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ScanmanDan

Vacuum Chamber or Pressure pot?

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Just mulling over the possibility of adding to my tool box.  I do a little casting ( a few times a year) and while with much practice I can usually get a good mould and make pretty decent/usable castings with the materials at hand I'd like to get better results.  I have access to getting either a Pressure pot ( but I'd also have to get a compressor so added cost) or a vacuum chamber.  While I have done quite a lot of research on all the bits I have yet to see any advice as to which to get first.  I was wondering what the caster fraternity advices or thoughts on each piece of gear.

Thanks in advance.

 

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A pressure pot will have tie-downs so you can tie down the top and create an over pressurization and a vacuum.  The roof of a vacuum chamber, at least the ones I've seen, doesn't have tie downs and as such is good for one purpose.

 

So, if you want to over-pressurize and vacuum, get a pressure pot.  You should already have a compressor, so all you need is a vacuum pump.  Know the volume of  your pot and buy a pump that can remove that air as soon as possible.  I'm not saying buy a room-sized vacuum pump, but the bigger the pump, the faster it will pull air, and the longer it will last.  If the pump is too small it will take too long to draw and it will burn out sooner.  Keep it's oil level up.

 

I mixed and matched stuff from a place called Harbor Freight, here in the States.  It works for me, but I wish it had a window.

 

Hope this helps.  Good luck.

Sincerely,

Mark

Edited by dodgem37

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Relying on vacuum systems as such for resin casting is like black magic. This is far more difficult than relying on the opposite as vacuum may result in trapping air in the resin. With a pressure pot, air bubbles go out of resin. Obviously using both techniques is generally recommended but this is depending on the type of compressor and pot you can find and purchase.

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Just now, nichenson said:

I was taught to use a vacuum chamber for your mold making after mixing but before pouring your rubber and a pressure pot for your casting resin.

That's exactly what I thought, too. 

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+1 nichenson and BiggTim.  Vacuum the RTV then pressurize the resin.  An important note.  One should use different chambers for vacuum and pressure especially aluminum types which are common for pressure.  IIRC the wide swing in pressure will work harden the aluminum resulting in a dramatic failure.  I would never use a garage sale pressure cooker for a vacuum pot.

 

Rick

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I've been down this rocky road for a while now.

The only way you will get the resin into every part of a mould  is to vacuum de gas the resin after mixing while in the mould, crucially in the short time before it starts to set so that you can quickly put the moulds with resin in into the pressure pot until it sets. The pressure pot is a paint spraying pot with the spray gun and hose taken off. The pressure is 60psi max ( the same as your bicycle inner tube) but is enough to squash down any bubble to nothing. Now the ideal thing would be a resin that sets in 5 mins and can be demoulded in half hour, To give you time to do the above at a reasonable pace.This does not exist! If it did I would be using it.

Most resins ( that are any good )are 2 mins working time and 45 to 60 mins demould. Also low viscosity is needed as well. So the process is unsuited for mass production.

For mould making you only need a vacuum chamber and you need to de gas the mixed silicon and then pour into the mould box and then de gas again, this process is easier because the silicon gives you at least 45 minutes working time but the setting is usually 12 hours.

So to re cap resin needs both processes. Your vac chamber needs a powerful pump mine gets down to full vacuum in 20secs, your pressure paint pot needs a compressor ( preferably a quiet one ).

 

Graham 

Edited by GrahamF
More info

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Interesting thoughts gentlemen, Thank you all.

I was aware that the best way forward was vacuum degassing of the mould material and then pouring the mould and setting it under pressure. Casting after degassing the resin and then setting under pressure but....

Lets look at cost. At present I have spent about $350 AUS to set myself up casting; Resin, Pinkysil mould material, digital scale and all the other bits and pieces. With this I can pretty confidently pull a decent casting( even if I have to make a couple of moulds because I'm still learning and casting is a 50-50 lottery) To add a vacuum pump of decent quality is another say $300-400 AUS and a pressure pot and compressor combo a bit more. Now that's a fair bit of cash to splash about for only a few castings a year. I'm not prepared or skilled enough at this stage to make anything like at paint pot pressure tank. I don't have access to Harbour Freight without an international airline ticket so.... I'm pretty much left with buying off the shelf.

It would appear that unless I want to commit some serious cash to stepping up to the next level I'm going to have to perhaps practice the basic craft of mould making ( Not such a bad thing as I do very much enjoy casting parts :) )

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'I was aware that the best way forward was vacuum degassing of the mould material and then pouring the mould and setting it under pressure.'

