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Casting investment crushing wax models


#1

Hi everyone. I’ve been encountering some problems with casting
sterling recently. It seems that when I set the investment, it tends
to crush some parts of the wax as it sets up. Does anyone encounter
the same problem? Is there a fix that I’m not aware of?

cheers
Don Humphreys


#2

Hi Don,

The investment is “crushing” part of the wax?? (!!??)

Shouldn’t be possible, under normal circumstances. The only thing I
can think of is that you’ve got some hollow areas in your wax model,
and the vacuum is causing them to implode. (They’ve got air in them,
the vacuum sucks out all the air, and then they crush when the
pressure is restored after vacuuming. Air can get out of a part much
more easily than investment can get back in, you see…) This would
also cause some outbound damage as the air leaves, but it could be
hard to tell what’s what, especially in the wreckage left after a
bad cast.

Try setting up one of your flasks, and filling it with water instead
of investment. Vacuum as per normal, and then look at the waxes. If
they’re squished or exploded, you’ve got trapped air pockets. (big
ones, probably.) If it survived OK, dry it out, and invest it. If it
fails, then the problem really is with the investment. (somehow.)

Best of luck,
Brian.


#3

Hi Don. How are you pouring the investment into the flask? I pour
the investment on the sides of the flask and let it fill up from the
bottom…

Teddy


#4

Wow! Don in 35 years I’ve had models do all sorts of tricks while
being invested… but have never seen one “Crushed” by
investment… Tell us some more about this. Is the model super super
thin?? Why do you say crushed??? Perhaps I’m misunderstanding. I
think we need more info… I’m really curious about this one!!

Dan


http://www.dearmondtool.com


#5

Don!

I have one simple idea with pouring of investment. Why don’t you use
a spoon to direct the investment flow towards the edges of the metal
flask. This way the pouring might be less aggressive, but not
endangering the waxes on the tree.

How much longer will it take? What is a few seconds more than to
pour directly unto the wax form and ruin your creations? Once the
investment has covered the wax, then you might be able to pour
anywhere you wish.

Like Teddy said, pour on the extreme outer rim of the inside of
flask…be gentle at all pouring times ! ! !

Gerry!


#6

Dear Don,

Dumb question… are you pouring the investment down the side of
the flask or into the center of the flask? The best way is to pour
it slowly against the side of the flask, let the investment level
slowly rise. Otherwise, you’re going to either break the wax off the
sprue or break the wax model itself.

Ruthie Cohen


#7
Wow! Don in 35 years I've had models do all sorts of tricks while
being invested... but have never seen one "Crushed" by
investment.... 

I’ve seen one situation that looked as though the model had been
crushed by the investment… It turned out that the model was thin
and hollow, and the vacuuming process had crushed it (sort of
collapsed it by sucking the air out of the inside). Could this
possibly be what is happening?

Noel


#8
It seems that when I set the investment, it tends to crush some
parts of the wax as it sets up. 

While investments expand during the heating/burnout process, at that
point, once the shape of the wax is fixed in the mold, it wouldn’t
matter. During initial setting up of the investment, it doesn’t
expand, so any damage to the models would be from physically pouring
the stuff or agitating so much as to shake the models and damage them
that way somehow. But there is another possibility. If you are using
a vacuum to de-air the investment, AND if there are substantial
trapped air pockets/bubbles imbedded in the wax, during vacuuming,
it’s possible that thin wax walls of such a bubble could burst, with
the investment then filling the bubble. The result would be a massive
defect in the casting.

And another possiblity. Improperly mixed or burned out investment
can have weak sections of the investment break off the mold surface
before or during casting. This can lead to very odd looking defects
in the finished casting, where the chunk prevented metal filling
where it ended up, leaving a shaped (not a random bubbled) cavity.
That might well look as though the wax had been crushed, or like some
section of the wax surface had moved (it did, but it was the mold
surface that broke off and moved, not the wax itself).

When this happens, often it means the water to powder ratio is
wrong, usually too much water. Or too fast a burnout, or not a long
enough setting time before you burn out the flask. It can also happen
with some wax models whose shape simply creates too fragile a section
of investment. Long drill holes in a wax, for example, create an
easily broken thin pillar of investment spanning a portion of the
mold cavity, which is easily broken and moved somewhere else when
the molten metal hits it.

