In process monitoring - as the name suggests - injection moulding processes are monitored on the basis of relevant process variables. Since the quality of the injection molded parts produced is not determined in the injection molding machine, but in the cavity of a mold, the cavity pressure and the cavity wall temperature in the cavity are the basis for the detection of good and bad parts. Specific characteristics of these signals correlate with the properties of the manufactured parts. Unfilled parts are detected in this way, as are deviations in mold temperature or differences in flow behavior.
As soon as the monitoring limits of these functions are violated in a validated process, this is automatically detected and forwarded in the form of a digital switching signal to the machine or peripheral devices for sorting out the bad parts.
Open-Loop Process Control
In open-loop process control we speak of an open loop. Here, parameters such as the switchover pressure to the holding pressure phase or the time recognition of the melt front are sent to the machine control system in the form of a switching signal in quasi real time.
This switching signal is transmitted during the current cycle, with no feedback for further process optimization. Not every switching signal provides the same effect. Switching over to holding pressure at a fixed pressure value may well lead to poorer quality under different process conditions. An automatic switchover to holding pressure based on the detection of the melt front is always carried out at the same filling level of the cavity, regardless of the viscosity of the melt and the filling speed.
Compared to closed-loop process control, the principle of open-loop process control is simpler and can be implemented with a much smaller system infrastructure.
Closed-Loop Process Control
In general, a closed control loop is used to bring a given physical quantity to a desired setpoint and to maintain it there by measuring and readjusting the actual value.
In closed-loop process control we speak of a closed control loop.
Compared to open-loop process control, the closed-loop process control parameters are not processed in real time but adaptively. This means that the parameters are not calculated until the end of the current cycle and are transferred to the machine control system via a host computer interface. This way, the new setting values of the injection molding machine for the following cycle are communicated via a protocol. In the process, setting profiles are transmitted such as for the injection speed, holding pressure, cylinder or hot runner temperatures or connected temperature control units.
Compared to open-loop process control, closed-loop process control takes a more holistic approach. Closed-loop process control permits much deeper intervention in the injection molding process, and considerably more parameters can be influenced. However, closed-loop process control also requires a much larger infrastructure.