a system for the rapid clamping of dies on a horizontal press for the injection moulding of plastics materials consists of a pair of structures fixed permanently along two opposite sides of the fixed and movable plates of the press and defining therewith two guides adapted slidably to receive the fixing flanges of the respective die . motor - driven clamping means are associated with these guides and are operable simultaneously to clamp the fixing flanges of the die to the attachment structures .