Late to the fray here.
I've not made a pen yet (but keep yer eyes open, heheh.. ), but played with several resins/stabilising/casting/'embedding' and general experiments of the sort quite intensively these last few years so, here's how I look at it.
I must also echo not to try and cast under vacuum - it's 100% counterproductive, as all you're doing is keeping the tiny voids that do remain inflated to their maximum size then locked there as it sets, and that will show in/on the final piece and the resin may not adhere well to anything being cast in it (DAMHIK*T*).
I tend to be using the much slower curing systems at the minute, so I degass all of them, difference is only in how deep a vacuum and how long I decide to or can leave it.
The particular '24hr' rigid/clear casting epoxy I use can take maximum vacuuming for even a few hours, and I often pre-pour degass, and then again in the mould if I have anything else in there (especially porous stuff like plant matter).
The PU50 urethane (stated 10-18 hours?.. ) and most of it's sisters in the range that are slow enough, seems best not to exceed 27Hg, and under 5 mins, or it suddenly starts to boil off an aromatic component, foams up, gets hot and then sets suddenly, sometimes still crackling (DAMHIKT).
But it^ still benefits from a quick pre-pour degass, and I have vacuumed it in the mould full of other things too.
For some of the things I cast, degassing in the mould seems to get the most air out and after releasing the vacuum, seems to give the resin a bit of 'bite' into the wood/whatever I'm embedding.
If I have a working pressure pot at the time I'll leave the moulds in a warm place until they are just about to start turning, them put them in the pressure pot, (I get about 40/45 psi max ATM) keep it warm until it sets.
Have read a lot but have no personal experience of the quicker uretrhanes, so I've nothing to add there, but I did many years ago play with one Polyester system and I hated it personally - didn't like so much heat generated when encapsulating some things, didn't like the feel or look, plus mine kept setting in the bloody can at the end of summer - so I have no good experiences with them to add nor did I have vac. or pressure but others definitely have.
But anyway - that's just what I do and why, but I'm the kinda guy who has this sh*t rattling around his skull all the time, so I could have some of it bass-akwards, who knows.
',;~}~
FlowolF is rambling as he's (off his meds and) on too much coffee <chuckle>
