A thermoplastic frame material that is molded instead of stamped out, and therefore returns to its original shape when heated is:

The thermoplastic frame material that is molded and retains its shape when heated is Optyl. This unique characteristic is due to its molecular structure, which allows it to be reshaped with heat and return to its original form once cooled. This feature provides flexibility in design and adjustments, making Optyl a popular choice in eyewear manufacturing.

Optyl is also lightweight, durable, and hypoallergenic, which enhances its appeal for both manufacturers and consumers. The ability to be molded rather than stamped also allows for more intricate designs and customization options, which is an advantage in creating stylish eyewear.

In contrast, other materials listed have different properties. Cellulose acetate, while a common choice for frames, cannot return to its original shape once it has been shaped, as it is typically stamped. Cellulose aceto-propionate is a similar material but still does not possess the thermoplastic qualities of Optyl. Nylon is strong and lightweight but is not known for the same heat reshaping characteristics that define Optyl.