JeffreyK Posted November 28, 2010 Share Posted November 28, 2010 (edited) Hi guys, I'd like to break into some more serious resin casting (fine detail, thin walled parts, consistent quality) and am therefore looking into pressure casting. I'd like to use a pressure pot, I've read that this is probably the best solution for high quality parts. I'm hoping that I can still use my airbrush compressor for pressurising the vessel and of course neither want to spend hundreds of pounds for the pot nor do I have the space for a monstrous contraption. However, I want to keep it safe and don't intend to bodge something together that might explode on me... Any suggestions? Many thanks for your help Jeffrey Edited November 28, 2010 by JeffreyK Link to comment Share on other sites More sharing options...
CarLos Posted November 28, 2010 Share Posted November 28, 2010 Googling by "pressure pot" brings many images of specialized pressure pots (as those used for dentists) strangely similar to those we use in the kitchen: http://www.dblabsupplies.co.uk/dental-pressure-pot-28-p.asp I have one very similar to the image in the link above... I am also thinking about this, and it's the way I will go. I don't know exactly how much pressure is needed, probably less than you can achieve safely with a kitchen pressure pot. Regards, Carlos Link to comment Share on other sites More sharing options...
Mike Posted November 28, 2010 Share Posted November 28, 2010 About 40psi will do for most circumstances. I'd like to find one too, but over here even the painter's pressure pots seem stupidly expensive for no apparent reason Link to comment Share on other sites More sharing options...
bootneck Posted November 28, 2010 Share Posted November 28, 2010 Googling by "pressure pot" brings many images of specialized pressure pots (as those used for dentists) strangely similar to those we use in the kitchen: I have one very similar to the image in the link above... How would that work then? I've dabbled with basic, one part, moulds but I can't get my head around what the principle of this method would be. Can anyone give a laymans description, or better a simple diagram please? Thanks Mike Link to comment Share on other sites More sharing options...
JeffreyK Posted November 28, 2010 Author Share Posted November 28, 2010 Thanks, I did a thorough google search before I posted here, the cheapest purpose-made pot I found was this one: SprayDirect The disadvantages are: it still costs £160.00 + VAT, is still quite big (takes up a lot of space and my compressor has to work too long and hard to fill it) and hasn't got quick release locks. This is an important issue because by the time you have mixed the resin, poured it into the mold(s), placed the mold(s) into the pot, tightened all screws, switched on the compressor and get it to ~40psi the resin will be cured long before the pressure can have any effect on your casting. I probably have to use resin with longer pot life and higher viscosity anyway, but even 4 minutes migh tbe too short to do all the tasks required. So ideally, the pot will be small and have some sort of quick release lid lock to get the pressure up as quickly as possible. That dental pressure pot you came up with Carlos looks good, but is way too expensive for me. However, cooking pots aren't safe enough, for resin and RTV pressure curing we need pressures around 50-70 psi (what I've read so far) and I've seen pictures on the web where someone's modified cooking pot exploded on him! Jeffrey Link to comment Share on other sites More sharing options...
xffw45343tg Posted November 28, 2010 Share Posted November 28, 2010 As a slight diversion off topic (sorry!) I thought the resin was put under vacuum rather than pressure? Wouldn't this implode the pressure pot? Apologies for not having the faintest idea where one would buy one... Link to comment Share on other sites More sharing options...
JeffreyK Posted November 28, 2010 Author Share Posted November 28, 2010 There are two different methods for reducing air bubbles in both the RTV mold and the resin. One is degassing my means of vacuum. After mixing the RTV silicone components, the mixing pot is places into a vacuum chamber and the low atmospheric pressure forces the trapped air bubbles out. This isn't suitable for resin because of the short pot life of mixed resin components - the resin would have started to cure before you even poured it into the mold. The other method is pressurising, basically you apply high pressure to collapse trapped air bubbles into tiny, microscopic bubbles as the trapped air has a lot less resistance to the applied pressure than the denser mediums of the resin and the silicome. This method works for both making the RTV mold (after pouring the RTV around your master you place the whole thing into a pressure pot, apply pressure and leave it until the RTV is cured) and for casting the resin (same principle). Neither principle has anything to do with injection molding or the like, you still have your plain old paper of plastic cup and pour resin or RTV by hand. You just put the whole thing into the pressure container afterwards. Still, does anyone have a source for a pressure pot? Jeffrey Link to comment Share on other sites More sharing options...
