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Selecting a Vacuum Pump

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Re: Selecting a Vacuum Pump

Postby blind pig » Wed Mar 21, 2012 11:26 pm

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.

:P
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Re: Selecting a Vacuum Pump

Postby Magnetic Duckling » Tue Mar 27, 2012 6:06 am

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
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Re: Selecting a Vacuum Pump

Postby Deadmeat30 » Tue Mar 27, 2012 6:56 am

For a lid, get some rubber gasket material, then 10mm polycarbonate sheet. Works a treat.
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Re: Selecting a Vacuum Pump

Postby geomod » Tue Mar 27, 2012 7:09 am

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,

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Re: Selecting a Vacuum Pump

Postby mangozac » Tue Mar 27, 2012 9:16 am

Magnetic Duckling wrote:I don't want to let it cure overnight without supervision.

Why not? It's a waste of money buying the vacuum pump just for degassing the silicone when you can use pressure to crush the bubbles anyway. I pressurise the mould while it cures, although the Pinkysil only needs about 30 minutes (I leave it under pressure for an hour)!
Oh yeah I can make that....
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Re: Selecting a Vacuum Pump

Postby Magnetic Duckling » Tue Mar 27, 2012 9:25 am

mangozac wrote:
Magnetic Duckling wrote:I don't want to let it cure overnight without supervision.

Why not? It's a waste of money buying the vacuum pump just for degassing the silicone when you can use pressure to crush the bubbles anyway. I pressurise the mould while it cures, although the Pinkysil only needs about 30 minutes (I leave it under pressure for an hour)!


The manufacturer's manual says not to let it on overnight *fear*
The main issue is that the silicon take HOURS to cure, I have 16 molds to make, 2 parts for each, or even 3 sometimes, so my pressure pot would be busy for days and days and days, efficiency fail if I cant cast any resin in the meantime.
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Re: Selecting a Vacuum Pump

Postby mangozac » Tue Mar 27, 2012 10:00 am

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.
Oh yeah I can make that....
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Re: Selecting a Vacuum Pump

Postby blind pig » Tue Mar 27, 2012 10:20 am

mangozac wrote: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.


Or get another pressure pot......

I leave my pressure pot under pressure for 12-18 hours whilst the silicone is curing.
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Re: Selecting a Vacuum Pump

Postby Anvils Hammer » Wed Mar 28, 2012 12:28 am

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.. :mrgreen:

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?

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Re: Selecting a Vacuum Pump

Postby Magnetic Duckling » Wed Mar 28, 2012 1:09 am

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).
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Re: Selecting a Vacuum Pump

Postby blind pig » Wed Mar 28, 2012 11:33 am

Magnetic Duckling wrote: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


There is a company called Forge World that makes resin models, called Titans. These models range in size from 20cm upto 70cm tall. They are made using split moulds.

Linky: http://www.forgeworld.co.uk/Warhammer-40000/Titans

The moulds of that size would need a wobbly/zig zag cut down the length of the model, to allow you an easier alignment of the split. Support the silicone mould when curing would be needed.

I'd contact people on ebay that are selling vacuum pumps, asks if they wouldn't mind posting/freighting it to you. Give contacting them a go, they will only say "yes" or "no" and it will have cost you nothing.
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Re: Selecting a Vacuum Pump

Postby Munkey Joe » Wed Mar 28, 2012 1:42 pm

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....
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Re: Selecting a Vacuum Pump

Postby SSB » Tue Apr 02, 2013 1:44 am

I have a doubt...

I have a :
51-201C C.A. Technologies Resin (Mold) Casting Pot - 2.5 gallon teflon lined steel pressure pot with wheels and single regulator. Pot is non-asme rated with maximum pressure up to 80 psi. Internal dimensions are 10-3/8" Dia. x 8-3/4" deep.

I am puzzled because when I do vacuum in this pressure pot with a 8 CFM (226 liters/Min) Vacuum pump, I get to -30 Hg in about 50 to 60 seconds... Which doesn't corresponds to the 2,5 Gallons (2,5 US gallons = 7.57082 L) indicated for the pressure pot... even in Imperial gallons (2,5 imperial gallons = 9.09218 Liters)

It is well over the 10L vacuum :
mangozac wrote:8CFM = 0.41 minutes (24.6 seconds)


But when I calculate the volume, it does not correspond to the "2.5 gallons" volume indicated...

10-3/8" = 26,3523 cm Diameter
8-3/4" = 22,225 cm deep

That makes a πR²H = π x (26,3523/2)² x 22,225 = 12115.882988447658 cm³ = 12,116 Liter

And with that, I do not include the spherical portion on the top of the pot that might be 2 liters worth... (I would have to calculate the volume it represents)

This might be why I do have to vacuum a lot more...
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Re: Selecting a Vacuum Pump

Postby mangozac » Thu Apr 04, 2013 10:16 am

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...
Oh yeah I can make that....
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Re: Selecting a Vacuum Pump

Postby SSB » Wed Apr 10, 2013 4:50 am

mangozac wrote: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.

Ok, I do have that in mind :)

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

I'll get more precise when I can...

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

I honestly don't know. I'll check out.
-27 is sufficient for degazing ?

mangozac wrote: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!).

I'll think about it !
This could really speed things up !
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