I have experienced casting defects and don’t know the reason.
Using high grade investment purchased in 2003 and stored in
carefully fastened plastic bag in a plastic pail bucket, the sterling
belt buckle casting was defective.
The casting exhibits a rough texture on surfaces that should have
been flat and smooth.
The molded investment surface seemingly broke down and the resultant
casting surface is rough over 50% of the total surface.
The 38/100 mixing ratios were carefully followed measured to the
gram. The water was pure rain water. The temperature was a little
cool with investment setting time ~ 15 minutes in a 65 F room.
The flask was burned out completely with the opening being white
with no trace of carbon. The flask was ramped to 300 f then 700 then
1350 and cooled to 850 and soaked for half an hour. The total cycle
was 5 hours. The burnout began approximately 16 hours after
investing. The invested flask was stored at approximately 45 F
overnight.
The flask was not dropped or jolted. The sprues were not tightly
bent. The casting is 66% new metal. The flask was cast with a good
solid 30 im on the vacuum gage
Although the soak temperatures are published, the ramp times are
not. Heating too rapidly is a cause of defects. Are there suggested
ramp times to 300, 700, 1350, cooling to 850?
Smaller castings with same investment and similar procedures but
lacking the extensive flat surfaces of a buckle don’t have the noted
problems.
Orchidian thoughts as to cause for the defects would be appreciated.
I have been casting both centrifugal and vacuum casting since 1971.
These are just some immediate thoughts.
Investment has the same storage characteristics as a bag of ready
mix cement. If you buy a bag of cement new right from the store and
open it up you will see a uniform mix of ingredients. Now if you let
the bag sit for a year or two open it up again and look at it. If
will have separated into its different ingredients. These ingredients
have different specific gravities. The heavier ones will settle to
the bottom of the bag with only gravity to blame.
Here is my point. Investment in a barrel does the same thing. As an
apprentice one of my jobs was to put my arm into the barrel of
investment and mix the stuff up from the bottom to the top. Many
times a new barrel will be fine until you reach the half way point.
Then the castings start to turn bad. I would have to guess that the
settled mix does not breathe as well during the casting process.
Vacuum casting does not seem to be as affected. You could FEEL the
difference from the top of the barrel to the bottom. It all looks the
same only the feel is different.
In the dental casting area, they solved this concern by having the
investment provided in smaller bags. It doesn’t separate in smaller
amounts as much.
Now if you combine the shipping of 100 pound barrels of investment
overland by truck, guess what happens? This shaking and vibration
can speed this process up.
I usually buy 100 pound barrels one at a time. The firm I
apprenticed at used to by 20 at a time. Every month they used to flip
the unused investment barrels upside down for storage until they used
them. This helped a bit, but I still had to mix up the investment by
hand.
I had been mentioned in AJM a while ago, because I store my
investment in a cement mixer with a clamp tight cover. I mix my
investment once a week and I have never had an investment related
problem.
My two cents,
Best Regards,
Todd Hawkinson
Instructor, Minneapolis Community & Technical College
I have a box of investment which was stored in a humid basement for
a few months, in theory tightly poly bagged. Your setting time and
surface breakdown sounds all too familiar Well the ‘investment’
is still good for patching walls. [remaining 23 pounds FREE to a good
home for only shipping]
Your 1350 is on the high side, extra stress on the investment. I
stop at 1250. If the buckle has a thick cross section try dropping
the 850, maybe drastically. Flat surfaces are also a problem, if you
make them slightly convex in wax, with the metal shrinkage they will
come out almost flat (better to err with convex metal than concave)
For me 1350 is a little too hot. I tend to hold at 1250 F to remove
carbon. How do you heat your metal? Do you have a temperature
controlled melting method? 850F may be too hot for your casting you
may need to drop it more. A large casting like a belt buckle can
hold so much heat that the investment will begin to break down and
you will get sulfur dioxide gas evolved and will get lots of gas
porosity at the surface that can lead to a rough surface this is made
worse by incomplete carbon removal (see Eddie Bells recent posts). If
this is the problem reducing either the metal temp or flask temp or
both may help as will longer dwell time at 1250F.
