We call in a generic way plastic resin the plurality of materials characterized by injection mold processing (some of which, historically used for fountain pens, have been reported in the side table). In fact also more traditional materials such as celluloid or galalith are plastic resins, but we will discuss briefly in this section only these new ones that could processed by injection mold, that were used in fountain pen productions from the 40s until around the 60s.
The pen that more than any other is considered the prototype of plastic resins fountain pens, is the famous Parker 51, created in 1939, but marketed extensively only since 1941. The body of the pen, so as the section and the shell that protects the hooded nib was realized in "Lucite".
But Parker was not the first manufacturer to use these new materials, well before the official launch of 51, in 1939, Waterman launched the Hundred Year model, also produced with the same material, Lucite, which was probably the first major model produced in plastic resin. Another great success plastic resin model was the Eversharp Skyline, made in polystyrene. Since the '50s, most manufacturers began to abandon celluloid, which until then was the dominant material, to switch to the use of different kind of plastic resins.
When plastic resins entered the market, they showed considerable advantages, first of which was the ease of the their processing. Parts which could be made with injection molded plastic favored the industrialization of the production at much lower prices. Furthermore, these new plastics were also much more resistant to the corrosive agents; in fact one of the reasons for which the Parker 51 resorted to Lucite was the need to resist the corrosive effects of a new fast drying ink, introduced together with the pen.
The plastic resins main disadvantage, at least at the time of their introduction, was the impossibility to produce them in anything but some solid colors. But this turned out to be a minor problem, because in the same period the stylistic trends, even for the birth of these new materials, began to be oriented towards a modernist and minimalist style, so colorful celluloid models began to be seen as a bit antiquated.
Furthermore some plastics could lend to the creation of mixed materials of considerable interest. Among them probably the most interesting was the Makrolon (the name used by Bayer) version introduced in 1966 by Lamy for his famous Lamy 2000 model, a mixed fiber glass polycarbonate with high mechanical strength, and a particular black/gray color coming from the weaving of the glass fibers.
Nowadays almost all pens are made in some kind of plastic resin, more or less shiny or resistant depending on the implementation. The evolution of technology also allows the creation of colorful plastics, whose brilliance and variety of colors has little to envy to celluloid. However, all of these are fully automated industrial productions, with a very low production cost, regardless of how someone can call them "precious". The celluloid long production times and the impossibility to process it by injection mold, made it a much more expensive material, and this is also why there was a return to it in recent years, as a distinctive feature of some luxury pens.