A plastic encased mandrel includes an open cell carbonaceous foam that is shaped to correspond to a core of a ceramic matrix composite (CMC) part. A polyvinyl butyral (PVB) plastic is injection molded into the carbonaceous foam, before or after shaping, to encase the carbonaceous foam core and form an encased mandrel. The open cell carbonaceous foam may be silicon carbide (SiC) foam or reticulated vitreous carbon (RVC) foam. The foam core may be incorporated into the CMC part when SiC foam is used for better distribution of thermal and structural loads or the foam core may be removed from the CMC part when RVC foam is used, such as when a CMC vane incorporates a metal spar.