The temperature shields obtain even temperature inside the target, when target is exposed to sunlight. The temperature shields are also worn by the projectiles, leaving a larger hole (normally in the centre section). 

Expect to replace rear temperature shield when the wear hole get to a diameter of 20% of the width of the temperature shield (general rule). The front temperature shield (already having a pre-cut hole) is replaced when the expanded plastic sheet insert no longer stay at place. 

The temperature shields are also worn by sunlight and other environmental factors. Expect to replace when the temperature shields don't maintain physical integrity.

Rear temperature shield rip-off section

The rear temperature shield start swelling when hit by several bullets. The swell will eventually start pushing target rubbers inwards. This may, in windy conditions, collapse target measuring chamber. 

To prevent collapse, the rear temperature shield is fitted with a rip-off section. Rip this section off and pour out debris when temperature shield starts swelling. The ripped-off section shall face the rear target rubber skin.