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Button or No Button


#1

(was: [Book] New Complete Metalsmith)

    Maybe that is because you don't need a button. A button is a
total waste of metal, take a look at the physics. It does not
increase pressure on the casting and by the time the button is
filling the casting is already solidified so it does not provide
shrink filling ability. 

Aren’t we to make a feeder sprue of such a size that it solidifies
after the piece? The button, being the very last to solidify, might
then have acted as a reservoir. However if the feeder sprue is under
that size, then I guess you’re right, the button is no more than
(perhaps) a hedge against underestimating the metal quantitiy.

Brian

B r i a n A d a m
e y e g l a s s e s j e w e l l e r y
Auckland NEW ZEALAND
www.adam.co.nz


#2

Jim,

In certain instances it is not possible to prove that a button
helps, but during vacuum casting of rather large 14K or sterling
parts I have direct evidence of better surfaces and denser castings
when I have been able to immediately flux a button after the metals
pour.

For the majority of folks utilizing this text they are probably not
casting trees so they will have no central core to deliver metal
during solidification. For these types of castings providing a
reservoir of molten material will help with shrinkage.

And depending on how folks were taught to sprue they may need the
excess so that during the pour they can quickly fill the button area
which will allow any sprues coming off it to smoothly fill and not
get plugged with a splash of metal.

Sure, I know and practice reducing the metal required in a casting
to an absolute minimum but I have the opportunity to do that because
of the quantities I have typically cast, but if someone is casting a
master or a one-of-a-kind it is surely best to error on too much than
not enough.

The text on page 174 instructs you to “Add about a third more to
allow for the button”

The java routine on the CD should take this into account. It gives
no option or mentions a button.

I have spent more time with the text lately and I like the new
format, I was just annoyed by the editing.

Regards,
James McMurray
@James_McMurray
Technical Instructor
Metals Program
University of Washington


#3
        Aren't we to make a feeder sprue of such a size that it
solidifies after the piece? The button, being the very last to
solidify, might then have acted as a reservoir. However if the
feeder sprue is under that size, then I guess you're right, the
button is no more than (perhaps) a hedge against underestimating
the metal quantitiy. 

I just saw the presentation of a paper on casting at the last Santa
Fe Symposium on Jewelry Manufacturing that included a FEA (Finite
Element Analysis) simulation of the casting process. This is the
kind of analysis that is used to simulate aerodynamic performance of
new aeroplane structures and many other extremely complex fluid
dynamics problems. It requires access to a super computer and the
staff to program it which is why it is rarely used for more mundane
things like lost wax casting of jewelry :-). The simulation showed
that most of the metal in the model cavity is solid before the model
cavity is filled so the ability of a sprue or gate to “feed” a
cooling model is basically nill. This idea of gates and sprues
feeding castings as they cool comes from larger scale foundry work
where the metal quantities can be hundreds of tons and the
solidification time is many orders of magnitude greater than in
jewelry casting. The solidification times in this simulation were on
the order of milliseconds.

One of the things that is starting to be obvious in the casting
research being done on jewelry casting is that the reduction of
turbulence is very important to good castings (maybe the most
important factor?) . This is backed up by the simulation that was
presented in that if the metal is slowed down by turbulent flow it
will not be able to feed the model cavity enough metal in a timely
manner to produce a sound dense casting due to this rapid
solidification.

Jim Binnion

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


#4

Hi James

    In certain instances it is not possible to prove that a button
helps, but during vacuum casting of rather large 14K or sterling
parts I have direct evidence of better surfaces and denser
castings when I have been able to immediately flux a button after
the metals pour. 

It will depend on how large a casting you are talking about (see my
reply to Brian) but I belive that what you may seeing here is the
effect of the reduction in oxygen absorbed through the button by the
cooling metal. In covering the button with a glass seal you greatly
reduce this absorption. But it is hard to guess without seeing it.
But I will be willing to bet that if you cast it without a button
and fluxed the top of the sprue you will not see any difference in
the castings.

    For the majority of folks utilizing this text they are
probably not casting trees so they will have no central core to
deliver metal during solidification. For these types of castings
providing a reservoir of molten material will help with shrinkage. 

Doesn’t matter if there is a tree or not, most jewelry sized models
are solid before the button starts to fill

    And depending on how folks were taught to sprue they may need
the excess so that during the pour they can quickly fill the button
area which will allow any sprues coming off it to smoothly fill and
not get plugged with a splash of metal. 

Turbulence is one of the main enemies of good casting results. A
tapered cone shape pouring opening goes a long way to reducing this
kind of problem. If the metal is poured slowly and with a lot of
splashing about in the button you will tend to end up with crappy
castings. Look for ways to support the pouring crucible so that the
entry of the metal is rapid, smooth and steady.

    Sure, I know and practice reducing the metal required in a
casting to an absolute minimum but I have the opportunity to do
that because of the quantities I have typically cast, but if
someone is casting a master or a one-of-a-kind it is surely best to
error on too much than not enough. 

I understand the level of concern in casting one of a kind or master
models I am always extra cautious with them but a simple scale will
remove all ambiguities in how much metal to use. Just weigh the
model, sprue and flask base and then after investing remove and
weigh the flask base , multiply the difference by the standard
values for the specific gravity for the alloy in question. If the
person casting cannot do this simple type of process they are going
to have problems in their castings no matter how much metal they
use. I have taught casting to a wide range of folks from high school
to students my grandparents age and I have always stressed the
importance of proper weights and measures to the success of casting
but it often takes loosing a prized creation for this lesson to sink
in. I am a firm believer in the axiom that we only truly learn from
our mistakes. (and sometimes not even then )

    The text on page 174 instructs you to "Add about a third more
to allow for the button" The java routine on the CD should take this
into account.  It gives no option or mentions a button. I have spent
more time with the text lately and I like the new format, I was
just annoyed by the editing. 

I’m sorry I haven’t read Tim’s new book yet ( I will soon I promise)
I was just up on my no button casting soapbox

Regards,

Jim Binnion

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


#5

Here is my Two Pence;

I have been casting 3 x 5 inch flasks with average of 80 pieces in
them, pieces that weigh on the average of 1.3 Grams each.

My sprue main sprue weight is 90 Grams, then my feeder sprues, (from
the tree to the part) weighs average of .6 Gram the pieces are
rectangular frames with lettering (Greek) in them and are open.

Some of the sprues weigh more than the charm (finished weight) most
have as many as 3 sprues some only two, and the pieces are
approximately 16 Ga. thick and are sprued with 14 Ga. wire, tapered
right at the connection point to the piece., some which are difficult
patterns are done with 12 Ga.

I have been using the Neutec Sprue from Rio, and have noticed a
SLIGHT improvement in casting rejects, which I attribute to less
turbulence. I still work in the stone age, and on these particular
items I’ve been using my old standby Centrifugal casting machine,
(too poor to buy long 3 inch perforated flasks) ;^)

With the Neutec flasks, it is extremely easy to get an accurate
metal weight, just unscrew the tree from the base, and weigh it, As
for the button what purpose could it possibly serve. I am using 14
Ga or 12 Ga. they are sprued to a tapered sprue that goes from just
guessing but the sprue section starts at about 3/8ts of an inch and
tapers to about 1/4 inch. So adding a button of lets say 100 grams
is going to help me HOW?

Now were I using lets say a 6 Ga sprue from the part to the tree,
and the part weighed 40 grams, the main sprue / tree was still the
same size. (which would be silly anyway) and I had 10 or 12 pieces
like that so that my total weight was 4 or 5 times that of the
sprue, then I might consider adding an extra bit of button. But
setting up a flask like that would be looking for disaster any way,
the main sprue should have been larger any way.

Now having been a Gear Head in a former life (around 40 or so years
ago) or BFI Before Fuel Injection) we used to go to great lengths to
minimize turbulence when flowing a combustible fuel/air mix from the
Carburetors to the combustion chamber. Things like eliminating rough
surfaces in the flow areas, (smoother sprues) eliminating as many
sharp angles as possible (filet the area where the part comes off the
tree) putting larger valves in the cylinder heads (make your feeder
sprues at least as big or larger than the thickest place on the part
being cast).

Now I hear there is this thing called Fluid Dynamics, heck I guess
we weren’t as dumb and stupid as the old people thought we were ;^o
. Also I think flask temperature has a lot more to do with complete
fills and surface quality than adding extra metal weight to a piece,
unless you are casting in a flat base, Then we are in a hole other
game and the weight of the button or base does become relevant.

This would be when casting Conchos or large belt buckles, several at
a time which also can be done on a ball base but you can’t cast as
many at a time, so having a 200 gram button/base is a good thing,

Kenneth Ferrell
www.shadras.com


#6
   I just saw the presentation of a paper on casting at the last
Santa Fe Symposium on Jewelry Manufacturing that included a FEA
(Finite Element Analysis) simulation of the casting process. 

