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Understanding Casting Shrinkage


#1

I’ve never been able to get my mind around what happens when casting
a wax with a bezel for a faceted stone. If I carve a snug fit for the
stone, then I do have to clean up the casting with a burr to allow
the stone to fit, but is this mainly because of “shrinkage” or
because the wax has give (elasticity) that the metal does not?

I’m not talking here about the shrinkage due to making a mold of the
wax. I understand that there are many factors that can contribute to
shrinkage in that case. This is just about wax to investement to
burnout to cast.

The problem is that if I think of the bezel setting as a glorified
donut, the shrinkage does not make any sense.

A reduction of surface area in a donut should result in a decreased
outer diameter but a bigger hole, not smaller!

The only thing that I can think of is that the investement shrinks,
thus the donut grows in all directions. Outer diameter would get
larger and the hole smaller. But wasn’t there once a thread about the
concerns of investement expansion?

I’ve always been vaguely troubled by this, is there a piece I am
missing?

Ben Steiger


#2

What you lack, Ben, like so many of us, is an advanced degree in
thermal dynamics.

Ideally speaking the shrinkage inherent in a wax model will be equal
in all directions for all thicknesses. So, like the universe
shrinking back towards the point of the big bang at some point in the
far and distant future, so do the volumetric units in your mold
shrink as cooling begins. Again, in a perfect universe, all negative
spaces (bezel and ring interiors) and positive elements (shanks and
all solid parts) will be reduced by the same factor. That factor will
depend on the material used for the investment plus its temperature
at the moment of casting.

In the real universe, of course, in most kilns there is a
temperature gradient in the invested flask decreasing from top to
bottom. By the time you’ve removed the flask and placed it in or on a
casting machine significant cooling in parts of the mold cavity
closest to the two ends or the metal wall will have cooled even more
significantly.

When the metal is thrown (which if torch heated and according to a
metalurgist I used to know) will itself be a mix of temperatures
(top of metal pool being hotter than bottom for the most part) all
swirling into the mold hitting different sections of differing
temperatures. If not torch heated there is still a major change in
entry temperatures just from the short trip through the atmosphere.

So, really, if you consider it carefully according to science
casting is virtually impossible. It is only by the power of art and
the artist’s will that it can and does occur.

As for your predicament I would suggest the following:

Maintain uniform procedures from investing through casting. Keep
notes. Make changes singly in order to judge the effect. Do a test
flask or two with pieces of known exterior and interior diameters and
thicknesses (close to your average work item preferabbly)and find the
average factor of increase you need to incorporate in your waxes to
get the final size you need.

You can do the same thing with pieces you intend to make molds of.
Again, be consistent in materials and procedures. Pieces to be molded
will generally have a greater increase factor than waxes going
directly to finished piece.

Namaste,
Les Brown


#3

Ben,

This was a tough concept for me, too. But here’s how I understand it
(which works for me).

When you cast metal, there is a small amount of shrinkage in the
casting process (5 - 10%, depending on the metal). When you then mold
the metal and run waxes from it, that molding process introduces even
more shrinkage into the process. So your finished pieces from molding
may be anywhere from 10 - 15% smaller than the original you started
with.

On the casting side, the shrinkage occurs because the metal is very
hot (heat expands because the molecules are moving around much more
in space) when it fills the cavities in the investment. As it cools,
those molecules slow down a lot and take up less space, so it shrinks
and becomes more dense. If you were to completely air-cool a flask
then cut it in half, you’d find that the metal has pulled away from
the investment a bit, representing that shrinking. As most of us
quench flasks hot (unless doing stone-in-place casting), we never see
that pulling away action.

For bezels, I’ve found that it generally works (for decent-sized
stones, at least) to wrap the diameter of the stone about 2 times
around with masking tape, then using that to carve the bezel. It’s
usually pretty much right on the money for silver, although I’ve had
to clean it up a tiny bit with burs on occasion.

Make sense?

Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry


#4
I've always been vaguely troubled by this, is there a piece I am
missing? 

