Hello Todd and Daniel, What great I have been using a
centrifuge for casting as I do primarily small production one of a
kind pieces. I now have the need to cast up to 255 pieces that will
be vacuum cast. Each pour will be approximately 2.5 oz. I cast the
prototype without the aide of any web or sprue wires and had no
problem. But for the production run I was considering using the old
trick of dipping the flask partialy into melted wax before
investing. Can either of you fill me in on the pros and con of this
method.
John Sholl
Dear John, I have never used the dip method. The easiest way to vent
is to position spru wax on the inside of the flask. Start about an
inch from the bottom of the flask. Melt it onto the inside of the
flask to position it. Put the wax wires where you might have the best
flow of air. Make sure the wax wires don’t touch a wax pattern or
you’ll flow the metal right into the machine. We have that happen in
class at least once a semester.
Here’s a little tip. We have at MCTC a commercial vac/investment
machine. This is the kind with the chamber for the perforated flasks.
I do teach this style also. But most of the time we put the flat
plate on top and cast with regular flasks. In the bottom on the
chamber pour about one half inch of investment and let it dry. If
there is a blow out or flask that doesn’t work and metal flows
through it, it will solidify on the bottom and be easy to remove. If
it solidifies on the metal bottom as supplied, you will always be
looking at a blob of metal in that chamber.
On the big vac system I built I made a brass trap that goes directly
under the table. This look just like a water trap under the sink.
Cost is about $15.00 for the fittings. I have had one blow out and
the metal drops into the brass pipe and solidifies. This part has to
be sent to the refinery but my unit is not ruined. The down side of a
powerful vacuum machine is that it will suck the bottom of the flask
off every once in a while. This usually is just a small fin of
investment. The popping sound it makes means I may have a bad flask.
It the kind of sound you never want to hear as you pour 200 ounces of
metal into a flask. Most of the time it happens right away so you can
stop and check the bottom of the flask.
It is also important to have a good filter between the vacuum table
and your machine. That investment on the bottom of the flask does
travel and can kill a good vacuum pump after a period of time.
Best Regards,
Todd Hawkinson
TR the Teacher
T.R. Hawkinson, Ltd.
Hello Todd, I also have a Welch Duo-Seal vacuum pump that I use for
de-airing RTV molds. I am planning to use the same pump for a vacuum
casting table, and would like to know what kind of filter you are
using between the pump and the table. All of the filters that i have
found are laboratory vacuum system filters, and frightfully
expensive.
Jack Reisland
The down side of a powerful vacuum machine is that it will suck
the bottom of the flask off every once in a while. This usually is
just a small fin of investment. The popping sound it makes means I
may have a bad flask. It the kind of sound you never want to hear
as you pour 200 ounces of metal into a flask.
Hi Todd, We use a 38 CFM high vac Busch Vaccuum pump… I’d say
that’s pretty powerful… We have never sucked the investment out of
the bottom of a flask unless the investment mix was wrong or if the
investment did not cover the upper most piece by at least 1/2 " .
This is Very rare in our case as all our investment measuring
capability is all done through digital scales for both water and
Investment ( 20 -30 lbs at a time) … This is also why we have no
Blow outs caused by investment problems since we went digital a
while back.
I don’t know if you meant to say 200 DWT or 200 grams… a 200 oz
melt in one flask is a bit much don’t you think 
? We have done a
200 oz melts… using a Johnson Gas blast furnace… But It didn’t
all fit in one flask…
You had mentioned in an earlier post that you did not need to use
wax or paper vaccuum liners as your pump was so powerful , it was
not neccessary… We have done this also with our large pump, but,
you still have to be at least 100 degrees hotter than if you used a
paper liner. We currently cast complex filigree wire work at 1150 oF
with the liner… With a 38 cfm high vac pump… They also cast at
the same temp with a 3 cfm pump… without the liner, we have to go
to 1300 oF. This is assuming metal temp of 1850 oF.
Any How, gotta stop writing … damaged my wrist and neck in an auto
accident Last week… typing left handed when you are right handed
is no fun. Best wishes, Daniel Grandi
We do casting and finishing for designers, sotres and people in the
trade contact sales@racecarjewelry.com
Hi Jack, I use a car/ truck oil filter… you buy the add on filter
kit fro a car which is mounted to the side of your case… then you
use regular oil car filters ( $3.50) keep a few in stock… when
they are dirty, Chuck them out the door I’ve been running my vaccum
pumps/ airlines and anything that needs to be filtered through car
oil filters for over 20 years and have not had a problem.
Daniel Grandi
Dear Jack,
My filter is a lab filter system. I was luck enough to by it at an
auction for about $50.00. It has a inner filter that can be washed
out. The inner filter is charcoal with a steel mesh liner. That whole
thing goes into a plastic shell I mounted under the bench before the
pump. The filter is a LABCONCO from MV Products of Mass. Their number
is 617-667-2393. I don’t know what they cost new, but they work like
a charm.
Best Regards,
Todd Hawkinson,
TR the Teacher
T.R. Hawkinson, Ltd.