pbhawkin Posted December 2, 2010 Share Posted December 2, 2010 Jeff, Hi. I use the following type of pressure pot BOTH for vacuum (degassing) the RTV rubber molds and also for pressurising (to 40-50PSI) the resin. See Ebay Item number: 130422918393 (yes it is in Australia (where I am)) but I am sure you can get these in the UK (although I did look at Ebay UK and see that your prices are higher and 2l pots are common!!!). I then bought a 15mm thick piece of clear polycarbonate from a glass supply store with a 12 inch diameter and had a thread to fit the adaptor cut in the middle. so I have a clear view of the degassing process as well as vacuum guage and a safety valve that acts as a air inlet to let air into the chamber when finished. I use a spare pressure pot seal that just sits omn the pot rim and the polycarbonate cover sits on top of that (the vacuum soon makes it a tight seal!). regards Peter Link to comment Share on other sites More sharing options...
JeffreyK Posted December 2, 2010 Author Share Posted December 2, 2010 Hi Peter, that's exactly what I thought I'll do. I've just found a new 10l pressure pot on ebay and bought it for 100pounds, so a bit cheaper than the one I found from the shop (not as cheap as yours in Aus though...). I'm looking for an affordable vacuum pump now and then will make the acrylic lid, as you say. I've also bought a new compressor, my only worry is that it may still take a bit too long until the pot is filled with 40-50 psi...My calculations say that it will take just over a minute - my resin has a pot life of 2-3 minutes and I'm casting mainly thin pieces, so it should be just ok. Jeffrey Link to comment Share on other sites More sharing options...
pbhawkin Posted December 2, 2010 Share Posted December 2, 2010 (edited) then will make the acrylic lid, as you say. NOooooo. It HAS to be Polycarbonate and NOT Acrylic. There is a BIG difference! Acrylic is brittle and will crack easily but polycarbonate is VERY tough (it is what bulletproof windows and decompression chamber windows are made from!). I'm looking for an affordable vacuum pump I also got mine from bay! and it is a refrigarator degassing pump. Mine is a 10CFM machine and will get to -28.5mmHg (29 is one atmosphere) for the 10litre pot in about 30sec. Mine is similar to this one: Item number: 190469822526 (but is 10CFM) the higher the number means how quickly it will remove the gas (air). They will ALL get down to close to -1 ATM but the smaller ones (like the common 2, 3 and 4.2CFM) will take longer!! I paid about A$350 I think. As for the compressor I use an old A$99 2Hp one from a local hardware shop. It gets the 10l pressure pot to 40-50 PSI in about 30sec. Peter Edited December 2, 2010 by pbhawkin Link to comment Share on other sites More sharing options...
JeffreyK Posted December 5, 2010 Author Share Posted December 5, 2010 (edited) Sorry!! my mistake, should have read more carefully. I've seen others user acrylic for that, but polycarbonate is probably much more suitable for this job. The maximum strength of sheets I could find was 12mm - do you think that's enough? My pot is still in the post so I don't know the exact diameter, but it's a 10l pot so it shouldn't be any more than about 40cm. Can you drill the polycarbonate yourself or do you need prefessional drilling equipment for that? Thanks Jeffrey Edited December 5, 2010 by JeffreyK Link to comment Share on other sites More sharing options...
pbhawkin Posted December 5, 2010 Share Posted December 5, 2010 Jeff, 12mm should be fine. you can drill and cut polycarbonate yourself but I got someone to do it for me as I didn't have the tools available to tap a thread for the fitting. I also got a second rubber seal so that I didn't have to keep removing the one from the lid when I wanted to degas (It also looked like it would be hard to put back in the lid as there appears to be 'crimps' around it to keep it inside the groove of the pressure pot lid). Link to comment Share on other sites More sharing options...
CarLos Posted December 5, 2010 Share Posted December 5, 2010 Easy vacuum pump: just use the air entry from a compressor. Link to comment Share on other sites More sharing options...
pbhawkin Posted December 23, 2010 Share Posted December 23, 2010 Hi CarLos, I had two problems with that method. Firstly my compressor would NOT give a vacuum over about 20mmHg (and was slow getting there) and secondly it would actually stop the compressor from running when it got down to that region! Admittedly I did not have it connected to a vacuum chamber first (and that is a good idea) as you then can open the valve and evacuate the air out of your second chamber with the mold in it very quickly. That is why I got the refigerator degasser. the larger CFM number means the quicker (greater 'pulling' power) it has. Note they will all get down to about the same negative pressure but the smaller CFM ones will take a Loooong time! Link to comment Share on other sites More sharing options...