The whole object shrinks in size due to the contraction of the metal
from liquid to solid state. There are many factors that influence
shrinkage and it is horridly complicated to model. It is not
uniform, thick areas shrink more than thin, and the speed of cooling,
and the amount of feed from still liquid areas and on and on. So the
simplest thing is to think of your whole piece being 1-2% smaller
from wax to metal and you will be close.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#5

Any time-- with the exception of water, of course-- a solid cools
from a liquid, there is shrinkage. (remember Seinfeld? :slight_smile: At any
rate, as others have stated, molecules are less excited and so less
"active" or energized and so quieter and more closely packed.

As far as bezels go, I don’t try to anticipate shrinkage too much.
There are so many variables such as flask temp, melt temp, mold
chamber volumes and masses. My thought is that I make it to size and,
knowing there will inevitably be some cleanup, I figure that the
removal of material during the clean up will account for the loss of
space due to shrinkage. Same holds true for rings, etc. I just make
sure that the bezel, ring-- whatever-- has enough mass when cast to
anticipate removal of material during clean up.

As an aside, when I used to make crowns and bridges we were quite
concerned about shrinkage, since the interior side of the crown-- the
"core" that mated with the tooth stump or prep–was custom fit to the
prep’s architecture (left by the dentist and his burs). Getting
inside this area and grinding with burs to fit the prep would
compromise the fit of the crown…

The crowns were built on plaster (dental “stone”) models cast from
impressions taken by the dentist chair-side. The plaster “prep” was
coated with a paint that stood in as a spacer in anticipation of the
crown cement which would eventually hold the crown in place.

Before investing, the flask (or “ring” as it was known) was lined
with a ceramic fiber sheet (kaolin) about 1.5mm thick or so. This
sheet traditionally was asbestos…This liner was there as a
cushion, to allow the expansion of the mold and so the mold chamber.

Investment is made with chrsitobalite quartz. As the burnout process
progresses, this material changes at just below 1100F-- a “quartz
inversion”. The particles of christobalite actually expand. So the
entire block of investment expands. It expands out the ends. It
expands in 3 dimensions. Since it cannot expand into the steel of the
flask, it expands into the mold chamber, effectively closing it in or
shrinking it. (This is independent of metal shrinkage.) Unless, of
course, there is a liner or cushion between plaster and steel,
absorbing a bunch of this expansion. So much so, in fact, that it
actually allowed for expansion of the mold chamber, since the
expansion of the christobalite crystals was shunted into the liner
and out the ends. This expansion displaced the inherent shrinkage of
the metal from liquid to solid allowing for a non shrunk casting.
Crowns just popped right on the models (and the patient) with only a
simple sand blast to remove plaster and oxides.

At least this is how I understood it from manuals and books.

Andy


#6
If you were to completely air-cool a flask then cut it in half,
you'd find that the metal has pulled away from the investment a
bit, representing that shrinking. 

This is how I have always pictured the process of shrinking, but
this model has problems when considering interior cavities. In a
casting of a simple ring shape, the volume in which the finger will
eventually go is filled with investement also. So if the metal pulls
away from all investement (granted to varying amounts due to the
complexities of cross section, cooling rate, etc), the result should
be a thinner walled ring with a slightly larger ring size.

But as we know, this is not what actually happens. The ring size is
slightly smaller, not larger. Thus we can clean up and polish the
interior curve of the ring and still end up at the originally
desired size.

Even stranger, this 1-2 percent of shrinkage in ring size means the
investment that occupied the center cavity was somehow displaced.
How does that happen? Does the investement shrink on it own, is it
being compressed and squeezed somehow?

Contraction of the metal itself due to cooling just does not explain
all this. As Les mentioned, it is astounding that this process works
at all, and with the accuracy it has. But it still seems that in the
ideal case negative spaces should end up larger, not smaller as they
do in reality.

One last question: if there was some kind of perfect investement
material that did not shrink or expand, had no elasticity at all, do
you think a casting of a ring could still result in a slightly
smaller ring size, or an uncompensated snug carving for a stone still
need metal removed for the stone to fit? If yes, how?

