hi all you casting mavens,
i’ve been experiencing bubbles on my castings and have trouble
shooted enough to ask for your help. i’ve had bubbles prior to
all the conditions i’m about to describe. i still have them, but
less of them. first i assumed that my mixer was introducing too
much air into the ivestment, so i bought a kitchen-aid (made by
hobart, model k5)super heavy duty mixer and mix with flat
spatula (no wisk!) or the dough hook. though i needed a new
mixer, this did not help much. after i bought the mixer, the
stainless bowl didn’t fit into my old bell jar so… i bought
a larger 10" bell jar, (the i.d of the old bell jar was about
8’’) then everything fit nicely. i put the stainless bowl under
the bell jar and debubble. but i still have bubbles.
my vacuum machine gauge indicates 27 inches of mercury, after i
tap it a few times, (thats all the vacuum is capable of, we are
at sea level) then i play punching bag with the investing table
for about 1 1/2 minutes. since the indicator gauge is pretty old
(over 20 years???), i didn’t think it was accurate, so i was
assuming the vacuum unit was fine. it is only about 9 years old,
a 3cfm. the next thing i did was to connect a direct line only
to the investing table. i got what seemed like a more vigorous
boiling, but… still bubbles on my castings, but not as bad.
all the bubbles are located on the inside of shanks at the 12
o’clock position, even though i tilt all my waxes a bit. i may
have short changed the time a bit on this batch. of course i
vacuum the flasks again after i pour the investment in them.
after i finish the second vacuum, i swirl the investment around
in the flask to hopefully dislodge any bubbles.
without investing anything i created a vacuum in the bell jar to
see how long it would hold the vacuum. without any indicator i
figured this would be inconclusive without comparitive data. but
it did hold for the few minutes it took for me to figure that
out. so my next step is to put a new gauge in line with the
direct line. this will tell me exactly what vacuum i’m drawing.
finally here are my questions:
do surface tension reducers get old? i pour the s.t.d. back
into the bottle after i’ve used it. i’ve had the same quart
bottle for about 2-3 years.
what is surface tension reducer? is it just denatured alcohol
with pink dye?
i’ve always read that 27 was an adequate vacuum. is 27 too
i put my old gauge indicator directly on the vacuum pump and
only got a 26 indication. assuming i got a good seal, does one
need an evacuation chamber (bell jar) to get an accuarate
i’m aware that the larger bell jar means more air space for
my 3cfm to remove air from. assuming the vacuum unit is good, am
i asking too much from this size vacuum unit.
if a 3cfm unit isn’t adequate enough, what could you guys
suggest that would be a cheap alternative to a 5 to 8 cfm vacuum
unit? i know about old car air conditioners working, but i
assume that they are more comparable to a 3cfm unit. perhaps a
large home air conditoner compressor would work?
has anyone tried to reverse the fittings on a standard air
compressor so it sucks instead of blows? i sort of put my finger
over the intakes of my compressor, WOW!, did it suck! i’m happy
to still have my fingers unbroken. no easy way to put a hose on
this particular compressor tho.
could some one explain ‘negative inches of mercury’ to me?
many thanks to all who respond to any of these questions! i know
it is asking a lot.