I am casting large silver cups - 16-24 each. overall about 3" high,
not including sprues and such.
using vaccuum assist into investment molding.
large gates from bottom 1/3 to top 1/3 of model, vented to mold
surface
high heat in mold, center is bright pink before casting
sprues to outside of cup, and also center
wall thickness is about 3 mm
silver melted in melting furnace, to about 1,875 deg F. recycled
scrap metal did not work, last pour used 100% new metal, this worked
best
So, here is what is not working, and I need to appeal to the
collective 10,000 years of casting experience of the group
a) many problems with air bubbles, the huge surface area of the
outside of the cup seems to collect the bubbles during vacuuming,
without lettign them top surface. I have tried different pour
techniques, mixing by eye, using distilled water
b) center 'core" of investment seems to be cracking, leading to some
interior defects
c) Would “shell” casting be an alternative than straight investment
casting? what would the trade-offs be?
thank you in advance
Mark Zirinsky, Denver, Colorado USA
"private cutter buying rouigh and collections"
or, as the case may be, “at it 35 years and I am still learning”
Mark, One thing I would try is lowering your flask temperature. With
castings that large and thick as the ones you describe you should be
able to cast at about 900F-1000F this will make for better castings.
You ideally want as cool a flask as will fill. You may need to do
some experiments to verify what temp is best. A lower flask
temperature will produce better surface finish and less porosity.
For your bubbles you may want to try hot water investing. This has
been used by casters at similar altitudes to Denver with some
success. Check out http://www.preciousmetalswest.com/ they have a
description of the process on their website. There are some folks
who don’t like this technique but it is one solution to bubble
problems at high altitude casting shops.
Jim Binnion James Binnion Metal Arts Phone (360) 756-6550 Toll Free
(877) 408 7287 Fax (360) 756-2160 http://www.mokume-gane.com @James_Binnion Member of the Better Business Bureau
Hi Mark, I cast large items fairly frequently . When I cast an item
like a Goblet, I usually use 1 sprue on the bottom of the stem…
Flask temp due to the length of the item around 1000 oF as a
starting temp and you can go up or down from that point.
I would use Deox Silver from United precious metals as you would
probably get the best results with no firescale.
I Torch melt my metal using either propane / Oxygen or Hydrogen
/Oxygen depending on the situation at hand.
As far as Investment is concerned, I would recommend using R&R Fiber
reinforced Investment ( available from Gesswein)
This material will give you extra strength for larger items and will
probably eliminate the cracking on the inside of the cup and may
solve a few other problems. I use this material when casting
customers hand carved waxes and all Large items .Have not had a
problem casting carved waxes or large items since.
Use a wax debbublizer made for vacuum investing… I get mine from
The Contenti co. 800-343-3364 . Othesr may also carry it. It is Pink
in color and has an alcohol base so it dries quickly. Use A large
tupperware container with a locking lid to keep the solution in and
also to dip large items like this. Even though the Investment
manufacturers claim that these debublizers are not neccessary… it
does help. Make sure the debbublizer is completely dry on the inside
of the cup before investing or it may cause investment breakdown in
that area.
These are some tips that may help… they tend to work for me. Good
luck
Daniel Grandi Racecar jewelry Co. Inc. we do casting,
Finishing, engraving , soldering and a host of other operations for
people in the jewelry field. Contact : sales@racecarjewelry.com
or Tel: 401-461-7803
Mark, I sometimes vacuum cast very large objects that might be
similar to your cups. I melt my silver in an electro melt furnace to
about 1860 degrees and use a 60 new to 40 scrap ratio. My mold
temperature is set to 860 degrees. Like you I usually sprue from
the top edge and center of the vessel. I like to have the metal
flow down against the design in the mold surface rather then be
lifted up against the design surface. I do not run gates to the
sprue end. Here in Phoenix I can pull a healthy vacuum.
I use R & R investment which I believe it is slightly stronger than
other products.
From you post I assume the bubbles are caused in the investment step
not the casting step. You might try one of the debubblizers on the
wax to eliminate the bubbles formed during the investment step. I
vacuum the mix for about 1 minute after the investment pancakes and
about 1 minute after the first bubble appears in the investment in
the flask.
You chance getting porosity in your castings if you mold temperature
is too hot. Jim said it right, “You ideally want as cool a flask as
will fill.”
Mark, you might want a copy of my anti-fire scale vacuum casting
procedure. The process eliminates most fire scale formed during the
cooling process. Its a very simple process and the paper is free.
Send me your snail mail address if interested.
the problem with air bubbles that tend to adhere to the wax surface
can be resolved by first “annealing” the wax by putting it for a few
hours in slightly warm water. Afterwards brush a light soapwater
onto it, there exist also special (expensive) liquids for this
purpose. As for the investment: do not mix with distilled water,
because some brands do not react as they should. However, some tap
water sources are sometimes not suitable too, but if the investment
gets hard in a normal way, there should not be a problem.
In my experience, vacuum casting is not my choice for very large
pieces. I have much better results by casting as the ancients did:
just by gravity. You have to have sufficient vents at the right
places, but this works just perfectly. The temperature of the mold
should be as low as possible. For casting pieces this large, I bake
the molds as usual first to a 720 C, but let them cool down to 300
C. This can take many hours before the inside of the mold also
reaches this temperature. The temperature of the silver can stay
normal, lets say about 50-100 C above melting point. Heating too
high causes to much attraction of oxygen and since you have a large
amount of silver, it stays quite long on this temperature, so your
casting can be carried out relatively calm. Just before casting,
putting in a small piece of zinc helps to eliminate excessive oxygen
from the cast. Melting below argon gas is also an option, but
perhaps not available.
Cracking of the core is the most difficult problem but can be
resolved by making a stainless steel reinforcement structure in the
core, or by putting fine silver (alloyed silver oxidizes because of
the copper content) through the wax model en thus holding the core.
I woudl recommend at least 1,5 mm diameter for each pin and would
put at least 10 of them in place to hold. A different solution can be
of casting upside down, the core stands on itself and you will have
much less problems of falling down pieces from cracking cores.
Also, the warming up of the molds is very important. This has to be
done as slowly as possible, certainly up to 500B0C (higher can go
quicker) and without opening the furnace now and then to check
things. These are often facts which cause cracking.
Succes.
Patrick Storme
Conservation Department of Metals
Royal Academy of Fine Arts, Antwerp
Belgium