Hi, I just had a situation where I put a flask on the vacuum and the
investment came unstuck all in one chunk and got sucked down onto the
silicone pad. I tried putting it back into the oven and pouring it
anyway but it wouldn't hold the vacuum and the casting was a failure.
I remember this happening once before, many years ago, and thinking
it may have been some oil on the inside of the flask that prevented
the investment from adhering to the steel. This time it was 2 custom
rings that I spent quite a while carving and my time was a total
loss. Has this happened to anyone else? Is my hypothesis correct
about oil on the flask? I really hope this never happensto me again.
The only time I had that happen was when it was a brand new flask
with perfectly smooth walls and I didn't fill the flask all the way
to the top with investment. What happened in my case was about half
the investment broke away and was sucked down to the pad. Ever after
that I filled the flasks all the way to the top, then after it dried
I took my spatula and scraped it off flush, then scraped it so it
was slightly concave. That created a little air space but didn't
allow the investment to shift. That did the trick and I only had the
problem that once. But I also think that once the flask is used a
few dozen times the inside surface becomes irregular enough that it
sort of holds on to the investment better.
Incidentally, I usually use 2x2.5 inch flasks and I noted that I can
do about 100 burnouts before the walls get too thin and I need to
replace them. I thought they would last longer than that, but I just
happened to keep track.
After I clean a flask after use, I spray liberally with auto lube
silicon spray (Liquid Wrench). It slows down the rusting of stainless
steel flasks. It does not keep the investment from sticking though. I
am meticulous about keeping the bottom of the flask or flange as
smooth as possible for a good seal. I also use the spray on silicone
pads to keep them smooth and clean. No adverse affects regarding
smoking or burning.
Keeps the spru bases very flexible. When they get really dry I use
Silicone 303 from an auto/marine store.
Perhaps the flask was not complete full of investment? I think the
top of the investment should be even with the top of the flask. Then
scoop out a little to create a good suctions spot if you are setting
the flask on top of a pad and not into a well. I'm still pretty new
at casting so someone will correct me I'm sure.
Do not know for sure about your problem. but in 40+ years never
experienced what you have. I also never allow oil to get close to my
flasks. So your thoughts might be correct.
I slightly over fill the flask.
I use the convex edge of a trowel to scrape away the extra
investment. Scrape the investment until the edge of the trowel
scrapes on the bottom edge of the flask.
This process forms a slight dome in the investment which distributes
the vacuum to a better degree.
The outside edge of the investment will press against the silicone
pad whenpulling a vacuum. The investment cannot slide out of the
If you are casting sterling check out the paper I wrote and posted
to the TIPS FROM THE JEWELERS BENCH on fire scale prevention during
vacuum casting. The process works and does not cost and will save a
large amount of work requires to remove fire scale.