Dynamic Feed Control
for Injection Molding

Dynamic Feed Control - First Generation (Stanford University, 1992-1996)

The thrust of thermoplastic materials into advanced technical applications has resulted in industry requirements which exceed the capabilities of available product development and manufacturing processes. The lack of robustness in these processes is evidenced by the long product development cycles, excessive tooling costs, low process yields, and inferior product quality. Dynamic Feed Control is a major revolutionary concept — an innovative industrial process that opens up new potential opportunities in the manufacture of plastic parts. With the cooperation of industry, government, and academia, this concept has been implemented and validated for the design and production of high quality, molded plastic parts.

The invention utilizes multiple valves in the feed system of a mold to selectively regulate the flow to each area of the cavity in response to real-time feedback from nearby cavity pressure transducers. A minimum complexity process model was identified for development of a closed-loop control strategy. Using an adaptive gain scheduling approach, the cavity pressures at each gate were controlled throughout the filling and packing stages. This enables the process dynamics to be modified so as to produce parts with the desired part properties without retooling mold steel.

The flexibility of the process was demonstrated by its ability to control flow rates and knit-line location in the filling stage as well as pack pressure and part dimensions in the packing stage. Moreover, the consistency of the proposed process was compared to the conventional process. This comparison was achieved by intentionally adding process noise in an experiment designed to simulate natural material, machine, and operator variation. Analysis of the experimental results showed an increase in the process capability, Cp, from 0.56 for the conventional molding process to 1.67 for Dynamic Feed Control.

Finally, a methodology (akin to axiomatic design) for the design of molded parts was introduced which leverages the degrees of freedom provided by Dynamic Feed Control. Product robustness was demonstrated by a test series based on the stochastic (probabilistic) distribution of material properties during the molding process. With this assumption, the production yield of various design and processing strategies were evaluated. The methods presented here are directly extensible to arbitrarily complex applications with a broad range of properties, requirements, and specifications.

Dynamic Feed Control, First Generation: Multiple Tapered (Needle) Valves

Dynamic Feed Control - Second Generation (Kona/Dynisco/Synventive, 1998-2002)

The second generation of Dynamic Feed Control used a reverse tapered valve to provide melt decompression when closing the valve to throttle the melt flow. This design improved the control response time and stability to address the fundamental issues related to observability and controllability of injection molding, thereby providing the molder with a more flexible and consistent molding process. Improved control of melt delivery can enhance productivity through higher number of cavities per mold, increased use of family and modular molds, and greater mold portability for production according to economic and production needs. Molding studies and results were presented for each of these productivity improvements, and substantiate how increased control in plastic melt delivery increases productivity.

While the benefits of increased melt delivery control are impressive, the justification of technology investment differs with each application, its requirements, supply chain, and manufacturing strategy. Quality and economic expectations should be carefully considered during application development.

Dynamic Feed Control, Second Generation: Reverse Tapered Melt Valve

Dynamic Feed Control, Third Generation (2003-2005, UMass Lowell/MoldMasters)

This research was motivated to provide a simple device for regulating the melt pressure in a polymer process operation. While extruders and injection molding machines are highly efficient for plasticizing and pressurizing melt for subsequent forming, uncontrolled pressure fluctuations can limit the consistency of the molded products. In response, practitioners are increasingly utilizing statistical analysis and auxiliary systems such as gear pumps and dynamic seals to improve process performance. Even so, each of these approaches have fundamental issues related to purchase cost, reliability, and support.

Previous research into the controllability of injection molding led to the development of a system for dynamic actuation of multiple valve pins in a hot runner system, thereby achieving individual and dynamic control of the melt pressure at each gate. If dynamic control of the polymer melt is to become common, it is necessary to design more compact valves that have improved dynamic response with lower actuation forces. Other important objectives include ease of use, ease of maintenance, and positive shut-off of the molten plastic at the gate as in a conventional valve gated hot runner system.
Recognizing these objectives, a self-regulating valve design is presented as shown at right. In application, polymer melt is delivered to the valve such that the pressure at the valve inlet is greater than the desired pressure at the valve outlet. The valve houses a valve pin that has an aperture for communicating the polymer melt from the inlet of the valve to the outlet of the valve. The valve pin is designed such that the melt pressure at the outlet of the valve acts on the exposed surface of the valve pin, and generates a force, Fpressure, that is proportional to the melt pressure at the valve outlet. The pressure force will act to close the valve and thereby reduce the pressure at the valve outlet. The valve also requires the use of a compressive control force, Fcontrol, that is applied to the valve pin which will act to open the valve and increase the pressure at the valve outlet. If the melt pressure force and the control force are not equal, then the valve pin will tend to move in a direction that corrects the imbalance. For example, if the control force exceeds the pressure force, then the valve pin will move to open the aperture and thereby transmit additional melt pressure to the valve outlet. The valve pin position will be continually and automatically moved until the melt pressure naturally adjusts such that the control force and the pressure force balance. In this way, the valve is self-regulating such that the pressure force equals the control force; no sensors or external corrective control signal is required to deliver the desired melt pressure.

The control force may be applied to the valve pin in a number of ways, including springs, pneumatic actuators, hydraulic actuators, electric actuators, and others. Given that the actuator is matched with a valve, an intensification ratio may be utilized to relate the control signal to the outlet melt pressure. For example, a valve with a 5 mm valve pin diameter may be utilized with a 50 mm pneumatic cylinder diameter. In this design, the pneumatic cylinder has a surface area one hundred times greater than the valve pin. This difference in the “push areas” leads to an intensification ratio similar to the melt pressure exerted by the injection cylinder on a hydraulic molding machine. If a pneumatic supply valve provides 0-1 MPa pneumatic pressure corresponding to a 0 to 10 V control signal, then a 10 V control signal would correspond to a 100 MPa pressure at the valve outlet; other intermediate voltages between 0 and 10 V would proportionally provide between 0 and 100 MPa pressure at the valve outlet. Higher melt pressures, if desired, can be achieved by utilizing a higher pressure hydraulic supply (hydraulic pressure is readily available to 20 MPa) or by utilizing a larger pneumatic cylinder with the same valve pin.

Dynamic Feed Control, Third Generation: Open Loop Valve Design

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