A simple machine (the only one discovered since prehistoric times) that operates on Pascal's principle (1650): pressure applied to a unit area of a confined liquid is transmitted equally in all directions throughout the liquid. A hydraulic press is comprised of a large piston in an enclosed chamber; its top is attached to a platen that rests on the members of a metal frame when the press is open. Water (or oil) is pumped into the chamber through a valve; once it has been filled, whatever pressure per square inch is applied at the valve will be transmitted to every square inch of the piston and of the walls of the chamber as well. Thus, for a piston whose cross-sectional area is 100 sq in., 10 psi at the valve will exert 1000 lb pressure on the bottom of the piston, causing it to rise and the press to close. The pressure on the object being pressed varies inversely with its area. Hydraulic presses exerting pressures up to 15 tons are used for shaping steel products. Less dramatic are those for molding rubber and plastics, compressing laminates, de-watering solids, and expressing vegetable oils. Some have up to a dozen platens (decks) for multiple-product work. The same principle is used to activate plungers on injection-molding presses.
