Charles, I bow down to you when it comes to the chemistry but 7 to 12 hours to me seems over kill. Once the PR jells are we really going to force more air out at 35-40 lbs pressure? I've been taking my castings out after 1 hr than into the toaster oven at 150 degrees. BTW I'm using a swing resin and 3 drops per Oz.and I also pre heat the resin in the oven for 30 min. @ 150 before adding the activator
Roy,
Please, no bowing, just unmarked cash.
I know the discussion has been that once pressurized and gelled that the disolved O2 has been trapped and cannot escape again however, I have seen
cured Silmar 41 form a vent in the inhibited surface and bubble after 7 hrs of pressure. If I remember correctly from a conversation with technical support, or a tech paper, uncured S41 resin is only very barely hydrophilic, and can/does absorb minute amounts of water. It is my untested theory that the water, as vapor(caused by the heat of curing), expands when the pressure is released, and bubbles out of the still soft resin.
The time to cure, using the recommended amount of MEKp, in the literature I have, and or read is that it requires 6-8 hrs, some brands list 12-24 hrs on 1/4" thick casting. So I leave the casts for 7hrs or so when I cast in the morning, 12hrs when I cast at night and unseal the pot in the morning.
Swing resins while they may be polyester resins are not the same as laminating resins(Silmar 41, EP4101, C'nC). They have extra/different promotors to help drive/accelerate the cure with the extra pigment or embedment load, up to 70% filler/pigments, so less load = faster cure. We really need to make a differentiation regarding which subtype of polyester resin we are using.
Silmar 41 has a viscosity between 400-600. When catalyzed at 1% has a time to gel of 20-30 minutes depending on temp, environmental conditions, thickness and load.
I typically catalyze at 0.5% of the resin volume in mL, I do not preheat the resin, but I do ultrasonic it for 16min, before pigmenting, and 8 min after pigmenting. I heat the dry/powder pigments, to drive off any moisture, before adding to the resin. I believe, perhaps anecdotally, that the US helps break up and mix the powdered pigments that may have lumped up. The US incidentally, does adds heat to the resin through molecular motion and absorbtion of the sound energy. The resin is significantly thinner after US than before.