Vacuum /pressure casting on a Low budget

[tango'd fett]

Has anyone built a vacuum casting setup without spending an arm and a leg on a vacuum pump? I have been searching around and all are real pricey :angry, has anyone found a cheaper alternative. I have found a small hand vacuum pump but it only pulls 25hg and 28-30 is deemed best especially for degassing silicone. I have lots of ideas for the chamber and have sourced guages etc but the pump is the expensive part so any alternatives would be a great help :cool

 

[cheech9898]

There's plenty of websites detailing how to rig a refrigerator or car a/c compressor as a cheap vacuum pump. If you have a appliance or HVAC repair place near you, you could see if they had a good used compressor or watch around college areas at the end of school year for little fridges that are getting tossed but may still work. Venturi setups are fairly cheap if you have a decent air compressor.

 

[zorg]

http://www.therpf.com/showthread.php?t=70274&highlight=degassing

rebelscum was kind enough to send me one of the harbour freight venturi pumps (thanks:thumbsup) , this seems to be the cheapest method.

just hook up a compressor and it will pull 28hg

-z

 

[tango'd fett]

Thanks guys , i will check into those, great thread Zorg I missed that one :thumbsup

 

[knightoftheroc]

Question for the silicone mold makers: which is better for removing bubbles in the silicone, pressure or vacume? I've seen some youtube tutorials, so I have a rough understanding of the process, and might try it out. I also own a vacume pump that will pull 29"Hg, and I've long ago converted a pressure cooker for matching my AC fittings onto. I figured it would work as a pressure chamber, OR a vacume chamber, depending on my mood, the only limitations being the interior size. So, since I can make this work either way, when I get to molding, which method gives the best results?

 

[exoray]

Question for the silicone mold makers: which is better for removing bubbles in the silicone, pressure or vacume? I've seen some youtube tutorials, so I have a rough understanding of the process, and might try it out. I also own a vacume pump that will pull 29"Hg, and I've long ago converted a pressure cooker for matching my AC fittings onto. I figured it would work as a pressure chamber, OR a vacume chamber, depending on my mood, the only limitations being the interior size. So, since I can make this work either way, when I get to molding, which method gives the best results?

You vacuum the bubble out of the silicone (aka boil) then pour very carefully so you don't introduce any new bubbles... This will work probably 99% of the time

Additionaly you can then mold the part under pressure, but if you mold it under pressure you should 'technically' cast every pull under equal pressure as well so that the silicone is under the same atmospheric pressure and not distorted by any changes in pressure... So if you plan to pressure cast resin pieces then the silicone should be cured under pressure as well... But if you only going to cast regularly not under pressure then let the silicone cure at normal atmosphere... In most casts the distortion and sorts is minimal to nothing you will ever see but in some cases I have seen quite noticeable distortions and warping, especially if there still ends up being trapped air in a softer silicone...

Generally you pressure cast resin parts to remove air, because of how hard they are when cured the expanding air bubble when brought back out to normal atmosphere can't expand and won't distort the casting...

 

[stapleton13]

Pressure works great, but if the part you are moulding is hollow there is a chance that the pressure will distort the piece being molded.

I did a mold of a tricorder door, and when I pulled it from the mold I discovered that the flat side had pushed in in the piece, and distorted the casting.

 

[exoray]

Pressure works great, but if the part you are moulding is hollow there is a chance that the pressure will distort the piece being molded.

This is universal with pressure and vacuum, trapped air pockets will distort as the surrounding atmospheric pressure changes...

This is why molding and casting should be done at a consistent surrounding pressure...

 

[knightoftheroc]

Ahhhh- I'm beginning to see the differences now- vacume is for making the MOLD, not the parts. And pressure for parts, not molds. If at all. When required.... I think I need a book.....and maybe a nap.

Thanks for the answers tho- it really did explain things better.

 

[exoray]

Ahhhh- I'm beginning to see the differences now- vacume is for making the MOLD, not the parts. And pressure for parts, not molds. If at all. When required.... I think I need a book.....and maybe a nap.

Not exactly, vacuum is for forcing the liquid to boil and expel the air trapped in it, pressure is used to squeeze the remaining air bubble to microscopic size...

Depending on the part and resin I will vacuum degas the resin before pouring into the mold... Just like I would silicone before pouring the mold... It all depends on what you are doing or want to accomplish...

Say for instance clear resin, to get the best casting you vacuum degas the silicone, then pour the mold and letting it cure under pressure so that the mold is 'normal' under pressure, you then vacuum degas the resin, fill the mold and then place the filled mold into a pressure take and let the resin cure under pressure... Many people will skip curing the silicone under pressure but I have experienced distortions not doing it this way and having the equipment it's really no hassle not to do it...

Vacuum and pressure are just two different tools that accomplish similar but slightly different task...

 

[tango'd fett]

Does it help to use a silicone with a denser shore hardness when pressure casting, depending on how you make the mold as well I pressume. Eg would a 2 part mold fold in in the join halves and distort the resin ?

Also how much pressure/psi is needed for pressure casting and what size compresser does the job? Am looking at a few at the moment but dont want to end up with something thats not powerful enough!

 

[zorg]

i dont think the shore hardness will have any effect on the degassing, the uncured silicones are in a liquid state anyway.

from memory i think that venturi pump needs 75 - 80 psi to run it successfully.

-z

 

[exoray]

Also how much pressure/psi is needed for pressure casting and what size compresser does the job? Am looking at a few at the moment but dont want to end up with something thats not powerful enough!

Well if you are using a venturi vacuum pump you are going to want as much air as you can get to pull a faster vacuum... Small air compressors will do OK but not as good as a high CFM compressor...

For simply pressure casting you are going to want a regulated 60 psi give or take...

 

[stapleton13]

Ahhhh- I'm beginning to see the differences now- vacume is for making the MOLD, not the parts. And pressure for parts, not molds. If at all. When required.... I think I need a book.....and maybe a nap.

Thanks for the answers tho- it really did explain things better.

Actually vacuum is used mostly for degassing the liquid before use. One thing I do know. If you intend to pressure cast the parts. You should also do it to the mold. The problem is that a mold may look great, but have bubbles just beyond the surface, and if you pressure cast the part. It will come out with little bumps all over it caused by small bubbles in the mold.

 

[tango'd fett]

Cheers guys thats a big help, all i need to find now is a cheap pressure pot if they exist !!

 

[exoray]

Cheers guys thats a big help, all i need to find now is a cheap pressure pot if they exist !!

Paint pressure pots, can't say how affordable they are on the other side of the pond but they are reasonable here under $100 new, I picked up both mine used for under $40...

They are stupid expensive on ebay.co.uk, there must be a cheaper Chinese outlet tool chain in the UK?

 

[tango'd fett]

Yeah they are stupid money here , am looking for 2nd hand as well as i have not found any cheap places for new ones yet .If anyone knows anywhere that would be MOST welcome :cool

 

[Disco58]

Ok, now I know a lot more than I did, thanks! I have a few questions though.
1. I was under the impression the resin/catalyst used was the same as for autobody work, polyester. In an article on HobbyTalk, it was mentioned the GK builders use a urethane resin. Why?
2. I have a small bronzed bust I'd like to recast in resin so I can paint it, and have both. Ballpark guess, it's about 30 cu.inches in volume. I've never cast anything, and have no supplies whatsoever. Any idea what it would cost to cast, having to buy everything? This is a copyrighted piece, and there will be only one recast for myself, so selling pieces to recoup expenses is not even a consideration.
3. When I make the mold is there a chance the silicone could attack any coatings on the original piece?