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Casting process questions


#1

I’ve been having a problem with my castings, in that little flakes
seem to be coming off the mold, and causing pits in the castings.

I think I figured out what was happening.

Because I’m working by myself, and have never actually seen anyone
else cast, I’ve had to figure most stuff out on my own. I’ve been
taking the mold out of the oven, and putting it in my casting machine,
then melting the metal.

Previously, my poor propane torch was barely able to bring any metal
up to casting temperature.

What I figure is happening is that heat from the torch is hitting the
mold, causing some differential heating, flaking, and blowing the
pieces into the mold.

After the two or so minutes it takes to melt the metal, a fair number
of flakes have gotten into the mold.

I’ve read that you’re supposed to get the metal ready for casting,
then put the mold into the machine. I’m not sure how I can do this
well without four arms, two keeping the torch on the metal, and two
loading the mold.

Anyone have any advice about how I should approach this?

Thanks!

  • darcy

#2

Hi Darcy, You can use a microphone stand from Radio shack to hold the
torch. When your metal is nearly melted, you will have both hands
free to grab your flask and put it in the casting machine. I have
pictures of my old casting workshop on my website if you’d like to
take a look… I use this for vacuum casting.
http://www.racecarjewelry.com then click on the word "workshop"
on the front page of my website.

Good luck
Daniel Grandi


#3

Darcy, Try leaving the flask in the oven until the very end. Using
your torch, melt the metal that you are working with in the crucible,
and then when the metal is molten, then put the flask into the machine
and then finish up torching the crucible again until it looks ready to
cast, and then let it rip.

I’m assuming that you are using a centrifugal casting machine, (maybe
a “LUCAS” brand).

Sincerely,
Richard Lucas,
Lucas Dental Company
Lucadent@webtv.net


#4

Darcy, your problem is more likely caused by to rapid of a burnout,
especially from room to 300deg.

If you rush this part of the burnout, you will cause steam to form
rapidly and this will pop and blow out flakes from the surface of the
investment. Follow the investment directions for burnout time. Also
make sure that you have let the investment dry the recommended time
before starting the burnout. Last thought is that your investment is
old and has absorbed moisture and this changes the way it sets up. I
have seen formulas for determining the moisture content of you
investment but all require a comparison to fresh investment, so why
not just try a new batch to begin with.

Don


#5

Hi Darcy; The problem you are discribing sounds like one referred to
as “spalling” although if I’m incorrect on that, someone here will
fix that. :slight_smile: Anyway, I’m not sure what is causing it, either taking
the mold up too quickly in the initial stages of the burnout, or
possibly too much water for the amount of investment. Our casting
experts here will inform you more accurately, I’m sure. But I have a
trick you can use to keep the metal molten in the crucible with the
flask in place in the casting arm. Simply back the crucible off from
the flask a bit and insert a piece of copper sheet between the
crucible and the flask, large enough to shield the sprue chamber.
When your metal is molten, flick the copper out of the way with your
tongs while keeping the torch on the metal. Then carefully slide the
crucible up snug to the flask and release. This will also help
minimize the amount of time that the torch is shooting through the
hole in the crucible into your sprue chamber, which can cause gases
to contaminate the investment.

David L. Huffman


#6

Generaly it seems better to cast a wet mold than a dry one. The steam
formed helps in the dewaxing. Investment breakdown cn be caused by
overheating it. direct heat from a torch or burnout with a “
flowerpot” kiln over a hot plate or a poorly controled burnout kiln.

Jesse


#7

Daniel - Enjoyed the tour of your workshop on your website. Thanks
for inviting us. I have a question. On workshop page 4, #14 you
mention “cooling down for 15 seconds before putting the flask on the
floor to cool for 20 minutes.” What’s the advantage over just
plunging it in a bucket of water after the button loses it’s red
color? And if you let it sit too long, isn’t it hard to remove the investment?


#8

Hi Darcy, I’m glad to meet another casting enthusiast! Dan’s idea, at
Race-Car, using a mic stand to hold your torch is the direction you
want to go in. I have done something similar. When I started out (in a
different epoch!)I had an assistant hold the torch while I got the
flask…very frustrating! But to get to the major reason I
replied…as David mentioned, what you are experiencing is probably
spalling. It can be caused by a few things. Assuming your investment
is healthy and that you weighed and measured acurately, you can narrow
it down to a few important steps in the burnout. After your investment
has hardened, there are two type of water in your flask. They are
called “free” and “chemically bound” water. The free water leaves the
flask as steam usually up to 500 F. This type can cause a problem with
carved wax because of the wax’s plastic content. Carving wax will sit
in the plaster as a gel even above the boiling point of water making
it vulnerable to the errosive pressure of the steam between the wax
and the plaster. The simple solution to this is to heat soak your
flask in the kiln for a couple of hours at 200F before proceeding with
your burnout. This will drive off the “free” water before it turns to
steam. The “chemically bound” water releases as the chrystobalite and
silica begin to expand at 700F. The combined pressures of the steam
and expantion of the investment material, if rushed through this
temperature range, can cause cracking and spalling. Make sure your
burnout heat-ramp stops and holds for an hour or so at 700F and then
proceed to the top of your ramp, which should not exceed 1350F. It is
a good idea to calibrate your kiln pyrometer to be sure your temps are
correct. Your can get some calibrating cones from a pottery supply
house. They melt or “slump” when they reach the temp written on their
side. Hope some of this helps. John, J.A.Henkel Co.,Inc, Moldmaking
Casting Finishing


#9

Attn. Darcy; First double check your -water to investment ratio,upon
mixing. The pits—

  1. Are you casting bronze,sterling or gold?

  2. Your metal could be dirty,

  3. you’re not using enough flux,

  4. your torch is too hot(metal too hot)–

  5. Your flask could be too cold—should be roughly 900deg.
    (dull orange color)…

NOTE: You really should be using oxygen with propane… Do Enjoy—

Dave.


#10

Hi David,

                  Depending of course on the metal, normally the
button looses its r ed color in around five minutes. When I have let
it cool for more than that it definitely has been much harder to
remove, even having to us e other tools like hammers, brass brushes
and sand blasting.It seams to me that 20 minutes is too long for
gold alloys and silver, but might be necessary for niquel alloys or
similar 

We simply break out in water after 20 minutes, Give the flask a light
taping with a rawhide mallet and re-quench the flask, Then, for
investment removal from the tree, we use a water blaster, home made
from an old sandblasting cabinet we bought for $50 used. We have a 30
gallon tank mounted under the flatbottomed sandblaster with a runoof
pipe at the top that drains into a 55 gallon drum, We recirculate and
reuse the water by having a sump pump in the 55 gallon drum.The sump
pump is hooked up to a hand water blaster that also has compressed air
hooked up to it to increase the water pressure.We don’t sandblast or
use anything that would mar the surface of the casting. Our investment
ratio is 40 /100 … if yours is not, then that may be where you are
having problems removing your investment. Using hotter water than the
investment manufacturer recommends will alsoincrease the strength of
the investmentmaking it harder to break out. Try wacking the flask
with a big rawhide hammer before quenching… when you see cracks in
the plaster on both ends, quench it and it should break away cleanly.
My recommendations for quenching times are the actual recommendations
from the metal suppliers we use . No, our alloys do not have nickel
or any similar alloys in it …

After removing 98 % of the investment, we put it in a hot ultrasonic
with a 30-40% vinegar and water mix… then we re-blast with water and
put the tree in pickle… presto… clean castings.

Best wishes to all,
Daniel Grandi