Cheshiretaurus Posted June 23, 2014 Share Posted June 23, 2014 Anyone tried making a pressure pot from one of these? http://www.a1plantsales.com/paint-shop-c84/fmt-professional-2-litre-air-paint-pressure-tank-pot-sprayer-c-w-gun-hoses-2-l-fmt6055-p1436 Or am I going to be the guinea pig? Link to comment Share on other sites More sharing options...
Cheshiretaurus Posted August 29, 2014 Share Posted August 29, 2014 Anyone tried making a pressure pot from one of these? http://www.a1plantsales.com/paint-shop-c84/fmt-professional-2-litre-air-paint-pressure-tank-pot-sprayer-c-w-gun-hoses-2-l-fmt6055-p1436 Or am I going to be the guinea pig? Just an update, I did make a pressure pot from one of these. Was a very simple conversion with no permanent modifications. Paint tube & handle simply unscrews, had to buy a 1/4" BSP blanking cap to seal off the spray gun air line (red hose on the pic) instructions say It has a max pressure of 50PSI. It also has a pressure relief valve set at just over 40 PSI so have been setting my compressor to 40PSI to run it. To release the pressure when complete I use the built in valve on the paint line (black hose on pic). It connects to the compressor via a 1/4" BSP fitting, I already had a hose for this. I'm using it on its side and have put a little marker pen line on main pot it to show which way is up for when I screw the lid on. Compressor tank (AS-186) is pre filled and the pressure pot connected to the my airbrush distribution manifold with its valve closed. When ready to pressurise I open the valve on the manifold and the tank fills to about 25 PSI in about 10sec then the compressor kicks back in and continues pressurising it to 40PSI. Its full in less than a minuet. Have had some good casts of small intricate items with no bubbles, very pleased with them! However its revealed that although ever care was taken in the mould making, my RTV is not bubble free and the pressure collapses the bubbles and forces the resin into them creating small bumps on the surface of some casts, annoying but can be sanded off. I've found that every time you movie up a step in this casting game you realise you then need more equipment. Seriously thinking of getting a vacuum degassing chamber in the next few weeks for the RTV Because it needed no permanent modifications I can still sell it on as a paint pressure pot if i wanted to. Mark Link to comment Share on other sites More sharing options...
kev67 Posted August 29, 2014 Share Posted August 29, 2014 I have an industrial vacuum pump for sale, but will need oil and a good clean up for £50 and may be some of the fittings but I can send pictures if interested, Link to comment Share on other sites More sharing options...
Cheshiretaurus Posted August 29, 2014 Share Posted August 29, 2014 Thanks for the offer Kev, Have actually just purchased one on ebay a few moments ago while you were typing probably. Link to comment Share on other sites More sharing options...
kev67 Posted August 29, 2014 Share Posted August 29, 2014 I might build one from 15mm-20mm thick MDF, just a box with a Perspex clear lid and just have a vacuum cleaner hose attached, I have a vacuum chamber but that really is for getting the air out of silicone mix and clear slow curing resin Link to comment Share on other sites More sharing options...
pbhawkin Posted September 8, 2014 Share Posted September 8, 2014 Cheshiretaurus, You can still use your current setup but would just need a vacuum pump. then you can degas the rtv rubber both after mixing and after you pour it in to make the mold. This will stop the small bubbles being in the mold and make the surface of your casts very smooth. Link to comment Share on other sites More sharing options...
Cheshiretaurus Posted September 8, 2014 Share Posted September 8, 2014 Cheshiretaurus, You can still use your current setup but would just need a vacuum pump. then you can degas the rtv rubber both after mixing and after you pour it in to make the mold. This will stop the small bubbles being in the mold and make the surface of your casts very smooth. Did think about that but considered it too high a risk to use it both ways with subjecting it to forces in a way that it was not designed to take. The metal is quite thin so would be some flexing and so weakening. Have seen on a number of occasions the result of pressure vessels giving way even at seemingly low pressures. A 2 litres volume at 40PSI is not something to be underestimated. Anyway have a brand spanking new vacuum chamber now, and the difference it makes to the RTV and resin is amazing. Many regards 2 Link to comment Share on other sites More sharing options...
robvulcan Posted September 16, 2014 Share Posted September 16, 2014 Kev I am interested My wages have not cleared yet but I am looking for one to degas my resin parts and rubber molds. However size is and issue as I have some quite large molds to use with. Cheers Rob Link to comment Share on other sites More sharing options...
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