Abstract: A pair of like mold parts are juxtaposed to form a mold cavity having a relatively large maximum cross-sectional size and an opening of relatively small cross-sectional size. A core is positioned in this cavity and has a body of cross-sectional size greater than that of the mold-cavity opening and having a neck extending out of the cavity through the opening and blocking this opening. A mass of hardenable material is introduced into the cavity around the core and is hardened. Thereafter the core is elastically deformed and withdrawn from inside the hardened mass whereupon the mold parts are displaced apart to demold the hardened mass.