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Casting questions


#1

In my new job, I oversee the preparation and clean-up of all our
cast pieces, though the boss does the actual casting off-premises. I
have not done a whole lot of casting in the past, and all of it was
in silver. At work, we cast silver and several colors and carats of
golds.

I am not very pleased with the way the castings are coming out, and
I’m hoping to get some help understanding what might be done to
improve them.

First-- almost everything we cast is injected. Is it hopeless to get
a good polish on injection waxes, pre-casting? I am not producing
these waxes myself, and I do not want to pressure the wax person
unfairly, but the finish is not what I am used to, on hard carving
wax. There is quite a lot of clean-up after casting.

Second-- Recent castings have had what the boss says is called
"spalling"-- networks of raised ridges that look almost like veins.
What can be done to correct this?

Last, today’s silver flask contained 8 pieces-- 2 rings and 6 flat,
coin-like charms-- and they came out with varying degrees of pitting
on the surface. Some were hardly affected; one was so bad in places
that the pits were too large/deep to be sanded out without wrecking
the piece. This was pretty evenly distributed through the flask, and
did not look like a typical porosity problem. The porosity I am
familiar with is generally close to the main sprue, and is not
confined to the surface. Though, in fairness, I don’t actually know
what causes the familiar porosity, either. Thoughts?

The silver is de-ox, 50-50 new and old; the investment is new; the
other flasks in the casting (various golds) did not have this
problem.

Oh-- and one more question-- Is it really an absolute no-no to get
even the tiniest bit of metal with solder on it in a casting? What
does it do, other than potentially lowering the karat or degree of
purity you can claim?

Thanks a lot,
Noel


#2

Hello,
Hope the ans help you.

Check the burnout cycle. Is it to rapid resulting in spalling. use a
manufacturer reccomended burnout cycle.

Could you please enclose some pictures.

G.Chavan

[Edit]

How can I share files and pictures with the list?
http://www.ganoksin.com/gnkurl/ftp

Or… send the files to the attention of service@ganoksin.com and
we will upload them for you…

[/Edit]


#3

Hello in my experience casting (by vacum and centrifuge) temperature
of the flask at casting is crucial, and the rule is as cold as
possible, of course you need it to be hot for metal to flow but the
more time metal stays liquid the more shrinking and porosity you get,
so if youre pieces are thin the flask needs to be hotter, and vice
versa if they are heavy so try To make trees with even thickness (as
much as posible) to hot metal is also a no no. My casting teacher
years ago taught me to visualize the metal as water flowing through
caves and to imagine where in the caves There would be turbulence
(this causes a lot of pits and bubbles) usually caused by protrusions
in the cave walls ( cavities next to sprues For example)


#4

Surely you will get much input, Noel…

First -- almost everything we cast is injected. Is it hopeless to
get a good polish on injection waxes, pre-casting? I am not
producing these waxes myself, and I do not want to pressure the
wax person unfairly, but the finish is not what I am used to, on
hard carving wax. There is quite a lot of clean-up after casting. 

Good waxwork means removing flashings and such. That’s easy to do in
wax, and much more work in metal. Since the rings are going to be
filed all over in finishing, we just don’t worry so much about
"polish" as long as it’s clean. You can use an old nylon hose to
polish injection wax, if needed. Just rub it.

Second-- Recent castings have had what the boss says is called
"spalling"-- networks of raised ridges that look almost like veins.
What can be done to correct this? 

That’s because the investment wasn’t thoroughly mixed, and there’s
free water in the mix. Mix it thoroughly, and then mix it some more.
I tshould be perfectly creamy and homogonous. There could be another
reason, but unmixed investment is almost 100% the answer.

Last, today's silver flask contained 8 pieces-- 2 rings and 6
flat, coin-like charms-- and they came out with varying degrees
of pitting on the surface. Some were hardly affected; one was so
bad in places that the pits were too large/deep to be sanded out
without wrecking the piece. This was pretty evenly distributed
through the flask, and did not look like a typical porosity
problem. The porosity I am familiar with is generally close to
the main sprue, and is not confined to the surface. Though, in
fairness, I don't actually know what causes the familiar
porosity, either. Thoughts? 

Sounds like incomplete burnout to me, but it’s hard to always pin
down the reason for raging porosity. Overheated metal, maybe,
probably incomplete burnout. Little bits of carbon scattered around
inside.

Oh-- and one more question-- Is it really an absolute no-no to get
even the tiniest bit of metal with solder on it in a casting? What
does it do, other than potentially lowering the karat or degree of
purity you can claim? We don’t use silver with solder on it, and we
always use “plumb” gold solder. We take a pretty casual view about
scrap metal and rarely have problems. I know what "the book"
says… I mean PRETTY casual - a bit of solder here and there
doesn’t usually mean much. We won’t cast chains… It’s the cadmium
repair solders that get you, and we don’t use those.

