Selecting a Vacuum Pump

[blind pig]

That's OK, those of us in Gods' Country, Queensland Australia, have to rub it in occasionally to those poor people in the rest of the world.

[Magnetic Duckling]

Heeey guess who wanna use vacuum along with pressure \o/
I made my first tryouts with pressure and it is how it feared, issues with the bubbles on the silicon making little spikes on the resin when they collapse. I have to get bubbles free silicon molds.
My only option right now apparently is to let the silicon cure under pressure but it consumes too much time (3 to 24h to cure) and I don't want to let it cure overnight without supervision.
All that to say, I wanna go the vacuum way to remove bubbles from my silicon.

I post here because it seems related enough to the topic (spank me if it is wrong).
Questions:
-I plan to use the pressure pot but get a transparent lid. So what is the name of said lid (so I can search for one) used to vacuum and what kind of gauge do I need?
-why the f**k don't I live near the sea => http://www.ebay.co.uk/itm/Robinair-1560 ... 4cffbebd92

[Deadmeat30]

For a lid, get some rubber gasket material, then 10mm polycarbonate sheet. Works a treat.

[geomod]

The clear lid I use for my vacuum work is 1cm lexan (bulletproof plastic used by Mythbusters), , although I have seen people use 1" thick perspex. I picked it up from a local plastics supplier and used epoxy to attach air fittings/taps etc. To seal it with the paint pot I have used sponge foam sealing strip I had lying around.

Under full vacuum, the lexan does flex but has not shown any indication of failing even after 12 months of irregular use.

Cheers,

Geomod

[mangozac]

[Magnetic Duckling]

[mangozac]

OK well I wouldn't hesitate to leave the pot under pressure overnight. In fact I used to do that frequently before I was using Pinkysil.

Your issue of time has only two solutions like you suggest:
- Use a pressure pump to degas the silicone. This will require you to paint the degassed silicone into all of the detailed areas of the part before you pour the rest over it.
- Get a faster curing silicone.

[blind pig]

[Anvils Hammer]

Buy a vacuum pump and learn to make single peice split molds asap.

I spent months working with two part molds and now I realise how much faster, easier and better split molds are, im kicking myself for not making the switch earlier.

If you are serious about doing a fair ammount of casting long term, get a vacuum pump and learn to vacuum/pressure cast.

If you buy my existing pump it would give me the excuse I need to drop a grand on a bigger and better one..

Incidentally, get a 6 hour cataltst! save a ton of hassel.. you can get 2H catalysts but the molds will deteriorate faster, where are you currently getting you silicone?

AH

[Magnetic Duckling]

Split molds looks cool but I am not sure it would work out with my hollow parts. I am still thinking about it. I am afraid of damaging my original when slicing ;_;

I still have some of the RTV 181 silicon I bought back then in Paris (I bought a 26kg bucket), if I use 2% of catalyst it cures in 24 hours, 5%: 2h45.

If you are serious we could work something out. I am unhappy to have to buy a new expensive pump when I see cheaper used ones on ebay but pick up only or guy with 1.002 transaction that I dunno if I can trust (for some reason I tend to trust you guys more than strangers on ebay xD).

[blind pig]

[Munkey Joe]

I also am looking into Vacuum for my casting..... As my first ones failed miserably as Bec said they would. Im talking to some of the welding contractors who work on our freezer units to get a good discounted pump. So far no one has come up with one But Im assured they are actually easy to find....

[SSB]

[mangozac]

Hey SSB,

OK for starters, the calculations and times I reported in my original post are theoretical only and there are several factors which make the actual evacuation time longer. The most prominent is probably slight leaks in the plumbing that allow air in as it is vacuuming.

Your calculation of the pressure pot volume sounds correct. It's pretty common for manufacturers to significantly round specifications to suit - the M54B25 engine in my car is branded a 2.5L engine by BMW, yet the actual displacement is only 2.494L. I know that's not a big difference, but I've seen other scenarios where there is as much of a difference as you have calculated and in an application such as the original purpose of the pressure pot nobody is going to complain if it's actually bigger than advertised!

I don't think there would be 2L in the dome at the top of the pot. Maybe 1L might be closer?

Still, getting to a full -30inHg in 50-60 seconds is not bad. I'm guessing it reaches -27 in around 20 seconds?

If the volume of the pressure pot is too large you could fill it with materials to take up space: lead weights, gravel, etc. I have mine half filled with beach sand (although it makes a mess if I vent the vacuum too quickly!).

Hope that helps...

[SSB]