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De-airing silicone?


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#1 Aaronw

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Posted 27 March 2007 - 07:48 PM

I have been trying to pressure cast, primarily for use with clear resin. The pressure side works fine but the problem I've found is the vacuum side. How do you use the vacuum chamber and how much vacuum do you use. Do you de-air the silicone then pour the mold or do you pour the mold then place it in a vacuum to cure?

Any help is appreciated.

Thanks

#2 Zoom Zoom

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Posted 28 March 2007 - 02:18 AM

I've only watched the process, but you de-air the silicone after mixing it and before pouring the mold. The silicone expands a tremendous amount, the mold box would be an utter mess if you poured the mold and then vacuumed it. The vacuum chamber my friend was using was clear-the silicone grows like bread dough when it's rising, but to a much higher degree. You'll never believe how much air is in the silicone until you see the vacuum chamber in action.

#3 Randy

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Posted 28 March 2007 - 02:30 AM

Yes, Bob is correct. Vacuum the silicon after mixing. You need to pull about 1 atmosphere (28bar?), make sure your vessel is at least 5 times bigger in volume. Then pour the silicon into your mold box in 1 corner and let the silicon fill from the bottom.

#4 PatRedmond

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Posted 28 March 2007 - 02:34 AM

Hey Randy .... Whose Beetle is that in your pic?

#5 Randy

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Posted 28 March 2007 - 03:06 AM

Picked that up last simmer pat. Runs good, but I need to bring it up to date. I have a narrowed beam, disc brakes, and Radars to go on it. I also found a good parts car for a stock interior, and steering wheel. Interior is in, just need to change the wheel. Body is not too bad for a Canadian car, but will need a once over at some point, but I can live with whats there now, maybe a hood ride :( . It's a '66 by the way, had the model.

#6 PatRedmond

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Posted 28 March 2007 - 03:28 AM

Right on ! Nice summer ride.

Long way from a bathtub Caprice wagon ... :( :)

#7 Randy

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Posted 28 March 2007 - 03:36 AM

Right on ! Nice summer ride.

Long way from a bathtub Caprice wagon ... :( :)


Got that out of storage last night, hope winter is over :) The bug is still up there.

#8 Modelmartin

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Posted 28 March 2007 - 02:40 PM

I have been resin casting for over 25 years and swore off of vacuum after my first experience with pressure casting about 20 years ago. Basically you need only a commercially available paint pot and an air compressor. When you pour the rubber for your mold and when you pour your castings you put it into the pot and pump it up to 30-40 psi and wait until it cures. All bubbles will be gone if you got it in before the resin kicks. It is easy and reliable and produces superior castings. With a slower resin you can cast finned cylinder heads with boltheads all over it and not get a single bubble. Vacuum sucks! :shock:

#9 Aaronw

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Posted 28 March 2007 - 05:25 PM

I have been resin casting for over 25 years and swore off of vacuum after my first experience with pressure casting about 20 years ago. Basically you need only a commercially available paint pot and an air compressor. When you pour the rubber for your mold and when you pour your castings you put it into the pot and pump it up to 30-40 psi and wait until it cures. All bubbles will be gone if you got it in before the resin kicks. It is easy and reliable and produces superior castings. With a slower resin you can cast finned cylinder heads with boltheads all over it and not get a single bubble. Vacuum sucks! :shock:


You describe my set up exactly, I have a paint pot and a compressor.

The problem I had when pressurizing the resin is it gets little "pimples" all over the surface. As i understand it de-airing the silicone is supposed to stop this.



Randy, Pat and Zoom Zoom, & MM thanks for the help.

#10 Randy

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Posted 28 March 2007 - 06:18 PM

Well the "pimples" are from the bubbles collapsing in the silicon when you add pressure. Andy's method works be cause has made the molds under pressure and they are returning to their correct size when he pressurizes for the resin pour.

#11 Modelmartin

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Posted 29 March 2007 - 02:36 AM

That is correct, Randy. I would add that the pressure actually gets rid of 99.99% of the bubbles. I have cut up molds to use for filler and I just can not find bubbles in the rubber if it has cured under pressure. It has to be kept under pressure the whole time. On occasion I will find a bubble that is pressurized in a casting but it is rare and easily prevented by getting the filled mold under pressure ASAP. If the resin is starting to kick it is harder for the bubble to leave the building! In the early days when I still screwed with vacuum I would get resin filling the bubbles in the mold's surface and then pressing into the surface of subsequent castings and I would get dimples on the surface of the castings. Pressurizing the molds and castings solved 90% of my casting problems. After that it is all part lines, sprues, vents, layout, and resin flow. Good luck Aaron! You are almost there.

#12 Aaronw

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Posted 29 March 2007 - 04:47 PM

Thanks, that was the part I missed. Everything is under pressure, I'll give that a try next time I make a mold.

#13 mr moto

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Posted 20 April 2007 - 05:42 AM

Could somebody post a pic or two of their pressurized paint pot setup so I can see how it's done? I've done a little resin casting of simple parts without pressure or vacuum but I'd like to take on some more challenging projects using pressure. This is just the kind of discussion I need. Thanks to everybody for sharing your "secrets". Also, if there are any specialized fittings or other parts needed for the setup, where can I get them?

#14 Gregg

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Posted 20 April 2007 - 09:07 AM

Hold on guys, so you do and can preasurize the RTV?
Do you do this after mixing, like the vacuum way? or after mold is poured?
This is a question in our Ask G&J going to press

#15 hookedonplastics

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Posted 20 April 2007 - 09:18 AM

Vacuum sucks!


I shouldn't......I just can't....... :?

#16 Modelmartin

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Posted 20 April 2007 - 11:39 AM

Gregg,

Yes. Everything is done in the pressure tank. Mix and pour RTV and put in pressure tank until it cures. When the mold is done you pour in your resin and put same in said tank again. 30-40 psi works for me. If you got it in and pressurized in time you will have some really fine bubble-free castings. :) :D

#17 Aaronw

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Posted 20 April 2007 - 02:30 PM

Could somebody post a pic or two of their pressurized paint pot setup so I can see how it's done? I've done a little resin casting of simple parts without pressure or vacuum but I'd like to take on some more challenging projects using pressure. This is just the kind of discussion I need. Thanks to everybody for sharing your "secrets". Also, if there are any specialized fittings or other parts needed for the setup, where can I get them?


I'll try to get a pic over the weekend. It is actually pretty simple, I bought a paint pot at harbor freight for about $30, some air fittings and a plug (for where the paint was supposed to come out).

#18 mr moto

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Posted 23 April 2007 - 05:01 AM

I'll try to get a pic over the weekend. It is actually pretty simple, I bought a paint pot at harbor freight for about $30, some air fittings and a plug (for where the paint was supposed to come out).


That's sounds great. I'll have to go to the local Harbor Frieght and check out the paint pots. I've looked at some on the web and E-bay but they're usually pricier than that!

#19 Lownslow

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Posted 29 April 2007 - 11:27 AM

i got one but i broke the pressure relief valve made with dinky metal

#20 BigPoppa

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Posted 30 April 2007 - 10:15 PM

Hold on guys, so you do and can preasurize the RTV?
Do you do this after mixing, like the vacuum way? or after mold is poured?
This is a question in our Ask G&J going to press


I mix and pour my mold and stick it in the pressure pot overnight. Works fine.

my set up. I made my gauge removable, but I can't remember why, maybe in case I have more pots someday. I'd like to make a base to set it sideways.

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