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