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Casting flat objects


#1

Hey Guys,

I am probably going back the basic problem of spruing and I am sure
it is something I am overlooking… But here is the problem. Picture
if you will a flat oval shaped peice of wax. About 1mm in thickness
and about 1 inch from the top of the oval to the bottom…

Now that we have pictured this. I can tell you that I am using a
flat sprue going straight into the peice from the side. so the srpue
is as thick as the actual wax itself. I taper the srue outwards
slightly at the point where it connects to the peice. I am putting
these peices on the tree at about a 45 degree angle.

The problem is two fold. First of all sometimes when they fill and
most always do. You can see the path that the metal has taken into
the peice. Now there will be what appear to be stains on the metal
where this path has shown. These stains are actually cracks. You can
litterally break the peices in two. This only happens with gold
though… Second of all . when they don’t crack they get a heavy
textured feel right at the point where the sprue is attatched.
However at the opposite end of the sprue the metal is very smooth
like it should be… This textured look actually ends up to be
porosity that cannot be chased away either…

Here is my burnout info… And yes I use a full burnout cycle, I try
not to rush anything. As I have heard alot of people do. 1 hour at
300, 1.5 hours at 600, 2.5 hours at 1350, then at least an hour or
more at the casting temp which is usually about 975 to 1000 on the
thin stuff.

I should note that this only happens on thin flat peices that I am
casting. I rarely ever have problems with rings or heavier type
peices…

Now that I have probably really confused you all I will let you try
to help if you will…

thanks in advance.
Thomas in SoCal


#2

Thomas,

Roughness on the surface of my castings tells me that something’s
too hot when I cast. Try lowering the temp of your flask 100 degrees
F. More if you are still getting completely filled castings.

Cracked pieces usually mean that I’ve been waaaay to eager to see
the castings after pouring the metal and that I haven’t allowed the
metal/flask to cool enough before I quench.

Sprueing the model could be part of the problem. If you decrease the
volume of the sprue where it connects to the model, you will create a
’venturi’. As a stream of molten metal flows through this narrowing
space, pressure increases and so does the speed with which the metal
flows through the volume. The increased speed may be creating
turbulence. I know there are some really knowledgeable folks on this
forum. I hope they will explain this better than I have.

I maintain a consistent sprue volume by not filing on my sprue. A
round sprue is always best. But in order to get a large enough sprue
onto the edge of the oval, you’ve got to squish the sprue down into a
fan shape. I hammer it flat. This way the metal that gets pushed out
to the side maintains the original volume of the round sprue. I file
the end of the sprue to conform to the edge of the oval, then solder
the two together. I don’t file on the top or bottom flat edges of the
sprue.

Since your flask temp is so high, I assume that you are vacuum
casting. If the rough texture at the gate won’t be ‘chased away’,
the metal may be so hot that it reacts with the investment at that
point releasing gases. Is the rest of your sprue and sprue trunk
rough? Try lowering the temperature of your whole system, flask and
metal and see what happens! Sometimes it’s that simple.

Just change one variable at a time. Next time you cast this piece,
lower the flask temperature. If the problem persists, lower the flask
temp again…until you start to get incomplete fills. But you are
probably already doing this. Hope what i’ve written is helpful.

Chuck in Asheville


#3

btw, casting flat smooth objects is one of the hardest things to do,
and come out with a smooth, dense, shiny casting… for me, anyway.
I’d much rather stamp simple shapes from sheet, especially if I have
a bunch of them to do!

Chuck from Asheville


#4

Hi Thomas, Your problem is a common one. The whole piece is cooling
before it can get enough feed to supply dense recrystalization. It is
cooling very rapidly near the sprue. Metal is in motion as the
crystals are forming (solidifying). Think of a bricklayer doing a job
inside that has a tender bringing the bricks in as he needs them to
continue his progress, but someone locks the door and he has to
finish the job with the bricks he has left in the room. You can
remodel the piece so it has a rim around the whole piece that will
act as a delivery system or you can use what I call a “comb” sprue
which has a series of short sprues down one side attached to a
larger sprue. If the flask is is filling at your kiln temp, even with
your current results, you are not too far off. You might add 50-100F
after you try the above. As an experiment, try casting the same piece
as you did, with fine silver.

John, J.A.Henkel Co. Inc., Moldmaking Casting Finishing, Producing
Solutions For Jewery Artists


#5

With using a thin sprue the metal streams into the cavity and fills
the far side of the piece before returning to the eddies. Where the
hotter incoming metal meets the backwash it seems the backwash must
already be too cool to recombine resulting in the “cracks”. The
cracks, under close observation, may have rounding to the backwash
side of their edges. The roughness where the sprue comes in could
be an indication that that part of the casting is solidifying before
the main body of the piece. The same process can also occur in wax
injection. Wax temp, mold temp., and too much talc are the factor
to look at. Use an inspection light to check the waxes.

It is difficult to establish directional solidification in flat
pieces.

Like you said, changing the sprue design is one easy thing you can
try. If you take the weight of one of your other pieces and compare
the cross section area where the sprue joins the piece, with similar
measurements on the flat pieces, you might find that the two samples
have different sprue cross section to weight ratios. Because
rectangular shapes cool faster than cylindrical ones, the ratio
should be more area for the rectangular, (or oval) sprue entering the
flat piece. Making sure the sprue solidifies after the piece will
prevent directional solidification coming from 2 directions and
pulling molten metal out of the piece.

One way that directional solidification is routinely established in
the foundry industry is through the use of cast iron “chills”.
Basically the mold maker sticks a big piece of cast iron into the
sand touching the area of the casting where it is desired that
solidification should begin. As the molten metal enters the sand it
starts to cool. Because of the much greater conductivity of the
cast iron over the sand, the metal touching the cast iron will start
cooling faster than a similar section touching only sand. In this
way mold makers can control where the shrinkage occurs, either by
hiding the shrinkage porosity in the interior of the piece, or
forcing the shrinkage to occur outside the piece in the risers
(sprues).

The dental catalogs sometimes sell reservoir sprues. They look like
a piece of sprue wax stuck through a ball of wax. The ball, which
is located several MM from the piece, is designed to keep the sprue
area next to the piece molten longer. while still allowing for
reasonable sprue removal.

Surface texture increases surface area, so a piece with texture will
have faster cooling than one without. Texture can be good for
pushing porosity problems into the inside of a piece, and can be
used a a directional shrinkage facilitator.

Sometimes we add small pointed nibs to the piece. These are later
ground off. Like the tiny strings of rubber on new tires. The nibs
cool first and localize the start crystallization, and aid in
filling both the casting and the wax mold. (No doubt this is wrong
in some circles)

In your post you didn’t mention the method of casting. Bronze
sculpture is usually cast from the bottom up, like filling a bath
tub from the drain rather than splashing down from the faucet. This
is why some casters put their sprues on at lower angles.

A recent AJM article claims that square sprues cast better because
"90% of the air goes back out the main sprue" and not through the
investment.

Bad sprueing works for some, just look at those adds for stone in
place products, where they show 10 inch trees of stone in place
rings all cast through the thinest cross section at the back of the
ring. (this is wrong)

I am still learning so correct away.
Cheers Marty.