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Casting problem - course grainy surface


#1

I do production casting and generally have no problem casting 3"x7"
flasks, 105 - 400 grams of silver. I do 2 1/2"x4" flasks for gold. I
do 4 to 6 flasks

2-3 times a week, no problem.

Every once in a while I do a small cast, 2 cans, one gold, one
silver. I invest one night, put them in the kiln next morning just
like I usually do.

My last casting , both gold and silver had some pieces with large
warts with really course grainy surface.

Burnout was 11am-8pm, 200 deg, 500, 700 1000 1350, cast at 1000 for
silver, 850 gold.


#2

The large warts you describe are caused by investment flaking off
from the surface of the mold.

You might find those flakes of investment, which are free floating
in the mold before pouring the metal, imbedded some where in you
castings. If you file off the warts you might find the investment
flakes imbedded under the surface of the warts.

If you used the same steps for you bad castings as you used for all
the good ones I would think the problem is in the investment.

Your investment might have been contaminated by moisture in the
air.

The investment to water ratio might not be to spec. It is easier
to mix to spec large amounts of investment that smaller amounts.
The measurement of water to investment becomes more critical when
mixing small amounts of investment. For instance, several ml of
water out of spec will have less impact on say 5 pounds of
investment then it would have on 1 pound of investment.

Small clumps of investment that were not mixed fully might have
moved against the wax in the flask. These clumps are weak because
they are not fully mixed. I pour my investment into the flasks
through a kitchen strainer. I mix with an electric mixer for 2
minutes and yet I still find small clumps of investment not fully
mixed in the strainer.

The flowing metal can hit thin sections of investment and break
away the surface.

Orchid members continually prove that lost wax casting is not a
science. For instance I pour silver into an 850 degree mold and gold
into a 1000 degree mold. Apparently the different casting
temperatures we use works for both of us. Metal temperatures could
compensate for the different mold temperatures.

Lee Epperson


#3

Hi;

Sounds to me like your not getting the investment totally mixed
making voids in the mold that fill and crystallize the gold as they
would act much like a vug in nature.

Ringman


#4

Hello Richard, Lee covered the health and mixture of the investment.
I can think of some other possible problems. Your burnout schedule
looks fine as long as you are holding the 200, 500, and 700 (F) where
damage can be done if hurried through. With too high of a climb rate,
steam can build up enough pressure inside the flask to produce
similar symtoms. Was the flask bench hardened for an hour before the
burnout? 1350 should be the top of the burnout. Investment can break
down with temperatures higher than that causing the same symtoms.
When is the last time you calibrated your kiln? The needle (or LED)
can read “1350” but can actually be much hotter. Another possibility
is air pockets in the wax near the surface that explode and fill with
investment during vacuuming. This occurs most often on one of a kind
waxes that have been carved, added to and recarved. It’s always the
details isn’t it ?!

John, J.A.Henkel Co., Inc., Moldmaking Casting Finishing,
Producing Solutions For Jewelry Artists


#5

Don’t forget to look at your burnout process If you burn out too
fast you can steam the wax in the flask and this can attach the
investment causing breakdown Follow the instructions of the
investment manufacturer based on the size flask you are using

Tino Volpe Metallurgist,
Technical Manager
Tiffany & Co
300 Maple Ridge Drive
Cumberland, RI 02864-8707 401-288-0124
Volpe@Tiffany com


#6

Richard my guess is that your kiln temperature went up too fast at
the low end and you are seeing spalling and surface erosion from
steam trying to escape from the interior of the flask before all the
wax has drained out. The reason I say this is because you report
that you have no problems with larger numbers of flasks in the same
kiln. With larger numbers of flasks the temperature rise will be
slower and will allow the majority of the wax to drain out before
you are producing large quantities of steam. If you heat too fast
the pressure developed by the wax still in the cavity and the steam
can fracture the surface of the mold and create this rough surface.
It was this problem that eventually made us shift to steam de-waxing
before placing the flasks in the kiln. This allows for removal of
most of the wax before placing in the kiln and typically results in
a better surface and cleaner burnouts.

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#7

I have a sheet of basic technical troubles on casting id interested
can fax or e-mail to you

Andy " The Tool Guy" Kroungold
Tool Sales / Technical
Stuller Inc
Phone 800-877-7777 ext. 94194
Fax 337-262-7791


#8

Andy,

Could you post the list on casting problems you mentioned? I am
sure many members would be interested in seeing it?!

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut2