because the flaws are indented into the castings this means that
some residue has been left in the void when the wax was burnt out.
Unless your dewax cycle is a little odd I would suggest that it is a
carbon residue. Shape is wrong for bits of investment to fall into
void and it would be a strange dewax cycle to cause gypsum crystal
growth from a secondary sol-gel process. just to make sure on the
last point, have you cooled and reheated the investment after
steaming/melting out?
It’s all about plumbing, you see. First off, looking at
ti-problem.jpg, the full pictured piece on the right side has
investment failure That patch is where a flake of investment just
flat fell off in burnout. Most likely it’s a chunk of unmixed
investment that was left in the bowl. Second is spruing, as it always
is. Plumbing is exponential - a 1" pipe is ~four~ times bigger than a
1/2" pipe, not twice. Your circular sprue base (more about that in a
bit) is roughly the size of your largest sprues on your pieces. So,
you are trying to fill four 1" pipes and 8 3/4" pipes (so to speak)
off of a single 1" pipe and that just ain’t gonna happen. Your ring
on the bottom should be a good three times the size of what it is, to
feed all of those sprues. That’s aside from the ring itself (the more
part) - you are asking molten metal to hit that ring, make a left
turn and go in a circle, then make another right turn and go up all
the sprues and the metal is not happy. Put everything so the metal
always goes forwards as much as possible. You have what I call, “The
coke bottle effect”. Get a full, open coke bottle and in one motion
turn it upside down. What happens is it goes “glug, glug glug” as the
liquid in the big bottle tries to push through the little opening.
Make that opening the same size and the body of the bottle and it
will j ust go splat.
Here are some avenues to explore. From what I understand this
machine spins pretty fast. Castings like this won’t need a high speed
to fill. Is it possible to change the speed at which the casting
machine spins?
Also, what water to powder ratio are you using? This type of casting
should be performed with a low water to powder ratio (a thicker
mix). This will provide you with a much stronger investment structure
that will better tolerate the volume of metal filling the cavity. A
slower speed might help too, but if you can’t adjust it, you’ll have
to rely on the strength of the investment alone. Keep in mind charts
provided by the investment manufacturers are based on medium detailed
castings, not heavyweight castings.
Are you putting any heat from the torch on the “button” after the
metal is cast? There seems to be a breakdown of investment around
the metal reservoir. I’ve seen that occur when torch heat is applied
to the button, but it could also be caused by a too thin mix of
investment.
One last detail I think you should attend to; Make the transitions
from the button to the sprue and from the sprue to ring smoother. A
smoother transition will hold down the turbulence caused when the
metal is forced into the cavity and keep bits of plaster from
breaking off into the mold.
Let me know if any of this resolves the problem. Good luck.
Second is spruing, as it always is. Plumbing is exponential - a 1"
pipe is ~four~ times bigger than a 1/2" pipe, not twice. Your
circular sprue base (more about that in a bit) is roughly the size
of your largest sprues on your pieces. So, you are trying to fill
four 1" pipes and 8 3/4" pipes (so to speak) off of a single 1"
pipe and that just ain't gonna happen. Your ring on the bottom
should be a good three times the size of what it is, to feed all of
those sprues. That's aside from the ring itself (the more part) -
you are asking molten metal to hit that ring, make a left turn and
go in a circle, then make another right turn and go up all the
sprues and the metal is not happy.
While I agree there is a sprue problem here I am wondering if you
are familiar with this type of casting machine it is quite different
in how the metal is introduced into the mold than a traditional
centrifuge. The flask spins on its central axis which passes
vertically through the center of that sprue ring you are pointing
out as being too small. The sprue opening is actually 1.25" in
diameter, so plenty of room for the metal to enter and feed the
rings. What you are pointing out is where the metal has frozen being
held against the wall of that opening by the centripetal force of
the spinning flask.One of the great advantages of this machine is
you don’t need a large sprue tree and button structure. You can see
more info and a video of it here
The issue I see is that the point where the main sprue for each ring
attaches to the feed button appears pinched off. That area should
have additional wax added to it to make it flare at the point of
attachment. I also think the biggest problem is that main sprue is
too small for that massive a ring. Both the main sprue and those
side feeds will freeze off before the massive center section of the
ring has solidified so there is going to be lots of shrink porosity.
