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Building a vacuum assisted caster for silver and gold.

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Author Topic: Building a vacuum assisted caster for silver and gold.  (Read 618 times)
samaeljaxon
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« on: January 19, 2016, 01:20:24 pm »

Okay guys, so this topic might be stretching the boundaries of this specific part of the forum a little bit, but I was hoping somebody here might have a little knowledge about this and this seemed to be the best place to ask this question on this forum.

I am building a vacuum casting machine out of a vacuum pump, silicon hoses and some metal canisters that are air tight and cut to fit the perforated vacuum flasks normally used with the actual retail machines.

In order to assure that the seals do not melt away when the hot flask is placed into it, I need a little information on where I can find the right type of seals for this so that the pump can create a vacuum inside the canister and pull the metal down into the mould.

Any ideas are welcome, thank you.
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slabbercabber
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« Reply #1 on: January 19, 2016, 01:44:34 pm »

The highest temperature polymer is PTFE (teflon) at around 600 degrees f.  If you need a higher temp seal then you need to start looking at microfinishing of the steel surfaces.
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« Reply #2 on: January 19, 2016, 02:24:45 pm »

Hi Sam
I think that the question fits in a forum about cutting stones. I know I have thought about casting setting for my stones that I cut. About 10 years ago I started gathering parts. What I remember is the sealing surface was a special piece of high temp rubber. I still have the piece.

I don't have time at the moment but I would like to revisit this and maybe build 1 myself.
I will try to get back to this tomorrow.

Bless
Shawn
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samaeljaxon
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« Reply #3 on: January 19, 2016, 02:25:37 pm »

I think they're using something other than a polymer to seal this since flask temperatures for casting are between 850F and 1200F. I've been looking online at other types of seals and there are some made from ceramic paper like this http://www.gasketsinc.com/product/g83-ceramic-paper-2300degf1260degc/1809

I was just hoping somebody might know the exact material used for these jewelry vacuum casters. It looks like red rings, but that doesn't say much besides color.

I did however find some plans for making a vulcanizer and it uses red silicone to insulate the coils from the wooden blocks used to build the press. I just don't know if red silicone is the way to go.
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samaeljaxon
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« Reply #4 on: January 19, 2016, 02:31:22 pm »

Thanks, Shawn. I'll actually be sharing all my successful DIY plans for creating both a vacuum castor, a investment vacuum chamber, a propane/ compressed air torch and a vulcanizer with everybody here once I am done creating my little foundry. Hopefully it'll help somebody else find out how to make all of this for cheap, since these big corporations have been getting the best of us all charging hundreds of dollars for parts that require nowhere near that much to assemble.

DIY all day.
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« Reply #5 on: January 19, 2016, 03:22:34 pm »

Hey Sam

I am in. I could of said the same words 10 yr ago. That was before kids. Now at least they can all blow there noses by them selfs I will try to help out on this project.

At this point the sealing gasket is the most important thing I will start looking into it. The second thing to look into is the amount of V is needed.

Go to go
Bless
Shawn
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bobby1
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« Reply #6 on: January 19, 2016, 07:29:45 pm »

The seal is a sheet of silicone rubber. Most of them that I have seen are red. Most jewelry tools suppliers should carry it.
Bob
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« Reply #7 on: January 19, 2016, 07:33:14 pm »

$15 for a 12" by 12" by 1/8" sheet on Ebay
Bob
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Debbie K
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« Reply #8 on: January 19, 2016, 09:52:24 pm »

They only last for a limited time. When they lose their ability to seal, you replace them.

I have a friends vacuum caster living in my studio, so I don't need to make one. But I, too, gave this some thought years back.

There are two approaches that made sense to me: One was to use a $15 air vacuum pump from Harbor Freight with a compressor and build a box out of plywood with hoses; the other to use a electric vacuum pump. What I found out recently was that the Gomco vacuums that get sold all over Ebay for relatively little money pull the right amount of atmosphere to do jewelry casting/investment work.

I found this out because I got a Gomco in a lot of dental supplies that I bought last year, and was researching them. For some reason, they cost a lot less than many other comparable vacuum pumps.

When I was looking into this years ago, I didn't have the money for a vacuum or compressor, so it was really lucky for me that my friend needed to find a home for his caster. I use it primarily to vacuum investment and on castings using more than 5 ounces of metal. I prefer the centrifigal caster to the vacuum; just think it gives you denser castings.

