Validation of moulding consistency with a self-regulating melt pressure valve

Abstract

Net shape manufacturing of plastic products through injection moulding, extrusion and other polymer forming processes has been limited by a lack of observability and controllability of the state of the polymer melt. For this reason, a self-regulating melt pressure valve has been developed that utilizes a valve pin to adjust the juncture loss to balance a provided 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 with natural laws, an open-loop control system design is feasible without the need for any instrumentation or process feedback for closed-loop control. The process capability of a hot-runner injection moulding process with two self-regulating melt pressure valves is validated with respect to part weight consistency. A resolution four fractional factorial design characterized the main effects for conventional hot-runner injection moulding and injection moulding with the self-regulating valve operated in both open and closed-loop control schemes. It was observed that the self-regulating valve operating in an open-loop control mode reduced the sensitivity of the part weight to changes in the moulding process by 73% compared with conventional injection moulding; short-term part-to-part variation was also reduced by 60%. While it might be expected that closed-loop control would provide even greater results, the relatively slow response of the pneumatic control valve actually tended to induce process variation compared with the open-loop control of the self-regulating valve.