Peter Rowe


#9

Hi are you moving the flask before it is completely “set up”? Maybe
thats it?


#10

Thanks to everyone who responded so far. I’ll try to clarify what I’m
doing and experiencing: When casting a fine filigree pattern, the
result tends to have large voids in the filigree. It almost looks
like a shattered piece with parts of it missing altogether. I’m
careful when pouring investment ie: down the side of the flask. The
waxes don’t seem to have bubbles in them when I examine them through
a loupe, but that’s not certain. I can understand what Brian’s
talking about. I’ll try the water test. To answer Dan’s question:
the filigree can have some very thin (1/3 mm) strands so that could
also be an issue. Question… should I add a couple of extra sprues to
the wax to allow metal to flow into the mold more smoothly? Sorry, I
don’t have a photo of the pieces… I melted them down to try again.

I’ll keep in touch
Thanks again

Don


#11

Sounds like either hot tearing or just a plain old no fill issue.
1/3 mm is awfully fine detail if it has any length. Try more sprues
or a taller flask with the filigree part as far from the sprue button
as possible. Taller column (main sprue) of molten metal more
hydrostatic pressure to fill finer detail.

James Binnion
James Binnion Metal Arts


#12

Hi Don,

If it’s filigree, made with commercial wax wire, the chances of
significant air bubbles in the wax are very low, so that’s probably
not it. When I first responded, I was picturing something larger,
probably marble sized, with large voids.

I think perhaps Peter’s suggestion about the shape of the pieces
causing long unsupported spans of investment to cross a large open
area in the mold is probably closer to the mark. Next time you get
one of these made & sprued, post pictures so we can see what the
piece looks like before you invest it.

Regards,
Brian.


#13
When casting a fine filigree pattern, the result tends to have
large voids in the filigree. It almost looks like a shattered piece
with parts of it missing altogether. 

That sounds more like the casting simply didn’t fill. In things like
filligree, where there may be multiple but inadequate paths for metal
to follow to an area, you can end up with both empty areas, and
places where a metal stream solidified just as it met up with another
one, giving you a gap or break in the middle of the piece of metal.
It can look like it broke, when in fact it just didn’t fill
completely. Try raising your flask temperature a hundred or hundred
fifty degrees, and maybe the metal pouring temperature a bit too, and
see if that helps. If you are vacuum casting, you might also try, if
you’re already sure your water to powder ratio is according to the
manufacturers directions, adding just perhaps one percent more water.
let the flask sit a bit longer before burning it out, and mix a
little longer since gloss off time for the investment will increase
(or use slightly warmer water to compensate for that). Especially
with thinner investment mixes, you want to make sure it doesn’t sit
for a long time before gloss off occurs, or you’ll get water marks.
Such investment mixes are slightly weaker (so not always a good idea
with centrifugal casting) but with vacuum casting, so long as you’ve
got enough investment thickness above the top of the model, so it
doesn’t blow out, the slightly increased gas permiability of the
thinner investment can help to reduce any resistance to metal flow
in thin areas. And as always, the more difficult or longer the pqth
the metal must tqke to fully fill the design, the more important
adequate spruing is. It’s almost never a mistake to add more sprues,
or heavier sprues, so long as you’ll be able to clean it up after
casting. The aim is to give the metal the simplest and shortest path.
And be sure that no heavy areas are fed by substantially thinner
areas. The heavy areas should get a sprue of their own if needed.

Cheers
Peter Rowe


#14
When casting a fine filigree pattern, the result tends to have
large voids in the filigree. 

Spruing, plain and simple. Like most here, I’ve been mystified by
what is seemingly an impossible situation - normal, proper investing
simply cannot crush the model.

I call it “internal spruing” - the piece itself must have a way for
metal to get everywhere after it comes from the sprue rod. It takes a
big sprue to feed several small sprues - meaning internally. Look at
your filagree - does each wire face the direction of flow? Is there
enough “plumbing " building up to the top that there’s a good feed”.
Think of it as a ladder - the metal is flowing along the uprights
easily. It needs to make a right angle turn to fill the steps, which
metal doesn’t like to do. Early on (down the ladder) it will work,
maybe, but the farther it goes the less likely they will fill. Does
your model have that situation - cross bars that need to fill at
strong angles from each end? That will do it.

