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Crummy castings


#1

Hi all!

I have a question about castings from lost wax casting that come
out bad. The last 2 times I went to my caster to have a wax model
cast into sterling silver, the pieces came back with awful pits
and extra silver caked on in some areas. Some of the caked on
silver flakes off easily, some is just like a rough skin over the
original surface. What causes this? I haven’t said any thing to
the caster about it yet because I usually run in, give the piece
a quick look over to make sure the basic detail is there, and
run out. In the past the castings I have gotten back were fine,
except one gold ring that came back pitted. I was told that it
was due to air bubbles in the wax - which was plausable. I did
break the shank towards the end and repaired it quickly. But
these last 2 pieces were carved from a block of wax with no
adding on. What could have happened? Is there anything the
caster could/should do to make up for this? Or is it just one
of those things that you have to accept with lost wax casting?
It is really frustrating to put many hours into a model and come
back with such poor results and be charged full price for the
job.

Thanks for any input on this.

Jill
@jandr
http://members.tripod.com/~jilk


#2

Hi Jill, All crummy castings have a reason. The symptoms you
describe do sound like a wax model problem. Good casters have
good vac pumps. A good vac pump will find the smallest air crack
or bubble in the wax (that is it’s job!) and replace that space,
however small, with investment. This investment carries though
and deposits and creates the symptoms you’ve described. As a
caster that advertises quality, I feel it is my job to locate
some of these problems at spruing time. Some times we can’t find
them all. You should know, block or slab carving wax is not
perfect. Prepare, take care, beware, and repair. Another possible
explanation could be a thing called spauling. This is investment
that breaks off inside the mold during the burnout phase and
re-deposits elsewhere. If I could see it I could tell for sure
what was happening. The best casting happens in private shops
where the attention to detail is lavished on. We have a very high
success rate because of our attention to detail and commitment to
quality. But, we are human and occasionally have to eat crow.
When we blow it, we admit it, learn from it, and try to soften
the blow to the customer with a free service or something of the
like. You can contact me with any questions you may have. J.A.
(207)729-3599


#3

Hi Jill,

Sorry to hear of the misfortune. If you carved the ring from a
solid block of say Mattwax there shouldn’t be any bubbles
because you’d see them on the surface. if they were subsurface
they wouldn’t matter.

Did you invest these pieces or did he? Right off the top of my
head, here are the possibilities I can think of:

  1. The investment is out dated, though in my experience this
    has caused investment to become incorporated into the final
    casting. It appears to have not set up completely.

  2. Bad debubblizer. In any case he didn’t dry the pattern
    sufficiently and it caused casting flash which, if it was
    tumbled, was peened over onto the design.

  3. Old used metal or metal with solder on it.

  4. Poor sprueing technique. The metal chilled in a thin area
    with the gasses still in the heavier, molten, parts of the
    pattern.

  5. Improper burnout technique. This can be caused by
    carelessness, accident or a burnout oven that is grossly out of
    calibration.

In any case let the caster know about the problem. Show him the
results.

Regards,

Skip

                                  Skip Meister
                                NRA Endowment and
                                   Instructor
                                @Skip_Meister
                                10/17/9701:15:48

#4

Hi Jill,

Only one piece of advice: find a new caster!

Dave

Dave Sebaste
Sebaste Studio
Charlotte, NC (USA)
dave@sebaste.com
http://www.sebaste.com


#5

sounds like your caster is not using the propper temperture in
his metals before the pour or vacume. if the metal cools to
quickly or the investment is not the right temperature the metal
cools to fast and crummy casting occur

Rick in Kc


#6

You seem to have a serious problem with your casters.
Personally, I would say that the casters you use are not very
good because a reputable lost wax caster would not allow lumps on
your casting or to use the excuse of bubbles in the wax. As you
are shortcutting the vulcanising process by giving the caster a
wax model there must be some problem with the plaster they are
using. The need to vibrate the plaster prior to casting to get
all the bubbles out. Perhaps their plaster dries too quickly or
they don’t vibrate it long enough. You should get a very high
quality surface from lost wax castings. I had some done recently,
they came back clean and white without any lumps or bumps.

