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Casting at high altitudes


#1

Hello, I am a twenty four-year old student at GIA; studying
comprehensive wax techniques and casting. My father has been a
jeweler for somewhere in the neighborhood of thirty years and I have
decided to follow in his footsteps and help out in the family
business. I am contacting you because I have been around the jewelry
industry my entire life. During my exposure to this industry I have
been exposed to a phenomenon which exists in the geographic region
where I live. This phenomenon is that of the highly gaurded trade
secret. Jewelers in the area learn through trial and error. There
are certain techniques which just not shared in order to maintain
competitive edge. It is somewhat of a paranoia which may be
understandable in some areas but in others it is plain absurdity.
This leads me to discuss one very problematic area in the workshop:
casting.

Through many years of trial and error we have tried and failed in
the specific procedure of investing for casting. We just seem to get
all of the air bubbles out of the investment; ulitimately leaving
bubbles of gold on the casted piece. What we have come to encounter a
majority of the time during casting cleanup is that when you remove
these bubbles you are left with a pit underneath which must then be
filled. This process of cleanup becomes quite time consuming and in
turn quite costly. It has always been this way sometimes you get
lucky and get less bubbles sometimes more; but almost
invaribly…bubbles. This assures us that the process has something
to with the investment process. We have also been told that when we
are vacuming the air bubbles out of our flasks that because we are at
a high altitude(6,984ft) that our vacum can not physically pull
enough pressure. The most pressure that the gauge on various
machines we have used is 24. We are currently using a vic-9 which is
manufactured by rio grande. Is it possible to get a machine with a
stronger vacuum or is it just not physically possible to achieve a
stronger vacuum than 24lbs at an altitude of nearly 7,000ft?

Please help
Sincerely,
Nicholas Griego


#2

Another thought… it maybe that you are pulling too deep of a
vacuum and are “boiling” the water out of the investment. Could you
try less of a vacuum but more volume of air?

Dan Wellman_


#3

The thing to remember is that you have less air pressure to push into
the vacuum. It is quite possible to pull a vacuum down to below 500
microns or less (25,400 microns to an inch of vacuum, with 0 being an
absolute vacuum) There are several “tricks” to doing so: One is to be
able to measure accurately. Most compound gauges are very inaccurate.
A simple and effective gauge is made using tubing bent in a U and
filling with mercury so that a 30 inch offset is possible; 15" up on
the vacuum side, 15" down on the atmospheric side. The reading may be
dependant on atmospheric pressure due to altitude. To get the most
out of your Vacuum pump run the pump for an extended time with the
gas ballast open and the output valved off. This helps to purge
moisture from the pump. Change the oil out frequently. The seals in
many pumps can be relapped by changing the oil and leaving the pump
running for 24 hours straight or longer. If you run as large a line
to your vacuum table as possible and install a vacuum chamber as
close as possible to your table that would aid in getting a quick and
hard pull at casting, making the most of your lower air pressure. I
dehydrate refrigerant systems using a vacuum pump and the moisture
pulls out at about 1500 microns. Sometimes it takes days. Also if
water is sitting under oil I can not pull it out unless I
heat or agitate the oil.


#4

Hi Nicholas, for strong vacuum pumps look in the GRAINGER Catalog
(Industrial Supplies) they have what ever you need.(www.grainger.com)
Do you use a debubblizer? (A soapy liquid you dip the wax tree in?)
Contenti Jewelry Supply in Rhode Island sells it.(
http://www.contenti.com/products/casting/160-363.html ) This helps
to get rid of bubbles. I know of guys that get rid of bubbles by
vibration only. Can you agitate your vacuum table? Is your investment
liquid enough? My be in higher altitudes you have to ad more water.
The mighty casting is a crazy science!! Hope I could help! Klaus, A
Jour Inc. http://www.ajourjewelry.com


#5
it just not physically possible to achieve a stronger vacuum than
24lbs at an altitude of nearly 7,000ft? 

