Mold question; bubbles in poured wax pattern?

Good evening to all, and I hope that you had a great Christmas.
As a hobby, I'd been doing centrifugal casting for a while, but I left the
hobby for a few years and I'm now trying to re-enter it.
I used to use a polysulfide molding compound (Smooth-On FMC-200), but I've
had a very difficult time locating the 1-quart cans I used to use. This time
around, I decided to try a platinum-based silicone (Smooth-On Smooth-Sil
920), and found it quite easy to work with. Molds that take a beating!
There's just a problem with the way the wax is settling into the pattern I'm
trying to overcome.
I've been using Kerr green Matt wax, slowly melted in a small ladle, and
poured into a open-faced mold to make the pattern. Before, using the
polysulfide mold, any bubbles in the wax would immediately come to the top,
even if the wax was overheated. However, with the new silicone no bubbles
seem to rise and they adhere to the face of the pattern, causing many pits
and design degradation.
I have not vacuumed the silicone, but no bubbles are on the face of the
mold. As well, the wax was not brought to a boil and there was no visible
bubbles as it poured.
After three days of trying to figure this out, I'm at my wit's end. That,
and my wife would really like me to get past this so we can change the topic
of our conversations. )
So far I have tried the following:

1. Suspecting possible outgassing of the silicone, I baked the mold at 200F
for four hours. No effect.
2. Thinking that the cause might be thermal, I heated the mold to 200F, and
experienced the same effect as I poured the wax.
3. I attempted to let the wax slowly melt into the mold after reducing the
wax to powder and chips. (Oven at 235F, wax melting point of 229F) Initially
it appeared to be working, but the bubbles were still there. (The molten wax
darkened as the pool deepened, hiding the bubbles.)
4. Just on a lark, and suspecting surface tension as the cause, I lightly
rubbed down the mold with canola oil and poured. The effect was reduced, but
still exists.

I believe I may be missing something basic here.
Would the use of a release on the silicone molds possibly solve the bubble
issue?
Should the silicone mold be brought to near the melting point of the wax
before the pour?
Is the wax I'm using (Kerr green Matt wax, P/N CA2719) completely
inappropriate to this application?
Are there any peculiarities of using silicone molds vs. polysulfides that
may be causing my issue? (All wax used was clean, no compounds from the
older polysulfide molds were introduced.)

Any information or recommendations at all regarding this would be sincerely
appreciated.

Thanks,

Rich

On Tue, 27 Dec 2005 02:05:08 -0800, in rec.crafts.jewelry "Rich"
wrote:

Good evening to all, and I hope that you had a great Christmas.
As a hobby, I'd been doing centrifugal casting for a while, but I left the
hobby for a few years and I'm now trying to re-enter it.
I used to use a polysulfide molding compound (Smooth-On FMC-200), but I've
had a very difficult time locating the 1-quart cans I used to use. Thistime
around, I decided to try a platinum-based silicone (Smooth-On Smooth-Sil
920), and found it quite easy to work with. Molds that take a beating!
There's just a problem with the way the wax is settling into the pattern I'm
trying to overcome.
I've been using Kerr green Matt wax, slowly melted in a small ladle, and
poured into a open-faced mold to make the pattern.

This is likely the bulk of your problem. This wax is simply not designedor
intended to be molded, and doesn't do it well. Use a proper injection wax. Kerr
makes a bunch of them. They include agents that will solve the adherance
problem. If it persists, try a little silicone mold release spray. Not Canola
oil. (It's not a lubricating oil, and is so viscous that it too will trap
bubbles) Silcone is compatible with the silicone rubbers. WD-40, by theway,
is NOT silicone. Castaldo makes a good mold release spray, and many machine
shop suppliers also sell silicone mold release sprays. Your problems are akin
to trying to figure out why the car is smoking, when your burning kerosene
instead of gasoline. Use the materials formulated for your use, and you'll have
fewer problems. Blue dental inlay wax is another one that will do this well.
All these waxes are characterised by low viscosity combined with a low melting
temperature so that then there is little shrinkage of the wax as it cools.

Before, using the
polysulfide mold, any bubbles in the wax would immediately come to the top,
even if the wax was overheated. However, with the new silicone no bubbles
seem to rise and they adhere to the face of the pattern, causing many pits
and design degradation.
I have not vacuumed the silicone, but no bubbles are on the face of the
mold. As well, the wax was not brought to a boil and there was no visible
bubbles as it poured.

