the depression 7 the pivoting movement of the mold cores 9 is achieved by a sloping cam slot control which comprises a drive rod 19 having a control pin 20 which is slidably received into a cam slot 21 on the mold core as shown in fig2 and 5 the drive rod 19 is rectangular in cross section and is slidably supported in bearings 22 on the outer side of the mold so as to be capable of horizontal movement as shown in fig2 a drive rod 19 is mounted on each mold side wall 3 and is drivingly connected to the mold cores 9 on a pertinent mold side wall by individual control pins 20 each drive rod 19 is connected to the inner side of a drive belt 23 which is guided around the mold 1 on deflection rollers 24 mounted on the four corners of the mold 1 the outer side of the drive belt 23 is connected to a piston rod 25 of a double acting hydraulic cylinder 26 which causes the drive belt 23 and thus all four drive rods 19 to move back and forth the stroke of the hydraulic cylinder 26 is so selected that the mold cores 9 are pivoted.