Nicholas, Vacuum is the absense of air pressure, so the maximum
vacuum reading you could get would be the actual air pressure at your
altitude. I don’t remember the precise readings for a given
altitude, but I’d guess that at 7000 feet, 24 inches of mercury would
be about the max. But this should not be a major factor. If you
were in outer space, your vac pump would not be able to pull anything
at all, since there would be no difference between the environment
and within the bell jar, yet there would be no air in the bell jar,
and bubbles of air would not be possible. In your situation, the key
is not the guage reading, but whether the pressure is low enough in
the bell jar to allow the investment mix to boil, which acts to more
fully mix it, as well as dislodging air bubbles. This behavier
depends on the actual pressure in the bell jar, which is NOT the
guage reading, and the higher you go in altitude, the EASIER task a
pump will have in getting the pressure in the bell jar down to those
needed levels. Examine the time it takes for the investment mix to
boil when you’re working, to determine if your pump is working
properly or not. If this seems in accordance with published
procedures, then adaquate vacuum is not your problem. You may also
need to examine factors such as the water quality where you are,
which may also cause investing problems, the temperature of the water
you’re using, the water/powder ratio, and perhaps just the brand of
investment powder you use. also, check things such as the cleanliness
of your wax. Dirt mixed in the wax can cause problems. I know of no
reason why an attached bubble of metal in the casting, caused by an
air bubble in investing, should automatically cause a pit, at least
with a small bubble. Dirt, on the other hand, might do it in some
cases…

Peter


#6

Hi Nicholas I also live in the neighborhood ( where the last of the
hummers have taken off for central america). Technique is simply
technique something to be shared. I’m sure Ken at MPG repairs has
some good info (incidentally, thanks Ken for helpful info on my
vacuuming problem if I didn’t thank you before). As Ken pointed out
it should be easier to pull suffcient vacuum at northern NM
altitudes. What puzzles me is; if you’re a student at GIA, don’t
they (GIA) know this stuff?


#7

Nicholas, you will lose 1" of mercury (on your gage) for every 1000
feet of altitude. Therefore, you should only be able to get to 23"
on your gage. You will need a vacuum that pulls at least 20 cfm to
get consistent results. I may have a setup for you if you call me
at 1-888-652-8994. Doug


#8

Nicholas,

 We have also been told that when we are vacuming the air bubbles
out of our flasks that because we are at a high altitude(6,984ft)
that our vacum can not physically pull enough pressure. 

This statement is not at all true. Who ever told you this doesn’t
know what they are talking about.

Altitude has an effect on the vacuum gauge reading. If you are
getting 24 in Hg at 7000 feet you are getting a full vacuum. Unless
you spend tens of thousands of dollars you will never get a more
complete vacuum. What is considered a full vacuum is different at
differing altitudes.

Full vacuum is different at all altitudes. It ranges from about 23
to 27 in Hg. The quality and accuracy of your gauge will also
determine what your full vacuum reading is. The gauge you have on
your Vic 9 is not a real good one and so if you were to put a better
more accurate gauge on you will find a different reading closer to
24 to 25 in Hg.

BTW, your vacuum gauge is reading in inches of mercury not pressure.

Since you are pulling a full vacuum that is not an issue. The issues
would be how fast are you investing? How thick is your investment?
Are you using de-bubblizer? How fast does the vacuum pump pull a
full vacuum?

Proper investing is really not that hard. Getting air bubbles on
your waxes means either the wax is holding on to trapped air because
of surface tension or your investment is solidifying before the
final vacuuming is complete.

First try a de-bublizer. if that doesn’t work then try a thinner
investment mixing.

Remember, only change one thing at a time or you won’t ever learn
what has caused a change in the outcome.

Also if you don’t do a process often be sure to keep a journal and
write down every variable you can so you can refer to your notes
later when there is another problem.

There are many factors involved in proper investing as well as every
step in jewelry manufacturing. For investing these are some factors
that can have an effect on the outcome.

Air temp., Humidity, Water temp., Water quality, investment temp.,
investment age, how investment is stored, mixing techniques, mixing
conditions, pouring technique, vacuuming times.

If your pump is taking more than 30 - 45 seconds to pull a full
vacuum you need to either clean the lines, filters and the pump by
flushing it and/or a complete disassembly and cleaning. Or, the pump
may be under sized for the volume of air being removed.