Hope that helps.
Richard W
UK


#7

Hi Jill, Skip is mostly right. However, sub-surface air bubbles
can and will burst and fill with investment, if your caster has a
good vac pump. Deep bubbles may not, but it’s better to be safe
than sorry, isn’t it? J.A.

J.A.Henkel Casting Co., Inc.
Veteran of hundreds of thousands of weird castings!


#8

Hi Jill,

I have found that most problems with casting are the result of
poor thermal management during the casting process. As metal
cools from a liquid to a solid, it undergoes about a 5% volume
change. As this shrinkage occurs, metal must flow into the area
that is becoming solid. Large pits are usually the results of
flow blockage that traps liquid metal. Once a segment of the cast
has been isolated from liquid metal flow, it will continue to
shrink. Since new liquid metal cannot be supplied, the shrinkage
results in pit formation.

Think about what happens as ice freezes. Ice is one of those
unique materials that expands upon turning from liquid to solid.
We all know how much force can be generated as the ice freezes. I
have seen ice generate enough force to fracture quarter inch
steel piping. The same forces are at work when a casting cools,
and the metal changes its state from a liquid to a solid.

The Japanese railroad companies were the first to recognize the
importance of the cooling process during casting. Their
engineers modeled the thermal cool-down of steel railroad wheels.
The common practice at foundries then was to pour molten metal
from a sprue directly into a wheel. The top part of the wheel
would be the last place to cool and would result in steel that
was more porous. Porous metal is a bad thing for railroad wheels.
The more porous metal wears faster, resulting in a short wheel
life. (No one likes an oval railroad wheel). They cured this
problem by redesigning the sprue to provide a reservoir that
could supply fresh molten metal while acting as a heat
reservoir.

There’s currently considerable research within the engineering
community on how to numerically model the casting process.
Automobile companies must cast engine blocks, and airplane
companies must cast jet engine parts by the millions. The
numerical models include fluid-flow analysis, thermal transport
between the flask and the fluid, and complex material models that
predict how the metal will behave under phase change. These
models currently require some of the world’s largest computers
and require many hours of computer time. If you really want to
use one these models, it is also good to have one or two spare
rocket scientists around to help with the simulation.

By now, you are thinking “I don’t have the world’s largest
computer, or any spare rocket scientists.” You do, however, have
experience and intuition to rely on. Any time that I get a porous
casting, I always think, "what’s the last place to cool ?“
Usually, when you think about the casting in this light, the big
"DUH” will occur, and you will be able to correct your problem.
The next time that you get a poor, pitted cast, ask the question:
could these pits be in the last place to cool?

Remember back to those big men’s rings that looked great until
the final polish. Just as you were starting to get a good shine,
you began to see those pesky little spherical pits. They are
always just inside the surface, waiting for you to remove the
outer skin and expose them.

Try this experiment. Cast a simple sphere with the sprue in the
center of a flask. Saw the sphere in half. Polish the surface,
and then examine the surface under a microscope. You should see
small voids near the center of the sphere.

Out-gassing of the plaster cannot form the pits located just
under the surface. Instead, they are formed as the metal shrinks.
Tensile stresses in the liquid become great enough to cavatate
the liquid and form nice spherical bubbles that are trapped in
the freezing metal. Sometimes, metal is kind of mushy, and the
shrinkage process will be able to pull chunks of material from
the surface. That’s bad.

Here are some things to remember. The rate of cooling is
proportional to the surface area to volume ratio. A sphere will
have the least surface area for any given volume. A plate,
however, will have a very high surface area compared to its
volume. Inside corners will always be hot spots.

There are several things you can do to help control the cooling.
Layout the wax and sprue such that the important design is
nearer the edge of the cylinder. You can also use wax vents to
accelerate the cooling process. A piece of metal inserted in the
plaster (a thermal shunt) near the wax will also locally
accelerate the cooling process. Remember that the flask has a
natural thermal gradient with the center portion of the flask
being the hottest. Exploit this.

