Abstract: Moulding tool (21), including an upper (22) and a lower (23) mound half, each with its recess, these recesses together forming a cavity (24) of the tool. The upper half (22) has a diaphragm (27) defining the cavity (24), on the rear side of which there is a space (31). Cores (25, 26) are fixed in the lower half (23), the ends (36) of theses cores engaging against the diaphragm (27). Raw material, e.g. plastics, is injected through a sprue (33) under high pressure into the cavity (24). Oil under pressure is pumped (34) through a duct (32) into the space (31) to keep the diaphragm (27) into engagement against the ends (36) of the cores. The plastics is thus prevented from penetrating between the ends (36) of the cores and the diaphragm (27), and friction forces between the diaphragm (27) and core (26) prevents the latter from being deflected laterally by the injected plastics. The manufactured products are free from flash, and disturbances in production and repair costs due to spoiled cores are small.