Waveguide exposure chamber for a microwave energy applicator

A microwave heating device for activating a microwave-activatable foam insulating material within the dual sidewalls of a beverage cup. The device has a waveguide that defines an exposure chamber for the articles. The waveguide has a plurality of 90-degree twisted rectangular sections, for rotating a perpendicular microwave field pattern along the length of the waveguide. A continuous flexible belt made of a dielectric material passes into and through the exposure chamber along a conveying path, and is formed and maintained into a curled shape in cross section through the exposure chamber to retain the untreated beverage cup upon the curled belt.