I bought a used Fine nibbed Phileas and while it writes wonderfully, there are tiny cracks on the cap that prevent me from securely posting it.

Anyone have any thoughts on plastic caps in general? I'd like to get another Phileas, but if this is a common problem, maybe I should stay away.

I'm also looking at the M200, and I can't tell if the bottom of the cap is plastic, or if there is a metal ring at the bottom of the cap.

Just a side note - my Phileas takes FOREVER to clean out, but perhaps for the same reason that it flows/writes so well.

I've had a Phileas for about 3 years now and though it's not used 100% of the time it is still in very good working order. Maybe the previous owner (as it was used) was not kind to it or you inherited the problem that already existed when it was purchased. To my knowledge, this is not a common problem.
Most of the pens I have are screw on not snap on as the Phileas but overall and in general, I have had no issues with plastic caps.

My Phileas never had problems with the cap cracking but then, I never posted it. Too heavy for me. I post my 200s/400s/600s all the time. They have cap bands in all cases so should not put any strain on the plastic when posted. None of my Pels have had cap cracks with the exception of the known problems with the White Tortoise.

I have other plastic caps that don't have cap bands and none of these have cracked either. Some get posted, some do not.

I have 3 Phileas pens and no problems with any of the caps. Given the reputation the Phileas has on this forum I'd say most folks generally have very good experiences with them, including the caps. I agree with EventHorizon above and think perhaps your cap has been abused in some way. If you like how it writes, and I love how my Fine Phileas writes, you might want to try another one instead of throwing out the whole line. Good luck.

I will agree with what others have said, that (unfortunately for you) your cap cracks appear to be the exception not the rule. I have owned several Phileas pens over the years and have never experienced a cap crack issue. This includes a couple fountain pens that have been in continuous use for over seven years and a roller ball that has been in continuous use for nearly ten and a half years.

mine didnt crack till my mom stepped on it...

I avoid posting, as there's some little protrusion in it that scuffs up the barrel (and the section, for that matter ). I find either setting the cap on a nearby surface, resting upon its mouth, or containing it in my non-pen hand a fairly adequate solution-- and one which I apply to a lot of my vintage pens, too.