The proportional pressure control of water hydraulic system for water-assisted injection molding (WAIM) is investigated through modeling and simulation. WAIM is an innovative process to mold plastic parts with hollow sections. A water hydraulic system is designed for WAIM and a differential pressure control strategy is provided to regulate the water injection pressure by a proportional relief valve. An accumulator and a pressure cylinder are applied in the system and modeled in MATLAB. The load characteristic is analyzed and the coupling relationship between load pressure and flow rate is approximately simulated. Based on the variable-parameter model with linearization, open-loop and closed-loop control with multi-point feedback are investigated. Integral compensation and PI control are used with slope control of injection pressure. In addition, the influence of load characteristic is also investigated. Results of the simulation show that the closed-loop control contributes to the stability and disturbance resistance performance under various load characteristics.

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