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Centrigal casting pewter fail


#1

I’m learning casting with a centrigal casting machine so I’m using
pewter with regular investment in a 2.5 X 2.5 inch flask. I’m
casting medallions 1 inch in diameter and about 1/8 of an inch thick.
The metal has flashed past the confines of the cavity. The medallions
have flashing all around the edge. I waxed three medallions around a
central wax trunk and the metal has flashed past and connected all
three medallions together. It’s almost like the pieces were too
close together and the investment was not strong enough to contain
the metal.


#2

It sounds like you used the wrong mixing ratio and your resulting
investment was too thin. If you are using one of the standard
jewelry investments the water to powder ratio should be precisely 40
to 100 by weight. This is most conveniently done in the metric system
because one ml of water weighs one gram. So you use 40 ml of water
for 100 g of powder. (Of course, this has to be scaled to mix enough
powder to fill your flask.) The resulting mixture has a consistency
of heavy cream and if it is more like the consistency of milk you
have too much water for the powder you used.

I have found that my students start to have problems with investment
when they are not careful when measuring powder or water.

Hope this helps. Let us know your results.
Fred


#3
I waxed three medallions around a central wax trunk and the metal
has flashed past and connected all three medallions together. It's
almost like the pieces were too close together and the investment
was not strong enough to contain the metal. 

could be that, if you mixed the investment wrong, but more likely,
rather than the casting process itself causing the problem, I suspect
your investment failed earlier, during the burnout process.
Investment not allowed to set long enough, and then too fast a
burnout, could have cracked the investment. The metal then just cast
into the crack. The crack would have followed the path of least
resistance, so if your medallions were all in one plane and close
together, stress on the flask might easily do that. Next time,
position the medallions at a slight tilt, positioned sort of like the
blades on a boat propeller, except maybe angled down as well from the
central sprue. That way the mold cavities are not all in line with
each other. But more important will be maintaining proper powder to
water rations and mixing/investing technique, allowing the
investment to fully set up properly (wait perhaps 2 hours after
investing to be sure), and then use a burn out method that ramps the
temperature up slowly. If you have a controller that can do it, ramp
slowly up to 300F, wait maybe 15 minutes, then slowly to 375F, wait
again, and then up to burnout temp. After it’s fully burned out,
allow it to cool gently to casting temp. Again, don’t shock it with
too fast a cooling. On heat up, though, that first 400 degrees is
where the most critical drying and expansion phases take place, and
where cracking of investment is most likely to happen if you’ve done
something wrong.

One other caution. After you’ve poured the investment in the flask
and vaccuumed it and all, be careful that during the time while the
investment sets, glosses off, and solidifies, perhaps the first 15
minutes at least, be sure that it’s not disturbed or bumped. It’s
weak at this point, and vibration or shocks can easily crack the
setting investment before it has a chance to gain sufficient
strength.

Pewter itself should be easy to cast, with the exception of the fact
that it melts at such a low temperature that it can take quite a
while to fully solidify in a thicker mold. If you stop the centrifuge
too quickly, the metal can run right back out… I doubt that the use
of pewter itself has anything to do with the failure of the mold.
It’s less dense than other jewelry metals, so even a thicker heavier
piece shouldn’t be putting much stress on the mold.

Hope that helps.
Peter Rowe


#4

It sounds like you have stale investment or you mixed it with too
much water. Most investment powders have to be mixed in the ratio of
40 ml of water to 100 grams of investment. Although some people can
judge whether the mixture is right by its look and feel (it should be
the consistency of a good crepe batter) it is usually best to
accurately measure the ingredients by weight. Some measure both the
water and powder by weight (1ml of water 1 gram). Others, including
myself measure the water by volume in a measuring cylinder and the
powder by weight using digital scales accurate to 1 gram.

The other problem might be that the investment powder is old and/or
has been exposed to humid air too long so that it absorbs water from
the air so that the plaster in the mixture no longer sets as
strongly. Investment must be kept in a well sealed container that is
opened only when necessary.

All the best
Jenny


#5
The medallions have flashing all around the edge. I waxed three
medallions around a central wax trunk and the metal has flashed
past and connected all three medallions together. 