Ben Steiger


#7
The problem is that if I think of the bezel setting as a glorified
donut, the shrinkage does not make any sense. A reduction of
surface area in a donut should result in a decreased outer diameter
but a bigger hole, not smaller! I've always been vaguely troubled
by this, is there a piece I am missing? 

Actually, the whole thing shrinks, the hole as well as the outside.
Barring irregular shrinkage, which can lead to distortion and
porosity, the cast shape should be an exact replica of the wax shape
(hole and all), just a few percent smaller. It’s metal shrinkage,
not investment, although there are dental investments which expand
slightly to yield a casting that’s the same size as the wax. Think
of it this way, Ben. If the hole really got bigger and the outside
smaller, then it wouldn’t take too much shrinkage for the “donut” to
disappear altogether, right? To compensate for shrinkage when setting
a stone in wax, using ordinary investment, I’ve had good luck using
a wrap of drafting tape (that’s like masking tape, but in a narrow
width) around the stone; it seems to compensate for about as much
shrinkage as I get on a medium-sized stone. Small ones will need a
little grinding; large ones get an extra wrap of tape.

Andrew Werby
www.computersculpture.com


#8

Hey Andy,

This expansion displaced the inherent shrinkage of the metal from
liquid to solid allowing for a non shrunk casting. Crowns just
popped right on the models (and the patient) with only a simple sand
blast to remove plaster and oxides. 

The only thing you left out is that the investment used by dental
techs is specially formulated and tested to control its expansion to
a precise percentage at a precise temperature and the dental alloys
are carefully engineered to have just the right amount of shrinkage
to match the investments properties.

So you will never get these kind of results with jewelers investment
or alloys.

Regards,
Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#9
This is how I have always pictured the process of shrinking, but
this model has problems when considering interior cavities. In a
casting of a simple ring shape, the volume in which the finger will
eventually go is filled with investement also. So if the metal
pulls away from all investement (granted to varying amounts due to
the complexities of cross section, cooling rate, etc), the result
should be a thinner walled ring with a slightly larger ring size. 

No you are trying to shrink each dimension individually. Think like
it is a photograph or a computer model and you reduce it either
optically or mathematically, the whole thing gets smaller.

Even stranger, this 1-2 percent of shrinkage in ring size means the
investment that occupied the center cavity was somehow displaced.
How does that happen? Does the investement shrink on it own, is it
being compressed and squeezed somehow? 

Yes the investment is porous and soft and the force from the
shrinking metal compresses it. If it did not compress the metal
would crack! The force of thermal expansion or contraction is huge.
Something will give the question is what, most mold material is much
softer than the metal so it gives as the metal contracts. Die
castings and permanent molds are a whole different topic but you can
rest assured that they take into consideration this shrinkage and
allow for it with their design or the mold would be one time use
only

One last question: if there was some kind of perfect investement
material that did not shrink or expand, had no elasticity at all,
do you think a casting of a ring could still result in a slightly
smaller ring size, or an uncompensated snug carving for a stone
still need metal removed for the stone to fit? If yes, how? 

The model would come out in pieces as it would tear itself apart as
it tried to contract and the mold did not allow it to do so.

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#10

Yes Jim,

You are absolutely right. Dental investments are more specifically
engineered. Some, such as Beauty Cast contain-- or at least
contained-- graphite as a reducer. We mixed these investments much
more precisely as well.

The alloys themselves were manufactured to produce micro fine
chrystal structures on cooling-- making for dense castings. The mouth
is a very challenging environment.

My point in offering the example is that some of that same stuff is
occurring when we cast jewelry. Kerr, the dental manufacture, also
produces Satin Cast which was and is a very popular jewelry
investment. This investment contains as a main ingredient
cristobalite, like the dental formulas. Though its aim is not
necessarily reduced shrinkage, there may be a gain from the quartz
inversion should one choose to maximize this possibility by lining a
small flask. I have no idea to what degree a large or treed flask
might benefit.

When I left the dental lab years ago some company had developed and
was marketing a flask-less casting system. You poured investment into
a plastic flask which was then removed after the investment set. This
left a chunk of plaster which was unrestricted by a steel ring.

