Final answer:
The part of the facepiece assembly made of clear safety plastic and mounted in a flexible rubber frame is known as the facepiece lens or visor, essential for visibility and protection.
Step-by-step explanation:
The part of the facepiece assembly made of clear safety plastic and mounted in a flexible rubber frame is commonly referred to as the facepiece lens or visor. This component is essential for providing visibility while protecting the wearer's eyes and face from hazards. It is a critical part of personal protective equipment, particularly in fields requiring respiratory protection, such as firefighting, chemical handling, or medical applications. The clear plastic allows for unobstructed vision, while the rubber frame ensures a snug and secure fit to prevent harmful substances from entering the facemask.