Pressure Casting & Compressors

[qc]

is there a cheap way around this top?

say buying (3) 1/4 sheets of plexi and bonding them together ...around the edges..and maybe a grid in the middle?

where might i get these thin sheets? may a hardware store?
and what is best to bond them?

[DX-SFX]

Forget laminating. That way lies disaster. There's a chance the bottom layer will be sucked from the upper layers if your bonding is less than perfect and they'll all fail in the same split second. Use one homogenous piece. Check out local illuminated sign manufacturers. They're good for all sorts of off cuts.

[Ein]

Hello guys!

I come bearing a truckload of questions about pressure casting that my amateur internet searching has not been able to answer. I'm hoping maybe the lot of you can help me clear up a few things so that I can get underway.

I already have what I believe to be the required tools for pressure casting. I have a Harbor Freight Painting Pressure Pot, which I have modified to work as a pressurized chamber. I also have a shop compressor that can put out 125 PSI, though the pressure pot's only rated to 60. I've already spent a bit of time working on the two and getting them hooked up together, and I am now able to pressurize the inside of the pot to a stable 60 PSI for an indefinite amount of time.

Now, here's a couple of my questions.

Is 60 PSI adequate pressure for pressure casting?

Is there a special kind of mold rubber, resin, or urethane that is required for pressure casting?

Is it correct in saying that the molds you use in a pressure pot need to be MADE while under pressure? I've read a few different places that say that the molds get deformed under pressure if they're not made IN that pressure.

If the answer to my previous question is yes, they must be made under pressure: How do you make a two part mold under pressure? Normally, when I make a two part mold, I use something like Klean Klay to mask off half of the model, and I pour the mold rubber overtop. While this works just fine and dandy for non-pressure casting, wouldn't the pressure applied in the pot also cause the Klay to deform? I can imagine there being an air bubble or two in the klean klay that I use to mask the model that screws the whole process up, but as I've never done it before I'm not entirely certain.

Does resin take the same amount of time to cast under pressure as it would normally?

Those are basically all of my questions for the moment. I've got a model I want to make a two-part mold in and cast in clear resin, but before I give it a go I want to make sure I understand the basics properly, and I am having trouble finding out the information I need on the net.

Thanks!

[Umi_Ryuzuki]

30-40 PSI is enough for pressure casting.

No special RTV is required for pressure casting.

Molds do not need to be made under pressure. However,
the RTV should be de-aired under vacuum to remove trapped
air bubbles. This is typically done in the tub just after mixing.
And also to your mold after just a thin layer has been applied to
the part and mold box. This releases any air trapped against the
part by the RTV or trapped in recesses or panel lines.

Any air bubble in your mold after it has cured will be compressed during
pressure casting. this will cause bumps on your cured part. Vacuum
de-airing helps minimize this issue.

The clay will also off gas during the vacuum de-airing process.
If your part is not securely mounted in the clay, it may pop loose and
float out of place. Cutting a hole beneath any air cavity under your
master will prevent this from occuring.

Resin will cure in the specified time whether it is on the work bench, or
under pressure.

[DX-SFX]

Be wary of using anything that contains water (like some clays) when vacuum degassing particularly if you have an efficient vacuum pump. Lowering the pressure can cause the water in the clay to boil particularly when it's warm and that will obviously create bubbles.