Marc if you have not tried the plastic webbing that goes on the
inside of the flask to vent gases I would reccomend that you do. I
use them for centrifical as well as vaccume and will not cast
without them. They seem to really help solve gas porousity.
If you want to try venting without purchasing the webbing or other
products made expressly for that application you can use soda straws.
To use soda straws, place a number of them in the flask before
pouring the investment. Place them in the flask so the bottom of each
straw is solidly on the bottom cap of the flask. The rest of the straw
should stand vertical & contact the inside of the flask all the way to
the top. If necessary, long straws can be cut off so the protrude a
little above the side of the flask. Double stick scotch tape can be
used to hold them in position until the investment is poured. Any size
straw can be used. If large diameter place them about 1 inch apart
around the inside of the flask. If small straws, link drink stirs, are
used place them closer together.
The plastic webbing or the straws provide surface area to the outside
of the investment that is exposed to the atmosphere. Any gases that
percolate through the investment have a shorter & unrestricted access
to the outside, they’re not held in by a solid metal sleeve (the
flask). Because part of the investment is still in contact with the
inside of the flask, it provides sufficient structural integrity to
prevent the investment from disintegrating when the vacuum is turned
on or the centrifuge is turning.