Ooops, I suppose I could be clearer in the wording ; OK let's put it this way ... most if not all of todays fountain pen, not using exotic materials like Celluloid , Ebonite or not being made of metal or other natural material are usually comprise of parts made from thermoplastic, but thermoplastic the term really mean nothing specific ; so do the term Resin or Acrylic
Since none of the types of plastic is in itself a single specific material but usually a family of materials ( in manufacturing ) it can be too generalized to say one is definitely better than the other or so .. say Lamy are famously known for using ABS for their Safari and marketed as tough, hard weathering and exotic among plastics but in reality ABS is nothing exotic. Its only exotic because few use it for pen bodies ( when Lamy introduce Safari back then ). Hero actually use ABS in some of their vintage hooded nib pens for the collector. Also PMMA was almost always marketed as material of choice for Demonstrator as it give reasonable strength and very good transparency but Parker 51 was then made in Lucite , and Lucie is just a trade name for a certain type of PMMA.
That stated, PMMA , the usual material we see in these Demonstrator is still having lower impact resistance and strength when compared to similar acrylic material Polycarbonate which itself can be made to be transparent ( though not as transparent as PMMA ) and these acrylic are the usual one we see in todays fountain pen of better made ( that is better than typical student supply pens ). The PMMA was used primarily for their transparency , not for their strength
One had to know that todays technology give us many different material in plastic and the modern day fountain pen body , if not required to be transparent can choose some that are economical to had, easy to mold, easy to color and return good strength in both tensile and imapct measurement. But as with all these often than not the material only made up half of the story, the other half is in the engineering and manufacturing