Pressure Casting??

[ajmadison]

Thought I'd start here then try this question in the Construction Forum, if I get just 'shrugs' here. *wink*

I'm contemplating casting up a variety of parts. I have a standard compressor, so I could do some pressure casting, once I have a pressure pot.

My question is, is it just a matter of making molds, pouring the resin in, popping mold w/resin into the pressure pot and applying air pressure? Of course I'm over simplifying the process a bit, but thought I'd ask. Does the mold need special reinforcement? I'll guess the mold needs to be designed a little bit special, like a place for the excess resin to go under pressure. Or is it more involved than that, like a special mold box that goes around the RTV?

Thanks in advance...

[Johnnycrash]

My question is, is it just a matter of making molds, pouring the resin in, popping mold w/resin into the pressure pot and applying air pressure?

Pretty much, that's it. Now, about molds...

Does the mold need special reinforcement?

No

I'll guess the mold needs to be designed a little bit special, like a place for the excess resin to go under pressure.

Yes and no. The yes part, you need to have a small reservoir of resin for the mold to draw in from to fill the bubble's ex space as they collapse. The no part is, this can be nothing more than a slightly larger pour stub.

Or is it more involved than that, like a special mold box that goes around the RTV?

No special mold box is needed. Well, not for pressure casting in itself. A mold support (mother mold) may be needed depending on the part, size, volume of resin and so on.

What you need to do that's different than NON-pressure casting: De-air you silicone (or urethane). For that, you will need a vacuum chamber, and a vacuum pump. The reason is, that without the de-airing, small bubbles of air get trapped inside the depths of the mold material. While this may not affect castings without pressure, it will affect castings that are pressured. Those small bubbles will collapse (just like in the resin), and that collapse will cause a small divot in the surface of the mold, and that divot will fill with resin, and you casting will look like it has the mumps. And that's a bad thing.

There are some silicones that are "self de-airing". They are OK, but not great. For best results, a vacuum pump (pulling 29") and a vacuum chamber are the best way to go.

[ajmadison]

Thanks for the response. Now I understand the need for de-airing the silicone. I've seen people looking for it, but it wasn't explained why or for what purpose.

I'll have to think/research the whole vacuum & de-airing process.