I should also add that I am talking about full metal crowns, onlays
or inlays. Not porcelain copings, etc. I am also talking about non
phosphate bonded investments of which I’m not sure of the
constituents.

Take care,

Andy


#11

Jim,

Your post and others have convinced me that my model for
understanding shrinkage is wrong. I was imagining the donut as being
made of a bunch of disks strung along a hoola-hoop. A change in
surface area would be equivalent to changing out the disks with those
of a smaller diameter.

But that doesn’t take into account surface tension. A better
understanding would be to think of the torroid as an inflated truck
inner tube. Letting out the air would decrease the thickness of the
tube, but surface tension of the shape would also want to decrease
the size of the hole in the middle.

Yes the investment is porous and soft and the force from the
shrinking metal compresses it. If it did not compress the metal
would crack! The force of thermal expansion or contraction is
huge. Something will give the question is what, most mold material
is much softer than the metal so it gives as the metal contracts.
Die castings and permanent molds are a whole different topic but
you can rest assured that they take into consideration this
shrinkage and allow for it with their design or the mold would be
one time use only 

So if I were to prop up a steel ring madrel in a sandbox with a
depression setup to make a 1-2mm thick ring around the mandrel, would
the resulting pour crack?

Ben


#12

Hi Ben,

if you have a collar or a bearing stuck on a shaft, to get it off you
heat the collar whilst keeping the shaft cool. It then expands,
including the hole, and comes off.

If you are starting with a hot shaft ( which is effectively what you
have with a hot mould) it will cool and shrink as the mould cools.
There is also transfer of heat from your cast metal to the mould,
which if the core is small will heat up considerably before the metal
freezes.

If the core is large and very enclosed as in a hollow sculptural
figure, the hot air which needs to escape will sometimes bubble out
through the still liquid sprue, unless you have provided vents.

I would say that if it does not crack,it will at least be very
firmly stuck around the rod.

There are so may possible variables which come into play, to a
greater or lesser extent, after a time you just have to make a guess
based on past experience,( or luck), this is why some people think of
casting as a black art.

regards Tim Blades.


#13
The alloys themselves were manufactured to produce micro fine
chrystal structures on cooling-- making for dense castings. The
mouth is a very challenging environment. 

Yes dental alloys are highly engineered materials that have many
requirements to deal with the corrosive, abrasive working conditions
in the mouth. Being the recipient of two crowns I was fascinated by
the precision of the fit and the whole process used to create them.
My dentist used to do his own lab work so we had several
conversations about the materials and processes. Or more precisely
he talked, I could not say much with all that stuff in my mouth :slight_smile:

My point in offering the example is that some of that same stuff is
occurring when we cast jewelry. Kerr, the dental manufacture, also
produces Satin Cast which was and is a very popular jewelry
investment. This investment contains as a main ingredient
cristobalite, like the dental formulas. Though its aim is not
necessarily reduced shrinkage, there may be a gain from the quartz
inversion should one choose to maximize this possibility by lining
a small flask. I have no idea to what degree a large or treed flask
might benefit. 

The same changes do occur but the since the investment is not sold
for this kind of purpose the manufacturers don’t necessarily use the
expansion characteristics as a quality control factor like they
would for a dental investment.

When I left the dental lab years ago some company had developed and
was marketing a flask-less casting system. You poured investment
into a plastic flask which was then removed after the investment
set. This left a chunk of plaster which was unrestricted by a steel
ring. 

I have seen this used for platinum jewelry casting. The new rapid
set platinum investments are dental investment repackaged for
jewelers.

Regards

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#14

Your analogy of the tire is a good one I think.

So if I were to prop up a steel ring madrel in a sandbox with a
depression setup to make a 1-2mm thick ring around the mandrel,
would the resulting pour crack? 

With a tapered ring mandrel maybe not, as the ring would move down
the taper as it cooled, with a straight rod it is highly likely it
would crack. If not you would have a hell of a time removing it from
the rod as it would be shrunk so tightly on the rod it would be like
you had soldered it on.

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#15

Here’s another two cents.

