US Patent 7832244 Controlling fluid pressure in a hydroforming process

A system for hydroforming a workpiece containing fluid at elevated pressure includes a die in which the workpiece is formed, a source of control pressure, a tank at low pressure, and a valve that includes a displaceable control element that alternately moves between an open state and a closed state, opening and closing, respectively, a passage that hydraulically connects the fluid in the workpiece and the tank in response to the relative magnitudes of a first force applied to the control element by the control pressure source and a second force applied to the control element by pressure of the fluid in the workpiece acting on the control element in opposition to the first force.