Casting is an art - we’ve been lucky or skilled enough to not have
serious problems in a long time. But it happens… Thanks a lot,
Noel


#5

Hi Noel,

I’ll try to go over the problems you are experiencing and offer some
probable causes with possible suggestions to fix them. Without
seeing the problems, it is often hard to nail them down.

First - Rough injection waxes - This is usually due to rough poorly
finished master models used to make the rubber injection molds. The
molds, waxes and subsequent castings will never be better than the
master model used to make them. Refinishing the models and making
new rubber molds will improve the surface finish. A secondary cause
can be a build up of mold release powder (talc or corn starch)on the
inside of the rubber mold. This can be solved by cleaning the rubber
mold in a warm gentle soap solution with a soft brush. Dry the
rubber molds well before using.

Second - Veins & rough spalled surfaces on the castings. This may be
due to water marks caused by using too much water in the investment
mix or not mixing for a long enough time. Check the investing
procedures being used, measure the water and weigh the investment.
Check the mix time, vacuum time and gloss off time being used.
(instructions are available from the investment supplier). Allow the
invested flasks a 2 hour set up time before burn out, this will
provide maximum green strength in the investment.

Last - Pitting on deox silver cast surfaces. There are several
possible causes, casting too cold, investment or oxide residues on
scrap added to the melt, lack of flux in open melting.

Regular sterling silver will cast well about 1760 to 1780F, the deox
silvercasts well at around 1850F, casting the deox silver too cold
could cause pitting. Check the condition of the scrap added to the
melt, remove any heavily oxidized metal and make sure the scrap is
free of investment residues.

If open melting is being done, flux & skim the surface of the metal
before casting.

As for using soldered material in your melt, it may alter the gold
or silver content of the castings and add more lower temperature
metals that may effect overall properties.

Good luck & Best regards,
Jim Sivertsen
United Technical Dept.


#6

Sounds to me like investment problems. The veining you referred to
sounds like what is sometimes called watermarking. It is usually
caused by using a mixture that’s too thin (too much water) or by some
other issue that causes the investment to take too long to set up
(too cold?), or it may be that the working time is a little too short
and the flasks are being allowed to set too long before gloss-off.

The rough texture can be caused by a number of things, but it sounds
like at least part of what is happening is that the investment is
breaking down and the little fragments of investment are getting
pinned between the walls of the cavity and the molten metal, forming
investment inclusions in the surface of the castings. These appear on
the divested casting as small holes and lumps and is sometimes called
spalling. It can be caused by some of the same investment issues that
cause the veining, basically anything that causes weakened
investment.

If the investment isn’t old and has been sealed properly so it
hasn’t absorbed any moisture, I’d look for measuring, timing, and/or
temperature errors made while investing and burning out. Errors like
these tend to compound as well. Start with cold investment, add a
little bit too much water that’s at 50 degrees, use thirty seconds
too little working time, start the burn-out too soon then raise the
kiln temp a little too fast to a little too high and you have an
almost guaranteed recipe for investment related problems like you
described. I have also gotten bad batches of investment brand-new
from the manufacturer, so that is definitely something to consider as
well.

Injection waxes do not have to be rough surfaced. Essentially, it
can be as simple as “garbage in - garbage out”. If the masters were
highly finished and the molds were made correctly, then a possible
cause of the roughness may be that the person shooting the waxes is
using too much powder or mold release, possibly in an attempt to help
correct a venting or release problem with the mold.

About using metal with solder in it - a tiny amount of solder
probably won’t give you too much trouble, but then again “a tiny
amount” to me might be an entirely different amount for someone else.
I try to always remove every bit of solder that I know about, the
occasional sized shank should be OK. The kind of trouble you will get
from too much solder in the mix can start with a slight discoloration
of the metal, sometimes the metal will come out extremely hard and
brittle (karated yellow golds seem most susceptible to this) and
there is a greater risk for what you properly described as porosity.

Hope this is helpful, Noel!
Dave Phelps


#7

Hi Noel,

Wow, a big question.

My experience with such things have been:

The initial model not finished to a fine point, little care taken in
making the mold, little care taken when the mold is out together for
injecting. Improper investment preparation, not mixed long enough,
not vacuumed properly. The use of cast silver too many time as the
non-new silver. Each time it is cast it picks up sulfur something.

(I was told by my metallurgous, (The guy I sell my scrap silver to),
that de-ox is only good for one time of casting after that it starts
to break down. I have actually got my silver too hot and it boiled and
it seemed like the copper separated from the silver.

I used to use many levels of silver, some solder, 3 or 4 times
re-used previous cast metals. I have found my castings results have
greatly improved since I started using all new metal and recycling my
old. You take a hit on the price, but it improves the castings and
less time is needed and time is money.