Hi Jim,

Thankyou for the report and the insight into cast metal
solidification.

I would however caution people not to assume that ‘feed’ in spruing
does not matter. Feed of material to the casting to my mind has to do
more with supplying material for shrinkage, and I know that shrinkage
reservoirs, thick short sprues to thicker areas, and other planning
for ‘feed’ during cooling all seem to matter when used.

The turbulence issue is an important one and there have been some
very good papers over the years on it.

best Charles

Charles Lewton-Brain/Brain Press
Box 1624, Ste M, Calgary, Alberta, T2P 2L7, Canada
Tel: 403-263-3955 Fax: 403-283-9053 Email: @Charles_Lewton-Brai1


#7

Thank you for the clarification, James McM. That post suggesting no
button would strike terror into those who don’t cast huge amounts at
a time. My students are taught to calculate for a necessary,
substantial button when we’re learning, or doing one piece at a
time, in a spin caster. It is helpful to know and understand the
different versions of New Complete Metalsmith available too. WE
would never have seen the professional edition or CD at school, and
my shelf of reference books at home runneth over. It points up the
range of casting that goes on in the world too, from one person
shops and hobbies, to the manufacturies. Isn’t it wonderful to have
such flow freely ?

Thank you all for such an enriched environment.

Pat


#8
    It will depend on how large a casting you are talking about
(see my reply to Brian) but I belive that what you may seeing here
is the effect of the reduction in oxygen absorbed through the
button by the cooling metal.  In covering the button with a glass
seal you greatly reduce this absorption.  But it is hard to guess
without seeing it. But I will be willing to bet that if you cast it
without a button and fluxed the top of the sprue you will not see
any difference in the castings. 

I don’t see how oxygen absorbed at the top of a button after casting
will affect a part way down inside the flask, particularly if, as
you say, the metal has solidified by then. Flux helps protect the
metal when it’s being melted, but in this case, what’s probably
happening is an insulation effect from the added material piled on
top. I usually cover castings with a piece of soft firebrick
immediately after casting; this helps keep the button fluid and
enhances its usefulness as a reservoir of semi-liquid metal to feed
the part.

    For the majority of folks utilizing this text they are
probably not casting trees so they will have no central core to
deliver metal during solidification. For these types of castings
providing a reservoir of molten material will help with shrinkage.
    Doesn't matter if there is a tree or not, most jewelry sized
models are solid before the button starts to fill 

“Solid” is a relative term, then. The metal inside the flask, which
is typically cast hot, is in the process of solidification, but it’s
still permeated with channels of liquid metal, like a wet sponge.
This will either draw metal from the button or sprue as it shrinks,
or it will draw it from the part. I’ve seen it happen too many times
to swallow this “no button” theory whole.

        Aren't we to make a feeder sprue of such a size that it
solidifies after the piece? The button, being the very last to
solidify, might then have acted as a reservoir. However if the
feeder sprue is under that size, then I guess you're right, the
button is no more than (perhaps) a hedge against underestimating
the metal quantitiy.
    I just saw the presentation of a paper on casting at the last
Santa Fe Symposium on Jewelry Manufacturing that included a FEA
(Finite Element Analysis) simulation of the casting process. This
is the kind of analysis that is used to simulate aerodynamic
performance of new aeroplane structures and many other extremely
complex fluid dynamics problems. It requires access to a super
computer and the staff to program it which is why it is rarely used
for more mundane things like lost wax casting of jewelry :-). The
simulation showed that most of the metal in the model cavity is
solid before the model cavity is filled so the ability of a sprue
or gate to "feed" a cooling model is basically nill. This idea of
gates and sprues feeding castings as they cool comes from larger
scale foundry work where the metal quantities can be hundreds of
tons and the solidification time is many orders of magnitude
greater than in jewelry casting. The solidification times in this
simulation were on the order of milliseconds. 

I’ve done sculpture casting as well as jewelry, and I have to say
that I’m unimpressed by this analysis, despite the hi-tech
trappings. When “most” of the metal is solid is when shrinkage
porosity does its damage, pulling metal from the surface as capillary
channels collapse. The effects are more easily appreciated in
larger-scale castings, but once one learns to recognize it, it can
easily be seen in smaller ones as well. I’ve also had the misfortune
to have castings disrupted by mishaps in a centrifugal casting
machine, and have seen that after quite a few milliseconds had gone
by, the castings were still fluid enough to be partially emptied
after they were full. When I was using an overloaded centrifuge that
slowed down after its first few spins, I’ve had the button drip out
and empty itself, with predictable bad results to the supposedly
solidified castings that remained.

