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High altitude vacume casting


#1

My ten year old Master Craft Vacuum caster worked just fine when I
lived at 1000 ft. Now I live at 6000 ft and I am wondering if this
is the problem with my sudden poor performance casts. Can I get a
more powerful vacuum pump? Any suggestions on suppliers?


#2

It is a mistake to think of it as a vacuum casting machine, it works
by removing the air in the mould and using the weight of the air
above you to aid gravity and push the metal into the mould.

You do not have as much air above you so much less push, you cannot
ge a pump that gives you more vacuum, only vacuum for a longer time
when casting or faster evacuation when degassing investment.

If you are getting surface bubbles, adding slightly more water or
increasing the water temperature will help, you will probably have
to adjust ,ie shorten the total work time as a result.

If you are not getting complete filling the extra water in the mix
may make the plaster slightly more porous, not sure about that, or
you may have to go the centrfuge route

regards Tim Blades.


#3

I live at 6500 ft and vacuum cast without any problems. That being
said, you will not be able to pull the same level of vacuum that you
could at 1000 ft so will have less margin for error. Make sure your
pump is in fine shape, filters are clear and there aren’t any leaks
in your system. Any and all vac pumps are limited by the atmospheric
pressure they are operating in so a different type, or bigger pump
won’t make a difference. (Unless your old one is shot)

Good luck!
Harry
www.harryhamilldesigns.com


#4

There may be some help if the problem with your castings is caused
in the mold. The vacuum you will see under the bell jar at high
altitude is less than you would see at 1000 foot elevation. This can
cause bubbles (small air pockets) to form on the wax and then
bubbles of the metal casting.

I remember way back when, and that is a long time ago, a fellow
caster suggested washing the waxes in green soap. Then shock the
vacuum table with short taps of a hammer when pulling a vacuum on
the wax. Don’t want to tap too hard as this might cause waxes to
break free form the sprue. This will cause the air bubbles to break
free form the wax and float to the top.

There are wax release sprays that can be sprayed on the wax that
will prevent air bubbles from sticking to the wax when investing

If you are getting surface bubbles, adding slightly more water Mixing
investment with too much water will cause bubble trails (like very
small flash) to form on the casting. At least I think that is what
thin vertical lines on that metal casting is caused by. This problem
is a greaterproblem than bubbles.

Lee Epperson


#5

Thanks you are for you recent helpful answers. You may have saved me
from spending 400 on a new vacuum motor. I will check and give it a
try.

Coit


#6
My ten year old Master Craft Vacuum caster worked just fine when I
lived at 1000 ft. Now I live at 6000 ft and I am wondering if this
is the problem with my sudden poor performance casts. 

Whoa, you call that high altitude? I have no problem at 8500
although my pump is a Ghast oil-less rotary vane style. This is not
like baking a cake at altitude (but your burnout might take a bit
longer). If anything, the lower air pressure should help not hinder
drawing a vacuum. Could be time to tear your pump down and rebuild.


#7

You might also want to just put a glass with an inch or so of water
under the bell and see if it boils vigorously in 30 seconds or so…a
good pump and system without leaks will boil in less than thirty
secs…smaller pumps might take almost a minute…Any fault with
either the pump or leaks will prevent the rapid boiling.

John