A crisping sleeve is intended for use in a microwave oven

Microwave oven

A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...

 to crisp and brown food items placed within it.

Crisping sleeves consist generally of paperboard

Paperboard

Paperboard is a thick paper based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper. According to ISO standards, paperboard is a paper with a basis weight above 224 g/m2, but there are exceptions. Paperboard can be single...

 affixed with a susceptor

Susceptor

A susceptor is a material used for its ability to absorb electromagnetic energy and convert it to heat . This energy is typically radiofrequency or microwave radiation used in industrial heating processes, and also occasionally in microwave cooking...

 that absorbs microwaves and converts them to heat. The purpose of the susceptor is to apply additional heat to a specific portion of the food, browning it. The paperboard is also often used as a rigid container to support the food items within.

They are generally intended for a single use.