From 1980 to 1985 I owned and operated a contract casting house for
several manufacturers in town here and out of state. I still love to
cast and find the most satisfaction in manufacturing my own line of
jewelry as well as teaching various jewelry techniques here in
Minneapolis.

This involved casting several hundred of the exact same ring time
and again as well as one of a king pieces. Needless to say casting
everyday required some consistency as well as strategy. Some of the
conclusions I formed and still maintain are heRe:

  1. INVESTMENT Be consistent. Do everything the same every time. A
    change from outside sources can really screw things up. This involves
    measuring and timing everything the same (don’t ever guess).
    Investing is one area that needs extreme consistency. I keep my
    investment in a sealed cement mixer to keep it mixed and uniform. A
    100 lb barrel of investment starts to separate traveling from the
    truck to any distributor. The worst casting you will ever have are
    from an old batch of investment that as sat for longer than three or
    four months. It feels thicker when you scoop it out. As an apprentice
    I used to have to put a mask on and with my bare hand reach in and
    stir the unused investment. I could feel the clumps of investment.
    Many casters will also flip the barrels every month to allow gravity
    to start the process the other way. The dental industry has solved
    this by packing the investment in small pouches.

  2. BURNOUT Take your time. I like to burn our overnight. I don’t
    care for the programmable rheostats. I just set my oven on the number
    of the dial that will eventually get to 1350 F. This is a fine slow
    ascent to the peak temperature. The larger the load the longer it
    takes, but overnight it works well with any size oven load. I don’t
    trust the programmable rheostats to go up and stop at the right
    temperature and then go down to the first flask casting temperature.
    In the morning I come in and just turn the dial down to start the
    ramp down to the various flask casting temperatures I use. I prefer
    electric to gas ovens. The city gas we are provided is not consistent
    enough. This is seen in the various color flames coming out of the
    oven hearth. Natural gas works great for bench work and building
    furnaces but not a casting oven on a cold winter day. As temperature
    starts to fall the small delicate pieces are cast first and the heavy
    more solid ones are cast last and much colder flask temperatures.

  3. SPIN VS VACUUM You will find success with both, but I prefer to
    spin smaller flasks, white gold especially. At about 100 dwt I start
    to consider vacuum casting for safety sake. I have been burned across
    my chest by molten spinning metal and it is not much fun. The max I
    vacuum cast in one pour is 250 dwt. I would never spin that much.
    Vacuum casting takes more heat because of the lack of thrust and the
    white gold metals chill quicker. Even the casters who use the fancy
    machines have a tougher time with the white gold, but the gold
    suppliers are developing allow that will compensate for that. The
    quench debate is a no brainer. It is safer to quench and leaves a
    nicely annealed casting. If you leave the flask until cold (like the
    stone in casters must) your venting system should handle this fine
    dust that may become air born. Here are my guidelines. Spin casting
    is to wait ten minutes for all castings to quench. Vacuum casting
    because of the higher flask temperatures is twenty minutes. Both of
    these times work well for all most metals gold and silver. Now here
    is another important technique. When you quench have a deep enough
    bucket of water so that when you plunge the flask into the water you
    go DEEP enough not to see any vapors or bubbles come out of the
    water. All the bad particles should stay under water, trapped and
    harmless to your lungs this way. The top water can go down the drain,
    but have an extra bucket or two. When the investment builds up in the
    quench bucket pour the top water off and let it dry. Most land fill
    accept this spent material this way, not a wet or mud like slurry. I
    usually line my buckets with a trash bag so when the time comes to
    dump the stuff it just drops out and goes to the trash. This is the
    EPA accepted process we use at the Minneapolis Community & Technical
    College.

One additional note. The one firm I cast the hundreds of the same
style complained once about the weight of the ring changing. The only
thing I could figure out that I did different is that I cast one
batch vacuum and another spin cast. They weight every ring to the
hundredth dwt. The vacuum cast rings were slightly heavier. Not much,
but the more dense casting produced by vacuum casting show up in this
ring.

