I never really kept track of what vacuum rating the chamber was at
when the rubber started to boil, but it was pretty well pumped out
before anything started to happen, so I’d say you need a reasonably
serious pump. Michael Knight reads Orchid, at least occasionally, so
I’m hoping he’ll hop in here and say what pressure you need to get
down to definitively.
The short form is that 30 inches of mercury (on the gauge) is
absolutely empty. (29.7 something, to be precise.) Zero inches is
room air. I’d guess the rubber didn’t really start to boil until
28-29 inches. I was doing mine in my investment chamber. So any pump
that’ll do for investment will pull hard enough for the rubber. But
for that, you need either a rotary pump, or a vane pump, which are
usually a couple of hundred dollars, even used. There are cheapo
little venturi pumps that use running water, and I’ve heard of
people using those for investment, but I don’t think they’d develop
enough vacuum to be really useful for rubber. (It being a lot
thicker than investment.)
On the other hand, if you spend money for a decent pump, you’ve got
the core of an investing system at least, and maybe a vac cast
system if you get a fast enough pump.
(For investing, you need low pressure. For casting, you need high
flow. (for just investing, you need 2(ish) CFM (Cubic Feet per
Minute) for vac casting, you want more like 5 or more CFM if you can
manage it, but you don’t need anything like as low of an absolute
pressure. I’ve seen vac casting done successfully with a shop vac.
Not low pressure, but very fast at what it does do.)
Actually, I just went and looked at ebay, to see what pumps you were
talking about. Found a couple of vane pump and chamber kits for
degassing silicone that look like they’d be just the thing, all for
only $160. Which is MUCH cheaper than I remember real pumps being.
I wouldn’t hold much hope that the pumps will last for more than a
few years, but they should get you down the road pretty well until
then, and it’s a cheap way to get your feet wet.