Can anyone give me some guidance on what I need in the way of a vacuum
pump for "de-airing" a resing casting? All I've been able to determine
so far is that I need one that will pull 29 inches of mercury/-1 Bar of
vacuum. Is that enough? Too much? What type of vessel do I use to hold
the resin? Do I need to modify something or buy them ready built. I'm
looking for help on capabilities, brand names, anything really. I'm a
"newbie" on this technique. TIA.
Gast makes some nice pumps and do google searches for "polycarbonate
vacuum dessicator"......the pump will run about $700 and the dessicator
about $120, vacuum tubing however will run about $100/20'.
As far as I am concerned, the air that is in the mold cavity when you
pour the resin is of more concern than the air dissolved in the resin.
Most bubbles in parts, in my opinion are due to air trapped in the mold
by poor mold design. The mold should have adequate height, especially
in the pour riser, to create some hydrostatic pressure head, the pour
riser should NOT enter the top of the mold cavity, it should enter at
the bottom (j-shaped riser) and there should be a vent tube for every
local high spot in the cavity. If you design the mold properly, you
will get good castings even without pressure or vacuum. This is not to
say that pressure or vacuum don't have their place, but proper mold
design is a simple and cheap thing to perfect first.
There is only one reason to de-air resin: If it is very thick and the
bubbles won't rise on their own. If this is the case, the resin had
better have a really long working time so the de-airing process can
actually be accomplished.
The best way I've found to get rid of surface bubbles is to coat the
mold with talcum powder. Just dump some in, whack the mold around
until you are satisfied the powder has come in contact with the entire
surface of the mold cavity, then dump out what's left. You just want a
very light coating, like it almost isn't there. Make sure no pockets
of powder are stuck in corners or recesses. For most resins, this will
do the trick and you won't have any surface bubbles. If this doesn't
work, the resin you are using is thicker than most and you need to use
a pressure chamber. However, a pressure chamber is useless unless you
have de-aired the rubber before pouring the mold. Since you obviously
don't have a de-airing chamber, your molds are not de-aired, so you're
stuck. Try the talc. It cheaper than buying a lot of equipment. If
it doesn't work, switch to a resin that is more viscous than what you
One last suggestion. If the talc doesn't work, try again. Use the talc
and once you've poured the resin in, dump it back out and let the mold
sit for about 10 seconds. The put the resin back in. You're only
trying to defeat the resin's surface tension, so in putting the resin
back in the possibility of more bubbles forming is very unlikely.
The last time I vacuumed resin was in fact the first and last time I
will ever vacuum resin. It came out of the mould in the blink of an eye
and made a big mess.
I vacuum my silicone rubber moulds but pressurise resin.
If you construct the mould correctly you don't really need to vac or
pressurise it .(as per other posted reply)
My vac/pressure pump is an old refridgeration compressor ,I just change
over the hose over from the suction side or the pressure side. 29inches
Hg is easily obtainable.
I bought a new Chinese vacuum pump that does 0.5 mbar absolute and 6 CFM
for slightly over 200 euros. To de-air silicone rubber you need 30 mbar
absolute or less in my experience, so this pump will do very nicely.
The vacuum chamber can be an old pressure cooker, something welded
together, or a polycarbonate desiccator. But be careful! An implosion can
be as dangerous as an explosion. Take a peek at the implosion photo
titled 'A sharp piece of wall/side punctured a water bottle':
Two more tips:
- become a member of YahooGroups 'casting' list; resin casting is one of
the main subjects in that group.
- 'Secrets Of Expert Mold Making & Resin Casting' by Karl Juelch is about
to be reprinted and the probably the best book on the subject.
Ah, that may be with thick resins. Personally, all the resins I use
(urethanes) are very thin (so called water pour- maybe they are not as
thin as water, really, but pretty close) and so do not need to be
de-aired. Is there some advantage to these thicker resins? I also do
not use the fast setting resins as I want plenty of working time, to
mix well and pour carefully.
Even with the thin resin I use, the main bubble problem I have is with
air already in mold when I pour. It can get trapped in mold,
especially if I do not design pour sprue and vent risers well enough.
I have a vacuum chamber that I used successfully for a dozen years.
Since I can't do casting anymore (that stuff is toxic and I got
slightly contaminated) I'll sell it. It's worth about $200, I'll sell
it for $150 plus actual shipping cost (whch may be expensive, this
sucker is heavy!).
Anybody interested drop me a line.
D> > There is only one reason to de-air resin: If it is very thick and the
Don- I use water-thin(ish) resin as well, but there have been some
occasions where I've used a clear urethane resin that was really thick.
It definitely needed to be de-aired and it had the working time to
make that possible.
Other than that, I stay away from thick resins.