An easy way to vent flasks for casting is to simply melt some wax
spru wires onto the inside of your flask. Melt it right at the top on
the inside. The position should start about 1/2 to 1 inch above the
bottom of the flask. I have used about 4 or 5 of them. You can look
down into the flask to determine the locations. The spru wax wires
should touch the inside wall of your flask and can be close to any
object to help air flow. Be careful not to have the wire touch any of
your wax items or you will have this wire cast with your object. Your
object will also be ruined. This process is fast and fairly
inexpensive if you buy the spru wax by the larger box.
I rarely use any vent system. At school we cast hundreds of dwt of
model metal brass every semester and at my own shop I cast about 25 to
50 oz of sterling and about 200 dwt of gold per week. I don’t use any
venting. And here is one reason why. This is one of the lessons in our
casting class here in Minneapolis. Casting investment is like a big
bag of cement. If you put the bag on the ground in your garage and let
it set, in a few months the ingredients start to separate. Without
touching it, the gravel, sand and concrete start to separate because
their specific gravities are very different. Just think of why a
cement mixer goes down the street and mixes so that all the
ingredients are well blended. Casting investment has some of the same
properties. Now add the truck transport of investment to your shop and
you have a vibratory method of investment separation. A barrel
sitting has the same thing happen. The investment powder looks all the
same, but believe me it is not!
When the investment comes to you it is designed to breath. Molten
metal goes in air flows out. It would be like a mouth full of smoke
going through a sponge. Now I would bet that if some of you had
problems casting on occasion and couldn’t figure out why, this may be
one reason. Are you towards the bottom of your barrell of investment?
Is your investment old? Has there been any moisture allowed to get
into the bag? Investment doesn’t come with a “use by” date you know.
And there is no manual on how to take care of the stuff. If you just
leave it alone I can guarantee you will eventually have problems.
As a caster, one of the jobs I did every week was to put a mask on
and shove my arm into the invest ment and mix it up. BY HAND! By doing
this I confirmed my suspicions about what happens. You can FEEL the
proper consistancy of the powdered investment. The bottom of the
barrel shold be just a fluffy as the top. When these ingredients are
mixed properly the investment will breath properly and your castings
turn out great. When doing this I have felt hard chunks of investment
towards the bottom of the barrel.
Now I hated to do this job. It was a mess and my arm was full of
investment. When I pulled my arm out there was powder everywhere.
Worse yet investment got under my fingernails. I hate stuff under my
fingernails. You can tell how much I live auto mechanics by that one.
So I thought all I needed was a small cement mixer. Well I found one
and hooked it up in my shop. We have one at school also. Harbour
Freight sells one for about $200.00. The only problem is that there
was no cover for it. I found a cardboard scrap recycling barrel with a
clamp on top that fit the cement mixer opening. After puting a strip
of edge protector on the metal edge of the clamp for a tight seal
(this was the stuff you put on your car door edge to keep it from
being damaged by bumping into another car when opening the door)., I
had my mixer and tight cover. EVERY TIME I mix investment, I stir the
whole supply. No more investment breathing problems. My castings turn
out great. Both spin and vacuum.
Vacuum casters have less of a problem if they have a super heavy duty
pump. The vac does not give the investment a choice. And it you
increase the size of the whole under the flask it works even better. I
never use the perferated flasks. With my investment preparation I
don’t feel I have to. And those perferated flasks a spendy.
One additional story. Back in 1979 I got 1500 pounds of investment
free. The casting foreman at the company I worked at said it was bad.
He taught me the same arm in investment method I described above. The
investment was going into landfill so I took it home. It didn’t seem
to be bad until one day in my arm mixing duties I pulled a mulched up
piece of a cardboard barrel out of the investment. An additional
ingredient! What some of you don’t know is that there have been
investment recalls in the past that most people don’t ever hear of.
Apparently a barrel had fallen into the mixing hopper and just been
mulched up and added to the batch. I used this investment for a few
years and gave quite a bit of it away to others casting buddies. But
from that time I alway mix my investment as soon as it arrives in my
shop. That way I kwow that this component of my process is consistent
and not a potential problem. Any change can create a big problem. By
doing this investment is never a problem.
All the Best,
TR the Teacher & Student