That may even be the cause of that large defect that looks a little
like investment spalling except it is as Nick noted going inward. I
don’t see how given its size that it could be a dry investment flake
but it might be where a whole section of the casting face was pulled
in by shrinkage below it.
I am not familiar with tri-research equipment. But the problem looks
like premature cooling of the mold or casting medium. Possibly want
to up the mold temp before casting. just a suggestion for a Kerr
type caster.
While I agree there is a sprue problem here I am wondering if
youare familiar with this type of casting machine it is quite
differentin how the metal is introduced into the mold than a
traditionalcentrifuge. Jo-Ann was looking over my shoulder and she
said, "Well yeah, maybe it IS a different machine but metal flow
is still metal flow." So, Jim is right, I'm not familiar with that
machine, though I've heard of it.
And I followed his link, though I didn’t watch the video. And right
away I saw one of the problems aside from those already mentioned -
by Jim also. You have that ring, which amounts to your sprue base,
and the parts are sprued onto it. One the website it’s clear that
the parts are sprued on the outer rim, more towards being the spokes
of a ship’s wheel. The OP’s casting was much more sprued on TOP of
the ring, though at some angle. That means the metal has to make a
turn to get to the part, which metal doesn’t like to do. The nearer
you can get to having metal go in a straight line, the better off
you’ll be. If the flask is spinning then try to put the parts in
line with the lines of force of the flowing metal. If casting wasn’t
an art, then anybody could do it…
It’s been said that you can cast anything if it’s properly sprued -
there are a lot of factors involved in casting, but spruing is top
of the list.
there is something that I do not understand. When he is done mixing
the investment, he connects the beaker to running water and then he
starts to mixit vehemently. Does the connection to the running water
creates a vacuum, if so, how does this work? I had never seen this
before. It looks like a great system, but very expensive!
yes This is the limit of Ti-casting system= you cannot play with
sprue larger than 6 mm because the outer rim is not larget than that.
better switch to a tradional vacuum casting for heavy ring where you
can have wax tree 12- 14 mm diameter and can have sprue to attach tho
the ring up thhis 14 mm.- T–research seems to be good only for thin
pieces as shown in their vdo.I will try with the german small vacuum
casting machine which has a vacuum pump of 21 m3/h. nat.
it appears that the investment was not mixed with the correct
thickness. just a guess but it looks like the force of the molten
metal has knocked the surface off of the cavity.
Yes the device the mixer is connected to is an aspirator pump. It
produces a vacuum via the venturi effect. A well designed aspirator
can produce vacuum of more than 29" of mercury. The speed of these
pumps is not very great but as long as the volume is small they work
very well. I am quite pleased with the Ti Research casting machine
although I did not buy the mixer because I have a several vacuum
pumps and bell jars but I thought it was well made when I examined
it.
I am a fan of the aspirator pump as an inexpensive method of
providing vacuum for investing and casting of small (less than
3"x3") flasks.
The running water creates a vacuum. If you click on “Appliances” then
"Waterjet" you’ll see a picture. The small tube that hooks to the
mixing bowl is the vacuum tube. We have Giovanni Battista Venturi to
thank for this.
I think the ring thickness is acting like the “button” reservoir,
(which there is none in this sprueing) the thick area staying liquid
while the thinner sections all around it are hardening. This causes
metal to be drawn from the thickest sections to replace metal to the
thinner cooler parts, causing the pitted spawled look. I don’t think
it’s an investment problem as I have made this mistake many time
when I was a younger man. Anytime you have a thick section being fed
by a thinner section, it becomes the “button”, cooling last. I don’t
see any need for the smaller additional side sprues either. Rather,
feeding a much larger sprue to the thick area makes more sense to me.
Even a small, say a 10mm ball feeding that thick area, would probably
alleviate all the problem, Then this larger sprueball becomes the
button and last to cool, depositing the shrinkage problems in that
area, on the ball. Temp control is critical too. 50 degrees too hot a
pour will increase this problem exponentially.