One of the things that is starting to be obvious in the casting
research being done on jewelry casting is that the reduction of
turbulence is very important to good castings (maybe the most
important factor?) . This is backed up by the simulation that was
presented in that if the metal is slowed down by turbulent flow it
will not be able to feed the model cavity enough metal in a timely
manner to produce a sound dense casting due to this rapid
solidification.

I do agree that a reduction in turbulence can help, but I’ll still be
casting with enough reserve metal to accomodate shrinkage. Shrinkage
is not a myth, it’s generally accepted as fact in the casting trade,
and I’m not the only one who believes it - see
http://kerrlab.com/JewelryProducts/technical/Glossary.htm or
http://www.sternleach.com/technical/articles/silvcast.php under
"shrinkage porosity".

Andrew Werby
www.unitedartworks.com


#9

Hi Charles,

    I would however caution people not to assume that 'feed' in
spruing does not matter. Feed of material to the casting to my mind
has to do more with supplying material for shrinkage, and I know
that shrinkage reservoirs, thick short sprues to thicker areas, and
other planning for 'feed' during cooling all seem to matter when
used. 

The ideas about “feeding” from the button and shrinkage reservoirs
are all valid if your casting is behaving like larger foundry
castings this is called progressive solidification. The problem with
progressive solidification theory is that it proposes that at one
point in time the whole mold , shrinkage reservoir, sprue , button
system is filled with molten metal. The reality in most jewelry
scale castings however is that the model is solidified before the
sprue is even partly filled. The most interesting thing from the
paper at this years symposium and a paper presented a few years ago
by some folks from Stuller is how fast the metal solidifies. All of
us had thought that the here was for at least a brief period of time
a situation all the mold cavities were filled with molten metal that
then solidified from the outside in with the thinnest sections
freezing first but it appears that solidification happens much
faster than we had thought with the model being almost totally solid
before the sprue feeding it is filled so we need to rethink how we
design our metal feed systems to make sure we produce as little
turbulence (which slows down metal feed) as possible and work on
getting the metal into the mold as rapidly as possible because the
solidification times are in milliseconds.

I am not saying that the sprue designs we are using are wrong, you
still need to feed the largest section on the model with a sprue of
thicker section than the heaviest section of the model and that
should attach to a sprue or pouring cup of even greater cross
section. But the reasons for this are not to control progressive
solidification but to speed the flow of molten metal into the model
cavity.

Warm regards
Jim

Jim Binnion

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


#10

Pat,

It doesn’t matter if you are doing one ring or a thousand at a time,
vacuum or centrifugal you don’t need a button and it is just wasted
metal. Use a tapered cone shape pouring cup and calculate enough
metal for the model and sprue anything more is just not necessary.

Jim Binnion

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

Hi Andrew,

    I don't see how oxygen absorbed at the top of a button after
casting will affect a part  way down inside the flask, particularly
if, as you say, the metal has solidified by then. Flux helps
protect the metal when it's being melted, but in this case, what's
probably happening is an insulation effect from the added material
piled on top. I usually cover castings with a piece of soft
firebrick immediately after casting; this helps keep the button
fluid and enhances its usefulness as a reservoir of semi-liquid
metal to feed the part. 

Because oxygen atoms are much smaller than metal atoms they are very
mobile in the metal crystal lattice at high temperatures. Silver
especially has an affinity for oxygen at elevated temperatures and
can hold great volumes of it at elevated temperatures, this is the
mecain.sim that creates firestain in sterling, the deep oxidation of
copper by the traveling oxygen atoms. The glass of the flux keeps
the oxygen out, not completely as it can still enter through the the
investment but at a much lower rate. I have done experiments with
this and can go into detail off line if you would like.

As for the insulation  if we are talking about jewelry sized items

here and not pours of pounds of metal then the models are solid
before you have even finished pouring the mold and trying to keep
the button /pouring cup molten via insulating material is not buying
you anything.

    Doesn't matter if there is a tree or not, most jewelry sized
    models are solid before the button starts to fill
    "Solid" is a relative term, then. 

No solid is definitely not a relative term it is a precisely defined
temperature at which the metal undergoes a phase change from solid
to liquid phase.