Your Vic 9 should have a 3 cfm pump which should work fine if it is
in proper working order but a 5 cfm pump would pull faster. Beware
of increasing the pump size just to pull a faster vacuum. You can
boil the water out of the investment causing it to cure very
quickly.

If you have any questions please feel free to email or call me.

Ken Kotoski
MPG Repair
www.mpgrepair.com
1-877-262-2185


#9

Hi Nicholas

     We have also been told that when we are vacuming the air
bubbles out of our flasks that because we are at a high
altitude(6,984ft) that our vacum can not physically pull enough
pressure. 

My suggestion is this.

  1. check the age of your investment… if it is very old and was not
    stored correctly ( sealed) that could be part of the problem.

  2. contact the investment manufacturer for the correct water ratio/
    temperature for the altitude that you are at. They should be able to
    give you the correct info .

3)Make sure your pump and table is in good shape…oil change/
filters cleaned lines not clogged.

  1. use a wax wash or wetting agent to dip the entire tree in . this
    will help reduce surface tension on the wax and release bubbles
    easier. they make some that are alcohol based… I prefer these as
    they dry quickly .

If all of this is correct, then I suggest you tap the investment
spring mounted table with your hands as you are investing… this
will help some bubbles rise to the top. In the final minute , tap on
it a lot more … this will go a long way to removing residual
bubbles from your waxes. We have an air operated vibrator attached
to the bottom plate of our investing table … this has reduced
bubbles to just about “0”.

We have also seen in the past a situation where a cast bubble is
removed from the casting and you have a small pit in that area…
what this means is that the bubble was almost released from the
wax… so the connection is not strong… this causes a tiny amount
of investment to break away when you are pouring your metal… this
would be a similar situation if you had a wax that was almost
touching another wax… the investment can break a little in this
case and you will have a few problems.

95 % of any bubble problem can be eliminated by using a wax wash and
vibrating the table by tapping or with a mechanical vibration
system.This , also goes without saying that all the other above
mentioned suggestions have been checked and the water/powder/
temperature and timing ratios are what they should be.

The Investment ratio chart you have received with your investment
was made assuming that you are at sea level or slightly above that
elevation.

You can easily add more water and increase your water temperature
by 10+ 0F and you will notice the investment will bubble better .
you may have to go 20 oF . Hotter water will cause your investment to
solidify faster , so you need to watch and time the situation
correctly.

Hope this is helpful.

Daniel Grandi Racecar Jewelry Co. Inc.

We do casting/ finishing and a whole lot more for designers,
jewelers, stores and people in the trade.Contact:
sales@racecarjewelry.com


#10

Vacuum is the absence of air. For our purposes it is measured in
inches of mercury with 30 inches of mercury being an absolute vacuum
at SEA LEVEL. This means that 14psi which is the air pressure at sea
level will raise a column of mercury 30 inches up a tube with vacuum
at the top of the tube and a reservoir of mercury at the bottom. The
higher you go the less air pressure there is therefore the shorter a
column of mercury you can force up the tube. What this means as to
you as a caster is that you can’t get as much expansion of the air
trapped in your investment slurry and the bubbles are less likely to
detach themselves from the wax so you get little metal spheres all
over your work. To prevent this you need to make the investment
thinner ( more fluid) so that the bubbles are freer to move in the
slurry and less likely to stick to the wax. The easiest way to do
this is to use hotter water to mix your investment with. If you use
110 degree F water you can make a very thin investment but your
setup time is greatly reduced. It will be about 4 min from adding
water to the gloss off time. This means you have to work fast and
can’t mix large batches because it will set up before you can pour
it. You can extend the set up time by adding citric acid to the
water before you add it to the investment powder. How much citric
acid to add is dependant on your investment and the water
temperature try 1% by weight as a starting point. You should be able
to extend the setup time so that you still have plenty of time to do
your investing.

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


#11

Altitude is a serious problem in this regard. Those who tell you
otherwi se fail to understand the physics of the process.

