when should I use acetyl engineering resin?

 Once, a student asked the Master, "when should I use the more expensive acetyl resins instead of say, amides or even vinyls, styrenes, or ureas?"

 To this, the Master replied, "when should you use quality materials in your work instead of, say, inferior materials or even sheep manure?"

 "But," persisted the student, "isn't Nylon nearly as good as Delrin?"

 At this, the Master used the stick he had been leaning on to smite the student on the head. "No!" he exclaimed, "that's why one is called an amide and the other is called an acetyl!  They have completely different properties!  You would know this if you had studied!"

 "Thank you Master," said the student, rubbing his head, "but what about low-surface-energy or low-friction applications where PTFE or HDPE are considered the bes- OW!"

 The student spoke in this way because the Master had just hit him on the head again. "NO!" he shouted, "Teflon and polyethylene are too soft!  They cold-flow!  Use graphite- or molybdenum-loaded acetyl!"

 "Thank *groan* you Master," said the student, rubbing the other side of his head, "but what about high temperature or chemical applications where thermosets or exotics are... um..." (the student raised his hands over his head) "the preferred resins?  Um... sir?"

 The Master glared silently at the student for a moment and then said, "you ask too many questions.  Go and meditate for three days on the value of reference books.  In the latrine.  Without a mask."

 After this exchange, the student was enlightened.  Very sore, but also enlightened.