The metal inside the flask, which is typically cast hot, is in the
process of solidification, but it's still permeated with channels
of liquid metal, like a wet sponge. This will either draw metal
from the button or sprue as it shrinks, or it will draw it from the
part. I've seen it happen too many times to swallow this "no
button" theory whole. 

You don’t have to swallow it just prepare several identical flasks
( there are so many variables in casting as you no doubt know that
basing statements of fact on one or two casts is not advisable) and
calculate your metal weights so that half are poured without the
button and half are cast with a button. statistically you will see
no difference between the two sets. Major manufacturers like Rio and
Stuller are not using buttons in their castings now because they
just don’t need them. It is not hocus pocus or something I just made
up to create controversy it as I said something you can easily test
in your own shop.

    I've done sculpture casting as well as jewelry, and I have to
say that I'm unimpressed by this analysis, despite the hi-tech
trappings. When "most" of the metal is solid is when shrinkage
porosity does its damage, pulling metal from the surface as
capillary channels collapse. The effects are more easily
appreciated in larger-scale castings, but once one learns to
recognize it, it can easily be seen in smaller ones as well. I've
also had the misfortune to have castings disrupted by mishaps in a
centrifugal casting machine, and have seen that after quite a few
milliseconds had gone by, the castings were still fluid enough to
be partially emptied after they were full. When I was using an
overloaded centrifuge that slowed down after its first few spins,
I've had the button drip out and empty itself, with predictable bad
results to the supposedly solidified castings that remained. 

Yes in large scale castings the process of solidification is
different in that is is slower. It is very much a matter of scale.
if you are talking about belt buckles or sculpture rather than rings
and earrings it is all about heat transfer, how large a mass of
molten metal is in the mold cavity gate and sprue. I am not trying
to make any assertions about larger castings as they have different
dynamics but I will stand by my statements as far as jewelry ( ie
rings, earrings small cross section items ) casting goes.

 One of the things that is starting to be obvious in the casting
 research being done on jewelry casting is that the reduction of
 turbulence is very important to good castings (maybe the most
 important factor?) . This is backed up by the simulation that was
 presented in that if the metal is slowed down by turbulent flow it
 will not be able to feed the model cavity enough metal in a timely
 manner to produce a sound dense casting due to this rapid
 solidification.
    I do agree that a reduction in turbulence can help, but I'll
still be casting with enough reserve metal to accomodate shrinkage.
Shrinkage is not a myth, it's generally accepted as fact in the
casting trade, and I'm not the only one who believes it - see
http://kerrlab.com/JewelryProducts/technical/Glossary.htm or
http://www.sternleach.com/technical/articles/silvcast.php under
"shrinkage porosity". 

Shrinkage is a fact of life in castings no mater what scale. Metal
volume changes significantly during cooling from the molten state.
Best practice is to provide sprues and gates that will allow for
rapid filling of the mold cavity so that a maximum amount of metal
will be present when the casting solidifies. The point that I am
trying to make is that trying to feed a casting that has already
solidified with a button is a waste of time and money. And the
research and practice is showing that a button in this scale
casting is not required.

Papers about this are available in the published proceedings of the
Santa Fe Symposium, for the most part this work is done by
metallurgists working in the jewelry industry. If you are interested
I can provide titles of some of the relevant papers for you to read.

Regards,

Jim Binnion

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


#12

Jim,

This is going to be interesting. I have been following your input.
Now it looks like I need to rethink my approach to casting. I was
taught to use 30% to 50% more metal for the button. I have always
been careful about flow, turbulence, and well placed sprues. I have
also used thicker sprues as a practice. The idea being that they
would also function as a reservoir. The idea about the metal in the
flask freezing before the button was in place never occurred to me.
When I have screwed up and put most of the metal other places than in
the flask, I still got good castings. I have always considered that
button metal to be a “butt” saver in those cases as the extra metal
let enough still get where it was needed.

So far I believe that the metals being talked about are assumed to
be silver and gold. Do the same principals apply to bronze on the
small scale? Say under 50 dwt?

I love learning new tricks and better ways to do things. Thanks for
the insights.

