Water-Assisted Injection Molding System Based on Water Hydraulic Proportional Control Technique Abstract: Water-assisted injection molding(WAIM), an innovative process to mold plastic parts with hollow sections, is characterized with intermittent, periodic process and large pressure and flow rate variation. Energy savings and injection pressure control can not be attained based on conventional valve control system. Moreover, the injection water can not be supplied directly by water hydraulic proportional control system. Poor efficiency and control performance are presented by current trial systems, which pressurize injection water by compressed air. In this paper, a novel water hydraulic system is developed applying an accumulator for energy saving. And a new differential pressure control method is proposed by using pressure cylinder and water hydraulic proportional pressure relief valve for back pressure control. Aiming at design of linear controller for injection water pressure regulation, a linear load model is approximately built through computational fluid dynamics(CFD) simulation on two-phase flow cavity filling process with variable temperature and viscosity, and a linear model of pressure control system is built with the load model and linearization of water hydraulic components. According to the simulation, model based feedback is brought forward to compensate the pressure decrease during accumulator discharge and eliminate the derivative element of the system. Meanwhile, the steady-state error can be reduced and the capacity of resisting disturbance can be enhanced, by closed-loop control of load pressure with integral compensation. Through the developed experimental system in the State Key Lab of Fluid Power Transmission and Control, Zhejiang University, China, the static characteristic of the water hydraulic proportional relief valve was tested and output pressure control of the system in Acrylonitrile Butadiene Styrene(ABS) parts molding experiments was also studied. The experiment results show that the dead band and hysteresis of the water hydraulic proportional pressure relief valve are large, but the control precision and linearity can be improved with feed-forward compensation. With the experimental results of injection water pressure control, the applicability of this WAIM system and the effect of its linear controller are verified. The novel proposed process of WAIM pressure control and study on characteristics of control system contribute to the application of water hydraulic proportional control and WAIM technology.70365

Key words: water-assisted injection molding, water hydraulics, proportional pressure control, linear control, load characteristic

1 Introduction

Water-assisted injection molding(WAIM), generally similar to gas-assisted injection molding(GAIM), is the newest way to mold hollow or partly hollow plastic    parts.

Water   can  be  supplied  by  water  hydraulic  system   and

of elongated parts, for example, automotive industry, white goods, office equipment, medical products and consumer goods, etc[4].

However, the WAIM process is complicated because there are many factors influencing molding pattern and quality. The load characteristic is coupling with pressure and flow. And it is necessary to control the pressure, flow

[5–6]

injected into the molten regions in mould. Then cavities are

rate and temperature of injection water

. In addition, the

generated and polymer melt is displaced to mold a hollow core. So the substitution of filtered water for pure  nitrogen

temperature   and   filling   percent   of   polymer   melt and

temperature  of  mould  need  to  be  regulated  by injection

[6–9]

used  in  GAIM  brings  many  advantages,  such  as    short