Capsules with integrated mouthpieces, heat-not-burn (HNB) aerosol-generating devices, and methods of generating an aerosol

A capsule for a heat-not-burn (HNB) aerosol-generating device may include a base portion, a first cover, a second cover, an aerosol-forming substrate, and a heater. The base portion includes an engagement assembly configured to couple with the first cover and the second cover. The first cover defines a first recess, and the second cover defines a second recess. When assembled, the first cover is aligned with the second cover such that the first recess and the second recess collectively form a chamber. The aerosol-forming substrate is within the chamber. The heater is configured to heat the aerosol-forming substrate to generate an aerosol. The heater includes a first end section, an intermediate section, and a second end section. The heater extends through the base portion such that the intermediate section is in the chamber, while the first end section and the second end section are external segments.