Well, George, it sounds to me the likely culprit is overheating
during burnout. Investment should go to around 1350F and much more
than that it just crumbles and self-destructs. I’ve had it happen
when the rheostat goes out… I’m assuming that you’re casting
into a cold flask, or maybe 100F or so? I don’t know what’s
recommended in the books, but 900F or socasting temp is for silver
and gold, not pewter…


#6

Hi George,

you normally cast pewter into rubber moulds which are held closed
under some pressure. Investment plaster is designed to be used hot,
I know you can let it get cold and then heat it up but it may be that
if you use it stone cold that it has shrinkage cracks which are what
is giving you flash.

regards,
Tim Blades


#7

Wow…

I can hardly believe what I’m seeing here. Can of worms time
again…

Pewter as well as other low temperature metals such as tin, lead,
and other white metal based materials are ALWAYS cast right into the
rubber or silicone mold. The equipment and process is totally
different. It is a separate industry from jewelry with lots of
technical available.

For more details go to:

http://www.ganoksin.com/gnkurl/tekcast

Read more abut the process here. These folks make all the equipment
to cast pewter and even plastic to cast into the same molds. They
made the pewter caster I used as well as sell all the supplies around
the process. Experts! Take a look at their sample products.

I HAVE CAST both pewter and plastic for several years and would
never consider going to the lost wax process to learn silver or gold
casting. It is a different process. If you want to learn casting with
a cheap metal you can cast a jewelers allow available at Stebgo
Metals for about $2.50 per ounce. This alloy casts, polishes and
stone sets with the same feel and look as 14 karat gold. If you want
to sell the material it has to be plated or lacquered.

Wow and best regards,
Todd Hawkinson
Southeast Technical College


#8

I worked at a factory casting pewter, and it was done in black
rubber molds, the mold spins around on a horizontal plane, metal was
poured in the center of the mold and centrifugal force spun the metal
outward and filled out 8-12 parts. John Donivan was closest in that
the investment mold does not need to be hot, but I do not know if the
metal will flow down thru sprues unless special gates are used to
allow the metal to go where you want it too and then past so you get
a complete fill using the investment normally used for gold or
sterling. Having no experience casting pewter in investment, I do
not know how the burnout of wax would affect the pewter.

I have used silicone molds at room temperature for a low temp white
metal from Atlas Metals. There were two sprues, one used to fill the
mold, the other is where you can see the metal rise up when
sufficient metal fills the mold. I melted the metal on a gas burner i
na small thrift store cooking sauce pan, never to be used for any
other purpose. I bent a spout on one side.

Richard Hart G.G.
Denver, Co.


#9
Pewter as well as other low temperature metals such as tin, lead,
and other white metal based materials are ALWAYS cast right into
the rubber or silicone mold. The equipment and process is totally
different. It is a separate industry from jewelry with lots of
technical available. 

Todd, while you’re correct that in the “industry”, pewter and other
white metals are spin cast into silicon rubber molds directly,
without wax models at all, your statement is just a little limited.
Only silicon molds work well for pewter or white metal casting.
Vulcanized standard rubber molds will not work so well as they’re
quickly damaged by even the low melting points of pewter. So
"always" into the rubber is wrong. You might get one or a few decent
castings, but standard rubbers won’t take the molten pewter without
damage. Silicon molds though, including the RTV liquid types that can
make a mold directly from a wax or other heat sensative model, do
work. Care needs to be taken in selecting an RTV rubber, since it
must be a silicon rubber. Some of the less expensive RTV rubbers
aren’t silicon, and won’t take the heat well.

Also, while the industry does this for pewter jewelry casting, that
does not mean that standard lost wax investment casting doesn’t also
work. It does. Just fine. If you have a case where you’ve made a
model in wax which you’d like to cast just one of, then simply burn
out the mold, same as you’d do to cast silver, gold, or bronze, etc,
but let the mold cool to a casting temp just warm enough so there
won’t be any moisture in the mold. That can range from lukewarm to a
bit more if needed to fill delicate details. The pewter will cast
just fine in an investment mold. The main limitation is that because
the investment is cool when cast, quenching doesn’t break away the
investment, which then has to be carefully removed manually. And just
hammering it to break it away can damage the soft pewter. This is
partly why rubber molds make more sense. But for a single piece,
that’s more work, and costlier.

Pewter is pretty simple to cast. Not only can you cast it into
silicon rubber molds, or investment molds, but molds made of plain
old plaster of paris (which cannot take burnout temps, though, so
lost wax casting doesn’t work with plaster of paris) or sand casting
also works. I recall at one point casting small handles for a pewter
cup in a mold made out of sawn and layered plywood. Worked fine.

So while you’re correct that the industry that casts white metals
for jewelry or other such small items uses a totally different
process (called spin casting) from that used to cast higher temp
metals, that doesn’t mean that the individual artist or craftsperson
HAS to do it that way. The industry does it because that method
allows very low cost and very high production rates for large
numbers of pieces. And it’s suited too for craftspeople who may wish
to do even limited production runs of only a few pieces. But one is
not limited to doing it ONLY that way.