The vacuum works by enlarging air bubbles which adhere to your wax
model so that they become large enough to detach from the model and
rise to the surface. Thus the important factor is the difference
between your starti ng pressure and the pressure your vacuum pump
can pull. Since at high altitude you are starting at a reduced
pressure, this pressure difference will be less.

You can use vibration, if you are not already, and painting your
models with a dilute soap solution may aid in bubble detachment.
Good luck.

Richard Bynum
Oakland, CA


#12

Hi Nicholas, Bubbles on you casting is normally caused by bubbles of
trapped air sticking to the wax during investing. The are usually
very easy to break off. This is what may cause your pitting. When
a bubble is broken of the casting it will take a little of the
casting with it. Try abrading the bubbles off. Be careful to not to
thin your investment too much. Too thin investment will cause very
thin ragged lines to form on your casting. They look like tracks
that bubbles made on the wax as the boiled to the top of the
investment. Vibrating your invested flask may help. Mixing and
vacuuming time are critical. Do not exceed the recommended time for
setup. Good luck Lee


#13

Hi All Increase the water temp and cut down on the mix time Regards
David Sheard England


#14
   Altitude is a serious problem in this regard.  Those who tell
you otherwi se fail to understand the physics of the process. The
vacuum works by enlarging air bubbles which adhere to your wax
model so that they become large enough to detach from the model
and rise to the surface.  Thus the important factor is the
difference between your starti ng pressure and the pressure your
vacuum pump can pull.  Since at high altitude you are starting at a
reduced pressure, this pressure difference will be less. 

I beg to differ. I do, in fact, understand the physics, and agree
with what you say, but not the seriousness you ascribe to this
aspect. Yes, at the reduced pressure of a higher altitutude, the
bubbles can’t expand as much, but the difference should be small, if
the pump is working well. The starting pressure difference differs
from seal level by less than 20 percent, if the pump guage shows an
indicated vacuum of 24 inches of mercury. . The amount of expansion
that bubbles can experience going from start pressure to almost zero
is many times their diameter. So while a few of the smallest bubbles
won’t expand enough to be dislodged, these should likely still be
extremely tiny, since a bubble would have to be very small indeed for
it not to still expand enough with the 80+ percent of sea level
pressure differential that is still experienced at the altitude
described.

And equally important, the agitation of the investment caused by
boiling is still just as effective at altitude, and in my experience,
may be more important than the absolute expansion of the bubbles. I
say this because there is a very marked difference in bubble
elimination between a pump that only barely is able to boil the mix,
and one which does so vigorously. Between the two there may be only a
half inch of mercury difference on the vacuum guage, but the
difference in effectiveness is dramatic.

And offsetting this in favor of altitude, is simply that a pump
doesn’t have to work as hard to achieve low pressures when the normal
pressure isn’t as high, so a smaller pump might then boil the mix
better and quicker, and achieve a slightly higher level of actual
vacuum than it would have done at sea level. I’d expect, actually,
that if the starting pressure is 20 percent less, then the ending
level of vacuum would also be 20 percent better for the same pump,
and if so, then the degree of bubble expansion a give pump could
achieve would be the same at either altitude. Remember that of you
cut the pressure in half from 28 inches of vacuum to what, 29 or
so(?)… then, the degree of expansion of the bubble caused by that
drop will be the same as that caused by a drop from sea level to 15
inches of mercury on the guage, also about a reduction by half in the
pressure. The upshot of that is that I believe it would take a
smaller pump to be effective at higher altitude, since the smaller
pump could still achieve the same pressure differential, if measured
in percentage of pressure reduction rather than raw inches of mercury
on the dial, and that since the vapor pressure of water hasn’t
changed, the smaller pump will be better able to more vigorously boil
and agitate the mix.

Am I missing something here?

Peter


#15

G’day; Just a brief simple suggestion if you have investment
bubbles. Why not try using boiled water for the investment. Water
always dissolves some air, but boiling removes it. So, boil the
water, and let it cool with the lid on the kettle. Haven’t you noticed
how a glass of tap water left standing soon has bubbles on the inside
of the glass? That is dissolved air coming out of solution. – Cheers
for now,

John Burgess; @John_Burgess2 of Mapua, Nelson NZ


#16

G’day; Just a brief simple suggestion if you have investment
bubbles. Why not try using boiled water for the investment. Water
always dissolves some air, but boiling removes it. So, boil the
water, and let it cool with the lid on the kettle. Haven’t you noticed
how a glass of tap water left standing soon has bubbles on the inside
of the glass? That is dissolved air coming out of solution. – Cheers
for now,

John Burgess; @John_Burgess2 of Mapua, Nelson NZ


#17

A very interesting thread developing on the altitude/investment
problem.

