Net shape manufacturing of plastic products through injection molding, extrusion, and other polymer processing methods has been limited by a lack of observability and controllability of the state of the polymer melt. A self-regulating valve is developed and validated that regulates the output melt pressure in proportion to an input control force. The valve relies on a valve pin that adjusts the juncture loss to balance the control force with the force exerted by the melt pressure on an exposed surface of the valve pin. Since the valve pin position is adjusted in accordance to natural laws, an open loop system design is feasible without need of any instrumentation or control system for closed loop feedback control. The design is analyzed using a three-dimensional flow analysis that utilizes independent shear and elongational viscosities for the polymer melt. Pressure drops and shear stresses through the valve are analyzed to estimate the steady state error in the output pressure when the valve pin is controlled in an open loop mode (i.e. without melt pressure feedback). Guidelines for the valve design are provided to achieve a reasonable trade-off between flow and structural requirements. Finally, experimental validation indicates an excellent level of response and consistency given the simplicity of the design.