Injection velocity control on a plastic injection molding machine

The injection velocity has always been considered the most important parameter during the filling phase of injection molding. Different dynamic characteristics of injection velocity are obtained when variations in melt temperatures and materials occur which can affect the plastic part quality. To address this problem a simplified predictive control (SPC) scheme was adopted for its effectiveness and ease of tuning. Initial investigations commenced on implementing an SPC position controller on a DC motor resulting in tight tracking of various position setpoint profiles. A user friendly interface was then developed to interface to a 150 tonne injection molding machine (IMM) in order to conduct open loop testing, setpoint profile generation and real time control. Several open loop tests were conducted using single and multi-input analog voltages to the hydraulic servo-valve. Time series models were derived from the experimental data in order to conduct control simulations and tuning. The results of these simulations indicate that an SPC controller to control injection velocity via position feedback provided good control performance in the form of relatively low IAE values in the order of 195 or less. The control structure was then made generic for use on the IMM since injection molding machines which can have various capacities in order to manufacture different plastic parts. Also, other plastic materials and processing conditions can be applied on any machine. The SPC controller was thoroughly tested on the 150 tonne IMM to track several velocity setpoint profiles with good results. The research demonstrates that predictive control can be readily applied to injection molding as a replacement of conventional ND controllers due to its ease of tuning.