that I use as a
>solder block, but all I found was hard, heavy fireplace firebrick.
>PLEASE, PLEASE, PLEASE help me find the right stuff. I tried several
>catalogs, but no luck. Please, email me any info. I think I need
>about 20 bricks.
>
> Do I sound desperate enough. ;)
>
Have you tried your local ceramics supply house? The
pottery folks use fire brick for making kilns.
Of course, this assumes you live near a reasonably
large city.
What you want is "insulating" fire brick. It's soft
enough to cut with a knife or saw.
I haven't bought any of the soft brick myself since
a friend of mine buys it in quantity.
Fred Sias
Clemson,SC
Steve Ramsdell
Some heating and airconditioning places carry fire brick. Be
careful of the price.
Steve Ramsdell
John Kelly
last...@ix.netcom.com (Charles Hofmeister ) wrote:
> I built one but never got it to work. I guess I used the wrong
>kind of firebrick. I wanted the light, porous stuff that I use as a
>solder block, but all I found was hard, heavy fireplace firebrick.
>PLEASE, PLEASE, PLEASE help me find the right stuff. I tried several
>catalogs, but no luck. Please, email me any info. I think I need
>about 20 bricks.
>
> Do I sound desperate enough. ;)
>
What you need is insulating firebrick such as that which is available from
a number of sources. Look up refractory in the phone book and there will be
places that list fire brick which will have or can order them. Not too
expensive, a couple of dollars each unless you are looking for something
exotic, such as for very high temp.
You used hard fire brick which absorbed all the heat your elements gave out.
Good luck.