Jim
James Binnion @James_Binnion
James Binnion Metal Arts
I have to agree here on both points. The 1350 is too hot 1250 is my
max temp and I would try casting around 600-650 for a heavy belt
buckle. You might also consider placing your sprue so that the metal
is directed toward the back of the buckle if you have not already
done this. This allows the most forceful part of the metal delivery
directed at the back instead of the smooth front. Less turbulance
means a cleaner casting. You might also add a vent sprue to the
opposite end of the casting to allow for gas expulsion. I
always(even when centrifical casting) use vaculiners to assist in gas
expulsion… my thoughts. Hope they help. Frank
Casting is a mystic art. I read all the post pertaining to casting
and always wonder how I get away with casting the things the way I
do. I burn out 1320 degrees, melt my sterling to 1870 degrees in a
100 ounce electro melt furnace and pour into molds at 860 degrees. I
find if I lower the mold temp or metal melt temp my castings loose
detail. I use scrap to new silver at a ration of 40 to 60.
It is difficult to determine what the problems are. My best guess is
that it might be one or more of the following:
Old investment. It will settle and absorb moisture. This will
cause the investment to break down and produce growth on the casting.
Do not use very old investment and stir it up before using.
Improper mixture of water to investment. If the investment is too
thin you will get thin parting lines that run up the casting. I
believe this is due to bubbles that travel up the wax to the surface
leaving parting lines. I know this because when I first started
casting I mixed investment without weighing and measuring. When I
got the thin parting lines I figured the investment needed to be
thinner to allow closing of the bubble paths. Wrong. Things got
worse.
Not mixing the investment long enough. There may be small clumps
of incompletely mixed invest that if they land on the wax will fail
during casting causing depressions in the casting. I pour my
investment into the flask through a kitchen strainer after mixing it
with an electric beater for 1.5 minutes. I always have small clumps
trapped in the strainer.
The combination of the metal and mold temperature too hot or too
cold. I started using scrap silver from a punching operation in my
electro melt furnace. I would let the silver melt then add more
silver until the crucible was full, then pour. Things went well until
is started using casting grain. I would pour as soon as the metal was
molten. The metal would chill as it passed over the lip of the
crucible leaving a thin tail of silver hanging from the crucible and
the casting missed detail. My conclusion was that the delay time of
heating scrap allowed the crucible ring to heat up much more that it
did when melting casting grain. The problem want away when I allowed
the furnace to sit at melt temp for several minutes before pouring.
Improper burn out cycle. Follow the investment manufactures
instruction. There are many errors in burn out that can cause
failure.
Quenching the casting too soon. You will get blow outs of metal.
Improper spruing. I sprue with about three no 8 gauge wax wires
welded together then welded to the center of the buckle at one end. I
then add a 8 gauge sprue to both corners. I then add sprues to the
center outside edge of the buckle. I add another 8 gauge sprue to the
center of the buckle near the end opposite the main sprue. The pour
is parallel to the buckle. If there is a thick spot on the buckle I
will add a sprue to the back side of the thick section. It takes a
lot of time to clean off sprues from the casting but that is nothing
to the time lost if the casting fails due to not enough sprues. I
can send a photo off line to anyone who is interested.
Use of dirty or too much scrap sterling. Its recommended to use a
50 50 ratio. Do not use silver with solder on it. Make sure that old
sprues are completely free form investment.
Incomplete burn out. The more wax in the burn out oven the longer
the temperature must remain at the burn out temperature.
Not vacuuming the flask for long enough time to let air bubbles
to leave the wax. This caused small bubbles of silver to form on the
casting.
Metal flowing directly against a part of the design might cause
the investment to erode.
Too much flux in the melt.
Hope this helps,
Hope to see many of you at the Orchid dinner.
Lee Epperson.
Well the ‘investment’