For some complex shapes, I form with wax a molten metal
reservoir using a shape that looks like a cherry. This spherical
cavity will be able to supply fresh metal and will sacrifice
itself by being the last place to cool.

Another technique that can be used to gain an understanding of
the solid freezing process is to use a wax injector with a clear
rubber mold. The wax that I use changes color and transparency as
it freezes. By looking at which parts of the wax freeze first, I
get in idea of how the molten metal will freeze.

Cooling isn’t everything. The alloy can also play a major role.
Most casting alloys contain an agent that will help with the flow
properties of the molten metal. Unfortunately for 0.95 silver, as
the metal cools, the copper can precipitate into small crystals
of pure copper surrounded by liquid silver. This mixture forms
what is known as a mushy zone. This mushy zone can choke the flow
in the casting process. Ingredients such as silica can reduce the
size in the mushy zone and improve the casting process. The one
drawback is that these ingredients make the metals hard to work.
Because these added ingredients can affect the annealing process,
you never want to use casting alloy for rolling.

If you really want to minimize the risks for your castings, try
pure metal. Pure silver or gold will have a much narrower mushy
zone.

For me, the proper placement and attachment of sprues, combined
with the placement of the wax within the flask, are the most
important factors in achieving a successful cast. Proper control
of the burnout process is also important to achieve a good cast.
A flask that is too hot or too cool will cause all kinds of
trouble for any casting. It’s got to be “juuuust” right.

Stephen and Nancy Attaway


#9

Everbody- thanks for your help on my casting problem. But with
all the suggestions, I’m still not sure what could be going on.
It would help much more if you all could see the problem, but
instead I will try to give more detailed description.

There are pits (regular pits, no explanation needed), and also a
sort of silver crud in patches on the piece. Almost as if some
of the silver flashed into the investment, or some of the
investment eroded in some areas. I don’t think it was that the
investment eroded, though. The silver crud was easy to flake off
in some spots, and I would think if the investment broke off that
the crud would be a solid part of the piece. And the metal was
also rather pourous in some areas. And not many bubbles on the
piece.

I really don’t think it was air bubbles in the wax. The piece
that was worse hit was carved from a single block with absolutely
no adding on. Unless the wax had bubble inclusions? I used
Ferris purple for one and a red ring wax for the other. I don’t
think it was that the metal was too cool either, it seemed to
have filled the cavity ok.

I will be mentioning this problem on my next trip to the caster.
They have given me great service in the past and are very
conscientious about keeping my designs safe, so I would like to
give them a chance to clear this up.

Thanks for your help.

Jill
@jandr
http://members.tripod.com/~jilk


#10

Jill,

First I would ask the caster what happened and see what is said.
As to responsibility for the bad casting/lost work I think it
depends on what your agreement is with the caster. We will
guarantee a casting from a <> piece and a new wax can be
made and recast. From one of a kind we do not guarantee the
casting. In either case, there is a factor of how much money you
are being charged (and for the caster how much potential profit
is in the work) as to weather there is a guarantee or not. Also
how complicated the piece is. How it was made (air voids in the
wax??) whos metal is being used and lastly how much pressure is
being applied to get the job done yesterday.

Dealing with a bad casting and recasting it when the client is
squirming over a $7.50 per piece charge, on a rush basis (3 days)
on a fabricated piece (l o t s of air spaces and bad jointing)
for 5 pendent and bracelet pieces total, using customer metal,
one of a kind, not molded pieces, doesn’t cause me to feel too
responsible for a bad casting especially when the client is told
of the potential problems in the wax and metal. There are just
far too many areas I have no control over that could cause
problems. If on the other hand, no rush, good solid waxes, good
clean metal, etc. I would feel responsible. No matter what the
piece to be cast is like, nobody would want to cast it poorly,
but there is a lot involved, and many places for the thing to go
wrong.

John and Cynthia/MidLife Crisis Enterprises
Maiden Metals/C. T. Designs/ Bloomin’ Wax Works. etc.

PO Bx 44, Philo
CA 95466
Ph 707-895-2635 FAX 707-895-9332

If your’re headed in the right direction,
each step, no matter how small, is getting you closer to your goal.