Silicones have different surface tension characteristics. No doubt this may be
part of the problem. Also, it's possible that the Matt wax is no longer the
same formula as it was years ago...

After three days of trying to figure this out, I'm at my wit's end. That,
and my wife would really like me to get past this so we can change the topic
of our conversations. )
So far I have tried the following:

1. Suspecting possible outgassing of the silicone, I baked the mold at 200F
for four hours. No effect.

No, the mold isn't outgassing. you can actually cast low temp metals, like
pewter, tin, and lead, directly in silicone molds with no problems with the
mold. Wax temps won't affect it. Once cured, silicones are pretty stable.

2. Thinking that the cause might be thermal, I heated the mold to 200F,and
experienced the same effect as I poured the wax.

It's just air bubbles sticking to the silicone or not escaping from details.
Less viscous wax, and release sprays should solve it. With a less viscous wax,
you can also try a bit of vibration of the mold as you pour it. (a portable
saber saw without it's blade held running against the table on which the mold is
sitting is what I used to do. Lightly vibrates the mold just fine. Might not
be needed, but might help if you've intricate models.

3. I attempted to let the wax slowly melt into the mold after reducing the
wax to powder and chips. (Oven at 235F, wax melting point of 229F) Initially
it appeared to be working, but the bubbles were still there. (The molten wax
darkened as the pool deepened, hiding the bubbles.)


I'd suggest you don't melt powdered wax. Forms a froth that takes forever to
settle out to clear liquid. If you use it, melt it seperately, pour it into a
chunk/ingot, and cut off any frothy part. Likely that part is also contaminated
with other junk. Normally, stick to melting just decent solid chunks, not
powder or filings.

4. Just on a lark, and suspecting surface tension as the cause, I lightly
rubbed down the mold with canola oil and poured. The effect was reduced, but
still exists.


See above. not a good lube.

I believe I may be missing something basic here.
Would the use of a release on the silicone molds possibly solve the bubble
issue?

It will help. Not the whole story.

Should the silicone mold be brought to near the melting point of the wax
before the pour?

No. room temp molds work best.

Is the wax I'm using (Kerr green Matt wax, P/N CA2719) completely
inappropriate to this application?\\

Yes. It's a carving wax. At least, if it's the one I'm thinking of... These
never mold well due to the higher percentage of plasticizers in them to make
them machinable.

Are there any peculiarities of using silicone molds vs. polysulfides that
may be causing my issue? (All wax used was clean, no compounds from the
older polysulfide molds were introduced.)

One other thing to remember is that the rubber is not gas permiable. Some blind
corners and details in a mold may need air release cuts. This is common in
molds used for injecting wax under pressure, and may be even more so for a
gravity poured one. Release cuts are shallow knife cuts make, usually along
edges so mold marks don't show. The cuts are held lightly open and dusted with
talcum powder or cornstarch, then blown clean again with compressed air. Just a
trace of the dust stays in the cuts, making a gap small enough that wax won't
enter, but backed up air can escape. The cuts don't need to go all the way to
the exterior of the mold, just to any other parting line or area from which the
air can continue to escape.

Hope this helps.

Peter Rowe

Excellent! I really appreciate it!
I'll give the new wax and release a try. Thanks again!

Rich

"Peter W.. Rowe," wrote in message
...
On Tue, 27 Dec 2005 02:05:08 -0800, in rec.crafts.jewelry "Rich"
wrote:

Good evening to all, and I hope that you had a great Christmas.
As a hobby, I'd been doing centrifugal casting for a while, but I left the
hobby for a few years and I'm now trying to re-enter it.
I used to use a polysulfide molding compound (Smooth-On FMC-200), but I've
had a very difficult time locating the 1-quart cans I used to use. This
time
around, I decided to try a platinum-based silicone (Smooth-On Smooth-Sil
920), and found it quite easy to work with. Molds that take a beating!
There's just a problem with the way the wax is settling into the pattern
I'm
trying to overcome.
I've been using Kerr green Matt wax, slowly melted in a small ladle, and
poured into a open-faced mold to make the pattern.