Lots more, but that’s all for now. I do have to go cast,

Best Regards,
Todd Hawkinson


#16

Ben,

First of all, outside of jewellery, this is a huge subject that
cannot be answered in one e-mail. However I will try to point out
some things out that may interest you.

Your post and others have convinced me that my model for
understanding shrinkage is wrong". 

I don’t agree.

You are all correct, just correct at different points and under
different circumstances. All of the responses have provided you with
what I consider to be the basics, and the basics being what we all
did in science class with expansion and contraction experiments with
the ring and ball apparatus and a bunsen burner. Very easy and very
predictable for a wedding band. More complex shapes become a little
more interesting.

When I say at different points, everyone else is looking at the
shrinkage factor from a solid perspective and as a finished casting,
and whether you relies it or not, you are looking actually and
thinking more like a foundry man than a jeweler, which is a good
thing. As jewelers or jewellery casters, what do we pay attention
to?, other than a few criteria’s relating to proper investment and
spruing techniques, flask and metal temperatures and burn out cycle
times, clean metal etc, pretty much nothing else of significance
seems to matter. A foundry man is a totally different science and
truly an art in itself. The pattern will come with shrink allowances
built in, much the same way a caster will receive a wax lets say. The
jewellery caster will cast without any further consideration to
shrinkage control what so ever. The foundry man on the other hand
will very carefully design a full flow system to efficiently deliver
that material, especially in precision casting. He will have gates,
sprues with multiple runners all connecting to risers. Not so easy
to now maintain everything to spec. The foundry man/engineer is
responsible to maintain the shrinkage factor within the toleranced
specifications and fully specified in the engineering drawings. The
jewellery caster has no such responsibility, and the attitude is, it
is what it is and shrinkage is to be expected. Yes shrinkage is to be
expected, however there are ways to influence shrinkage, and other
industries use it to their advantage.

The flow of material will initially hit all surfaces and for
arguments sake, create a layer/crust of material against all the
surfaces and then molten metal will flow through the casting much
like a lava tube. Here is where you will get your thoughts played out.
The ID and OD surfaces will move away from the mold surface creating
what is known as an air gap between the casting and the mold and it
happens due to volumetric shrinkage. The easiest way to explain it,
is think of a sink in a particular surface of a casting. It created a
sink hole in the surface, because of the lack of material, hence
volumetric shrinkage. At this point and in relation to the mold, the
OD is smaller than the mold and the ID is larger than the mold core,
and therefore is also thinner in relation to the mold pattern. When
the whole casting as a solid cools, the whole solid will of course
become smaller. It is at this point of pre-solidification, that it is
critical to maintain a good flow of material through the casting to
not only avoid further volumetric shrinkage, but to just as
importantly maintain consistent heat distribution throughout the
material to avoid hot or chill spots. The foundry man, will rely
heavily on risers, especially on large and complex castings.These
risers are designed to solidify after the main casting therefore
allowing additional material to feed the casting for a denser and
fully filled final product. To collaborate the volumetric shrinkage
point, and that the cast surface separates from the mold, observe the
direction that the sinking/shrink happens, the shrinkage moved or
shrank towards the core of the material and not towards the center
point of the object. An approximate calculation for volumetric
shrinkage is around 1% for every 210 degrees (F) depending on the
metal. It can be more or less.

On another tangent, Draft angles in die cast molds and even plastic
injection molds do more than just aid in release. They also play a
role in many instances of influencing shrinkage. Because the thinnest
area of a drafted surface will cool faster than the heavy base it
creates what is called directional shrinkage. Many features in a mold
create directional shrinkage in a controlled manner to distort the
piece in a way to come as close to the desired end product as
possible. When everything is ideal, and the part is uniform and
simple, well it’s not a big deal, but when dealing with thick and
thin sections, how that part solidifies and cools is critical. A
casting engineer with good flow analyses can add or delete features
to push pull and distort sections and somewhat control how that end
product ends up, and therefore using shrinkage to his advantage.
Depending on the size of the ID and its relation to the OD and flow
length which is rated at twice the section thickness from points of
entry, the size and shape of that hole within reason can be
manipulated by calculating the area of cooling surface at that feature
etc.

