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Subject:
Re: I don't understand the pressure Part....

David Walser
Barry -- The confusion might partly be my fault. Up thread, I mentioned that some use pressure for resin stabilising wood. That is the process whereby pressure is used to force stabilsing resin deep inside the wood. Very few hobbyists use this approach. Instead, they use a vacuum to remove the air from inside the wood, and then let atmospheric pressure force the resin into the wood. Both approaches work, but pressure stabilizing is quicker and is the process most often used commercially. However, the pressures involved are quite high and the cost of large pressure chambers and pumps is much more expensive than a similarly sized vacuum set-up.

As you noted, pressure doesn't remove the air from the wood. But, since the resin is not yet cured, once the resin is forced into the wood and the pressure is released, the air will quickly migrate to lower pressure -- exiting the wood. Not all of the air leaves the wood and some the air movement takes resin out of the wood. But, because the resin is much thicker than the air, much of it stays in the wood where it can be cured in place.

Where most hobbyists use pressure is in pressure casting resin. Casting resins are entirely different from resins used for stabilising. After the A and B parts of the casting resin is mixed, the resin is going to cure. (Stabilizing resins are heat cured.) Some casting resins harden within minutes, which doesn't give enough time for any trapped in the mold to escape before its encased in hardened resin. So, pressure is used to shrink the air bubbles to microscopic size -- so small they typically cannot be seen.

As you suggest, the bubbles are still there in the resin. But, the cured resin is hard and -- for the most part -- the resin prevents the bubbles from expanding.

One last thing to note: Resin stabilising and resin casting are two distinct processes. They are often used together, but they don't need to be. Stabilized wood can be used as part of a resin casting, but it can be used in other ways, too. Unstabilised wood can be used in a resin casting. And, of course, a cast blank need not contain any wood at all.

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