The ability of an object to mirror infrared radiation is called reflexion.
The amount of the reflected radiation depends on the surface of a given object.
Plastics have usually a low amount, while shining metals (for example an insert in an injection mold) have a high amount of reflexion.
The higher the proportion of reflexion, the more difficult it is to measure temperature using an infrared camera.
IR Temperature Measurement
This glossary of plastic industry is provided by PLEXPERT Canada Inc.