Abstract: A sintering device includes separable mould walls which together define a mould cavity and a mould core which is positioned within the cavity to define an undercut mould surface. The core includes a hollow frusto-pyramidal central support formed with guideways and a plurality of core segments mounted for slidable movement within the guideways whereby the core is radially expandable and contractable. The segments include a plurality of arc-surface segments alternating with trapezoidal segments. The mould walls and the core include passages for introducing steam and air into the cavity. The passages of the core place the hollow interior of the central support in communication with the mould cavity.