Why are you making airtight cans for your flasks? I primarily set my flask on top of the silicone ring over the intake hole and pour away. The vacuum pulls through the investment.

Debbie K

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samaeljaxon
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« Reply #9 on: January 19, 2016, 10:18:53 pm »

Thank you guys for all your input on this subject. Hopefully I can get this little project to work so I can share helpful information with anyone that's interested.

Debby, I'm using a vacuum cavity to fit the perforated flasks used for vacuum casting. This allows for the vacuum to pull on the investment from all sides, allowing the flow of the molten metal to fill every little recess in the mould perfectly. I've been doing a lot of research on the subject, and it appears that when you use a vacuum caster that only pulls on the flask from the base, that it doesn't fill the horizontal cavities in the plaster very well.

I've been reading that centrifugal casting has a higher initial fill temperature, which helps it all solidify more homogeneously, but that vacuum assisted casting fills the moulds more precisely than any other form of casting when using the perforated flasks.

I found some of these silicon gaskets for real cheap on eBay. They have a notably lower max temperature than the casting temperature for the both silver and gold, so like Debby said, they will deteriorate with use. Apparently they also make graphite gaskets to use in combination with the red silicon gaskets and this really extends the life of the silicon since graphite is such a poor thermal conductor.

I'll be ordering parts today and tomorrow and hitting the Home Depo for some extra fittings and such and I'll share my progress with you guys as I go along. Again, thanks for all your input, guys. 
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T P
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« Reply #10 on: January 20, 2016, 02:51:25 pm »

Built a vacuum table 4 yrs ago.  Purschased a used HVAC air conditioner service vacuum,  you can find them on Craigs list quite often.  used a sheet of 1/4" aluminum for the table, drilled and tapped a hole in the center and use a sheet of red hi temp silicone gasket material for the seal.  there is an additional tapped hole with a valve off to the side to be used with a plexi bell jar that I use for debubblizing investment.

In use the standard flask is filled short of the rim by 3/8 to 1/2"  with a series of short holes drilled around the outside edge of the flask for additional surface area for the vacuum to draw on.  do not dill more the a inch deep and watch out for the location of your wax pattern.  when the flask has calcined it is placed on the gasket, the pump is started and as soon as it draws a vacuum the molten metal is poured in. 

I have pictures of the table and will see if I can find them for posting. 
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Debbie K
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« Reply #11 on: January 20, 2016, 06:59:04 pm »

In hopes that this will save some of you having a vacuum casting failure I will share a piece of information that I didn't know about until a smarta$$ told me after I had problems.

In centrifigal casting, the spueing is done like a tree, with the feeder sprues coming off in an angle like a "Y" where the base of the Y is where the button would be.

When sprueing your wax for vacuum casting when you are placing the flask in a cavity, the feeder sprues should be horizontal, like a "T" to the main feeder sprue. This is because the vacuum does not exert the same amount of force ramming the metal into the investment and the metal has a chance to freeze before it can reach the bottom and go out to the sides.

Also, all vacuum cast flasks should have a minimum of 1' investment on the top. If you blow out the investment in a centrifigal caster, the metal generally will hit the back of the cradle and stop there. But with a vacuum caster, you can actually pull it into the caster. The man who "loaned" me his caster made this mistake and there's about 3-4 ounces of silver in the inside of the caster that's welded to the steel. If you sucked this into plastic tubing you could have a potential hazardous situation.

I know that your vacuum caster will work just fine and am looking forward to seeing pictures from all of you how you've made your own machines. But I would recommend that you give a thought to what would happen with the molten metal if you do have a blowout and where it is likely to go. The same thing can and should be said about centrifigal casters, which is why I bought a NeyCraft. The NeyCraft caster has a barrel that moves around with the crucible so if a blowout happens, the metal hits one spot on the barrel instead of being flung all the way around and bouncing around and out. And with casting, it seems, whatever can happen, does.

Debbie K
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samaeljaxon
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« Reply #12 on: January 20, 2016, 11:40:21 pm »

Built a vacuum table 4 yrs ago.  Purschased a used HVAC air conditioner service vacuum,  you can find them on Craigs list quite often.  used a sheet of 1/4" aluminum for the table, drilled and tapped a hole in the center and use a sheet of red hi temp silicone gasket material for the seal.  there is an additional tapped hole with a valve off to the side to be used with a plexi bell jar that I use for debubblizing investment.