As I said before, this is a huge subject with a lot of complexities,
at least at the industrial level. Items many times will have ribs in
place and are integral features that impart strength into machined
parts with thin and hollow sections, but are also designed at the
same time, to be conducive for casting. Ribs can have straight walls,
they can have a draft to enhance directional shrinkage, the point
being is at the machining level they have a purpose, and at the
casting level, additional purposes come into play. Same thing with
Radii that are machined at the intersection of adjoining faces, they
help to reduce the possibility of shearing in parts. Add casting into
the mix and they help reduce turbulence and sharp changes in direction
in casting. The direction of features many times and even their
position can alter a casting significantly. A straight flow direction
of material travel will have totally different effects on the part
than a cross flow metal delivery system, and it can become quite a
task to calculate.

Anyway, enough for now.

Best Regards.
Neil George
954-572-5829


#17
The foundry man/engineer is responsible to maintain the shrinkage
factor within the toleranced specifications and fully specified in
the engineering drawings 

Very nice bit, there, Neil - a little too condensed for some maybe.
Yes, when a jeweler casts something that’s 1mm, 10% shrinkage is
only.1mm. If the part is a foot deep, 10% is more than an inch -
very significant. The US Navy foundry manual reprint here:
http://www.lindsaybks.com/bks/foundry is an excellent start for the
concepts you discussed, too…

http://www.donivanandmaggiora.com


#18
There is really no chance for the material to vaporize. The flask
goes the eight or so inches immediately, rumbles to a stop and is
then removed and cleaned. I've never really seen anything rise to
breath with this procedure. 

This is an unfortunate example of what I call hookie-boo. It
reflects lack of understanding and is exactly how inaccurate and
sometimes dangerous gets into the hands of newly trained
jewelers, who rely heavily on the words of wisdom of the old-time
"expert" jewelers.

Jewelers, please be very careful with dispensing advice relating to
technical materials. This is not the realm of the jeweler. It is the
realm of trained engineers at manufacturing companies, who would be
stunned by the foregoing statement of “fact”.

The quench area is not a place for this kind of homily. The fact is
that it is the UNSEEN silica particles that float freely in the air
that do the most damage. It is not possible to plunge the flask so
FAST that no vapor or particles get free. The instant the flask hits
the cold water, even if at the speed of sound, miniscule silica
particles enter the air, and they do not go to ground quickly.

This is like the nugget I found in AJM that theorizes that if you
place a small flask right into an oven already at burnout
temperature, the resin model will not have time to expand before it
softens and begins to melt. Hello–it just expands faster, but it has
to expand folks. Please think about getting technical information
about materials such as investment, wax, rubber, RTV from the
manufacturer, not from another jeweler (in most cases). There are
many jewelers out there who really understand materials. Just don’t
take any chances.

Wear your breathing and eye protection.

Regards
Bill
Zero-D Products, Inc.


#19

I agree with Bill that it is the unseen particles that are the most
dangerous.

Methods of reducing the danger include having local (right at the
source, within inches of where it is being generated) ventilation,
and allowing the object to cool to room temperature and then removing
the investment. (a royal pain in the neck and time consuming).

An abatement method is to use ‘floaters’ in the quenching container.
This means you have a bunch of floating objects on the surface of the
liquid so that material vented into the air is reduced. For
electroforming you can buy floating nalgene ‘cross-like’ shapes which
overlap. The purpose there is to reduce evaporation rates.

Cheap jewelers working with quenching a flask might for instance use
a whole lot of wine corks, so many that they are about 2-3 thick on
the surface. You quench through this and the emitted particles are
reduced. So ask your friends to collect corks for you or have a
really big party…

best
Charles


#20
Cheap jewelers working with quenching a flask might for instance
use a whole lot of wine corks, so many that they are about 2-3
thick on the surface. You quench through this and the emitted
particles are reduced. So ask your friends to collect corks for you
or have a really big party. 

Or cooking oil, which floats.

I use acetone and methylene chloride soaks in my work and use water
as a “cover” liquid."

Wayne