Man, everytime this subject comes up (centrifugal vs vacuum)
there’s always a different outcome. Last time this thread ran
vacuum won out. It seems more professional casters are using
vacuum from what I read then. I have a Neycraft Spincaster and
have never perfected the process. I think most of you guys are
casting objects of same thickness type things like rings etc.
that have no real thin parts where things join etc. My castings
almost always had subsurface pits and despite alot of excellent
advice was never able to completely eliminate them. I have
recently tried my vacuum table to do vacuum casting and gotten
better results than I ever did with centrifugal.
My suspiscion is that the violent force of the metal being
thrown into the flask breaks bits of investment off, not to
mention turbulent gases from all that sloshing around. I got
better results when I added vents to the outside of the flask,
but still I think centrifugal might work better for thick
roundish types of things like rings. My most perfect cast was a
large roundish curvy simple ring with good venting. Anyway, I
plan on continuing to work with vacuum for now as it seems more
logical and holds detail as good as my centrifugal casts ever
did. Also, I tried that dentist’s recommendation of fast
burnout, which he’s done for years with success, and it worked
perfectly, I was shocked! On a 2 1/2x 2/12 inch flask burned out
at 400F for an hour and an hour at 1350 then down to 925 for
silver. I set the flask on the silicone pad and then heat my
metal to melting and then hit a footswitch to start the vacuum.
Works great so far…Dave
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Dave,
Yes, centrifugal casting machines are more violent, but with
dental high heat investments it is a moot point! In fact I have
never experienced a problem with the investments meant for low
heat burnout either. I regularly cast gold alloys and
palladium/silver/gold alloys in dental castings to .2 mm
thinness! Always! The pal/sil/gold needs to be thicker or
have multiple sprues.
Are you using that Ransom and Randolph investment? It is an
o.k. investment in pre-weighed packets but in bulk form it
goes bad faster than a hornet can find a Coke can at a picnic!
The packets also go bad quickly but not as fast. It is not wise
to mix techniques unless you really know what you are doing.
If you are going to use the dental burn out schedule than use
dental investment.(You don’t need to cool the flask to 925 F
before casting. Put the hot ring in the cradle before you start
to melt the metal(2 1/2 oz. or less of silver). Melt it then
bring it to casting temp. and let her fly.
Have you ever taken into consideration your melt? It’s really
easy to under heat the metal when using a bulk of several
ounces. If you have metal left in the crucible, you never
reached casting temp. and thin areas won’t cast. On the other
hand a small amount of metal can be overheated and burned and
IN-GASSED producing sub-surface porosity and possibly a brittle
casting.
I have been doing this for so long it is second nature to me.
There are so many variables in any casting process that you need
to actually have control of, or have experienced first hand that
it is difficult if not impossible to diagnose on this forum. If
you or anyone else wishes, E-mail me and I’ll send you my phone
number and you can call me. I’ll be happy to spend as much time
as you need. I work from my home so you can call me at most
times. I’m usually up at 9 am central and go to bed at 1-2 am.
Please don’t call after midnight:).
Regards,
Skip
Skip Meister
NRA Endowment and
Instructor
@Skip_Meister
10/11/9703:33:46
Hi Dave, I suspect your problem w/centrifugal has more to do with
spruing than turbulance. Spruing is 95% of the battle. Until you
really under stand this you will have problems with centrifugal
and vac in varying degrees. There are three types of porosity. 1.
Hot Tear… caused by improper spruing, the most common kind,
solve this first. 2. Gas… caused by overheating the metal,
usually the zinc (or another constituant) gets fried and turns to
vapor and continues in the investment mold ,watch the heat!. 3.
Inclusion… broken investment, too much flux, or other foreign
particles carried through into the mold, if you are careful this
should be the least of your problems. Vac casting can by-pass
some of your problems, but understanding the metal and spruing
will eliminate them, whether you use vac or centrifugal. e-mail
me directly if you like. J.A.