I saw this article in this month's Rock Auto newsletter, and thought it was good enough to republish:
Tired of breaking the old, brittle plastic clips that hold on exterior and interior trim? Worried that the plastic thermostat housing will either leak or crack after being over tightened? Hoping for a way to rejuvenate the yellowed plastic gears, slides and rollers in that window regulator? Having trouble sliding the plastic handles onto the metal rods that came with that new foosball table? The answer to these and other plastics conundrums is an elixir available at the nearest kitchen sink.
My dad was a plastics engineer for 30+ years. One of the best tips he has given me thus far is to rejuvenate and limber up old and new thermoplastic polymers by soaking them for a minute or so in boiling water. The results with nylon can be especially dramatic. Yellowish nylon goes into the water brittle and comes out supple and milky white. New composite thermostat housings, foosball table handles, and other plastic pieces that must be mounted to metal become slightly more flexible and are an easier and better fit.
The boiling water does not melt the plastic. Soaking nylon in room temperature water for a long time has the same impact as a short bath in boiling water. The chemistry is too complex for just the son of a plastics engineer to explain. Basically the moisture releases tension between polymer molecules that was created when the plastic was first made (molded, extruded, etc.) or that built up over time as the plastic was exposed to sunlight, heat, chemicals or otherwise aged.
Over the years I have only seen good results from putting my plastic parts in boiling water. At worst the plastic seems unaffected, probably a thermoset plastic (rigid body parts, distributor caps, Bakelite, etc.). However, there are myriad plastic resin recipes and plastic products out there and I must include a disclaimer and encourage common sense and caution. Do not soak plastic pieces that include electronics, gaskets, lubricants, paint, adhesives, decals, etc. that are not supposed to be exposed to water. Do not bring a plastic part out of a freezing garage and immediately dunk it into a boiling pot of water. Thin, molded plastic pieces like interior trim or milk jugs might lose their shape if exposed to heat. Heat and moisture from boiling water might not be uniformly transferred through very thick plastic pieces. If you are at all concerned about the temperature of boiling water, then maybe instead try soaking the plastic piece in unheated water for a day or two. Don’t boil a greasy composite valve cover in your spouse’s favorite spaghetti kettle…