In use the standard flask is filled short of the rim by 3/8 to 1/2"  with a series of short holes drilled around the outside edge of the flask for additional surface area for the vacuum to draw on.  do not dill more the a inch deep and watch out for the location of your wax pattern.  when the flask has calcined it is placed on the gasket, the pump is started and as soon as it draws a vacuum the molten metal is poured in. 

I have pictures of the table and will see if I can find them for posting. 

That sounds very interesting. I would certainly love to take a look at some pictures to get a better idea of what you created here. Thanks for your input on the subject!
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samaeljaxon
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« Reply #13 on: January 20, 2016, 11:44:29 pm »

In hopes that this will save some of you having a vacuum casting failure I will share a piece of information that I didn't know about until a smarta$$ told me after I had problems.

In centrifigal casting, the spueing is done like a tree, with the feeder sprues coming off in an angle like a "Y" where the base of the Y is where the button would be.

When sprueing your wax for vacuum casting when you are placing the flask in a cavity, the feeder sprues should be horizontal, like a "T" to the main feeder sprue. This is because the vacuum does not exert the same amount of force ramming the metal into the investment and the metal has a chance to freeze before it can reach the bottom and go out to the sides.

Also, all vacuum cast flasks should have a minimum of 1' investment on the top. If you blow out the investment in a centrifigal caster, the metal generally will hit the back of the cradle and stop there. But with a vacuum caster, you can actually pull it into the caster. The man who "loaned" me his caster made this mistake and there's about 3-4 ounces of silver in the inside of the caster that's welded to the steel. If you sucked this into plastic tubing you could have a potential hazardous situation.

I know that your vacuum caster will work just fine and am looking forward to seeing pictures from all of you how you've made your own machines. But I would recommend that you give a thought to what would happen with the molten metal if you do have a blowout and where it is likely to go. The same thing can and should be said about centrifigal casters, which is why I bought a NeyCraft. The NeyCraft caster has a barrel that moves around with the crucible so if a blowout happens, the metal hits one spot on the barrel instead of being flung all the way around and bouncing around and out. And with casting, it seems, whatever can happen, does.

Debbie K

This is some very important information here, Debbie. Thanks a bunch. This could save us all some serious trouble here.
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samaeljaxon
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« Reply #14 on: January 20, 2016, 11:50:40 pm »

I'm having trouble finding a machined aluminum box for this vacuum chamber to fit around the flanged/perforated flask. I read somewhere about people converting pressure cookers into vacuum chambers, so I'm wondering if I can take an old pressure cooker and install a vacuum gauge and a line in for my vacuum pump.

I'll be hitting a few shops tomorrow to see what I can find that's large enough for this and still cost effective. I'm going to be using a 3" diameter flask with the flanged edge. Anyone know a good place to find one for cheap?
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T P
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« Reply #15 on: January 21, 2016, 11:52:56 am »

why does the vacuum chamber have to be an aluminum box?   A pc. of large diameter steel pipe with a suitable pc of plate welded to the bottom would be much easier to find.  A circular chambe would also be subject to less stress when the vacuum is pulled on the chamber.
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« Reply #16 on: January 21, 2016, 01:12:33 pm »

here are attached photos of the table I built.  the last photo is of the tuyere,  a safety valve if the bottom of the flask fails and molten metal is sucked into the system,  the plastic bottom melts out rather then plugging the machine


* casting table top.JPG (38.1 KB, 480x320 - viewed 24 times.)

* casting table bottom.JPG (39.45 KB, 480x320 - viewed 18 times.)

* casting table tyre.JPG (33.67 KB, 480x320 - viewed 20 times.)
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Debbie K
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« Reply #17 on: January 24, 2016, 10:31:12 am »

Darn it! Can't view the pictures. I really want to see what you made, especially with the safety feature.

I've had problems getting on to this site for the last few days; anyone else?

Debbie K
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« Reply #18 on: January 24, 2016, 05:48:59 pm »

here are attached photos of the table I built.  the last photo is of the tuyere,  a safety valve if the bottom of the flask fails and molten metal is sucked into the system,  the plastic bottom melts out rather then plugging the machine


* casting table top.JPG (38.1 KB, 480x320 - viewed 9 times.)

* casting table bottom.JPG (39.45 KB, 480x320 - viewed 8 times.)

* casting table tyre.JPG (33.67 KB, 480x320 - viewed 7 times.)
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Debbie K
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« Reply #19 on: January 24, 2016, 08:03:06 pm »

Thanks for the pictures! I like the valve on the bottom. I might have to end up making one in the future if the commercial one gives out, so this is good to know.

Debbie K
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