Mould side portions (10; 110; 210) of three moulds are supported by a mechanism arranged to equalize mould closure force between the three moulds. Three mould side portions are supported by a triple support (50, 52; 150, 152; 250; 350) centrally pivotally mounted. The opposed mould side portions are mounted two on a double support (32; 232; 332) which is pivoted on a pivot (24; 224; 324) aligned with a line passing centrally between the center mould and one of the other moulds and one on a single support (40, 42; 240; 340) which is pivoted on a pivot (26; 226; 326) aligned with the center-line of the other of said other moulds.