Peter, you are quite right. My reply was too hasty. In addition to
the points you quite rightly bring up, the difference pulled by a
given pump (at least, expressed as a percent) would probably be the
same at various altitudes.

Very good thinking (as always!) by John Burgess: bubbles not being
left on the mold but forming there, as in a warming glass of cold
water–which reflects on the suggestion to use a warmer investment,
not to decrease the setup time, but to decrease bubble formation.
Bubbles don’t form in a glass of hot water, do they?


#18

A very interesting thread is developing on the altitude/investment
question.

First, Peter, you are absolutely correct. I was wrong, I spoke too
hastily. As you rightly point out, the starting pressure difference
is n ot a signifant percentage of the difference developed by the
pump. Not only that, but the pump difference (as a percentage of
the difference between starting pressure and zero) would be constant
for a given pump, wouldn’t it?

Brilliant thinking (as always!) by John Burgess. What if the
bubbles are not retained on the model, but form there, like bubbles
forming in a glass of cold water? And that, in addition, casts a new
light on David’s suggestion of increasing the temperature of the
investment mixture. This might work not because of decreasing the
setup time, but by decreasing formation of air bubbles (bubbles do
not form in a glass of hot water, do they?).

Actually, my experience with casting is many years old. I gave up
lost wax when I went into art. :slight_smile: (Now I’m really in trouble).

Richard Bynum
Oakland, California


#19

The metal bubbles on your casting are probably the result of gas
bubbles produced by mixing the investment, dissolved gases in the
water used or gases entrained in the slurry making the mold, and
poor wax preparation.

Castings are done above 8000 ft successfully. There is something
wrong with your procedures. The metal balls with pits underneath is
suggestive of a wax origin problem . In larger scale castings the
metal balls are caused largely by poor wax surface treatment which
is eliminated by having a clean wax surface and the use of a
debubleizer – wetting agent. If there is water in the wax (
shouldn’t be in jewelry scale ) you do get something that is
closer to the metal ball and pit situation. The wetting agents stop
the balls but not the pits. This originates in home reclaimed or
blended wax.

When you pump on a system the pump is removing condensible vapor
which includes water. As long as there is any liquid water in the
system the system pressure is limited by water vapor pressure. The
pressures achieved in the usual vacuum pump do not approach the
boiling point of water so you will never see actual water vapor
bubbles. They are air related. The slurry is viscous so
bubbles move slowly . You will not correct major process errors
with a vacuum.

In practice it might be better to pressurize , not vacuum, the mold
as it cures as is done sometimes in polymer mold making.

Jesse


#20
     The upshot of that is that I believe it would take a smaller
pump to be effective at higher altitude, since the smaller pump
could still achieve the same pressure differential, if measured in
percentage of pressure reduction rather than raw inches of mercury
on the dial, and that since the vapor pressure of water hasn't
changed, the smaller pump will be better able to more vigorously
boil and agitate the mix. 

Apparently I was typing too fast, here. A smaller pump, I believe,
might well be able to boil water effectively at higher elevations as
well as would the required larger pump at lower elevations. That
part of my statement is OK, I think, since the smaller pump would not
have so much work to do to achieve the vapor pressure of water (which
is the same at either lower altitude or higher). But obviously, a
smaller pump won’t achieve the same pressure differential as the
larger pump in the same time frame, if at all. What I wonder is
whether this would matter. In my experience, once the water is being
boiled well, and within a decent time frame, the investment should be
reasonably bubble free, if other factors are also done right. If
this is so, then perhaps at higher altitudes, one might get a good
boiling mix in the right amount of time, with a smaller vac pump than
would be needed at sea level. At least that’s how it seems to me…

Peter