large cavity when pressure casting

[checkonetwo]

I've been casting parts, and experimenting for several months. I use fast setting resin for some parts, and slow setting for others. I have a vacuum chamber, and recently added a pressure chamber to the arsenal.

I have degassed slow setting resin prior to pouring, with good results, but have to add a ridiculous amount of microballoons to the mix to get proper buoyancy (fishing lures).

I recently gave my new pressure chamber a try. Using 7 minute, slow set pu. I have my pressure chamber and compressor set up in separate rooms, as a safety measure. I can remotely pressurize, and depressurize my chamber. I use 50 psi as per the pressure chamber manufacturer's suggestion.

My first three casts have huge voids at the top. I've even cut my pour hole Much Bigger to give a larger reservoir of resin, but this doesn't seem to help. The resin is shrinking, and obviously pulling large amounts of air into the mold.

My first question is, how fast/slow should you pressurize your chamber? As fast as you can? Or should you do it more slowly? The resin will of coarse start to set up if you go too slow. Am I filling the chamber to fast/slow?

Do I need more vents? At some point it's gonna get ridiculous, and I'll have a huge cleanup/sanding job to look forward to with too many vents.

Also, I noticed that I have a slight leak. Over the course of 45 minutes (cure time) I have a drop in pressure of about 5 psi. Could this be a big part of my problem?

Keep in mind, my current pour hole and vent hole are situated in a manner that I'm capable of getting air-trap-free castings when I cast without the pressure chamber. I have the pour hole lower than the vent, and the vent is at the high point of my mold. I also squeeze out the air, and slightly vibrate the mold prior to placing in the chamber.

Any suggestions will help, thanks

[Fleafa]

I'd be interested to see photos.

Don't worry about your leak, that is fairly normal and won't be the cause.
If you were getting good casts without pressure, you don't need more vents.
Your description makes it sound very unlikely to be trapped air, but more likely the shrinkage. Pressure will compact the resin significantly, especially as it is filled, and is likely causing it to pull back from the feed. This also means you will have a denser cast. How's the buoyancy on the failed casts?
How big is the reservoir for the resin?
Try mixing up two batches of an identical amount of resin with the microballoons in two measuring cups. Cast one without pressure and one with. This should tell you a lot about the amount of shrinkage.
I'd also try another under pressure without the microballoons.

I pressurize as fast as I can without causing resin to be flung everywhere by the airflow (not installed a silencer/muffler on my new pot yet).

[mangozac]

[checkonetwo]

Thanks guys...

Yes, it definitely seems like a shrinkage issue. But oddly enough, despite the fact that I tripled the size of my pour hole/reservoir, the void in the cast stays virtually the same size. It looks as though the resin in the mold is shrinking, pulling in air, and the resin in the veservoir is losing the battle, not given a chance to get into the mold...When I open the pressure chamber, I immediately notice that the pour hole/reservoir is completely coated with cured resin. Resin that didn't get a chance to flow into the mold.

I hesitate cutting another vent hole or pour hole, in fear that it won't solve the problem, and make my future trimming/sanding a nightmare on non-pressure cast parts. I could always place my master back in the mold, build a box, and make repairs with silicone...but what a pain.

When you guys say "fast", how fast do you mean? Getting your pot up to pressure in 5 seconds? 10? I bring mine up slowly, over about 20 seconds. But maybe that's part of my problem...the resin might be starting to set a little.

The resin I use is Alumilite slow set 7. I will take some pics of the casts and my mold, you guys might immediately see my problem.

[checkonetwo]

Oh, and just for future reference...A "crushed" air bubble has less buoyancy than the same air bubble uncrushed. The actual volume of the bubble has a direct affect on buoyancy. This was somethingI was unsure of, until I experimented.

[mangozac]