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.
Related topics:
IR Temperature Measurement
This glossary of plastic industry is provided by PLEXPERT Canada Inc.