The use of TR-90 (Polyamide), Ultem and other lightweight and flexible materials for eyeglasses frames construction have contributed to the popularity of plastic frames. These materials are been advertised as lighter and more flexible than conventional acetate materials. Demonstration of super flexibility and the ability to twist the frame without breaking it has added to the confidence of these materials.
Its flexibility and ability to bend without breaking is good. However, when the temple stiffness is too low, this will reduce its wearability. The temple of an eyeglasses frame needs to have a minimum stiffness in order to provide sufficient force to grip onto the side of your head. Without sufficient stiffness, the glasses will certainly slip when you bend your head down. On the other hand, if the stiffness is too high, it will put too much pressure on the head.
Stiffness of the temple depends on the material property and its cross-sectional shape. In the simplest sense, a thicker temple will have a greater stiffness than a thin temple. Ultem and TR-90 both have good strength and flexibility but the cross-section of the temple need to be designed in such a way that it provides sufficient stiffness for normal usage.
A simple way to test its stiffness is to hold the frame by pushing against the side of the temples such that it stretches apart. Just like how your head would push them apart. If you need to push the temples very wide apart to support the frame and the prescription lenses, the stiffness is too low.
For existing eyeglasses users, another option to secure the glasses is to use temple earhooks. These act as mechanical support by tucking behind your ears. Due to sensitivity of the ear to constant excessive pressure, most earhooks in the market need to be carefully adjusted such that it just touches the back of the ear without applying too much force but still holding the glasses in place without slipping. Such fine adjustment is often difficult to get it right. FOCUS and Flex Discreet earhooks are designed to absorb excessive force such that it is easy to get the glasses to stay in position without hurting the ears. FOCUS uses a forward pointing cushion to reduce the pressure and Flex Discreet has a flexible tail/hook that limits the amount of force applied.
If your eyeglasses temple stiffness is too low, you may not have to ditch it and buy another pair. There are accessories that can be used to compensate for its low stiffness. Friction sleeve may be used to provide more friction to the temple so that a lesser force is necessary to stop the glasses from slipping. For a better grip, eyeglasses temple earhook may be used. This will provide additional support by holding your glasses from behind your ears.
When selecting your next eyeglasses frame, do not be too fascinated by its super flexible temple. Just do the simple test to check its stiffness. If the frame design has failed, even your optician may not be able to help stop it from slipping.