Bill Churlik
@Bill_Churlik
www.earthspeakarts.com


#13

Dear Charles and Jim,

I thought I would toss in my two cents on this string. In my humble
opinion we still don’t know much about feed sprues and what is
really going on in the wink of an eye. We have all seen sound
castings that were cast on sprue systems that should not have
produced a sound casting according to the generally accepted wisdom.
The example of a wax pattern broken loose from a tree during
investing and lodged next to another pattern with pinpoint contact
that filled completely comes to mind. On the other hand we have all
seen casting with gross porosity that were fed with a sprue system
that conformed to generally accepted wisdom. My analogy is the math
puzzle you may have seen before presenting proof that bumblebees
cannot fly. The only thing the puzzle ultimately proves is that
something is wrong with our math. We don’t know enough about sprue
systems yet. With this in mind I will venture into dangerous ground.

The sprue system feeding any casting has thermal and hydrodynamic
functions. These are truly dynamic relationships that are changing
at different rates relative to each other and to the passage of time,
even if the time is short. But to simplify it so my meager mind can
wrap around it I would consider it enough to think about pressure,
temperature and flow rate. The higher the velocity the faster the
metal flows but the more turbulent it will be. Higher head pressure,
from standing metal or other wise applied, the better the form
filling will be. The higher the temperature the longer the metal will
stay liquid but the more likely thermal decomposition of the
investment will cause gas porosity. A larger sprue and less torturous
path to the pattern will reduce heat dissipation from the metal on
the way to the pattern. Such a sprue will allow casting at a lower
temperature to avoid gas porosity. The sprue system is all-important
in managing shrinkage porosity. This is the generally accepted wisdom
about casting. So now we come to the button or no button question.

Considering that many who will read this are casting one off pieces,
I think we should clarify a few things here if we can. The FEA
casting model Jim saw at the Santa Fe Symposium was of a tree that I
would estimate to be 500 grams of 18K gold. In the tree there was a
central sprue and feed sprues were attached to it. As stated above,
this is a very complex model and I feel sure that Jim will agree with
me that any FEA system available today would have to hold some of the
variables static and therefore does not tell the whole story. In any
case, the central sprue in the FEA model is the reservoir of heat
that keeps the metal flowing. When casting a tree with a central
sprue and patterns with a high surface area and low volume, the sprue
button is of little use as a heat reservoir. You can tell if the
button is beneficial by how much sink or piping is there after
solidification. If the shrinkage evident in the button were equal to
about 5% of the button volume, then I would judge the sprue button
not useful. If the shrinkage is greater than 5%, and I have seen many
examples of both, than the button is working in your favor.

If you are casting two or three patterns you generally don’t have a
central sprue and you need some mass of metal that will remain
liquid longer than the pattern and feed sprue, in that order. The
classic way to do this is to attach the feed sprue to the button and
therefore the button is the heat reservoir.

A while back I started to think about the volume of the button as it
relates to pressure. Any production caster knows that incomplete
filling defects are more prevalent at the bottom (sprue button end)
of the tree than the top. The explanation for this is the head
pressure is lower because of less sprue height above the feed sprue
inlet. I was at the Hong Kong Show and Tyler Teague was in the booth.
I showed him that if I cut the button off of a NeuSprue (which is
mostly hollow) and weighted it, the volume was the same as about 4
inches of sprue. In relationship to pressure the height of a liquid
column is more important than the diameter, so I surmised that
instead of a 1 inch tall button, casting with a four inch buttonless
sprue would use the same amount of metal and provide more pressure.
He took the idea and expanded on it as he reported at the SFS two
years ago. Keep in mind that the castings he was making were large
trees cast on NeuSprues with the sprue button weight subtracted from
the total metal weight. In my opinion, there is no proof yet that the
button can be eliminated if there is no central sprue or some other
heat reservoir.

Another factor that I have not seen anyone discuss is volume to
surface ratio. Beside the already mentioned influence of temperature,
etc., the volume to surface ratio of the pattern controls how quickly
metal solidifies and therefore how long the feed sprue needs to feed
metal. As the ratio of volume increases relative to surface, the
solidification time will increase. Another factor is the melt range
of the alloy. The FEA model was for 18K if I remember correctly. Most
18K alloys have a narrow melt range and little or no silicon and
solidify quickly. A low karat alloy (8, 9, 10K) loaded with silicon
will have a much longer solidification time as reported by Stewart
Grice at SFS a few years ago. My point is that we need to be careful
to consider all the variables before making a decision to change a
process and experimental casting to prove anything new is wise before
committing production to a change.

I can provide illustrations to demonstrate these factors if that
would be helpful. I welcome all comment and opinions.

Eddie Bell
Neutec/USA