Ron -- I didn't know that the manufacturers did that.? Good tip! I'll check with K2 here and see if they can do that.
In the meantime, I've done a little bit of checking and calling around about helmet construction, and there appear to be two kinds of builds:
Type 1) The first type has a thin layer of light polycarbonate material that gets vacuum formed over, and bonded to, a thicker layer of expanded polystyrene beads (EPS), which is the shock absorbing layer.? This construction is used because it is light weight and has a slim profile.? However, because the outer layer is not overly rigid or strong, it is very susceptible to damage from impact, even moderate ones.? The outer polycarbonate shell and will buckle in on impact, compressing the EPS beneath.? These type of helmets probably need to be replaced after a moderate or worse impact.
Type 2) The second type has a rigid, hard outer shell made of injection molded ABS, to which the EPS liner is glued.? This construction is much stronger, as the ABS is much less susceptible to deforming on impact -- it' hard stuff, so your head will probably just bounce without the helmet compressing.? The downside is that these helmets are heavier and look bulkier.? According to my local shop, these do not necessarily have to be replaced after every impact, unless it's a really hard one on ice or against a tree or branch.? The hard shell will prevent the EPS liner from compressing.? However, if in doubt, toss it and replace it, or get it checked out like Ron suggested.
This is by no means an exhaustive review, and there is a lot that I don't know yet.? So take the above as just a quick overview.
FYI, I have the second type of helmet -- with the ABS hard shell.? It's a K2, and is about the only one I could find that fit my cranial shape (no squarehead jokes, please!).? It's, well, pretty darn unattractive, but so comfortable that I can wear it all day and not even know it's there.? Jim -- it also has the click wheel in the back to adjust -- great feature. If I have to replace it, I'd buy another just like it, it fits so well.