In this section I hole to give you an insight into pressure casting.  This is not for beginners as quite a lot of equipment is needed, so itís not just a spur of the moment have a go at type thing. 

Also working with high pressure is dangerous, so you canít scrimp and scrap on the equipment needed.  A metal container full of air at 80psi is not something you want to explode.

So in this picture it shows what basic equipment you will need.  Obviously there is a pressure pot, and these come in various shapes and sizes.  I have a large and small one; the larger one gets used far more, but the smaller one is handy for one offs, or doing stuff in clear resin where it takes 12 hours to cure.  Itís small enough not to get in the way, and means the big pot is not tied up for long periods. You will also need a compressor, and its best to use an inline pressure gauge/dryer.  This is a very simple item that spins the air removing the water out of it before letting it onto the pot.  In the case of my pressure pot, I removed the wing nuts that hold the lid down, and replaced them with bolts and washers.  This allows the top to be done up with a torque wrench equally all the way round  Under high pressure the air will find the smallest weakness and escape, and the wing nuts were just too hard to do up the same all round.  Also with the wing nuts the highest pressure I could get was around 40psi before it hissed out, with the nuts, it goes up to 85psi no problems.

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So in this picture it shows what basic equipment you will need.  Obviously there is a pressure pot, and these come in various shapes and sizes.  I have a large and small one; the larger one gets used far more, but the smaller one is handy for one offs, or doing stuff in clear resin where it takes 12 hours to cure.  Itís small enough not to get in the way, and means the big pot is not tied up for long periods. You will also need a compressor, and its best to use an inline pressure gauge/dryer.  This is a very simple item that spins the air removing the water out of it before letting it onto the pot.  In the case of my pressure pot, I removed the wing nuts that hold the lid down, and replaced them with bolts and washers.  This allows the top to be done up with a torque wrench equally all the way round  Under high pressure the air will find the smallest weakness and escape, and the wing nuts were just too hard to do up the same all round.  Also with the wing nuts the highest pressure I could get was around 40psi before it hissed out, with the nuts, it goes up to 85psi no problems.

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Any mould you intend to use really needs to have been made with pressure casting in mind.  Most people normally mix the silicon and pour it over the object at hand, which is fine, but if you place that under high pressure, the air bubbles trapped in the silicon will compress under the pressure and distort the mould.

To stop this the silicon really needs degassing, to remove as much air as possible. A simple degassing chamber can be made for next to nothing, as seen on another page of my web site in this section.  If you don't have any means to degas the silicon, add silicon fluid to the silicon rubber, and use a fraction less catalyst.  The silicon fluid thins the rubber, and less catalyst means it will take longer to cure.  The longer it takes to cure the more air can escape. 

Also the mould if not an open one, it needs a resin reservoir at its pour in point. Under pressure all the air is compressed so the resin level drops.  If you fill it just enough it wonít be enough when under pressure. Allow a few extra ml just to make sure.

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 So with the pressure pot sorted out, and the moulds made, its time to sort out the resin.  There is not one resin better than others, and in fact some of the cheaper ones are better to work with than the really expensive ones, so itís more down to what you use or can get.

The only thing you need to remember is chose a resin with a minimum of a 15 minute gel time.  Anything less than this you will be hard pushed to fill the moulds, get the lid on, and do it up tight and then pressurise it.

Also work out exactly how much resin you will need to fill the moulds you are using, as this way you donít waste any.  I make a chart of things I have made and write down how much resin each one takes to fill.  However with pressure casting allow a few ml extra for compression.

I always mix the resin with a high speed whisk, and then degas it to remove the air.  While the air is compressed tiny bubbles in the resin, they will rise to form a bigger bubble, which even under pressure will leave a small bubble mark.  Degassing the resin gives a much cleaner cast.

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     So here you have a finished item, in this case its a coin return that will go on the main body or R2 D2 when finished.

The thing you notice the first time you do this is compared with none pressure casting, the hugely improved quality of your cast.  Normal open casting comes out well if done properly, but if you spray paint that, you can then see tiny little bubbles. However with pressure casting first thing you notice when spraying, it the lack of tiny air bubbles and the super smooth finish.

Also it allows for far thinner sections than normal to be cast, as there are no worries about whether or not the resin will run into the thin section, air bubble free or not.

While itís not the cheapest set up to get going, it is worth it if you do a lot of casting, and especially of you cast very small things, as you are guaranteed a full bubble free cast each time.

Hopefull that has given you an insight into the very basics of pressure casting.

Hopefully I have pointed out while itís not that hard to do, it does require more equipment than most casting techniques, but not the sort that is out of reach of most peopleís budgets.