Peter Rowe


#10
Having no experience casting pewter in investment, I do not know
how the burnout of wax would affect the pewter. 

Pewter will cast in investment quite nicely if the flask is cool. But
as mentioned by others the investment is designed to be used hot so
low melt pewter may not be the best metal to experiment with when
learning investment casting.

George


#11
Investment plaster is designed to be used hot, I know you can let
it get cold and then heat it up but it may be that if you use it
stone cold that it has shrinkage cracks which are what is giving
you flash. 

Investment is designed to be used hot; excellent point, and one I
never considered. I was using the molds cold to allow for the low
melt point of pewter. It is very probable that the investment may
crack once allowed to go cold and then quickly heated again with hot
metal. It may be best to practice with a high temp metal and hot
molds as suggested by someone else.

George


#12
I can hardly believe what I'm seeing here. Can of worms time
again... 

The purpose was to learn investment casting; and the process of
creating a burned out mold. The intention was never to use investment
and flasks to cast pewter. The one usefull comment from the lecture
was the reference to Stebgo and cheap metal for casting. For that I
thank you.


#13
Pewter as well as other low temperature metals such as tin, lead,
and other white metal based materials are ALWAYS cast right into
the rubber or silicone mold. The equipment and process is totally
different. It is a separate industry from jewelry with lots of
technical available. 

This is a little over the top Todd. Yes the industrial casting of
white metals is done in rubber but that is because it is so much
cheaper to do so in a production environment. And there has been a
huge amount of jewelry (costume) cast in these metals and yes it
often is plated but it is jewelry nonetheless. Yes there can be some
issues with allowing investment to cool down to the temperatures
needed for pewter but most of the time you can get away with it with
no real problems. It is by far easier if you have the investment
casting setup to go ahead and cast pewter into it than trying to
acquire the casting equipment needed to properly do rubber mold white
metal work. But there is nothing intrinsically wrong with what the OP
was trying, a little better mixing procedure a cool flask temp and it
is no big deal.

Jim

James Binnion
James Binnion Metal Arts


#14
Pewter will cast in investment quite nicely if the flask is cool.
But as mentioned by others the investment is designed to be used
hot so low melt pewter may not be the best metal to experiment with
when learning investment casting. 

Investments are designed to ALLOW being used hot. Standard plater of
paris, for example, won’t take such temperatures as needed for
burnout or casting without breaking down unacceptably. But there’s
nothing in the design of casting investments that makes them less
suited to being used at lower temperatures. Unless cooled WAY too
rapidly, properly mixed investment molds won’t crack just from
cooling, nor from having molten pewter poured into the pretty much
cold mold. The main thing would be to have the investment warm enough
so it’s not likely to have too much moisture, as that could cause the
metal to spit back out, or incompletely fill. Heat shock from pewter
won’t bother the investment, any more than pouring molten 18K white
gold at 1700 degrees into a 900 degree investment mold would do.

When I was in grad school, teaching the undergrad classes in
casting, we’d routinely invest flasks up to several days before
burnout, moisten them again before going into the kiln if they’d
gotten a bit dry, and then, if needed, such as if some student didn’t
happen to have their metal available for casting when the flasks were
ready to cast, we’d just let the molds cool down again, and they’d go
back into the kiln for the next burnout (without the wetting bit).
This always worked just fine.

While a number of factors can lead to investment cracking as
experienced by the OP, the most likely cause was improper
water/powder ration and/or incomplete mixing, going into the oven too
quickly and too hot, so it hadn’t yet reached full strength before
burnout, and then too much heat shock from going into a preheated
oven when the first two conditions also existed. With smaller flasks
especially, you can bend many of the rules. I’ve put flasks into a
300 degree preheated kiln after sitting just an hour when I was in a
hurry, but these are 1.5 inch x 2 inch small flasks. Usually, but not
every time, they survive this. Proper mixing and ratio and letting a
flask sit long enough, usually give modest size flasks enough
strength to withstand a moderately preheated kiln (but not over 300
please, to start) or a fast ramp speed (just turn it on high and let
it go from a cold start) But the more corners you cut, the more
likely you’ll have a failure. Follow all the rules and you might be
accused of being more careful than really needed. And that might be
true, but you’ll also not have to worry about blown out flasks
either.