I hope this helps,

Ken Moore
kenworx.com


#8
Recent castings have had what the boss says is called "spalling"--
networks of raised ridges that look almost like veins. What can be
done to correct this? [snip]
Last, today's silver flask contained 8 pieces-- 2 rings and 6
flat, coin-like charms-- and they came out with varying degrees of
pitting on the surface. Some were hardly affected; one was so bad
in places that the pits were too large/deep to be sanded out
without wrecking the piece. This was pretty evenly distributed
through the flask, and did not look like a typical porosity
problem. The porosity I am familiar with is generally close to the
main sprue, and is not confined to the surface. Though, in
fairness, I don't actually know what causes the familiar porosity,
either. Thoughts? 

That “familiar” porosity is caused by shrinkage of the metal as it
cools. The mass of cooling metal resembles a sponge, with a thin
skin of solid metal on the surface next to the mold. If there’s no
reservoir of liquid metal adjacent for the sponge to draw from as the
metal shrinks, it will draw it from all its channels, producing a lot
of pits and tunnels just below the surface and sometimes collapsing
the surface itself causing soft detail or crumpled, distorted areas.
This is the sort of porosity that finishers hate, since the more they
grind, the more they find. The solution is usually better spruing,
and designing cast pieces so that thick areas aren’t surrounded by
thin ones without a thicker sprue to shrink from.

The silver is de-ox, 50-50 new and old; the investment is new; the
other flasks in the casting (various golds) did not have this
problem. 
Oh-- and one more question-- Is it really an absolute no-no to get
even the tiniest bit of metal with solder on it in a casting? What
does it do, other than potentially lowering the karat or degree of
purity you can claim? 

Solder has zinc in it, which can cause gas bubbles in the castings,
similar to what you describe above.


#9

Could it be that your “masters” are low quality?

If you wish to do it properly, take a silcone RTV mould of your
master.

Cast red polyester resin into the RTV mould and NOW you can see the
imperfections.

Clean up and polish resin master with stainless steel polish.

Now make your wax injection mould from the resin master.

Of course you as experienced professionals can allow for shrinkage.

Now you flame your waxes to give a high sheen.

If I was getting such low quality castings I would just send them
back to the casters.

I have not found professional casters always produce quality
castings without quality masters.

Richard


#10

I spoke with Dennis the head caster at Stuller and here is what he
said… Hope it helps…

Good fresh investment properly weighed and measured.

Two hour bench set before burning out for best ‘green strength.’

One hour to two hours at 250-300F to remove most of the pattern
material.

Gradual ramp to max burn temp (keep close to 4degrees/minute.) This
is assuming he has normal injection waxes on the tree.

Andy “The Tool Guy” Kroungold


#11
If I was getting such low quality castings I would just send them
back to the casters. 

Haha I can’t do that-- the caster is the boss! I can work with the
wax carver to try to improve the waxes, the spruing, and the
investing. I already do my best to make sure the models are good. I
think our wax originals need much better polishing and finishing. The
actual casting is beyond my control.

I very much appreciate the help and suggestions I’ve been getting.
You guys are the best resource anyone could have.

Noel


#12

Noel, I recently had to do some casting for friends who needed
heavier components for flute production cast. With their stuff, we
were also getting the pitting and it was a real problem. After some
research, I found that, at least with silver, oxygen absorption into
the silver is one of the problems…silver sucks in oxygen when it
is melted, and it’s that oxygen which causes the pitting. When I
learned to cast, it was centrifugal with a blow torch to melt in an
open crucible. we always added flux… When I got my own set up, I
bought vacuum and an electromelt from Rio. The electromelts use
graphite crucibles, and theoretically, the graphite will burn up the
oxygen during heating rather than it being absorbed by the
silver…problem was, I was still adding borax/boric acid like I had
with the blow torch/open crucible, and this was coating the inside of
the graphite crucible, preventing it from absorbing the oxygen. I’d
not had much trouble with this with my filigree, but it is so fine,
that bubbles didn’t get trapped as easily as with the heavier parts.
After discontinuing the flux, the pitting almost completely stopped.

The injection waxes should come out with a reasonably glossy
finish… or at least as glossy as the original metal masters.
However, if they’ve been used over and over again, the molds can
wear out, making them less shiny. It may be time to make new ones…
silicon based rubber is great because it makes a much shinier finish
and does not require mold release spray. The molds seem to last
longer as well.

If the wax is old, or too many times recycled through the injection
pot, it can also loose quality. Not only can heating the plaster too
quickly cause the hairline fractures… but you may want to check
how long they are letting it set before dewaxing…too long or too
short can affect the quality of the mold as well. You said the
plaster is new? how new is new? some estimates say the shelf life can
be as short as 6 months when you start loosing quality on the
investment… though I’ve cast with older plaster…it’s always a
risk it won’t cure properly.

considering this problem seems to be silver only, with the pitting,
I’d look at the oxygen issue… it could be a matter of too little
or too much flux, or how it’s melted…pouring temperature/mold
temperature can also affect things in terms of how much time the
oxygen has to escape from the silver… Rio even suggested leaving
the vacuum on a little longer before removing the flasks…though I’m
not sure that does as much as cutting out my flux did.