Abstract: A mold closing unit for an injection molding machine having a double acting mold closing piston, a closable valve interconnecting both sides of the piston, an auxiliary piston, and a piston rod for each piston, each connecting one side of the pistons to the movable mold part wherein the cylinder spaces on mold closing side of the two pistons are permanently communicated whereby the change in volume of the combined working space is proportional to the areas of the piston rods. The cylinder for the auxiliary piston is placed laterally of the main cylinder so that the overall length of the machine is held to a minimum. An additional return piston is also provided.