And 1/3mm wires aren’t going to fill without some really particular
spruing, especially if they are far from the button…

You can make any filagree ring you like, once - if you’re going to
mold and cast it, it needs to be designed with that in mind…


#15

Since filigree is not truly a casting technique, but rather a
fabricating one, IMHO one must make allowances to adapt a filigree
type of design for casting. My guess is that portions of the design
are not thick enough, or spaces in the design are so thin, that it
cannot be properly cast, due to the reasons which members have
already stated. Not everything is made for taking a mold and
casting! There are limits, and I am afraid you have just met some of
them. I’d look at changing the designs to avoid tiny, thin air spaces
first. Then move on to beefing up the general thickness of the metal
models, if that doesn’t work. Sorry, I know that is not what you want
to hear, but I believe it may be a model problem, not a casting
procedure one. Model making for casting is a learned skill; welcome
to school!

M’lou Brubaker
Minnesota, USA
http://www.craftswomen.com/M’louBrubaker


#16
Spruing, plain and simple. Like most here, I've been mystified by
what is seemingly an impossible situation - normal, proper
investing simply cannot crush the model. I call it "internal
spruing" - the piece itself must have a way for metal to get
everywhere after it comes from the sprue rod. It takes a big sprue
to feed several small sprues - meaning internally. 

(etc…)

A few years ago, a friend, now departed, helped me with a similar
problem. I was trying to cast some small pierce work pieces, about
0.70mm thick. One of the pieces is a lunette (half-circle)
approximately 30mm long by 13mm wide, pierced with 28 small holes.
The other is an outline only of a Celtic cross, with the outline
varying from 0.80mm to 1.2mm in width. The “wire” making up these
pieces is then roughly similar to filigree.

It took several tries, even with his expertise, before consistent
castings were made.

Each piece was connected with a wax wire about 1mm in diameter to
the end of a central sprue of considerable size - about 10mm to12mm.
This central sprue extended into the flask about 20mm in addition to
the rather large button. The models were placed in a single layer, as
if they were standing on end by their sprue. Great care should be
taken to ensure a smooth entry into each model as well as into the
central sprue. The lunettes had a sprue from each corner which joined
into a “Y” shape before being connected to the central sprue.

A section of wax mesh (such as used on the inside of a flask next to
the flask wall for venting purposes) was suspended 2mm to 3mm above
the wax models with vents running to the surface, i.e. the inside of
the rubber flask base. This mesh was not connected to the models or
the central sprue in any way. The theory is that air and other
gasses permeate the investment when the hot metal is forced in. These
gasses then collect in the cavities left by the mesh where they may
be vented to the outside, relieving pressure in the model cavity,
allowing metal to flow better.

This worked for me. I did several batches of each of 25-30 models
being cast. Only one or two castings were lost in each batch,
compared to over half in early attempts. The spruing process,
however, was like building a house of cards!

This may not work for you, but perhaps you might like to try this
technique.


#17

The person to ask about casting filigree is Jeanne Moen. She has a
sampleon her home page.

http://www.jeannius.com/websys/store_mainpage.php


#18
Since filigree is not truly a casting technique, but rather a
fabricating one 

My opinion is that filigree is a design style. It can be cast or
fabricated. I have cast fine filigree. Experience over time allows
for compensations. The design may be thin and fine, but the depth may
be greater to allow the metal to flow to complete the design. At the
school I went to, the instructor said if you can see it, you can
cast it and if you can make a mold and remove the wax intact, it will
cast it successfully. How the piece is sprued is critical. It is not
only how thick the sprue is, it is understanding the flow of metal,
the metal temp for casting, and the effect of gravity or centrifugal
force on the molten metal.

Richard Hart G.G.


#19

Don, Thanks for the info.

Perhaps the investment isn’t crushing the model… especially since
you are pouring carefully. Mabey you aren’t throwing the flask hot
enough and thats causing the voids. Those super thin pieces like
that need to be cast hotter than anything else. What metal & How
Hot??? Feel free to drop me an e-mail if you like, I’m glad to help
if I can.

Dan.


http://www.dearmondtool.com


#20

Hi Don,

Look at the ends of the piece next to the missing areas. Are they
convexly rounded off? If so, you’re getting an incomplete casting
because the metal is cooling before it can flow into the narrow
sections. You will have to add sprues, or maybe raise your flask
temperature, or perhaps both. That’s some pretty thin wax.

Bruce Morrison