This happens because the plastic shrinks a bit when it gets warm, kind of like shrink wrap.  Very similar plastic edging, T shaped and designed to press into a saw kerf, is available at stores that serve cabinet makers and other serious woodworkers (not a regular home center or lumber yard).  Is is usually available in several colors.
  ----- Original Message ----- 
  > This is probably a not-often-asked-for item, but the Westfalia gray
  > plastic (rubber?) moldings on many of my cabinets appear to have shrunk
  > in size, in that they no longer extend over the length of the corner,
  > joint, or door aperture as they originally did/were supposed to. Thus,
  > my cabinet trim has an odd, ill-fitting appearance. The molding on my
  > A/C housing is even rippled as if temperature changes caused it to warp
  > in place.

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