This is likely the bulk of your problem. This wax is simply not designedor
intended to be molded, and doesn't do it well. Use a proper injection wax.
Kerr
makes a bunch of them. They include agents that will solve the adherance
problem. If it persists, try a little silicone mold release spray. Not
Canola
oil. (It's not a lubricating oil, and is so viscous that it too will trap
bubbles) Silcone is compatible with the silicone rubbers. WD-40, by the
way,
is NOT silicone. Castaldo makes a good mold release spray, and many machine
shop suppliers also sell silicone mold release sprays. Your problems are
akin
to trying to figure out why the car is smoking, when your burning kerosene
instead of gasoline. Use the materials formulated for your use, and you'll
have
fewer problems. Blue dental inlay wax is another one that will do this
well.
All these waxes are characterised by low viscosity combined with a low
melting
temperature so that then there is little shrinkage of the wax as it cools.

Before, using the
polysulfide mold, any bubbles in the wax would immediately come to the
top,
even if the wax was overheated. However, with the new silicone no bubbles
seem to rise and they adhere to the face of the pattern, causing many pits
and design degradation.
I have not vacuumed the silicone, but no bubbles are on the face of the
mold. As well, the wax was not brought to a boil and there was no visible
bubbles as it poured.

Silicones have different surface tension characteristics. No doubt this may
be
part of the problem. Also, it's possible that the Matt wax is no longer the
same formula as it was years ago...

After three days of trying to figure this out, I'm at my wit's end. That,
and my wife would really like me to get past this so we can change the
topic
of our conversations. )
So far I have tried the following:

1. Suspecting possible outgassing of the silicone, I baked the mold at
200F
for four hours. No effect.

No, the mold isn't outgassing. you can actually cast low temp metals, like
pewter, tin, and lead, directly in silicone molds with no problems with the
mold. Wax temps won't affect it. Once cured, silicones are pretty stable.

2. Thinking that the cause might be thermal, I heated the mold to 200F,
and
experienced the same effect as I poured the wax.

It's just air bubbles sticking to the silicone or not escaping from details.
Less viscous wax, and release sprays should solve it. With a less viscous
wax,
you can also try a bit of vibration of the mold as you pour it. (a portable
saber saw without it's blade held running against the table on which the
mold is
sitting is what I used to do. Lightly vibrates the mold just fine. Might
not
be needed, but might help if you've intricate models.

3. I attempted to let the wax slowly melt into the mold after reducing the
wax to powder and chips. (Oven at 235F, wax melting point of 229F)
Initially
it appeared to be working, but the bubbles were still there. (The molten
wax
darkened as the pool deepened, hiding the bubbles.)


I'd suggest you don't melt powdered wax. Forms a froth that takes forever
to
settle out to clear liquid. If you use it, melt it seperately, pour it into
a
chunk/ingot, and cut off any frothy part. Likely that part is also
contaminated
with other junk. Normally, stick to melting just decent solid chunks, not
powder or filings.

4. Just on a lark, and suspecting surface tension as the cause, I lightly
rubbed down the mold with canola oil and poured. The effect was reduced,
but
still exists.


See above. not a good lube.

I believe I may be missing something basic here.
Would the use of a release on the silicone molds possibly solve the bubble
issue?

It will help. Not the whole story.

Should the silicone mold be brought to near the melting point of the wax
before the pour?

No. room temp molds work best.

Is the wax I'm using (Kerr green Matt wax, P/N CA2719) completely
inappropriate to this application?\\

Yes. It's a carving wax. At least, if it's the one I'm thinking of...
These
never mold well due to the higher percentage of plasticizers in them to make
them machinable.

Are there any peculiarities of using silicone molds vs. polysulfides that
may be causing my issue? (All wax used was clean, no compounds from the
older polysulfide molds were introduced.)

One other thing to remember is that the rubber is not gas permiable. Some
blind
corners and details in a mold may need air release cuts. This is common in
molds used for injecting wax under pressure, and may be even more so for a
gravity poured one. Release cuts are shallow knife cuts make, usually along
edges so mold marks don't show. The cuts are held lightly open and dusted
with
talcum powder or cornstarch, then blown clean again with compressed air.
Just a
trace of the dust stays in the cuts, making a gap small enough that wax
won't
enter, but backed up air can escape. The cuts don't need to go all the way
to
the exterior of the mold, just to any other parting line or area from which
the
air can continue to escape.

Hope this helps.

Peter Rowe