What would be the purpose of pressurizing the mold?

 

'At present I have spent about $350 AUS to set myself up casting'

I bought a 2-1/2 gallon pressure pot ($99 USD), a vacuum gauge ($20), premium air hoses that won't collapse under vacuum ($5, maybe $10), quick disconnect brass fittings ($12?), and a 3 CFM 2 stage vacuum pump ($160).  It was all off of the shelf from Harbor Freight.  There must to be something like that in Melbourne.  Harbor Freight is an inexpensive tool store.

 

There is debate that the home hobbyist needs both vacuum chamber and a pressure pot.  I think that the home hobbyist doesn't need both.

 

Depending on the part, I either vac the resin before or after I pour it into a mold.  Small parts I vac before I pour.  Vac'ing causes anything liquid you put in a chamber to heat up.  Period.  Vac'ing out air creates friction which creates heat.  Heated resin sets faster.  Heated RTV silicon sets faster.  With small parts there is time enough to fill the part cavity.  For larger parts, not so much.

 

For larger parts I vac after I pour.  I make sure the mold has a casting block so I have a resin reservoir.  I vacuum out trapped air, then when I release the vacuum the rise in air pressure forces the resin that is in the reservoir into the mold which fills the void left by the air.  This saves me from having to purchase a second chamber.

 

'With a pressure pot, air bubbles go out of resin.'

Thierry, don't you mean under pressure bubbles are compressed in resin?

 

I have the T-shirt as well.  I molded and cast parts for years before it came time to upgrade.  I went through the same thing you are going through before I bit the upgrade to chamber bullet.  Which will it be?  Hobbyist Grade vs Professional Grade?  My head swirled.  I don't want to waste good money.  Throw good money after bad correcting a mistake.  I chose Hobbyist Grade and solved problems with my brain instead of my wallet.  It's only a hobby.  I don't even need it.  It's just something I want.

 

Anyway.   This is just my 2 cents.  Don't break the bank.

Best of luck.

Sincerely,

Mark

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18 hours ago, MikeMaben said:

HPH uses only vacuum (apparently)  ...

 

Watch about 1 minute in https://www.youtube.com/watch?time_continue=86&v=NGeN1DcrSlE

 

 

 

The industrial rapid prototyping shop I worked at in the '90s also solely used a vacuum system. I recall that for vac chambers we used oil industry pipeline (14 and 20 inch diameter sections, about half inch thick steel), and that our casting guru claimed that the vac pump pulled so much vacuum, the pump's cooling water was slightly radioactive and really shouldn't have been discharged into the public sewer system.

 

As I was mainly concerned with finishing LOM and SLA masters, I wasn't interested enough to ask too much about how that happened in case something legal came to bite, but it did sound impressive! And I don't recall ever having to fill any bubble holes in hard or flexible resin or wax castings, not ever, on castings up to a foot long or more. Aluminium castings, which was what the wax masters were for, were on the other hand, a total pain. They were foundry cast, but for those who complain about cleaning up badly cast and dimensionally unstable resin, be thankful yo're not dealing with ali.

Edited by Chek

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Hey Mark - I have been told that best practise was to make the mould under exactly the same conditions you intended to cast the mould.  Now to my mind that makes some sense but what do I know? Whether it is strictly necessary is another matter, though if you didn't degas the silicon wouldn't the pressure try and crush the bubbles left in the mould?  Your milage may vary :) What's the feedback from those that cast a lot? 

Please take this next bit in the spirit it's intended-  Most Americans have no idea what basic materials and equipment costs in other parts of the world.  Please don't advise me to look around for a cheaper supplier because they don't exist in Australia.  We pay prices much higher for almost everything.  As a transplanted Yank it's taken years to get used to but that is the way of the world.  I constantly chuckle when Americans complain about high prices. Most haven't a clue what the world price may be.  ( And why should they know as the market supples them well whereas we tend to look overseas a lot to source stuff. We have to, it ain't in the shops round here ;)  ) Americans have access to a wide range of goods from multiple sources whereas I often either have one supplier or have to go to an overseas supplier and pay a substantial amount for shipping.  My point in bringing up pricing was that I need to spend a fair bit more than say someone in the US to go to this next level and to do so maybe out of reach if both setups were needed.  As I said at the beginning this is neither a dig at Americans lack of knowledge nor Australian higher prices just a fact of life to be contended with.

 

Thanks for adding your insights and advise it's been most helpful.  I'm going to mull everything over and see what I can afford both in time and cash.

 

Chek - Interesting life you have lead. 

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