The apparatus disclosed comprises a barrel (12) in which there is a piston (42) on a shaft (36). The shaft (36) can be moved axially and is surrounded by a feed screw (14). There is a shut-off valve (70) in the barrel (12). The feed screw (14) is used to fill a holding cavity (C) between the piston (42) and the valve (70). The valve is then opened and the piston (42) forces the charge from the cavity (C) into the moulding cavity (30). In its forward position, the piston (42) has its front face forming part of the bounding wall of the moulding cavity (30).