By the way, one other factor I’ve not mentioned before nor seen
others do, has to do with investment working time. If you take too
long, you may have a situation where you’ve vaccuumed the investment,
poured the flask, vaccuumed it too, and now go to top off the flask,
but find that by now, the investment still in the mixing bowl has
started to thicken. When that happens, there can be an incomplete
bonding between what is already in the flask and the topping off
layer. You can’t see it, but that boundry is more prone to cracking
later. It’s a good idea to mix a small text batch of any new lot of
investment, using the proper ratio of water to powder, and paying
attention to water temperature (use water that’s been allowed to sit,
so it’s at room temp). Then time the amount of time between water
first hits the powder, and gloss off. Normally, gloss off should
occur somewhere around the 13 to 14 minute mark. That will mean your
working time is roughly nine minutes. You want everything done, and
the flask sitting quiet by that time, or a bit less (not too much
less or you can get “water marks”). If the investment glosses off
more quickly that this, use cooler water, or adjust your working time
to match. If it takes longer, you can increase the water temperature
to get the gloss off time shorter. Make notes, and do the same for
any use of that same batch of investment.

cheers
Peter


#15
Only silicon molds work well for pewter or white metal casting.
Vulcanized standard rubber molds will not work so well as they're
quickly damaged by even the low melting points of pewter. 

The silicon rubbers certainly are much more heat resistant but much
of the white metal casting is done in organic rubbers. It is a
different formulation from the rubber you use for wax injection but
it works just fine see

and other suppliers of rubber for the white metal industry

James Binnion
James Binnion Metal Arts


#16

To all:

Perhaps my experience with many kinds of rubber, silicone and metal
molds gives me a different perspective. I always like to think any
recommendation I have is meant to reduce the time and expense of
trying something.

I think my original response was that I cannot mix these two
different processes together. I am pragmatic with most everything I
do and teach. If it takes you more time and expense to do something
quite simple I would recommend a different approach. If we are to
survive these hard economic times we have to be cost effective with
what we do. At least in my opinion.

I have tried to cast pewter once or twice with the lost wax process
years ago and gave up because the quality was so compromised.
Production this way was a waist of time for me. When considering the
product, you use the proper technique what works. I’m not writing the
book here either. The master models I made however were always made
with the lost wax process for multiple models in the production
molds.

When I was casting the pewter and plastic in the spin casting
machine I had, I did have a large vulcanizer for the molds and made
some gum rubber molds with it. The vulcanizer weighed about seven
hundred pounds. These molds were a gum rubber in their composition.
Making these molds was quite a trip! I had about three hundred and
fifty black rubber molds for casting pewter belt buckles at that
time. I just could not do both the fine sterling and gold product
side by side with the pewter and plastic items. It was too expensive
to do both. That’s just me and my experience. If anyone has the
capital and patience to do both, more power to them!

I currently use silicone for almost all my molds.

Maybe I should have kept all the machinery and brought it back into
production because of the current cost of metals.

I would only wish anyone success in what they do. I continue to
learn form my students, my peers and forums like this.

Good luck & Best regards,
Todd Hawkinson
Southeast Technical College


#17

Dear All,

It’s just that casting the low melting temperature metals directly
into the molds is so much cheaper and less complicated IMO.

I modified an old wax spinner to hold a jewelry style rubber mold
and cast the metal right into the mold at a much much lower cost that
the lost wax casting cost.

I think the original post was for learning lost wax casting and I
cannot associate pewter casting and the lost wax casting process as
anything remotely the same for learning.

I’m also amazed and the loss of and the lack of
in some areas.

Best regards,
Todd Hawkinson
Southeast Technical College


#18
It's a good idea to mix a small test batch of any new lot of
investment, using the proper ratio of water to powder, and paying
attention to water temperature (use water that's been allowed to
sit, so it's at room temp). Then time the amount of time between
water first hits the powder, and gloss off. 

This is a critical point. There is mention in the archives of other
investors who use warmer water in the mix but adjust their other work
time values to allow for the quicker set-up. The point seems to be
that there is a narrow window that is optimal for pouring the
investment into the flask. The timing process may be innacurate if
every new batch of investment is not timed for it’s gloss-off time at
a measured water temperature.

George


#19

Hi George:

The investment in equipment to cast pewter directly into silicone
molds is probably prohibitive for a guy who wants to cast with
pewter a couple of times each year. It is perfectly fine to cast
pewter in gypsum investment.

If you ever decide that you wish to expand into higher production of
costume jewelry made with pewter or zinc, contact Len Schaer at
Tekcast in New Rochelle, New York. He and his associates are
experts. We have a lot of contact with Len, as we make silicone
rubber mold sets for Tekcast.

Regards,

Bill Mull
Zero-D Products, Inc.