Casting reheating investment molds - flask blow-out

Jewelry Discussion
1 
    If you try with the vacuum, as soon as you put the vacuum on,
listen for a popping sound. If you hear it, the flask has just blown
out. This will leave an open path for the molten metal to go
through the flask and either into the  vacuum line, flask chamber
or even vacuum pump. Most of the time the molten metal just gets to
the brass vacuum table fittings and freezes up. All in all a lot of
work. Not only lost metal but rebuilding a vacuum system can be
time consuming and expensive.

This happened to me a couple months ago. I had not re-heated the
flask, but apparently the top layer of the investment wasn’t thick
enough, and the vacuum pulled it apart when I turned it on. I heard a
brief metallic “thunk” noise as I turned on the vacuum machine. I had
no idea what it was. So I poured 60 grams or so of molten sterling
into the flask. The vacuum machine made a straining sound when I
poured. I STILL didn’t realize what had happened until I lifted the
flask to put it aside to cool, and saw flames shooting up from the
top of the vacuum machine! I sat the flask back on the machine to
smother the flames, and grabbed the fire extinguisher. A few seconds
of panic ensued–how could I handle a hot flask that needed to be
moved, pull the pin on the extinguisher and shoot it at the machine
all at once? Fortunately when I moved the flask again, the flames
were out–it was just the silicone rubber casting pad that had burnt
a bit.

I put the flask on the cooling area and inspected the machine. I
feared that something inside was still burning, but found nothing. I
quenched the flask that broke, and found that the entire mold inside
had been “lined” with silver–I had some really pretty sterling lace
that was all hollow and crumbled when touched. I then left the shop
for a while to go calm down. Later I found that the metal had gone
through the vacuum lines and ended up in the clear plastic external
filter cup. The inner metal filter was permanently welded to the
cup, which had melted around the filter. And the plastic vacuum line
leading to the filter had been shriveled up from the heat of the
molten metal. A call to Swest produced a new filter and cup, and a
trip to the hardware store got more pneumatic tubing to replace the
damaged vacuum line. $25 in parts and the machine worked fine.
(actually it worked before we replaced the parts–we found that we
could swap around the external casting and investing filters, since
only one is used at a time). I’ve thanked the deities that watch over
idiots repeatedly since then that nothing worse happened to the
machine. And now I listen very carefully for weird noises coming from
the flasks every time I cast.

–Kathy Johnson
Feathered Gems Jewelry

2

tell me about the investing filters?? have had metal go down the
vacuum lines before, not as dramatic as yours, but still had to
replace the lines. lucky it did not get to the pump. a filter
sounds like a good thing. Rick

3 
    tell me about the investing filters?? have had metal go down
the vacuum lines before, not as dramatic as yours, but still had to
replace the lines.  lucky it did not get to the pump.  a filter
sounds like a good thing.

The filters are part of the design of my vacuum machine, which is a
Swest-Vac model. You can sort of see the filters on the right front
corner of the machine in the photo from the Swest online catalog (the
photo’s rather dark). I do not know if you can add similar filters to
other machines.
http://catalogs.google.com/catalogs?dq=&hl=en&lr=&ie=UTF-8&issue=1367
2&catpage=3 2&zoom=3

–Kathy Johnson
Feathered Gems Jewelry