Pressure Casting - how?

[Przemas]

After some encouragment I`ve decided to give casting in pressure pots a try. So far I`ve used vaccum chamber and got perfect, bubble-free casts.
But the idea of using faster resins is so tempting...
The problem is I can`t get it right. So far I got good casts in pressure pot only with simpler/bigger pieces, where injection hole (sorry, can`t find better term) was quite large (for example scenery bits), or when I was casting in open mould (for example bases, medals).
When I was trying to cast small pieces like 28mm miniatures or small conversion pieces the resin haven`t filled the mould completely and I got huge air pockets.
I`ve tried building the pressure slower (up to around 60 psi) but that still hasn`t helped .
Any ideas what do I do wrong? How do you do it? Any tips, step-by-step instructions, links? Your help would be greatly appreciated.

 

[Tommi]

If you are using a vacuum chamber and getting perfect results why not carry on using this method but use a faster curing resin. I use Biresen G26 & G27LV which has a pot life of only a few minutes, I get perfect results even on very small parts.
Tommi

 

[Einion]

Very high pressure?

Einion

 

[Przemas]

@tommi: the problem is that to get perfect results when using vacuum chamber I need to keep the mould in it for at least 3 minutes. I can only shorten this time with simple casts. Faster resins (Biresin g27 fe) have a pot life under this time and when you add the time for mixing, pouring etc it turns out it is not possible to work with them.
On the other hand in pressure chamber, the pressure difference that forces the resin to fill the mould starts at the very beggining of the process thus making it possible to work with fast resins. At least this is the theory - Ijust can`t get it right :/ .
Normally I wouldn`t care that much about pressure casting and would stick to vacuum which gives me perfect results. The thing is that each month I have more and more things to cast so short demold time (which is typical for resins with short pot life) would make my life a whole lot easier .

@Einion: as for pressure I think 60psi should be enough - this is at least what I`ve been told. And to be honest I believe it - during vacuum casting the pressure difference is much lower and it still works.
AT first I thought that I`m building the pressure too fast thus making not only resin to flow into the mould, but air as well. Still, building the pressure much slower hasn`t helped a lot.
Maybe I should pour even more resin that I do when I`m vacuum casting?
Any suggestions would be greatly appreciated .

 

[Tommi]

When you state that you have to leave the mould in the chamber for three minutes is this because it is a large chamer or the pump does not extract the air quick enough?

I use a 5 litre tank and small pump for small part and a 30 litre tank with a hi-vac pump for large parts, they both work fine with fast cure resins and degass the moulds in less than one minute.
Cheers
Tommi

 

[Przemas]

@Tommi: how big minis do you cast? are you getting the foam build / foam fall effect within this 1 minute?

 

[Calvin]

I`ve tried building the pressure slower (up to around 60 psi)

Slower? 60 psi is a tremendous amount of pressure, I cast at 1,5 bar (21/22 psi) using a 2 min. resin with no problems and no air bubbles at all. Most of people think the pressure reduces the air bubbles size putting them on the deepest so the more the better but that is wrong, the only effect of pressure, as for the vacuum, is to help the trapped air bubbles flowing out the mold. It is all about the mold design and how/were you place the vents.

 

[Przemas]

@Calvin: the thing is that with the same mould I get perfect results when using vacuum chamber and can`t get the same effects with pressure.
I agree that 60psi is quite a pressure. By "slower" I simply meant I haven`t let the air in rapidly - I`ve started from 0psi and I`ve gradually built it to 60psi.
I assume it is the same as with vacuum - you can`t be too fast or the mould will "suck" not only resin but air too.

 

[Calvin]

You cannot use a mold designed for vacuum to cast with pressure and viceversa. When you build a mold to cast with pressure it is all about (air) flowing, I mean you should carefully consider how to place the piece into the mold, how to incline the mold while pouring and where to place the vents, which may also vary depending on what kind of mold you are using (2 parts or 1 back cut). How fast you pressurize your chamber doesnt matter at all while the final amount of pressure you apply is crucial to avoid any kind of distorsion and shrinkage.

 

[Przemas]

@Calvin: do you have any pics/vids etc? I fail to see the difference between concept of casting in vacuum chamber and pressure pot.
As far as my thinking and observation goes it is all about building difference in pressure.To explain how I see things clearer:
- when vacuum casting I`m pouring the resin on top of the mould where i have injection points. When I start the pump the air from inside the mould flows away but as the pressure above the mould (or I should rather say above resin layer) is around the same very little resin pours in. After a while I`m stopping the pump and I`m slowly letting the air flow into the chamber. Now the difference in pressure between the inside the mould and outside does the work - the pressure inside the mould is lower so it naturally tries to get equal with that outside so the resin is sucked into it
- with pressure casting the difference in pressure starts to build as soon as you start the pump. the pressure outside the mould is higher and it tries to get equal so it forces the resin to flow in.

Regardless I got it right or wrong I`d love to see how other do it .