vacuum pump questions...

I made a vacuum chamber out of small trash can, a couple scraps of lexan, and piece of flat silicone rubber (pictures below). I want to use it to de-gas poured urethane that I plan to use to make some Hobie turbo fins (pictures to follow...).

Today, I tested it out, using a vacuum pump that I borrowed from the local O'Reilly auto parts store (18 CFM, as I recall). It worked fine, except that oil got sucked into the hoses, which was a mess to clean up. Actually, the pump itself was very oily before I started, so I think there might have been some problem with it.

I'm considering buying a cheap vacuum pump, and would prefer that it be oil-free, both to prevent the oily mess I had today, and so that I can run it indoors. The oil-free pumps seem to be expensive and not as powerful. This one is suspiciously cheap (several of the oily kind are in this same price-range):

https://www.amazon.com/Vacuum-Oil-Free-Oilless-Diaphragm-Silencer/dp/B0D9BPC79V/

However, online it says that an airflow of 8 CFM is needed, and you need to be able to pull 29 inches of mercury (https://www.smooth-on.com/support/faq/111/) and I'm sure that oilless pump won't do either. So, maybe I'll have to go with an oily pump.

Any thoughts on a cheapo pump that might work?

Fwiw
"Usually "
Those rental pumps get used to pull vacuum on an ac unit.
Possibly someone fouled it priming the compressor on the car.
I've used the same pump you did and it was dry.
Good luck, post progress pics, please

Maybe three years ago I bought an amazon oil lubricated pump for degassing 2 part silicone rubber -- specs attached below. Sounds like my vac chamber is substantially smaller than yours; mine's diameter is ~6" and depth is ~4". It's pretty decent for my use cases, but I suspect I'm degassing significantly smaller amounts of goo than you intend to degass. It's still going strong, but I don't use it all that much.

Never had any issues with oil leaking. I didn't realize I shouldn't be running mine indoors.... crap, thanks for the heads up.

You may already be aware but I've noticed other websites are often substantially cheaper than amazon even when shipping is figured in.

Just spitballing but if your urethane takes some time to set you could maybe degass in multiple batches?

Not to X-Y your question, but have you considered a pressure pot instead of a vacuum chamber for this?  With moderately careful mixing and a pressure pot, bubbles are usually squeezed small enough to not be an issue structurally or aesthetically and it's far easier to pressurize than depressurize using home equipment.

Fwiw
"Usually "
Those rental pumps get used to pull vacuum on an ac unit.

Yes, I’ve actually done that, as I installed a mini split AC unit (twice, actually, as the first unit got fried in a power surge shortly after it was installed). I never noticed this oil issue previously.

Not to X-Y your question, but have you considered a pressure pot instead of a vacuum chamber for this?  With moderately careful mixing and a pressure pot, bubbles are usually squeezed small enough to not be an issue structurally or aesthetically and it's far easier to pressurize than depressurize using home equipment.

I’d read that the rule of thumb is vacuum chamber for flexible stuff and pressure pot for solid casting. Since this is rubber for Hobie-like fins, it’ll be flexible. Also, I was under the impression that you have to cure under pressure, and my mold is about 7”x 16” x 1”, which would require a massive pressure pot. Anyways, thanks for the suggestion and I’ll do a bit more research…

Maybe three years ago I bought an amazon oil lubricated pump for degassing 2 part silicone rubber -- specs attached below. Sounds like my vac chamber is substantially smaller than yours; mine's diameter is ~6" and depth is ~4". It's pretty decent for my use cases, but I suspect I'm degassing significantly smaller amounts of goo than you intend to degass. It's still going strong, but I don't use it all that much.

Never had any issues with oil leaking. I didn't realize I shouldn't be running mine indoors.... crap, thanks for the heads up.

You may already be aware but I've noticed other websites are often substantially cheaper than amazon even when shipping is figured in.

Just spitballing but if your urethane takes some time to set you could maybe degass in multiple batches?

FWIW these are the same type of pumps all chamber vacuum sealers use, and theyre regularly used in commercial kitchens indoors..  I replaced the pump in my Chinese sealer with a $50 amazon unit and there identical (*and still running strong 1000s of lbs later)

Maybe three years ago I bought an amazon oil lubricated pump for degassing 2 part silicone rubber -- specs attached below. Sounds like my vac chamber is substantially smaller than yours; mine's diameter is ~6" and depth is ~4". It's pretty decent for my use cases, but I suspect I'm degassing significantly smaller amounts of goo than you intend to degass. It's still going strong, but I don't use it all that much.

Never had any issues with oil leaking. I didn't realize I shouldn't be running mine indoors.... crap, thanks for the heads up.

You may already be aware but I've noticed other websites are often substantially cheaper than amazon even when shipping is figured in.

Just spitballing but if your urethane takes some time to set you could maybe degass in multiple batches?

FWIW these are the same type of pumps all chamber vacuum sealers use, and theyre regularly used in commercial kitchens indoors..  I replaced the pump in my Chinese sealer with a $50 amazon unit and there identical (*and still running strong 1000s of lbs later)

Good to know. I see several in the $50 range, so I'll give one of them a try...

I've used a couple of the cheap-o piston pumps for de-gassing, and they all consumed oil and sprayed it out. So, don't use it in the kitchen. I was skeptical, having worked with industrial ones before, but they do work pretty well for hobby stuff like this.

Quality diaphragm vacuum pumps (Edwards, Gast, etc) pop up on Craigslist and local liquidation auction sites fairly often, but are often $200+ and run on 220 single phase.

most of them have a simple filter on the oil fill cap so any aerosolized oil vapor is trapped