Three tricks we use in dentistry are:
1.) After the investment sets, scrape the top of the exposed
investment with a lab knife. The smallest particles float to the
top during the vibration process, and literally seal off the
ring. The investment has a micro-porosity to enable any trapped
gasses to be dissipated through the investment.
2.) About 15 minutes before you cast the flask, invert it in
the oven so that the sprue holes are up. This also will let
entrapped gasses escape. Be sure to remember that you
inverted the flask before you place it in the cradle in your
casting machine or you will think a meteorite landed in your
casting tub! A very expensive meteorite indeed!
3.) A long thin vent of 14ga. sprue wax from the thickest
portion of your pattern down to the sprue former base and
attached low on the base can also be a useful vent.
In dentistry we use a flask liner of some sort of inert mineral
fiber as a liner for the flask ( we used to use asbestos liner).
It comes with the packets of investment right in the case from
the Mfgr. (Jelenko Complete). It allows plenty of venting.
Could you possibly be in gassing your metal? Remember that you
are not trying to set a speed record for how fast you can melt
the metal. The flame, as Bernini said, should "kiss the metal".
With enough experience, you will be able to tell when the metal
goes from melting point to casting temperature. DON'T BOIL IT!
NRA Endowment and