Consulting my US Navy Foundry Manual pages 221-222 [see #20072 in
http://www.lindsaybks.com/bks/foundry/index.html ] indicates two
possible problems. [I highly recommend this book -- it is low
cost and filled with practical information.]
(1) Your ladle and/or mold may not be completely dry allowing
the melt to pick up hydrogen from the water vapor.
(2) Your melting procedure is allowing the metal to pick up
excessive hydrogen from somewhere. This can be caused by too
slow/long a melt or by a reducing [hydrogen rich] atmosphere.
Manual suggests an oxidizing atmosphere. In addition, your melt
might be at too high a temperature when you are making the pour.
You may need to degas your melt before pouring, especially if you
are using scrap/salvage metal. If the top of your casting is
domed or convex when it has cooled this is an indication of
excessively gassy metal. There should be a pronounced dent or
dip [called a pipe] on the top when the casting cools with good
metal.
Which bronze alloy are you using? There are several cheap things
you can try to degas the melt depending on the composition.
You may also need to add more vents to the mold to allow the air
to escape. As my foundry teacher hammered home to me "if the air
can't get out, the metal can't get in."
=======================
On Fri, 26 Aug 2005 18:46:33 GMT, "jr"
wrote:
Mmmm, I think it would be not the problem, because the mold is an open
mould, so there is a lot of metal over it. Think it's a cylinder, so the
mold only have an entry. I will try to draw with characters:
******** *********
******** *********
******** *********
******** ********* Think in the figure as a box with a
cylinder hole in the middle.
***********************
***********************
The * are the mold, and the [spaces] the cavity of the mold. I fill it all
with melt bronze. The result piece I get is like that:
--------------------
--------------------
-------------------[]
--------------------
-------------------- Think in the figure as a Cylinder
--[]-----------------
---[]-----[]---------
--[][]----[]------[]--
Being the [] holes in the bronze resulted piece. Of course the "-" are solid
bronze. It seems there were captured some bubbles in the piece.
I will try to make some pictures right now and post here.
Thanks
JRL
"F. George McDuffee" escribió en el mensaje
.. .
On Fri, 26 Aug 2005 16:01:50 GMT, "jr"
wrote:
Hi,
I'm just trying to make a mold with plaster + kaolin (25% + 75 %) to hold
bronze. The mold works fine but the final piece have a lot of big bubbles
inside the bronze , so the piece it's totaly unusable.
any idea?
Thanks
JRL
==============
That the bubbles are inside the casting indicates that these may
not be bubbles but rather voids left when the metal solidified
and shrank. Water expands when it freezes but metal shrinks by
quite a large amount in volume when it changes from liquid to
solid. In casting it is common to provide a large volume of
liquid metal in the mold above the casting [the riser] to feed
metal into the casting as it cools and solidfies. The sprues and
runners that feed the metal into the mold cavity must be large
enough so these will not solidify first so that additional liquid
metal [from the riser] can fill any voids in the mold cavity.
Design and creation of molds to allow ouring of sound castings is
an art. Lindsay books has a good selection. see
http://www.lindsaybks.com/prod/sub/foundry.html