The natural gum rubber with stabilizer additives or silicon alternative equivalency is compression molded after the material is hot extruded to ensure uniform material bonding composition and thickness prior to shaping. Pinball rubber rings are not injection molded as this is not as cost effective, and the rings have to have higher elasticity strength in comparison to sealing style gasket rings.
The process has remained unchanged since the 1940s except for updated modern materials.
People do not realize you can make your own rubberized silicon rings at home, but they may not be as strong as commercial rings.
Rubber maker marks are stamped on the rings that are included in the molds.
The main comparison of difference is elasticity to durability but their are other factors, such are hardness which measured on the durometer scale. Other critical factors are temperature response, pressure response, ductility, tensile strength, and elastic modulus even for rubber or silicon. A common mistake is overstretching a ring beyond recommended length leading to a shortened lifespan, especially on slingshots. Use the correct size.
Keep flipping.

Added over 6 years ago: Injection molded rings ARE used for other industries, based on specific requirements.
Pinball rubber rings are cheap, not for purposes of areas such as high temperature or pressure.