This forum is great for sharing I have learned more
from the folks on Orchid than I have learned from any book. To
all of you who contribute to this list, I am grateful. To Dr
Aspler and Hanuman, Thank You! Hopefully one day the jewelry
business will not be as secretive as it is today.
With that said, I would like to share some of my past
experiences when I had a failure or a bad casting, and what I did
to correct it. Don’t get me wrong, my castings are still not
perfect, and occasionally I still have a failure. But I have
learned; by using a process of elimination; how to correct many
of my own mistakes and bad habits. I don’t pretend to know it all
about anything when it comes to jewelry. I have been in this
business for over 16 years now and there is not a day that goes
by when I don’t learn something new.
No one method is right or wrong when carrying out a specific
procedure for casting, investing or anything else for that
matter. If it works for you, then don’t change it.
When I was taught this business by my father, we were using a
centrifugal casting setup almost exclusively. We were having
major problems with shrinkage porosity in the heavier castings.
It was getting to be a major pain spending hours filling and
burnishing those pesky little pits so I decided we needed to find
out what we were doing wrong. So, I started from scratch and went
back to the basics.
The first mistake was my sprueing methods. I would always sprue
up on the bottom of the ring or just use the injected model from
the rubber mold that was sprued up at the bottom.
Well, it seems that gold doesn’t like to flow from a thin area
to a thick area without solidifying unevenly. When gold
solidifies, it contracts. If the model is sprued incorrectly, the
thicker parts of the model cant properly draw metal from the
sprue and tend to draw metal from itself. This was what was
happening. If the thin areas solidify first, the thicker parts
have nothing to draw from, except for the thinner parts of the
model itself; which causes shrinkage porosity. Now, I always
sprue from the thickest area of the model and even use resivuoir
sprues on very heavy items. Sometimes I will even attach feeder
sprues on the opposite end of the pattern so the metal can draw
from it too. This seemed to eliminate about 95% of the porosity
problems I was having.
The next problem I had was only happening with carving waxes
(especially the green wax). I would often times get cancer like
nodules on my castings.Sometimes I would have little bits of
investment trapped in the casting itself (especially on corners
or sharp bends). It appeared the investment was having little
"explosions" inside the mold cavity. I figured this was either
caused by an improper burnout cycle or improper investment/water
ratio.
So, I started with the investing procedure. I was using Satin
Cast 20 (and often still do) investment and was not being real
precise on the weighing. I was using an old postal scale that
only had a readability of 1/4 lb. increments. I did have a
graduated cylinder in which I measured the water so I would go
about mixing up what I thought was a 100/40 investment/water
mixture and go through the usual vacuuming process. Timing? what’s
that? Gloss off? Never heard of it… No, I never timed this and
if the investment looked to thick or too thin, I would add a
little water or investment 'till it “looked” right.
Folks, don’t try this! Follow the manufacturers instructions
that comes with the investment and please use a good respirator
when mixing this stuff.
Well, I wised up and got a good gram scale to weigh the
investment and started timing the mixing process. Since then I
have had no problems with the “micro explosion” cancer looking
things in the mold cavity…Don’t get any water marks or bubbles
in my castings anymore either. But, I was still getting
investment breakage on sharp corners with the carving waxes. So I
turned to the burnout cycle.
I was using a 5 hour burnout starting with a preheated oven at
300deg F, ramping to 1350 deg then back to casting temp
(variable).
Carving wax expands more than injection wax, so I thought I was
getting investment breakage from the wax expanding before it
reached melting temp, so I changed the burnout cycle to start
with a cold oven, then I ramp to 200 deg for 1 hour and then to
300etc…Seems this helped a whole lot on the carving waxes. I
guess it softens up the wax a bit before it melts and the
expansion doesn’t affect the investment near as bad. But on the
real heavy pieces, I was still getting investment breakage on the
corners. The only other thing I guessed it could be was the
centrifugal caster slinging the gold into the mold with such
force and turbulence that it was breaking bits of investment off
on the corners. I hear there is a fiber reinforced investment out
there that is formulated specifically for casting heavy items,
but I haven’t tried it.
Skip Meister gave me some great advise on dental investments. I
ordered some called Fastfire 15 and now I can invest, burnout and
cast in less than 1 1/2 hours. This stuff is extremely hard and
has to be hammered out of the flask because it doesn’t dissolve
when its quenched, but what a time-saver! The invested flask goes
straight into a 1600deg oven for 30 mins and then dropped down to
cast temp and cast. The only problem with this stuff is that it
swells three times as much as regular investment under vacuum
(during the investing process) so you have to use REAL tall flask
extenders or pull a vac till it swells to the top of the flask
extender and release, then repeat the process untill it doesn’t
rise anymore and all the air is evacuated. Or I guess you could
buy one of those fancy vac/mixers that dental techs use. This
investment is also great for casting platinum because it is
phosphate bonded and has a compressive strength of 500 psi (hard
as concrete). Also, you can vary the amount of shrinkage or
expansion by varying the amount of binding liquid/water ratio you
use. Great stuff!
One of the last things I did was switch over to vacuum
casting…All my final problems disappeared. No more investment
breakage.
Now I use vacuum almost exclusively except for the occasional
filigree item or platinum piece (which I am just learning to do).
I do quite a bit of casting with stones in place and only use
vacuum for this.
I tried centrifugal casting once and the force from the metal
entering the mold caused some of the stones to break loose in the
investment and get lodged in a place where they weren’t supposed
to be. A real nightmare! Someone told me that you can get a denser
casting by using vacuum, but I don’t know this for a fact. I’m
not a metallurgist, but I was told that when the gold cools and
crystallizes, the slower cooling process from vacuum casting
allows for a better or tighter grain structure; giving one a
denser casting…Maybe Peter Rowe or John Burgess could clarify
this.
I am not a volume caster or a teacher. I am just a little guy
with a little retail store. If anyone has other suggestions that
could help my casting process, please let me know. I try to put
out the best product I can for my customers. I can’t compete with
the big boys when it comes to price on mass produced jewelry, but
I can give great customer service and give my customers a better
value for their dollar. As I stated earlier, I am still in the
learning phase of this business and will eagerly accept any tips
or suggestions from anyone else. Sincerely, Ken Sanders