Every piece of this pen except for the spring and the nib is plastic. Well, 'acrylic', but not acrylic in the way that Sailors or Viscontis are acrylic--acrylic in the way that fake nails are acrylic. It feels cheap, almost disposable, in the hand. But it also IS cheap, so that's not really a flaw, it's a feature.
The cap snaps in place with a solid click, and contains an inner cap designed to keep the nib from drying out. Mine has been very effective. The pen doesn't dry out, even after a few days without use. The cap has no clip, but has a very small lanyard loop that also serves as a roll-stop. I put a bit of cotton cord on mine, so I could wear it around my neck when I'm at work. I could only stand to do that for a few days, though, because it's just too odd-looking. I'd prefer a clip. The rim of the cap is not reinforced in any way, so I do worry about cracking. Even my sturdier TWSBIs have cracked, and this one is distinctly more lightweight. I have knocked it about a fair bit, though, in pockets, purses, around my neck, closed up in notebooks--and it doesn't look any worse for the wear. The cap does post securely, and it's light enough that posting doesn't add any extra weight, but I worry about the stress posting puts on the cap rim.